SPINAL CORD REFLEXES
When you hear the term reflex, you may think of an action that “just happens,” and in part this is so. A reflex is an involuntary response to a stimulus, that is, an automatic action stimulated by a specific change of some kind. Spinal cord reflexes are those that do not depend directly on the brain, although the brain may inhibit or enhance them. We do not have to think about these reflexes, which is very important, as you will see.
A reflex arc is the pathway that nerve impulses travel when a reflex is elicited, and there are five essential parts:
1. Receptors—detect a change (the stimulus) and generate impulses.
2. Sensory neurons—transmit impulses from recep-tors to the CNS.
3. Central nervous system—contains one or more synapses (interneurons may be part of the pathway).
4. Motor neurons—transmit impulses from the CNS to the effector.
5. Effector—performs its characteristic action.
Figure 8–5. Patellar reflex. The reflex arc is shown. See text for description.
QUESTION: Why is this reflex called a stretch reflex?
Let us now look at the reflex arc of a specific reflex, the patellar (or knee-jerk) reflex, with which you are probably familiar. In this reflex, a tap on the patellar tendon just below the kneecap causes extension of the lower leg. This is astretch reflex, which means that a muscle that is stretched will automatically contract. Refer now to Fig. 8–5 as you read the following:
In the quadriceps femoris muscle are (1) stretch receptors that detect the stretching produced by strik-ing the patellar tendon. These receptors generate impulses that are carried along (2) sensory neurons in the femoral nerve to (3) the spinal cord. In the spinal cord, the sensory neurons synapse with (4) motor neu-rons (this is a two-neuron reflex). The motor neurons in the femoral nerve carry impulses back to (5) the quadriceps femoris, the effector, which contracts and extends the lower leg.
The patellar reflex is one of many used clinically to determine whether the nervous system is functioning properly. If the patellar reflex were absent in a patient, the problem could be in the thigh muscle, the femoral nerve, or the spinal cord. Further testing would be needed to determine the precise break in the reflex arc. If the reflex is normal, however, that means that all parts of the reflex arc are intact. So the testing of reflexes may be a first step in the clinical assessment of neurologic damage.
You may be wondering why we have such reflexes, these stretch reflexes. What is their importance in our everyday lives? Imagine a person standing upright—is the body perfectly still? No, it isn’t, because gravity exerts a downward pull. However, if the body tilts to the left, the right sides of the leg and trunk are stretched, and these stretched muscles automatically contract and pull the body upright again. This is the purpose of stretch reflexes; they help keep us upright without our having to think about doing so. If the brain had to make a decision every time we swayed a bit, all our concentration would be needed just to remain standing. Since these are spinal cord reflexes, the brain is not directly involved. The brain may become aware that a reflex has taken place, but that involves another set of neurons carrying impulses to the brain.
Flexor reflexes (or withdrawal reflexes) are another type of spinal cord reflex. The stimulus is something painful and potentially harmful, and the response is to pull away from it. If you inadvertently touch a hot stove, you automatically pull your hand away. Flexor reflexes are three-neuron reflexes, because sensory neurons synapse with interneurons in the spinal cord, which in turn synapse with motor neurons. Again, however, the brain does not have to make a decision to protect the body; the flexor reflex does that automatically. The brain may know that the reflex has taken place, and may even learn from the experience, but that requires different neurons, not the reflex arc.
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