Peripheral Nervous System
From brain and spinal cord, nerves divide and subdivide,
supplying almost every cranny of the body. Anatomists have
devised various methods of grouping these nerves that run outside
the central nervous system. The simplest way of looking at them
is to use the name "peripheral nerves" for all twelve
pairs of cranial nerves projecting from the brain.(though some
are strictly speaking brain tracts) and all 31 pairs of nerves
rooted in the spinal cord. The peripheral nerves rooted in the
spinal cord. The peripheral nerves in turn may be divided into
two main groups making up the so-called
autonomic("self-regulating") nervous systems.
SOMATIC NERVOUS SYSTEM
---The somatic system features
two kinds of nerves. Motor nerves branching from the central
nervous system make muscles act on orders from the brain or
spinal cord. Sensory nerves taking signals in the opposite
direction home in on the central nervous system, bringing
information from sensors in skin, eyes, tongue, nostrils, joints
Subconsciously bombarded by this artillery of information, we use the knowledge it brings about our posture and surroundings to control how we hold ourselves or move around.
AUTONOMIC NERVOUS SYSTEM
---The autonomic system serves the different function of automatically controlling glands and structure like the lungs, heart, blood vessels and pupils of the eyes. Linked with the spinal cord and brain's hypothalamus, this system is influenced by the central nervous system, but most people tend to think of it as beyond the brain's conscious control. In fact the autonomic system harks back hundreds of millions of years to our backboneless ancestors whose nervous system comprised two poorly integrated parts - one sensitive to stimuli outside the body, the other coping with events inside.
---The autonomic nervous system
consists of two sub-systems: the sympathetic and parasympathetic
nervous systems. Each acts in opposition to the other.
SYMPATHETIC NERVOUS SYSTEM
---Springing from all but top and
bottom of the spinal cord, nerve fibres of the sympathetic
nervous system supply eyes, nose, salivary glands, heart, lungs,
stomach, liver, kidneys, genitals and other glands or muscles.
Output from the sympathetic nervous system steps up internal
bodily activity, increasing heart rate, dilating pupils, and
switching blood supply from the intestines to the brain and
PARASYMPATHETIC NERVOUS SYSTEM
---The parasympathetic system serves most of the same organs as the sympathetic system. But nerves supplying organs are far down as the intestines are cranial nerves, sprouting from the brainstem. The rest - supplying bladder, kidneys and reproductive organs - spring from the bottom of the spinal cord. Unlike the sympathetic nervous system, the parasympathetic system puts a brake on heart rate, contracts pupils, and feeds blood away from brain and muscles to intestines.
---Normally the two opposing systems keep some kind of balance in the body. But, under stress, the sympathetic system dominates, while relaxation sees the parasympathetic system in command.