Neurones are excitable cells that make up the nervous system. On average a neurone is 20-30µm in diameter, however this varies depending on the type of neurone. A neurone has four main sections; the dendrites, cell body, axon and axon terminals. The axon is where saltatory conduction causes the propagation of an action potential to move down the axon towards the axon terminal. The axon terminal is where presynaptic modulation occurs, and where neurotransmitters are released causing the action potential to be passed to another neurone across the synapse. The dendrites are where graded potentials occur. These can either be excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP). A graded potential is a subthreshold voltage (below -55mV) however if a number of graded potentials sum together at the axon hillock this can lead to a suprathreshold value and and an action potential is generated.
Neurones have a resting membrane potential (RMP), this is set up by a potassium ion leak channel. This allows potassium to move by facilitated diffusion from inside the cell out, down its concentration gradient, creating a slightly positive charge on the outside of the membrane compared to the inside. RMP is also set up by an ATP-ase pump, sodium and potassium. This uses ATP to pump, against their concentration gradients, three sodium outside the cell and two potassium back in. This is a symport protein channel. Both of these ion movements causes the inside of the cell to be slightly negative when compared to the outside, usually around -70mV. This is the resting membrane potential for a neurone.
References === Mathews G. (2003) Cellular Physiology of Nerve and Muscle, 4th Edition, Oxford: Blackwell Pub.