EXAM 4 TAMU COHN

Describe the details of muscle function in the detail gone over in class.

Movement, Posture, Joint Stability, Thermogenesis

Describe the details of muscle function in the detail gone over in class. Movement

Muscle cells shorten to create tension and pull (never push). Most skeletal muscles move the skeleton, but some (like facial muscles) move skin.

Describe the details of muscle function in the detail gone over in class. Posture

Muscles constantly maintain body position against gravity, even when "still." Torso, neck, and limb muscles create continuous tension; loss of consciousness → collapse. Posture is continuously adjusted by the brain and spinal cord.

Describe the details of muscle function in the detail gone over in class. Joint Stability

Stability and mobility are inverse. The hip is more stable (deep socket, weight-bearing), while the shoulder is more mobile but less stable

Describe the details of muscle function in the detail gone over in class. KEY EX:

Muscles: subscapularis, supraspinatus, infraspinatus, teres minor

Function: Tendons form a “cuff” around the humeral head

Action: Contract to snug the humeral head into the glenoid cavity, stabilizing the shoulder

Always slightly active; especially important when the arm is pulled back (prevents dislocation)

Describe the details of muscle function in the detail gone over in class. Thermogenesis

Muscle generates heat (ex: shivering = involuntary contraction)

Are muscle cells "excitable". If so, how so

Muscle cells are stimulated by a neuron and generate a muscle action potential along the sarcolemma, which is required for contraction.

define "excitable".

ability to produce an action potential (a wave-like change in voltage across the cell membrane).

What does the term "potential" often mean in muscle and nerve physiology?

“Potential” = voltage.

In muscle and nerve physiology, it refers to an electrical difference across the membrane.

Why is an action potential called an "action" potential.

It’s called an “action” potential because the change in voltage actually moves in a wave-like way along the membrane and takes action (spreads down the cell).

Describe the relationship between contraction of a muscle cell and excitation of a muscle cell. The word "predicated" is as good word to put in your answer.

Contraction is predicated on excitation.

A muscle cell must first be electrically excited (action potential) before it can contract.

This is called excitation–contraction coupling: excitation comes first, then contraction follows.

What is a "muscle potential"?

a muscle action potential—a wave-like change in voltage that travels along the sarcolemma.

Is skeletal muscle tissue (and therefore skeletal muscle as a whole) relatively (compared to other tissues and organs) elastic and extensible.

Yes.

Skeletal muscle is relatively elastic and extensible compared to other tissues.

describe how thow elastic and extensible are related.

Relationship: Muscle is both—it can stretch (extensible) and then return to normal length (elastic).

Define elastic

can return (snap back) to its original resting position after being stretched or compressed

Define extensible

can be stretched from its resting position without tearing

Describe the relationship between the phrase "muscle fiber" and "muscle cell".

Muscle fiber = muscle cell.

They are exact synonyms—a muscle fiber is a single muscle cell (just very large).

Describe relative size (length and thickness) of skeletal muscle cells compared to other muscle cells contribute to forming the fully mature skeletal muscle cell.

Much larger than other cells—can be many mm to several cm long (even ~30 cm) but only ~100 μm thick, so still not visible individually.

Describe striations contribute to forming the fully mature skeletal muscle cell.

Striated (due to internal organization of contractile structures)

Describe number of nuclei contribute to forming the fully mature skeletal muscle cell.

Multinucleated (many nuclei per cell)

Describe stem cell name and function and how these stem cells contribute to forming the fully mature skeletal muscle cell.

Myoblasts

Hundreds of myoblasts fuse together

Each contributes cytoplasm + nucleus → forms one large muscle cell

This is why mature skeletal muscle cells are very large and multinucleated

Describe the synapse between a neuron and a skeletal muscle cell

Neuromuscular junction (synapse):

A chemical synapse where a neuron communicates with a skeletal muscle cell

An action potential travels down the neuron → causes release of a neurotransmitter → stimulates the muscle cell to generate its own muscle action potential

how an action potential propagates down the muscle cell while also propagating through the muscle cell.

