Neuromuscular Communication

3 Key Signals

  1. Electrical

  2. Chemical

  3. Mechanical

Electrical

  • neurons & muscle fibers carry electrical signals = action potentials (AP)

    • AP is created by a shift in membrane ions

    • AP measured as a voltage change

      • 4 distinct phases

        • 1. resting membrane, -70mV

        • 2. depolarization, -55mV threshold. More Na+ ions move inside the cell membrane

        • 3. repolarization/hyperpolarization, shifting back towards -70mV but oftentimes overshoots and ends up more negative

        • 4.restoration, back to -70mV

  • measured by electromyography (EMG)

    • tells us when muscle is active vs. not active. Allows study of magnitude & timing of contraction

    • limitations of EMG: not exact, cannot compare signals from one person to another, for ideal readings the electrodes need to be in-line with muscle fibers

Chemical

  • APs cannot cross the neural synapse, so they get transferred into a chemical/neurotransmitter (for movement, usually Acetylcholine (ACh))

    • we have synapses to have back ups in case one neuron is damaged, and it allows for more connections

  • Synaptic transmission: nerve → muscle

    • APs travel down motor neuron axons, releases ACh into neuromuscular junction resulting in generation of APs along muscle fibers themselves

  • signals measured by tissue samples

Mechanical

  • Cross-bridge formation: myosin attaches to actin to generate contractile force

  • measured by force output using dynamometer

Motor Units & Properties

  • Motor unit: a single alpha motor neuron+axon+all the muscle fibers it innervates

    • smallest functional sub-division of NM system

    • an AP in the alpha MN produces contraction of all the innervated fibers (all or none principle)

3 Motor Unit Properties

  • Innervation ratio: number of muscle fibers innervated by a single alpha MN

    • innervation ratio varies among MUs w/in the same muscle

    • larger muscles usually have larger overall innervation ratios

    • larger ratio = more force produced by that unit

    • smaller ratio= more precise movement

  • Territory

    • how are the muscle fibers of a single MU distributed? (ex: even dist. or clustered)

    • can cause more smoother movements/mitigate injury risk

  • Type

    • 3 types of MUs

      • S- slow, non fatigable

      • FR- fast, fatigue resistant

      • FF- fast, fatigable

    • differ in innervation ratio& fiber x-sectional area (force) , & fiber type (fatiguability)

      • slow MUs have lower innervation ratio and don’t fatigue, vis versa