The peripheral nervous system (PNS) is made up of three types of neurons: sensory, relay, and motor. Sensory neurons carry action potentials from the receptor (e.g. pressure on the skin) to the brain; they relay from the sensory neuron to the motor neuron, and motor neurons from the brain to the effector (e.g. a muscle).
These three neurons differ in structure to best aid their function. For example, a sensory neuron has a smaller cell body compare to a relay and motor neuron.
The junction at which these neurons connect is called a synapse. They are sites at which information is transmitted from one neuron to another. The preceding neuron is called the presynaptic neuron and the proceeding neuron is called the postsynaptic neuron.
The information is transmitted in chemical messengers called neurotransmitters (NT). The incoming action potential in a presynaptic neuron triggers neurotransmitter release. The NT’s cross the synapse and bind to the postsynaptic neuron. This will trigger the action potential to continue along the next neuron.
Figure: Image of the synaptic cleft. Wadsworth Inc, “Synapse”, 1992. Source: https://www.printablediagram.com/synapse-diagrams-to-print/ (Accessed 8/11/2018)
Action Potential – the rise and fall of membrane potentials resulting from changes in electrical potential across muscle or nerve cells.
Synapse – the junction between two nerve cells which action potentials are transmitted across.