- PSYCHOBIOLOGY – NEUROSCIENCE – #101 Basic Cell Structures & Physiology (In 2 Parts) – Part 1: NEURONS
The average adult brain weighs a little more than 2.2 pounds (1 Kg), is mostly lipids (Fat – yes its a compliment to be called a “Fat Head”), and has an outer “cortex” that is split into 2 hemispheres that interact with their opposite side of the body. It has a spinal chord with a “cerebellum” at the top where the spinal chord joins the brain. On top of this and inside there is the “reptilian brain” that is dominated with emotional centers. Yes, if you look at brains from ancient species of animals up to modern animals you see how the same basic underlying structures are persevered and new additional layers keep growing on top. The cortex actually takes information from that reptilian brain, reflects on it (processes it) and re-innervates the emotional centers to modify the initial sensory reaction, a chance to re-evaluate ones situation based on experience. This is how “learning” initiates and is how we have over come our “instincts” that reside in those reptilian brains that those animals are enslaved to. Creating the structures of the brain are 2 types of cells, Neurons and Glial Cells. There is a 3ed type, a “Schwann Cell”, but they function only when properly attached to neurons. Part 2 will cover the Glial and Schwann Cells.
There are several types of neurons, but they all share a basic structure:
- Cell Body
The web of dendrites is where the neuron receives input from other neurons. There may be thousands of neurons that provide input to one neuron. The cell body, along with production everything the cell needs to function, adds up all the input from the incoming neurons and decides whether or not its neuron/cell should fire. If the cell body receives enough excitatroy input the it will fire a signal down the axon to the terminals where signals will be transited to the next cell, which does the same process of summing up its inputs to make the same decision to fire or not. The Schwann Cells that make up the Myelin Sheath that covers the axon allows for faster transmission of the signal down the axon. It does this by allowing the signal to jump from “Node of Ranvier” (the gap between Schwann Cells) to Node instead of having to travel the entire distance of the axon. Some neurons, if very short, might not have any Schwann Cells, were as neurons with long axons (like those going to muscle tissue – “motoneurons“) will surely be covered with many.
BASIC NEURON TYPES
Of these 4 basic types of neurons, “Bipolar, Unipolar, Multipolar & Anaxonic ” (without an axon), the “interneurons“, perkinje (in the cerebellum), Amacrine (in the eye) the pyrimidal cells are usually very short in axon length, where as the sensory neurons and motoneurons can be extremely long, even meters long.
A very special class of neurons are “Receptor Neurons“. These are the ways the physical stimulation gets converted into neuronal signaling (aka Firing of the “Action Potential“). There is a type for each of the 5 senses we have as well as 2 types for vision, the Cone and the Rod. Cones have a one of three special coatings that only allow certain frequencies of light to stimulate them. By the neurons comparing which of the 3 cone types are firing and how often they are firing, the brain can determine color differences. Rods are very sensitive and are good for night vision, but they distinguish no colors (aka photon frequency differences) and therefor only reveal black and white images.
PHYSIOLOGY & ANATOMY OF A NEURON
THE ACTION POTENTIAL
Neurons fire by a process of diffusion. Diffusion is the process of an substance getting distributed evenly in a liquid, in this case in the cerebral spinal fluid (CSF) and the internal cellular fluid. An example would be putting table salt (Na+Cl-) in a glass of water, once it is dissolved the salt is distributed evenly, all parts of the water taste the same “salty”. There are 3 super important ions (charged elements) Sodium (Na+), Potassium (K+) and Calcium (Ca+2), right now we are only going to discuss K+ and Na+.
In the dendrite, where all the incoming signals from other neurons are being added up and a decision to “fire” is being made. There are special proteins known as “Ion Channels” and “Ion Pumps” along the cell wall (cellular membrane) of neurons. Of these the Na+/K+ Pump is functioning during the rest state of the of the neuron. It pushes the Na+ inside of the neuron to the outside of the neuron and pumps K+ the opposite direction into the neuron. Theoretically this should not change the electrical charge across the membrane, but, as the K+ concentration builds up inside, a small leak lets some K+ diffuse back out of the neuron. This changes the relative electrical charge on both sides of the cell wall from what would be 0 to a charge difference of -70micro-volts. Those incoming signals to the dendrite cause Na+ channels to open or close, if enough channels on the dendrite open and the micro-voltage across the membrane reaches -55mv, then an “Action Potential” will initiate.
During this process, the the Na+ Channels will open because the “threshold” of -55mv was reached. As these Na+ channels open there is a rush of Na+ into the neuron (from diffusion) until micro-voltage across the cell membrane becomes +30mv. When that happens the K+ channels open and the K+ rushes out (do to both diffusion and the + charge pushing it) of the neuron dropping the voltage a little more negative than the resting state, which the neuron quickly returns to as the Na+/K+ Pump resumes its actions. This process continues down the axon as each successive group of Ion Channels does its thing.
