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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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By injecting into muscle groups visible tracers that are transported by the axons of the reduced motor neurons back to your cell bodies, the lower motor neurons that innervate the body"s skeletal muscles have the right to be seen in histological sections of the ventral horns of the spinal cord. Each reduced motor neuron innervates muscle fibers in ~ a solitary muscle, and also all the motor neurons innervating a solitary muscle (called the motor neuron swimming pool for that muscle) room grouped together into rod-shaped swarm that operation parallel to the long axis of the cord for one or an ext spinal cord segment (Figure 16.2).


Figure 16.2

Organization of lower motor neurons in the ventral horn of the spinal cord demonstrated by retrograde labeling from individual muscles. Neurons were figured out by put a retrograde tracer right into the medial gastrocnemius or soleus muscle that the cat. (A) (more...)

An orderly relationship between the location of the motor neuron pools and also the muscle they innervate is obvious both along the size of the spinal cord and throughout the mediolateral measurement of the cord, an arrangement that in result provides a spatial map of the body"s musculature. Because that example, the engine neuron pools that innervate the arm are located in the cervical enlargement that the cord and those the innervate the leg in the lumbar enlargement (see chapter 1). The mapping, or topography, that the exact same motor neuron pools in the mediolateral measurement can be evaluate in a cross ar through the cervical enlargement (the level depicted in number 16.3). Thus, neurons that innervate the axial musculature (i.e., the postural muscle of the trunk) are located medially in the cord. Lateral to this cell teams are engine neuron pools innervating muscles situated progressively much more laterally in the body. Neurons the innervate the muscles of the shoulders (or pelvis, if one were to look at a similar section in the lumbar enlargement; see figure 16.2) space the next many lateral group, whereas those the innervate the proximal muscles of the arm (or leg) are located laterally come these. The motor neuron pools that innervate the distal components of the extremities, the fingers or toes, lie farthest from the midline. This spatial organization provides clues about the attributes of the descending upper motor neuron pathways defined in the following chapter; few of these pathways terminate mostly in the medial region of the spinal cord, which is pertained to with postural muscles, whereas various other pathways terminate an ext laterally, wherein they have accessibility to the lower motor neurons that regulate movements the the fingers.


Figure 16.3

Somatotopic company of lower motor neurons in a cross ar of the ventral horn in ~ the cervical level of the spinal cord. Motor neurons innervating axial musculature are located medially, whereas those innervating the distal musculature are located (more...)

Two species of lower motor neuron are discovered in this neuronal pools. Tiny γ motor neurons innervate specialized muscle yarn that, in mix with the nerve fibers the innervate them, room actually sensory receptors dubbed muscle spindles (see chapter 9). The muscle spindles are installed within connective organization capsules in the muscle, and are therefore referred to together intrafusal muscle fibers (fusal way capsular). The intrafusal muscle yarn are also innervated by sensory axons the send info to the mind and spinal cord around the length and also tension of the muscle. The function of the γ engine neurons is to manage this sensory entry by setup the intrafusal muscle yarn to an ideal length (see following section). The second kind of lower motor neuron, called α motor neurons, innervates the extrafusal muscle fibers, which are the striated muscle yarn that in reality generate the pressures needed for movement.

Although the following discussion focuses top top the lower motor neurons in the spinal cord, comparable sets of motor neurons responsible because that the regulate of muscles in the head and neck are situated in the brainstem. The latter neurons are dispersed in the eight motor nuclei the the cranial nerves in the medulla, pons, and midbrain (see thing 1). Rather confusingly, but quite appropriately, this motor neurons in the brainstem are additionally called lower motor neurons.

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