Examples:
- During intense exercise the body’s temperature is increased, and this initiates several
automatic responses. Heat or thermal receptors convey stimuli to the sympathetic control
centers of the brain. An inhibitory message is then sent along the sympathetic chain of nerves
to the blood vessels of the skin, causing a dilatation of the cutaneous blood vessels. This now
greatly increases the flow of blood to the surface of the body, where heat is lost by radiation and
evaporation from sweating. This is the body’s cooling mechanism. - The SNS responds to environmental heat in another important way. The rise in body
temperature is sensed by the hypothalamic center. Stimuli are then sent, via sympathetic nerves
to the sweat glands, resulting in appropriate sweating. This serves to cool the body by the loss
of heat resulting from evaporation of the sweat aided by a cool breeze. We cannot voluntarily
influence the dilatation of our blood vessels or the adequacy of our sweating in response to heat
in other ways. - Control of the rate and strength of cardiac contractions is also under the predominant control
of the SNS. During intense exercise, there is a greater cardiac demand for blood and oxygen, so
as the intensity of the exercise increases, the SNS sends transmissions to the heart to increase
cardiac output by increasing the intensity and frequency of the heart rate.
The Parasympathetic Nervous System is sometimes referred to as the “Wine or Dine”
pathway, meaning that it is a very slow, relaxing and moderated process when stimulated. It
calms the body back down and returns it to a normal state of operation.
Examples:
- When a stimulus is applied to an organ, such as a bright light flashed into the eyes, a message
is conducted through sensory fibers to the midbrain. This creates a stimulus that travels through
the parasympathetic fibers of the oculomotor (third cranial) nerves to the pupils, causing
automatic contraction of the pupillary muscles to constrict the pupil. This reduces the amount of
light reaching the sensory cells in the retina on the back of the eyeball. - Food enters into the stomach, creating a stimulus that is conveyed by afferent fibers of the
vagus nerve to a specific nucleus of the vagus nerve in the brain. Here, messages are interpreted
and then transported through efferent fibers of the vagus, back to the stomach. These stimulate
the secretion of gastric juices and contraction of the stomach muscles to mix the food with the
secreted digestive juices, and slowly move the digested food contents into the intestines.
The peripheral nerves of the afferent division contain sensory receptors. The sensory
receptors are specialized structures located through the body that are designed to transform
environmental stimuli (heat, light, sound, taste, motion, etc.) into sensory information that the
brain and/or spinal cord can then interpret to produce a response.