Propagation in the muscle cell:

The muscle action potential travels along the sarcolemma

When it reaches a T-tubule, it splits:

One continues down the surface

One travels into the cell (through T-tubules)

This allows the signal to spread both along and deep into the muscle cell, activating the entire fiber

Define myofilament

protein filaments that slide past each other to cause contraction

Define sarcoplasm

the cytoplasm of a muscle cell

Define sarcolemma

the cell membrane of a muscle cell

Define sarcoplasmic reticulum

specialized smooth ER in muscle that has a key role in contraction (stores/releases Ca²⁺)

Given a muscle like the biceps or triceps, describe the "cylinders within cylinders" paradigm of organization, beginning with the muscle itself all the way down to the level of the myofibril. The terms fascicles, fibers, myofibrils should be used in your description.

“Cylinders within cylinders” (muscle organization):

The whole muscle (ex: biceps) is a large cylinder

Inside are smaller cylinders called fascicles

Each fascicle contains many muscle fibers (muscle cells)

Each muscle fiber contains even smaller cylinders called myofibrils

→ So: muscle → fascicles → fibers → myofibrils (each level nested inside the next)

What is the relationship between myofibrils and sarcomeres.

Myofibrils are made of repeating sarcomeres.

Sarcomeres are arranged end-to-end (in tandem) to form a myofibril.

Each sarcomere is the contractile unit of the myofibril.

Which of these organizational unit functioning as the anatomical/physiological "atomic unit" of skeletal muscle contraction and why is this so?

Sarcomere.

The sarcomere is the anatomical/physiological “atomic unit” of contraction because it is the smallest independent unit that can contract.

When all sarcomeres shorten, the entire muscle shortens.

Describe/define the terms epimysium

dense connective tissue that surrounds the entire muscle (outer “skin”)

Describe/define the terms perimysium

connective tissue that surrounds each fascicle (bundle of fibers)

Describe/define the terms endomysium

thin connective tissue that surrounds each individual muscle fiber (cell)

Describe the role epimysium, perimysium and endomysium have in muscle organization

These layers organize the muscle into cylinders within cylinders and support/hold all structures together.

myofibrils, mitochondria, nuclei, myofibrils, the triad (what are the components of the triad), sarcoplasmic reticulum

Which of the above are large contractile organelles that take up almost all of the volume of the cell? 

Myofibrils are the large contractile organelles that occupy most of the cell’s volume

Describe the ultrastructural relationship of myofibrils in general

long contractile organelles that run the length of the cell and take up almost all of the cell’s volume

Describe the ultrastructural relationship of nuclei & mitochondria in general

pushed to the edges/periphery of the cell, fitting into small spaces between myofibrils

Describe the ultrastructural relationship of the triad (what are the components of the triad) in general

1 T-tubule (inward extension of the sarcolemma)

2 terminal cisternae (enlarged regions of SR on either side)

Describe the ultrastructural relationship of the sarcoplasmic reticulum in general

a network surrounding (draped over) the myofibrils

Describe the ultrastructural relationship of the terminal cistern(s)/cisterna(e) in general

specialized parts of SR that store high concentrations of Ca²⁺ and release it during stimulation

 Describe an action potentials path down the sarcolemma and the impact that T-tubules have on its path.  

The action potential travels along the sarcolemma (muscle cell membrane).

When it reaches a T-tubule, it splits:

One continues down the surface

One goes into the cell through the T-tubule

Describe the impact that T-tubules have on its path.  

T-tubules allow the action potential to spread deep into the muscle cell, so the entire fiber is activated at the same time.

What effect does an action potential down a T-tubule have on the adjacent terminal cisterns?  

An action potential in a T-tubule causes the adjacent terminal cisternae to release Ca²⁺ into the sarcoplasm.

What is released from these terminal cisterns into the sarcoplasm upon action potential stimulation? 

Calcium ions (Ca²⁺) are released into the sarcoplasm.

What does the increased spike of sarcoplasmic calcium interact with and what is the significance of this interaction?

The increased Ca²⁺ binds to the contractile machinery of the sarcomere (thin filament components).

Significance: this initiates contraction by allowing the filaments to interact (sliding filament mechanism).

