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Organization of the Nervous System

The nervous system is divided into the
Link to discussion of the central nervous system.

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

The Cranial Nerves

Nerves Type Function
sensory olfaction (smell)
sensory vision
(Contain 38% of all the axons connecting to the brain.)
motor eyelid and eyeball muscles
motor* eyeball muscles
mixed Sensory: facial and mouth sensation
Motor: chewing
motor* eyeball movement
mixed Sensory: taste
Motor: facial muscles and
salivary glands
sensory hearing and balance
mixed Sensory: taste
Motor: swallowing
mixed main nerve of the
parasympathetic nervous system (PNS)
motor swallowing; moving head and shoulder
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

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 (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:

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

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

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

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.

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.

Link to discussion of the central nervous system.

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18 November 2022