Human Physiology Exam 3 Material

Skeletal Muscle Structure at the Cellular Level: Components of a Muscle Fiber

Sarcolemma Sarcoplasm Sarcoplasmic Reticulum Myofibrils

Recruitment

1 of 2 factors of regulation of the force generated by whole muscles An increase in the number of active neurons Motor neuron activation = all muscle fibers in motor unit

The Size Principle

1 of 2 factors of regulation of the force generated by whole muscles The correspondence between size of motor units and the order of recruitment Small units (delicate movement) recruited first -when small forces are needed Large units (strong movement) recruited last -when larger forces are needed When contraction is sustained over a long time, motor units are activated asynchronously--as one becomes active, another ceases its activity -in this manner, the total force of the muscle is maintained at a constant level without overworking any of the individual motor units

Tropomyosin

1 of 2 regulatory proteins in the thin filaments One of the two regulatory proteins in striated muscle; a long fibrous molecule that acts to block myosin-binding sites on thin filaments when a muscle is not contracting A long fibrous molecule that extnds over numerous actin monomers in such a way that it blocks the myosin-binding sites in muscles at rest.

Troponin

1 of 2 regulatory proteins in the thin filaments One of the two regulatory proteins in striated muscle; binds calcium reversibly and is responsible for starting the crossbridge cycle by moving tropomyosin out of its blocking position It is a complex of three proteins--one that attaches to the actin strand, another that binds to tropomyosin, and a third containing a site to which calcium ions can bind reversibly.

Fiber Diameter

1 of 3 factors affecting the force generated by individual muscle fibers Force-generating capacity depends on: -more crossbridges/sarcomere -> more force -more sarcomeres in parallel -> more force

Fiber Length

1 of 3 factors affecting the force generated by individual muscle fibers Length of fiber at the onset of contraction Optimal length vs. non-optimal length

Number of Stimuli

1 of 3 factors affecting the force generated by individual muscle fibers Summation and tetanus -depends on amount of Ca^2+ bound to troponin

Events in Excitation-Contraction Coupling

1. Acetylcholine (ACh) is released from the axon terminal of a motor neuron and binds to receptors in the motor end plate. This binding elicits an end-plate potential, which triggers an action potential in the muscle cell. 2. Action potential propagates along the sarcolemma and down T tubules. 3. The action potential triggers Ca^2+ release from SR. 4. Ca^2+ binds to troponin, exposing myosin-binding sites. 5. Crossbridge cycle begins (muscle fiber contracts). 6. Ca^2+ is actively transported back into lumen of SR following the action potential. 7. Tropomyosin blocks myosin-binding sites (muscle fiber relaxes).

5 Steps of the Cross-Bridge Cycle

1. Binding of Myosin to Actin 2. Power Stroke 3. Rigor 4. Unbidning of Myosin and Actin 5. Cocking of the Myosin Head

Sliding-Filament Theory

A band does not change in length -A band spans the length of the thick filaments in a sarcomere, this means the thick filaments do not change length when the muscle cell contracts I Band and H Zone shorten -the shortening of I bands (which contains only thin filaments) occurred not because thin filaments contract, but because they slide past the thick filaments, moving deeper into the H zone and decreasing its width -as this occurs, adjacent A bands move closer together, which decreases the width of the I bands The end result is that the Z lines at either end of a sarcomere move closer together, thereby shortening the sarcomere As sarcomeres shorten, myofibrils also shorten, as do muscle fibers and ultimately whole muscles Process of muscle contraction whereby thick and thin filaments slide past each other

Structure of a Thick Filament

A myosin molecule. Note the actin-binding and ATPase sites in the head region. Two myosin molecules joined tail to tail. A portion of a thick filament showing myosin heads (crossbridges) protruding at either end but not in the middle region (the bare zone). A detailed view of a sarcomere showing the relative positions of thick and thin filaments and the protein titin, which anchors the thick filaments in place. An electron micrograph of a sarcomere showing thick and thin filaments and crossbridges.

