A&P Exam 2 (10, 11)
Which of the following causes myosin to detach from actin? A calcium ion binds to myosin. Troponin binds to myosin. An ATP molecule binds to myosin. An ADP molecule is hydrolyzed to ATP.
An ATP molecule binds to myosin.
The continuous maximum tension generated by the tetanus toxin produces fatigue in muscle cells. Which of the following would be observed in Joe's muscle cells?
An increased demand for oxygen by the muscle fiber
The resting membrane potential is maintained by Na+-K+ pumps that actively transport K+ into and ______ out of the cell.
Na+
What prevents the Na+ and K+ gradients from dissipating?
Na+-K+ ATPase
What type of conduction takes place in unmyelinated axons?
continuous conduction
After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment?
ATP binds to the myosin head.
The progressive stiffening of muscles after death, known as rigor mortis, is due to:
ATP depletion, which leads to high cytosolic calcium and inability of crossbridges to detach.
List the sequence of events during excitation-contraction coupling.
1.) AP propagates along the sarcolemma 2.) AP travels down T tubules to triads 3.) Voltage-sensitive proteins open Ca2+ channels 4.) Sarcoplasmic reticulum releases CA2+ 5.) CA2+ levels in sarcoplasm increase
List the sequence of events at the neuromuscular junction.
1.) Ca2+ enters the axon terminal 2.) Synaptic vesicles release ACh 3.) ACh binds to SCh receptors 4.) Ligand-gated cation channels open 5.) Na+ enters and K+ exits 6.) Membrane potential is less negative
List the steps of propagation of an action potential.
1.) Local current flows to axon segment 2.) Axon segment depolarized to threshold 3.) Voltage-gated Na+ channels open 4.) Influx of Na+ 5.) AP generated in adjacent axon segment
List the steps in a cross bridge cycle.
1.) The activated myosin head binds to actin, forming a cross bridge. 2.) ADP is released and myosin slides the thin filament toward the center of the sarcomere. 3.) ATP binds to the myosin head and detatches it from actin. 4.) ATP is hydrolyzed to ADP and Pi and the energy released re-cocks the myosin head.
What are the steps to generating an action potential?
1.) The membrane is at the resting membrane potential of -85 mV and the sodium and potassium voltage-gated channels are closed. 2.)The sodium voltage-gated channel opens in response to a stimulus allowing sodium ions to enter the cell, making the membrane potential more positive. 3.) Repolarization of the cell membrane occurs when sodium voltage-gated channels close and potassium voltage-gated channels open, allowing potassium ions to leave the cell. 4.) The membrane potential is now negative again.
List the steps of generating an action potential.
1.) Threshold stimulus Na+ channels open 2.) Na+ influx; depolarization 3.) Na+ channels close; K+ channels open 4.) K+ efflux; repolarization 5.) Hyperpolarization; K+ channels close
Which of the following steps of the crossbridge cycle occurs immediately before the power stroke?
A crossbridge forms.
The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge?
A myosin head bound to actin
A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected?
A series of proteins that control calcium release.
Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction? Select all the correct answers. ACh binds to ACh receptors. ACh diffuses away from the synaptic cleft. ACh is taken up by the axon terminal via endocytosis. ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE).
ACh diffuses away from the synaptic cleft. ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE).
Action potential propagation in a skeletal muscle fiber ceases when acetylcholine is removed from the synaptic cleft. Which of the following mechanisms ensures a rapid and efficient removal of acetylcholine? Acetylcholine is transported into the postsynaptic neuron by receptor-mediated endocytosis. Acetylcholine is transported back into the axon terminal by a reuptake mechanism. Acetylcholine is degraded by acetylcholinesterase. Acetylcholine diffuses away from the cleft.
Acetylcholine is degraded by acetylcholinesterase.
The neuromuscular junction is a well-studied example of a chemical synapse. Which of the following statements describes a critical event that occurs at the neuromuscular junction?
Acetylcholine is released by axon terminals of the motor neuron.
The cross bridge cycle starts when _________. Select the best answer. Ca2+ is actively transported into the sarcoplasmic reticulum acetylcholine diffuses away from the synaptic cleft Ca2+ from the sarcoplasmic reticulum binds to troponin Ca2+ from the sarcoplasmic reticulum binds to tropomyosin ATP binds to troponin and is hydrolyzed to ADP and Pi
Ca2+ from the sarcoplasmic reticulum binds to troponin
Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber?
Calcium ions are stored in the sarcoplasmic reticulum.
What specific event triggers the uncovering of the myosin binding site on actin?
Calcium ions bind to troponin and change its shape.
Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? Hints Sodium ions. Calcium ions. Action potentials. Acetylcholine.
Calcium ions.
Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction?
Calcium release from the sarcoplasmic reticulum initiates the contraction.
Which energy source would you expect to be depleted first during continuous contractions caused by the tetanus toxin?
Creatine phosphate
When does cross bridge cycling end?
Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.
Which of the following can cause relaxation? Increase in the calcium ion concentration inside the muscle fiber Inhibition of acetylcholinesterase (AChE) End of neural stimulation Release of ACh from the synaptic terminal
End of neural stimulation
Excitation-contraction coupling is a series of events that occur after the events of the neuromuscular junction have transpired. The term excitation refers to which step in the process?
Excitation, in this case, refers to the propagation of action potentials along the sarcolemma.
Depolarizing local potentials are caused by an influx of:
Na ions via gated ion channels
As previously mentioned, this toxin can cause a fatal disease known as botulism. If the toxin becomes systemic, it can block the release of ACh from nerves in other locations. Why do you think botulism can be fatal?
If it affected the phrenic nerve, the diaphragm muscle would be paralyzed.
What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization?
Inactivation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open.
The concentration of __________ is higher inside than outside the cell.
K+
The membrane is more permeable to _____.
K+
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels
The concentration of________ is higher outside than inside the cell.
Na+
Some nerve agents have been used as bioweapons. Unfortunately, Sarin gas has been used as a bioweapon several times in other countries. Sarin is an acetylcholinesterase (AChE) inhibitor and even exposure to low levels can be fatal. Based on your knowledge of muscle physiology, why do you think Sarin has these effects?
Muscles will contract and be unable to relax, known as spastic paralysis. This can cause spasms of the diaphragm and bronchi, which lead to asphyxiation.
Acetylcholinesterase inhibitors have therapeutic value in the treatment of some medical conditions and diseases. Which of the following do you think would be a clinical application of these drugs?
Myasthenia gravis, a disease in which the ACh receptors are damaged and the muscle becomes progressively weaker.
In which type of axon will velocity of action potential conduction be the fastest?
Myelinated axons with the largest diameter
What characterizes repolarization, the second phase of the action potential?
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.
Continuous stimulation of motor neurons by the tetanus toxin generates maximum muscle tension given time. This is similar to the normal response known as _____________, where greater muscle tension is generated by activating more motor units to accommodate a heavy external force.
Recruitment
Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?
Resting membrane potential (RMP)
Which of the following is a characteristic of smooth muscle? It can be controlled voluntarily. Smooth muscle cells contain actin and myosin. Muscle contraction does not depend on calcium ions. It has striations and sarcomeres.
Smooth muscle cells contain actin and myosin.
Calcium entry into the axon terminal triggers which of the following events?
Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine.
In the absence of ATP in the muscle, which of the following is most likely to occur? Hints The crossbridge cycle will be much slower. All myosin heads will continuously be in the "cocked" position. The crossbridge cycle will continue as normal, because ATP is not necessary for muscle contraction. Some myosin heads will remain attached to actin molecules, but are unable to perform a power stroke.
Some myosin heads will remain attached to actin molecules, but are unable to perform a power stroke.
Knowing what tetanus toxin does to motor neuron activity, predict what would happen to the concentration of acetylcholine in the synaptic cleft.
The concentration of acetylcholine would increase in the synaptic cleft.
How does the myosin head obtain the energy required for activation?
The energy comes from the hydrolysis of ATP.
Why does regeneration of the action potential occur in one direction, rather than in two directions?
The inactivation gates of voltage-gated Na+ channels close in the node, or segment, that has just fired an action potential.
On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?
The inside surface of the plasma membrane is much more negatively charged than the outside surface.
Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why?
The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients.
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value
What event triggers the generation of an action potential?
The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.
Botulinum toxin is a very powerful toxin produced by the bacteria Clostridium botulinum. It is the cause of the sometimes fatal disease, known as botulism. This toxin prevents the release of ACh from the motor neuron. What effect will it have on muscle function?
The muscle will be unable to contract, known as paralysis.
The neurons of patients with multiple sclerosis are unable to transmit action potentials down the axon. How will that affect skeletal muscle stimulation?
The muscle will not be stimulated and therefore will not contract.
What is the function of the myelin sheath?
The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals.
The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.
The presence of concentration gradients and leak channels
What structure is the functional unit of contraction in a skeletal muscle fiber?
The sarcomere
The plasma membrane is much more permeable to K+ than to Na+. Why?
There are many more K+ leak channels than Na+ leak channels in the plasma membrane.
