(1) The pathway for the conscious sensation of light touch begins with the usual afferent (sensory) neuron as the first neuron. The afferent neuron carries the information to the CNS by way of the appropriate nerve.
(2) In the CNS, the afferent neuron synapses with the interneuron, the second neuron of the pathway. After crossing to the opposite side of the CNS, the interneuron ascends the neuraxis in the fiber tract known as the anterior spinothalamic tract. This is in the anterior funiculus of the spinal cord (Figure 12-6).
(3) In the thalamus, the second neuron synapses with the third neuron. The axon of the third neuron then projects to the appropriate location in the postcentral gyrus of the cerebral hemisphere. There, it is interpreted as the conscious sensation, light touch.
b. Pathway for Conscious Sensation of Deep Touch. The pathway for deep touch is quite different from that for light touch.
(1) Still, the first neuron is the afferent neuron from the deep touch receptor to the CNS via the appropriate nerve. When the axon of the afferent neuron enters the CNS, it turns upward and ascends the neuraxis in the posterior funiculus (Figure 12-6) of the same side that it entered. In other words, it does not yet cross the midline of the CNS.
(2) In the lower hindbrainstem, the axon of the first neuron synapses with the cell body of the second neuron. The axon of the second neuron then crosses to the opposite side of the brainstem. This axon then continues the ascent through the neuraxis to the thalamus, where it synapses with the third neuron.
(3) Again, the axon of the third neuron projects to the appropriate location in the postcentral gyrus of the cerebral hemisphere. There, impulses are interpreted as conscious sensations of deep touch.
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