Muscle Physiology: Excitation-Contraction Coupling

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Questions and Answers

What is the primary neurotransmitter released by postganglionic neurons in the sympathetic nervous system?

  • Dopamine
  • Serotonin
  • Acetylcholine
  • Noradrenaline (correct)

Which response is predominantly associated with stimulation of the sympathetic nervous system?

  • Inhibits heart rate
  • Increases metabolic function
  • Accelerates digestion
  • Relaxes airways (correct)

Which of the following functions is primarily governed by the parasympathetic nervous system?

  • Increasing peripheral resistance
  • Digesting food (correct)
  • Accelerating heart rate
  • Relaxing airways

What role does acetylcholine play in the sympathetic nervous system?

<p>Released by preganglionic neurons (B)</p> Signup and view all the answers

What is a likely outcome of administering adrenaline during a cardiac arrest?

<p>Inhibits digestion (C)</p> Signup and view all the answers

What occurs during relaxation in muscle contraction?

<p>Decrease in Ca2+ or ATP levels (D)</p> Signup and view all the answers

What is the primary mechanism that increases muscle contraction strength?

<p>Recruitment of additional motor units (B)</p> Signup and view all the answers

Which statement correctly describes a reflex action?

<p>It is an involuntary movement not requiring the brain (C)</p> Signup and view all the answers

What role do interneurons play in reflex actions?

<p>They facilitate communication between sensory and motor neurons (A)</p> Signup and view all the answers

How do the muscles work during reflex actions to maintain stability?

<p>Through coordinated activity of flexors and extensors (A)</p> Signup and view all the answers

What role does calcium ion (Ca2+) play in the process of muscle contraction?

<p>It binds to troponin, allowing myosin binding sites on actin to be exposed. (D)</p> Signup and view all the answers

Which statement best describes the sequence of events at the neuromuscular junction during muscle contraction?

<p>The presynaptic action potential leads to ACh release, causing postsynaptic depolarization. (D)</p> Signup and view all the answers

What is the primary function of the sarcoplasmic reticulum in muscle fibers?

<p>To store calcium ions required for muscle contraction. (A)</p> Signup and view all the answers

Which of the following accurately describes the relationship between motor neurons and muscle fibers?

<p>Multiple muscle fibers can be activated by a single motor neuron. (B)</p> Signup and view all the answers

What initiates the sliding filament mechanism during muscle contraction?

<p>Release of calcium ions from the sarcoplasmic reticulum. (A)</p> Signup and view all the answers

What is the role of acetylcholine (ACh) in muscle contraction?

<p>It binds to receptors on the muscle fiber’s sarcolemma, triggering depolarization. (C)</p> Signup and view all the answers

What occurs directly after the postsynaptic depolarization in the muscle fiber?

<p>Calcium ions are released from the sarcoplasmic reticulum. (C)</p> Signup and view all the answers

What happens to myosin after it binds to actin during muscle contraction?

<p>The myosin head pivots, pulling the actin filament toward the center of the sarcomere. (D)</p> Signup and view all the answers

What is a direct consequence of the parasympathetic nervous system activating acetylcholine receptors?

<p>Constricted airways (B)</p> Signup and view all the answers

Which statement correctly describes the function of somatic motor neurons?

<p>They generate behavior through voluntary control of skeletal muscles. (B)</p> Signup and view all the answers

How are lower motor neurons organized within the ventral horn of the spinal cord?

<p>Neurons for axial muscles are more medial than distal muscle neurons. (A)</p> Signup and view all the answers

What describes the relationship between alpha motor neurons and muscle fibers?

<p>One alpha motor neuron can synapse with many muscle fibers. (A)</p> Signup and view all the answers

What is a key characteristic of the motor neuron pool?

<p>It consists of all motor neurons innervating one muscle. (D)</p> Signup and view all the answers

What role do lower motor neurons play in muscle contraction?

<p>They initiate contraction by interacting with local spinal cord circuits. (B)</p> Signup and view all the answers

Which statement is true regarding the contraction of large versus small muscles?

<p>Large motor neurons ensure more significant force generation for large muscles. (D)</p> Signup and view all the answers

What is the main effect of the housekeeping function of action potentials in parasympathetic neurons?

<p>They enable long-distance propagation of action potentials. (A)</p> Signup and view all the answers

Flashcards

Sympathetic nervous system

Part of the autonomic nervous system that prepares the body for action, often called the "fight-or-flight" response.

Parasympathetic nervous system

Part of the autonomic nervous system that promotes rest and digestion, often called the "rest-and-digest" response.

Excitation-contraction coupling

Process connecting nerve signals to muscle contractions.

