Muscle Physiology and Reflexes: Motor Units, NMJ, and Contraction

Motor unit

A motor neuron and all the muscle fibers it innervates; all fibers contract together when the neuron fires.

Neurogenic contraction

Skeletal muscle contraction that depends entirely on stimulation by somatic motor neurons.

Higher motor centers

Motor cortex, basal nuclei, and cerebellum; plan and coordinate movement.

Lower motor centers

Brainstem and spinal cord; execute and control reflexes.

Proprioception

The sense of limb and body position and movement, providing feedback to the CNS.

Muscle spindle

Sensory receptor detecting muscle stretch or length.

Golgi tendon organ

Sensory receptor detecting muscle tension to prevent damage.

Somatic motor neuron

Neuron originating in the ventral horn that directly innervates skeletal muscle.

Motor unit size

Fine control = small motor units; gross movement = large motor units.

Neuromuscular junction (NMJ)

Synapse between motor neuron and muscle fiber where acetylcholine (ACh) is released.

Steps at NMJ

AP arrives → Ca²⁺ influx → ACh release → binds nicotinic receptors → Na⁺ influx → EPP → muscle AP → Ca²⁺ release → contraction.

End-plate potential (EPP)

Depolarization of the motor end plate due to ACh binding to nicotinic receptors.

Acetylcholinesterase (AChE)

Enzyme that breaks down ACh to terminate the signal at the NMJ.

Somatic reflex

Automatic, involuntary response mediated by spinal circuits.

Reflex arc components

Receptor → sensory neuron → integration center → motor neuron → effector.

Monosynaptic reflex

One synapse between sensory and motor neuron; e.g., patellar reflex.

Polysynaptic reflex

Multiple synapses with interneurons; e.g., withdrawal reflex.

Stretch reflex (myotatic)

Tendon tap stretches muscle → spindle activates → same muscle contracts → maintains posture.

Reciprocal inhibition

Inhibition of antagonistic muscles during reflex or voluntary contraction.

Withdrawal (flexor) reflex

Polysynaptic reflex withdrawing a limb from a painful stimulus.

Crossed-extensor reflex

Extends the opposite limb during withdrawal to maintain balance.

Hyperreflexia

Exaggerated reflexes due to upper motor neuron damage.

Hyporeflexia

Reduced reflexes due to lower motor neuron or sensory damage.

Spinal shock

Temporary loss of reflexes following spinal cord injury.

Knee-jerk reflex spinal levels

L2-L4 segments.

Biceps reflex spinal levels

C5-C6 segments.

Achilles reflex spinal levels

S1-S2 segments.

Spinal level motor control

Reflexes and basic movement patterns via α- and γ-motor neurons.

Brainstem motor control

Controls posture and balance through reticular formation and vestibular nuclei.

Cortical motor control

Initiates voluntary movement through the motor cortex.

Cerebellum and basal nuclei

Provide coordination, precision, and initiation of movement.

Properties of muscle tissue

Four key properties: electrical excitability, contractility, extensibility, and elasticity.

Electrical excitability

Ability to generate and propagate action potentials in response to stimuli.

Contractility

Ability to shorten and produce tension or force.

Extensibility

Ability to be stretched without damage.

Elasticity

Ability to return to original shape after contraction or stretch.

Epimysium

Connective tissue surrounding the entire muscle.

Perimysium

Connective tissue surrounding a bundle (fascicle) of muscle fibers.

Endomysium

Connective tissue surrounding each individual muscle fiber.

Myofibril

Rod-like contractile structure composed of repeating sarcomeres.

Sarcomere

Basic functional unit of contraction in striated muscle, between Z-lines.

Thick filament

Composed of myosin molecules with ATPase heads forming cross-bridges.

Thin filament

Composed of actin, troponin, and tropomyosin proteins.

Actin

Double-stranded protein containing binding sites for myosin.

Tropomyosin

Rod-shaped protein that blocks actin binding sites at rest.

Troponin

Complex that binds Ca²⁺ and moves tropomyosin to uncover binding sites.

Sliding filament theory

Filaments slide past each other to shorten sarcomeres; A-band constant, I-band and H-zone shrink.

Cross-bridge cycle step 1

Energized myosin head binds to actin forming a cross-bridge.

Cross-bridge cycle step 2

Power stroke occurs as Pi is released and the head pivots, pulling actin.

