The nervous system as a whole is made up of input systems (afferent division) and output systems (efferent division)- the motor system itself makes up the efferent division which is responsible for coordinating muscle function.
The motor system can be further divided into the somatic motor system controlling all voluntary muscle movement and the autonomic motor system controlling all non-voluntary movement.
The human motor system is made up of four subsystems that help mediate function.
The first of the four sub-systems is comprised of lower motor neurons and the local circuit neurons that are associated with them. The lower motor neurons are found in the spinal cord and in the brain stem, and they innervate skeletal muscles in the body and in the head, respectively. The local circuit neurons innovate the lower motor neurons and receive sensory inputs from the periphery and descending motor signals from the brain. The main function of the local circuit neurons is to coordinate the different muscle groups and facilitate organized movement, but otherwise it is the lower motor neurons that are in direct contact with the muscles- they are the final common paths for all communication with the muscle.
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Upper motor neurons originate in the brain (cortex and brainstem) and descend to the spinal cord or brainstem. LMNs, on the other hand, have their cell bodies in the spinal cord or cranial nerve nuclei and their axons extend to the target muscles or glands.
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The second sub-system is the descending systems which are derived from the motor cortex and from the brainstem centers. This subsystem is made-up of upper motor neurons which mostly innovate the local circuit neurons and, very rarely, directly innovate the lower motor neurons to control movement. Upper motor neurons from different parts of the cortex are important in the planning and initiation of voluntary movement of muscles, the movement of eyes, the skilled and learned movements of our distal muscles whereas upper motor neurons from the brainstem are important for control mainly of posture as well as basic navigational movements that involve the axial muscles.
Then there's a third and a fourth sub-system which are not in direct contact with lower motor neurons, but send massive communication to the upper motor neurons in the cortex and in the brainstem to regulate and refine movement. These two sub-systems include the basal ganglia and the cerebellum.
The grey matter of the spinal cord is made up of ten layers known as Rexed’s Laminae and each layer has a specific function for handling different types of sensory information.
The ventral horn of the spinal cord (layers 7-9) contains very large neurons called alpha motor neurons which directly innervate skeletal muscles. These neurons are multipolar and have prominent nuclei.
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Motor nuclei: clusters of motor neurons that innervate the same muscle (also called motor pools, usually hundreds of motor neurons)
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The neurons that supply a given muscle do not come from just one part of the spinal cord straight to that muscle. They actually extended over some distance across multiple segments of the spinal cord.
An experiment was conducted on an animal model looking at gastrocnemius and the soleus. These muscles were injected with a dye substance which was taken up by the neuromuscular axons of alpha motor neurons that innervate that muscle. When the dye is taken up, it is transported back to the cell bodies of the neurons which are found in the ventral horn of the spinal cord. The experimenters then looked at the spinal cord and did histological sections to plot the location of the cells that were labelled with the specific dye.
