Muscle Anatomy and Function Quiz

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Questions and Answers

Match the following terms with their definitions:

Insertion = Attachment to movable bone Origin = Attachment to immovable or less movable bone Aponeurosis = Sheet-like fibrous tissue connecting muscles Fleshy attachment = Epimysium fused to periosteum of bone

Match the following terms with their associated connective tissue sheath in skeletal muscle:

Epimysium = Surrounds the entire muscle Perimysium = Wraps around fascicles Endomysium = Surrounds individual muscle fibers Fascicle = Bundle of muscle fibers

Match the following muscles with their functions:

Occipitofrontalis = Responsible for raising eyebrows Auricular muscles = Responsible for moving the ears Fascicles = Structural units of muscle tissue Aponeurosis = Connects muscles to other body parts

Match the following types of muscle attachments:

Direct (fleshy) = Epimysium fused to periosteum Indirect attachment = Uses tendons or aponeurosis Tendon = Rope-like structure connecting muscle to bone Aponeurosis = Sheetlike connective tissue for wide attachment Signup and view all the answers

Match the following anatomical structures with their locations:

External occipital protuberance = Origin of the epicranial aponeurosis Blood vessel = Found within perimysium Muscle fiber = Basic functional unit of muscle Fascicle = Grouped by perimysium Signup and view all the answers

Match the following characteristics with their corresponding connective tissue wrapping:

Epimysium = Surrounds whole muscle Perimysium = Encases bundles of muscle fibers Endomysium = Surrounds muscle fibers Tendon = Extension of connective tissue beyond muscle Signup and view all the answers

Match the following muscle attachment concepts with their description:

Direct attachment = Fleshy connection to bone Indirect attachment = Utilizes tendons for connection Insertion point = The moving end of muscle attachment Origin point = The fixed end of muscle attachment Signup and view all the answers

Match the following connective tissues with their descriptions:

Epimysium = Outer layer covering a muscle Perimysium = Surrounds a collection of muscle fibers Endomysium = Thin layer enveloping individual fibers Fascia = Connective tissue that surrounds muscles Signup and view all the answers

Match the steps at the neuromuscular junction with their descriptions:

1 = AP arrives at axon terminal 2 = Voltage-gated calcium channels open 3 = Calcium causes release of ACh 4 = ACh binds to receptors on sarcolemma Signup and view all the answers

Match the components involved in muscle contraction with their functions:

ACh = Neurotransmitter released for muscle contraction Calcium = Binds to troponin to initiate contraction Troponin = Regulates the position of tropomyosin Myosin = Forms cross bridges with actin during contraction Signup and view all the answers

Match the process with its description during muscle excitation:

End plate potential (EPP) = Local depolarization at the sarcolemma Action potential (AP) = Electrical signal that travels along the sarcolemma Excitation-contraction coupling = Process linking electrical activation to muscle contraction Cross bridge cycling = Mechanism that results in muscle contraction Signup and view all the answers

Match the sequence of events during muscle contraction:

ACh binding = Opens Na+ gates on sarcolemma Calcium release = Triggers muscle contraction T tubules = Transmits the AP deeper into muscle fiber Acetylcholinesterase = Degrades ACh to terminate signal Signup and view all the answers

Match the structures in skeletal muscle with their roles:

Sarcolemma = Plasma membrane of muscle fibers Sarcoplasmic reticulum = Stores calcium ions T tubules = Facilitates transmission of the AP Actin = Thin filament involved in muscle contraction Signup and view all the answers

Match the description of the neuromuscular junction with the action performed:

Calcium entry = Triggers release of neurotransmitter ACh diffusion = Crosses synaptic cleft to bind receptors Receptor binding = Opens Na+ channels in muscle fiber ACh degradation = Ends neuromuscular signal to prevent over-contraction Signup and view all the answers

Match the ion involved in muscle function with its role:

Na+ = Enters muscle fiber to create EPP Ca2+ = Initiates contraction by interacting with troponin K+ = Helps repolarize the membrane after contraction Cl- = Inhibitory ion that can prevent muscle contraction Signup and view all the answers

