Slowly but steadily… Lecture 11, Neuroscience - part 2.
Last time you heard about neuroscience, now something in your head has changed and you remember that you heard about neuroscience before.
Stephen Wiltshire - an autistic savant. He can draw a very precise city view only after looking from an airplane for some time. Savants take spacial tours through their memories.
Memory (learning) happens (to the best of our knowledge) in synapses. People used to assume that each new memory leads to a new neuron. That is learning was perceived as equal to growing new neurons. Then people discovered synapses.
Memory is synaptic plasticity. When your presynaptic neuron fires a lot in a given time, eventually this will cause a stronger response in the post synaptic neuron. I.e. a simplified version of Hebbian Plasticity - neurons that fire together - wire together.
Excitating neurotransmitter is glutamate. It is excitatory and information is transmitted in the brain through activation.
Long term potentiation
Long term potentiation (LTP) - getting more output for the same input. This can be achieved through e.g. releasing more glutamate, reacting stronger in each of receptors, having more receptors.
There is a way for post-synaptic neuron to communicate back - retrograde neurotransmitter, e.g. nitric oxide gas.
Hippocampus is the site of memory and LTP.
There is adult neurogenesis and adult plasticity. In the last 20 years it was discovered that adult brain forms new neurons.
HM - person with their hippocampi surgically removed. As a result he could no longer form any new memories.
But LTP happens all over the brain e.g. for motor and emotion memory.
LTP can also go wrong, e.g. post traumatic stress disorder, when context brings undesired memories.
There are also mechanics for intentional disrupting LTP:
- hypoglemic states (i.e. when one is really hungry) lead to insulin cascades, which in turn reduces LTP. In other words, if you starve, it is not a good time to remember something, but to find food instead.
- stress hormones - improve short term memory (e.g. one may be able to remember tinniest details after a car crash as if in slow motion). However, if the impact is chronic, this damages LTP and memory.
- alcohol directly disrupts LTP.
Networks & inhibition
When you connect each neuron to e.g. 10k others and overall you have 100B of them, small changes in each synapse start having large effects.
There can be random and spontaneous generation of signal. It is important for the brain to filter out the noise.
So we need to consider neurons as groups instead. They also have capability to inhibit. A neuron can inhibit itself. This allows temporal sharpening. A neuron can inhibit its neighbors: “this is my signal, ignore my neighbors”. This creates spatial sharpening.
There can be fast sharp pain and dull aching throbbing pain. There are two types of neurons and each type is responsible for each kind of pain. Fast pain generates slow pain. Slow pain can inhibit first spike of fast pain. If you get stung by a scorpion, the fast pain forces you to pay immediate attention, perhaps the scorpion is still there. But there is no sense to keep this forever. Instead slow pain comes to remind you that you still have the issue and have to do something about it.
This also allows retinal cells to be responsible for specific types of receptive fields. Each neuron in your eye just gets a certain angle of light. This allows e.g. edge and contrast detection.
If you activate a neuron in your retina, you get spatiotopic (oriented and aligned in a similar way) field in the back of your visual cortex. This was detected by tracing back which neurons a given retina neuron was connected to. Then this has been repeated for the next level. However, no single neuron activation was propagated. Instead a group of neurons forming a line lighted up a neuron in the next layer. This was e.g. a neuron responsive to a vertical line.
You can construct visual world in layers by extracting features. The further you get, the higher order abstractions you create (e.g. orange, banana). However, no one has found orange neuron.
You can simplify your brain to a neural network. Once a neuron is activated, it doesn’t know where the input came from. I.e. it is a one-of function. So you can have concepts and categories. E.g. you have separate neurons for impressionist painters. Then all of them are connected to one neuron. This one neuron can represent impressionism. This is also how you remember stuff. You first remember a category and then try to narrow it down.
Emotionally salient memories last longer. Context (like emotion) can get represented neurally too.
Memory is encoded in such a neural network, not a single neuron.
Memory variation can come from variation in:
- how much glutamate gets released
- how strongly post-synaptic receptors respond
- ways in which neural networks are constructed
As a result there is spectrum from HW to Stephen Wiltshire. We are in the middle.
Autonomic Nervous System
It controls anything that happens automatically. E.g. heart beating, digesting, goosebumps, orgasm.
The nervous system can be divided into Central and Peripheral. Peripheral divides into Somatic and Autonomic. Somatic system is responsible for transferring sensory information from periphery to Central Nervous System (CNS) and voluntary control (e.g. moving muscles). It is fast and myelinated (covered with myelin which allows faster signal propagation).
Autonomic - moves organs, unmyelinated and, thus, slow.
Autonomic system is further divided into Sympathetic and Parasympathetic.
Responsible for arousal, alertness, emergency, flight or fight. Releases norepinephrine (NE) into target organs.
Responsible for growth, repair, calm vegetative function. Releases acetylcholine (ACh) into target organs.
Sympathetic and Parasympathetic have opposing functions. Homeostasis - dynamic balance between the two.
There is one exception - in male reproductive system they work together. Erection is controlled by Parasympathetic (and that’s why one needs to be stress free). Then Sympathetic overtakes and culminates in ejaculation.
In case of erectile dysfunction - 60% of cases are due to stress and not organic basis in the body.
Premature ejaculation - transition from Parasympathetic to Sympathetic happens too quickly.
When Parasympathetic system is active, immune system works. In Sympathetic - contrary. As a result, it is easier to get sick when you are stressed out (Sympathetic gets activated).
NE & ACh can inhibit and excite depending on the organ. E.g. in case of heart, there is excitatory NE reception and, thus, it is excited by NE. Gastrointestinal tract has inhibitory NE receptors.
The center of regulation of Autonomic Nervous System is hypothalamus. Even reptiles have it. Then there is limbic system, which is responsible for emotions in mammals. As a result, seeing someone you hate can cause sympathetic response through limbic system (emotion) and then hypothalamus. In primates, cortex is responsible for thoughts and memories. As a result, just a thought can cause a sympathetic response (from cortex to limbic system and so on).
So when you think about a test, this activates your cortex, then limbic system, then hypothalamus (so-called triune system of the brain). This cases sympathetic response. It can be similar to a reptile’s body response to losing blood, but caused only by your thought.
Autonomic Nervous System is highly plastic.
Biofeedback can be used to cure high blood pressure. You think of something pleasant. This decreases blood pressure. You repeat. Get further decrease. This makes the connection between this cortical thought and parasympathetic activation even stronger.