This column of neural tissue only about 2 centimeters in diameter runs downward from the brain through the hollow bones of the spine like a thread through a string of beads. A Cross-section of the spinal cord (see my articles) shows two areas that differ somewhat in appearance: a but- terfly-shaped area of ”gray matter” in the middle surrounded by “white matter.” The grayish area consists mostly of cell bodies; the whitish area of myelinated axons that carry neural impulses upward downward or across the cord.
The spinal cord has two major jobs: to carry information back and forth between the body and the brain and to provide the necessary connections between the sensory and motor neurons involved in spinal reflexes.
These are automatic motor responses to stimulation that don’t involve neurons in the brain.
During a physical examination your leg may be struck with a rubber hammer just below the kneecap. Unless you inhibit it by tensing your muscles, your leg swings up in the familiar “knee-jerk” reflex. This is a good example of the simplest kind of reflex: the monosynaptic reflex arc. The doctor’s hammer strikes a tendon that pulls a muscle and causes it to stretch. The movement fires sensory neurons that go from the muscle all the way to the spinal cord. In the spinal cord the sensory neurons synapse directly onto motor neurons, which lead back to the knee. Thus the impulses received in the spinal cord are ”reflected” right back to the muscle causing it to contract one of my articles shows the neural pathway of the knee- jerk reflex.
Because the knee-jerk reflex is so easily tested it serves as a ready, if rough, measure of nervous-system health. It is one of a large number of reflexes that make it possible to stand and walk erect. Such reflexes enable you to keep your balance when for example someone pushes down unexpectedly on your shoulders or knocks you sideward.
Few reflexes, however, are as simple as (he knee-jerk. Most involve at least one additional neuron an interneuron. For instance when you touch something that is hot and your hand withdraws automatically, the sensory neurons from the finger carry the message to the spinal cord The message to the arm to withdraw the hand comes out of a different level of the spinal cord. To transmit the signal from the sensory neuron at one level to the motor neuron on another at least one more neuron-and two more synapses-are required. Interneurons are also needed whenever the reflex crosses the spinal cord to the opposite side of the body. If you’re barefoot and you step on something sharp, not only does the hurt foot withdraw. but the other leg stiffens so that you won’t fall down.
The signals from the burnt hand or the hurt foot do not stop at the spinal cord. They continue upward to the brain Only when (he message gets to the brain do you become aware of the sensation of pain-by which time your hand or foot has already jerked back. A person whose spinal cord is severed in an accident cannot feel any stimulation from the areas of the body innervated (supplied with nerves) by the part of the spinal cord below the break Yet many spinal reflexes remain intact because they do not involve neurons in the brain.
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