Organization of the Nervous System

The nervous system is divided into the The PNS consists of The CNS consists of the The peripheral nervous system is subdivided into the

The Sensory-Somatic Nervous System

The sensory-somatic system consists of All of the spinal nerves and most of the cranial nerves are "mixed"; that is, they contain both sensory and motor neurons. All our conscious awareness of the external environment and all our motor activity to cope with it operate through the sensory-somatic division of the PNS.
Link to a discussion of the mechanism by which the commands of the motor neurons of the sensory-somatic system are executed by skeletal muscles.

The Autonomic Nervous System

The autonomic nervous system consists of sensory neurons and motor neurons that run between the central nervous system (especially the hypothalamus and medulla oblongata) and various internal organs such as the:

It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. The contraction of both smooth muscle and cardiac muscle is controlled by motor neurons of the autonomic system.

The actions of the autonomic nervous system are largely involuntary (in contrast to those of the sensory-somatic system). It also differs from the sensory-somatic system is using two groups of motor neurons to stimulate the effectors instead of one.
The autonomic nervous system has two subdivisions, the

The Sympathetic Nervous System

The preganglionic motor neurons of the sympathetic system arise in the spinal cord. They pass into sympathetic ganglia which are organized into two chains that run parallel to and on either side of the spinal cord.

The preganglionic neuron may do one of three things in the sympathetic ganglion:

The neurotransmitter of the preganglionic sympathetic neurons is acetylcholine (ACh). It stimulates action potentials in the postganglionic neurons.

The neurotransmitter released by the postganglionic neurons is noradrenaline (also called norepinephrine).

The action of noradrenaline on a particular gland or muscle is excitatory is some cases, inhibitory in others.

The release of noradrenaline

In short, stimulation of the sympathetic branch of the autonomic nervous system prepares the body for emergencies: for "fight or flight".

Activation of the sympathetic system is quite general because

The Parasympathetic Nervous System

The main nerves of the parasympathetic system are the tenth cranial nerves, the vagus nerves. They originate in the medulla oblongata. Other preganglionic parasympathetic neurons also extend from the brain as well as from the lower tip of the spinal cord.

Each preganglionic parasympathetic neuron synapses with just a few postganglionic neurons, which are located near - or in - the effector organ, a muscle or gland. Acetylcholine (ACh) is the neurotransmitter of both the pre- and the postganglionic neurons of the parasympathetic system.

Parasympathetic stimulation causes In short, the parasympathetic system returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the danger is over.

Although the autonomic nervous system is considered to be involuntary, this is not entirely true. A certain amount of conscious control can be exerted over it as has long been demonstrated by practitioners of Yoga and Zen Buddhism. during their periods of meditation, these people are clearly able to alter a number of autonomic functions including heart rate and the rate of oxygen consumption. These changes are not simply a reflection of decreased physical activity because they exceed the amount of change occurring during sleep or hypnosis.

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18 April 1999