spinal reflexes Flashcards
spinal reflexes are stereotyped suggesting…..
‘occur the same way each time’
underlying circuitary is very simple
spinal reflexes require stimulation (not spontaneous) suggesting…
they need sensory input
spinal reflexes are automatic suggesting ….
little input from higher centres
spinal reflexes are quick suggesting….
few synapses involved
the stretch (myotactic) reflex- knee jerk
knee jerk experiment is not a repsonse to pain
the tap stretches the thigh extensor muscle and associated tendon and sets in motion a process to correct the stretching
important in maintaining body posture
charles sherrington
monosynaptic relfex
simple proprioceptive feedback pathway
muscle spindle
special sensory receptor that detects muscle stretch
pouring a drink (stretch reflex)
1) sensory firbes sense muscle stretch and send signals to spinal cord
2) direct monosynpatc connection to motor neurone fires action potential which contract biceps
3) simultaneously, a distinct connection to an inhibitory interneruone inhibit during of motor neurones connected to triceps thus relaxing the antagonistic muscle
(reciprocal inhbition)
where are muscle spindles found
in most striated muscles
and particualry in muscles in fine motor control e.g. hand
what are muscle spindles innervated by
Ia sensory fibres
which provide feedback to the MN innervating the surrounding muscle (alpha motor neurons- called this because of their conduction velocity) on the amount muscles stretch that is occurring (as in knee jerk)
the muscle spindle has muscles- they are innervated by…
axons from gamma (y) motor neurones
gamma motor neurons do what
stimulate the intrafusal (muscle) fibres to adjust the tension in the spindle as the extrafusal (muscles) fibres of the surrounding muscle contract (so that the muscle spindle is never slack)
Ian waterman
viral infection destroyed the nerve in properioceptio
also lost moving in space(kinaesthesia)
types of proprioceptors
muscle spindle
golgi tendon organ
goligi tendon organ detects
muscles tension due to muscle contraction (not muscle stretch)
muscle spindle detects m
muscle stretch
activation of GTO sensory (Ib) afferents leads to
activation of inhibitory interneurons which in turn inhibit alpha motor morons that innervate the same muscle
the golgi tendon reflex
is a negative feedback circuit that regulates muscle tension and protects the muscle (and tendon) from damage when large forces are generated
flexor (withdraw) reflex
quick contraction of flexor muscles to withdraw a limb from an injurious stimulus
results from the activation of nociceptors
despite speed of response it is actually a polysynaptic reflex
‘parallel after discharge circuit’
‘parallel after discharge circuit’
Activation of multiple excitatory interneurons sustains the response:
Assume time taken to cross each synapse is the same.
Therefore, stimulus initiated by A will take different times to reach output neuron Z.
Result is that initial signal is sustained over extended period
imbalance esp in leg with rapid withdrawal of limb in flexor reflex
there is a contralateral element to account for this
crossed extensor reflex provides postural support
local circuits that generate the pattern of alternating flexion and extension are known as
central pattern generators
as a cat walks
each limb has extensors and flexors alternating between active and inhbited as the cat walks
even if spinal cord connections to the brain are severed the basic alteration of acuity and the coordination between legs continues
different neurones exhibit distinct patterns of rhythmic firing
also evidence in lamprey (worms)
critical feature: inhibition across the midline
nematode worm - C. elegans
proprioception at the molecular level
loss of TRP-4 ‘stretch’ receptor in DVA neuron leads to ‘loopy’ swimming
DVA neuron detects body bending directly
repetitive spikes seen in swimming
DVA appears to inhibit motor neurone
