Nov 13, 2009

A vertebrate neurone

Arising from the cell body are two types of extensions: numerous dendrites and a single axon. Dendrites (from the Greek dendron, tree) are highly branched extensions that receive signals from other neurones. The axon is typically a much longer extension that transmits signals to other cells, which may be neurones or effector cells. Some axons, such as the ones that reach from your spinal cord to muscle cells in your feet, may be over a meter long. The conical region of an axon where it joins the cell body is called the axon hillock; as we will see, this is typically the region where the signals that travel down the axon are generated. Many axons are enclosed by a layer called the myelin sheath. Near its end, an axon usually divides into several branches, each of which ends in a synaptic terminal. The site of communication between a synaptic terminal and another cell is called a synapse. At most synapses, information is passed from the transmitting neurone (the presynaptic cell) to the receiving cell (the postsynaptic cell) by means of chemical messengers called neurotransmitters.