2. Your Brain ... Tabitha M. Prowledge

Macmillan Publishing company, 866 Third Avenue, New York, NY 10022 USA

This is an intermediate level book on Brain and Neurophysiology. A very nice readable book with information presented in a way be understood by a novice or an expert. The book introduces brain beautifully but not inspirational or challenging to the reader. The following summary of the book presents the information hopefully to spark an interest in Neurons and neural systems.

ABSTRACT
Neurons are generated at the center of the brain and travel to different locations in the brain. While traveling to their destination, they pickup information of the neurons in their path way for possible synaptic connections. After reaching their destination, they change their shape and structure depending upon their location. They make synaptic connections as necessary. The synaptic connections could be; between cells and neurons, dendrites and dendrites, dendrites and axons or between axons.
The receptors at our five senses create action potentials (electrical pulses) which in turn emit chemical neurotransmitters. They swim the synaptic path way and gets attached to receptors of the dendrites of the other neuron. The receptors in turn generate second messages which in turn generate some more second messages. The action potential travels like a wave through the neuron-synaptic pathway and reach its ultimate destination in the cerebral cortex of the brain. The message is analyzed and brain responds.The action message travels through the neural pathway and reaches the destination( a muscle cell, a hormone making site etc).

How these neurons make choice synaptic connections, how they store information , how they retrieve information; these are all questions to be answered. The players are action potentials, neurotransmitters, second messages and yet to be discovered unknowns.

Summary
For all existing organisms the name of the game is survival and improving survival skills as the mantra of nature is survival of the fittest. when the survival skills are not adequate they become extinct. We are surrounded by lot of information of organisms which survived and the ones which did not survive. From this information, obtained from fossils, we try to understand how the evolution is evolving.

When uni-cellular organisms appeared on the planet billions of years ago one single cell took care of all the survival skills. They do not have any built in thinking system for survival. They started improving their existence by evolving into multi cellular organisms where different cells do different things at the same time, action coordination became critical. Nerve Cell(neuron) appeared in evolution whose sole purpose is coordination between cells in the organism.

Neuron's basic function is to communicate and exchange information between various parts of the organism for survival coordination. So it has dendrites to receive information and axons to transmit information to other cells. When two neurons are communicating, the neural message moves through the gap between an axon of one neuron to the dendrite of other neuron. This gap is called the synapse. This basic synaptic communication system called nerve net, which we find in Jelly fish and corals is the precursor of our human nervous system, .

As the organisms started improving their survival skills, sensual receptors started appearing in multi cellular organisms. Rapid communication between cells became a necessity and basic Nerve net's simple system is inadequate. The existing Nerve net's communicating channel was tapped at different points for information sharing. The simple nerve net became a nerve cord running along the front of the body with swelling of neuron structures for information exchange. Such a swelling is called a nerve ganglion. The nerve cord is the precursor to spinal cord in humans. An earth worm has a nerve cord with a ganglion in each of its body segments so that it can move in one direction. The ganglion in its head - the cerebral ganglion - is connected to its mouth so as to coordinate its movement in one direction for food.

As the new kind of animals appeared, the head end of an animal became a prominent organ. Improvements in survival skills are developed around the mouth (eyes, ears) to hunt for food. Visual skills, hearing skills, smelling skills, touching skills, tasting skills started to appear in organisms. These new skill system were melded with the existing spinal neural network as they appear. As a result we see organisms with unique specialized skills, like Bats with sonar system(sound) , dogs for olfactory(smell) etc.

In evolution, incorporating multiple specialized skills started with the African ape line (Chimpanzees and Gorilla) and led to the human line. Brain development took place to coordinate all the messages coming from the senses. There are two classes of cells in brain, neurons and glial. Neurons do the neural message processing and glial cells do the structural supportive work for neurons.

Neural development in humans is interesting. Three weeks after conception, the embryo is about 1/10 of an inch and has a hollow channel called neural tube running the back of the embryo. At one end of the neural tube is a small bulge. The neural tube grows up to be the spinal cord and the bulge transforms into 3 bulges which eventually become 3 important parts of the brain, Hind brain, Mid brain and Fore brain. One of the bulges which eventually become fore brain divides into three fluid-filled spaces in the cerebrum, known as ventricles. Ventricles lie at the center of the brain. The cerebral cortex neural cells are created by the cell division at ventricles.

The neurons created at the center of the brain travel to the various areas of the brain through the help of glial (glue in Greek) cells. while on their way to reach their destinations they pickup information from the settled neurons in their path, probably to make connections (synapses) later. Once they reach their final destination, neurons get bigger, axons lengthen and dendrites branch out seeking synaptic connections with other cells.

Hind brain is the one closest to the spinal cord. It controls many automatic functions, such as breathing and heartbeat. It has Cerebellum and Medulla oblongata.

The fore brain is involved in thinking, planning, language and problem solving. It consists of Hippo campus, Amygdala , Hypothalamus, Thalamus and Cerebrum.

Hippo campus processes memories. Amygdala is involved in our emotions. Hypothalamus governs the system of hormones in the body. Thalamus is the brain's central message relay station; almost all information passes through here on its way to other parts of the brain. Cerebrum is the top most part of the brain where we think and plan. It has the Cerebral cortex where all the signals from the senses are stored and analyzed.

