An Introduction to the Nervous System
By the Paul Wittle
The human body is a complex mass of interlinked processes. Each part of the human body relies on all other parts to function correctly. This means that each part of the body must be able to communicate with the other parts. There are two systems which are responsible for this communication, the endocrine and nervous systems.
The Central Nervous System (CNS)
The nervous system is the fastest of these two systems. The brain and spinal cord are described as the central nervous system. These are like the processor in a computer, all signals are sent to the CNS where they are processed and effected. Firstly we will only look at "normal" responses which are handled by the brain. This is located in the back of the human skull/head. As you can see from the picture below, it is made up of three areas, the forebrain, midbrain and hindbrain. Each area of the brain is used to deal with different instructions.The brain
Hindbrain
The hindbrain is at the back of the brain and is made up of two main parts, the Medulla oblongata (Green) and the Cerebellum (Red). The Medulla oblongata is responsible for looking after many of the autonomic (involuntary) responses. It deals with breathing and heart rates. Also it processes information about blood pressure and some of the glandular responses (like saliva in the mouth). The spinal cord is attached to the Medulla oblongata!
Midbrain (Light Blue)
The midbrain is a small area of the brain which primarily links the forebrain to the hindbrain. It is also the processor centre of the audio/visual system. This means that responses to sound (from ears) and light (from eyes) are controlled by the midbrain.
Forebrain (Blue)
The forebrain makes up the largest part of the brain. It contains the thalamus, hypothalamus,cerebrum and the corpus callosum.
The thalamus is another linking area. It is located in the centre of the brain and is in effect the reception of the brain. It is able to interpret the signals and send them to the appropriate region of the brain for processing. It is also thought to link signals so that they can be interpreted, ie the eyes see a fire and the nose smells it, the thalamus can connect the two signals. Experiments have suggested that pleasure and pain produced in this part of the brain.
The hypothalamus is responsible for controlling most of the autonomic responses. It has two routes from it, the sympathetic and the parasympathetic route. Many of the systems controlled by this centre are endocrine systems.
The cerebrum is divided into two cerebral hemispheres are makes up the bulk of the brain size. It carries out numerous tasks, but can store information (memories) and allows learning (recall of memories). !It is a vital part of the brain! Removal of certain areas will result in the loss of certain body functions.
The corpus callosum connects the two halves of the cerebrum!
All through the brain is a system known as the reticular activating system. It is responsible for waking a person up. It can inhibit or encourage signals to the brain. It is a bit like an electronic organiser which regulates what the brain is doing and when.
The Spinal Cord
The spinal cord is a mass of long neurons which runs from the medulla oblongata at the base of the brain to the base of the spine. The spinal cord runs inside the vertebrae of the spine. There are neural junctions at each vertebrae which send or receive signals from the body. It consists of two types: grey matter and white matter.
The grey matter is composed of nuerons which are unmyelinated and hence grey in colour. The myelin sheaths of the myelinated nuerons give them a white colour and they are the main components of the white matter. The term ganglion is used to refer to a clump of nuerons.
The spinal cord is usally a fast route to the brain for signals, like a highway. However some responses must be so quick that it is too slow. These are known as reflex responses! A reflex signal can be processed by the spinal cord. The process for a reflex is shown in red in the flow chart below.
This bypasses the brain and is therefore quicker. It is the reason that you will often carryout a reflex response before you feel it (feeling is processed by the brain).
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Further Reading: Understanding Biology, glenn and Susan Toole