In this lecture, you are going to learn about
the organisation of the nervous system. The
structure of the nerve cell or neuron and
also the structure of the spinal cord.
At the end of this lecture, you should have a
very good understanding of the structure of
the nerve cell or nerve fibre. You should
know the anatomical and functional divisions
of the nervous system. And finally you should
be able to describe the histological structure
of the spinal cord and also be able to identify
its functional components.
Well, the nervous system is really important and
its essential function as you would all be
aware of is one of communication. And through
the next two lectures, I will be explaining
to you structural and functional components
that allow us to perceive various information,
both consciously and subconsciously. And how
the brain then receives that information
and then evokes responses, either motor responses
bringing about movement or responses that we
are unaware of that control many of our body
processes, secretion of glands, movement of
food through the gut, blood through the vascular
system etc. Well, it is very important that we first
of all start with the description of the divisions
of the nervous system. And although this is
very complex I am going to try and take it
slowly and trying to explain it in a number
of different sections.
First of all, we can divide the nervous system
anatomically. It consists of two parts.
One is the central nervous system. We abbreviate
CNS, and that consist of the brain and the spinal
cord. The brain is protected inside the cranium,
the skull and the spinal cord is protected
by the vertebral column. The peripheral nervous
system, abbreviate PNS, consists of cranial
nerves, spinal nerves and peripheral nerves.
There are twelve cranial nerves and they leave
the brain or brain stem to supply structures
mainly in the head. Now although I said there
are 12 cranial nerves, the first two are really
nerve tracts rather than nerves and we would
describe those as we look at the structure
of the nasal cavity and the sense of smell
and also vision in a later lecture. Spinal
nerves are named according to the segments
in which they exit the spinal column. For
instance C1, C2, C3 or T1, T2 from the thoracic
region and those spinal nerves can then combine
together and form a number of plexuses. For
instance the brachial plexus in the neck region
that provide nerves supplying the upper limb,
or the lumbosacral plexus in the lumbar region
providing nerves that supply muscles in our
lower limb. And then some of those peripheral
nerves can be named. The sciatic nerve, for
instance, in our lower limb or the median ulna
and radial nerve in our upper limb.
Besides nerves, the peripheral nervous system
consists of structures we call ganglia.
They are structures that helds the nerve cell bodies of
sensory neurons. Those neurons that pick up sensations
in the periphery and those ganglia also helds
nerve cell bodies to do with the autonomic
nervous system. And I will describe the autonomic
nervous system later on in this lecture.
As well as having nerves and having locations
for cell bodies in the ganglia in the peripheral
nervous system, of course we also have the
nerve endings. The motor endplates, they transmit
the nerve impulse onto skeletal muscles or
smooth muscles through the motor endplates
of skeletal muscles or the myoneural junctions
they are often called. And then there are
all the sensory nerve endings in all the
sensory receptors we have perhaps in the skin.
They carry all the information about touch,
pain, pressure etc back into the central nervous
system. So we are aware of those sensations
and we can deal with them by various means.
Besides the anatomical divisions of the nervous
system, there are also functional divisions.
The somatic nervous system and the autonomic
nervous system. The somatic nervous system
consists of that part of the nervous system
that we are consciously aware of, such as the
movement of our body, the movement and contraction
of skeletal muscle in our limbs and sensory
information such as temperature, touch and
pain. All that sort of information that we
are aware of, muscle movement and sensory perception.
The autonomic nervous system is that part
of the nervous system that controls parts
of our body that we are unaware of. And it
includes nerves that provide stimulus to smooth
muscle cells and also cardiac muscle fibres
and also the secretion of glands and those
subconscious nerve fibres are of two sorts.
There are the parasympathetic fibres or the
sympathetic fibres. So they form part
of the sympathetic division and the parasympathetic
division of the autonomic nervous system and
usually those two divisions are antagonists.
They do the opposite effects on their target
tissues. They might increase the rate of the
heart beat or they might decrease the rate
of the heartbeat. They might increase contraction
of smooth muscle moving food along the gut
or decrease it. There are also sensory neurons
that travel from viscera. By viscera, I referred
to the internal organs of the body. We are
unaware of that sort of sensory information
going to our central nervous system and being
interpreted, sort of movement of food through
the gut for instance. The contraction of smooth
muscle changing the dimensions of all the
blood vessels to affect the flow of blood
to different parts of our organs when they
are required. We are unaware of those sorts
of controls. That is all part of the autonomic
nervous system. Then finally the enteric division
is a totally different system. It is separated
from both these somatic nervous system and
also the autonomic nervous system.
It refers to a group of neurons or a network of
neurons or nerve cells that we see along the
gut tube. And those neurons control the activity
of the muscle layers of the gut and they work
independently of the central nervous system.
I am not going to refer to those neurons or
that division in this lecture, but I will
refer to these neurons and this network of
nerve fibres when I talk about the gut.