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  • The basics of neuroscience

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Neuroscience is the study of the nervous system. The nervous system is divided into two broad classes, the central nervous system (CNS) and the peripheral nervous system (PNS). Unlike the CNS, the PNS is not protected by the bone of spine or skull and its function is to connect the CNS to the limbs and organs. The PNS consists of nerves, whilst the CNS consists of the spinal cord and the brain.

The brain is the most complex part of the human body, this 1.4 kg (3 pound) organ is the seat of intelligence, interpreter of the senses, initiator of body movement, and controller of behaviour. Lying in its boney shell and bathed in protective fluid, the brain is the source of all the qualities that define our humanity.



Contents

Introduction to Neuroscience

We have five senses: sight, smell, hearing, touch and taste. Through these senses, our brain receives messages, often many at one time. Our brain controls our thoughts, memory and speech, the movements of our arms and legs and the function of many organs within our body. Different parts of the brain are responsible for coordinating and performing specific functions. There is a clear correlation between the structures and function in the brain. Differences in appearance predict differences in function.

The arrival of functional brain imaging machines allowed scientists to look inside the living brain and see its mechanisms at work. In the last 20 years, positron emission tomography, functional magnetic resonance imaging (FMRI) and most recently, magnetic encephalography (MEG) have between them produced an ever more detailed map of the brain's functions.

The word neuroscience is quite young, The Society for Neuroscience, an association of professional neuroscientists, was founded as recently as 1970. The study of the brain, however is as old as science itself. The Society for Neuroscience is the largest and fastest-growing association of professional scientists in all of experimental biology.


Web-Links for Further Learning:


Video Materials:

Introduction to Neuroscience Lecture by Stanford University

HHMI Video Series on Neuroscience Lectures


Reading Materials:

Neuroscience for Kids

Society for Neuroscience (as mentioned above)

Neuroscience: Exploring the Brain by Mark F. Bear, Barry Connors & Michael Paradiso

Neuroscience at a Glance by Dr. Roger Barker & Stephen Barasi

The Brain Book by Rita Carter & Medi-Mation

History of Neuroscience

As early as 7000 years ago, people were boring holes in each others skulls (a process called trepanation[1]), with the goal to cure. This procedure is speculated to have been used to treat headaches or mental disorders, perhaps by giving the evil spirits an escape route.

The view of the brain in the ages of ancient Egypt was that the heart was the seat of consciousness and thought instead of the brain, this view was challenged until the time of Hippocrates[2]. The most influential scholar was Hippocrates (460-379 B.C.), the father of Western medicine, who stated his belief that the brain not only was involved in sensation but also was the seat of intelligence. However, this view was not universally accepted. The Greek philosopher Aristotle (384-322 B.C.)[3] clung to the belief that the heart was the centre of intellect; he proposed the brain to be a radiator for the cooling of blood that was overheated by the heart.

The Greek physician and writer Galen (130-200 A.D.)[4], embraced the Hippocratic view of brain function, his opinions were influenced more by his many careful animal dissections. Galen's view of the brain prevailed for almost 1500 years. More detailed drawings[5] of the brain was added by anatomist Andreas Vesalius (1514-1564)[6] during the Renaissance[7].

A chief advocate of this fluid-mechanical theory of rain function was the French mathematician and philosopher René Descartes (1596-1650)[8]. He reasoned that unlike other animals, people possess intellect and a God-given soul.

One of the most significant observations in the early ages that that brain tissue is divided into two parts: the gray matter[9] and white matter[10]. It was correctly believed that white matter contains fibers that bring information to and from the gray matter, because it was continuous with the nerves of the body. By the end of the eighteenth century, the nervous system had been completely dissected (cut), and its gross anatomy had been described in detail.

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By the turn of the 1751’s century, Italian scientists Luigi Galvani[11] and German biologist Emil du Bois-Reymond[12] had shown that muscles can be caused to twitch when nerves are stimulated electrically and that the brain itself can generate electricity. These discoveries displaced the notion that nerves communicate with the brain by the movement of fluid. Scottish physician Charles Bell[13] and French physiologist Francois Magendie[14] found that the fibers divide into two branches, or roots called the dorsal and ventral roots. The dorsal root enters toward the back of the spinal cord, and the ventral enters towards the front. Bell tested the possibility that these two spinal roots carry information in different directions by cutting each root separately and observing the consequences in experimental animals. He found that cutting only the ventral roots caused muscle paralysis, later Magendie was able to show that the dorsal roots carry sensory information into the spinal cord.

