Neuroanatomy I: The Nervous System

The nervous system is a structure possessed by all living animals. It is a complex structure that when put together is responsible for all we do, feel and experience. The human nervous system is a particularly complex system. Of course many people spend their lives with the understanding that they have nerves and that nerve endings are located all over the body, they are sensitive areas which are particularly susceptible for both pain and pleasure, they then think nothing more of it. Yet the nervous system is a structure so complex that it involves every organ in the body to function, in return it helps every organ in the body to work as well as it can. But what is involved in the nervous system or is it just the sensor points on our skin?

The nervous system can be split primarily into two sections, the central nervous system (CNS) – built up of the brain and the spinal cord, and the peripheral nervous system (PNS) made up of everything else. This seems nice and simple, so the CNS is the governing body of the nervous system and has all the power and control, if the CNS dies then the person is essentially dead, so it's the important part. The PNS is it's minion. That's great, so why am I going through this? It is for the PNS that I bother to explain this, for the minion to the CNS is a surprisingly complex little structure of ins and outs and subsections dividing into subsections. So awfully confusing.

To the left, one can see what the PNS and CNS are built up of, though the diagram is surprisingly complex and covers all areas of the brain, it is only for those with a particular interest in neuroscience to pay attention to such points at the moment. More important is to know that the central nervous system consists of both the brain and spinal cord and the peripheral nervous system splits multiple times.

When the PNS splits, it starts by splitting into the Somatic Nervous System (SNS) which interacts with the external world and the Autonomic Nervous System (ANS) which interacts with the internal bodily affairs. Both the SNS and ANS then split into Afferent and Efferent Nerves. It is the role of Afferent nerves (remember its role by attraction) that work with the organs and skin (etc) to take information from the PNS over to the CNS for confirmation and action, the CNS then uses the efferent (read escape) nerves to feedback information. One will notice from the diagram it is then only in the ANS and efferent nerves which it splits again into Sympathetic and Parasympathetic nerves. These nerves have many interesting properties, firstly they both use secondary neurons (which will be explained in further articles) to send information. In basic terms this means that they only travel part way to their target of communication before they synapse from another neuron.

Sympathetic and parasympathetic nerves have some interesting roles, namely they hold the following roles:

Parasympathetic Nerves

  • Conserve energy
  • Suggest that the person is psychologically relaxed

Sympathetic Nerves

  • Will use energy to organise reaction in dangerous situations
  • Suggest person is psychologically aroused.

Both nerve types then interact with the target location sending opposing messages so that reaction happens built up of all nervous input.

To the left is a simpler diagram of the nervous system showing only the components of the CNS and the splits of the PNS explaining all briefly.

As it has been suggested that all nerves in the PNS grow from the spinal cord, it is more or less accepted, however there are known to be 12 exemptions from this. These being the cranial nerves (12 pairs of nerves) which directly stem from the brain and so, are essentially by nature parasympathetic nerves.

The Cranial Nerves are as follows:

  • I.Olfactory Nerve (responsible for smell)
  • II. Optic Nerve (Quite obviously responsible for vision)
  • III. Oculomotor Nerve (responsible for eye movement, including the constricting of pupils)
  • IV. Trochlear Nerve (only responsible for eye movement)
  • V. Trigeminal Nerve ('Somatosensory information' which includes pain and touch but only in the face and head.)
  • VI. Abducens nerve (Eye movement)
  • VII. Facial Nerve (taste, somatosensory info from the ears)
  • VIII. Vestibulocochlear nerve (hearing and balance.)
  • IX. Glossopharyngeal nerve (taste, somatosensory info from tongue, tonsil and pharynx and so controls muscle movement when one swallows)
  • X. Vagus nerve (sensory, motor and autonomic functions of heart, glands and digestion)
  • XI. Spinal accessory nerve (head movement)
  • XII. Hypoglossal nerve (controls muscles of the tongue)

As you will notice, the cranial nerves are responsible only for movement above the neck and spinal cord and responsible for little else which occurs within the rest of the body, and falls under the autonomic nervous system because of its internal and not external association. Many defects are due to breakdowns (or impairments) of the cranial nerves and as stated by Pinel are a key point in diagnosis and pathology.

As one would expect, the CNS is the protected area of the nervous system and is protected by the various methods shown in the more simple one shown to the right. Essentially, it is protected by bone - obviously in the form of the skull, which itself is protected. Following this there are layers of meninges and fluids all designed to protect the brain from damage and then the vertebrates and cerebrospinal fluid (the liquid in the spine) which protect the spinal cord. Remember, the CNS is essential in communication around the body and the processing of all brain and nervous system processes, we could not survive without with structure.

References

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