Test 3
Cardiac cycle Aortic valve Phase 1
Closed
Cardiac cycle Mitral valve Phase 3
Closed
Z disc
Coin-shaped protein structure that anchors the thin filaments and marks the ends of a sarcomere.
Systole
Contraction of the heart chambers. Ventricular systole is contraction of the ventricles. Atrial systole is contraction of the atria. A part of the cardiac cycle.
the ventricular filling phase. Pulmonary valve
closed the semilunar valves are closed until pressure in the ventricle exceeds the pressure in the aorta and pulmonary trunk.
the ventricular filling phase. aortic valve
closed the semilunar valves are closed until pressure in the ventricle exceeds the pressure in the aorta and pulmonary trunk.
This depolarization is called an
end plate potential and represents a small voltage change that only occurs at the junctional folds.
twitches can be
isometric, in which case the muscle generates force but does not shorten or isotonic, in which case the muscle shortens
A reduction in the number of active crossbridges is responsible for a decrease in force-generating capacity of a muscle fiber that is significantly (longer/shorter)than its optimum length.
longer
Calcium ions released from the terminal cisternae bind to troponin, causing a
shape change that pulls tropomyosin away from the myosin binding sites on actin
The reduced level of calcium ions causes troponin to return to its original shape, causing
tropomyosin to glide over and cover the myosin-binding sites on actin.
Troponin holds tropomyosin in place over the binding sites. Calcium binding to troponin causes
tropomyosin to shift away from the binding sites.
Each actin has a binding site for the myosin heads, but in the absence of calcium ions, these sites are covered by
tropomyosin.
When the muscle is not being stimulated to contract, these binding sites are covered by
tropomyosin.
In skeletal muscle, when calcium is released from the sarcoplasmic reticulum, it binds to (troponin/tropomyosin) to initiate the crossbridge cycle.
troponin
Which protein is telling the truth? Select the truth teller. troponin: I am the regulatory protein that calcium binds to. tropomyosin: No, I am the regulatory protein that calcium binds to.
troponin Hallelujah! You can see the truth. I am Troponin, the regulatory protein that calcium ions bind to. When calcium ions bind to me, I change shape, causing tropomyosin to shift, and exposing the myosin binding sites on actin.
Calcium ions released from the terminal cisternae bind to
troponin.
They prevent the backflow of blood into chambers that have recently
emptied.
I band during contraction
As the thin filaments slide inward over the thick filaments, the I zone gets narrower and may even disappear.
two types of isotonic contractions are possible
1) concentric 2) eccentric
The darker central areas of the sarcomere are the
A bands.
Action potential
A long distance electrical signal transmitted along an axon also called a nerve impulse
Voltage gated proteins
A protein that is activated in response to reaching a threshold change in the membrane potential.
motor neurons
A single nerve cell that extends from the brain or spinal cord to a muscle or gland and carries movement instructions to muscle.
Acetylcholine receptors
A type of chemically gated ion channel located on the junctional folds of the neuromuscular junction Chemically-gated ion channel on the junctional folds to which the neurotransmitter binds
Membrane potential
A voltage difference across the plasma membrane created by a difference in ion distribution.
Ventricular filling
As the AV valves open at point (e), ventricular filling starts. The initial rapid filling is mainly augmented by ventricular suction which results from ventricular untwisting and the return of each ventricular muscle fiber to its slack length.
Cross bridge cycling continues as long as the myosin-binding sites on actin are exposed and
ATP is available.
One myosin binding site binds to
ATP, which will supply the energy used for muscle contraction.
The electrical impulse leaves the AV node via the
AV bundle and rapidly propagates through the bundle branches.
H zone during contraction
As the thin filaments slide inward over the thick filaments, the H zone gets narrower and may even disappear.
Ca2+ channels
Allow ions that trigger neurotransmitter release to diffuse into the axon terminal
ECG
An electrocardiogram (ECG) is a recording of the heart's electrical activity.
H zone
Area at the center of the A band of a sarcomere where only myosin is present.
I bands
Area of the sarcomere where the thick filament is absent.
AV bundle
Atrioventricular bundle. Impulse-conducting structure located in the superior part of the interventricular septum. The only electrical connection between atria and ventricles. Also called bundle of His.
AV node
Atrioventricular node. Mass of pacemaker cells located in the inferior portion of the interatrial septum above the tricuspid valve. Action potentials are briefly delayed here, allowing atria to finish contracting before ventricular systole.
myosin binding site
Attachment site for myosin during muscle contraction
in the first step the action potential arrives at the
Axon terminal
tropomyosin
Blocks myosin binding sites on actin when muscle fiber is at rest
Electrical impulse
Brief, large, all-or-none change in membrane potential that occurs in muscle and nerve cells. Regenerative signals that can propagate long distances along the membrane of a cell. Also called impulses or spikes.
Impulse
Brief, large, all-or-none change in membrane potential that occurs in muscle and nerve cells. Regenerative signals that can propagate long distances along the membrane of a cell. Also called impulses or spikes.
action potential
Brief, large, all-or-none change in membrane potential that occurs in muscle and nerve cells. Regenerative signals that can propagate long distances along the membrane of a cell. Also called impulses or spikes.
Cardiac cycle Mitral valve Phase 2
C
Cardiac cycle Mitral valve Phase 4
C
Gap junctions
Cell membrane structure connecting the cytoplasm of adjacent cells via channels called connexons. Gap junctions electrically couple cells by allowing intracellular ions to pass back and forth.
depolarization
Change in membrane potential such that the cell interior becomes less negative (more positive). Characteristic of the initial phase of an action potential.
Repolarize
Change in membrane potential toward its resting value from a more depolarized potential. Characteristic of the later phase of an action potential.
Neurotransmitter
Chemical messengers usually released from synaptic vesicles in neuron axon terminals that communicate with target cells
Curare can be used as a skeletal muscle relaxant with general anesthesia for certain types of surgeries. Curare has also been used by South American native peoples as an arrow poison for hunting wild game.
Curare is a plant extract that can bind to acetylcholine receptors, but in a way that does not cause the chemically-gated channels to open.
Cardiac cycle Phase 1 Ventricle
D
Cardiac cycle Phase 2 Atrium
D
Cardiac cycle Phase 3 Atrium
D
Cardiac cycle Phase 4 Atrium
D
Cardiac cycle Phase 4 Ventricle
D
Cardiac cycle Phase 1 Atrium
D to S
M line
Dark band at the center of the sarcomere to which myosin filaments attach.
Terminal cisterns
End sacs of sarcoplasmic reticulum, where it is adjacent to a transverse tubule.
Excitation-Contraction Coupling
Excitation-contraction (E-C) coupling refers to the series of events that link the action potential (excitation) of the muscle cell membrane (the sarcolemma) to muscular contraction. Although E-C coupling in myocardium is similar in many ways to skeletal muscle and smooth muscle, there are also critical differences.
T tubules
Extensions of the muscle cell plasma membrane (sarcolemma) that protrude deeply into the muscle cell.
Internodal pathway
Groups of atrial cardiac cells that conduct action potentials from the SA node to the AV node.
Mechanism of how heart valves open and close
Heart valves are all one-way valves. The atrioventricular valves only open towards the ventricles, the semilunar valves only open toward the aorta and pulmonary trunks. These valves open and close as a result of pressure differences on opposite sides of the valve.
troponin
Holds tropomyosin over the myosin binding sites in the absence of calcium
When viewed with a microscope, the skeletal muscle tissue has a striped or striated appearance, with alternating light and dark bands. Let's examine what causes these stripes. The lightest areas are called
I bands
Botulinum toxin is used in a clinical setting to weaken muscle for treating spasms and neurological movement disorders. It is also used in cosmetic procedures to prevent wrinkles by paralyzing facial muscles.
