Organization of the Spinal Cord for Motor Functions

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The cord gray matter is the integrative area for the cord reflexes. Figure 54-1 shows the typical organization of the cord gray matter in a single cord segment. Sensory signals enter the cord almost entirely through the sensory (posterior) roots. After entering the cord, every sensory signal travels to two separate

We have already noted that sensory information is integrated at all levels of the nervous system and causes appropriate motor responses that begin in the spinal cord with relatively simple muscle reflexes, extend into the brain stem with more complicated responses, and finally extend to the cerebrum, where the most complicated muscle skills are controlled.

Wide Dynamic Range Neuron
Figure 54-1

Connections of peripheral sensory fibers and corticospinal fibers with the interneurons and anterior motor neurons of the spinal cord.

destinations: (1) One branch of the sensory nerve terminates almost immediately in the gray matter of the cord and elicits local segmental cord reflexes and other local effects. (2) Another branch transmits signals to higher levels of the nervous system—to higher levels in the cord itself, to the brain stem, or even to the cerebral cortex, as described in earlier chapters.

Each segment of the spinal cord (at the level of each spinal nerve) has several million neurons in its gray matter. Aside from the sensory relay neurons discussed in Chapters 47 and 48, the other neurons are of two types: (1) anterior motor neurons and (2) interneurons.

Anterior Motor Neurons. Located in each segment of the anterior horns of the cord gray matter are several thousand neurons that are 50 to 100 per cent larger than most of the others and are called anterior motor neurons. They give rise to the nerve fibers that leave the cord by way of the anterior roots and directly innervate the skeletal muscle fibers. The neurons are of two types, alpha motor neurons and gamma motor neurons.

Alpha Motor Neurons. The alpha motor neurons give rise to large type A alpha (Aa) motor nerve fibers, averaging 14 micrometers in diameter; these fibers branch many times after they enter the muscle and innervate the large skeletal muscle fibers. Stimulation of a single alpha nerve fiber excites anywhere from three to several hundred skeletal muscle fibers, which are collectively called the motor unit. Transmission of nerve impulses into skeletal muscles and their stimulation of the muscle motor units are discussed in Chapters 6 and 7.

Motor Sensory Motor a g Ia II g

Alpha motor / Sheath / Primary \ \ Extrafusal ending / / ending \ \ fibers

Gamma motor Fluid Secondary Intrafusal ending cavity ending fibers

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