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In 1811, Bell proposed that the origin of the motor fibers is the cerebellum and the destination of the sensory fibers is the cerebrum, the approach in which parts of the brain are systematically destroyed to determine their function is called the experimental ablation method[15]

In 1823, the esteemed French physiologist Marie-Jean-Pierre Flourens[16] used the experimental ablation method in a variety of animals (particularly birds) to show that the cerebellum does indeed play a role in the coordination of movement. He concluded that the cerebrum is involved in sensation and perception, as Bell and Galen before him had suggested.

In 1861, French physician Paul Broca[17] described a patient who he named "Tan", as it was the only word "Tan" could say. When Tan died, Broca examined his brain and found a lesion in the left frontal lobe. This part became Broca's Area[18]. He concluded that this region of the human cerebrum was specifically responsible for the production of speech.

In 1876, German neurologist Carl Wernicke[19] found that damage to a different part of the brain which became known as Wernicke's Area[20] also caused language problems. He concluded that it is responsible for the comprehension of speech, language development or usage can be seriously impaired by damage to the Wernicke's Area. Paul Broca and Carl Wernicke were the first scientists to clearly define functional areas of the brain.


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In 1870, German physiologists Gustav Fritsch[21] and Eduard Hitzig[22] showed that applying small electrical currents to a circumscribed region of the exposed surface of the brain of a dog could elicit discrete movements. Scottish neurologist David Ferrier[23] repeated these experiments with monkeys.

In 1881, Ferrier showed that removal of this same region of the cerebrum causes paralysis in the muscles. Similarly, German physiologist Hermann Munk[24] using experimental ablation presented evidence that the occipital lope of the cerebrum was specifically required for vision.

The German neuroanatomist and physiologist Franz Joseph Gall[25] invented phrenology[26], which is the pseudoscience of correlating the structure of the head with personality traits. Phrenology was the idea different bumps on the skull could reveal our mental abilities and character traits.

Technical advances in microscopy[27] during the early 1800s gave scientists their first opportunity to examine animal tissues at high magnifications. In 1839, German zoologist Theodor Schwann[28] proposed what came to be known as the cell theory[29]: all tissues are composed of microscopic units called cells[30].

However, scientists faced another problem, brain tissue has a uniform, cream-coloured appearance under the microscope; the tissue has no differences in pigmentation to enable histologists to resolve individual cells. The final breakthrough in neurohistology was the introduction of stains that could selectively colour some, but not all, parts of the cells in brain tissue.

One stain, still used today, was introduced by the German neurologist Franz Nissl[31] in the late 19th century. Nissl showed that a class of basic dyes would stain the nuclei of all cells and also stain clumps of material surrounding the nuclei of neurons. The clumps are called Nissl bodies[32], and the stain is known as the Nissl stain. The Nissl stain helps to distinguish neurons and glia from one another, as well as enables the study the arrangement, or cytoarchitecture[33] of different parts of the brain (the prefix cyto- is from the Greek word for “cell”). A thin slice of brain tissue has been stained with cresyl violet, a Nissl strain.

The Nissl stain doesn’t show the whole features of neurons, until the work of Italian histologist Camillo Golgi[34] was published. In 1873, he discovered that by soaking brain tissue in a silver chromate solution, now called the Golgi stain[35], a small percentage of neurons became darkly colored in their entirety. The Golgi stain shows that neurons have at least two distinguishable parts: a central region that contains the cell nucleus, and numerous think tubes that radiate away from the central region.

Golgi invented the stain but it was the Spanish histologist Santiago Ramon y Cajal[36] who used it to its greatest effect. Cajal used the Golgi stain to work out the circuitry of many regions of the brain. Golgi and Cajal drew completely opposite conclusions about neurons. Golgi argued that the neuritis of different cells are fused together to form a continuous network whereas according to Cajal the neurites of different neurons are not continuous with one another and must communicate by contact, not continuity. This idea is known as the neuron doctrine[37]. Final proof had to wait until the development of the electron microscope[38] in the 1950s.

Functions of Brain Parts

The brain consists of three categories namely; Cerebellum, Brain Stem and Cerebrum.

The Cerebellum sits at the back of the brain. It helps to learn something new, for example if you learn for the first time to play piano or basketball then you’re going to make mistakes, the cerebellum helps to correct those mistakes. It’s also involved in balance, equilibrium, muscle tone, and the coordination of voluntary motor movement. The cerebellum, like the cerebrum, has a cortex or outer covering of gray matter.

