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Ch. 15: Lower Motor Neuron Circuits and Motor Control

- a model for understanding the nervous system

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Neural centers responsible for movement

• 4 interactive subsystems contribute to motor control

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Neural centers responsible for movement

1) Gray matter of spinal cord and brainstem - local motor neurons innervate skeletal muscle cells

- motor neurons of cranial nerves from cell bodies in the brain stem

- local circuit neurons are the major source of synaptic input to the lower motor neurons.

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Neural centers responsible for movement

2) Upper motor neurons

- cell bodies in brainstem or cerebral cortex

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Neural centers responsible for movement

3) Cerebellum- regulate activity of upper motor neurons

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Neural centers responsible for movement

• 4) Basal Ganglia

• Regulate upper motor neurons

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Basal ganglia - caudate, putamen, globus pallidus

-subthalamic nucleus, substantia nigra

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1) Gray matter of spinal cord and brainstem

• Lower motor neurons send axons out of the brain stem and spinal cord to innervate skeletal muscles of the head and body.

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Gray matter of spinal cord and brainstem

• Local circuit neurons– Synaptic input to lower motor neurons

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Gray matter of spinal cord and brainstem

• Lower motor neurons convey voluntary and reflex commands to skeletel muscle

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Gray matter of spinal cord and brainstem

• Local circuit neurons– Receive descending projections from

higher brain centers– Also receive sensory input from higher

brain centers

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Gray matter of spinal cord and brainstem

• Circuits provide co-ordination between muscle groups for movement

– Are the final common path for movement.

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2) Upper motor neurons

• Cell bodies in the brainstem or cerebral cortex

• Axons descend to synapse with local circuit neurons, sometimes directly onto lower motor neurons

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2) Upper motor neurons• In the cortex

– Initiate voluntary movements– Initiate complex spatiotemporal sequences

of skilled movements– E.g. descending projections from the

frontal lobe (primary motor cortex and lateral pre-motor cortex)

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2) Upper motor neurons

• In the brainstem– Regulate muscle tone for orienting eyes,

head and body wrt sensory information (balance, somatic, sound, vision input)

– Imp. For movement and posture

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3) Cerebellum

• Does not directly access local circuit neurons or lower motor neurons

• Regulate the upper motor neurons

• Located on the dorsal suface of the pons

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3) Cerebellum

• Detects the difference between an intended movement and the movement that is actually performed (servomechanism)

• Reduces movement error in short term and in long term (learning) eg. Riding a bike.

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4) Basal ganglia

• No direct access to local circuit neurons or to lower motor neurons

• Regulate upper motor neurons

• Suppress unwanted movement• Prepare upper motor neurons for

movements to start

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How does thought lead to movement?

• Still poorly understood

• Clear picture at the level of muscles themselves.

• Start with the lower motor neurons and their targets.

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Motor neuron - muscle relationships

• Neuronal tracer injections into the cell body show the pathway on motor neuron axons

• motor neuron pools are all of the motor neurons that innervate a muscle.

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Motor neuron - muscle relationships

• Motor neuron pools run together in the spinal cord.

• An orderly arrangement between the motor neuron pools and the muscles that they innervate*** - a rule for the nervous system (mapping, topography)

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Motor neuron - muscle relationships

• Fig. 15.3– Topography– Neurons for postural muscles are medial in

the cord– Neurons for the shoulders are next most

lateral– Neurons for upper arms are next lateral.– Neurons for distal extremeties are most

lateral.

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Motor neuron - muscle relationships

• Spatial organization gives information about functions

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Motor neuron - muscle relationships

• Two types of lower motor neurons in the motor neuron pools– Small motor neurons motor neurons

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Small motor neurons

• Innervate sensory structures in skeletal muscle (muscle spindles)

• Muscle spindles are embedded in connective tissue– Are called intrafusal muscle fibers

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Small motor neurons

• In Intrafusal muscle fibers– Sets the intrafusal muscle fiber to an

appropriate length for the sensory neuron to function.

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motor neurons• Large• Innervate extrafusal muscle fibers.

These generate force for movement and posture.