4. Unbidning of Myosin and Actin

A new ATP enters the ATPase site on the myosin head, triggering a conformational change in the head, which decreases the affinity of myosin or actin, so the myosin detaches from the actin.

The Twitch

A twitch is the mechanical response of an individual muscle cell, a motor unit, or a whole muscle to a single action potential. It is reproducible: -repetitive stimulation of a muscle produces several twitches in a row (same magnitude and shape) -however, when the frequency of stimulation is high enough that the twitches follow one another closely -as the stimulation frequency increases, muscle move from twitch contractions to treppe, to summation, and finally to tetanus

A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected? A. A series of proteins that control calcium release. B. Voltage-gated sodium channels. C. Potassium leak channels. D. Myosin cross-bridge binding sites.

A. A series of proteins that control calcium release. When action potentials propagate along T-tubules, a voltage-sensitive protein changes shape and triggers a different protein to open it's channels, resulting in the release of calcium from the terminal cisternae.

Calcium entry into the axon terminal triggers which of the following events? A. Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine. B. Cation channels open and sodium ions enter the axon terminal while potassium ions exit the axon terminal. C. Acetylcholine binds to its receptor. D. Acetylcholine is released into the cleft by active transporters in the plasma membrane of the axon terminal.

A. Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine. When synaptic vesicles fuse to the plasma membrane, acetylcholine is released via exocytosis.

Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________. A. extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber B. extend from the brain to the sarcolemma of a skeletal muscle fiber C. arise in the epimysium of a skeletal muscle and extend to individual skeletal muscle fibers D. extend from the spinal cord to the sarcolemma of a skeletal muscle fiber

A. extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber The cell bodies of motor neurons to muscles in the head and neck are located in the brain. The cell bodies of motor neurons to the rest of our muscles are located in the spinal cord.

Transverse Tubules

AKA T tubules Continuous with the sarcolemma and penetrate into the cell's interior Lateral sacs (terminal cisternae) are near them Each is associated with 2 terminal cisternae, forming a triad This along with the sarcoplasmic reticulum play important roles in the activation of muscle contractions: they help transmit signals from the sarcolemma to the myofibrils, enabiling a muscle cell to respond to neural input

Terminal Cisternae

AKA lateral sacs Store calcium Enlargements on the sarcoplasmic reticulum located near the T tubues Each T tubule is associated with two of these to form a triad

Muscle Metabolism: How Muscle Cells Generate ATP

ATP require for work Sources of ATP -phosphorylation of ADP by creatine phosphate (substrate level phosphorylation) -oxidative phosphorylation of ADP (mitochondria) -anaerobic glycolysis Exercise: light to moderate intensity -oxidative phosphorylation: =initially, glycogen stores supply glucose =up to 30 min, glucose and fatty acids in blood =O2 supply must be kept adequate =transient increase in GLUT4 Exercise: heavy intensity -substrate level phosphorylation -anaerobic glycolysis =lactate =only 2 ATP molecules per glucose molecule

Conformation of Myosin: Low-Energy Form

After the stored energy is released to drive the movement of the thin filaments Step 3 of the cross-bridge cycle Rigor

When do cross-bridges generate force? When does a muscle cell generate force?

Although a given crossbridge generates force only part of the time while it is active (during the power stroke), a muscle cell generates force continually during a contraction. -this is because many crossbridges go through the cycle simultaneously but out of phase ("out of step") with each other Thus, at any given time, some crossbridges are starting the cycle, others are finishing it, and still others are at various stages in between If the cycles were in sync and all the crossbridges detached from actin at the same time, then the thin filaments would passively slide back to their original position, making the crossbridge cycle inefficient (or having movements like a robot). During actual contraction, there are always several crossbridges attached to actin at any given time--an arrangement that prevents this effect from occurring

Each myosin head posesses two sites that are critical to its force-generating ability: what are they?