Match the neurotransmitter with its function in muscle activation:

ACh = Main neurotransmitter at the neuromuscular junction Dopamine = Influences movement but not directly in muscle contraction Serotonin = Regulates mood rather than muscle function Glutamate = Primary neurotransmitter for excitatory signals in the brain Signup and view all the answers

Match the following steps of the neuromuscular transmission process with their descriptions:

<ol> <li>Action potential arrival = Action potential arrives at axon terminal of motor neuron.</li> <li>Voltage-gated Ca2+ channels = Voltage-gated Ca2+ channels open.</li> <li>Ca2+ entry = Ca2+ entry causes ACh release by exocytosis.</li> <li>ACh binding = ACh diffuses and binds to receptors on the sarcolemma.</li> </ol> Signup and view all the answers

Match the following components of the neuromuscular junction with their roles:

Axon terminal = Site of action potential arrival. Synaptic cleft = Space where neurotransmitter diffuses. ACh = Neurotransmitter released at the junction. Sarcoplasm = Interior of the muscle fiber involved in contraction. Signup and view all the answers

Match the following events with their sequence in neuromuscular transmission:

A. Ca2+ entry = Occurs after voltage-gated channels open. B. ACh release = Occurs after Ca2+ enters the axon terminal. C. ACh diffusion = ACh moves across the synaptic cleft. D. ACh receptor binding = ACh binds to receptors on the muscle fiber. Signup and view all the answers

Match the following structures with their functions in the neuromuscular junction:

Motor neuron = Transmits action potential to axon terminal. Synaptic vesicle = Stores and releases ACh. Sarcolemma = Membrane that receives the neurotransmitter. Neuromuscular junction = Site of communication between neuron and muscle. Signup and view all the answers

Match the following terms with their definitions in the context of neuromuscular transmission:

Axon of motor neuron = Pathway for action potential to travel. Synaptic cleft = Gap between neuron and muscle. Fusing synaptic vesicles = Process that releases ACh. Receptors on sarcolemma = Bind ACh to initiate muscle contraction. Signup and view all the answers

Match the following ions involved in neuromuscular transmission with their roles:

Ca2+ = Triggers ACh release through exocytosis. Na+ = Involved in post-synaptic depolarization. K+ = Helps return cell to resting potential after activation. Cl- = May inhibit action potentials in some contexts. Signup and view all the answers

Match the neuromuscular junction terms with their definitions:

ACh = A neurotransmitter released from motor neurons End plate potential = Local depolarization caused by Na+ influx Depolarization = The process of becoming less negative inside the cell Repolarization = The return of the membrane potential to resting state Signup and view all the answers

Match the following terms related to neuromuscular transmission with their correct pairs:

Action potential = An electrical signal that initiates neurotransmitter release. Exocytosis = Process by which synaptic vesicles release ACh. Diffusion = Movement of ACh across synaptic cleft. Receptor binding = Event that triggers muscle contraction after ACh binds. Signup and view all the answers

Match the following phrases with their corresponding actions during neuromuscular transmission:

Arriving at axon terminal = Initiates the series of events for muscle contraction. Opening of channels = Allows Ca2+ ions to enter the axon terminal. Diffusion of ACh = Occurs after its release into the synaptic cleft. Binding to receptors = Leads to muscle depolarization and contraction. Signup and view all the answers

Match the diseases with their characteristics:

Myasthenia gravis = Autoimmune disease affecting ACh receptors Multiple sclerosis = Disease affecting nerve signaling in the CNS Amyotrophic lateral sclerosis = Motor neuron disease leading to muscle weakness Guillain-Barré syndrome = Autoimmune disorder attacking peripheral nerves Signup and view all the answers

Match the ions involved in action potential generation with their roles:

Na+ = Diffuses into muscle fiber causing depolarization K+ = Diffuses outward but less significant during EPP Ca2+ = Triggers the release of ACh from synaptic vesicles Cl- = Typically stabilizes resting membrane potential Signup and view all the answers

