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The Nervous System


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    The nervous system is probably the most complex of all systems. It is the control center for most of the functions accomplished by our body. It is also the centre for our sensations, our emotions and our thoughts.

Brain Cerebellum Pons Medulla oblongata Sagital view of the brain.

Sagital view of the brain.

    The figure on the left shows you a lateral view of the brain and its twelve pairs of cranial nerves (just click on figure for more details). The brain is the part of the nervous system that is located in our skull; it is the major portion of the central nervous system. On the outer surface, you can see the cortex which is folded to increase the surface of organized elements (like a cloth folded in our hand). The cortex is the palce of conciousness. It integrates sensory information and triggers appropriate responses. It is also the place dealing with language, mood and spacial representation.

    Our five senses: sight, hearing, smell, taste and touch, send information to the brain which built a mental representation of the environment. The brain must incorporate and correlate this information so that the environment is interpreted as a whole. There are three parameters of the sensory information, that participate to the sensory perception: these are the time (the duration of the sensory stimulus), the space (the physical extent of the signal: the tip of a needle or the surface of a tennins ball, the location a person's voice in a noisy crowd), and the intensity (the glow of a candle or the brightness of the Sun. In addition, these parameters influence each other. For example, two objects of identical weights but having different shape may seem of different weight depending on the duration of the stimulus or the surface onto which their weights are distributed. I hope, I have given you an idea about the integration of the sense's parameters, and that I was not too confused.

    Under the cortex, you find the thalamus which is mainly a relay centre for the sensory-motor information and pain. Beneath the thalamus, the hypothalamus controls several unconscious functions such as thirst, hunger, sexual/martelnal functions. At the base of the brain, in the back, the cerebellum, even more folded than the brain, is involved in the coordination and automation of movements. The last portion of the brain, before the spinal cord, is the brain stem that consists of the Pons and the medulla oblongata. In these regions you find the control cernters for our vegetative functions (primitive and unconscious). Examples of these vegetative functions are the control of respiration, heart rate, blood pressure, and body temperature.

Two segments of the spinal cord (thoracic level).
Two segments of the spinal cord (thoracic level).

    Following the medulla begins the spinal cord, housed and protected at the centre of the vertebral column. The figure on the left shows the network of spinal nerves for two thoracic vertebrae. As a generality, the sensory information enters the spinal cord via the dorsal roots, and the motor commands exit through the ventral roots. This figure does not show details about the specific nerves innervating the different organs.

    The figures below show specific branches that originate from different levels of the spinal cord. These include the cervical and brachial plexus on the left, and the lumbar and sacred plexux on the right.

Cervical and brachial plexus
Cervical and brachial plexus.

Lumbosacral and coccygeal plexus
Lumbosacral and coccygeal plexus.

    These figures do not show the autonomic nervous system, the nerves that control the vegetative functions such as the blood pressure and the digestion. The autonomous system is divided into two part: the sympathetic system which is primarily activated under stress conditions and, upon excitation, increases blood pressure and heart rate, and the parasympathetic system which is mostly involved for the digestive functions.



Website about the nervous system:

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Top of the page.      TEXT© 2000-2015 René St-Jacques