An actin-binding site -which is capable of binding to the actin monomers in the thin filaments An ATPase site -which has enzymatic activity and hydrolyzes ATP

Titin

An extraordinarily elastic protein that can be stretched to more than three times its unstressed length Strands of titin extend along each thick filament from the M line to each Z line, anchoring the thick filaments in their proper positions relative to the thin filaments When an external stretching force is applied to a muscle, titin strands elongate as the sarcomeres lengthen, and these strands begin to exert an opposing force, just as a spring resists stretching -when this external force is removed, this opposing force pulls the Z lines and thick filaments closer together and causes the sarcomeres to shorten, allowing the titin strands to spring back to their original length -as this occurs, individual muscle fibers shorten, as does the whole muscle

3. Rigor

As the power stroke ends, ADP is released from the myosin head and the myosin molecule goes into its low-energy state. In this state, myosin and actin are tightly bound together, a condition called rigor (rigor mortis--the stiffening of the body that occurs after death--occurs because the crossbridge cycle gets stuck at this step due to (1) excess of calcium when cell membranes are damaged and (2) a lack of ATP due to the termination of energy production). Rigor mortis continues until enzymes leaked by disintegrating cellular components begin to break down the myofibrils.

After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment? A. ADP and inorganic phosphate are bound to the myosin head. B. ATP binds to the myosin head. C. Acetylcholine binds to receptors in the junctional folds of the sarcolemma. D. Calcium ions bind to troponin.

B. ATP binds to the myosin head. The binding of ATP to the myosin head weakens the bond between myosin and actin, forcing the myosin head to detach. ATP also provides the energy for the next power stroke.

What structure is the functional unit of contraction in a skeletal muscle fiber? A. The triad B. The sarcomere C. The cross bridge D. The junctional folds of the sarcolemma

B. The sarcomere A sarcomere is a regular arrangement of thin and thick myofilaments that extends from one Z disc to the next. A myofibril consists of a series of sarcomeres.

Acetylcholine binds to its receptor in the sarcolemma and triggers __________. A. the opening of ligand-gated anion channels B. the opening of ligand-gated cation channels C. the opening of voltage-gated calcium channels D. the opening of calcium-release channels

B. the opening of ligand-gated cation channels These channels permit sodium ions to diffuse inward and potassium ions to diffuse outward.

Z Lines

Borders of sarcomeres Run perpendicular to the long axis and anchor the thin filaments at one end Located in the center of the I Band

What specific event triggers the uncovering of the myosin binding site on actin? A. Calcium release channels open in the sarcoplasmic reticulum, and calcium levels rise in the sarcoplasm. B. Sodium ions bind to troponin and change its shape. C .Calcium ions bind to troponin and change its shape. D. Calcium ions bind to tropomyosin and change its shape.

C .Calcium ions bind to troponin and change its shape. The shape change caused by the binding of calcium to troponin shifts tropomyosin away from the myosin binding sites on actin.

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? A. Calcium bound to troponin B. ATP bound to a myosin head C. A myosin head bound to actin D. Troponin bound to tropomyosin

C. A myosin head bound to actin As soon as the activated myosin head forms a cross bridge with actin, the power stroke begins.

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? A. Acetylcholine binds to its receptor in the junctional folds of the sarcolemma. Its receptor is linked to a G protein. B. When the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal. C. Acetylcholine is released by axon terminals of the motor neuron. D. Acetylcholine is released and moves across the synaptic cleft bound to a transport protein.

C. Acetylcholine is released by axon terminals of the motor neuron. Acetylcholine is released into the synaptic cleft via exocytosis.

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? A. Acetylcholine. B. Action potentials. C. Calcium ions. D.Sodium ions.

C. Calcium ions. Action potentials propagating down the T-tubule cause a voltage-sensitive protein to change shape. This shape change opens calcium release channels in the sarcoplasmic reticulum, allowing calcium ions to flood the sarcoplasm. This flood of calcium ions is directly responsible for the coupling of excitation to contraction in skeletal muscle fibers.

