Control of movement Four subsystems: Lower motor system- Grey matter of spinal cord and brainstem-contain lower motor neurons and lower circuit neurons-the.

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

– Familial/genetic component in 10% of cases