12-26. TOUCH -- GENERAL SENSES
Throughout the body are a variety of sensory receptors which detect varying
degrees of pressure. For example, the pacinian corpuscles are typical of the receptors
which detect deep pressure. In addition, an individual can usually identify the location
of a touch on his body; in fact, he can usually distinguish two simultaneous touches to
adjacent areas (the "two-touch test"). As usual with the general senses, sensory inputs
for touch can also result in immediate reflex actions.
a. Pathway for Conscious Sensation of Light Touch.
(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,
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
(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.