03 - Proprioceptors and Spinal Reflexes

proprioceptors

provide unconscious awareness of limb position and movement

• muscle spindles → length

• Golgi tendon organ → tension

• joint angle receptors → position

muscle spindle

proprioceptor that senses muscle length

• extrafusal fibers → function in force generation

  • innervated by ⍺-motor neurons

• intrafusal fibers → transduction mechanism for sensing length

  • lives within the spindle

  • innervated by ɣ-motor neurons (efferent)

  • group I & II fibers (afferent) → send sensory information from muscle to spinal cord

    • nuclear bag (Ia) → rapid and slow adaptation

    • nuclear chain (II) → mostly slow adaptation

muscle spindle afferents

• group Ia → dynamic length

  • thick, myelinated

  • fastest propagation velocity

  • combination of rapid and slow adapting receptors

  • proprioceptors of skeletal muscle

  • nuclear bag fibers

• group II → static length

  • slightly smaller

  • fast propagation velocity

  • mostly slow adapting receptors

  • mechanoreceptors of skin

  • nuclear chain fibers

• group III → pain, temperature

• group IV → temperature, pain, itch

ɣ motor neurons

motor neurons of muscle spindle in intrafusal fiber

• adjust spindle sensitivity during muscle contraction

• if muscle shortens but spindle doesn’t, spindle would go slack and lose signal

• fires with ⍺ motor neuron in extrafusal fiber during voluntary movement

Golgi tendon organ (GTO)

proprioceptor that senses muscle tension in tendons, which are in series with muscle fibers

• free nerve endings encapsulated by collagen fibers

• increase in muscle tension squeezes on nerve endings to elicit response

• during passive stretch:

  • no response in GTO due to tension spread over many different fibers (Ib)

  • higher firing rate in response to muscle length (Ia)

• during active contraction:

  • tension increases when muscle is contracted, causing high firing rate in GTO (Ib)

  • muscle gets shorter, so no response from spindle (Ia)

spinal reflex

involuntary and nearly instantaneous movement in response to a stimulus

• simple circuit

• building block for complex movements

• clinical diagnosis of pathological conditions

myotatic (stretch) reflex — knee jerk

homeostatic reflex to restore homonymous muscle to original condition

• tapping tendon stimulates spinal cord to extend/stretch muscle

• muscle spindle activated by stretch → Ia efferent to spinal cord

• two synapses:

  • ⍺-motor neuron excited, causing muscle to contract → monosynaptic path

  • inhibitory interneuron excited, inhibiting antagonist muscle → disynaptic path

convergence + divergence

excitation of one fiber isn’t enough because it will not generate enough force to elicit motion of a muscle

• EPSP too small and does not reach threshold to generate AP

• must activate many spindles to reach threshold → convergence

• one motor neuron innervates one fiber and cannot generate enough muscle force

• must activate many muscle fibers → divergence

descending modulation

reflexes are automatic but modifiable

• descending inputs make contact to enhance or inhibit reflex from occurring

inverse myotatic reflex / Golgi reflex

in high muscle tension, the reflex inhibits motor neurons to extensor muscle to prevent muscle damage from excessive force

• active muscle contraction activates GTO

• 1b afferent synapses into spinal cord, inhibitory interneuron, and motor neuron

• inhibits homonymous muscle, while exciting antagonist muscle

flexor (withdrawal) reflex / complex reflex

in the leg that feels pain, the reflex inhibits the motor neurons to the extensor muscle and stimulates the motor neurons to the flexor muscle

• in the opposite leg, the reflex stimulates the motor neurons to the extensor muscle and inhibits the motor neurons to the flexor muscle

• multisynaptic, multisegmental, bilateral, contralateral extension

• reflex responses depend on context

  • different outputs depending on mental preparation and task goal

  • reflex circuits are pre-biased by brain

central pattern generators

spinal circuits that generate rhythmic movements

• walking is not commanded step-by-stem by cortex

• animals continue to talk after spinal cord transection

  • alternating left-right stepping preserved

  • speed changes cause gait changes

• oscillating response to steady stimulation → graded response and dependence on initial conditions

  • does not require sensory feedback

  • sensory feedback can modulate reflex