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Module 5: Pathways of the Human Nervous System

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Temperature and Touch

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Temperature
There are two categories of temperature in the body - warmth and cold.

However, these are relative entities. For example, a given temperature seems cool when compared to a much higher temperature and seems hot when compared to a much lower temperature.

Sensing Temperature
In addition, the body has two different mechanisms for sensing temperature:

• Specific sensory receptors detect warmth and especially cold in the periphery of the body.

• Special heat-sensitive neurons in the hypothalamus detect increases in the temperature of the blood that flows through the hypothalamus (portion of the forebrainstem). By this means, the body monitors the core temperature, the temperature in the central part of the body.

Neurons for the general sense of temperature use pathways similar to those discussed for pain.

They include both nerves and fiber tracts.

Touch

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.

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, light touch.

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.