Match the phases of action potential with their descriptions:

End plate potential = Initial depolarization due to neurotransmitter binding Depolarization = Rapid interior change from negative to positive Repolarization = Return of the membrane potential to its resting state Hyperpolarization = Further increase in negativity after repolarization Signup and view all the answers

Match the following events with their sequence during action potential generation:

Release of ACh = First step at the neuromuscular junction Na+ influx = Causes end plate potential Wave of depolarization = Spreads to the adjacent sarcolemma Return to resting potential = Final step in the action potential process Signup and view all the answers

Match the types of synaptic vesicles with their contents:

Excitatory synaptic vesicles = Contain neurotransmitters like ACh Inhibitory synaptic vesicles = May contain GABA or glycine Modulatory synaptic vesicles = Release neuropeptides affecting neurotransmitter activity Membrane-bound vesicles = Contain receptors for various ligands Signup and view all the answers

Match the components of the neuromuscular junction with their functions:

Axon terminal = Location where neurotransmitter is released Synaptic cleft = Gap between neuron and muscle cell Sarcolemma = Muscle cell membrane that responds to neurotransmitters ACh receptors = Bind ACh to facilitate muscle contraction Signup and view all the answers

Match the mechanisms of muscle contraction with their descriptions:

Excitation = Action of ACh binding to receptors Contraction = Shortening of muscle fibers due to myofilament interaction Relaxation = Process of returning the muscle to its resting state Transmission = Spread of electrical signal along the sarcolemma Signup and view all the answers

Match the following steps with their corresponding descriptions in the neuromuscular junction process:

Action potential arrives = 1. The initial signal to release neurotransmitter Ca2+ channels open = 2. Allows calcium ions to enter the axon terminal ACh release by exocytosis = 3. Neurotransmitter is released into synaptic cleft Synaptic vesicles fuse = 4. Vesicles containing ACh merge with membrane Signup and view all the answers

Match the terms with their respective descriptions regarding excitation-contraction coupling:

Tropomyosin = Blocks active sites on actin at low Ca2+ concentration Troponin = Binds to Ca2+ to allow muscle contraction Sarcoplasmic reticulum = Stores and releases Ca2+ during contraction Cross bridge = The connection formed between myosin and actin Signup and view all the answers

Match the terms with their definitions in the context of nerve impulses:

Acetylcholine (ACh) = 1. The neurotransmitter used at the neuromuscular junction Exocytosis = 2. Process of neurotransmitter release Motor neuron = 3. Neuron that transmits impulses to muscles Voltage-gated channels = 4. Ion channels that open in response to voltage change Signup and view all the answers

Match the steps of the cross bridge cycle with their correct order:

Cross bridge formation = 1. High-energy myosin head attaches to actin Working stroke = 2. Myosin head pivots, pulling thin filament Cross bridge detachment = 3. ATP binds to myosin head causing detachment Cocking of myosin head = 4. Hydrolysis of ATP prepares myosin for next cycle Signup and view all the answers

Match the following mechanisms with their roles in muscle contraction:

Action potential = A. Initiates the sequence of events at the junction Calcium entry = B. Vital for ACh release into the synaptic cleft Neurotransmitter binding = C. Triggers muscle fiber contraction Muscle fiber response = D. Final action leading to contraction Signup and view all the answers

Match the intracellular events with their resulting outcomes:

Low Ca2+ concentration = Muscle fiber remains relaxed High Ca2+ concentration = Troponin changes shape, exposing binding sites Nervous stimulation ceases = Ca2+ is pumped back into the sarcoplasmic reticulum Binding of ATP = Causes myosin to detach from actin Signup and view all the answers

Match the following elements with their locations in the neuromuscular junction:

Synaptic vesicle = 1. Contains ACh at the axon terminal Neuromuscular junction = 2. The synapse between a motor neuron and muscle fiber Axon terminal = 3. Where the motor neuron ends Sarcolemma = 4. Membrane of the muscle fiber Signup and view all the answers

