Lecture 10, Neuroscience - part 1.

Broader context

The course tries to show how various sciences answer “why chicken crossed the road”:

  • evolution - how behavior evolved, why it is adaptive
  • molecular genetics - genes controlling the evolution, how they change over time
  • behavioral genetics - what individual genes are responsible for variation in behavior
  • ethology - studying behavior in nature, stimulus

Today we consider neuroscience - what happens in brain before the behavior and why.

Chief function of the body is to carry the brain around. (attributed in Internet to Thomas A. Edison)

Nervous system & brain

Nervous system is divided into central and peripheral. Central consists of brain and spinal cord.

Brain consists of:

  • stem - responsible for communication with spinal cord

  • cerebellum - motor movements (especially when learning new stuff and correcting mistakes). There is a peculiar experiment with prism glasses (everything appears normal, but in reality shifted to the side). When wearing such glasses, one struggles with touching a target, because their brain calculates object position as if there were no glasses. After some time (~minutes) one gets used to the glasses and learns to touch the object as precisely as without glasses before. I.e. their brain learned how to correct for the shift. Once one takes off the glasses, they have the same “problem” again, but in the other direction, and it takes time to autocorrect back.

  • cortex
    • frontal lobe - plans actions, controls movements, each body part has its own part here and they are located the same way as in the body.
    • parietal lobe - sensory touch info. Area per body part can differ, e.g. fingertips are overrepresented.
    • temporal lobe - auditory info + memory formation
    • occipital lobe - visual info (back of the brain)
  • limbic system
    • hippocampus - forming new memories.
    • amygdala - fear and anxiety - sweat of scared people (experiment described in a previous lecture) lights up amygdala.
    • hypothalamus & pituitary gland - hormone secretion

A joke about 4 F’s of brain - flight, fight, feeding and reproduction.

Spinal cord has specialized areas too.

Peripheral system - all motor and sensory nerves + nerves for heartbeat, digestion, breathing

Brain cells

Santiago Ramón y Cajal is considered by some a grandfather of the modern neurobiology.

Before people thought of a brain as a web. He stained individual cells in the brain with black and drew pictures of them. This way he investigated brain structure on cellular level.

Cell examples:

  • 90% of cells in brain are glia (means glue).
  • astrocytes supply nutrients to neurons.
  • myelin wraps around neuron wires and makes the signal go faster.
  • microglia - immune system.
  • neurons - computational units (100B in average, each has 10k synapses-connections).

Neuron

Consists of dendrites (inputs), nucleus (cell core), soma (cell body), axon hillock (start of output), axon (output) and terminal (end of output branch).

Neuron is binary (on or off) instead of gradual (i.e. there is no middle level of activation). This helps to keep resting potential. Sending of information happens through movement of ions (charged chemicals). Neuron keeps itself quiet by keeping positive ions outside of the cell by using pumps. Net charge inside is as a result negative.

When dendrites receive neurotransmitter, channels for positive ions are opened. This changes the charge. If there is a lot of positive charge, neuron may fire further. If there is enough, channels on axon hillock open to let more positive ions in, this creates a positive feedback loop. Positive ions in the terminal trigger sending the signal further to the next neuron. As it was said above, this is also binary.

To stop, cell opens other channels to let some positive ions out. Axon terminal stores large amounts of neurotransmitter packaged in vesicles (discrete structures). After that a channel can be opened in the next neuron to let ions in.

Neurotransmitter might induce transcription factor. Neurotransmitters are located in terminals. Examples:

  • dopamine - commonly associated with reward, pleasure, but it has many other functions too
  • epinephrine (adrenalin) - involved in flight or fight response
  • norepinephrine - pretty much interchangeable with epinephrine
  • serotonin - e.g. regulation of sleep, appetite and mood.
  • gaba - inhibitory
  • glutamate - excitatory

You have to remove used neurotransmitter from receptor (re-uptake), otherwise it will keep signaling.

Neuropharmacology

External manipulation of synaptic events, e.g. for research or to correct a disease state. Often this involves something resembling a neurotransmitter, but not fully. Hallucinogens often work like that (e.g. LSD resembles serotonin).

One can weaken synaptic response.

People with Parkinson’s have low level of dopamine in brain areas responsible for movement. One can increase level of dopamine globally, but this will cause other issues (e.g. schizophrenia-like symptoms).

HM

Patient HM - a man who had terrible seizures. As a fix, his hippocampus was removed. As a result, he couldn’t form any new memories. He could remember some stuff from childhood.