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6 Cards in this Set
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Spinal cord |
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. we discuss the control of muscle function by the spinal cord. Without the special neuronal circuits of the cord, even the most complex motor control systems in the brain could not cause any purposeful muscle movement. To give an example, there is no neuronal circuit anywhere in the brain that causes the specific to-and-fro movement of the legs that is required in walking. Instead, the circuits for these movements are in the cord, and the brain simply sends commandsignals to the spinal cord to set into motion the walking process. Let us not belittle the role of the brain, however, because the brain gives directions that control the sequential cord activities—to promote turning movements when they are required, to lean the body forward during acceleration, to change the movements from walking to jumping as needed,and to monitor continuously and control equilibrium. All this is done through “analytical” and “command” signals generated in the brain. But it also requires the many neuronal circuits of the spinal cord that are the objects of the commands. These circuits provide all but a small fraction of the direct cont |
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Spinal cord |
The cord gray matter is the integrative After entering the cord, every sensory signal travels to two separate 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 the other neurons are of two types: (1) anterior motor neurons and (2) interneurons. |
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Anterior motor Neuron |
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. |
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Alpha motor neuron |
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.
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Gamma motor Neurons |
Along with the alpha motor neurons, which excite contraction of the skeletal muscle fibers, about one half as many much smaller gamma motor neurons are located in the spinal cord anterior horns.
These gamma motor neurons transmit impulses through much smaller type A gamma (Ag) motor nerve fibers, averaging 5 micrometers in diameter, which go to small, special skeletal muscle fibers called intrafusal fibers.
These fibers constitute the middle of the muscle spindle, which helps control basic muscle “tone,” |
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Interneurons |
Interneurons are present in all areas of the cord gray matter—in the dorsal horns, the anterior horns, and the intermediate areas between them,
These cells are about 30 times as numerous as the anterior motor neurons.
They are small and highly excitable, often exhibiting spontaneous activity and capable of firing as rapidly as 1500 times per second.
They have many interconnections with one another, and many of them also synapse directly with the anterior motor neurons,
The interconnections among the interneurons and anterior motor neurons are responsible for most of the integrative functions of the spinal cord that are discussed in the remainder of this chapter.
Essentially all the different types of neuronal circuits d are found in the interneuron pool of cells of the spinal cord, including diverging, converging, repetitive-discharge, and other types of circuits.
In this chapter, we examine many applications of these different circuits in the performance of specific reflex acts by the spinal cord.
Only a few incoming sensory signals from the spinal nerves or signals from the brain terminate directly on the anterior motor neurons. Instead, almost all these signals are transmitted first through interneurons, where they are appropriately processed. Thus, in |