Match the components of the muscle contraction process with their roles:

Actin = Thin filament which myosin pulls Myosin = Thick filament with heads that bind to actin M line = Center of a sarcomere, where filaments pull towards ATP = Energy source for the myosin head to function Signup and view all the answers

Match these steps of muscle activation with their order:

Arrival of action potential = 1. First step in the activation process Release of Ca2+ = 2. Triggers ACh release Binding of ACh = 3. Initiates muscle fibers to contract Calcium's role = 4. Essential for the exocytosis of ACh Signup and view all the answers

Match the concepts with their corresponding processes:

Excitation-contraction coupling = Sequence leading from action potential to muscle contraction Voltage-sensitive proteins = Change shape to stimulate Ca2+ release Myofilaments sliding = Result of cross bridge cycling Sarcoplasmic reticulum release = Triggered by electrical stimulation of T tubules Signup and view all the answers

Match the muscle contraction terms with their definitions:

Relaxation = Muscle fiber returns to its resting state Sarcoplasmic reticulum (SR) = Organelle that regulates calcium ion concentration Cross bridge cycling = Repeating process of myosin heads pulling actin Cytosol = Fluid component where calcium is released for contraction Signup and view all the answers

Match the following ions with their actions in the neuromuscular junction:

Calcium ions (Ca2+) = A. Open channels allowing calcium entry Sodium ions (Na+) = B. Not directly involved in ACh release Potassium ions (K+) = C. Important for repolarizing the membrane Chloride ions (Cl-) = D. Regulates excitability of the neuron Signup and view all the answers

Match the changes of myofilaments with their corresponding conditions:

Tropomyosin moving away = Allows myosin heads to bind to actin Ca2+ binding to troponin = Initiates muscle contraction Cocking of myosin head = Prepares for the next power stroke Cross bridge detachment = Occurs when ATP binds to myosin Signup and view all the answers

Match the following neurotransmitters with their specific functions:

Acetylcholine (ACh) = 1. Activates muscle fibers Glutamate = 2. Main excitatory neurotransmitter in the brain Dopamine = 3. Modulates movement and reward Serotonin = 4. Regulates mood and sleep cycles Signup and view all the answers

Match the following muscle contraction phases with their characteristics:

Latent phase = Time between stimulus and contraction Contractile phase = Actual shortening of the muscle fiber Relaxation phase = End of contraction; muscle fiber returns to rest Recovery phase = Restoration of Ca2+ levels in SR Signup and view all the answers

Study Notes

Chapter 09 Part A: Muscles and Muscle Tissue

Muscles make up nearly half of the body's mass
Muscles transform chemical energy (ATP) into mechanical energy, capable of exerting force
To understand muscles, consider:
- Types of muscle tissue
- Characteristics of muscle tissue
- Muscle functions

Types of Muscle Tissue

Terminologies: Myo, mys, and sarco are prefixes for muscle. Example: sarcoplasm (muscle cell cytoplasm)
Three types: Skeletal, Cardiac, Smooth
Skeletal muscle:
- Packaged into skeletal muscles (organs attached to bones/skin)
- Longest of all muscle fibers
- Striated (striped)
- Voluntary (consciously controlled)
- Contracts rapidly; tires easily; powerful
- Key words: skeletal, striated, voluntary
Cardiac muscle:
- Found only in the heart
- Makes up most of the heart walls
- Striated
- Involuntary (cannot be controlled consciously)
- Contracts at a steady rate, controlled by the heart's own pacemaker, but the nervous system can increase rate
- Key words: cardiac, striated, involuntary
Smooth muscle:
- Found in the walls of hollow organs (e.g., stomach, urinary bladder, airways)
- Not striated
- Involuntary (cannot be controlled consciously)
- Key words: visceral, nonstriated, involuntary

Table 9.3-1 Comparison of Skeletal, Cardiac, and Smooth Muscle

Provides a comparison table of skeletal, cardiac, and smooth muscle, detailing body location, cell shape/appearance, and other characteristics.

