Neurotransmitter molecules

Nerve impulses are propagated by membrane depolarization

Life is electric. Neurons, of which we each own approximately 1012, conduct electrical signals to other nerve or muscle cells. To maximize the speed of conduction, certain human neurons are up to a meter long. The signal terminates in a specialized intercellular junction termed a synapse which, when electrically excited, releases neurotransmitter molecules into a cleft and thus chemically activates adjacent neurons. The long (transmitting) part of the nerve is called the axon, whereas the branched (receiving) parts are termed dendrites.

Differences between intracellular and extracellular ionic charge give rise to a transmembrane potential. Neuronal membrane resting potential is usually around –70 mV. Stimuli such as voltage or neurotransmitter release induce the depolarization of neural tissue – that is, conversion of the transmembrane potential to a less negative level such as 0 mV. Membrane depolarization activates voltage-gated sodium channels, leading to an inward flux of sodium ions (Na+) down an electrochemical gradient. Since sodium influx leads to further depolarization, a self-amplifying wave of electrical excitation is created; nerve impulses triggered in this way are termed action potentials.