Ch. 15: Lower Motor Neuron Circuits and Motor Control - a model for understanding the nervous system 12/3/2018 55-485

Neural centers responsible for movement 4 interactive subsystems contribute to motor control 12/3/2018 55-485

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. 12/3/2018 55-485

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Neural centers responsible for movement Upper motor neurons - cell bodies in brainstem or cerebral cortex 12/3/2018 55-485

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Neural centers responsible for movement 3) Cerebellum - regulate activity of upper motor neurons 12/3/2018 55-485

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Neural centers responsible for movement 4) Basal Ganglia Regulate upper motor neurons 12/3/2018 55-485

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Basal ganglia - caudate, putamen, globus pallidus -subthalamic nucleus, substantia nigra 12/3/2018 55-485

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. 12/3/2018 55-485

Gray matter of spinal cord and brainstem Local circuit neurons Synaptic input to lower motor neurons 12/3/2018 55-485

Gray matter of spinal cord and brainstem Lower motor neurons convey voluntary and reflex commands to skeletel muscle 12/3/2018 55-485

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 12/3/2018 55-485

Gray matter of spinal cord and brainstem Circuits provide co-ordination between muscle groups for movement Are the final common path for movement. 12/3/2018 55-485

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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 12/3/2018 55-485

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) 12/3/2018 55-485

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 12/3/2018 55-485

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 12/3/2018 55-485

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. 12/3/2018 55-485

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 12/3/2018 55-485

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. 12/3/2018 55-485

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. 12/3/2018 55-485

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) 12/3/2018 55-485

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. 12/3/2018 55-485

Motor neuron - muscle relationships Spatial organization gives information about functions 12/3/2018 55-485

Motor neuron - muscle relationships Two types of lower motor neurons in the motor neuron pools Small  motor neurons  motor neurons 12/3/2018 55-485

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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 12/3/2018 55-485

Small  motor neurons In Intrafusal muscle fibers Sets the intrafusal muscle fiber to an appropriate length for the sensory neuron to function. 12/3/2018 55-485

 motor neurons Large Innervate extrafusal muscle fibers. These generate force for movement and posture. 12/3/2018 55-485