Characteristics of Muscle Tissue

All muscles share four main characteristics:
- Excitability (responsiveness): ability to receive and respond to stimuli
- Contractility: ability to shorten forcibly when stimulated
- Extensibility: ability to be stretched
- Elasticity: ability to recoil to resting length

Muscle Functions

Produce movement: responsible for all locomotion and manipulation (e.g., walking, digesting, pumping blood)
Maintain posture and body position
Stabilize joints
Generate heat as they contract

Skeletal Muscle Anatomy

Skeletal muscle is an organ made up of several tissues with these features:
- Nerve and blood supply
- Connective tissue sheaths
- Attachments

Nerve and Blood Supply

Each muscle receives a nerve, artery, and veins
Control skeletal muscle activity by supplying nerves to each fiber
Contraction requires oxygen and nutrients, thus needing quick removal of waste products.

Connective Tissue Sheaths

Each muscle and muscle fiber is covered by connective tissue
Support cells and reinforce the whole muscle
Three layers from external to internal:
- Epimysium (surrounds entire muscle or blends with fascia)
- Perimysium (surrounds fascicles or groups of muscle fibers)
- Endomysium (surrounds each muscle fiber)

Skeletal Muscle Attachments

Muscles span joints and attach to bones in at least two places.
Two points of attachment:
- Origin: attachment to immovable or less movable bone
- Insertion: attachment to movable bone
Types of attachments:
- Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage
- Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis

Aponeuroses

Sheet-like, pearly-white tissues similar to tendons
Connect sheet-like muscles, providing wide areas of attachment to other body parts (bones, cartilage, other muscles)

9.3 Muscle Fiber Microanatomy and Sliding Filament Model

Skeletal muscle fibers are long, cylindrical cells with multiple nuclei
Sarcolemma: muscle fiber plasma membrane
Sarcoplasm: muscle fiber cytoplasm
Myofibrils: densely packed, rod-like elements within muscle fiber, responsible for ~ 80 % of muscle volume
- Contains many glycosomes (for glycogen storage) and myoglobin (for O2 storage)
Sarcoplasmic reticulum: network of smooth endoplasmic reticulum surrounding each myofibril
T (transverse) tubules: formed by sarcolemma protrusions into the muscle fiber, increasing surface area and allow electrical nerve transmission to reach deep into the fiber.

Myofibrils (1 of 7): Features and Composition

Myofibrils are packed, rod-like elements within muscle fibers.
Key features include striations (striped pattern), sarcomeres (functional units), and myofilaments (actin and myosin).

Myofibrils (2 of 7): Striations and Structural Components

Striations are formed by repeating series of dark (A bands) and light (I bands) bands along the length of each myofibril.
Components within each myofibril include: A bands, H zone, M line, I bands, and Z discs.

Myofibrils (3 of 7): Sarcomere (Functional Unit)

Sarcomeres are the smallest contractile units of a muscle fiber.
Each sarcomere is the region between two Z discs.
Sarcomeres align end to end along a myofibril, similar to boxcars.

Myofibrils (4 of 7): Myofilaments (Actin and Myosin)

Myofilaments (protein fibers) are responsible for muscle contraction.
Thin filaments (primarily actin) extend across the I band and partway into the A band.
Thick filaments (primarily myosin) extend the entire length of the A band.

Myofibrils (5 of 7): Molecular Composition of Myofilaments (Thick Filaments)

Thick filaments are made of myosin protein.
Myosin contains two heavy and four light polypeptide chains.
Heavy chains form the myosin tail.
Light chains form the myosin globular head..
Myosin heads bind to actin during contraction forming cross bridges.

Myofibrils (6 of 7): Molecular Composition of Myofilaments (Thin Filaments)

Thin filaments are made up of actin protein.
G actin subunits bear active sites for myosin head attachment.
F actin is a filamentous actin consisting of two strands of G actin twisted together.
Tropomyosin and troponin are regulatory proteins bound to actin.

