The human eye

A nervous system is made up of interconnected neurones specialised for the rapid transmission of impulses throughout the organism. They carry impulses from special receptor cells to special effector cells which bring about the appropriate response. The simplest nervous systems are made up of recptor cells, neurones and nerve endings, however many are more complex. Groups of receptors have evolved to work together in sensory organs such as the eye or ear. Some neurones only carry information from the internal or external environment into the central processing areas of the nervous system. These are sensory neurones. Animals of a large size have more specialised concentrations of nerve cells which form the central nervous system (CNS). This is an area where incoming information is processed and from where impulses are sent out via a motor neurone which carries an impule to the effector organ. 

Neurones are individual cells each containing a nerve fibre which carries the nerve impulse. Nerve are bundles of nerve fibres. Some only carry motor fibres and so are motor nerves whilst others only carry sensory nerve fibres so are sensory nerves. Neurones have a cell body which contains the nucleus, mitrochondria and other organelles. The cell body has slender finger like processes called dendrites which connect to neighbouring nerve cells. Nerve fibres which carry impulses away from the cell body are called axons, ones carrying it towards the cell body are dendrons.

Most neurones are associated with another kind of cell, the Schwann cell whose membrane wraps itself repeatedly around the fibre forming a layer known as the myelin sheath. The gaps between the Schwann cells are known as nodes of Ranvier. The myelin sheath protects the nerves from damage and speeds up the transmission of the impulse. The speed is also affected by the diameter of the nerve fibre, the thicker the fibre the quicker the impulses travel along it. Invertebrates don't have myelin sheats and so have devloped giant axons which allow quick reactions. Vertebrates have both myelinates and unmyelinates nerves.

The concentration of sodium ions, potassium ions and other charged particles is different outside the axon from that inside. The membrane of the axon is partially permeable. At rest it is relatively impermeable to sodium ions but permeable to potassium ions. It also contains sodium/potassium pumps which use ATP to move sodium ions out of the axon and potassium ions in. This lowers the concentration of sodium ions inside the axon. Eventually the movement of positively charges potassium ions out of the cell along the concentration gradient is opposed by the electrochemical gradient and the cell is left slightly negative relative to the outside, it is polarised. There is a potential difference across the membrane of around -70mV which is known as the resting potential.

When an impulse travels along an axon it changes the permeability of the cell membrane to sodium ions. This change occurs in response to a stimulus in a sensory neurone or the arrival of a neurotransmitter chemical in a…