b. Pain Receptor. The pain receptor is not a specific receptor organ, as with
most senses. This receptor is referred to as a free nerve ending.
c. Excessive Stimulation. If any of the other senses receives excessive
stimulus, pain results. Examples are excessive light and excessive noise.
d. Pain Reflex Arc. Generally, a pain sensory input causes a reflex action long
before the information reaches the cerebral cortex and the pain is consciously
perceived. For example, you will remove your hand from a hot object before you realize
you have been burned.
e. Pathway for Conscious Sensation of Pain. As usual, the pathway leading
to conscious sensation of pain consists of three neurons.
(1) The first neuron is the afferent (sensory) neuron from the free nerve
ending. Within the CNS, it synapses with the interneuron.
(2) The axon of the interneuron crosses to the opposite side of the CNS. It
then ascends the neuraxis in a fiber tract known as the lateral spinothalamic tract. This
tract is found in the lateral funiculus (see Figure 12-6). In the thalamus, the interneuron
synapses with the third neuron.
(3) The third neuron projects to the appropriate location of the postcentral
gyrus of the cerebral hemisphere. Here, this information is interpreted or recognized as
a pain sensation from a particular part of the body.
12-25. TEMPERATURE -- GENERAL SENSES
There are two categories of temperature in the body--warmth and cold.
a. 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.
b. In addition, the body has two different mechanisms for sensing temperature.
(1) Specific sensory receptors detect warmth and especially cold in the
periphery of the body.
(2) 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.
c. Neurons for the general sense of temperature use pathways similar to those
discussed for pain (para 12-24e). They include both nerves and fiber tracts.