The Brain

The cerebral hemispheres are the two main portions of the brain, and make up its majority. An important thing to note is that the left side of the body is controlled by the right cerebral hemisphere, and the right side of the body is controlled by the left cerebral hemisphere.

The different parts of the cerebral hemisphere are sized according to the complexity of their innervation. This means that a more complicated function requires a larger portion of the cerebral hemisphere to perform it. For example, the visual areas take up a large chunk of the back of the brain.

The brain is made up of sensory, association and motor areas.

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   Sensory areas receive and process information from sensory organs

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   There are different sensory areas for the different sensory organs.

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2. 2
   Association areas interpret the sensory information by relating it to memory

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   For example, the visual association area interprets what you see, by comparing it to things you've seen before.

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3. 3
   Motor areas are also localized with certain areas for different movements.

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As way of example, the diagram below outlines the different areas that relate to speech.

The autonomic nervous system is a branch of the nervous system that deals with things under subconscious control. It has two types of antagonistic (opposite and opposing) nerves:

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   Sympathetic - which prepares the body for action, such as increasing heartbeat.

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2. 2
   Parasympathetic - which does the reverse and returns the body to normal.

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In the below table, we look at some reflexes and how they are controlled by the autonomic nervous system.

Reflex	Action	Explanation
Pupil diameter	Pupil dilates to allow more light into the eye and constricts to reduce it.	See the eye
Tear production		The tear duct sits below the eyebrow. When the sympathetic system is stimulated, tears are produced and secreted by the eye to lubricate and protect it, the film of tears is spread across it by blinking. Extra tears are drained through the nasolacrimal duct to the digestive system. When the parasympathetic system is stimulated, tear secretion is increased.
Bladder emptying	The internal and external sphincter muscles open and the detrusor muscle contracts to allow urine out via the urethra.	When the bladder is stretched, impulses are sent by the parasympathetic system to contract the detrusor muscle which surrounds the bladder, and tightens it. In order for the urine to exit; the internal (autonomic control) and external (voluntary control) sphincter muscles must open. Sphincter muscles form a ring, and when they relax, this ring opens. The sympathetic system relaxes the bladder.

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