Index to this page

The Sensory-Somatic Nervous System
- The Cranial Nerves
The Autonomic Nervous System
- The Sympathetic Branch
- The Parasympathetic Branch

Organization of the Nervous System

The nervous system is divided into the

peripheral nervous system (PNS) and the
central nervous system (CNS)

Link to discussion of the central nervous system.

The PNS consists of

sensory neurons running from stimulus receptors that inform the CNS of the stimuli
motor neurons running from the CNS to the muscles and glands - called effectors - that take action.

The CNS consists of the

spinal cord and the
brain

The peripheral nervous system is subdivided into the

sensory-somatic nervous system and the
autonomic nervous system

The Sensory-Somatic Nervous System

The sensory-somatic system consists of

12 pairs of cranial nerves and
31 pairs of spinal nerves.

The Cranial Nerves

Nerves	Type	Function
I Olfactory	sensory	olfaction (smell)
II Optic	sensory	vision (Contain 38% of all the axons connecting to the brain.)
III Oculomotor	motor	eyelid and eyeball muscles
IV Trochlear	motor*	eyeball muscles
V Trigeminal	mixed	Sensory: facial and mouth sensation Motor: chewing
VI Abducens	motor*	eyeball movement
VII Facial	mixed	Sensory: taste Motor: facial muscles and salivary glands
VIII Auditory	sensory	hearing and balance
IX Glossopharyngeal	mixed	Sensory: taste Motor: swallowing
X Vagus	mixed	main nerve of the parasympathetic nervous system (PNS)
XI Accessory	motor	swallowing; moving head and shoulder
XII Hypoglossal	motor*	tongue muscles

*Note: These do contain a few sensory neurons that bring back signals from the muscle spindles in the muscles they control.

The Spinal Nerves

All of the spinal nerves are "mixed"; that is, they contain both sensory and motor neurons [View].

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 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:

heart
lungs
viscera
glands (both exocrine and endocrine)

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 first, the preganglionic neurons, arise in the CNS and run to a ganglion in the body. Here they synapse with
postganglionic neurons, which run to the effector organ (cardiac muscle, smooth muscle, or a gland).

The autonomic nervous system has two subdivisions, the

sympathetic nervous system and the
parasympathetic nervous system.

The Sympathetic Nervous System

The preganglionic motor neurons of the sympathetic system (shown in black) 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:

synapse with postganglionic neurons (shown in white) which then reenter the spinal nerve and ultimately pass out to the sweat glands and the walls of blood vessels near the surface of the body.
pass up or down the sympathetic chain and finally synapse with postganglionic neurons in a higher or lower ganglion
leave the ganglion by way of a cord leading to special ganglia (e.g. the solar plexus) in the viscera. Here it may synapse with postganglionic sympathetic neurons running to the smooth muscular walls of the viscera. However, some of these preganglionic neurons pass right on through this second ganglion and into the adrenal medulla. Here they synapse with the highly-modified postganglionic cells that make up the secretory portion of the adrenal medulla.

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. (At excitatory terminals, ATP may be released along with noradrenaline.)

The release of noradrenaline

stimulates heartbeat
raises blood pressure
dilates the pupils
dilates the trachea and bronchi
stimulates glycogenolysis — the conversion of liver glycogen into glucose
shunts blood away from the skin and viscera to the skeletal muscles, brain, and heart
inhibits peristalsis in the gastrointestinal (GI) tract
inhibits contraction of the bladder and rectum
in mice exposed to stress, it stimulates the interaction between T-helper cells and B cells so the B cells can develop into antibody-secreting plasma cells [Link]
and, at least in rats and mice, increases the number of AMPA receptors in the hippocampus and thus increases long-term potentiation (LTP).

In short, stimulation of the sympathetic branch of the autonomic nervous system prepares the body for emergencies: for "fight or flight" (and, perhaps, enhances the memory of the event that triggered the response).

Activation of the sympathetic system is quite general because

a single preganglionic neuron usually synapses with many postganglionic neurons;
the release of adrenaline from the adrenal medulla into the blood ensures that all the cells of the body will be exposed to sympathetic stimulation even if no postganglionic neurons reach them directly.

The Parasympathetic Nervous System

The main nerves of the parasympathetic system are the tenth (X) cranial nerves, the vagus nerves. They originate in the medulla oblongata. Parasympathetic neurons also extend from three other cranial nerves (III, VII, and IX). Recent work (in mice) indicates that there are no pre-ganglionic parasympathetic nerves extending from the tip of the spinal cord. If this is true for humans, then the bottom-left part of the above figure is incorrect.

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 at all the pre- and many of the postganglionic neurons of the parasympathetic system. However, some postganglionic neurons release nitric oxide (NO) as their neurotransmitter, and some release noradrenaline.

The Nobel Prize winning physiologist Otto Loewi discovered (in 1920) that the effect of both sympathetic and parasympathetic stimulation is mediated by released chemicals. He removed the living heart from a frog with its sympathetic and parasympathetic nerve supply intact. As expected, stimulation of the first speeded up the heart while stimulation of the second slowed it down.

Loewi found that these two responses would occur in a second frog heart supplied with a salt solution taken from the stimulated heart. Electrical stimulation of the vagus nerve leading to the first heart not only slowed its beat but, a short time later, slowed that of the second heart also. The substance responsible was later shown to be acetylcholine. During sympathetic stimulation, adrenaline (in the frog) is released.

Parasympathetic stimulation causes

slowing down of the heartbeat (as Loewi demonstrated)
lowering of blood pressure
constriction of the pupils
increased blood flow to the skin and viscera
peristalsis of the GI tract

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.

The vagus nerves carry signals involving many other autonomic actions. Two examples.

In mice, three types of receptors that detect sugar, fat, and amino acids respectively signal the brain (by cholecystokinin, CCK) when food enters the small intestine.
The vagus nerves also help keep inflammation under control. Inflammation stimulates nearby sensory neurons of the vagus. When these nerve impulses reach the medulla oblongata, they are relayed back along motor fibers to the inflamed area. Release of acetylcholine suppresses the release of inflammatory cytokines, e.g., tumor necrosis factor (TNF), from macrophages in the inflamed tissue.

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.