Predict the effect of Tetrodotoxin, a powerful blocker of voltage-gated sodium ion channels:
There will be no action potential generated in the muscle fiber.
____________________ consist of two intertwined actin strands, covered by long, ropelike strands of tropomyosin, which is held in place by the smaller globular protein troponin.
Thin filaments
What are the components of a triad?
Two terminal cisternae of the sarcoplasma reticulum (SR) and one T-tubule
What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated?
Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron.
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
The end-plate potential is:
a depolarization caused by sodium ion movement into the cytosol.
A crossbridge forms when:
a myosin head binds to actin.
An action potential is self-regenerating because __________.
depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment
Involuntary control of smooth and cardiac muscles and glands is provided by the __________.
autonomic nervous system
A type of neuron that has one axon and one dendrite.
bipolar neuron
Compared to Type I fibers, Type II fibers__________.
can generate faster, more powerful contractions
what contains intercalated discs and is found in the heart?
cardiac muscle
The brain and spinal cord compose the __________.
central nervous system
Multiple sclerosis is an autoimmune disease that destroys myelin in the CNS. Which of the following would be a symptom of multiple sclerosis? abnormally high rate of cellular division of neuroglia delay in neural conduction inability to synthesize neurotransmitters increased synaptic transmission
delay in neural conduction
Loss of K ions from the cytosol will have the same effect as:
gain of Cl ions
Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________.
extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber
Excitation-contraction coupling results in an increase of calcium ion concentration:
in the cytosol of the muscle fiber.
Where do most action potentials originate?
initial segment
Neurons that relay information within the CNS and are the location of information processing are called __________.
interneurons
During an __________________________ the muscle is contracting but is neither lengthening nor shortening.
isometric contraction
Which type of contraction causes a skeletal muscle to shorten?
isotonic concentric contraction
The ________________________ is the time between stimulation and the beginning of contraction. It is the time it takes for excitation-contraction coupling to occur.
latent period
The active ingredient of Botox, botulinum toxin, blocks the release of ACh from the axon terminal. An overdose of Botox will most likely result in:
muscle paralysis at the site of injection.
Saltatory propagation occurs in _________ axons, in which action potentials _________.
myelinated; move from one node of Ranvier to another myelinated
The protein that binds to actin when tropomyosin moves off of the active sites of actin, is:
myosin.
If a neuron simultaneously receives a stimulus that results in local depolarization of 10 mV and a stimulus that results in a local hyperpolarization of 10 mV, the net effect would be:
no net change in membrane potential
If a patient is given a drug that inhibits acetylcholinesterase (AChE), you would expect to see__________.
prolonged muscle contraction
What is the plasma membrane of skeletal muscle called? sarcolemma sarcoplasmic reticulum sarcomere sarcoplasm
sarcolemma
During excitation contraction coupling calcium is released from the__________.
sarcoplasmic reticulum
What has multinucleate cells, contains a well-developed sarcoplasmic reticulum, and is long and cylindrical in shape?
skeletal muscle
As myosin heads complete the power stroke, actin filaments:
slide toward the M line of the sarcomere.
What is found in digestive organs and blood vessels?
smooth muscle
Stimulation of acetylcholine (ACh) receptors by ACh will primarily result in:
sodium ions entering the muscle fiber.
The narrow space between the axon terminal and the motor end plate is called the:
synaptic cleft
When the sarcomere contracts and shortens__________. the I-band becomes larger the zones of overlap become smaller the Z-disks move further apart the A band stays the same
the A band stays the same
When the sarcolemma repolarizes and returns to rest:
the inside of the sarcolemma is more negatively charged than the outside.
Acetylcholine binds to its receptor in the sarcolemma and triggers __________.
the opening of ligand-gated cation channels
What is name given to the regularly spaced infoldings of the sarcolemma? Hints terminal cisternae transverse or T tubules motor endplates sarcoplasmic reticulum
transverse or T tubules
In preparation for contraction, calcium ions bind to:
troponin.
The type of contraction represented by a single stimulus/contraction/relaxation sequence is a(n)__________.
twitch
What opens first in response to a threshold stimulus?
voltage-gated Na+ channels
The generation of an action potential in a neuron requires the presence what type of membrane channels?
voltage-gated channels
During the repolarization stage of a muscle action potential__________. voltage-gated sodium ion channels open and sodium ions leave the cell voltage-gated potassium ion channels open and potassium ions enter the cell voltage-gated sodium ion channels open and sodium ions enter the cell voltage-gated potassium ion channels open and potassium ions leave the cell
voltage-gated potassium ion channels open and potassium ions leave the cell
