reflexes Flashcards
what is function of alpha motor neurones
they control muscle contractions
what are extra/intrafusal fibres
extrafusal muscle fibres innervated by alpha motor neurones, standard muscle fibres
intrafusal muscle fibres are innervated by gamma motorneurones, they are specialised sensory organs, they make up muscle spindle
describe a motor unit
the motor neurone, its axon and the muscle fibres it controls is a motor unit
in small motor units a single motor neurone may contact 10-20 muscle fibres, in large motor units they may contact over 1000
small motor units have small motor neurones with high resistance, larger units have larger efferents with lower resistance
low levels of activation will activate small high resistance neurones of the motor units, constant low levels of activation of gamma fibres is called gamma bias
higher levels of activation will also activate the larger lower resistance motor neurones
what is proprioceptive muscular feedback controlled by
proprioceptive (sensory) muscular feedback to brain is controlled by muscle spindles and golgi tendon organs
what are extensor/flexor muscles
extensor muscles extend a joint and flexor muscles bend/flex the joint
describe function and physiology of the muscle spindle
detect both amount and rate of change of muscle length muscle spindles are in parallel with the main extrafusal fibres of the muscle
muscle spindles act as stretch receptors lengthwise for the muscle
muscle spindle afferents are type 1a (very large diameter fibres) and type 2 (medium diameter)
muscle spindle sensitivity is controlled by gamma efferents synapsing onto intrafuscle muscle fibre end plates
describe the golgi tendon organ
golgi tendon organs are located in the musculo-tendinous junction, in series with the extrafusal fibres
golgi tendon organs are tension receptors
afferent golgi tendon organ fibres are type 1b (slightly smaller than 1a)
what causes reflexes
reflex movements are produced by muscle and cutaneous receptor afferent input, these reflexes are organised within the spinal cord and can be modulated by descending pathways from the higher motor centres in the brain
describe the stretch reflex
short latency response is monosynaptic: 1a afferent directly inputs to motorneurones, can be elicited in relaxed muscles via tendon jerk
long latency component evoked by stretch during active contraction, involves polysynaptic spinal pathways and a long loop via supraspinal structures
function of stretch reflex is to control intended muscle length during contractions against loads
if agonist muscles (working together) e.g (elbow extensor-triceps) muscle is stretched the 1a afferent also excites the 1a inhibitory interneurone in the spinal cord. This interneurone inhibits the antagonistic muscle at the joint, this reflex is disynaptic
what is alpha-gamma coactivation
if alpha motorneurones were excited alone, then extrafusal muscle fibre contraction would unload the muscle spindles and reduce or silence their output, for most movements, alpha and gamma motorneurones are activated together, however to maintain sensitivity of the spindle alpha and gamma motorneurones can be seperately controlled so that supraspinal systems can control the sensitivity of the spindle system if necessary
describe the tendon organ reflex
also known as inverse myotatic reflex
tendon organ afferents (1b fibres) terminate on one group of 1b interneurones that are inhibitory to agonist muscles (disynaptic pathway) and on other interneurones that are excitatory to antagonist muscles, di or tri synaptic pathway
these actions oppose spindle stretch reflex and allow fine control of muscle tensions
spinal reflexes can involve several spinal segments; these interactions involve interneurones and may run up and down spinal cord to influence other muscles acting in synergy
what are spinal reflexes modulated by
5% of descending fibres from motor cortex excite alpha motorneurones, remaining 95% of fibres act upon spinal interneurones and gamma motorneurones.
supraspinal control of movement is largely by modulation patterns of spinal interneurone activity
many spinal reflexes only evident when not controlled by supraspinal influence; in babies the babinski sign and grasp reflex are seen before the descending spinal tracts have fully developed their myelin. they disappear within the first year but return if there is damage to corticospinal tract.
describe the vestibular reflexes
the vestibular system (inner ear) responds to ration of head, short latency pathways run from the vestibular nuclei in the brainstem to cranial oculomotor neurones that control extraocular muscles (which rotate the eye).
vestibulo-ocular reflex stabilises eye position in space during head movements
vestibulospinal pathways pass down the spinal cord to control other muscle systems., in particular to neck muscles, drives the vestibulo-collic reflex that inputs to axial (trunk) muscles controlling postural reflexes
describe the cranial reflexes
cranial reflexes; do not go through spine, operate through afferent and efferent pathways in the head, include blink reflex and gag reflex
describe the H reflex
H reflex: spinal reflex elicited by artificial electrical stimulation of the nerve
electrical stimulation of a motor nerve first activates largest diameter 1a fibres which initiate monosynaptic reflex (seen as H wave on EMG)
increasing stimulus begins to activate motor fibres which activates muscle EMG directly and at shorter latency an M wave seen on the EMG
M wave comes after stimulus and before H reflex (H wave on EMG)
as stimulus strength increases M wave predominates as antidromic effects cancel the reflex orthodromic drive through the motor fibres
