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Spinal cord circuit and motor control
• Overview of the motor system
• Topographic relationship between spinal
motor neurons and muscles
• Motor unit and muscle force
• Reflex circuit
Spinal cord circuit and motor control
Planning, initiation
of voluntary
movement
Sensory-motor
integration,
motor learning
Simple movement,
reflex
Basic movement, posture
Dorsal horn
Ventral horn
Motor neuron pool
Motor neuron pool
• In a mature animal, each
muscle fiber receives input
from a single motor neuron
• Each motor neuron
innervates multiple muscle
fibers
• All motor neurons
innervating a muscle is
called a motor neuron pool
• All motor neurons in a
motor neuron pool are
clustered together in spinal
cord
muscle
Topographical relationship between
motor neurons and muscles
• Motor neuron pools are organized
within the ventral horn with those
innervating distal muscle groups
located laterally and those innervating
the proximal muscles located medially
• Different segments of the spinal cord
innervate muscles in different parts of
the body (see diagram on the right)
Motor Unit
• A motor unit consists of a
motor neuron and all the
muscle fibers it innervates
• Three types:
• Fast fatigable (FF)
• Powerful, but fatigue with repetitive
stimulation
• muscle fiber thick, large number
• Motor neuron large, fast conduction,
but higher firing threshold
• Slow (S)
• (opposite to FF)
• Fast, fatigue-resistant (FR)
• Intermediate between FF and S
Muscle force
• Firing rate of motor
neurons
• Progressive
recruitment of motor
units
• S  FR  FF
Stretch reflex
Muscle stretch  activation of dorsal root ganglion
neuron

{
(1) Alpha motor neuron activation  contraction of
same or synergistic muscle
(2) Interneuron activation  inhibition of another motor
neuron  reduced contraction of antagonist muscle
The dorsal root ganglion neuron (proprioceptor) is activated
when intrafusal muscle fiber (spindle) detects changes in
muscle length
Another example of stretch reflex: Knee jerk