lecture 37 - proprioception 1+2 - PoNF

What are proprioceptors?

UNCONSCIOUS information received. Detect BODY POSITION in space and movement

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

- tension is produced by muscle contraction so monitor muscle tension also

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

- when gamma motor neurones fire, the 2 ends contact and shorten but central area get stretched out

- small diameter compared to alpha motor neurones

Ia sensory fiber

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

- wrap around centre of intrafusal fibres

- secondary endings form type II afferents

- ENDS of intrafusal fibres contain contractile sarcomere BUT NOT CENTRE

spindle stretch receptors

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

Alpha fired without gamma

extrafusal (alpha) muscle shortens and contracts

intrafusal fibres stay same length - slack spindle

1a spindle sensory firing decreases when muscle shortens

alpha and gamma firing together

muscle and muscle spindle shorten

spindle stretch reset

no drop off in action potential