Unconscious Control

Many functions of the nervous system are under conscious, or voluntary, control. When a cat sees prey, it voluntarily controls its muscles so that it can pounce on its prey.

Sensory nerves carry messages to the brain, and motor nerves carry messages back to the muscles, stimulating them to work in the controlled way necessary to pounce accurately.
 
But other activities are involuntary. These usually involve the internal organs, regulating heartbeat and breathing and the many digestive processes.

Such involuntary activities are controlled by the autonomic nervous system.
The autonomic system consists of two parts: sympathetic and parasympathetic.

The former stimulates activity; the latter damps it down. When a cat is relaxed, the parasympathetic part controls involuntary activity: the pupils are relaxed, and the heart rate and breathing are slow and regular.

When a cat becomes stressed, the sympathetic system takes over, triggering the hypothalamus and the pituitary in the brain to stimulate the adrenal glands into the "fight-or-flight" response.

This chain of response, although complex, is instantaneous. Blood flows from internal organs into muscles, tiny subcutaneous muscles cause body hair to stand on end, the heart speeds up, and pupils dilate for better vision.

Even a perceived threat will trigger this cascade of activities in the autonomic nervous system. If the alarm is false, the sympathetic system is deactivated, and the parasympathetic system takes charge of involuntary activity once more.