lecture 37 - proprioception 1+2 - PoNF

What are proprioceptors?

mechanoreceptors that signal body or limb position

muscle spindles

monitor muscle length and rate of change of muscle length

controls reflexes and voluntary movements

Golgi Tendon Organ (GTO)

specialised sensory receptor that monitors tension on tendons and therefore muscle tension

joint receptors

monitor joint angle, rate of angular movement and tension on the joint

what is the function of the 3 aforementioned receptors (muscle spindles, GTO and joint receptors)

1. send sensory info to the spinal cord and on to the brain so that it can control VOLUNTARY movement

2. muscle spindles and GTO provide sensory info to spinal cord neurones which can generate spinal reflex movements

3. kinaesthesia - provide sensory info to perceive limb and body position + movement in space

Muscle spindles are

intrafusal muscle fibres contained within a capsule

lie in parallel with muscle fibre

types of intrafusal fibres

nuclear bag fibres - collected nuclei

nuclei chain fibres - nuclei lined up in a chain

gamma motor neurones

innervate and cause contraction of the contractile ends of intrafusal fibres

fire and 2 ends contact and shorten, central area is stretched out

gamma motor neuron diameter compared to alpha sensory

smaller

Ia sensory fiber

a large, fast sensory axon that connects a muscle spindle to neurons in the spinal cord

- Ia wrap around centre of intrafusal fibres

- secondary endings form type II afferents

spindle stretch receptors (muscle spindle)

stimulated by muscle stretch

stretch sensitive ion channels open

local generator potential

regenerative action potential in Ia afferent fibres

action of GTO

nerve endings of GTO mingle with tendon bundles of muscles

tendons are inelastic so passive stretch doesn't affect them much

muscle contraction increases tension in tendons - stretches the nerve endings of GTO and initiates APs in 1b afferent fibres

GTO lies parallel to muscle fibres (lies in series)

gamma motor neurone innervation

innervate contractile ends of muscle spindles so that muscle spindles contract with muscle and the brain is told about change in muscle length

what would happen when Alpha motor neurons fired if gamma motors didn’t exist

extrafusal (alpha) muscle shortens and contracts

intrafusal fibres stay same length - slack spindle

1a spindle sensory firing stops when muscle shortens

alpha and gamma firing together

muscle and muscle spindle shorten

spindle stretch reset

no drop off in action potential