Dendrites convey signals towards the cell body.  Axons, conversely, conduct signals away from the cell body.  Dendrites, in general, are short and numerous and branch extensively.  Most neurons have only a single axon which is usually  very long.  Axons stem from the axon hillock which is a cone-shaped region of the cell body.  Schwann cells are arranged along the length of the axon.  All together these Schwann cells form an insulating layer which is called the myelin sheath.

     Axons can be branched and each branch can terminate in hundreds or thousands of small branchlets called telodendria which end in a small bulbous end called the synaptic knob.  Synaptic knobs relay messages to other cells via neurotransmitters.  Between the synaptic knobs are gaps called synapses.

     In the central nervous system supporting cells are called glial cells, glial roughly means glue.  Astrocytes are glial cells that line capillaries in the brain and contribute too the blood-brain barrier.  The blood-brain barrier restricts the passage of most substances into the brain.  It helps prevent dangerous fluctuations in the chemical environment of the central nervous system.

     In the body, cells usually carry a negative charge.  Nervous cells are able to alter their charge in order to conduct signals.  Cells that are able to do this are called excitable cells.  Neurons are able to alter their charge largely due to ions.  Neuron plasma membranes have an abundance of sodium potassium pumps.  These pumps actively transport Na+ out of the cell and K+ into the cell.  This generate steep gradients of these ions.  Na+ is then more concentrated outside the cell and K+ is more concentrated on the inside of the cell.  This in turn polarizes the cell, meaning that there is a difference in the electrical charge between the outside of the cell and the inside of the cell. 

     The unstimulated, polarized state of a neuron is called the resting potential, which is usually about -70 millivolts.  When the cell is stimulated gated ion channels in the membrane open and allow Na+ to enter the cell.  This depolarizes the cell meaning that it is now more positive on the inside.  If the stimulus is powerful enough, more Na+ gates open which causes complete depolarization or an action potential.  This in turn opens K+ gates and the K+ on the inside of the cell moves out. 

     This causes repolarization of the cell which is accomplished by restoring the original membrane polarization.  Soon after these gates open the Na+ gates close.  When the K+ gates finally close, more K+ has exited the cell than necessary to reestablish the original polarization.  This makes the membrane become hyperpolarized.  During the refractory period, K+ and Na+ return to there original sides of the membrane.  During this period the neuron is not able to respond to a new stimulus.

     Cells in which a signal begins are called pre-synaptic cells.  Cells which receive the signal are called post-synaptic cells.  There are two types of synapses: electrical and chemical.  An electrical synapse allows the action potential to spread directly from the pre-synaptic to the post-synaptic cells.  Chemical synapses allows for cells that so not have an electrical connection to spread their action potential.  This happens by converting the electrical signal to a chemical signal that then travels across the synapse and is then converted back to an electrical signal on the other side of the synapse.  These chemicals are called Neurotransmitters.