Muscle Contraction: Nerve, Neuromuscular Junction, and Excitation-Contraction Coupling

What is the role of nerves in skeletal muscle contraction?

Skeletal muscle never contracts unless stimulated by a nerve.

What happens to a muscle if nerve connections are severed or poisoned?

The muscle becomes paralyzed.

What is denervation atrophy?

The shrinkage of paralyzed muscle when the nerve connection is not restored.

Where are somatic motor neurons located?

In the brainstem and spinal cord.

What are somatic motor fibers?

The axons of somatic motor neurons that lead to skeletal muscle.

How many muscle fibers does an average motor unit innervate?

About 200 muscle fibers.

What is a motor unit?

One nerve fiber and all the muscle fibers innervated by it.

What is the function of small motor units?

They provide a fine degree of control, such as in eye and hand muscles.

What is the function of large motor units?

They provide more strength than control, allowing powerful contractions.

What is a neuromuscular junction (NMJ)?

The synapse where a nerve fiber meets a muscle fiber.

What is contained within the synaptic knob at the NMJ?

Synaptic vesicles filled with acetylcholine (ACh).

What is the synaptic cleft?

The tiny gap between the synaptic knob and the muscle sarcolemma.

What role does acetylcholinesterase (AChE) play at the NMJ?

It breaks down ACh after contraction, causing relaxation.

What is the basal lamina?

A thin layer of collagen and glycoprotein that separates Schwann cells and muscle cells from surrounding tissues.

What is the significance of ACh receptors in muscle contraction?

They are proteins in the muscle cell membrane that bind ACh; a lack of receptors can lead to paralysis in myasthenia gravis.

What happens to synaptic vesicles during muscle stimulation?

They undergo exocytosis, releasing ACh into the synaptic cleft.

How do muscle fibers of one motor unit behave?

They contract in unison and produce weak contractions over a wide area.

What is the role of motor units in postural control?

Motor units take turns contracting to sustain long-term contraction.

What is the effect of multiple motor units contracting simultaneously?

It usually results in effective contraction.

What are electrically excitable cells?

Cells, such as muscle fibers and neurons, that exhibit voltage changes in response to stimulation.

What is electrophysiology?

The study of the electrical activity of cells.

What is resting membrane potential (RMP)?

The stable voltage of about -90 mV maintained by the sodium-potassium pump in a resting cell.

What happens to the membrane potential when a muscle fiber is stimulated?

Ion gates open, Na+ diffuses into the cell, causing depolarization, and then K+ exits, leading to repolarization.

Define depolarization.

The process where the inside of the plasma membrane becomes briefly positive due to Na+ influx.

What is repolarization?

The process where the membrane potential returns to a negative value after depolarization.

What is an action potential?

A quick up-and-down voltage shift from the negative resting membrane potential to a positive value and back.

What are the four major phases of muscle contraction?

Excitation, excitation-contraction coupling, contraction, and relaxation.

What occurs during excitation of a muscle fiber?

Nerve action potentials lead to muscle action potentials through the release of acetylcholine (ACh).

What is the role of calcium in muscle contraction?

Calcium binds to troponin, causing a conformational change that exposes active sites on actin for myosin binding.

What is the end-plate potential (EPP)?

A quick voltage shift in the end-plate region caused by Na+ entering the cell after ACh binds to receptors.

What initiates the action potential in muscle fibers?

The voltage change (EPP) opens nearby voltage-gated channels, producing an action potential that spreads over the muscle surface.

What happens during excitation-contraction coupling?

The action potential spreads down T tubules, opening voltage-gated channels and allowing Ca2+ to enter the cytosol.

What is the role of myosin ATPase during contraction?

It hydrolyzes ATP, activating the myosin head and allowing it to bind to actin, forming a cross-bridge.

What is a power stroke in muscle contraction?

The process where the myosin head releases ADP and Pi, flexes, and pulls the thin filament past the thick filament.

How often does each myosin head perform power strokes?

Each head performs five power strokes per second.

What happens when myosin binds to a new ATP molecule?

Myosin releases actin, breaking the cross-bridge, and the contraction cycle can repeat.

What is the significance of the sodium-potassium pump?

It maintains the resting membrane potential by pumping Na+ out and K+ into the cell.

What causes the inside of a resting cell to be negatively charged?

Excess anions such as proteins, nucleic acids, and phosphates in the intracellular fluid.

What triggers the release of acetylcholine (ACh) at the neuromuscular junction?

The entry of calcium ions (Ca2+) into the synaptic knob after a nerve signal.

What happens to the voltage during the action potential?

It shifts from the negative resting membrane potential to a positive value and then back to negative.

What is the role of troponin in muscle contraction?

Troponin binds calcium, causing a shift in the tropomyosin complex to expose actin's active sites.

