Organization of the Nervous System

00:02 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.

00:15 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.

00:42 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.

01:50 First of all, we can divide the nervous system anatomically. It consists of two parts.

01:58 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.

03:50 Besides nerves, the peripheral nervous system consists of structures we call ganglia.

03:56 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.

04:20 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.

04:55 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.

05:11 Besides the anatomical divisions of the nervous system, there are also functional divisions.

05:18 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.

06:20 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.

07:48 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.