Neurons are electrically excitable cells in the nervous system that process and transmit information by chemical signals within the neuron. While variable in size and shape, all neurons have three parts. Dendrites receive information from another cell and transmit the message to the cell body. The cell body, or soma, contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body. Sensory neurons respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the spinal cord and brain. Motor neurons receive signals from the brain and spinal cord and cause muscle contractions and affect glands. Interneurons are found only in the central nervous system where they connect neuron to neuron. The junction between two neurons is called a synapse.
There is a very narrow gap between the neurons called the synaptic cleft. This is where the nerve impulse is transmitted from one neuron to the next. Transmission is done by relaying the message across the synaptic cleft using neurotransmitters, which diffuse across the gap. The neurotransmitters then bind to receptor sites on the neighboring neuron, which in turn produces its own electrical/nerve impulse. This impulse is sent to the next synapse, and the cycle repeats itself.
Neurons are also sometimes classified by the direction in which they conduct nerve impulses. Afferent (sensory) neurons carry impulses to the CNS. Efferent (motor) neurons carry impulses from the CNS.