I wrote this note while reading the book Behave by Robert Sapolsky where I had to have some sense of the terminology within the field of neuroscience. It is a ridiculously complicated topic which is why this note may come across as lacking a sense of depth as I wrote it just to have a quick overview of all of the terms.
Neuron: The so-called “brain cell” that communicate with one another. There are about a hundred billion of them in our brains which communicate with each other, forming complex circuits.
The function of a neuron is to talk to other neurons and cause one another to get excited to transmit information.
To receive information (Metaphorical ears dendrites): At the end of each neuron are ears that receive information from other neurons called metaphorical ears. These ears are called dendrites.
To send information (Axon): Transmits information to different neurons and the axon connects the dendrites of the next neuron in line.
axon terminal-> dendrites -> cell -> axon -> axon terminals -> dendrite
Flow of information – what is sent?
Ions: Negatively and positively. Function as the information flow.
Inside the neuron are various positively and negatively charged ions. When a neuron has gotten an exciting signal from the previous neuron at the end of one single dendritic fiber, channels in the membrane in that dendrite open, allowing various ions to flow in and others to flow out, and the net result is that the inside of the end of that dendrite becomes more positively charged.
Neurons thus have two states. It is all about contrast.
1. Action potential: Neuron is positively charged and has something to say.
2. Resting potential: Neuron is negatively charged and has nothing to say.
When a neuron in resting potential and receives excitatory signal then this does not mean that the entire neuron becomes action potential instantly.
You’ve taken a nice smooth calm lake, in its resting state and tossed a little pebble in. It causes a bit of a ripple right there, which spread outward, getting smaller in its magnitude until it dissipates not far from where the pebble hit. And miles away, at the lake’s axonal end, that ripple of excitation has had no effect whatsoever.
To get sufficient excitation, a spine must repeatedly be stimulated, or a bunch of the spinal neurons must be stimulated at once.
You can not get a wave, rather than just a ripple, unless you throw in a lot of pebbles.
Two different types of signaling systems
Analogue signal: Dendritic spines to axon hillock
Digital system: Axon hillock to axon terminals
The more neurons that neuron A projects to, by definition, the more neurons it can influence; however, the more neurons it projects to, the smaller its average influence will be at each of those target neurons. There’s a trade-off
This is not a problem in the spinal cord where it is only one neuron that sends all its projections to the next in line. This only applies to brain neurons where one neuron can send to a lot of others.
Your average neurons has bout ten thousand dendritic spines and about the same number of axons terminals. Factor in a hundred billion neurons, and you see why brains, rather than kidneys, write poetry.
Trans-synaptic communication – How excitation is passed to the next neuron in line.
Neurons are independent units and the axon terminals don’t actually touch the dendrite spines.
Synapses: Microscopic gaps between axon terminals and dendrite spines.
Neurotransmitter: Chemical messengers
Vesicles: Little balloons filled with neurotransmitters that sit insides each axon terminal.
When the information is being sent then these vesicles, then they release the neurotransmitters which are then transmitted across the synapse reaching the dendrite spine on the other side.
Receptor: Sits on the membrane of the spine and receives neurotransmitters.
Neurotransmitters bind to a receptor, which triggers those channels to open, and the currents of ionic excitation begin in the dendrite spine.
After delivering the information to the receptor, the neurotransmitter will float off the receptors and will have to be cleaned up.
Two ways of cleaning up neurotransmitters:
1. Re-uptake pumps: Pumps in the axon terminal that take up the neurotransmitters and recycle them, putting them back into the vesicles.
2. Degraded: The neurotransmitters are degraded in the synapse by an enzyme.
Types of Neurotransmitters
Some neurotransmitters have more excitation effects than others. This allows for a lot more complexity in information being passed from one neuron to the next.
Depolarizing effect: Increase the likelihood of the next neuron in line having an action potential.
Hyperpolarizing effect: Increase the likelihood of the next neuron in line having an resting potential.
Inhibitory neurotransmitters: Transmitting hyper polarizing effect.
Each neurotransmitter binds to a unique receptor site that is complementary to its shape.
Examples of different neurotransmitters:
- Glutamate (the most excitatory)
- GABA (the most inhibitory)
It’s at this point that medical students are tortured with all the multi syllabic details of how each neurotransmitter is synthesized – its precursors, the intermediate forms the precursors is converted to until finally arriving at the real thing, the painfully long names of the various enzymes that catalyze the syntheses.
The insight into neurotransmitters allowed scientist to understand how drugs and medicines work. Drugs fall into two categories.
- Increase signaling across a particular type of synapse
- Decrease signaling across a particular type of synapse
Strategies for increase signaling
- Stimulate more synthesis of the neurotransmitter
- Administer a synthetic version of the neurotransmitter
- Stimulate the post-synaptic neuron to make more receptors.
- Inhibit degradative enzymes so that more of the neurotransmitters stick around in the synapse.
- Inhibit the reuptake of the neurotransmitter, prolonging its effects in the synapse.
A final, very relevant point – just as the threshold of the axon hillock can change over time in response to experience, nearly every facet of the nuts and bolts of neutrotransmitterology can be changed by experiences as well.
Circuits of neurons
Neuromodulation: The below illustration shows how Neuron B connects to Neuron C to send neurotransmitters. But then neuron A connects it’s axon to neuron B. This has an inhibitory effect and removes any action potential from the axon. Neuron A is having a neuromodulatory effect on neuron B.
There are many other types of circuitry. More will be added to the note.