Autonomic functions do not require the involvement of consciousness, and involve things like circulation and digestion. Autonomic functions usually involve autonomic neurons, which some people call general visceral efferent or visceromotor, which send efferent axons through the peripheral nervous system to contact and control smooth muscle cells, cardiac muscle cells, and some gland cells. Smooth muscles cells are widespread in the tissues of the body, such as the airways of the lungs and the walls of blood vessels, and they often play a major role in how most of the organs function. Cardiac muscle cells make us most of the heart tissue, and they contract to push blood through the circulatory system.
The autonomic nervous system is divided into two parts called the sympathetic and parasympathetic nervous systems; these usually cause opposite effects on the tissues they influence. These two parts are usually both active to some degree, with the balance of activity shifting one way or the other in response to stimuli. Certain areas of the cerebrum and brainstem connect to and influence the activity of autonomic neurons. Both parts of the autonomic nervous system consist mostly of chains of two neurons connecting the central nervous system with target cells in the periphery. The first neuron is in the central nervous system and its axon travels in nerves to synapse in an autonomic ganglion, so these are called preganglionic autonomic neurons. The second neuron is in an autonomic ganglion and its axon travels in nerves to synapse with the target cell, so these are called postganglionic autonomic neurons.
Next:
Introduction to Neurology 