An animal's response to a stimulus is coordinated by its central nervous system (CNS). Neurons carry electrical impulses, and are connected by synapses. Some drugs and toxins affect how impulses pass from one neuron to the next across a synapse.

Neurons.

Neurons carry impulses from one place to another, around the many parts of the nervous system. They connect receptors to the central nervous system and also connect one part of the nervous system to another, for example in the brain and spinal cord. They also carry impulses from the nervous system to effector organs, such as muscles and glands. When neurons are stimulated they transmit an electrical impulse. The diagram below shows a motor neuron. It has a nucleus surrounded by cytoplasm. The cytoplasm forms a long fibre that is surrounded by a cell membrane. This is called an axon. The axon carries the electrical impulse and is protected by a fatty sheath - a bit like the plastic coating around an electrical wire. The fatty sheath increases the speed at which the nerve impulse is transmitted. The nerve ending is branched to make good contact with other neurons or the effector organ. Two neurons do not make direct contact. Where they meet, there is a very small gap called a synapse. The impulse needs to cross this gap to continue on its journey to, or from, the CNS. This is done by means of chemicals which diffuse across the gap between the two neurons.

Central nervous system.

An animal's response to a stimulus is coordinated by its central nervous system (CNS). The CNS consists of the brain and the spinal cord. It gathers information about, and responds to, changes in the environment. Receptors detect a stimulus and send impulses along sensory neurons to the CNS. The CNS coordinates the information and sends impulses along motor neurons to the effectors, which bring about a response. The sequence is as follows:

1. Stimulus
2. Receptor
3. Sensory neuron
4. Central nervous system
5. Motor neuron
6. Effector
7. Response

The peripheral nervous system.

The peripheral nervous system (PNS) consists of motor and sensory neurons that carry impulses from the receptors to the CNS, as well as impulses from the CNS to the effectors.

Reflex arc.

Reflex reactions in humans are controlled by the reflex arc. When the safety of an organism demands a very quick response…