Acetylcholine

Neurotransmitter released by both parasympathetic and preganglionic sympathetic neurons.

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Noradrenaline (Norepinephrine)

Neurotransmitter released by postganglionic sympathetic neurons, causing rapid target effect.

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Muscle Relaxation

Muscle relaxation happens when calcium ions (Ca2+) and ATP levels decrease.

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Reflex

An automatic, quick response to a stimulus, usually without conscious thought.

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Motor Unit Recruitment

Increasing the number of motor units activated, resulting in a stronger muscle contraction.

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Myotatic Reflex

A reflexive response that stretches a muscle and causes it contraction, involves coordination of opposing (flexor and extensor) muscles.

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Crossed-Extensor Reflex

A reflex that coordinates movement on the opposite side of the body from the stimulus

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Parasympathetic effect on airways

Binding to specific receptors in the airways causes constriction.

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Parasympathetic effect on digestion

Stimulates digestive processes.

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Parasympathetic effect on heart rate

Causes a decrease in heart rate.

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Somatic motor system function

Controls all skeletal muscles in the body.

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Lower motor neuron role

Directly command muscle contraction.

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Motor unit definition

An alpha motor neuron and all the muscle fibers it controls.

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Motor neuron pool

All alpha motor neurons controlling a specific muscle.

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Lower motor neuron distribution

Arranged spatially in the spinal cord according to the body part controlled; axial are medial, distal are lateral, flexors are dorsal to extensors

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Neuromuscular junction

The point where a motor neuron connects to a muscle fiber.

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Action potential (motor neuron)

Electrical signal that travels down a nerve to make a muscle contract.

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Sarcolemma

The cell membrane of a muscle fiber.

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Myofibrils

Contractile units within a muscle fiber.

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Sarcoplasmic reticulum

Stores calcium ions crucial for muscle contraction.

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Calcium (muscle contraction)

Triggers the sliding of actin and myosin filaments, leading to muscle shortening.

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Sliding filament theory

Actin filaments slide along myosin filaments during muscle contraction.

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Excitation-contraction coupling

Process that links nerve signals to muscle contractions.

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Study Notes

Excitation-Contraction Coupling and Reflexes

  • Excitation-Contraction Coupling:

    • Muscle contraction is initiated by the sliding of actin filaments along myosin filaments.
    • Calcium ions bind to troponin, exposing sites on actin for myosin binding.
    • Myosin heads then bind to actin and complete a power stroke to move actin filaments.
    • Myosin releases actin when ATP is used.

  • Muscle Contraction:

    • Action potential travels along the alpha motor neuron.
    • Acetylcholine (ACh) is released and binds to receptors on the muscle fiber.
    • Depolarization of muscle fiber occurs.
    • Calcium ions are released from the sarcoplasmic reticulum.
    • Calcium ions bind to troponin, initiating the sliding filament mechanism.
    • Actin filaments slide along myosin filaments.
    • Muscle contraction occurs.
    • Muscle relaxation occurs when calcium levels return to normal.

  • Motor Units:

    • A motor unit consists of a motor neuron and all the muscle fibers it innervates.
    • Small motor units control fine movements, large motor units control powerful movements.
    • The motor neuron pool is the collection of alpha motor neurons that activate a single muscle.

  • Single Muscle Twitches:

    • A single twitch is the response of a muscle to a single action potential.
    • Sustained contractions result from repeated action potentials from motor units.
    • Increased firing rate of motor neurons increases the strength of contraction.
    • Multiple motor units firing in succession increase contraction further.

  • Reflexes:

    • Reflexes are involuntary responses to stimuli.
    • Stretch reflexes involve reciprocal innervation of flexor and extensor muscles.
    • Knee-jerk reflex is an example of a stretch reflex.
    • Crossed-extensor reflex coordinates movement in response to painful stimuli.

  • Neurotransmitter Release at Neuromuscular Junctions:

    • Muscle contraction is initiated by the release of acetylcholine.
    • Acetylcholine binds to receptors on the postsynaptic muscle fiber.
    • One pre-synaptic action potential is sufficient to induce a postsynaptic action potential.

  • Structure of a Muscle Fiber:

    • Sarcolemma: cell membrane surrounding the muscle fiber.
    • Myofibrils: contractile units in the muscle fiber.
    • Sarcoplasmic reticulum: stores calcium ions.

  • Molecular Basis of Excitation-Contraction Coupling:

    • Organization of the components involved in muscle contraction.

  • Vestibulo-ocular Reflex (VOR):

    • Coordinated movement of eye muscles to maintain stable vision during head movement.
    • Sensory input from the vestibular system, signal to muscles, muscles contract to maintain vision.

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