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27 Cards in this Set

  • Front
  • Back

stretch reflex is aka

myotatic reflex


key for posture


*Max convergence and divergence of thispathway

*Changes in stretch reflex are involved inactions commanded by the brain

* Each 1a fiber will contact virtually all alpha motor neurons, and each alpha get input from EVERY spindle in that muscle

Phasic stretch reflex occurs in response torapid, transient stretch of the muscle (reflex hammer) *1a*

•or by an unexpected impediment to an ongoing movement

•Tonic stretch reflex occurs in response to a sloweror steady stretch applied to a muscle


Phasic(or 1a) Stretch Reflex

•Elicited by the primary(1a) endings of muscle spindles

Phasic Stretch Reflex

•excitatory (+) synapses directly (mono-synaptically) on virtually all a motor neurons that supply the same (homonymus) muscle and with many of its synergistic muscles

**1a: donot contact the gamma motor neurons: to avoid a positive-feed back loop ! **

In all other cases, excitation causes contraction

1a fibers can end on a variety of interneurons

reciprocal 1a inhibitory neuron innervates antagonist muscles

Monosynaptic nature

little opportunity for direct control ofactivity through this reflex arc BUT

* CNS overcomes this problem by controllingmuscle spindle sensitivity via the g motor neuron system*

knee reflex- stretch muscle spindle

AP enters spinal cord and joins at dorsal column pathway. Other branch synapses with motor neurons in ventral horn inhibiting flexors and activating extensors

Tonic Stretch Reflex

elicited by passivelybending a joint

Tonic Stretch Reflex

circuit includes both group 1a and II afferent fibers frommuscle spindles

group II= monosynaptic connections with alpha motor neurons and excite through di/polysynaptic pathways

Howis muscle tone innervated? **

Test question

Normally there is ongoing activity in the 1a and IIafferents and help maintain a baseline firing rate of alpha motor neurons.

Therefore tonic stretch reflexes contributes to muscle tone. Activity also contributes to posture maintenance

soldier standing

tonic stretch elicited to maintain posture.

•If the muscle length is ↑, then there is an ↑ group 1a and II firing to cause contraction to reverse stretch

•Passive shortening of the muscle will unload the spindle which ....

↓ excitatory drive to the motor neurons

and thus relaxation of the muscle

stretch reflex is essentially a

feedback system to regulate muscle length

Hyperactive stretch reflex can lead totremors and clonus

EXAM Question. What is clonus

•Clonus is a series of involuntary,rhythmic, muscular contractions and relaxations

Inverse Myotatic or 1b Reflex

•Feedback system to help maintainforce levels in a muscle

•Afferent 1b fibers branch as they enter the spinal cord and end on 2 interneurons

Receptor organ are GTO

•There are no mono-synaptic connections to a motor neurons

2 interneurons afferent 1b end on

•Interneurons that inhibit a motor neurons that supply the homonymus muscle

•excitatory interneurons that activate a motor neurons to the antagonistic muscle

2 synapses in series

disynaptic reflex arc

1b activity should have the opposite action of the 1a stretch reflex

EXAM- what is the difference in mono and disynaptic

Arm lifting bicep and weight is too much, bicep drops

GTOs detect excessive force and discharge action potentials --> spinal cord inhibitory neurons --> turn off motor neurons to the biceps and to other flexors synergistic to thebiceps and turn on motor neurons to the extensors that are antagonistic to the biceps to extend the arm and thebarbell drops

1a and 1b synergy

maintain posture

muscles fatiguing-

coordinated action of both musclespindles and GTO

Afferent volleys

•excitatory inter-neurons to activate the a motor neurons that supply the flexor muscles in the ipsilateral limb

•inhibitory interneurons to inhibit thea motor neurons that supply the antagonistic extensor muscles

•Collaterals of interneurons (Commissural) cross the midline to excite contralateral extensor motor neurons and inhibit flexors

Results in extension of the opposite limb (Crossed extension Reflex)

•Collectively the spinal reflexes provide for stability and postural support (the myotatic and inverse myotatic) and mobility (flexor withdrawal)


•Central Pattern Generator (CPG)

a set of neurons and circuits capable of generating the rhythmic activity that underlies motor acts, locomotion.

(cat with brainstem cut)