It is a powerful neurotoxin produced by the bacterium Clostridium botulinum and related species. It works by preventing the release of acetylcholine from the axon terminal.
Aorta
Largest artery in the body; receives oxygen-rich blood from left ventricle.
Ventricle
Lower chamber of the heart that contracts to eject blood into pulmonary or systemic circulation.
Exocytosis
Mechanism by which substances are moved from the cell interior to the extracellular space as a secretory vesicle fuses with the plasma membrane
Synaptic vesicles
Membranous organelles containing neurotransmitter substances found within the axon terminals of neurons Membranous sacs in the axon terminal that contain the neurotransmitter acetylcholine
Contractile cells
Muscle cell containing the cellular components necessary to generate tension (i.e., contract). Also called myocardial cell. Compare with pacemaker cell.
Cardiac cells
Muscle cell of the heart. Cardiomyocyte.
Step three is cross bridge detachment.
Myosin remains attached to actin until an ATP molecule binds to the empty ATP-binding site on the myosin head. This weakens the link between the myosin head and actin, and the myosin head detaches. The detached myosin head remains in its low-energy state.
Neostigmine helps to improve muscle tone and strength. It is sometimes used after surgery to help reverse the effects of muscle relaxants.
Neostigmine and related drugs prevent the breakdown of acetylcholine by acetylcholinesterase.
You have learned how the activity of acetylcholine at the neuromuscular junction stimulates an action potential along the adjacent sarcolemma. But what causes this electrical signal for muscle contraction to stop?
Once signals along the motor neuron cease, additional acetylcholine will no longer be released into the synaptic cleft at the neuromuscular junction.
Bundle branches
One of two impulse-conducting structures in the interventricular septum. Connection between AV bundle and Purkinje fibers within the walls of the left or right ventricle.
Atrium
One of two superior chambers of the heart.
The process known as the cross bridge cycle consists of four steps:
One: cross bridge formation, two: the power stroke by the myosin head, three: cross bridge detachment, and four: reactivation of the myosin head.
Cardiac cycle Mitral valve Phase 1
Open
Cardiac cycle Mitral valve Phase 3
Open
calcium release channel
Opens when triggered by voltage-gated proteins on the T tubule
ATP
Organic molecule that stores and releases chemical energy for use in body cells.
Depolarization of the SA node occurs immediately before which segment of the ECG wave?
P wave The P wave occurs immediately after depolarization of the SA node, and is caused by depolarization of the atria.
Ventricular ejection
Phase of cardiac cycle when the ventricles are in systole, and blood is pumped into the systemic and pulmonary circuits.
Isovolumetric relaxation
Phase of cardiac cycle when ventricles are in diastole, but the volume of blood in the ventricles remains the same.
sarcolemma
Plasma membrane of the muscle fiber
Pulmonary and systemic circuits
Pulmonary circulation moves blood between the heart and the lungs. It transports deoxygenated blood to the lungs to absorb oxygen and release carbon dioxide. The oxygenated blood then flows back to the heart. Systemic circulation moves blood between the heart and the rest of the body.
Rapid conduction continues throughout the
Purkinje fibers, depolarizing the ventricles.
Contraction of the ventricles occurs as a result of the signal that causes which portion of the ECG?
Q-R-S complex The QRS complex is caused by ventricular depolarization, which stimulates ventricular contraction.
The repolarization of the atria occurs during which portion of the ECG?
Q-R-S complex The QRS complex is caused by ventricular depolarization, which stimulates ventricular contraction.
Repolarization of the atria normally coincides with the
QRS complex and is not visible.
junctional folds
Region of the muscle fiber membrane that forms part of the neuromuscular junction
Tropomyosin
Regulatory protein in the sarcomere; blocks myosin binding sites on actin.
Troponin
Regulatory protein in the sarcomere; holds tropomyosin in position on actin.
Diastole
Relaxation. Specifically, relaxation of the ventricles (ventricular diastole). Atrial diastole is relaxation of the atria. A part of the cardiac cycle.
Step Two is the power stroke.
Release of inorganic phosphate from the myosin head causes the head to pivot, pulling the thin filament toward the center of the sarcomere. ADP is then released as well. The myosin head is now in its low-energy state, but it remains attached to actin. The energy for the power stroke is ultimately provided by the ATP that binds to the myosin head.
motor neuron
Releases neurotransmitter to stimulate the muscle fiber
voltage gated Na+ channel
Responsible for conducting an action potential along the sarcolemma
Myofibril
Rodlike contractile elements that occupy most of the muscle cell volume.
Cardiac cycle Phase 2 Ventricle
S
Cardiac cycle Phase 3 Ventricle
S
During a normal heartbeat, action potentials originate in the pacemaker cells of the SA node.
SA node.
terminal cistern
Sac-like bulge of sarcoplasmic reticulum near the T tubule where calcium release channels are abundant
Sarcoplasmic reticulum
Specialized endoplasmic reticulum of muscle cells.
The AV node causes a delay in the signal between the atria and the ventricles. Select the section of the ECG that occurs as the signal travels through the AV node.
Segment between the P wave and Q-R-S complex The segment between the P wave and QRS complex represents the delay in conduction from the atria to the ventricles.
Cardiac cycle
Sequence of events in the heart that takes place from one heartbeat to the next.
SA node
Sinoatrial node. Mass of pacemaker cells located in the right atrial wall near the entrance of the superior vena cava. Normally sets pace for the entire heart.
Which channel is telling the truth? Select the truth teller. Sodium channel: I am the channel that's responsible for conduction of an action potential. Calcium channel: No, I am the channel that's responsible for conduction of an action potential.
Sodium channel You are perceptive and correct! I am the Sodium Channel who is responsible for conduction of an action potential by opening and allowing Na+sodium to diffuse into the muscle fiber.
Pacemaker cells
Specialized cardiac cells within the intrinsic conduction system that initiate impulses. Also called autorhythmic cells. Pacemaker cells within the SA node and AV node are also called nodal cells.
Crossbridge
Structure formed from the connection of myosin heads to binding sites on actin.
Intrinsic conduction system
System of pacemaker and conducting cells in the heart that generates and distributes action potentials. Stimulates the heart to contract.
The sarcoplasmic reticulum or SR extends outwards from the
T tubules forming a weblike network surrounding each myofibril.
A band during contraction
The A band remains the same size whether the muscle is contracted or relaxed because the length of the thick filament remains unchanged.
A bands
The area of the sarcomere where the thick filament is present.
blood flow through the heart
The blood enters the heart's right atrium and is pumped to your right ventricle, which in turn pumps the blood to your lungs. The pulmonary artery then carries the oxygen-poor blood from your heart to the lungs. Your lungs add oxygen to your blood.
on the other side of the synaptic cleft the junctional folds contain
acetylcholine receptors
thick filament
The contractile filament made up of myosin
thin filament
The contractile filament primarily made up of actin
Actin
The contractile protein of a muscle sarcomere that is the main component of the thin filaments of skeletal muscle myofibrils.
Myosin
The contractile protein of a muscle sarcomere that makes up the thick filament in skeletal muscle myofibrils.
Step One, cross bridge formation.
The cycle starts when excitation-contraction coupling causes the myosin-binding sites on actin to be exposed. Myosin heads bind to these sites, forming cross bridges. Myosin starts in its high-energy state, in which the ATP binding site on the myosin head is occupied by ADP and inorganic phosphate. The myosin head is ready to pivot for the power stroke.
which filament generates the force for muscle contraction.
The heads of the thick filaments contain ATP binding sites that power muscle contraction.
Sarcolemma
The plasma membrane of a muscle cell. the cell membrane surrounding the muscle fiber Plasma membrane of the muscle fiber
Depolarization
The reversal of the membrane potential due to more positive ions diffusing into the cell or less positive charge leaving the cell.
Sarcomere
The smallest contractile unit of muscle.
Sarcomeres
The smallest contractile unit of muscle.