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Neuromuscular Junction (NMJ):

A chemical synapse between a neuron and a skeletal muscle cell where the signal is transmitted

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Action potential down a neuron

A wave-like voltage change that travels along the neuron to the axon terminal

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Axon terminal

The end of the neuron where neurotransmitter is released

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Voltage-gated calcium channels

Open when the action potential arrives → allow Ca²⁺ to enter the axon terminal

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Acetylcholine (ACh) vesicles

Vesicles that store ACh and release it into the synapse

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Synaptic cleft

The gap between neuron and muscle cell where ACh diffuses

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: ACh-gated sodium channels

Channels on the muscle that open when ACh binds → allow Na⁺ in

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Motor endplate

Specialized region of the sarcolemma with ACh receptors

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Motor endplate depolarization

Na⁺ entry causes a local voltage change in the muscle membrane

Be able to define the structure/function of the following components of the neuromuscular junction to the extent discussed in class.: Action potential down the sarcolemma

The depolarization triggers a muscle action potential that spreads along the sarcolemma and initiates contraction

Describe how the sarcomere is the fundamental atomic unit of muscle contraction.

The sarcomere is the fundamental atomic unit of muscle contraction because it is the smallest unit that can contract independently.

Myofibrils are made of sarcomeres arranged end-to-end

When each sarcomere shortens, the myofibril shortens

This causes the muscle fiber → fascicle → whole muscle to shorten

→ Therefore, whole muscle contraction occurs because all sarcomeres contract

Relaxed (100% default) sarcomere: Z-discs

at each end (define the sarcomere boundaries)

Relaxed (100% default) sarcomere: Thin filaments (actin)

attached to Z-discs, extend inward ~1/3 of the sarcomere

Relaxed (100% default) sarcomere: Thick filaments (myosin)

centered in the middle of the sarcomere

Relaxed (100% default) sarcomere: A-band

entire length of thick filaments (middle region)

Relaxed (100% default) sarcomere: Half I-bands

regions near each Z-disc with only thin filaments (no thick)

Relaxed (100% default) sarcomere: H-zone

center of A-band with only thick filaments (no thin overlap)

Relaxed (100% default) sarcomere: M-line

middle of sarcomere, holds thick filaments together

Relaxed (100% default) sarcomere: Titin

spring-like protein connecting thick filaments to Z-discs

Fully contracted sarcomere: Z-discs

move closer together

Fully contracted sarcomere: Thin filaments

pulled further inward toward center

Fully contracted sarcomere: Thick filaments

stay in same position/length

Fully contracted sarcomere: A-band

unchanged

Fully contracted sarcomere: Half I-bands

shorten dramatically (almost disappear)

Fully contracted sarcomere: H-zone

collapses/disappears (now overlap with thin filaments)

Fully contracted sarcomere: M-line

still at center

Fully contracted sarcomere: Titin

becomes compressed (spring compressed)

Which components change length and how?

Half I-bands → shorten

H-zone → disappears

Sarcomere overall → shortens (~2/3 resting length)

What component DO NOT change length and how?

A-band (thick filament length) stays the same

How do the changing of these components support the hypothesized mechanism in which the filaments move past one another (i.e., the sliding filament hypothesis).

→ These changes support that thick filaments stay stationary and pull thin filaments inward, causing the sarcomere to shorten (sliding filament hypothesis).

Describe the role of titin in endowing the sarcomere with its ability to shorten, extend and snap back into position. The term "spring" should be in your description.

Titin acts like a spring in the sarcomere.

It connects the thick filament to the Z-disc

When the sarcomere is stretched, titin extends like a spring

When released, it snaps the sarcomere back to its resting position

During contraction, it can also be compressed like a spring

→ This “spring” property allows the sarcomere to shorten, extend, and return to default position

Explain why skeletal muscles on average are capable of only shortening to about 2/3 of their resting length. The word "sarcomere" should be in your answer.

Skeletal muscle can only shorten to about 2/3 of its resting length because each sarcomere can only shorten that much.

During contraction, the sarcomere shortens until the filaments are maximally overlapped and the titin “spring” is fully compressed

At this point, the Z-discs cannot move any closer

→ Since a muscle is made of many sarcomeres in series, the whole muscle can only shorten as much as each sarcomere allows (~2/3 length)

Describe myosin protein structure to the extent discussed in class. The terms dimer, polypeptide, globular head and hinge region should be in your description.

