Anatomy and Cell Biology 3319 Lecture Notes - Lecture 10: Alpha Motor Neuron, Flaccid Paralysis, Stretch Reflex
1 May 2018
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Lecture 010: Descending Motor Pathways
Objectives
● Name and function
● Summary of motor controls
Motor Control
● Reflexes
○ Constantly operative
○ Thus cerebral cortex has to
override/suppress it
● All motor control is affected through
the motor unit
● A motor unit is composed of
○ Alpha motor neuron in the
ventral horn
■ Goes to the
extrafusal skeletal
muscle to signal
contractions
● 1a afferent
○ Takes in dynamic information about stretch and relaxation of the muscle
○ stretch reflex
○ monosynaptic reflex
■ 1a sensory nerve has an excitatory synapse with the alpha motor neuron
■ Activation of interneurons to Inhibit antagonist muscles
● Gamma motor neurons
○ terminate on the intrafusal fibres of the muscle spindle
○ control the length and tension of the spindle
○ Determines the frequency of firing
○ Participate in the spinal stretch reflex.
● lower motor neuron lesion causes flaccid paralysis, loss of stretch reflex, and atrophy.
Descending Motor Pathways
● Corticospinal
● Corticobulbar
● Reticulospinal
● Vestibulospinal (postural)
Things to Know for the Descending Motor Pathways
● Where they originate
● Where they synapse
● Where they cross the midline
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Corticospinal pathway
● for lower motor neurons
● main/critical pathway that takes
commands from the cerebral cortex
(through the spinal cord) and send to the
lower motor neurons
○ Precision and speed of skilled
movemets
● Primary motor cortex feeds into the
internal capsule then descend
○ Midbrain -> Cerebral peduncle ->
Pons (pontine nuclei)
○ Continues ipsilateral to until the
medulla
○ No synapses there
● At the lower medulla it decussates
○ 85-90% of the fibers cross the
midlines
○ Why cerebral cortex control is
contralateral
○ Descends through the lateral
corticospinal tract to the lower
motor neuron
● 15% of the fibers do not decussates at
the lower medulla
○ Continues ipsilateral down
the anterior corticospinal
tract (much smaller)
● The lateral corticospinal neuron
terminals
○ Directly on a lower motor
neuron
○ On a lateral internal neuron
■ This neuron in turn can
connect to a motor
neuron
● The anterior corticospinal neuron
decussates (in the spinal cord) at the
level where it will make a connect with
a motor neuron
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Thus cerebral cortex has to override/suppress it. All motor control is affected through the motor unit. Alpha motor neuron in the ventral horn. Goes to the extrafusal skeletal muscle to signal contractions. 1a afferent lower motor neuron lesion causes flaccid paralysis, loss of stretch reflex, and atrophy. Things to know for the descending motor pathways. Takes in dynamic information about stretch and relaxation of the muscle. 1a sensory nerve has an excitatory synapse with the alpha motor neuron. Activation of interneurons to inhibit antagonist muscles. Gamma motor neurons terminate on the intrafusal fibres of the muscle spindle. Control the length and tension of the spindle. Main/critical pathway that takes commands from the cerebral cortex (through the spinal cord) and send to the lower motor neurons. Primary motor cortex feeds into the internal capsule then descend. 85-90% of the fibers cross the midlines. Descends through the lateral corticospinal tract to the lower motor neuron.
