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PostPosted: Mon Jan 07, 2013 3:29 pm 
Broken Crown Panelist
Broken Crown Panelist

Joined: Thu Nov 15, 2012 5:09 pm
Posts: 13
It's a classic line, from Star Trek: The Original Series episode "Spock's Brain", in which we discover that if you just connect the vocal chords, the patient can direct you in connecting the rest of the nerves!

To introduce this forum and the brain science topics supporting Escaping Titan, it is necessary to discuss a little bit of the brain's structure. I usually use this picture Image or one like it to introduce the structure of the brain – either that, or I tell people to hold their right hand at arm's length in front of their face, fingers together, palm flat and facing directly away from the face. The thumb is the temporal lobe (yellow) and that's where a lot of the structures that help us make memory are located. Fingertips are the frontal lobes (red). That's where a lot of decision-making and "conscious thought" seems to take place. Neuroscientists call that "executive function" and it is mainly the conscious control of other brain functions. The outside edge of the hand is the parietal lobe (blue), there's a lot of motor and sensory structures there. Just forward of the junction of frontal and parietal lobes is the "motor strip" which controls voluntary muscle movement throughout the body (including the Dorsal Premotor Area which will be the subject of the next couple of blogs). Next to that, within the parietal lobe, is the somatosensory area, which is structurally a near match of the motor strip, but receives the sensory input from the body. Down toward the temporal lobe are areas that handle hearing, speaking and reading. The occipital lobe (green) is about where the heel of the hand would be in our model. It is almost exclusively related to vision and visual functions. However, some visual processing areas are also in the parietal lobe, since seeing quite frequently connects with hearing, speaking and reading. The cerebellum (orange) and brainstem (light blue) are equivalent to the wrist - in many ways. The cerebellum coordinates muscle movement throughout the body, and the brainstem connects the brain to the body (via the spinal cord).

Here's another illustration for the structures inside the brain. Image It seems almost as if for every function on the *outside* (surface) of the brain (we call it the "cortex" or "neocortex"), there's a structure deeper within the brain that acts as a relay, switch or preprocessing junction. By the way, we often call the deep structures "nucleus" or "ganglion", or even "fiddly bits" (with apologies to Howard Tayler), although many have their own names such as "thalamus," or "hippocampus."

The thalamus is the major relay for sensory information – particularly feedback from the rest of the body that is used to assess the results of movements. The primary function is one of switching or routing the inputs to the appropriate cortical (surface) areas (vision, hearing, touch, etc.). There are also connections between cortical areas that are routed through thalamus, and it is essential at directing information that needs to be combined – i.e. vision plus muscle movement for visual tracking, hearing plus vision for speech. The lentiform nucleus – also called "corpus striatum" (striped body) or "striatum" – manages movement. Parkinson's Disease affects neurons in this part of the brain. Striatum also serves as an essential component of learned behaviors, by providing information about reward and motivation. It is the neurons in this region that are involved in the behaviors leading to drug abuse, as well as positive/negative reinforcement training.

Hypothalamus is not well labeled in this diagram, but is the yellow area under ("hypo") the thalamus. This region is involved in all of the hormonal and "autonomic" (think of it as "automatic") functions of the body – hunger, satiety, temperature regulation, metabolism, menstrual cycle… Pons and medulla are relay points in the nerve and muscle connections between brain and body (via the spinal cord). They also form the muscle control equivalent of the hypothalamus: heart rate, respiration rate, eyeblink. Some of the "reflex" actions such as control of muscles that push vs. pull at the same joint are handled here as well as deeper within the spinal cord. Hippocampus is prominently involved in memory, but also in looking for patterns in our sensory input. Amygdala is commonly called the center of emotion. While this is not strictly true, amygdala – in fact the whole "limbic system" – mammillary bodies, fornix amygdale and hippocampus (yes, they form a loop – when viewed from the side, it forms almost a complete cycle of a spiral) are involved in processing emotion. More appropriately, amygdala is involved in processing the emotional content or context of information. To do so requires reference to memory – and that requires hippocampus, and the temporal and frontal cortex.
Finally, to finish off this discussion, we'll look at the building blocks of brains and nervous systems: Neurons. Yup, neurons. Not nerves. Neurons are brain cells and the cells that make up nuclei, ganglia, fibers, tracts, roots, and even nerves. But please, do not make the mistake of calling brain cells “nerves”. A neuron is a singular cell that has several very important properties which will be described below. All brain tissue is made up of neurons, but not all brain tissue contains nerves. A nerve is a bundle of neurons – in particular the portion of the neuron called an “axon”, and it specifically connects the brain and spinal cord with sites in the rest of the body – such as muscles and sensory organs.