Myofibrils (7 of 7): Additional Proteins

Elastic filaments (titin): hold thick filaments in place, recoil after stretch, resist excessive stretching.
Dystrophin: links thin filaments to proteins of sarcolemma.
Nebulin, myomesin, C proteins: bind filaments/sarcomeres together; maintain alignment.

9.1 Overview of Muscle Tissue (cont.): Muscle Fiber Microanatomy

Muscle fiber microanatomy: Describes the internal structure of muscle fibers (cells), including myofibrils, sarcomeres, and the proteins involved in contraction.

Clinical – Homeostatic Imbalance 9.1 (Duchenne Muscular Dystrophy)

Duchenne muscular dystrophy (DMD) is a sex-linked recessive disease, affecting primarily males.
Symptoms start in childhood, progressively affecting muscles (from extremities upward) and can lead to cardiac muscle issues.
Caused by defective gene for dystrophin, a protein linking thin filaments with extracellular matrix/sarcolemma
Muscle fibers tear easily, allowing excess calcium entry, causing damage to contractile fibers, inflammation, and increased apoptosis (cell death) leading to muscle loss and disease progression.

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic reticulum (SR): network of smooth endoplasmic reticulum surrounding each myofibril.
Terminal cisterns: form perpendicular cross channels at A-I junction. Important for regulating intracellular Ca2+ levels. Stores/releases Ca2+ during contraction
T tubules are extensions of the sarcolemma, traveling deep into the muscle fiber. Increase membrane surface area and allow electrical nerve transmissions to reach deep into fiber.

Sarcoplasmic Reticulum and T Tubules (Triad Relationships)

Triad is the junction formed by t tubule and two terminal cisterns
T tubule and SR integral protein interactions trigger Ca release from SR cisterns.
When electrical impulse passes through t-tubules, SR proteins are affected causing Calcium release into the cytoplasm causing muscle contraction.

Sliding Filament Model of Contraction (1 of 3)

Contraction: Activation of cross-bridges generate force. Shortening occurs when tension generated on thin filaments exceeds opposing forces. The contraction ends when cross-bridges become inactive.
Sliding filament model: In a relaxed state, thin and thick filaments slightly overlap. During contraction, thin filaments slide past thick filaments causing more overlap. Lengths of filaments do not change.

Sliding Filament Model of Contraction (2 of 3)

Nervous system stimulation allows myosin heads to bind to actin.
This binding causes the sliding process to begin.

Sliding Filament Model of Contraction (3 of 3)

Cross-bridges attach and detach, pulling thin filaments towards the center of sarcomeres.
I bands shorten, Z disks become closer together, H zones disappear, A bands move closer together.

9.4 Muscle Fiber Contraction: Background

Decision to move is activated by brain, signal transmitted down spinal cord
Neurons activate muscle fibers by generating action potentials
AP crosses from neuron to muscle cell via the neurotransmitter acetylcholine (ACh)

Ion Channels

Play a major role in changing membrane potentials
Two types:
- Chemically gated channels: opened by chemical messengers (e.g., neurotransmitters, like ACh)
- Voltage-gated channels: open or close in response to membrane potential changes

Anatomy of Motor Neurons and the Neuromuscular Junction / Motor End Plate

Skeletal muscles are stimulated by somatic motor neurons via axonal projections (long extensions) called axon.
Each axon branches into many branches as it goes into the muscle
Axon branches end on muscle fiber, forming neuromuscular junction (or motor end plate)
Each muscle fiber has one neuromuscular junction with one motor neuron

Overview of Skeletal Muscle Contraction

Signal from brain travels down spinal cord to motor neuron, triggering axon release of neurotransmitter Acetylcholine
ACh binds receptors on muscle fiber surface
Resulting depolarization triggers an action potential and muscle contraction through steps of excitation, contraction coupling, and cross-bridge cycling.

Events at Neuromuscular Junction

Action potential arrives at axon terminal
Voltage-gated Ca2+ channels open, Ca2+ enters axon terminal.
Ca2+ entry causes ACh release into synaptic cleft.
ACh diffuses across synaptic cleft, binds to ACh receptors
ACh binding opens ion channels in muscle fiber membrane, leading to end-plate potential.
Acetylcholinesterase degrades ACh, causing termination of signal.

