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 movements- contains 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.
Transcript
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 movements- contains 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 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.– Progressive muscle weakness
– Skeletal muscle wasting
– Usually die within 5 years of onset
– Hypothesized to be defect in axonal transport mechanisms
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