When does cross bridge cycling end? A. Cross bridge cycling ends when ATP binds to the myosin head. B. Cross bridge cycling ends when calcium release channels in the sarcoplasmic reticulum open. C. Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. D. Cross bridge cycling ends when calcium ions are passively transported back into the sarcoplasmic reticulum.

C. Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. The sarcoplasmic reticulum contains Ca2+-ATPases that actively transport Ca2+ into the SR. Without Ca2+, troponin returns to its resting shape, and tropomyosin glides over and covers the myosin binding sites on actin.

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? A. Excitation refers to the release of calcium ions from the sarcoplasmic reticulum. B. Excitation refers to the shape change that occurs in voltage-sensitive proteins in the sarcolemma. C. Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. D. Excitation refers to the propagation of action potentials along the axon of a motor neuron.

C. Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. These action potentials set off a series of events that lead to a contraction.

What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated? A. Motor neurons always innervate thousands of skeletal muscle fibers. B. A motor neuron typically innervates only one skeletal muscle fiber. C. Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. D. A skeletal muscle fiber is innervated by multiple motor neurons.

C. Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. There are many more skeletal muscle fibers than there are motor neurons. The ratio of neurons to fibers varies from approximately one to ten to approximately one to thousands.

What is name given to the regularly spaced infoldings of the sarcolemma? A. terminal cisternae B. sarcoplasmic reticulum C. transverse or T tubules D. motor endplates

C. transverse or T tubules T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior.

Termination of Contraction

Ca^2+ -ATPase (sarcoplasmic reticulum) -Transports Ca^2+ from cytosol into the sarcoplasmic reticulum Acetylcholinesterase

Muscle Contraction Process (Overview)

Calcium ions are released in response to electrical signals that travel from the carcolemma to the T tubules, and they serve as chemical messengers that carry these signals to the myofibrils to initiate contraction.

H Zone

Center region of the A Band that is lighter than the sides Only the thick filaments are present; there are no thin filaments overlapping the thick filaments here

Voluntary Motor Control

Completion of voluntary motor action

Skeletal Muscle Structure

Connected to two or more bones by tendons Striated muscle

The Mechanism of Force Generation in Muscle

Crossbridge cycle

Sarcoplasm

Cytoplasm of a muscle cell

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? A. Acetylcholine is transported into the postsynaptic neuron by receptor-mediated endocytosis. B. Acetylcholine is transported back into the axon terminal by a reuptake mechanism. C. Acetylcholine diffuses away from the cleft. D. Acetylcholine is degraded by acetylcholinesterase.

D. Acetylcholine is degraded by acetylcholinesterase. Acetylcholinesterase is an enzyme that degrades acetylcholine. This degradation results in a rapid cessation of the acetylcholine signal and a swift removal from the cleft.

Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber? A. Calcium ions are stored in the mitochondria. B. Calcium ions are stored in the transverse tubules. C. Calcium ions are stored in the nuclei. D. Calcium ions are stored in the sarcoplasmic reticulum.

D. Calcium ions are stored in the sarcoplasmic reticulum. Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle fibers. The sarcoplasmic reticulum is very elaborate in skeletal muscle fibers, allowing for significant storage of calcium ions.

Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction? A. Action potentials propagate into the interior of the skeletal muscle fiber. B. Voltage-sensitive proteins change shape. C. Sodium release from the sarcoplasmic reticulum initiates the contraction. D. Calcium release from the sarcoplasmic reticulum initiates the contraction.

D. Calcium release from the sarcoplasmic reticulum initiates the contraction. Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle cells. It is especially abundant and convoluted in skeletal muscle cells. It functions in the storage, release, and reuptake of calcium ions.

How does the myosin head obtain the energy required for activation? A. The energy comes from the hydrolysis of GTP. B. The energy comes from the direct phosphorylation of ADP by creatine phosphate. C. The energy comes from oxidative phophorylation. D. The energy comes from the hydrolysis of ATP.

