Sensory Nerves and Muscle Function

Sensory Nerves and Muscle control

Muscle Spindles

  • Muscle spindles sense stretch in muscle fibers.
  • They relay information about muscle length to the spinal cord and brain.
  • They have both afferent (sensory) and efferent (motor) innervation.
  • Intrafusal fibers lie within the muscle fibers, parallel to extrafusal fibers (typical muscle fibers).

Intrafusal vs. Extrafusal Fibers

  • Intrafusal fibers: Lie between muscle fibers and contain muscle spindles within capsules.
  • Extrafusal fibers: Typical muscle fibers responsible for muscle contraction.

Gamma Loop

  • The gamma loop controls muscle position.
  • Descending signals from the brain activate both alpha and gamma motor neurons.
  • Stretch on the spindle provides feedback on muscle position via gamma afferents.
  • The gamma loop affects the firing of the alpha motor neuron cell body, influencing reflexes like the stretch reflex.

Type 1a (Primary) and Type II Muscle Spindles

  • Both types respond to muscle stretch but differ in sensitivity.

  • Type 1a fibers:

    • Greater dynamic sensitivity (detect changes in length).
    • Fire when the muscle is stretched, including during movement.

  • Type II fibers:

    • Absolute length detectors.
    • Primarily fire when the muscle is static.

  • There is overlap in function between Type 1a and Type II fibers.

Intrafusal Muscle Fiber Types in Cats

  • Muscle spindles in cats contain:
    • Two to three bag fibers (nucleic bag fibers).
    • Four to six chain fibers (nucleic chain fibers).
    • Complex motor innervation.

Tendon Jerk Reflex (Knee Jerk Reflex)

  • Tests muscle spindles and the integrity of the system.
  • Components:
    • Receptor: Muscle spindle
    • Gamma afferent neurons
    • Alpha motor neurons
    • Muscle fibers
  • Process:
    • Tapping the patellar tendon activates muscle spindles.
    • Information is sent to the spinal cord.
    • Alpha motor neurons are activated, signaling muscle stretch.
    • Excitatory signals cause the muscle to contract, resulting in the knee jerk.

Muscle Spindle Activity and Vibration

  • Vibrating a tendon (e.g., patellar tendon) causes tiny muscle stretches, activating muscle spindles.
  • Even without conscious effort, vibration can lead to increased EMG activity and passive force generation.
  • Muscle spindles send information to the motor neuron, causing muscle fiber contraction without voluntary command.

Golgi Tendon Organs (GTOs)

  • Located at the transition between muscle fibers and tendons.
  • Run in series with muscles.
  • Each GTO is innervated by a single 1B afferent.
  • Respond to active tension of muscles, providing information about force production to the spinal cord.
  • Not innervated by efferent fibers.

GTOs and Autogenic Inhibition

  • Historically, GTOs were thought to inhibit muscle contraction when tension is too high.
  • Limited evidence supports the idea that strength training reduces this autogenic inhibition.
  • Adaptability of GTOs is unclear.
  • Potential for greater muscle activation and force production.

Group Three and Four Afferents

  • Sensory fibers sensitive to metabolism and work (metabo- and ergosensitive).
  • During exercise, convey sensations of work and pain to the nervous system.
  • May reduce voluntary muscle activation and impair motor neuron output during intense exercise.
  • May modulate activity in the motor cortex.

Experimental Evidence

  • Marcus Amar's work uses group three and four afferent blocks (e.g., fentanyl).
  • During cycling tasks, blocking these afferents:
    • Initially shows greater central motor drive (EMG activity).
    • Over time, power output decreases more significantly.

Afferent and Efferent Fiber Characteristics

  • Summary:
    • Larger fibers typically have faster conduction velocities.