Impulse Transmission

In this lesson we’re going to talk about nerve impulse transmission.

The functional unit of the nervous system is the neuron. All impulses, also called action potentials, will travel down the neuron. It initiates in the neuron body, which is where the nucleus is, and gets sent down the axon, the long part you see here. Around the axon, we have the myelin sheath which helps insulate the axon from damage and between each of these is the node of ranvier. What happens is the impulse is actually able to skip across from node to node because of this myelin sheath, so it helps to speed up the transmission. Then the impulse gets to the end of the neuron into these finger-like projections called dendrites. That dendrite will be in contact with another neuron – that’s called a synapse. The impulse will cross the synapse and into the next neuron and then it will continue from neuron to neuron until it reaches its destination. It’s like doing the wave at a baseball game – it moves from one section to another and makes its way all the way around. If any section doesn’t do its part, the whole wave stops.

So how does it get across that synapse from one neuron to another? Neurotransmitters. These are chemicals that help to facilitate transmission across the synapse. They are housed in these little vesicles at the end of the dendrites and when the action potential comes, they are released into the synaptic cleft – that’s this space here. They travel across the synaptic cleft and bind with receptors on the next nerve. When that receptor is activated, it initiates the next action potential. In order for that receptor to be inactivated, an enzyme will come and inactivate the neurotransmitter. Each neurotransmitter has its own specialized receptors. Noradrenaline has noradrenergic receptors. Dopamine has dopaminergic receptors. Histamine and serotonin have their own receptors as well. And Acetylcholine has cholinergic receptors. Let’s look at an example impulse transmission with acetylcholine.

So here’s our synaptic cleft. An action potential shows up and releases the acetylcholine out of the vesicles to cross the synaptic cleft. Once one of them binds with the cholinergic receptor site, it stimulates the receptor and initiates a new action potential. To avoid overstimulation, the enzyme called cholinesterase (the -ase tells you it’s an enzyme) will then come in and remove acetylcholine from this receptor – therefore inactivating it. This happens over and over as long as the action potentials keep arriving. As long as we have a good balance of acetylcholine and cholinesterase, the system remains beautifully balanced. If we give a drug that affects any of these steps, we change the result. It could increase the amount of neurotransmitters, it could block or activate the receptors, or it could affect the enzyme activity. It just depends on what our desired outcome is.

Key points to remember. Neurons function to transmit impulses throughout the nervous system. The myelin sheath and nodes of ranvier help to insulate and speed up the transmission. Neurotransmitters are chemicals that facilitate the movement of impulses across synapses. The synaptic cleft is where all transmission must cross – this is where you’ll find many receptors that drugs will actually block or activate to create their effects.

So those are the basics of nerve impulse transmission. Let us know if you have any questions. Happy nursing!