Neurons

Motor neurons transmit action potentials (impulses) from the central nervous system (CNS) to an effector such as a muscle or gland (cause the muscle to contract or the gland to secrete a particular hormone). The cell body of a motor neurone lies within the spinal cord or the brain. Many thin cytoplasmic processes extend from the cell body and all but one of these are quite short (looks a bit like a spider and its web). The short cytoplasmic processes (called dendrites) conduct impulses towards the cell body. The longer cytoplasmic process (called the axon) conducts impulses away from the cell body (e.g in motor neurones it carries the impulse to an effector. Large numbers of mitochondria are present at the tips of the terminal branches of the axon, together with many vesicles containing transmitter substances. These are involved in passing nerve impulses from the neuron to the effector (see Synapses to learn all about this).

Sensory neurons carry impulses via a dendron from sense organs (such as the eyes and ears) to the brain or spinal cord. Intermediate neurons (sometimes called relay neurons) have their cell bodies and dendrites inside the brain or spinal cord. They are adapted to carry impulses to and from various other neurons.

Test yourself:
1. Describe two differences between the structures of a motor neuron and a sensory neuron.