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Control of movement
Four subsystems:
• Lower motor system- Grey matter of spinal cord and brainstem-contain lower
motor neurons and lower circuit neurons-the final common path of all motor
output.
• Upper motor systems- send info to spinal cord, initiate voluntary movementscontains motor cortex and some brainstem centers.
• Cerebellum- No direct access to lower motor systems-connects to upper
motor systems. Responsible for motor learning.
• Basal ganglia. Suppresses unwanted movements and primes neurons for the
initiation of movements. Involved in Parkinson’s disease.
Motor pools
• Cell bodies of motor neurons are found in ventral horn of the spinal cord.
• Each motor neuron innervates muscle fibers within a single muscle.
• All the neurons innervating a single muscle are grouped together in clusters
called motor pools.
• Motor pools are located near their targets (in nearby segments of the cord).
Lower Motor Neurons in the Ventral
Horn of the Spinal Cord
The motor unit
• Extrafusal skeletal muscle fibers are
innervated by a single a motor neuron.
Called a motor unit.
• A single axon branches to synapse on
several fibers of the muscle.
• An action potential generated by a motor
neuron normally brings to threshold all of
the muscle fibers it contacts.
The Motor Unit
Force and Fatigability of the Three Types
of Motor Units
A: muscle tension after a single
action potential of the motor
neuron.
B: Tension after repetitive
stimulation.
C: Repetitive stimulation evokes
maximum tension.
Recruitment of motor neurons to medial
gastrocnemius (leg muscle):the size principle
Force measured from muscle tendon.
Force as a function of stimulation
frequency
A. low frequency:
each action potential>
single twitch.
B. higher frequencies,
Summation.
C. greater force, individual
twitches still apparent.
D. fused tetanus – max rate
of stimulation, individual
twitches not apparent.
Central Pattern Generators Organize the Cycle
of Locomotion for Terrestrial Mammals
You don’t need your brain to walk
Amytrophic Lateral Sclerosis
(ALS)
• Neurodegenerative disease affecting alpha motor
neurons of ventral horn and brainstem (lower motor
neurons) and upper motor neurons of motor cortex.
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Progressive muscle weakness
Skeletal muscle wasting
Usually die within 5 years of onset
Hypothesized to be defect in axonal transport mechanisms
Familial/genetic component in 10% of cases