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Anatomy and function of the peripheral nervous system.
The nervous system is a complex system of nerve tissue that transmits nerve impulses around the body. Vital to the normal functioning of the human body, this complex system coordinates both voluntary and involuntary actions and is responsible for the transmission of signals to and from different parts of the body. The nervous system can be broken down into the central nervous system which is made up of the brain, the brainstem, the cerebellum and the spinal cord; and the peripheral nervous system which is made up of all the nerve fibers in the remainder of the body. The peripheral nervous system facilitates communication between the central nervous system and different parts of the body and is of course the main topic of our tutorial today.
The peripheral nervous system is comprised of two different types of nerves – the thirty-one pairs of spinal nerves and their branches and on the right you can see an image of the C3 to C4 spinal nerves which are two of these thirty-one pairs; and the twelve cranial nerves, and on the right here you can see an image of the two paired vagus nerves which is the tenth cranial nerve.
The peripheral nervous system can also be divided functionally into the somatic nervous system which is primarily involved in the voluntary movement of the skeletal system like the deltoid, the autonomic nervous system which is primarily involved in the involuntary movement of the smooth muscle of the viscera like the cardiac apex on the right, and the enteric nervous system which is primarily involved with the gastrointestinal system and as you can see here we have the ascending part of the duodenum as an example of this system. And in the following slides, we'll look at a few key points and features of each of these three subdivisions of the peripheral nervous system.
Let's start with the somatic nervous system. The word somatic means relating to the body. As we mentioned, the somatic nervous system transmits signals from the central nervous system to skeletal muscle and is associated with the control of voluntary body movement such as the lateral cutaneous branches of the intercostal nerves which we can see here on the right. It is also associated with involuntary movements known as reflex arcs, but we'll not discuss that topic in this tutorial. In addition, the somatic nervous system transmits external stimuli such as sight, touch and sounds to the CNS such as this image of the olfactory nerve on the right.
The second part of the peripheral nervous system known as the autonomic nervous system or the ANS is associated with the control of involuntary movements such as those carried out by our internal organs such as this ciliary ganglion on the right here which controls the innervation of the ciliary muscle and the iris sphincter. In other words, the autonomic nervous system controls the smooth muscle of internal organs thereby controlling unconscious bodily functions such as digestion, breathing or regulation of the heart rate and so on. The autonomic nervous system can be further divided into the sympathetic nervous system and the parasympathetic nervous system. So let's take a quick look at these two subdivisions of the autonomic nervous system before moving on to the third part of the peripheral nervous system.
Very simply put, we can say that the sympathetic nervous system acts in sympathy with our motions. It is this part of the autonomic nervous system and indeed the peripheral nervous system that is in charge of the fight, flight and fright response or the three F's. It basically prepares the body for physical and/or mental activity, and on the right here is an image of the superior mesenteric ganglion which sends postsynaptic autonomic fibers to the small and large intestines.
While the sympathetic nervous system is responsible for the three F's, the parasympathetic nervous system fulfills our so-called rest and digest functions. In other words, it slows down the heart rate and increases bowel contraction. Within the ANS, parasympathetic innervation is mainly restricted to four cranial nerves namely the oculomotor nerve or CN III whose image we can already see on the right here, the facial nerve or CN VII, the glossopharyngeal nerve CN IX and the vagus nerve CN X. in addition, the pelvis splanchnic nerves which arise from the sacral spinal nerves S2, S3 and S4 also provide parasympathetic innervation to the large intestine after the vagus nerve has completed its innervation.
Finally, the third part of the peripheral nervous system is the enteric nervous system or the ENS. This part of the parasympathetic nervous system is also known as "the brain in the bowel" and its components are found in the walls of the gastrointestinal tract. In particular, there are two groups of plexuses – Meissner's plexuses found in the submucosal tissue and Auerbach's plexuses which are also known as the myenteric plexuses and are located between the circular and muscular layers of the gastrointestinal tract and these plexuses cause the contraction of the bowel wall. Note that the enteric nervous system works independently, however, there is some complex interaction that exists between it and the autonomic nervous system which is why you may come across it being referred to as quasi-autonomic in some literature.
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