Notes on Neurons

Preparing for a deeper dive into Neuroscience

(Context: I thought I should practice effectively learning something that's both difficult and useful, just to prove to myself I can. I have a textbook "Neuronal Dynamics" that I had sitting around for a while now, but while it seems promising, I'm immediately faced with a lot of terms I'm not familiar with. Rather than set a goal to finish the first chapter of the textbook and drive myself into the mud, I figured it was more effect to review the basics of neurons first. The secondary goal is to produce a glossary of terms that I can easily lookup in the future as I continue to read the textbook. This is that review/glossary)

Sources: Crash Course The Nervous Systems Parts 1-3

Nervous System

• All machines/computers/systems/processes can be understood fundamentally as Input -> Process -> Output

• In the human body, we have Sensory Input -> Brain -> Motor Output

• Taxonomy:

  • Central nervous system (Process)

    • Brain and spine

  • Peripheral nervous system

    • Sensory division (Input)

    • Motor division (Output)

      • Autonomic nervous system (Short feedback loop)

      • Somatic nervous system (Long feedback loop)

• Ultimately, not very in the context of "Neuronal Dynamics", which seems to be focused exclusively on the neuron, but I thought it was cool the nervous system could be broken down into these fundamental roles

Neurons

• Input -> Process -> Output

• Dendrite -> Soma -> Axon

• There are multiple types of neurons (and other cells in nervous tissue), but we'll be focusing on the most general type of neuron, the multipolar neuron (multiple dendrites + 1 axon)

• We can visualise the neuron abstractly as a kind of chicken claw; three pointy bits (dendrites), a center bit where they join (soma), and a straight bit (axon)

• Neurons can be imagined as a fleshy battery, membrane separates insides and outsides, maintaining electric potential difference (membrane potential)

• Membrane potential is from different concentrations of sodium and potassium ions inside and outside the neuron

• Suppose we have a resting neuron, membrane potential is -70 mVs (there are more positive sodium ions outside the neuron than positive potassium ions inside the neuron, resulting in the potential difference)

• Neuron has several mechanisms to depolarise (decrease membrane potential) and repolarise (increase membrane potential). Signals are sent between neurons when membrane potential crosses a threshold of -55 mVs

  • Neuron has ion channels and pumps

  • Channels can be ligand-gated (by neurotransmitter), voltage-gated (the the threshold for activation), and mechanically-gated (no idea how this works)

  • When channels are open, ions flow diffuse through the membrane due to disequilibrium ``

  • Pumps go the opposite direction, from equilibrium to disequilibrium

  • Suppose a spider jumps on your leg, sodium ligand-gated channels are opened by a neurotransmitter, sodium ions flow into the neuron, decreasing membrane potential

  • Sodium voltage-gated channels open when they reach the threshold membrane potential, more sodium ions flow in

  • The potential difference "moves through" the neuron (action potential)

  • So many sodium ions flow in there is a temporary positive potential difference (depolarisation)

  • Potassium voltage-gated channels open, potassium ions flow out of the neuron (repolarisation)

  • So many potassium ions flow out that membrane potential temporarily drops to < -70 mVs (hyperpolarisation)

  • The sodium-potassium pump moves three sodium out for every two potassium in, and the membrane potential is restored to -70 mVs

  • While all of this is happening, neuron is unable to receive any new signals, refractory period (I'm not sure if it's strictly physically impossible, and if so why, but I don't want to go too deep at this stage)

  • The electrical signal is of constant strength, what differs is the frequency of the signal (and arrangement of the neuron, I guess)

• Synapses

  • Electrical potential has arrived at the end of the axon (synapse seems to be more the location of the juncture between two neurons than a specific thing)

  • (Note than the axon of one neuron can be connected to like 1,000 other neurons, so the transmission is actually complex)

  • Neuron which is transmitting is presynaptic neuron, receiving is postsynaptic neuron

  • Synapses can be electrical or chemical

    • Electrical - Fast, coordinated, "group text", e.g. heart muscles, ions flow through gap junctions

    • Chemical - Slow, moderated, "direct message", action potential activates calcium voltage-gated channels open, calcium ions flow in, synaptic vesicles fuse with cell membrane and release neurotransmitters, which diffuse across synaptic cleft, binding to receptors on post-synaptic neurons