Clinical – Homeostatic Imbalance 9.2 (Myasthenia Gravis)

Myasthenia gravis is a disease characterized by muscle weakness (drooping eyelids, difficulty swallowing/talking).
Involves a shortage of ACh receptors due to the immune system attacking the receptors.
Suggests it is an autoimmune disease, based on ACh receptor shortage.

Generation of an Action Potential Across the Sarcolemma

Resting sarcolemma is polarized (negative inside, positive outside)
Action potential is caused by changes in electrical charges (generation of end plate potential, depolarization, and repolarization)

Generation of an Action Potential Across the Sarcolemma (End Plate Potential)

ACh released from motor neuron binds to ACh receptors
Neurotransmitter binding opens chemically gated ion channels (Na+ channels open)
Na+ diffuses into muscle fiber, causing interior of sarcolemma to become less negative (more positive)
This local depolarization is called end-plate potential (EPP), leading to action potential.

Generation of an Action Potential Across the Sarcolemma (Depolarization)

If end-plate potential causes enough change in membrane voltage to reach threshold (a critical level), voltage-gated Na+ channels open.
Large Na+ influx triggers unstoppable action potential, leading to muscle fiber contraction.
The action potential spreads from one voltage-gated Na+ channel to the next, depolarizing adjacent areas.

Generation of an Action Potential Across the Sarcolemma (Repolarization)

Voltage-gated Na+ channels close, and voltage-gated K+ channels open.
K+ efflux out of the cell rapidly returns membrane to its initial resting potential.
Ionic conditions (Na+/K+) of resting state are restored by the Na+/K+ pump.

Excitation-Contraction (E-C) Coupling

Events that transmit AP along sarcolemma (excitation) are coupled to the sliding of myofilaments (contraction).
AP is propagated along sarcolemma and down T-tubules.
Voltage-sensitive proteins in T-tubules stimulate Ca2+ release from SR.
Ca2+ release leads to contraction but AP ends before contraction actually starts.

Muscle Fiber Contraction: Cross Bridge Cycling

At low intracellular Ca2+ concentration, tropomyosin blocks active sites on actin, preventing contraction. Myosin heads cannot attach to actin, and the muscle fiber remains relaxed.
Voltage-sensitive proteins in T tubules change shape, triggering SR release of Ca2+ into the cytosol.

Muscle Fiber Contraction: Cross Bridge Cycling (cont.)

At higher intracellular Ca2+ concentrations, Ca2+ binds to troponin, causing tropomyosin to move away from myosin-binding sites on actin.
Myosin heads can then bind to actin, initiating the cross-bridge cycle, which leads to sarcomere shortening and muscle contraction.
When nervous stimulation ceases, Ca2+ is pumped back into the SR, contraction ends, and tropomyosin returns to block active sites.

Muscle Fiber Contraction: Cross Bridge Cycling (Four Steps)

Four steps of the cross-bridge cycle:
1. Cross-bridge formation: Energized myosin head attaches to actin.
2. Power (working) stroke: Myosin head pivots and bends, pulling actin toward M line.
3. Cross-bridge detachment: ATP attaches to myosin, causing detachment.
4. Cocking of myosin head: Energy from ATP hydrolysis "cocks" myosin into high-energy state, preparing for the next cycle.

Clinical – Homeostatic Imbalance 9.3 (Rigor Mortis)

Rigor mortis: Stiffening of muscles after death, lasting 3-4 hours after death, peaking about 12 hours postmortem.
Caused by the lack of ATP synthesis, preventing calcium from being pumped back into the SR.
This leads to continued cross-bridge formation, resulting in muscle contraction until muscle proteins break down and myosin is released.

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Test your knowledge on the terms, structures, and functions associated with skeletal muscle. This quiz covers connective tissue sheaths, muscle attachment concepts, and the steps involved in muscle contraction. Perfect for students studying anatomy or physiology!