why do nerve cells have projections????
One is the synaptic nob on the presynaptic membrane. The arrival of an action potential opens calcium channels in the membrane, allowing calcium ions to diffuse into the nob. Afterwards these are pumped back out using the energy provided by ATP.
The increased concentration of calcium ions in the nob causes the vesicles containing the neurotransmitter (acetylcholine) to move up to and to fuse with the presynaptic membrane. The vesicles release their neurotransmitter into the synaptic cleft by exocytosis.
The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane of the neurone. This stimulates an action potential in the post synaptic membrane by opening sodium channels.
Because the receptors are only on the post synaptic membrane the signal is unidirectional - the impulse is only sent in one direction, e.g. from the receptor in the eye to the sensory neurone in the central nervous system.
Some nerve cells have fibers that grow out of the cell, which are called axons. Axons allow a nerve cell to connect to distant parts of the body, so that cells in the brain can send messages to, and receive messages from a toe, for example, which might be six feet away from the brain. Other nerve cells just connect to their immediate neighboring nerve cells, and therefore do not require axons; they instead have smaller extensions called dendrites
They have fibres to send messages from the brain throughout the whole body. Don't get confused with fibers and fibres. They are extremely different. Anyways, the reason why nerve cells have long fibers are because they have to send messages from the brain to other parts of the human body......