Synaptic cleft
The space between the axon terminal and the membrane of the target cell Gap between the axon terminal and junctional folds
which filament slides in towards the center of the sarcomere during contraction.
The thin filaments move towards the M line during contraction.
PR interval
The time from the beginning of atrial depolarization to the beginning of ventricular depolarization.
Step four is the reactivation of the myosin head.
This occurs when the ATP bound to the myosin head undergoes hydrolysis to ADP and inorganic phosphate. The energy released during hydrolysis reactivates the myosin head, pivoting it back to its high-energy state.
P wave
Tracing on an ECG that results from depolarization of the atria.
QRS complex
Tracing on an ECG that results from ventricular depolarization.
voltage gated protein
Triggers the opening of the calcium release channel when stimulated by an action potential
Cross bridge cycling at all of these sarcomeres causes the entire
muscle to contract.
T tubules
Tube-like infoldings of the sarcolemma that conduct the action potential throughout the interior of the muscle fiber
Semilunar valves
Valves at entrance to aorta and pulmonary trunk; prevent backflow into ventricles. Includes the aortic and pulmonary valves.
Na+ channels
Voltage-gated ion channel on the sarcolemma
Isovolumetric
Volume remains the same.
sarcoplasmic reticulum
Web-like membranous network surrounding the myofibrils
To understand how acetylcholine activates the muscle fiber, we need to look at step 4, in which
acetylcholine binds to acetylcholine receptors
The signal for muscle contraction stops when the concentration of
acetylcholine in the synaptic cleft decreases.
calcium ions will cause vesicles to release
acetylcholine into the synaptic cleft through exocytosis
axon
a long thin cytoplasmic process that extends from a neurons cell body each neuron has a single axon which transmits action potentials
action potential
a long-distance electrical signal transmitted along an axon. also called a nerve impulse
As we saw in muscle excitation at the neuromuscular junction, a somatic motor neuron stimulates
a muscle fiber at the junctional folds initiating an action potential on the adjacent sarcolemma.
The thick filaments (myosin) are located in which region? a. A bands b. Z discs c. M lines d. I bands
a. A bands The A bands are the darker regions where the thick filaments are present.
Which of the following functions as the electrical connection between the atria and ventricles? a. AV bundle b. SA node c. AV node d. Purkinje fibers
a. AV bundle
What is the primary method for terminating the acetylcholine signal at the neuromuscular junction? a. Acetylcholine is degraded by the enzyme acetylcholinesterase. b. Acetylcholine combines with curare. c. Acetylcholine is reabsorbed into the axon terminal. d. Acetylcholine diffuses away from the synaptic cleft.
a. Acetylcholine is degraded by the enzyme acetylcholinesterase. The rapid breakdown of acetylcholine by the enzyme acetylcholinesterase quickly terminates the signal.
Add the proper substance from the supply room that will cause the regulatory proteins troponin and tropomyosin to uncover the myosin-binding sites. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
a. Ca2+ Calcium ions bind to troponin causing it to change shape and shifting tropomyosin off of the myosin-binding sites. Cross bridges can now form and contraction can proceed as usual.
Add the proper substance from the supply room that is released from the terminal cisterns of the sarcoplasmic reticulum. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
a. Ca2+ Calcium ions diffuse out of the terminal cisterns through the open channels. Now excitation-contraction coupling can occur!
Add the proper substance from the supply room that would cause the synaptic vesicles to undergo exocytosis. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
a. Ca2+ Calcium ions stimulate synaptic vesicles to undergo exocytosis. We're back in business!
What causes tropomyosin to cover the myosin-binding sites on actin? a. Removal of calcium ions from the sarcoplasm b. ATP binding to the myosin head c. Diffusion of sodium ions through voltage-gated channels d. Release of calcium ions from the terminal cisterns
a. Removal of calcium ions from the sarcoplasm Removal of calcium ions lowers the calcium ion concentration below the threshold for binding to troponin. Without adequate calcium binding, troponin shifts tropomyosin back over the myosin-binding sites.
Which event allows the formation of cross bridges by myosin? a. Shifting tropomyosin away from binding sites on actin b. Release of calcium from the terminal cisterns c. Sodium ions diffusing through voltage-gated channels d. Depolarization of the junctional folds
a. Shifting tropomyosin away from binding sites on actin Shifting tropomyosin away from binding sites allows for the formation of cross bridges.
Neostigmine and related drugs prevent the breakdown of acetylcholine by acetylcholinesterase. What happens when neostigmine and related drugs prevent the breakdown of acetylcholine by acetylcholinesterase? Make a prediction! a. The junctional folds will remain depolarized and the muscle fiber will remain contracted. b. The membrane potential of the junctional folds will remain unchanged, and the muscle fiber will not contract. c. The concentration of acetylcholine in the synaptic cleft will decrease, and the muscle fiber will relax.
a. The junctional folds will remain depolarized and the muscle fiber will remain contracted. Your prediction is correct! Neostigmine binds to acetylcholinesterase preventing the breakdown of acetylcholine. Consequently, released acetylcholine stays around the junctional folds longer and can activate the acetylcholine receptors multiple times, initiating several action potentials. As a result, the muscle fiber will remain contracted for a longer time.
How does the presence of pacemaker cells affect the electrical activity of the cardiac contractile cells? a. The pacemaker cells are able to stimulate action potentials in the cardiac contractile cells. b. The pacemaker cells increase the frequency of normally occurring action potentials in the cardiac contractile cells. c. The activity of the pacemaker cells cancels out the action potentials of the cardiac contractile cells.
a. The pacemaker cells are able to stimulate action potentials in the cardiac contractile cells.
Conduction of an action potential along the sarcolemma requires the opening of what type of ion channels? a. Voltage-gated sodium channels b. Calcium-release channels c. Chemically-gated calcium channels d. Chemically-gated sodium channels
a. Voltage-gated sodium channels The conduction of an action potential along the sarcolemma of the muscle fiber requires voltage-gated sodium channels to open.
Now that the synaptic vesicles have fused with the membrane of the axon terminal, what substance diffuses into the synaptic cleft? a. acetylcholine b. sodium ions c. calcium ions d. potassium ions
a. acetylcholine The synaptic vesicles release the neurotransmitter acetylcholine into the synaptic cleft.
Which ion diffuses out of the terminal cisterns for excitation-contraction to occur? a. calcium b. sodium c. potassium d. acetylcholine
a. calcium Calcium is the key to excitation-contraction coupling. You will learn how the calcium ions allow the sarcomere to contract on the next page.
What would happen to cross bridge cycling if a drug is present near the end of contraction that prevents calcium pump activity from returning calcium ions to the sarcoplasmic reticulum but does not affect the production of ATP? a. cross bridge cycling continues and the muscle remains contracting b. cross bridge cycling stops and the muscle relaxes c. cross bridge cycling stops but the cross bridges remain attached
a. cross bridge cycling continues and the muscle remains contracting High calcium ion concentration and normal ATP production results in the continuation of cross bridge cycling, and the muscle remains contracting.
Binding of acetylcholine to the acetylcholine receptors causes these chemically-gated ion channels to a. open b. close c. move inside the junctional folds
a. open Binding of acetylcholine to its receptor opens these chemically-gated channels.
What kind of membrane potential does the monitor record when the electrode is in the contractile cells in Dish 2? a. resting membrane potential b. rhythmic action potentials c. single action potential
a. resting membrane potential Notice that the membrane potential remains constant in the absence of stimulation.
Most of it is broken down by the enzyme
acetylcholinesterase which is located in the synaptic cleft. However, some acetylcholine just diffuses away from the synaptic cleft.
Most acetylcholine is broken down by
acetylcholinesterase; some diffuses away from the cleft.
Now, sodium ions can no longer diffuse
across the junctional folds
A sarcomere shortens when myosin heads on the thick filaments form cross bridges and pull on the
actin molecules in the thin filaments.
