Cells making up the central nervous system and peripheral nervous system are classified as nervous tissue. In the central nervous system, nervous tissue forms the brain and spinal cord. In the peripheral nervous system the nervous tissue forms the cranial nerves and spinal nerves, which include the sensory and motor neurons.
The function of nerve tissue is to transmit nerve impulses around the body. Nerves consist of a cell body (soma), dendrites, which receive impulses, and axons which send impulses. The axons of neurons are surrounded by a myelin sheath. The myelin sheath consists of layers of myelin, a white fatty substance. The myelin sheath’s main function is to insulate nerve fibres and it also increases the speed of the impulses transmitted by the nerve cell. There are three types of nerve cells: sensory neurons, interneurons and motor neurons.
|Sensory neuron||Motor neuron||Interneuron|
|Sensory neurons are responsible for transmitting various external stimuli from the environment into internal chemical messages (called stimuli). They are activated by touch, hearing etc. Sensory nerve cells (or sensory neurons) carry impulses (electrical signals) from a receptor to the central nervous system (CNS).||Motor neurons carry impulses from the CNS to the effectors (muscle or glands). In most cases the effector is a muscle whose contraction is directed by a particular impulse. Other effectors could include glands. The motor neuron facilitates muscle contraction in response to specific impulses and chemicals known as neurotransmitters.||These neurons are very short compared to the sensory and motor neurons. The connectors or inter neurons connect a sensory neuron with a motor neuron. The impulse travels from the cell body at the head end along the short axon to the dendrites.|