D. The energy comes from the hydrolysis of ATP. Myosin is a large, complex protein with a binding site for actin. It also contains an ATPase. The energy released during the hydrolysis of ATP activates the myosin head.

Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why? A. The outside surface of the sarcolemma is negatively charged compared to the inside surface. Sodium ions diffuse outward along favorable chemical and electrical gradients. B. The outside surface of the sarcolemma is negatively charged compared to the inside surface. Potassium ions diffuse outward along favorable chemical and electrical gradients. C. The inside surface of the sarcolemma is negatively charged compared to the outside surface. Potassium ions diffuse inward along favorable chemical and electrical gradients. D. The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients.

D. The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients. The resting membrane potential of all cells is negative (inside compared to outside). Therefore, given the direction of the chemical and electrical gradients, more sodium ions diffuse inward than potassium ions diffuse outward.

Dihydropyridine Receptors & Ryannodine Receptors

DHP Receptors & Ryanodine Receptors are two proteins that physically link the T tubule and SR membranes together DHP receptors are found in the T tubule membrane and function as voltage sensors Ryanodine receptors are found in the SR membrane and connected to the DHP receptors -they are also calcium channels that are gated by DHP receptors When an action potential travels down the T tubules, DHP receptors undergo a conformational change that transmits a signal to the ryanodine receptors, causing the calcium channels to open Calcium then moves out of the SR and into the cytosol Some calcium binds to specific sites on other SR calcium channels and causes them to open -in this manner, the initial release of calcium triggers the release of even more calcium from the SR: calcium-induced calcium release

A Band

Dark striation due to the presence of thick and thin filaments In the center, there is a region that is lighter than the sides, known as the H Zone

Completion of Voluntary Motor Action

Development of idea to move Program of motor commands Correct activation of proper muscles Feedback to ensure movement is carried out smoothly and successfully

The Neuromuscular Junction

End-plate potential occurs at motor plate end

Diabetes and Skeletal Muscle

Long-term impact Contractile weakness, fiber-type changes, decreased oxidative activity and peripheral insulin resistance How does insulin impact glucose transporters? Glucose metabolism? 1. 2.

Anatomy of the Somatic Nervous System

Motor Unit

Factors Affecting the Force Generated By Individual Muscle Fibers

Fiber diameter Fiber length Number of stimuli

Thin Filaments

Filaments composed of actin that form part of the contractile machinery of a muscle cell; also contain roponin and tropomyosin in striated muscle cells The basic component of each thin filament are actin monomers called G actin ("G" because they are globular proteins), each of which has a myosin-binding site

Thick Filaments

Filaments composed of myosin that form part of the contractile machinery of a muscle cell Made of hundreds of myosin molecules, each of which looks a bit like two golf clubs wrapped around each other Within a thick filament, the myosin molecules bind to each other at their tail ends so that their heads extend in opposite directions away from the center The tails of the adjacent myosin molecules are also arranged in a staggered fashion so that their heads protrude from the thick filament in an orderly helical pattern Because he middle of the thick filament is devoid of crossbridges, this region is appropriately called the bare zone Connected by M lines

F Actin

G actins are linked together end to end, like pearls in a necklace, to form strands called F actin ("F" because they are fibrous proteins). Two F actins are arranged in a double helix to form the actin strands found in thin filaments

Excitation-Contraction Coupling

How muscle contractions are turned on and off In a muscle cell, the sequence of events that links the action potential to the contraction Input from motor neurons always has an excitatory effect on muscle cells and serves to trigger contraction of those cells. Like neurons, muscle cells are excitable, meaning that they are capable of generating action potentials if their plasma membranes are depolarized to a sufficient degree. When a muscle cell receives input from a motor neuron, the cell depolarizes and fires an action potential that then stimulates contraction. The sequence of events that links the action potential to the contraction is referred to as excitation-contraction coupling

Steps in Voluntary Movement

Idea -> Program -> Execution -> -Movement of skeletal muscle -Feedback Feedback loops back to idea, program and execution separately

