There are 100 different explanations of this on the www and none of them make sense. There even are several cute animations but they leave one even more confused than one was before one started. So this is my attempt to explain it as clearly as possible.
Neurons consist of: soma; dendrites; axons.
Soma = cell body.
Dendrites = conduct impulses towards neuron. There are many per neuron. The more dendites, the greater the ability to process information. Studies show that a rich learning environment actually increases the number of dendrites.
Axon = conducts impulses away from neuron. One per neuron.
Axon terminals = at the end of the axons.
Axon hillock = where the soma is connected to axon. Propogates action potential (AP) – where the summation of excitatory post-synaptic potential (EPSP) and inhibitory post-synaptic potential (IPSP) from the dendrites occurs.
Nodes of Ranvier = space between myelin sheaths were AP is retransmitted; where ion flow across the membrane occurs. The AP jumps along the axon from node to node.
Synapses = junctions between neurons.
Synaptic vesicles = sacs filled with neurotransmitters.
Synaptic cleft = physical space between axon and dendrites.
Schwann cells – surround the axon – insulate the axon to help AP transmit more quickly.
Electrical transmission of action potential
Resting membrane potential (RMP)
The neuron is covered with a membrane.
There is intracellular fluid inside of it – made up of both Na+ and K+ together with negatively-charged proteins.
There is extracellular fluid outside of it – also made up of both Na+ and K+.
The fluids are filled with ions = atoms bearing a charge.
The inside is negatively charged at -72mV.
How? The membrane has many channels or gates, which only allow Na+ or K+ to move through them. The gates are the way the Na+ and the K+ move back and forth between the inside and the outside. When the neuron is resting, Na+ and K+ ions move down their concentration gradients through their membrane channels to the opposite sides of the membrane (they drift to the other side and gradually become less concentrated). The membrane is “hemipermeable.”
In the meanwhile there ALSO is a specialized pump (a protein channel) that maintains the concentration gradient by using energy to force Na+ and K+ back to the sides they came from. For every three Na+ pumped outside only two K+ are pumped inside. At rest the outside concentration of Na+ is higher than the inside, while inside the K+ is higher than the outside. This difference results in the inside of the axon being negatively charged relative to the outside; the number of positive charges on the outside is greater. The pump works constantly to ensure that more + ions remain outside the cell than inside. This is called the resting potential (RP).
Action Potential (AP)
Electricity is created by a sudden (milliseconds) reversal in charge that travels (propagates) down the length of axon. Each node regenerates a new AP at full strength, “bucket-brigade” style. This is called the action potential (AP). Usually the outside is + (because there is more Na+) and the inside is – . The AP travels down the axon and reverses the charge so that the outside is – and the inside is +. They flip polarity.
1. A stimuli alters the RP. It comes from upstream neurotransmitters.
2. Depolarization = the inside becomes +. The Na+ channels open; the K+ channels close. Na+ flows in and makes the inside +. So the mV graph ascends (upstroke). NOTE – if the upstream neurotransmitter is net minus (–) then the cell will become hyperpolarized, i.e. more negative (–) and there will NOT be an AP.
3. Resting potential of -72mV depolarizes to an activation threshold of -55mV. At threshold all the Na+ gates open and Na+ floods the cell. The “all or none” principle – the neuron will fully depolarize once threshold is crossed.
4. Goes up to +30mV at the top of the cycle. The membrane is completely depolarized and this is transmitted along the axon.
5. When the AP reaches its peak the Na+ gates close and the K+ gates open. So K+ flows outside of the cell and it repolarizes, i.e. the + charge goes back to being on the outside and the – charge is on the inside. The mV graph descends back to the RP of -72mV (downstroke)
6. Refractory period = brief period of hyperpolarization after repolarization; the AP overshoots -72mV; more K+ move out; as a result the membrane cannot be stimulated; which prevents reflux (the message from being transmitted backwards along the axon). The firing of APs briefly raises the threshold for generating additional APs.
7. But then the sodium-potassium pump moves the Na+ out and the K+ in to repolarize at -72mV.
8. The AP is constant along the entire axon. The difference in the information transmitted depends on neural pathways and destinations, not any difference in the AP.
Chemical transmission between cells
AP moves along the axon. AP is regenerated at each node along the axon.
AP arrives at vesicles to spill neurotransmitter into synaptic cleft.
Some of the neurotransmitters bind to receptors on the post-synaptic dendrite.
This excites or inhibits it.
This is a passive process of diffusion.
The properties of the neurotransmitter determine if it is excitatory post-synaptic potential (EPSP) or inhibitory (IPSP). EPSP is + (+ ions flow into the post-synaptic cell and it depolarizes) and IPSP is – (- ions flow into it and it hyperpolarizes).
The cell neuron sums the EPSP and the IPSP. NOTE – AP results only if EPSP is +. E.g. dopamine is excitatory, GABA is inhibitory.
Each neuron can be activated either + or – depending on the properties of the neurotransmitter. NOTE – classifying neurotransmitters as + or – is technically incorrect, as there are several other synaptic factors that help determine a neurotransmitter’s excitatory or inhibitory effects. Generally: if + it results in EPSP; if – results in IPSP.
Each neuron is both presynaptic and postsynaptic depending on relation in chain.