Neurons transmit signals to neurons and to other cells (muscles or glands) at the synapse (CNS Clinic, 2007). The outer membranes of the transmitting neuron and the receiving cell are separated by a fluid-filled synaptic gap, typically 20 nm wide. So in order for the transmitting neuron to affect the receiving cell, chemical events bridge that gap and then create an electrical event in the receiving cell.
A variety of neurotransmitter chemicals can be found at the terminal end of the transmitting neuron. These neurotransmitters can be released to cross the gap and attach to specific receptor sites on the receiving cell. When the neurotransmitter binds to the receptor site, specific ion channels open in the membrane of the receiving cell, ions flow in and out creating a flow of electrical charge, and the membrane either depolarizes (excitatory synapses) or hyperpolarizes (inhibitory synapses).
For neurons in some invertebrates, the synaptic gap is smaller, and electrical impulses from the transmitting neuron can affect the receiving cell directly, with the same results (activation or deactivation). However, a 20 nm gap is too wide for direct electrical transmission, so most vertebrate cells require chemical transmission of the signal.
The effects will depend on the type of receiving cell. Depolarization/excitation causes the conduction of an electrical impulse to the next neuron, or contraction of a muscle, or excretion from a gland. Hyperpolarization/inhibition prevents impulse conduction, prevents muscle contraction, or prevents gland excretion. Consider, for example, taking a step forward. You not only have to excite forward-moving muscles, but inhibit backward-moving and side-moving muscles, as well as use a combination of excitation and inhibition to stabilize the other leg for balance.
Neurotransmitters need to be inactivated after release, so that the signal stops propagating across to the receiving cell. In some cases, neurotransmitters simply diffuse away from the synaptic gap. In others, they are taken back up by the transmitting neuron (reuptake). In still others, enzymes in the gap or on the membrane quickly degrade the neurotransmitters. Neurons may send dozens of signals per second and reuptake the chemicals in about five seconds (Welsh, 2011).
- CNS Clinic-Jordan. (2007). The synapse. CNS Clinic. Retrieved from http://www.humanneurophysiology.com/synapse.htm
- Welsh, J. (2011). Speed of brain-cell chatter clocked for first time. Live Science. Retrieved from http://www.livescience.com/14735-neuron-synapse-speed-clocked.html