Muscle Contraction and Nervous System Control

Muscle Tension

  • Force generated by muscle contraction or sarcomere shortening.
  • Two types: isotonic and isometric contractions.

Types of Isotonic Contractions

  • Tension remains constant; muscle length changes.
  • Concentric: Muscle shortens to move a load (e.g., biceps brachii contracting when lifting a weight).
  • Eccentric: Muscle lengthens as tension diminishes (e.g., lowering a weight in a controlled manner).

Isometric Contractions

  • Muscle produces tension without changing joint angle.
  • Sarcomeres shorten, increasing tension, but load doesn't move.
  • Important for posture, bone, and joint stability.

Motor Units

  • A single motor neuron and all the muscle fibers it innervates.
  • Small motor units: fine motor control (e.g., eye muscles).
  • Large motor units: gross movements (e.g., thigh muscles).

Neural Control of Muscles

  • Small motor units with low-threshold neurons fire first.
  • Larger motor units with high-threshold neurons are recruited for more strength.
  • Recruitment: increasing activation of motor units for stronger contraction.
  • Prevents fatigue by alternating active and resting motor units.

Length-Tension Relationship

  • Sarcomere length affects force generated during shortening.
  • Maximal tension: 80-120% of resting length.
  • Optimal overlap of actin and myosin.

Muscle Twitch

  • Single contraction from a single action potential.
  • Myogram measures tension over time.
  • Phases: latent period, contraction phase, relaxation phase.

Graded Muscle Response

  • Varies with frequency of action potentials and number of motor neurons.

Wave Summation

  • Increased tension when fibers are stimulated before previous twitch ends.
  • More Ca++ ions released, activating additional sarcomeres.

Tetanus

  • Incomplete tetanus: quick cycles of contraction with short relaxation phases.
  • Complete tetanus: continuous contractions with no relaxation phase due to high stimulus frequency.

Treppe (Staircase Effect)

  • Initial contractions generate less force than later contractions after dormancy.
  • Muscle tension increases in graded manner.
  • Believed to result from higher concentration of Ca++Ca^{++} in the sarcoplasm.

Muscle Tone

  • Muscles are slightly contracted even when not producing movement.
  • Stabilizes joints and maintains posture.

Hypotonia and Hypertonia

  • Hypotonia: absence of muscle tone, leading to flaccidity (e.g., damage to CNS).
  • Hypertonia: excessive muscle tone, often with hyperreflexia (e.g., damage to upper motor neurons).