Tetanus

In a muscle being stimulated at a high frequency, the plateau phase of the contraction, during which the tension is relatively constant At higher frequencies of stimulation, summation reaches a peak called tetanus (also refers to a disease in which toxins produced in a bacterial infection cause motor neruons to stimulate muscle contraction inappropriately) In unfused (or incomplete) tentanus, the force demonstrates small oscillations with brief periods of relaxation between peaks -the peaks are reached when calcium levels are great enough to saturate troponin, exposing all myosin-binding sites on actin -at even greater frequencies, calcium evels are great enough to continually saturate troponin such that all myosin-binding sites on actin are continually exposed, resulting in a plateau called fused (or complete) tentanus.

The Roles of Troponin and Tropomysin in Excitation-Contraction Coupling

In its normal (resting) conformation and, as a consequence, tropomyosin is positioned on the thin filaments in such a way that it blocks actin's myosin-binding sites, so the crossbridge cycle cannot occur As cytosolic calcium concentration rises, some of the calcium binds to one of the three proteins making up each troponin complex, which then undergoes a conformational change that causes tropomyosin to shift out of its normal resting position, thereby exposing the myosin-binding sites on the actin monomers

Summation

In neurophysiology, the adding together of graded potentials that occurs within a neuron; in muscle physiology, the adding together of twitches that occurs when a muscle is stimulated at high frequency Summation and tetanus occur at greater frequencies of stimulation as twitches overlap in time When a muscle is stimulated repetitively such that additional action potentials arrive before twitches can be completed, the twitches become superimposed on one another, yielding a force greater than that of a single twitch; this process is called summation Occurs whenever twitches occur at a frequency such that calcium cannot be removed from the cytosol as rapidly as it is released from the SR -calcium removal is necessary for relaxation; thus the muscle fiber cannot relax completely between twitches

Treppe

Increase in muscle tension with repeated twitch contractions Occurs at a frequency of muscle stimulation where independent twitches follow one another closely such that the peak tension rises in a stepwise fashion with each twitch, until eventually it reaches a constant level Causes is unknow, but may result from an increase in cytosolic calcium between twitches or from a "warming" of the muscle fiber that occurs with work

What is the calcium-induced calcium release subjected to?

It is subjected to a sort of negative feedback: as the cytosolic calcium concnetration increases, calcium ions begin to bind to certain sites on the SR calcium channels, causing them to close (These sites are distinct from those that trigger channel opening and have a lower affinity for calcium, so they do not com into play until the cytosolic calcium level has become sufficiently high) The closure of these channels turns off the release of calcium and enables the ative transport of calcium back into the SR (an ongoing process) to clear calcium from the cytosol, which causes the calcium concentration to fall.

Phases of the Twitch

Latent Period Contraction phase Relaxation phase

I Band

Light striation comprises areas where there are thin filaments with no overlap with thick filaments The Z line that connects thin filaments is located in the center of the I band

Idea

Limbic system Association areas Supplementary motor area

Conformation of Myosin: High-Energy Form

Myosin heads go into this conformation after they hydrolyze ATP It is called this because the myosin molecule stores energy that is released in the hydrolytic splitting of ATP Step 5 of the Cross-bridge cycle Cocking of the myosin head

Motor Unit

One motor neuron and all of the muscle fibers that it innervates

The design of the SR and T tubule network surrounding the myofibrils permits what?