Tropomyosin blocks the binding sites on
actin when the muscle is not stimulated.
thin filaments are composed of three types of molecules:
actin, tropomyosin, and troponin.
Cross bridge cycling is the sequence of events where the myosin of the thick filaments pulls on
actin, which slides the thin filaments toward the center of the sarcomere, and results in the shortening of the sarcomere.
When voltage gated channels open, sodium ions diffuse inward, depolarizing the adjacent membrane and triggering an
action potential in this new segment.
Voltage-gated sodium channels in the sarcolemma conduct the
action potential over the whole muscle fiber.
The diffusion of sodium ions initiates an
action potential, which spreads outwards from the neuromuscular junction in all directions along the sarcolemma.
in cardiac muscle, contractions are triggered by
action potentials initiated in pacemaker cells
when a motor neuron fires an action potential
all fibers in the motor unit contract together
Voltage-gated sodium channels in the T tubules allow them to conduct the action potential throughout the muscle fiber interior, ensuring that
all the myofibrils contract simultaneously.
A sarcomere consists of
alternating thick and thin filaments.
Finally, in step 6, the depolarization of the junctional folds triggers
an action potential on the adjacent sarcolemma that spreads outward from the neuromuscular junction in all directions.
The end plate potential triggers the opening of voltage-gated sodium channels on the adjacent sarcolemma, resulting in
an action potential, which propagates in all directions across the sarcolemma of the muscle fiber, causing muscle contraction.
The sarcomere is now ready for the next stimulation at the neuromuscular junction to trigger
another muscle contraction.
muscle excitation at the neuromuscular junction
any time you move your body your brain or spinal cord sends electrical impulses called action potentials along the axon of a motor neuron to the muscle fiber
The result is a coordinated contraction of the heart muscle, with first the
atria and then the ventricles contracting.
These waveforms of the ECG precede the corresponding contraction or relaxation of the
atria and ventricles.
At the AV node, where cells are small and have few gap junctions, conduction of the impulse is slowed as
atrial contraction proceeds.
The P wave results from
atrial depolarization.
smooth muscle is regulated by
autonomic neurons, sympathetic neurons, parasympathetic neurons, or both, the effect may be excitatory or inhibitory
After a brief period of time, acetylcholine that was already present at the neuromuscular junction diffuses
away from its receptor site, and the ion channels close.
Which of the following is a valid generalization regarding the properties of smooth muscle? a) NT's can either excite or inhibit smooth muscle contraction but any given NT is always excitatory or inhibitory, regardless of where it is located. b) A given smooth muscle cell can respond to more than one type of NT . c) Smooth muscle cells are generally unresponsive to NT's of all types. d) Smooth muscle cells can respond to neural input from the somatic or autonomic nervous systems.
b) A given smooth muscle cell can respond to more than one type of NT .
Which of the following statements concerning the characteristics of different types of muscle fibers is false? a) The higher the myosin ATPase activity, the faster the speed of contraction. b) Muscles that have high glycolytic capacity and large glycogen stores are more resistant to fatigue. c) Oxidative types of muscle fibers contain myoglobin. d) Oxidative fibers have a richer blood supply. e) Larger-diameter fibers can produce greater tension.
b) Muscles that have high glycolytic capacity and large glycogen stores are more resistant to fatigue.
Which of the following is the first step in cross bridge cycling, once the myosin-binding sites are exposed? a. Cross bridge detachment b. Cross bridge attachment c. Cross bridge power stroke d. Myosin head reactivation
b. Cross bridge attachment Cross bridges attach myosin to actin once the myosin sites are exposed.
Which ion diffuses into the muscle fiber through the open acetylcholine receptor? a. Ca2+ b. Na+ c. K+
b. Na+ see image Opening the acetylcholine-gated receptor channel enables sodium ions to diffuse into the muscle fiber.
Which part of the intrinsic conduction system initiates the depolarizing impulse and normally sets the pace for the entire heart? a. AV bundle b. SA node c. Purkinje fibers d. AV node
b. SA node
Botulinum toxin prevents the release of acetylcholine from the axon terminal. What happens when botulinum toxin prevents the release of acetylcholine from the axon terminal? Make a prediction! a. Depolarization of the junctional folds will initiate an action potential in the sarcolemma and the muscle fiber will contract. b. The membrane potential of the junctional folds will remain unchanged and the muscle fiber will not contract. c. The concentration of acetylcholine in the synaptic cleft will increase and the muscle fiber will remain contracted.
b. The membrane potential of the junctional folds will remain unchanged and the muscle fiber will not contract. Your prediction is correct! The botulinum toxin prevents the release of acetylcholine. Therefore, the junctional folds are not stimulated, the membrane potential remains unchanged, and the muscle fiber will not contract.
Curare is a plant extract that can bind to acetylcholine receptors, but in a way that does not cause the chemically-gated channels to open. What happens when an action potential reaches the axon terminal if curare is present? Make a prediction! a. Depolarization of the junctional folds will initiate an action potential in the sarcolemma, and the muscle fiber will contract. b. The membrane potential of the junctional folds will remain unchanged, and the muscle fiber will not contract. c. Acetylcholine will not be broken down by acetylcholinesterase, and the muscle fiber will not contract.
b. The membrane potential of the junctional folds will remain unchanged, and the muscle fiber will not contract. Your prediction is correct! Curare binds to the acetylcholine receptor without opening the chemically-gated ion channels. Therefore, the membrane potential remains unchanged and the muscle is prevented from contracting.
What effect does the diffusion of sodium ions through the chemically-gated ion channels have on the membrane potential across the junctional folds? a. The voltage becomes larger (more negative on the inner surface). b. The voltage becomes smaller (less negative on the inner surface). c. The voltage does not change because calcium ions are also diffusing inward.
b. The voltage becomes smaller (less negative on the inner surface). Diffusion of sodium ions into the muscle fiber causes partial depolarization of the junctional folds.
What would happen to cross bridge cycling if a drug is present near the end of contraction that binds free calcium ions but doesn't affect ATP concentration? a. cross bridge cycling continues and the muscle remains contracting b. cross bridge cycling stops and the muscle relaxes c. cross bridge cycling stops but the cross bridges remain attached
b. cross bridge cycling stops and the muscle relaxes Cross bridge cycling will stop and the muscle will relax because without calcium ions, tropomyosin covers myosin-binding sites, preventing further cross bridge formation.
What causes the indicated sodium channels along the sarcolemma to open? a. contact with voltage-gated proteins b. depolarization reaches their threshold value c. binding to calcium ions d. depolarization of the T tubules
b. depolarization reaches their threshold value Local depolarization of the sarcolemma stimulates adjacent voltage-gated sodium channels to open. This allows the action potential to spread across the surface of the muscle fiber.
What kind of membrane potential does the monitor record when the electrode is in the pacemaker cells in Dish 1? a. resting membrane potential b. rhythmic action potentials c. single action potential
b. rhythmic action potentials Notice that the membrane potential does not rest; rather, it rhythmically depolarizes and repolarizes. Each of these spikes is an action potential.
What kind of membrane potential does the monitor record when the electrode is in the contractile cells in Dish 3? a. resting membrane potential b. rhythmic action potentials c. single action potential
b. rhythmic action potentials The contractile cells now fire rhythmic action potentials because they are stimulated by the pacemaker cells.
What kind of membrane potential does the monitor record when the electrode is in the pacemaker cells in Dish 3? a. resting membrane potential b. rhythmic action potentials c. single action potential
b. rhythmic action potentials The pacemaker cells fire rhythmic action potentials, just as they did on their own.
First, the "hinge" portion of the tail allows the myosin head to reach the
binding sites on actin.
during ventricular filling
blood from the pulmonary and systemic circuits flows passively into each atrium, through the open atrioventricular valves, and then into the corresponding ventricles. During this process, the semilunar valves are closed. Atrial systole finishes ventricular filling.
