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Physiology Exam 4, Lecture 9
Organization of Motor System; Spinal Cord
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Physiology
Professional
11/10/2011

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Term
What is the lowest level of the motor system?
Definition
The α-motoneurons that innervate skeletal muscle (final common pathway), originating in spinal cord and brainstem
Term
What is the middle level of the motor system?
Definition
Brain regions that directly or indirectly influence the lowest level

sensorimotor cortex (areas giving rise to the corticospinal tract), brainstem nuclei giving rise to descending pathways (reticulospinal and vestibulospinal pathways), cerebellum, and basal ganglia system
Term
What is the highest level of the motor system?
Definition
Brain regions that initiate the desire to move

Premotor and supplementary motor cortices, association areas
of cortex, and regions involved in memory and emotion (limbic cortex).
Term
Describe the basal ganglia circuit in the motor system
Definition
Cerebral cortex → basal ganglia → ventrolateral/ventroanterior nuclei → cerebral cortex
Term
Describe the cerebellum circuit in the motor system
Definition
Cerebral cortex → pontine deep nuclei → cerebellum → ventrolateral/ventroanterior nuclei → cerebral cortex
Term
What are the two main locations of alpha motor neuron innervation?
Definition
Cerebellum and cerebral cortex
Term
Define motor unit
Definition
Motoneuron and all its muscle fibers
Term
What is the size principle?
Definition
Smaller motorneurons are recruited first, then large ones to increase force
Term
What do spindles sense?
Definition
Muscle length
Term
What is the name for the specialized muscle fibers in spindles?
Definition
Intrafusal fibers

(extrafusal fibers would be the outer ones that generate muscle force)
Term
What sensory fibers innervate intrafusal spindle fibers?
Definition
Group Ia and II
Term
What innervate the ends of intrafusal spindle fibers?
Definition
Small motoneurons, the γ-motoneurons
Term
How do spindle fibers function?
Definition
When the muscle is lengthened, action potentials in the sensory fibers alert the CNS

γ-motoneurons set the sensitivity, or gain, of the muscle spindles by determining the end of the spindle length
Term
What do golgi tendon organs sense?
Definition
Muscle tension
Term
What sensory fibers innervate golgi tendon organ fibers?
Definition
Large sensory fibers (group Ib)
Term
Describe the pathway of the stretch reflex
Definition
Muscle lengthens → spindle
stretch → activate Ia fibers → excite α- motoneurons → contract muscle

The fastest and only monosynaptic reflex

Also activate extension and antagonistic muscles as needed to keep balance; and brain is made aware of reflex
Term
Describe the tendon reflex
Definition
Muscle tension increases → GTO stretches → activate Ib fibers →
excite inhibitory interneuron → inhibit α-motoneurons → muscle relaxes

Also, excites antagonistic muscles and makes brain aware of muscle tension
Term
Describe the sequence for withdrawal reflex
Definition
Activation of pain fibers (flexor reflex afferents) → excitation of excitatory interneurons innervating flexor α-motoneurons and inhibitory interneurons innervating extensor α- motoneurons
Term
What reflex does the withdrawal reflex work together with?
Definition
The Crossed extensor reflex, to maintain balance
Term
Describe the sequence for the crossed extensor reflex
Definition
On opposite side of the flexion response, activation of pain fibers (flexor reflex afferents) → excitation
of excitatory interneurons innervating extensor α-motoneurons and inhibitory interneurons innervating flexor α- motoneurons
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