In the peripheral nervous system, each myelin sheath is formed by an individual Schwann cell which wraps itself around the axon length. In the central nervous system, oligodendrocytes send out numerous processes (arms!) to wrap multiple axons. So myelin is actually just layers and layers of phospholipid membrane!

The insulating properties of myelin allow the action potential to be conducted along the axon at high velocity. This is because action potentials are formed only at the points along the axon which are not myelinated (called the nodes of Ranvier). Voltage-gated Na+ channels are located in these gaps. An action potential at one node causes an action potential in the neighboring node, thus “skipping” the myelinated section. This propagation of action potential is called saltatory conduction.

The loss of myelin can cause severe physical problems such as blindness, motor problems, imbalance and speech impairments. Multiple sclerosis is one such disease.

Related posts:

1. Neurotransmitters
2. Amino acids

These chemicals are released in response to an electrical stimulation (the traveling action potential). Another neuron then detects the neurotransmitters by specific receptors at its postsynaptic membrane.

Activation of these receptors by an excitatory neurotransmitter causes Na+ channels to open, which starts of another action potential in the second neuron.
Activation by an inhibitory neurotransmitter stops the flow of action potential.

• Examples of excitatory neurotransmitters are such as acetylcholine, glutamate, dopamine, epinephrine and norepinephrine.
• An example of an inhibitory neurotransmitter is GABA.

To recap, an electrical flow that reaches the end of a neuron causes the release of neurotransmitters. A second neuron detects the neurotransmitter, which could cause an action potential to start and flow in the second neuron.

* Neurotransmitters are NOT taken up by the second neuron. They are either destroyed or recycled back to the presynaptic terminal of the first neuron, after release from the receptor on the second neuron.

* At the presynaptic terminal: When an action potential reaches the terminal, it opens up voltage-gated Ca2+ channels and causes Ca2+ ions to enter into the terminal of the neuron. Ca2+ then causes the synaptic vesicles to fuse with the presynaptic membrane, thus releasing the neurotransmitter into the synapse.

Related posts:

1. Myelin
2. Electron Transport Chain