The brain stem sits at the bottom of the brain and at the top of the spinal cord. It’s responsible for relaying (act of passing) information to and from the spinal cord. Parts in the brain stem include Pons, Medulla & Midbrain etc. are those necessary for survival (breathing, digestion, heart rate, blood pressure) and for arousal (being awake and alert).

The Cerebrum is the evolutionary (and largest) part of the brain. It is here where things like perception, imagination, thought, judgment and decision occur.

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The wrinkled outer layer of the brain is called the cortex. The cortex constitutes of 4 different lopes, each lope has a different location and its own functions.

The frontal lope is up at the front of the brain, the functions are amongst many others; control of movement, plans of actions, problem solving, memory, language, judgement and social and sexual behaviour. Different locations of the frontal lope control different parts of the body, for instance one location may be to control the right hand while another location controls the left hand but they are organized which means the part that controls the feet sits at the bottom of the frontal lope while facial expressions are controlled from the top. It is important for voluntary and planned motor behaviours - such things as voluntary movement of eyes, trunk, limbs and the many muscles used for speech.

The parietal lope sits in the top middle of the brain; its function is to sense the sensory touch/pain information from the outside world. Different parts of the parietal lope are specialised for different parts of the body and the sizes of the parts of body may differ, for example the finger have a lot of sensory nerves which means the part on the parietal lope controlling the fingers is also bigger.

The temporal lope function is to receive auditory information (hearing). Deep inside the temporal lope are the parts of the brain which help in memory formation and speech.

At the back of the brain lies the occipital lope, it’s responsible for visionary information and processing. They visual information from the eye moves all the way to the back of the brain to the occipital lope.

The Neurons and Glia

A neuron is a nerve cell[39] which allows us to think and move our body parts. The average brain constitutes of approximately 100 billion neurons.

The thin tubes that radiate away from the cell body are called neuritis and are of two types: Axons[40] and Dendrites[41]. Histologists[42] immediately recognized that axons must act as “wires” that carry the output of the neurons as they travel great distances, dendrites on the other extend more than 2mm in length. The cell body (soma) is about 20 micrometre in diameter. The watery fluid inside the cell body, called the cytosol[43], is a salty, potassium-rich solution that is separated from the outside world.

A neuron is either switched on or off; neurons communicate and send signals through the movement of chemicals called ions[44]. The inside of the neuron is separated from the outside by the limiting skin, the neuronal membrane; which lies like a circus tent on an intricate internal scaffolding, giving each part of the cell its special three-dimensional appearance.


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Dendrites receive information/signal from the neuron before it and pass it through the soma. The location before the first myelin sheath is called the axon hillock; this will decide whether the information should be passed on to the next cell. If the information gets passed then it moves through the axon to the terminal to another neuron through the synapse/axon terminal.

The neuron that sends the information is called the Presynaptic Neuron and the neuron that receives that information is called the Postsynaptic Neuron. The junction at which both of those neurons communicate is called the Synapse[45].

The Schwann cells produce the myelin sheath which are long flat structure that wrap around the axon; they insulates the axon, increasing also the signal velocity. The gaps between the Myelin sheaths are called the Node of Ranvier.

The myelin sheath increases the speed of transmissions across the axon and provides insulation for protection. The main purpose of myelin is to cover neuron cells so that it can conduct action potential (transferring signals) more quickly. Also myelin is composed of lipids (fat) which makes its conductance very low or not at all. 10% of the brain is fat, this is because many of the brain's nerve fibres are wrapped in a fatty sheath. This fatty sheath, called myelin, is vital as it insulates the nerves.

The neurons have pumps that pump positive ions[46] outside the neuron in order to switch it off so the net charge of the neuron on the inside of the nucleus will be negative.

When the synapse sends chemical (neurotransmitter) signals and hits a receptor at the dendrite, that receptor will open up a channel/pump. This channel will let the positive ions inside the neuron which results in a change of net charge. When enough positive cells are at the axon hillock it will fire the information to the next neuron. A so called action potential[47] will be fired if there are enough positive ions at the axon hillock.


Resting and Action Potential

Contents will follow....

The Structure of the Nervous System

Contents will follow....

Basic Neuroanatomy

Contents will follow.... for now here's Pinky with the Brain

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