The special properties of neurons are: (1) ability to separate chemical ions to produce an electrical charge gradient across its membrane, (2) the ability to “gate” diffusion of those ions across the cell membrane so that ionic charge can be built up, then discharged selectively, (3) the ability to perform this gating in response to an input stimulus (electrical, chemical, mechanical), and (4) the ability to transfer a signal to adjacent cells via direct electrical contact, or by release of a chemical that will replicate the electrochemical process in the next neuron in a sequence.

This next figure Image is an excellent diagram of a model neuron. Note the section marked “dendrites” – this is the region that collects inputs from hundreds and thousands of other (input) neurons. The soma contains the metabolic “machinery” of the neuron; the axon is the conductive channel that transfer a signal at distances of microns to meters; and the synaptic terminal is the output zone of the neuron. These terminals can connect again to tens, hundreds or even thousands of “downstream” neurons. By the way – a “nerve” is a bunch of axons, all running in the same direction, with the cell bodies back in the brain (or spinal cord), and the synaptic terminals out in the body somewhere. There are also nerves running in reverse, with the cell bodies and dendrites out in the muscles, fingertips, etc, and the synaptic terminals in spinal cord and brain.

I could go on at length about how the process works (in fact, I teach exactly this subject to graduate students) but instead I want to describe how different specializations of these 4 characteristics produces the variety of neurons that serve specific functions in the brain and nervous system.

(A) If the trigger to gate the ions and release the stored electrical potential is chemical, and the synaptic terminal releases chemicals called neurochemicals onto other neurons – that is a common output or projection neuron.

(B) If the synaptic terminal is on a muscle cell, it is a motor neuron.

(C) If the trigger to release the electrical potential is light, it is a "photoreceptor" (visual) neuron.

(D) … if mechanical, it may be a tactile (touch) or pain neuron or a “proprioceptor” (joint and muscle position) neuron.

(E) … if vibratory, it may be an auditory (hearing) neuron.

(F) … if complex chemicals, it may be a taste or olfaction (smell) neuron.

(G) If the synaptic terminal release chemicals into the bloodstream instead of onto another neuron, it is a “neurosecretory" neuron such as those that produce hormones.

There are other specialized cell types in the brain which provide support, metabolism, chemical synthesis, insulation, waste removal, etc. These are generally called “glia” but also have more specific names based on shape and function – such as “astrocytes” and “oligodendrocytes.”

The signals transmitted by neurons can increase or decrease the activity of the targets. It is commonly *misstated* that these are excitatory or inhibitory neurons. The truth is that the combination of neurotransmitter, the specific receiving molecule (receptor) on the target neurons, and the position of the receptors on the target neurons all combine to determine whether a particular synapse of a neuron is excitatory or inhibitory. Thus a *lot* of different functions can be performed by the same basic building blocks with only minor variations.

In fact, put enough neurons together, and you can even build … a brain!

Over the next few blogs we'll cover specific brain areas and their relation to Escaping Titan - especially in regard to "hacks" and/or abilities that will improve the performance of your characters in the game. Join me next month for a discussion of the function of Dorsal Premotor Cortex, and the "Self-Buff" ability.

-RH


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