Physiology 3120 Lecture Notes - Type Ia Sensory Fiber, Alpha Motor Neuron, Stretch Reflex

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26 Nov 2011
Human Physiology
Friday, November 13, 2009
“Neuro X”
Spinal Reflexes
Knee-jerk reflex
Tapping the quad muscle, you produce a reflexive quadriceps contraction
Ia afferents enter the spinal cord and stimulate its own efferent alpha motor-neuron
Also inhibit the antagonist muscle (i.e. the hamstring) uses a branch (collateral) of the
same Ia afferent fibre
Collaterals also feed up the dosal-column medial lemniscal system (not shown in
Reciprocal inhibition
Property of all spinal reflexes
Agonist muscle excited, but the antagonist inhibited
Functions as the first defence in overcoming unexpected muscle stretches (i.e. think of someone
pushing you over; you want to contract the same muscle that was stretched by the perturbation);
second defence is the vestibular postural reflex
Extraordinarily fast (contraction 30ms after the stretch); voluntary contractions caused by a bone
tap take 150ms
Electromyogram (EMG)
Record electrical activity of entire muscle because all the individual muscle fibres in the motor
unit simultaneously send action potentials
Extracellular recording
Much easier than recording from the nerve
Different speeds of stretch
Can’t manually generate a stretch reflex in the biceps because they can’t go fast enough to
summate & reach threshold
Medium = as fast as we can go manually
Note: IA afferent responds to velocity of stretch
Frequency of discharge proportional to velocity of stretch
Get greater summation of EPSPs with faster stretch
With tendon tap, summation is such that they summate to reach threshold
Patient has lesion to motor cortex due to a stroke
Patients with spasticity have a stretch reflex in response to muscle stretch
Proportional to velocity of the stretch
Said to be a velocity-dependent increase in muscle tone
Spastic patients have RMPs depolarized from normal controls; due to loss of inhibition from
motor cortex
The same stretch brings spastic patients to threshold & they fire action potentials
Spinal flexion (withdrawal) reflex
Still functions in quadriplegic (i.e. don’t need a brain, because it is a spinal reflex)
Receptors are nociceptors; a number of different chemical substances released (bradykinin,
potassium, histamine)
Polysynaptic reflex (at least 3 synapses involved); the only monosynaptic reflex is the
stretch reflex
Contraction of flexors, relaxation of extensors (reciprocal inhibition)
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