1. When a muscle cell is relaxed and intracellularATP levels are normal, a crossbridge will remain in which of the following states? a) Bound to actin and in the low-energy form b) Bound to actin and in the high-energy form c) In the high-energy form, with ADP and Pi bound to it d) In the high-energy form, with ATP bound to it e) In the low-energy form with nothing bound to it
c) In the high-energy form, with ADP and Pi bound to it
Which of the following is not a determinant of whole muscle tension? a) The number of muscle fibers contracting b) The tension produced by each contracting fiber c) The proportion of each motor unit that is contracting at any given time d) The extent of fatigue e) The frequency of action potentials in the motor neurons
c) The proportion of each motor unit that is contracting at any given time
During a muscle contraction, which of the following does not change length? a) The distance between Z lines b) The width of I bands c) The width of A bands d) None of the above
c) The width of A bands
Which of the following causes the cross bridge to detach from the thin filament (actin)? a. Calcium ion binding to troponin b. Hydrolysis of ATP to ADP and inorganic phosphate c. ATP binding to the myosin head d. ADP and inorganic phosphate being released from the myosin head
c. ATP binding to the myosin head ATP binding to the myosin head causes the cross bridge to detach from actin.
Which part of a sarcomere is connected to the Z discs? a. Myosin b. Troponin c. Actin d. Tropomyosin
c. Actin The actin of the thin filaments are connected to the Z discs at the ends of the sarcomere.
What occurs after movement of tropomyosin exposes the binding sites? a. Thin filaments enlarge to become thick filaments. b. Binding of calcium ions to troponin. c. Cross bridges form between the myosin heads and the binding sites. d. Tropomyosin connects actin of the thin filament to myosin of the thick filament.
c. Cross bridges form between the myosin heads and the binding sites. Cross bridges can form between the thick filament and thin filament when the binding sites are exposed.
Contraction of the atria is triggered by electrical activity that is recorded on the ECG tracing as the __________. a. T wave b. QRS complex c. P wave d. A wave
c. P wave
Which one of the following components conducts an action potential throughout the interior of the muscle fiber? a. Myosin b. Actin c. T tubule d. Sarcoplasmic reticulum
c. T tubule T tubules conduct the action potential throughout the interior of the muscle fiber.
Predict what will change as the action potential travels along the axon before it reaches the axon terminal. a. The axon terminal moves into contact with the muscle fiber b. The acetylcholine receptors open c. The axon depolarizes, and the charge on the inside becomes positive
c. The axon depolarizes, and the charge on the inside becomes positive
Why is it important that the AV node delays depolarization? a. The delay allows the ventricles to finish contracting before the atria begin to contract. b. The delay triggers depolarization of the ICS. c. The delay allows the atria to finish contracting before the ventricles begin to contract. d. The delay allows the atria to fill with blood before contraction.
c. The delay allows the atria to finish contracting before the ventricles begin to contract. The smaller cells of the AV node have fewer gap junctions, slowing down the impulse and giving the atria time to contract.
How does the presence of pacemaker cells affect the electrical activity of the cardiac contractile cells? a. The pacemaker cells increase the frequency of normally occurring action potentials in the cardiac contractile cells. b. The activity of the pacemaker cells cancels out the action potentials of the cardiac contractile cells. c. The pacemaker cells are able to stimulate action potentials in the cardiac contractile cells.
c. The pacemaker cells are able to stimulate action potentials in the cardiac contractile cells. Without the pacemaker cells there would not be a stimulus for generating action potentials in the contractile cells, and the heart would not contract.
Calcium ions bind to which regulatory protein during excitation-contraction coupling? a. Myosin b. Actin c. Troponin d. Tropomyosin
c. Troponin Troponin is the regulatory protein of the sarcomere to which calcium ions bind during excitation-contraction coupling.
An action potential at the axon terminal of a motor neuron opens what type of ion channels? a. Voltage-gated sodium channels b. Acetylcholine receptors c. Voltage-gated calcium channels d. Chemically-gated potassium channels
c. Voltage-gated calcium channels When the action potential reaches the axon terminal, this causes the opening of calcium channels.
What would happen to cross bridge cycling if a drug is present near the end of contraction that releases calcium ions from the SR but prevents the production of ATP? a. cross bridge cycling continues and the muscle remains contracting b. cross bridge cycling stops and the muscle relaxes c. cross bridge cycling stops but the cross bridges remain attached
c. cross bridge cycling stops but the cross bridges remain attached Cross bridge cycling would stop and the cross bridges would remain attached in a rigid state because cross bridge detachment cannot occur without ATP. This is called rigor mortis.
During ventricular filling, pressure in the left ventricle should be _____ aortic pressure. a. greater than b. the same as c. less than
c. less than Because pressure in the ventricle is less than the pressure in the aorta, the aortic valve remains closed.
The specialized cardiac cells of the SA node are most precisely known as __________. a. intrinsic cells b. atrial cells c. pacemaker cells d. contractile cells
c. pacemaker cells
What kind of membrane potential does the monitor record when you stimulate the contractile cells in Dish 2? a. resting membrane potential b. rhythmic action potentials c. single action potential
c. single action potential An action potential is generated when the contractile cells are stimulated.
Together, the voltage-gated proteins and the calcium release channels control the release of
calcium into the sarcoplasm.
Which ion is telling the truth? Select the truth teller. sodium ion: I am the ion that's responsible for coupling the action potential signal to contraction of a muscle fiber. calcium ion: No, I am the ion that's responsible for coupling the action potential signal to contraction of a muscle fiber.
calcium ion Right! You are not easily fooled. I am the Calcium Ion who is responsible for coupling the shift from an action potential signal to contraction of a muscle fiber when I bind to the regulatory protein.
muscle action potential causes the release of
calcium ions from the sarcoplasmic reticulum.
In addition to voltage-gated sodium channels, the T tubule membrane has voltage-gated proteins that interact physically with
calcium-release channels in the membrane of the terminal cisternae.
neuron
cell of the nervous system specialized to generate and transmit electrical signals or action potentials
According to the sliding filament mechanism, a muscle contracts because the thin filaments slide past the thick filaments toward the
center of the sarcomere from both sides.
The thick filaments are anchored to the M line, which marks the
center of the sarcomere.
These voltage-gated proteins respond to depolarization by
changing shape opening the calcium release channels.
neurotransmitter
chemical messengers usually released from synaptic vesicles in neuron axon terminals that communicate with target cells
The left and right atria contract to
complete filling of their corresponding ventricular chamber.
Alterations in the heart rhythm, changes in the QRS complex, or an abnormal PR interval may indicate problems with the
conduction system.
The impulse spreads across the atria both via the internodal pathway and through gap junctions between
contractile cells.
The thin filaments consist primarily of the
contractile protein actin.
The thick filaments are made of the
contractile protein myosin.
The cycle alternates between periods of
contraction (or systole) and relaxation (or diastole) of the atria and ventricles.
By understanding how one sarcomere shortens, we can understand how the simultaneous contraction of thousands of sarcomeres along hundreds of myofibrils leads to
contraction of one muscle fiber.
Which of the following would tend to reduce the concentration of lactic acid that accumulates in a muscle cell as a result of contractile activity? a) Increasing the concentration of glycolytic enzymes b) Decreasing the oxygen supply to the cell c) Increasing the diameter of the cell d) Increasing the number of mitochondria in the cell e) All of the above
d) Increasing the number of mitochondria in the cell
During contraction of a skeletal muscle fiber, a) The thick filaments contract. b) The thin filaments contract. c) The A band becomes shorter. d) The I band becomes shorter. e) All of the above.
d) The I band becomes shorter.
