cerebral hemisphere (para 12-6c(3)(c)). The postcentral gyrus is the site of conscious
sensation of a stimulus. Between the point of stimulus reception and the postcentral
gyrus, there is a minimum of three neurons in series.
(1) The first neuron is the afferent (sensory) neuron. It picks up the
information from the sensory receptor organ and carries it to the CNS via the
appropriate peripheral nerves.
(2) The second neuron is the interneuron, located within the spinal cord or
brainstem. It crosses the midline of the CNS to the opposite side. It then ascends the
neuraxis to the forebrainstem, where it reaches a mass of gray matter called the
thalamus. In the thalamus, the interneuron synapses with the cell body of the third
(3) The axon of the third neuron projects up through the cerebral
hemisphere to the appropriate location in the postcentral gyrus.
c. Homunculus of Conscious Sensations. There is a specific location in the
postcentral gyrus which corresponds to each location in the body. For example, a
location in the postcentral gyrus near the midline of the brain (at the top of the cerebral
hemisphere) receives information from the hip region. On the other hand, information
from the tongue and the pharynx projects to the lowest part of the postcentral gyrus, just
above the lateral sulcus.
d. Visceral Sensory Inputs. Visceral sensory inputs follow pathways different
from those of other general sensory pathways. The inputs for visceral reflex actions
usually travel via the parasympathetic nerves. The visceral inputs for pain usually travel
via the sympathetic nerves.
12-24. PAIN--A GENERAL SENSE
Pain is an ancient protective mechanism which generally helps us to avoid
injury. However, tolerance for pain varies from one individual to another.
a. Means of Reducing Pain (Analgesia).
(1) Endorphins ("morphine from within"). Endorphins are chemicals found
naturally within the body which tend to block the sensation of pain.
(2) Drugs. Clinically, a number of drugs are used to block or reduce the
sensation of pain.
(3) Competing inputs. Competing pain stimuli tend to minimize each other.
The body usually recognizes one pain stimulus at a time. Thus, an individual may "bite
his lip" when he anticipates a painful experience.