The knee jerk reflex is a simultaneous action of
all muscle spindle sensory neurons and activation of all motor neurons.
stretch receptor neurons are
Ia afferents
each afferent in particular muscle innervates all motor neurons in that muscle
motor reflex
- negative feedback system
- muscle is stretched–> Ia afferent endings are distorted–> initiates AP
- APs propagate in SC–> synapse on dendrites on that muscle’s motor neuron
- a-motor neurons fire AP and produce muscle contraction
- relieves stress so Ia neurons are silenced
axons DO NOT make a
single synapse at a single target
Some target of Ia afferents
- excitatory synapses on motor neurons of synergistic muscles
- through interneurons, inhibit the motor neurons of antagonistic muscles.
- motor neurons of muscle in which spindle is located
muscle spindle arrangement
in parallel with extrafusal fibers
golgi tendon organ arrangement
- in series with muscle
2. when muscle contracts or is passively stretched, 1b afferents fire APs
muscle spindle info transmitted
length
golgi tendon organ info transmitted
force
muscle spindle innerv
- afferent and effecert innervation
- sensory and motor innerv
- peripheral DRG neuron processes enter spindle and wrap around intrafusal fiber middle
peripheral DRG neuron processes enter spindle and wrap around intrafusal fiber middle
- fibers get stretched, nerve endings get deformed and membrane is depol, initiating AP (afferents fire)
- when muscle contracts, spindle becomes relaxed because its parallel orientation (afferents can’t fire)
- intrafusal fibers are excited by gamma motor neurons and contract
Golgi tendon organs innerv on motor neurons
1b afferents
- slower than 1a afferents
- depol to stretch or deformation
- synapse on SC interneurons that inhibit alpha motor neurons
- stim by active contraction of muscle
size principle for recruiting motor neurons
- grade tension on skeletal muscle, this changes the # of active motor neurons that innervate muscle
- 1 motor neuron–> motor unit size that produces 5% increase in tension
- smallest motor neuron are recruited first and largest ones last
size principle for recruiting motor neurons allows the
force of contraction.movement to be varied based upon the specifics of a particular situation/movement
flexor withdrawal reflex
- causes you to withdraw limb when activate flexors
- pain pathways excited flexors through polysynaptic connections with spinal interneurons
- simultaneously excited contralateral extensors