As a result calcium ions are responsible for
coupling electrical excitation to the contraction of skeletal muscle fibers.
muscles rely on
creatine phosphate as their initial source of ATP
But not all myosin heads are bound or disconnected at the same time. They can be in different steps of the
cross bridge cycle.
Muscle contraction can now occur through
cross bridge cycling.
Once the binding sites are exposed, the myosin heads can bind to them, forming
cross bridges between the thick and thin filaments.
The thin filaments are pulled by the myosin heads of the thick filaments, which continuously form and break thousands of
cross bridges with the actin molecules of the thin filaments.
this sliding is driven by the
crossbridge cycle, in which the motion of crossbridges is coupled to their cycle binding and unbinding to actin molecules in adjacent thin filaments
Which of the following is true for the excitation-contraction coupling of all muscle types (skeletal, cardiac, and smooth)? a) An action potential causes calcium levels in the cytosol to increase. b) Calcium binds to troponin. c) Thick and thin filaments slide past each other. d) Both a and c. e) All of the above.
d) Both a and c.
Add the proper substance from the supply room that will allow for cross bridge detachment and continuation of cross bridge cycling. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
d. ATP ATP is the energy source that powers cross bridge cycles and allows the myosin heads to detach from the actin to start a new cross bridge cycle. You're good to go!
Which of the following characteristics of the intrinsic conduction system best explains why atrial contraction finishes before ventricular contraction begins? a. Pacemaker cells are autorhythmic. b. Purkinje fibers have the slowest rate of spontaneous depolarization. c. The SA node initiates electrical impulses. d. Action potentials are delayed at the AV node.
d. Action potentials are delayed at the AV node.
How does neurotransmitter binding to it's receptor activate a muscle fiber? a. It causes exocytosis of synaptic vesicles from the axon terminal. b. It opens voltage-gated channels that generate an action potential in the sarcolemma. c. It opens voltage-gated channels that allow calcium ions to diffuse into the axon terminal. d. It opens chemically-gated ion channels that allow sodium ions to diffuse into the junctional folds.
d. It opens chemically-gated ion channels that allow sodium ions to diffuse into the junctional folds. Diffusion of sodium ions changes the membrane potential of the junctional folds.
Which protein forms the thick filaments in a sarcomere? a. Actin b. Tropomyosin c. Troponin d. Myosin
d. Myosin Myosin is the protein that forms the thick filaments in a sarcomere.
Which of the following slide(s) inwards toward the M line during muscle contraction? a. Myosin b. A band c. Thick filaments d. Thin filaments
d. Thin filaments During cross bridge cycling, the thin filaments slide inwards, shortening the sarcomere.
Which structure of the neuromuscular junction is a chemically-gated ion channel that the neurotransmitter binds to? a. synaptic vesicle b. voltage-gated calcium channel c. voltage-gated sodium channel d. acetylcholine receptor
d. acetylcholine receptor Once released from the motor neuron, the neurotransmitter acetylcholine diffuses across the synaptic cleft and binds to an acetylcholine receptors on the junctional folds.
Where in the neuromuscular junction do you find voltage-gated calcium ion channels? a. synaptic cleft b. junctional folds c. sarcolemma d. axon terminal
d. axon terminal The voltage-gated calcium ion channels allow calcium into the axon terminal to trigger the exocytosis of neurotransmitter.
Which one of the following components stimulates contraction of a skeletal muscle? a. muscle fiber b. acetylcholinesterase c. synaptic cleft d. motor neuron
d. motor neuron Motor neurons are needed to stimulate a skeletal muscle to contract.
Ventricular contraction follows
depolarization.
The Purkinje fibers
depolarize the contractile cells of both ventricles. When that happens, ventricular contraction immediately occurs.
Depolarization of conduction system components can be determined from the ECG. When the SA node functions as the pacemaker, it
depolarizes right before the P wave.
In step 5, the diffusion of sodium ions into the muscle fiber
depolarizes the junctional folds.
Finally, between heartbeats, there is a period when the atria and ventricles are both in
diastole.
This is followed by depolarization of the ventricles and ventricular systole while the atria are in
diastole.
ventricular filling, during which both the atria and the ventricles are in
diastole.
In an isotonic contraction, a) Muscle length shortens. b) Muscle tension exceeds the force of the load. c) The load is moved. d) Both a and c. e) All of the above.
e) All of the above.
Which of the following muscle types contain gap junctions? a) Skeletal muscle b) Smooth muscle c) Cardiac muscle d) Both a and b e) Both b and c
e) Both b and c
Add the proper substance from the supply room that enters through the voltage-gated channels along the sarcolemma to propagate the action potential. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
e. Na+ As sodium ions enter through voltage-gated channels, the action potential spreads along the T tubules. We are back on track and this muscle can now contract. Dynamite!
Add the proper substance from the supply room that enters the muscle fiber causing depolarization of the junctional folds. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
e. Na+ Sodium ions diffuse through the channels in the acetylcholine receptors to enter the muscle fiber, causing depolarization. Now, muscle contraction can continue.
The cross bridge cycle starts again when the
energized myosin head forms another cross bridge.
However, the myosin-binding sites are exposed when calcium ions bind to troponin during
excitation-contraction coupling.
This calcium transport reduces the calcium ion concentration in the sarcoplasm below the threshold necessary for
excitation-contraction coupling.
Add the proper substance from the supply room that is the neurotransmitter that is released into the synaptic cleft. a. Ca2+ b. ADP Pi c. K+ d. ATP e. Na+ f. ACh
f. ACh Acetylcholine is the neurotransmitter that we need. Now the muscle can contract and Logan can be on his way to lifting that free weight.
According to the size principle, the force generating capacity of a muscle fiber increases in direct proportion to its length.(true/false)
false
Even though some of the myosin heads are in the high energy state, they can no longer
form cross bridges.
The four steps of the cross bridge cycle are the
formation of cross bridges, the power stroke, cross bridge detachment, and myosin head reactivation.
the influx of calcium ions during step 2 causes synaptic vesicles adjacent to the membrane to
fuse with the plasma membrane releasing acetylcholine into the synaptic cleft by exocytosis
action potentials travel from cell to cell through
gap junctions, so the entire network of cells contracts as a unit
the heads (crossbridges) of myosin molecules are responsible for
generating the motion that drives contraction and possess two important sites, an actin-blinding site and an ATPase cite
This ends depolarization of the junctional folds and prevents the
generation of additional action potentials along the sarcolemma.
The timing is coordinated by the intrinsic conduction system of the
heart.
Contraction and relaxation of the heart chambers create pressure differences that drive blood through the heart. Blood flows from areas of
high pressure to areas of low pressure.
Second, the myosin head can pivot back and forth between
high-energy and low-energy positions.
calcium ions diffuse into the Axon terminal through the open channels because there is a
higher concentration of calcium ions outside than inside the membrane
The key to excitation-contraction coupling is an increase
in calcium ions within the sarcoplasm.
To prevent re-stimulation of the junctional folds, the acetylcholine
in the synaptic cleft must be eliminated.
During an (isometric/isotonic) muscle contraction, a muscle develops contractile force but does not change in length.
isometric
The greater diffusion of sodium ions into the muscle fiber than potassium ions out, makes the
inner surface of junctional folds less negative, meaning that the junctional folds are partially depolarized. We call this depolarization an end plate potential.
a motor neuron plus the muscle fibers it
innervates constitutes a motor neuron
in skeletal muscle, each fiber receives
input from one motor neuron, which branches and innervates more than one fiber
ion channel
integral membrane proteins through which ions diffuse across the cell membrane
smooth muscle is found in
internal organs and other structures that are not under voluntary control and is regulated by autonomic neurons
chemically gated ion channel/ligand gated ion channel
ion channel stimulated to open by binding with a chemical ligand
voltage gated ion channel
ion channel that is stimulated to open or close by changes in voltage across the plasma membrane
Acetylcholine binds to acetylcholine receptors on the
junctional folds, opening these chemically-gated ion channels
Contraction increases the pressure in a chamber, whereas relaxation
lowers it.
glycolytic fibers synthesize
most of their ATP via glycolysis and generate lactic acid, which makes them fatigue rapidly
Each skeletal muscle fiber is stimulated by a
motor neuron at the neuromuscular junction
If the SA node slows down or fails, the pacemaker cells of the AV node take over to set the heart rate. However, this results in a
much slower heart rate.
muscle fiber
muscle cell
once the muscle fiber is stimulated these channels open initiating an action potential on the sarcolemma this action potential will trigger
muscle contraction
Multiplied over all the sarcomeres in the myofibrils, this results in
muscle contraction.
