Physiology: Muscle Contraction & Reflexes Muscle Structure Skeletal muscle: bundles of contractile, multinucleated fibers that move the body and hold posture.…

Physiology: Muscle Contraction & Reflexes

Muscle Structure

  • Skeletal muscle: bundles of contractile, multinucleated fibers that move the body and hold posture. Properties: excitability, contractility, extensibility, elasticity.
  • Organization: muscle (epimysium) → fascicle (perimysium) → fiber (endomysium, sarcolemma) → myofibril → sarcomere → myofilaments.
  • Terms: sarcolemma = cell membrane; sarcoplasm = cytoplasm; sarcoplasmic reticulum (SR) = Ca²⁺ store; T-tubule = inward fold of the sarcolemma; triad = one T-tubule + two terminal cisternae.
  • Sarcomere (Z disc to Z disc) is the smallest functional unit. During contraction the A band stays the same; the I band and H zone shorten.
  • Proteins: contractile (actin, myosin), regulatory (troponin, tropomyosin), structural (titin, nebulin, myomesin, dystrophin). Troponin C binds Ca²⁺; TnI inhibits; TnT anchors to tropomyosin.
  • Muscle types: skeletal (striated, voluntary, troponin, fibers independent, fastest); cardiac (striated, autorhythmic, gap junctions); smooth (no sarcomeres, calmodulin instead of troponin, slowest).

Contraction Mechanism

  • Neuromuscular junction: ACh binds nicotinic receptors → end-plate potential → muscle action potential. Acetylcholinesterase ends the signal. A single fiber is all-or-none.
  • Excitation–contraction coupling: the AP runs down the T-tubule; the DHP receptor is mechanically coupled to RyR1 on the SR, releasing Ca²⁺. (Calcium-induced calcium release is the cardiac mechanism.)
  • Ca²⁺ switch: Ca²⁺ binds troponin C → tropomyosin moves off actin → myosin binds.
  • Crossbridge cycle: rigor (45°) → ATP binds, head detaches → ATP hydrolysis cocks the head (90°) → Pi release, power stroke toward the M line → ADP released → rigor.
  • ATP is needed for the power stroke, for detaching myosin and for SERCA to pump Ca²⁺ back. No ATP = rigor.
  • Sliding filament theory: filaments slide, they do not shorten.
  • Relaxation: SERCA returns Ca²⁺ to the SR (bound by calsequestrin), Ca²⁺ leaves troponin, tropomyosin re-covers actin.

Energy for Contraction

  • Stored ATP: about 6 s.
  • Creatine phosphate (direct phosphorylation, creatine kinase): 1 ATP per CP, about 10–15 s. Sprint, weightlifting.
  • Anaerobic glycolysis: 2 ATP per glucose, lactic acid, about 30–40 s. Tennis, football, 100 m swim.
  • Aerobic respiration: about 32 ATP per glucose (also fatty acids, amino acids), hours. Marathon, jogging.
  • Clinical: hypertrophy (more actin and myosin), atrophy (disuse, denervation, sarcopenia), rigor mortis (no ATP to break crossbridges), cramp (motor neuron hyperexcitability; relieved by stretching).

Phenomena of Contraction

  • Three factors set force: degree of stretch, number of motor units, frequency of stimulation.
  • Length–tension: maximal active tension at sarcomere length 2.0–2.2 µm. Passive tension (titin) rises with stretch. Total = active + passive.
  • Motor unit = one motor neuron + all the muscle fibers it supplies. More units = more force; small units recruited first (size principle).
  • Twitch: response to one stimulus. Latent period (Ca²⁺ release) → contraction → relaxation.
  • Treppe: stimuli after complete relaxation give progressively stronger twitches (staircase of Bowditch).
  • Summation: a new contraction before the last one ends; tension adds up.
  • Unfused tetanus: partial relaxation, wavy plateau with valleys. Fused tetanus: no relaxation, smooth line at maximal tension. (One practicum slide swaps these.)
  • Fatigue: force falls despite stimulation. Central (CNS) or peripheral (NMJ, E–C coupling, Ca²⁺ release, depletion of CP/ATP/glycogen, accumulation of Pi, H⁺). Rises as glycogen falls.
  • Isotonic (length changes; concentric shortening, eccentric lengthening) vs isometric (tension without length change). Heavier load = longer latent period, slower shortening.

Reflexes

  • Reflex: rapid, predictable, involuntary motor response. Adequate stimulus: suits the receptor, reaches threshold, sudden, unanticipated.
  • Reflex arc: receptor → sensory neuron (dorsal root) → integration center → motor neuron (ventral root) → effector.
  • Types: cranial vs spinal; somatic vs autonomic (two-neuron efferent); monosynaptic (only stretch reflexes) vs polysynaptic; innate vs learned (learned = longer reaction time).
  • Stretch reflex: spindle → Ia afferent → alpha motor neuron of the same muscle; reciprocal inhibition of the antagonist. Golgi tendon organ relaxes a muscle under high tension.

Reflex Examination

  • Pupillary: light in one eye constricts both pupils (direct and consensual); CN II in, CN III out.
  • Biceps C5–C6 (tap your thumb on the tendon) → elbow flexion.
  • Triceps C7 (tap above the olecranon) → elbow extension.
  • Abdominal T8–T12 (stroke toward the midline) → umbilicus moves toward the stimulus.
  • Patellar L3–L4 (legs hanging, tap the patellar ligament) → knee extension.
  • Achilles S1 (foot slightly dorsiflexed) → plantar flexion.
  • Distract the subject, compare both sides, grade 0–4+, use the Jendrassik maneuver before calling a reflex absent. Hyperreflexia = UMN; hyporeflexia = LMN.

Practicum

  • Wet lab: frog (poikilothermic, large gastrocnemius). Pith the frog → prepare gastrocnemius with the sciatic nerve → keep moist with Ringer lactate → mount on a kymograph → stimulate and label the tracing.
  • PhysioEx 9.1 Exercise 2: latent period stays constant; threshold about 0.8 V and maximal stimulus about 8.5 V; beyond the maximal stimulus only frequency raises force; 50 stimuli/s gives unfused tetanus and about 140–150 gives maximal tetanic tension; sustained tetanus shows fatigue; length–tension peaks near 75 mm; heavier loads slow shortening until the contraction becomes isometric.