The function of neurons is to process and transmit information. Without input, most neurons have a stable electrical charge difference across the cell membrane called the resting potential.
Neurons receive excitatory or inhibitory input information from other cells, or from physical stimuli such as odorant molecules. Inputs usually enter the dendrites, or to a lesser extent the soma or the axon.
The information from inputs is transmitted to the axon with membrane potential changes called graded potentials, which are small in size and brief in duration, and which travel short distances. The size and duration of a graded potential is proportional to the size and duration of the input. Summation (combining) of all the excitatory and inhibitory graded potentials at any moment in time occurs at a functional area called the trigger zone, which is at the axon initial segment.
This is the way neurons process information from their inputs.
If the membrane potential at the trigger zone crosses a value called the threshold potential, a membrane potential change called an action potential usually occurs, which is then conducted the entire length of the axon.
An action potential is large in size and brief in duration, and it may travel a long distance. Action potentials are usually similar in size and duration for any given neuron. Action potentials are conducted faster along thicker axons, and those sheathed in myelin.
When an action potential reaches an axon terminal, molecules called neurotransmitters are usually released at the synapse to bind to receptors on the target cell, which may change its behavior.
Neurotransmitter is then removed from the synapse. The input information contained in the size and duration of graded potentials is converted into the temporal pattern (timing) of action potentials conducted along the axon. This information is then converted into the amount and temporal pattern of neurotransmitter release at the synapse. This is the way neurons transmit information to their target cells.
There are multiple functional types of neurons. Afferent neurons transmit information from the periphery into the central nervous system, and efferent neurons transmit information away from the central nervous system toward the periphery.
Afferent neurons are also called sensory neurons because they bring information about stimuli in the internal or external environment into the central nervous system. Motor neurons, also called somatomotor or neurons of the somatic nervous system, are efferent neurons that control skeletal muscle. Autonomic neurons, also called visceromotor or neurons of the autonomic nervous system, are efferent neurons that control smooth muscle, cardiac muscle, or gland cells.
Most neurons of the central nervous system are interneurons, which are neurons that connect other neurons together.
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Introduction to Neurology 