Sensory impulses not only participate in reflex activity, as previously described, but also give rise to conscious sensation, calibrate body position in space, and help regulate internal autonomic functions like blood pressure, heart rate, and respiration.
A complex system of sensory receptors relays impulses from skin, mucous membranes, muscles, tendons, and viscera. Sensory fibers registering sensations such as pain, temperature, position, and touch, pass through the peripheral nerves and posterior roots and enter the spinal cord. Once inside the cord, sensory impulses reach the sensory cortex of the brain via one of the two pathways: the spinothalamic tracts or the posterior columns.
Within one or two spinal segments from their entry into the cord, fibers conducting the sensations of pain and temperature pass into the posterior horn of the spinal cord and synapse with secondary sensory neurons. Fibers conducting crude touch—a sensation perceived as light touch but without accurate localiza-tion—also pass into the posterior horn and synapse with secondary neurons. The secondary neurons then cross to the opposite side and pass upward in the spinothalamic tract into the thalamus.
Fibers conducting the sensations of position and vibration pass directly into the posterior columns of the cord and travel upward to the medulla, together with fibers transmitting fine touch—touch that is accurately localized and finely discriminating. These fibers synapse in the medulla with secondary sensory neurons. Fibers projecting from secondary neurons cross to the opposite side at the medullary level and continue on to the thalamus.
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