Permits nearly simultaneous delivery of calcium to all sarcomeres of a muscle fiber, so the sarcomeres contract in unison, as does the entire muscle fiber

Sarcolemma

Plasma membrane of a muscle fiber

Regulatory Proteins

Present in thin filaments that enable muscle fibers to start or stop contracting Tropomyosin Troponin

Cross-Bridges

Protrusions on both ends of the thick filament that bind to actin and are responsible for generating the motion that causes musce contraction Under certain conditions they bridge the gap between the thick and thin filaments

Execution

Pyramidal tract Extrapyramidal tract Motor neuron

Regulation of the Force Generated by Whole Muscles

Recruitment The size principle

Myofibrils

Repeating sarcomeres Banded, rodlike elements that contain a muscle fiber's contractile machinery Comprises a bundle of overlapping thick and thin filaments made of the proteins myosin and actin

Overview of Excitation-Contraction Coupling: How Muscle Contractions are Turned On and Off

Requires: 1. neural input (motor neuron) 2. Ca^2+ release (sarcoplasmic reticulum) Termination of Contraction

The binding of calcium to troponin is what?

Reversible This concentration change causes calcium to dissociate from troponin, which allows both troponin and tropomyosin to revert to their original positions

Thick and Thin Filaments

Run parallel to the muscle cell's long axis Exist in a 2:1 ratio

Sarcoplasmic Reticulum

Sac-like membranous network Surrounds each of the myofibrils and is closely associated with other structures such as the transverse tubules It has enlargements called lateral sacs or terminal cisternae near the T tubule The function of this structure is to store calcium ions (Ca^2+) This along with the T tubules play important roles in the activation of muscle contractions: they help transmit signals from the sarcolemma to the myofibrils, enabling a muscle cell to respond to neural input

Feedback

Sensory systems Cerebellum Thalamus Basal nuclei Brainstem

5. Cocking of the Myosin Head

Soon after it binds to the myosin's ATPase site, ATP is plit by hydrolysis into ADP and Pi, which releases energy. Some of the energy is captured by the myosin molecule as it goes into its high-energy conformation. Although AtP has been hydrolyzed at this point, the end-products of the reaction (ADP and Pi) remain bound to the ATPase site. If calcium is present, the cycle will continue by revisiting step 1.

Triad

T tubule and two terminal cisternae

Strength of contraction ultimately depends on what?

The amount of calcium in the cytosol With more calcium exposing more binding sites on actin for myosin, it allows more crossbridge cycling to occur (more recruitment???)

Structure of a Thin Filament

The backbone of a thin filament consists of two strands of polymerized actin molecules wound together to form a double helix. Myosin-binding sites on individual actin molecules (G actin) are represented by dark dots. A portion of a thin filament showing troponin and tropomyosin in their normal resting positions on the actin strands. Notice that actin's myosin-binding sites are covered by tropomyosin when a muscle cell is at rest.

G Actins

The basic component of each thin filament are actin monomers called G actin ("G" because they are globular proteins), each of which has a myosin-binding site G actins are linked together end to end, like pearls in a necklace, to form strands called F actin ("F" because they are fibrous proteins).

2. Power Stroke

The binding of myosin to actin triggers the release of the Pi from the ATPase site. During this process, the myosin head pivots toward the middle of the sarcomere, pulling the thin filament along with it.

Velocity of Shortening

When muscle contracts isotonically -latent period of shortening increases with increasing load -duration of shortening decreases with increasing load -velocity of shortening decreases with increasing load =rate of change of the distance is shortened =reaches 0 when load >/= maximum tension =greatest when no load on muscle

When do the voltage-gated ryanodine receptors close?

When the membrane potential returns to normal

Sarcomere

The fundamental repeating units that make up myofibrils Bordred on either end by Z lines

Crossbridge Cycle

The mechanism that drives muscle contraction -the mechanism that drives the sliding of thick and thin filaments past one another At the heart of this mechanism is an oscillating, back-and-forth motion of myosin crossbridges that is powered by ATP hydrolysis Coupled with this activity is cyclic binding and unbinding of crossbridges to the thin filaments, which occurs in such a way that the motion of the crossbridges pulls the thin filaments toward the center of the sarcomere The back-and-forth movement of crossbridges is due to changes in the conformation of myosin molecules -conformational changes not only cause the heads to change position, but also alter both their ability to bind to actin monomers in the thin filaments and the energy content of the myosin molecules Conformation of Myosin: -High-Energy Form -Low-Energy Form Involves 5 steps

