Introduction
The nervous system is our system of receiving, processing and transmitting nerve impulses within our body. In this way it may be seen as a communication system. The nervous system is made up from billions of nerve cells called neurons.
The human brain is made up of around 10,000,000,000 neurons.
The total collection of nerve cells in the body is called the nervous system and the major part of the nervous system, consisting of the brain and the spinal cord, is called the central nervous system (CNS).
Neurons outside the CNS, which take information to and fro between the CNS and various regions of the body, are called peripheral neurons.
Neurons are the basic building blocks of the nervous system and come in a variety of shapes and sizes, however they share a similar overall structure (see fig 1).
Figure 1 - The motor neuron
Neurons have two major functions:
(1) to transmit sensory information, via sensory neurons, from the sensory receptors in our body to the central nervous system. There are sensory neurons in the skin, muscles, joints, and organs that indicate pressure, temperature, and pain. There are more specialised neurons in the nose and tongue that are sensitive to the molecular shapes we perceive as tastes and smells. Neurons in the inner ear provide us with information about sound. And the rods and cones of the retina allow us to see.
(2) to transmit motor information, via motor neurons (fig 1), from the central nervous to effector organs (e.g. muscles) to initiate action.
How do neurons function?
The cell body is in effect the controlling centre of the neuron. Dendrites branch out from the cell body, receiving information from other neurons and carrying information towards the cell body. The axon transmits messages away from the cell body towards other neurons, muscles or glands. Some neurons are coated with a myelin sheath, the function of which is to insulate the axon from interference from other neurons. The myelin sheath also speeds up the transmission of information from one cell to another.
Neurons do not actually touch each other, the small gap between one neuron and the next is known as the synapse. A chemical messenger (or neurotransmitter) transfers the electrical impulse at this point from one cell to the next, an example of which is serotonin.
Information is transmitted along the length of the neuron by means of an electrical charge.When this electrical voltage doesn’t change, when it remains constant, the cell is said to be resting. However, when the voltage changes up and down, the cell is said to be active. Many neurons can rapidly change their electrical state, where the cell suddenly changes its voltage and then just as suddenly returns to its original condition. Changes of this kind are called action potentials. The more frequently the cell changes its voltage in this way (also termed firing), the more active it is said to be. Changes in electrical voltage constitute the signalling system, or language, of the nervous system.
A pathway consisting of neurons, over which information travels, is termed a neural pathway. Neurons that carry information to the CNS are called afferent neurons and neurons that carry information from the CNS to other parts of the body are called efferent neurons.
Neurons handle information in the body, taking messages from one part of the body to another. Another way of transmitting information in the body is via the endocrine system. The vehicle used to transmit information through the endocrine system is provided by hormones. Hormones are chemicals that are carried by the blood from one location to another, more distant location. These chemical messengers take longer to transmit their information than the nervous system, but their effects are usually longer lasting.
It is obviously important to Psychology that the nervous system is understood: the information that reaches our CNS concerning events both in the outside world and in the internal environment of the body does so in the language of electrical changes in the neurons. Everything we feel, everything we do, everything we know has, as its physical basis, the neural structures of the central nervous system. BUT how do we come to possess our nervous systems? What factors determine the form of the nervous system? To answer these questions we need to go further into the structure and properties of cells to help us to understand the most important unit of information in the human body, the gene.