The work in the brain is performed by the neurons. There are 100 billion neurons actively participating in the transmission and storing of information in the body. Neurons tirelessly work from the time we were born to the time we die. About 250,000 neurons are created every minute during pregnancy so as to reach its limit 100 billion towards the end of pregnancy.

After the baby is born, it has to struggle to acquire skills for survival in the world, like struggling to walk and talk. The more struggle in repetitive tasks the more intense are the synaptic connections it makes and the more ease of performing them throughout their life. That is how our walk and talk became almost automatic. We never pause to think that we struggled to acquire those skills for months when we are babies.

Human body senses are triggered by physical stimulus occurring outside the brain. These physical stimuli act upon the specialized receptor cells. These receptor cells in turn generate nerve impulses. These nerve impulses are partly chemical and partly electrical. The main function of the neurons is to receive and transmit these impulses. The nerve impulse travels from the dendrite to axon for transmission to the next neuron through a synapse.

The transmission of nerve impulses is well written in the book but I will make my humble attempt to describe.

Cell membranes have perforations for ion transportation. These are called ion channels. These channels control the movement of ions in and out of the cell. The cell has a stable charge configuration and because of the movement of charged ions in message transmission, parts of the cell charges are altered. Immediately a counter migration of ions are initiated to counteract the altered charge. For example it is slightly negative inside a cell but when positive ions get into the cell it becomes more positive and immediately some positive ions are allowed to get out of the cell membrane ion channels for restoring the negative charge. This process generates a tiny electrical impulse called action potential. These action potentials have about 1/10 of a volt and travel like waves with speed around 100 miles per hour. Potassium and Sodium ions participate in impulse transmission.

The nerve impulse after reaching the axon's terminus, forces it to eject what are called neurotransmitters thereby transforming an electrical signal to a chemical signal. The chemical signal travels in the synapse and attaches to a receptor on the dendrites of the neighbouring cell. The signal changes to an electrical signal in the next neuron. The travel continues until it reaches the destination. Some of the neurotransmitters are Glutamate, Aspartate, Dopamine, Serotonin, Nitric Oxide.

When a neurotransmitter attaches itself to a receptor on a dendrite, it initiates the release of chemicals within the cell called second messengers. They carry the message to the interior of the cell and trigger reactions. These reactions could change the cell behavior or may produce additional second messages some of them may reside in the cell permanently.

Axons and dendrites branch out like trees. An average Neuron forms thousand synapses. Some neurons have 200,000 synapses where they can exchange neurotransmitter messages with thousands of other neurons. Synapses are the gaps where either chemical or electrical impulse information is exchanged between neurons or neurons and cells .

We have five senses, seeing, hearing, smelling, tasting and touching. There are specific receptors for specific senses spread all over body. They produce electrical impulses called action potentials. First destination for all these impulses is the thalamus, which is in the middle of the brain. Thalamus is the central processing unit for sensory impulses where they were analyzed and sent to proper locations on cerebral cortex. Visual Cortex takes care of Optical signals coming from the eye, auditory cortex for hearing, somatic sensory cortex for identifying and locating touching signals. Final signal analysis was made at the cerebral cortex and response signal was sent to the appropriate area through the neural network.

For example you are walking and suddenly saw your friend whom you want to meet going in the other way. Signals in your cortex know your location, optical and auditory signals sending all about traffic on the street, brain knows your intention to move, signals sent to your muscles so that you can cross the street safely and meet your friend. Your optical system, auditory system, muscular system coordinated your street crossing and your lips are there ready to greet your friend.

The main function of the brain is to be aware of the body's location and what is happening around it and respond appropriately. The continuous routine functions like breathing, heart beat, temperature monitoring etc are handled by the brain stem. The sensual signals coming from receptors have all the necessary information, like location of the sending receptor(eye); location of the object, color, shape, texture etc. ( from taste buds sweet or sour etc.; from nose type of smell etc; from ear what type of sound and where it is coming from etc; from touch texture of the object, temperature etc.). The signals take two paths to the brain. One goes there directly and the other visits the spinal cord on the way. Spinal cord takes care of the emergency signals by sending reflexive signals and informs brain later ( We take away our hands after touching a hot plate).

All signals go to their respective areas on the cerebral cortex through the controlling area on the thalamus. Some signals my be stopped at the thalamus level if they are just routine signals. For example you get used to ocean sound and do not here at all after a while; the sensory signals being sent by the rubbing of your clothes to the skin, you will not be aware of it; the perfume smell we ware, we will not be aware of it after a while; Sometimes conflicting signals reach the brain and brain gets confused resulting in dizziness, or seasickness etc. In almost all these cases you will be doing two opposite things at the same time, you are moving and yet you are sitting stationary ( going in a plane, traveling on a ship, whirl around and stop etc).

Brain will analyze all these signals reaching the cortex and responds by sending action signals to the proper location. We do not know how it makes this analysis but it happens all the time. There are so many mysteries in our own body. May be sometime someday in the evolutionary cycle there comes a super human who will solve all the mysteries.