Cross-bridge cycle step 3

New ATP binds to myosin → cross-bridge detachment.

Cross-bridge cycle step 4

ATP hydrolysis re-cocks the myosin head for the next cycle.

ATP roles in muscle contraction

Powers cross-bridge cycling and is required for relaxation (detachment and Ca²⁺ reuptake).

Neuromuscular junction (NMJ)

Site where a motor neuron communicates with a muscle fiber via ACh.

End-plate potential

Depolarization of the motor end plate that triggers a muscle AP if threshold is reached.

Excitation-Contraction (E-C) Coupling

Sequence linking sarcolemma depolarization to Ca²⁺ release and tension development.

T-tubules

Invaginations of the sarcolemma that carry the AP deep into the fiber.

Triad

Structure formed by a T-tubule and two terminal cisternae of the SR.

DHPR (L-type Ca²⁺ channel)

Voltage sensor in the T-tubule that mechanically opens RyR in skeletal muscle.

Ryanodine receptor (RyR)

Ca²⁺ release channel on the SR membrane.

SERCA pump

Ca²⁺-ATPase that actively transports Ca²⁺ back into the SR for relaxation.

Calsequestrin

SR protein that binds and stores Ca²⁺.

Calcium at rest

Low cytoplasmic [Ca²⁺] (~10⁻⁷ M); tropomyosin blocks actin sites.

Calcium during contraction

Ca²⁺ (~10⁻⁵ M) binds troponin-C → moves tropomyosin → cross-bridges form.

Isometric contraction

Tension develops but muscle length remains constant.

Isotonic contraction

Muscle shortens while tension remains constant to move a load.

Concentric contraction

Isotonic shortening while lifting a load.

Motor unit recruitment

Activation of more motor units to increase total muscle force.

Size principle

Small, fatigue-resistant units recruited first; large, powerful last.

Fiber diameter

Larger fibers contain more myofibrils → generate greater force.

Length-tension relationship

Optimal sarcomere length (~2.0-2.2 µm) maximizes cross-bridge formation.

Too short sarcomere

Excessive overlap reduces force generation.

Too long sarcomere

Limited overlap reduces cross-bridge formation and force.

Twitch

Single contraction response to one action potential.

Summation

Increased tension from repeated stimuli before full relaxation.

Incomplete tetanus

Sustained but fluctuating contraction due to moderate stimulus frequency.

Complete (fused) tetanus

Maximal, steady tension at high stimulation frequency.

Fusion frequency

Frequency of stimulation at which tetanus is achieved.

Force-velocity relationship

Higher load → slower shortening velocity (inverse relationship).

Frequency-tension relationship

Higher stimulation frequency → greater Ca²⁺ buildup → greater tension.

Slow-twitch (Type I) fibers

High endurance, oxidative metabolism, fatigue resistant.

Fast-twitch (Type II) fibers

High power, glycolytic metabolism, fatigue quickly.

Muscle fatigue

Caused by ATP depletion, lactic acid accumulation, and Ca²⁺ handling inefficiency.

Recovery from fatigue

Replenish ATP and phosphocreatine; remove metabolites.

Main function of circulatory system

To transport gases, nutrients, wastes, and hormones throughout the body.

Heart role in circulation

Pressure generator that drives blood flow.

Arterial system

High-pressure vessels that distribute oxygenated blood to tissues.

Venous system

Low-pressure, high-volume return system to the heart.

Capillaries

Sites of nutrient and gas exchange between blood and tissues.

Pulmonary circulation

Right heart → lungs → left heart; low pressure (~22/8 mmHg).

Systemic circulation

Left heart → body → right heart; high pressure (~120/80 mmHg).

Heart location

In the mediastinum of the thoracic cavity, enclosed by the pericardium.

Pericardium

Serous membrane surrounding the heart; cushions and stabilizes it.

Four chambers of the heart

Right atrium, right ventricle, left atrium, left ventricle.

Right heart function

Pumps deoxygenated blood to the lungs.

Left heart function

Pumps oxygenated blood to the body.

Atrioventricular (AV) valves

Between atria and ventricles; tricuspid (right) and mitral (left).

Semilunar valves

Between ventricles and arteries; pulmonary and aortic valves.

Valve mechanism

Open and close passively based on pressure differences.

Unidirectional flow

Ensured by one-way valves preventing backflow.

Cardiac cycle

One complete heartbeat: atrial systole, ventricular systole, and diastole.