Y1-BMF-2024-Neural Stimulation of Muscle (1)

MOTOR UNIT

  • A motor unit is defined as a motor neuron and all of the muscle fibers it innervates.

  • Types:

    • Large Units: >100 fibers; used for posture (e.g., postural muscles).

    • Small Units: <10 fibers; precise control (e.g., extraocular and hand muscles).

  • Key Properties:

    • All fibers in a motor unit contract simultaneously when the neuron is excited.

    • Muscle contraction force increases via:

      • Recruitment: Activating more motor units.

      • Frequency Increase: Higher stimulation frequency causes summation or tetanus.

RECRUITMENT OF MOTOR UNITS

  • Motoneurons that connect to slow muscle fibers are smaller and more excitable, recruited first during moderate contractions.

  • Fast muscle fibers are recruited during intense contractions.

  • Generally, motor units consist of one type of muscle fiber only.

TETANUS (PIC)

  • Twitch: A single action potential results in a single muscle contraction

  • Tetanus: Clostridium tetani infection leads to uncontrollable muscle spasms due to blocking inhibitory synapses in the CNS, causing increased motor nerve activity.

  • Summation:

    • High-frequency stimulation prevents muscle relaxation between contractions.

    • Results in increased tension.

SUMMATION AND TETANUS

  • Various phenomena:

    • Single twitch: Complete relaxation occurs between stimuli.

    • Summation: Incomplete relaxation occurs, resulting in increased muscle tension.

    • Complete tetanus: Muscle reaches maximum tension and does not relax, despite continuous stimulation.

STAIRCASE PHENOMENON (TREPPE)

  • Repetitive stimulation at low frequencies increases tension with each successive twitch.

  • A maximum tension is eventually reached.

Electromyography (EMG)

  • Technique: Records electrical activity in skeletal muscles using electrodes.

  • Applications:

    • Diagnosing muscle disorders (e.g., muscular dystrophy).

    • Assessing nerve-muscle junction diseases (e.g., myasthenia gravis).

    • Identifying motor neuron diseases (e.g., ALS, polio).

Denervation

  • Effects:

    • Flaccid Paralysis: Due to lower motor neuron lesions.

    • Fasciculations: Visible twitches from ACh release by degenerating neurons.

    • Denervation Hypersensitivity: Spread of ACh receptors causes fibrillations.

MUSCLE TYPES

  • Skeletal Muscle: Most abundant; attached to bones for movement.

  • Smooth Muscle: Found in organs; controls internal movements of body.

  • Cardiac Muscle: Heart muscle; responsible for pumping blood.

CLASSIFICATION OF MUSCLE TYPES

  • Appearance:

    • Striated (skeletal and cardiac)

    • Unstriated (smooth)

  • Innervation:

    • Voluntary (skeletal)

    • Involuntary (smooth and cardiac)