The sarcomere is the basic unit of
muscle contraction.
Myosin heads can now bind to actin, and the
muscle fiber contraction can occur.
the junctional folds are the region of the
muscle fiber membrane or sarcolemma with specialized structures for receiving signals from a motor neuron
When voltage-gated sodium channels open they help conduct the action potential from the
muscle fiber surface down into the muscle fiber interior.
The sarcomeres return to their original resting length, and the
muscle relaxes.
within muscle fibers are
myofibrils that contain the contractile machinery
the sarcoplasmic reticulum surrounds the
myofibrils, stores calcium ions, and is closely associated with transverse (T) tubules, which penetrate into the cell interior from the sarcolemma
The velocity of contraction of a muscle fiber is directly related to its (diameter/myosin ATPase activity).
myosin ATPase activity
The actin of the thin filament has binding sites to which
myosin attaches during muscle contraction.
This causes troponin to change shape in a way that shifts tropomyosin away from the
myosin binding sites on actin.
Each actin molecule has a myosin-binding site to which a
myosin head can attach.
During muscle contraction, ATP hydrolysis is catalyzed by (myosin head groups/ actin monomers).
myosin head groups
The thick filaments are made up of many
myosin proteins.
Cross bridge cycling is the sequence of events where
myosin pulls on actin.
This causes tropomyosin to shift back to cover the
myosin-binding sites on actin.
the site where the action potential excites the muscle fiber is called the
neuromuscular junction
most skeletal muscles contain
numerous elongated cells (muscle fibers) that generate contractile force using energy from ATP hydrolysis
The second myosin binding site binds to the myosin-binding site
on actin. The result is a cross bridge between thick and thin filaments.
The valves of the heart open and close which insures that blood can only flow in
one direction through the heart.
These valves ensure that blood flows in only
one direction through the heart.
the ventricular filling phase. Mitral valve
open The atrioventricular valves are open to allow blood into the ventricle
the ventricular filling phase. Tricuspid valve
open The atrioventricular valves are open to allow blood into the ventricle
Does the depolarization of the junctional folds cause the voltage-gated sodium channels to open or close?
open see image
in Step 2 the action potential causes the calcium channels to
open allowing calcium ions to enter the cell
Changes in pressure within the chambers also cause the heart valves to
open and close.
Sodium ions enter the muscle fiber through the
open channels, generating an end plate potential.
the acetylcholine receptors are chemically gated ion channels that
open when they bind to their ligand acetylcholine
When depolarization of the junctional folds reaches the threshold voltage for the sodium channels on the sarcolemma adjacent to the neuromuscular junction, those channels
open, allowing sodium ions to enter the muscle fiber.
As you'll discover, the cardiac cycle is divided into four phases on the basis of ventricular activity and the
opening and closing of valves.
In the T tubules, the action potentials cause voltage-gated proteins to undergo a shape change that
opens calcium-release channels of the terminal cisternae.
the neuromuscular junction, there is a resting membrane potential across the sarcolemma that results from the distribution of ions across the membrane. In particular, sodium ions are more abundant
outside the muscle fiber than inside it.
Let's look at how this works. Sodium ions are present in a much higher concentration
outside the muscle fiber than inside. Potassium ions have a higher concentration inside than outside.
(Glycolytic/Oxidative) fibers contain high concentrations of the oxygen-binding protein myoglobin.
oxidative
(Oxidative/Glycolytic) muscle fibers are more resistant to fatigue.
oxidative
oxidative fibers synthesize ATP mostly via
oxidative phosphorylation and are more resistant to fatigue
contractile cells do the work of the heart by contracting, but they need
pacemaker cells to tell them when to do it. The pacemaker cells function like an officer giving orders to soldiers telling them to move. Let's now look at how those orders are communicated.
smooth muscle cells exhibit two types of spontaneous action potentials
pacemaker potentials and slow-wave potentials
the action potential that travels along the Axon causes
positive sodium ions to enter the cell and briefly make the membrane potential more positive
this change in the membrane potential to a more
positive value is called depolarization
the end-plate potential is followed by
propagation of the action potential through the T tubules, release of Ca2+ from the sarcoplasmic reticulum, binding of Ca2+ to troponin, movement of tropomyosin away from actins myosin-binding sites and initiation of the crossbridge cycle
The Z discs also mark the ends of the
sarcomere.
As long as the myosin-binding sites on actin remain exposed and ATP is available, the cross bridge cycle repeats,
pulling the thin filaments toward the center of the sarcomere.
The SA node is the pacemaker of the heart because it spontaneously fires action potentials at a
rate faster than any other component of the conduction system.
in step 3 synaptic vesicles
release acetylcholine into the synaptic cleft
The contraction ends as the ventricles
repolarize and relax.
The calcium ions from the sarcoplasmic reticulum activate the
sarcomeres of each myofibril.
Each myofibril is made up of thousands of
sarcomeres.
Cross bridge cycling ends when the concentration of calcium ions in the
sarcoplasm decreases to the resting level of the unstimulated muscle.
Calcium ions rapidly diffuse out of the terminal cisternae through the open channels and flood the
sarcoplasm of the muscle fibre.
Pacemaker cells of the SA node spontaneously generate action potentials that spread through the
rest of the conduction system and to all cardiac contractile cells.
In fact, the distribution of these and other ions creates a
resting membrane potential or voltage across the sarcolemma similar to that of the axon, with the inner surface of the membrane negatively charged.
Cross bridges are now prevented from forming, so the thin filaments slide back to their
resting position.
in the case of an action potential the membrane charges actually
reverse so that the outside of the Axon is briefly negative and the inside is briefly positive
The plasma membrane of a muscle cell is also known as the ________.
sarcolemma
the action potential initiated on the
sarcolemma
Stimulation at the neuromuscular junction triggers action potentials that spread across the
sarcolemma and down the T tubules
As sodium ions enter, depolarization spreads to affect nearby voltage-gated sodium channels on the
sarcolemma. These channels are called "voltage-gated" because they open when the voltage across the membrane reaches a specific threshold value.
Second, calcium ions are actively transported out of the sarcoplasm back into the
sarcoplasmic reticulum by specialized calcium pumps.
Now let's see how the action potential on the T tubules stimulates calcium release from the
sarcoplasmic reticulum.
ATP → ADP + Pi cycle
see image
Calcium ions cause several synaptic vesicles to mobilize and fuse with the membrane of the axon terminal. Drag the highlighted calcium ion to an appropriate synaptic vesicle to stimulate exocytosis.
see image
Components of the ICS
see image
Components of the thick and thin filaments of a myofibril
see image
Heart anatomy
see image
Microscopic structures of a muscle fiber
see image
Now that the action potential has reached the axon terminal, it allows calcium ions to diffuse. Select the structure that will allow calcium ions to diffuse.
see image
pathway of depolarization
see image
To begin, drag an acetylcholine molecule to its correct receptor.
see image Binding of acetylcholine to acetylcholine receptors causes activity at the junctional folds.
Now that the calcium channel is open, calcium ions can diffuse through it. Drag the calcium ion to the arrow that indicates the correct direction of travel.
see image Once the voltage-gated calcium channels open, calcium ions diffuse into the axon terminal because there is a higher concentration of calcium ions outside than inside the membrane.
