Sensory Pathways Help
Introduction to Sensory Pathways
Just as motor pathways descend or come down from both higher and lower motor neurons, sensory pathways ascend or rise up from sensory neurons . A lower or first-order sensory neuron is a nerve cell that has part of its dendrites modified to form sensory receptors . Sensory receptors are specialized nerve endings that are sensitive to a particular kind of “goad” (poking prod) called a stimulus ( STIM -yuh-les).
Consider, for example, certain onion-shaped sensory receptors in the skin of the right hand (Figure 14.11). They are sensitive to touch or pressure sensations. A nerve impulse is created, and then it travels over a peripheral nerve, then a spinal nerve, and into the spinal cord. A higher or second-order sensory neuron is then excited. Its axon crosses the body midline to the left side of the spinal cord, and then ascends towards the thalamus ( THAL -uh- mus ).
The thalamus is an egg-shaped “bedroom” ( thalam ) – an oval region nestled deep within the cerebrum. The thalamus is often described as a sensory relay center . This is because sensory nerve impulses (action potentials) are often switched over (or relayed) up onto third-order sensory neurons , whose cell bodies lie within the thalamus. And from the thalamus, the axons of the third-order sensory neurons finally carry the information about the stimulus all the way up to the postcentral gyrus .
As its name clearly indicates, the postcentral gyrus is a raised ring or fold of cerebral cortex that runs up and down just behind or “after” (post-) the central sulcus. The postcentral gyrus is alternately called the primary or general sensory cortex. This name reflects the fact that the postcentral gyrus area is the final destination for most of our general or primary body senses, such as touch, pressure, temperature, vibration, and pain.
Remember that we imagined there was a hidden puppeteer within the left precentral gyrus, directing much of the motor pathway descending on the right side of the body. Likewise, we can draw a sensory homunculus (hoh- MUNG -kyuh-lus) – a “little feeling man or dwarf” (homuncul) – just above the left postcentral gyrus. This little feeling dwarf is a rather bizarre visual method by which anatomists have commonly mapped the final destinations of sensory nerve fibers ascending from various parts on the right side of the body.
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