Download Nervous System (Human): Introduction

Nervous System (Human): Introduction
The nervous system controls and correlates basic bodily functions and behavior. There are two main
parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (cranial
nerves, spinal nerves, and the nerves of the autonomic nervous system).
Neurons These basic units of the nervous system
intercommunicate electrochemically via synapses
(junctions) between their projecting axons and
dendrites – processes whose number and pattern
divide neurons into three types: unipolar, bipolar, and
multipolar. From unipolar or bipolar receptor
neurons, sensory (also called afferent) nerve bundles
(nerve fibers) conduct impulses to the central nervous
system. From there, motor (also known as efferent)
neurons conduct impulses to muscles.
Brain An outgrowth of the spinal cord, the brain is
protected by the cranium. It has three main regions:
the hindbrain, midbrain, and forebrain. The hindbrain
features the medulla oblongata, pons, and
cerebellum, which between them automatically
control respiration, consciousness, and coordination.
The midbrain acts largely as a relay station. The
forebrain, comprising the diencephalon (between
brain) and telencephalon (endbrain), is the part of the
brain that handles higher mental functions.
Spinal cord From the brain the spinal cord extends
down inside the vertebral column (spine), bulging at
intervals where pairs of spinal nerves branch out to
other parts of the body. Its main role is to provide a
highway for nerve impulses passing to and from the
brain, but it can also itself process some sensory
information and initiate appropriate motor responses
without recourse to the brain.
Cranial nerves These 12 pairs of peripheral nerves
emerge directly from the brain. Their names and
numbers are: olfactory nerve (I), optic nerve (II),
oculomotor nerve (III), trochlear nerve (IV), trigeminal
nerve (V), abducens nerve (VI), facial nerve (VII),
vestibulocochlear nerve (VIII), glossopharyngeal
nerve (IX), vagus nerve (X), accessory nerve (XI) and
hypoglossal nerve (XII).
Spinal nerves These 31 nerves sprout from the
spinal cord. In descending order they are grouped as:
eight cervical, 12 thoracic, five lumbar, five sacral, and
one pair of coccygeal spinal nerves.
Autonomic nervous system This system’s neuron
chains, linked to the central nervous system, control
involuntary processes in the body. There are two
subsystems: the sympathetic and parasympathetic,
which adjust bodily activity respectively during stress
and relaxation. Circumstances determine which of
these systems prevails at a particular time.
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