This depolarization spreads to trigger additional voltage-gated sodium channels in nearby
segments, and the action potential spreads along the sarcolemma.
The cardiac cycle is the
sequence of events that propels blood through the heart during one complete heartbeat.
In doing so, they pull the Z disc at either end of the sarcomere toward the central M line,
shortening the sarcomere. This occurs at all the sarcomeres along the length of the muscle fiber.
During a muscle contraction, sarcomeres shorten because the thin filaments are pulled past the thick filaments toward the center of the sarcomere. This is called the
sliding filament mechanism.
the three major classes of skeletal muscle fibers are
slow oxidative fibers, fast glycolytic fibers, and the somewhat rare fast oxidative fibers
These receptors are chemically-gated ion channels that open when acetylcholine binds to them, allowing
sodium and potassium ions to diffuse across the membrane.
When acetylcholine receptors open, large numbers of
sodium ions rapidly diffuse into the muscle fiber, while a smaller number of potassium ions diffuse out.
The intrinsic conduction system consists of
specialized cardiac cells that establish the heart rate.
First, the terminal cisterns cease releasing calcium ions. This occurs when signals from the motor neuron
stop and no additional action potentials conduct along the sarcolemma and T tubules.
skeletal and cardiac muscle are
striated, reflecting the orderly arrangement of thick and thin filaments in the myofibrils, which are made up of fundamental force-generating units (sarcomeres) that are joined in an end-to-end fashion
stimulation at high frequencies causes
summation of twitches, such that the force eventually reaches a plateau (tetanus)
Acetylcholine molecules released from the axon terminal diffuse across the
synaptic cleft and bind to acetylcholine receptors on the junctional folds
The calcium ions cause synaptic vesicles to release acetylcholine into the
synaptic cleft by exocytosis
the pattern of alternating periods of
systole and diastole of the atrium and ventricle.
Which structure is telling the truth? Select the truth teller. t tubule: I am the structure that stores and releases calcium. terminal cisterns: No, I am the structure that stores and releases calcium.
terminal cisterns Nobody can put one on you! You are correct in identifying me. I am Terminal Cisterns that stores and releases calcium ions. Calcium ions, once released, can bind to the regulatory protein.
Adjacent to the T tubules are sac-like bulges called
terminal cisterns of the sarcoplasmic reticulum.
These proteins link to calcium release channels in the
terminal cisterns.
Excitation-contraction coupling is the sequence of events by which
that muscle fiber action potential (the "excitation") leads to muscle fiber contraction.
contractions are triggered by
the binding of Ca2+ to calmodulin, which activates myosin kinase, resulting in the phosphorylation of myosin crossbridges
thick and thin filaments contain
the contractile proteins myosin and actin, respectively
axon terminals
the enlarged distal end of an axon contains a neurotransmitter substance within synaptic vesicles
the neuromuscular junction includes
the enlarged distal ends of the axon called axon terminals and the junctional folds of the muscle fiber
junctional folds
the folded portion of the sarcolemma in close contact with the synaptic ending of the axon terminal Specialized region of the sarcolemma at the neuromuscular junction
the force generated by an entire muscle is determined by both
the force generated by individual fibers (which depends on the frequency of stimulation, fiber diameter, and changes in fiber length) and the number of fibers that are contracting
excitation-contraction coupling triggers calcium ions binding to troponin, causing
the myosin-binding sites on actin to become uncovered.
within the Axon terminal or membrane bound synaptic vesicles that contain
the neurotransmitter acetylcholine
an action potential in a motor neuron triggers
the release of acetylcholine, which binds to receptors in the muscle fibers motor end plate, the result is an electrical signal (end-plate potential) that triggers an action potential in the sarcolemma
the structures of a muscle fiber that are responsible for excitation-contraction coupling. These structures include:
the sarcolemma, the transverse tubules, the sarcoplasmic reticulum with terminal cisterns, and the sarcomeres of each myofibril
With the chemically-gated channels closed and acetylcholine removed,
the signal for muscle contraction stops.
two regulatory proteins (troponin and tropomyosin) present on
the thin filaments serve to initiate and terminate contractions
Two regulatory proteins, tropomyosin and troponin, are also associated with
the thin filaments.
specifically when the action potential arrives at the Axon terminal the change in the membrane potential opens
the voltage gated calcium channels
fast-twitch fibers and slow-twitch fibers differ in
their speed of contraction which is related to the type of myosin they contain
the Axon terminal and the junctional folds do not touch
they are separated by a gap called the synaptic cleft
During muscle contraction, cross bridge cycles occur between the
thick and thin filaments along the entire length of each sarcomere.
when a muscle contracts
thick and thin filaments slide past one another
Each sarcomere consists of an arrangement of
thick and thin filaments.
Myosin molecules have two important features that enable the
thick filament to bind to and move the thin filament.
The central region of the A band is slightly lighter than the rest of the A band because it contains
thick filaments but no thin filaments.
The A band is the region occupied by
thick filaments.
This pivoting motion constitutes the "power stroke" that pulls the
thin filament along the thick filament.
A Z disc in the center of each I band anchors the
thin filaments.
The only filaments in the I band are
thin filaments.
Let's examine how a nerve impulse can stimulate an end plate potential on the muscle fiber to trigger contraction
this process takes place in six sequential steps right now our focus is on the first three steps leading to narrow transmitter release
Remember that there are thousands of sarcomeres in a myofibril and
thousands of myofibrils in each muscle fiber.
Muscle relaxation occurs when the calcium ion concentration drops below its
threshold value for binding to troponin.
voltage gated calcium channels will open in response
to an action potential allowing calcium ions to diffuse into the cell
Glycolytic fibers generate more force than oxidative fibers because they are larger in diameter. (true/false)
true
When a muscle fiber contracts, the I bands shorten. (true/false)
true
The transverse tubules, or T tubules, are regularly spaced,
tube-like infoldings of the sarcolemma. They branch within the muscle fiber to reach each myofibril.
the mechanical response of a motor unit to a single action potential is a
twitch, which is reproducible in size
Each myosin molecule is shaped a bit like a golf club, with
two heads and a tail. Each myosin head contains two important binding sites.
skeletal muscles contain different
types of fibers in various proportions
the central nervous system regulates muscular force by
varying both the action potential frequency in motor neurons and the number of active motor units (recruitment). as muscular force increases, motor units are recruited in order of increasing size, a phenomenon referred to as the size principle
The sinoatrial, or SA, node initiates depolarization and systole of the atria while the
ventricles are in diastole.
Conduction through the intrinsic conduction system can be determined from the PR interval, the time from the beginning of atrial depolarization to the start of
ventricular depolarization.
The QRS complex is generated by
ventricular depolarization.
And the T wave results from
ventricular repolarization.
the membrane of the Axon terminal contains
voltage gated calcium channels
outside the junctional folds the sarcolemma contains
voltage gated sodium channels
to understand this step let us look briefly at how motor neurons work when the neuron is at rest the inner surface of the membrane is negatively charged while the outer surface is positively charged this unequal distribution of charge creates a voltage across the plasma membrane we call this
voltage the resting membrane potential
An action potential arriving at the axon terminal opens
voltage-gated calcium channels, allowing calcium ions to diffuse into the axon terminal
The membranes of the T tubule contain
voltage-gated proteins.
Activity at the NMJ triggers an action potential in the adjacent sarcolemma by opening
voltage-gated sodium channels.
Remember that the sarcolemma, outside of the neuromuscular junction, contains
voltage-gated sodium channels.
Since they are infoldings of the sarcolemma, the t tubules also contain
voltage-gated sodium channels.
when the ventricle fills with blood and the volume increase and when blood is pumped out of the ventricle and the
volume decreases.
The actin molecules form a pair of strands that
wind around each other.
