Biology 1 Nervous System

Hormones are chemicals that are produced in endocrine glands and are released directly into the blood. Even though they can travel around the body and can make contact with any cell, they can only affect particular cells that they target as they are the cells that have the correct receptors to respond to the hormone. The affects of hormones are long lasting, though they are slow acting compared to nerves.

When an impulse is detected by a receptor cell in a sense organ it is transmitted as an electrical impulse to the central nervous system. It is recieved by the dendron of a nerve and travels down the axon of the nerve, insulated by the myelin sheaths that span the length of the axon. The myelin sheath not only stops the impulse from being lost but it also speeds up the impulse. When the impulse reaches the end of the axon, it travels to the synapse, where it is transmitted by neurotransmitters, which are chemicals, that diffuse over the gap to the dendrons of the next nerve cell. Nerves are faster acting than hormones and they act on a precise area.

The central nervous system consists of the brain and the spinal chord. It coordinates the respose to a stimulus that is detected by a sense organ, such as the eyes (light), nose (smell) or tongue (taste). When the stimulus is detected by the sense organ, it is changed into an electrical impulse by the receptor cells which make up the sense organ. The impulse travels down the axon of a sensory neurone until it reaches the synapses, where it is transmitted to a relay neurone which is in the spinal chord. It then travels down the (short) axon of the relay neurone until it reaches the synapses, where it is transmitted to the motor neurone. The impulse travels along the motor neurone until it is detected by an effector muscle which initiates a response.  

The refle arc consists of sensory, relay and motor neurones and goes through the spinal chord. When a stimulus is detected, it travels as an impusle along a sensory neurone to the spinal chord where it is transferred to a relay neurone which relays the impulse to a motor neurone. The motor neurone then transfers the impulse to the effector which responds to the  stimulus. The reflex occurs so fast that it is an unconscious action (you don't have to think about doing it) and happens quicker than a normal response, which you are aware you are doing.