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|A reflex is a direct connection between stimulus and response, which does not require conscious thought. There are voluntary and involuntary reflexes. It is the voluntary reflexes we are considering here. As discussed earlier, a reflex involves at least 2 or 3 neurons. The typical components of a reflex are shown in Figure 13.12. The reflex shown in this figure is called a 3-neuron reflex because it requires three types of neurons: a sensory, an interneuron, and a motor neuron. It is also called a withdrawal reflex because it is commonly involved in withdrawing from painful stimuli. Withdrawing from painful stimuli does not require thought. But the interneuron does send a fiber through the spinothalamic tract to the brain where the pain is perceived. This occurs at virtually the same instant you are withdrawing from the stimulus. For example, let's say you accidentally touch a hot stove. Instantly you withdraw your hand from the stove, at the same time you are feeling the pain.|
|The Stretch Reflex: (See Figure
The stretch reflex in its simplest form involves only 2 neurons, and is therefore sometimes called a 2-neuron reflex. The two neurons are a sensory and a motor neuron. The sensory neuron is stimulated by stretch (extension) of a muscle. Stretch of a muscle normally happens when its antagonist contracts, or artificially when its tendon is stretched, as in the knee jerk reflex. Muscles contain receptors called muscle spindles. (See Figure 13.13) These receptors respond to the muscles's stretch. They send stimuli back to the spinal cord through a sensory neuron which connects directly to a motor neuron serving the same muscle. This causes the muscle to contract, reversing the stretch. The stretch reflex is important in helping to coordinate normal movements in which antagonistic muscles are contracted and relaxed in sequence, and in keeping the muscle from overstretching.
Since at the time of the muscle stretch its antagonist was contracting, in order to avoid damage it must be inhibited or tuned off in the reflex. So an additional connection through an interneuron sends an inhibitory pathway to the antagonist of the stretched muscle - this is called reciprocal inhibition.
|The Deep Tendon Reflex: (See Figure 13.16)
Tendon receptors respond to the contraction of a muscle. Their function, like that of stretch reflexes, is the coordination of muscles and body movements. The deep tendon reflex involves sensory neurons, interneurons, and motor neurons. The response reverses the original stimulus therefore causing relaxation of the muscle stimulated. In order to facilitate that the reflex sends excitatory stimuli to the antagonists causing them to contract - reciprocal activation.
|The stretch and tendon reflexes complement one another. When one muscle is stretching and stimulating the stretch reflex, its antagonist is contracting and stimulating the tendon reflex. The two reflexes cause the same responses thus enhancing one another. [See Complementarity of Stretch and Tendon Reflexes]|
|The Crossed Extensor Reflex - (See
The crossed extensor reflex is just a withdrawal reflex on one side with the addition of inhibitory pathways needed to maintain balance and coordination. For example, you step on a nail with your right foot as you are walking along. This will initiate a withdrawal of your right leg. Since your quadriceps muscles, the extensors, were contracting to place your foot forward, they will now be inhibited and the flexors, the hamstrings will now be excited on your right leg. But in order to maintain your balance and not fall down your left leg, which was flexing, will now be extended to plant your left foot (e.g. crossed extensor). So on the left leg the flexor muscles which were contracting will be inhibited, and the extensor muscles will be excited. The textbook illustration uses the arms. Study it to see how the same process works there, even though it isn't necessary to maintain balance.
Revised: March 28, 2005