Ch 15 lower motor neuron circuits and motor control
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Ch. 15: Lower Motor Neuron Circuits and Motor Control. - a model for understanding the nervous system. Neural centers responsible for movement. 4 interactive subsystems contribute to motor control. Neural centers responsible for movement. Gray matter of spinal cord and brainstem

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Ch. 15: Lower Motor Neuron Circuits and Motor Control

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Ch 15 lower motor neuron circuits and motor control

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

Neural centers responsible for movement

  • 4 interactive subsystems contribute to motor control

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Neural centers responsible for movement1

Neural centers responsible for movement

  • 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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Ch 15 lower motor neuron circuits and motor control

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Neural centers responsible for movement2

Neural centers responsible for movement

  • Upper motor neurons

    - cell bodies in brainstem or cerebral cortex

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Ch 15 lower motor neuron circuits and motor control

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Ch 15 lower motor neuron circuits and motor control

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Neural centers responsible for movement3

Neural centers responsible for movement

3) Cerebellum

- regulate activity of upper motor neurons

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Ch 15 lower motor neuron circuits and motor control

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Neural centers responsible for movement4

Neural centers responsible for movement

  • 4) Basal Ganglia

    • Regulate upper motor neurons

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Ch 15 lower motor neuron circuits and motor control

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Ch 15 lower motor neuron circuits and motor control

Basal ganglia - caudate, putamen, globus pallidus

-subthalamic nucleus, substantia nigra

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1 gray matter of spinal cord and brainstem

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

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 brainstem1

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 brainstem2

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 brainstem3

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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Ch 15 lower motor neuron circuits and motor control

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2 upper motor neurons

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 neurons1

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 neurons2

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

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 cerebellum1

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

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

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

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 relationships1

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 relationships2

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 relationships3

Motor neuron - muscle relationships

  • Spatial organization gives information about functions

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Motor neuron muscle relationships4

Motor neuron - muscle relationships

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

    • Small  motor neurons

    •  motor neurons

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Ch 15 lower motor neuron circuits and motor control

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Small motor neurons

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 neurons1

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

 motor neurons

  • Large

  • Innervate extrafusal muscle fibers. These generate force for movement and posture.

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