Myosin is a dimer, meaning it is made of two polypeptide chains twisted together

Each polypeptide forms a globular head at one end

The heads are connected to the tail by a hinge region

→ The globular heads extend outward from the thick filament and are able to interact with thin filaments during contraction

What filament has myosin proteins as the component?

Thick filament.

Describe the following components of the thin filament: helical arrangement of actin

thin filament is a helical (twisted) chain of actin proteins

Describe the following components of the thin filament: active sites on actin

specific sites where myosin heads can bind

Describe the following components of the thin filament: tropomyosin

protein that covers (blocks) the active sites on actin in an unstimulated muscle

Describe the following components of the thin filament: troponin.

protein that holds tropomyosin in place on actin; when Ca²⁺ binds, it changes shape and moves tropomyosin to expose active sites

Which of these is the core scaffolding of the thin filament?

Actin

Which is the site that myosin heads can potentially bind to?

Active sites on actin.

Which blocks myosin globular heads from binding active sites in an unstimulated muscle?

Tropomyosin

Which attaches the tropomyosin to the actin in an unstimulated muscle so that active sites on actin are blocked?

Troponin

Which subunit of troponin binds calcium released from the sarcoplasmic reticulum and thus changes shape, pulling tropomyosin out of the way so that myosin heads can bind to the thin filament and pull?

The calcium-binding subunit of troponin.

-Know the names of all steps of crossbridge cycling and describe the order of each step and what happens during each step.

Crossbridge formation

Power stroke

Crossbridge detachment

Reactivation (cocking of myosin head)

→ This cycle repeats, causing the thin filaments to slide past thick filaments, shortening the sarcomere

-Know the names of all steps of crossbridge cycling and describe the order of each step and what happens during each step: Crossbridge formation

Myosin globular head binds to exposed active sites on actin

-Know the names of all steps of crossbridge cycling and describe the order of each step and what happens during each step: Power Stroke

Myosin head pivots (hinge region) and pulls the thin filament inward

-Know the names of all steps of crossbridge cycling and describe the order of each step and what happens during each step: Crossbridge detachment

Myosin head releases from actin

-Know the names of all steps of crossbridge cycling and describe the order of each step and what happens during each step: Reactivation (cocking of myosin head)

Myosin head returns to its original position and is ready to bind again

Between what steps can the cycle be disrupted by insufficient amounts of a sarcoplasmically soluble factor? What are these factors?

Between crossbridge detachment and reactivation.

The cycle can be disrupted if there is not enough ATP (sarcoplasmically soluble factor).

Without ATP, the myosin head cannot detach from actin, stopping the cycle, creating rigor mortis.

what will the effect be on the muscle if that particular soluble factor is not present in sufficient quantities?

If ATP is not present in sufficient amounts, the muscle will remain contracted (rigid).

Myosin heads cannot detach from actin

Crossbridges stay locked → no relaxation

→ This results in muscle stiffness/rigor

Which factor's diminishment leads to fatigue causing forced relaxation?

Calcium (Ca²⁺).

Which factor's diminishment leads to fusion of myosin heads with thin filaments causing "seizing up" of the muscle? If that happens, what is the name of the condition that results?

ATP.

When ATP is insufficient, myosin heads cannot detach from actin → they remain bound (“seize up”)

This results in rigor (rigor mortis)

Between which two steps is it critically important for there to be a sufficient amount of ATP?

Between the power stroke and crossbridge detachment.

ATP is required for the myosin head to detach from actin

Without ATP, the cycle cannot continue

What happens if ATP falls below a sufficient amount?

If ATP falls below sufficient levels:

Myosin heads cannot detach from actin

Crossbridges remain locked → muscle stays contracted

→ This causes rigor (muscle stiffness/seizing up)

Between which two steps is it critically important for there to be enough sarcoplasmic Ca++ to continue subsequent crossbridge cyclings after the first crossbridge cycle?

Between reactivation (cocking) and crossbridge formation.

Sufficient Ca²⁺ is needed to keep active sites on actin exposed

Without Ca²⁺, tropomyosin covers the sites → no new crossbridge can form