Schwann Cells, aka Myelin Sheaths, allow the neuron to send the signal down the axon faster with less effort by making it possible for the Action Potential to skip from Node of Ranvier to Node of Ranvier. This is very useful when signals have to travel large distances, say to muscles in the extremities, as well as it keeps neurons in the brain from “short circuiting” each other when they fire.
THE SYNAPTIC CLEFT & TERMINAL BUTON
It seems a little strange at first, but no two neurons actually touch each other. They are always separated by a gap into which chemicals are released to either excite the adjacent “post-synaptic neuron” to, or inhibit the post-synaptic neuron from firing (the action potential). ANY and ALL drugs that have an effect on the psyche (psychotropic compounds) have an affect on these chemicals and or the receptors for these chemicals, and they usually act within the synapse.
To the left is an image of a synapse. It shows the “Terminal Buton” (Boo-tawn) of the firing “pre-synaptic neuron” which releases its chemicals (neurotransmitters) into the “synaptic cleft” where they diffuse to the dendrite of the post-synaptic neuron and either excite it towards firing or inhibit it making it harder for other incoming stimulating pre-synaptic neurons from causing it to fire. The synapse is where all the magic, as relating to information processing, takes place. There are also “secondary messengers” that are stimulated by these neurotransmitters that change the metabolism and other functions of the neurons allowing for very complex changes in signaling in very short periods of time.
When a neurotransmitter is released into the synapse, a few different things can happen:
- It can interact with the post-synaptic receptor molecule for a short period and then be released back into the synapse.
- It can be broken down by other chemicals (like Mono-amine-oxidase [MAO]).
- It can be “re-uptaked” into the terminal buton of the pre-synaptic neuron to be repackaged into another “vesicle” (see image)
- It can attach to a receptor on the terminal buton causing a feedback loop to the firing cell telling it to “calm down”
- or lastly it can diffuse out into the CSF to migrate towards other neurons in its general vicinity.
Some drugs that are used for depression inhibit the functioning of MAO (Mono-Amine-Oxidase Inhibitors [MAOI]) thereby slowing the destruction of certain desired neurotransmitters considered deficient in depression and causing them to have a continued (longer lasting) affect on the dendrite of the post-synaptic neuron.
Neurotransmitters basically have two functions, to either EXCITE the post-synaptic neuron to fire or to INHIBIT it from firing, hence the categories of EXCITATORY and INHIBITORY Neurotransmitters. Traditionally there are said to be 7 main categories of neurotransmitters, but as time goes on these distinctions have grown and the lines blurred a little bit. These 7 neurotransmitters are:
- Acetylcholine (ACh)
- Dopamine (DA)
- Gama-aminobutyric Acid (GABA)
- Serotonin (5-HT)
- Norepinephrine (NE)
Of these, #1 was the first discovered, as it is the main neurotransmitter for talking with muscle tissues, even though it is also found in the sensory neurons and plays a roll in dreaming. The “inhibitory transmitters” are #2–>#5, and the “excitatory transmitters” are #6 & #7. But this is a big over simplification, a VERY SMALL example of the complexity is shown below (not a complete list):
|Small Molecule Neurotransmitter Substances||Amino Acid Transmitters Substances||Neuroactive Peptides
partial list only!
|Soluble Gases Affecting Receptors|
|beta-endorphin, calcitonin, enkephalin, insulin, substance P, glucagon, somatostatin, vasopressin, oxytocin, prolactin, angiotensin II, neuropeptide Y, thyrotropin-releasing hormone, gonadotropnin-releasing hormone, growth hormone-releasing hormone, luteinizing hormone||Nitric Oxide (NO),
Carbon Monoxide (CO)
Not to mention the Cannabinoid Receptors and all of its variants that are becoming so popular in medicine today for its many beneficial affects. The cannabinoids have receptors on the pre-synaptic neuron which affects the re-uptake of the primary neurotransmitter being released into the synapse.
The common description of a Neruotransmitter is that it is like a key, with a specific shape, and the receptors are like a lock. Of course it takes a key made for (or very close to) the lock you want to open (activate) for the pair to interact with each other.
The cell body of a neuron is basically like every other animal cell body. It contains the Nucleolus (where the genes and Deoxyribo-nucleic-acid [DNA] are kept) and the Mitochondria (where energy in the form of adenosine-tri-phospate [ATP] is made) and all the organelles that turn the DNA code into proteins (aka enzymes) that preform the cell’s functions.
As mentioned earlier, axons can come in many lengths. Some almost non-existent and some as long as a meter or more that go to the extremities and communicate with our muscles. This poses a big problem, how do you manufacture neurotransmitters in the cell body but have them present in the terminal buton ready to be released into the synapse upon arrival of the action potential? The answer is “Micro-Tubules“.
These tubes made of 2 different proteins are the amazing transport system of the neuron. It is these tubules that become entangled by “alumina-silica plaques” and result in Alzheimer’s Disease (senile dementia). After the manufacturing of the neurotransmitter in the nucleolus, these neurotransmitters and other nutrients are piped down the axon to supply the terminal buton with all of its needs. They also send waist products and other signaling molecules back to the cell body and nucleolus for regulation purposes.
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