The Role of the Neuromuscular Junction in Excitation-Contraction Coupling

The motor neuron (presynaptic cell) transmits an action potential and secretes the neurotransmitter acetylcholine upon its arrival at the axon terminal. After release, acetylcholine diffuses to the plasma membrane of the muscle cell (postsynaptic cell), where it binds to specific receptors, triggering a change in ion permeability that results in a depolarization. At the neuromuscular junction, the motor neuron's terminal boutons fan out over a wide area of the sacolemma -opposite these boutons is a specialized region of the sarcolemma called the motor end plate, which is highly folded and contains a high density of acetylcholine receptors An action potential in the motor neuron triggers the release of acetylcholine from each of its many terminal boutons, which causes many acetylcholine receptors to become activated. As a consequence, the resulting depolarization (called an end-plate potential) is much larger than an ordinary postsynaptic potential--so large, that it is always above threshold and triggers an action potential the in the muscle cell If an action potential occurs in a motor neuron, it is always followed by an action potential in the muscle cells it innervates Once an action potential is initiated in a muscle cell, it propagates through the entire sarcolemma and down the T tubules, which triggers the release of calcium from the lateral sacs of the SR The calcium serves as the signal that initiates the crossbridge cycle

As calcium is being actively transported back into the SR, what happens to the exposed sites on the actin filament?

The number of exposed sites on the actin filament decreases, leading to a decline in the number of active crossbridges. Eventually, as calcium concentration returns to normal, the muscle contraction ends.

The Role of Calcium in Excitation-Contraction Coupling

When a muscle cell is relaxed, the concentration of calicum in the cytosol is very low, and little binding occurs between calcium and troponin The cytosolic calcium level is normally low because the membrane of the SR is equipped with calcium pumps that actively transport calcium ions from the cytosol into the SR Due to the action of these pumps, the SR is unable to accumulate calcium against the concentration gradient and, therefore, can function as a calcium storehouse In addition to calcium pumps, the membrane of the SR contains voltage-gated calcium channels that are normally closed (prevent calcium inside the SR from leaking out) -when an action potential travels through the T-tubules, it causes these channels to open briefly, allowing calcium to flow out into the cytosol and increase cytosolic calcium concentration

M Lines

The thick filaments in a sarcomere are connected by these, which also run perpendicular to the long axis

Myosin Myofilament

Thick filament Myosin head binding sites -actin binding site -nucleotide binding site for ATP and ATPase The contractile protein found in thick filaments in striated muscle 1 of 2 contractile proteins -constitute the machinery that generates contractile force -made up of structures arranged in an orderly, repeating fashion Each myosin molecule is a dimer consisting of two intertwined subunits, each having a long tail and a fat, protruding head -these heads are called crossbridges Contains: additional proteins -titin

Actin Myofilament

Thin filament The most common microfilament; found in thin filaments in muscle fibers; provides structural support for microvilli 1 of 2 contractile proteins -constitute the machinery that generates contractile force -made up of structures arranged in an orderly, repeating fashion Contains: Regulatory Proteins: -Tropomyosin -Troponin

1. Binding of Myosin to Actin

We start with myosin in its energized form; that is ADP and Pi (inorganic phosphate) are bound to the ATPase site of the myosin head. In this state, myosin as a high affinity for actin, and the myosin head binds to an actin monomer in the adjacent thin filament. This step can occur only in the presence of calcium.

When does a muscle cell stop contracting?

When it no longer receives input from its motor neuron, and action potentials no longer occur in the sarcolemma. When an action potential triggers the release of calcium from the SR, this release does not continue indefinitely.

Striated Muscle

muscle in which the cells have a striped appearance due to the presence of sarcomeres; includes skeletal and cardiac muscle close-up view shows that these striations are created by the orderly arrangement of protein fibers in the myofibrils called thick filaments and thin filaments, which run parallel to the muscle cell's long axis

Program

supplementary motor area Premotor area Primary motor cortex