INTRODUCTION
Throughout history Man has been fascinated by
what actually makes us what we are. In the biological sciences discoveries by
scientists merely seem to reveal more of the complexities of the organisational
patterns intimately involved with the function of being alive.
Perhaps one of the areas which has fascinated
us most is the study of Homo sapiens ability to comprehend, learn and act
independently. Man has the ability not only to perceive the wonders of the world
but also to question, study and manipulate both the internal and external
environment.
The biological system, above all, which allows
this is the Nervous System. This allows the human body to process, interpret and
act on information received both from within and from the outside world.
What sort of chaos would we be in if we did not
possess a Nervous System? We would not receive any messages from the outside
world, unable to perceive light and dark, colours, sounds, smells, touch, heat,
cold etc. We would have no concept of the danger we were in if we fell off a
cliff and would feel no pain on hitting the rocks below.
Internally we would be a complete mess,
different organs acting entirely independently and at random. Totally without
harmony with other systems and without regard for the needs of the body. Blood
couldn't be distributed around the body according to changing demands. Body
temperature would fluctuate wildly according to the outside environment. Groups
of muscles could not work together in a smooth and co-ordinated manner making
any regular motion impossible. We would be about as able to express feelings,
emotions or thoughts as a lump of concrete.
Fortunately we do possess a nervous system
which takes care of this problem.
These pages aim to provide nursing students
with a basic knowledge of how the nervous system works. Also you will find out
what can go
wrong, and what forms of treatments are available.
OVERVIEW
The nervous system therefore has a tremendous
part to play in enabling us to survive in the world.
It can be thought of as the telecommunications
system of the body. Sending information from various sources to a vast computer
network ( the brain ) which analyses and solves the problems presented to it and
then passes the appropriate information out to the field workers ( muscles,
glands etc.) enabling the appropriate actions to take place in a co-ordinated
and logical fashion,
The nervous system works in close conjunction
with another body system known as the endocrine system. This has effects on the
body's function by producing organic chemical substances known as hormones,
Taking the analogy above and relating this to
the endocrine system, it can be likened to a road & rail network
communication system transporting bulk items (hormones) around the body to
enable essential works to be carried out.
Simple Systems
Although all many-celled animals have some kind
of nervous system, the complexity of its organization varies considerably among
different animal types. In simple animals such as jellyfish, the nerve cells
form a network capable of mediating only a relatively stereotyped response. In
more complex animals, such as shellfish, insects, and spiders, the nervous
system is more complicated. The cell bodies of neurons are organized in clusters
called ganglia. These clusters are interconnected by the neuronal processes to
form a ganglionated chain. Such chains are found in all vertebrates, in which
they represent a special part of the nervous system, related especially to the
regulation of the activities of the heart, the
glands, and the involuntary muscles.
Vertebrate Systems
Vertebrate animals have a bony spine and skull
in which the central part of the nervous system is housed; the peripheral part
extends throughout the remainder of the body. The brain
is the part of the nervous system located in the skull; the spinal
cord is that found in the spine. The brain and spinal cord are continuous
through an opening in the base of the skull; both are also in contact with other
parts of the body through the nerves. The distinction made between the central
nervous system and the peripheral nervous system is based on the different
locations of the two intimately related parts of a single system. Some of the
processes of the cell bodies conduct sense impressions and others conduct muscle
responses, called reflexes, such as those caused by pain.
In the skin
are cells of several types called receptors; each is especially sensitive to
particular stimuli. Free nerve endings are sensitive to pain and are directly
activated. The neurons so activated send impulses into the central nervous
system and have junctions with other cells that have axons extending back into
the periphery. Impulses are carried from processes of these cells to motor
endings within the muscles. These neuromuscular endings excite the muscles,
resulting in muscular contraction and appropriate movement. The pathway taken by
the nerve impulse in mediating this simple response is in the form of a
two-neuron arc that begins and ends in the periphery. Many of the actions of the
nervous system can be explained on the basis of such reflex arcs, which are
chains of interconnected nerve cells, stimulated at one end and capable of
bringing about movement or glandular secretion at the other.
The Nerve Network
The nervous system is divided principally into
2 major divisions,
The Central Nervous System ( CNS ) which
comprises the brain and the spinal cord.
The Peripheral Nervous System ( PNS )
which comprises sensory receptors and the nerves which provide communication
lines to and from the CNS.
Figure
1. Major divisions of the nervous system
The cranial nerves run from the head and neck
to the brain by passing through openings in the skull, or cranium. Spinal nerves
are the nerves associated with the spinal cord and pass through openings in the
vertebral column. Both cranial and spinal nerves consist of large numbers of
processes that convey impulses to the central nervous system and also carry
messages outward; the former processes are called afferent, the latter are
called efferent. Afferent impulses are referred to as sensory; efferent impulses
are referred to as either somatic or visceral motor, according to what part of
the body they reach. Most nerves are mixed nerves made up of both sensory and
motor elements.
Within the PNS there are 12 pairs of cranial
nerves (which link directly to the brain) and 31 pairs of spinal nerves ( which
link, to the spinal cord and then to the brain).
Cranial nerves are distributed to the head and
neck regions of the body, with one conspicuous exception: the tenth cranial
nerve, called the Vagus. In addition to supplying structures in the neck, the
Vagus is distributed to structures located in the chest and abdomen. Vision,
auditory and vestibular sensation, and taste are mediated by the second, eighth,
and seventh cranial nerves, respectively. Cranial nerves also mediate motor
functions of the head, the eyes, the face, the tongue, and the larynx, as well
as the muscles that function in chewing and swallowing.
Figure 2. Underside view of brain showing
origin of cranial nerves
In contrast the spinal nerves mainly connect to
the limbs, torso and internal organs.
After they exit from the vertebrae, spinal
nerves are distributed in a bandlike fashion to regions of the trunk and to the
limbs. Interconnecting extensively, they form the brachial plexus, which runs to
the upper extremities, and the lumbar plexus, which passes to the lower limbs.
Peripheral nerves are in the business of
continually transmitting information to and from the specific parts of the body
they serve.
For convenience, the PNS can be further
sub-divided into three systems:
The Sensory Nervous System ( SeNS) made
up of nerves known as afferent nerves or sensory nerves. These transport
information from sensory receptors in the body to the CNS.
The Somatic Nervous System ( SoNS) this
is made up of efferent nerves or motor nerves transporting information
from the CNS to parts of the body concerned with voluntary functions such as
walking, talking, manipulating with the fingers etc.
The Autonomic Nervous System (ANS) again
made up of efferent nerves but this time concerned with the regulation of
those movements which do not normally require conscious thought such as heart
beat, breathing, peristalsis, release of digestive juices etc.
The ANS itself is further subdivided into the Sympathetic
Nervous System (SyNS ) and the Parasympathetic Nervous System ( PaNS ).
Figure 3 (above) and 3a (below).
Relationship of nervous system divisions.
Note that the sympathetic and parasympathetic divisions are only shown on the left side of the body and the peripheral division is only shown on the right side of the body in the interests of clarity. In reality they are all present on both sides.