What is the resting state of a muscle fiber characterized by?

More anions inside the plasma membrane than outside, with excess Na+ in ECF and K+ in ICF.

What is the function of voltage-gated calcium channels in muscle contraction?

They allow Ca2+ to enter the cytosol, initiating the contraction process.

What is the sequence of events from nerve signal to muscle contraction?

Nerve signal → ACh release → EPP → Action potential → Ca2+ release → Contraction.

What is the role of tropomyosin in muscle contraction?

It blocks the active sites on actin until troponin binds calcium and shifts tropomyosin.

How does the action potential propagate along the muscle fiber?

It triggers adjacent voltage-gated channels, causing a wave of depolarization along the membrane.

What stops nerve stimulation and ACh release during muscle relaxation?

The breakdown of ACh by acetylcholinesterase (AChE).

What is the role of AChE in muscle relaxation?

AChE breaks down ACh, leading to the cessation of nervous stimulation.

How is calcium (Ca+2) involved in muscle relaxation?

Ca+2 is pumped back into the sarcoplasmic reticulum (SR) by active transport.

What binds to calcium ions (Ca+2) while in storage in the sarcoplasmic reticulum?

Calsequestrin.

What is required for both muscle contraction and relaxation?

ATP (adenosine triphosphate).

What happens to tropomyosin during muscle relaxation?

Tropomyosin reblocks the active sites on actin.

What is the result of calcium removal from troponin?

The muscle fiber ceases to produce or maintain tension and returns to its resting length.

What is the length-tension relationship in muscle physiology?

The amount of tension generated by a muscle depends on how stretched or contracted it was before stimulation.

What happens if a muscle is overly contracted at rest?

A weak contraction results because thick filaments are too close to Z discs and cannot slide.

What is muscle tone?

A state of partial contraction maintained by the central nervous system to keep muscles ideally ready for action.

What is the optimum resting length for muscle contraction?

The length at which the muscle produces the greatest force when contracted, typically between 2.0 to 2.25 µm.

What is a myogram?

A chart that shows the timing and strength of a muscle's contraction.

What is the threshold in muscle physiology?

The minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction.

What is a twitch in muscle physiology?

A quick cycle of contraction that occurs when the stimulus is at threshold or higher.

What is the latent period in muscle contraction?

A 2 ms delay between the onset of stimulus and the onset of twitch response.

What occurs during the contraction phase of a muscle twitch?

Filaments slide, and the muscle shortens as it begins to produce external tension.

What happens during the relaxation phase of a muscle twitch?

The sarcoplasmic reticulum quickly reabsorbs Ca+2, myosin releases thin filaments, and tension declines.

What is the duration of an entire muscle twitch?

It lasts from 7 to 100 ms.

How does stimulus frequency affect muscle twitch strength?

Closer stimuli produce stronger twitches.

What is the all-or-none law in muscle physiology?

Electrical excitation of a muscle follows an all-or-none law, but muscle fibers can vary in twitch strength.

What factor affects the strength of muscle contraction before stimulation?

The stretch of the muscle before it is stimulated.

How does temperature affect muscle contraction?

Warmed-up muscle contracts more strongly because enzymes work more quickly.

What effect does a lower than normal pH of sarcoplasm have on muscle contraction?

It weakens contraction and contributes to fatigue.

How does hydration state affect muscle contraction?

It affects the overlap of thick and thin filaments.

Why do muscles need to contract with variable strengths?

To perform different tasks effectively.

What happens when the nerve is stimulated with higher voltages?

Stronger contractions occur as more nerve fibers in the motor nerve are excited.

What is recruitment or multiple motor unit (MMU) summation?

The process of bringing more motor units into play to increase contraction strength.

How does stimulus frequency affect twitch strength?

Twitch strength can vary with the frequency of stimuli; up to 10 stimuli per second produce identical twitches.

What phenomenon occurs with 10-20 stimuli per second?

Treppe (staircase phenomenon) occurs, where each twitch develops more tension than the previous one.

What causes the treppe phenomenon?

Increased Ca2+ concentration in the cytosol and heat released by each twitch enhance muscle enzyme efficiency.

What happens with 20-40 stimuli per second?

Incomplete tetanus occurs, where new twitches ride on previous ones, generating higher tension.

What is temporal summation?

It results from two stimuli arriving close together, leading to increased tension.

What is wave summation?

It results from one wave of contraction added to another, producing higher levels of tension.

What occurs at 40-50 stimuli per second?

Complete tetanus occurs, where muscle has no time to relax, leading to a smooth, prolonged contraction.

How much tension does a muscle in complete tetanus produce compared to a single twitch?

About four times the tension of a single twitch.

Why is complete tetanus rare in the body?

The body rarely exceeds 25 stimuli per second, and motor units function asynchronously to maintain smooth contractions.