Getting our heads around the brain

Key text

What is it about the human brain that gives us the edge? Neuroscientists (scientists who study the brain and nervous system) and philosophers have learned plenty about the functioning of the brain. But they admit there are aspects of brainpower that remain among humanity's most enduring mysteries.

An introduction to the brain

The basic facts about the brain are well known. Weighing in at around 1.3 kilograms, it is one of the largest organs in the human body. It is nothing remarkable to look at – a wrinkled object about the size of a number 13 chicken – but it consists of a complex and apparently hopelessly tangled mass of nerve cells, or neurons. It sits inside the skull immersed in a fluid that cushions it from sudden impacts to the head.

Neurons are the basic unit that makes up the brain and nervous system. They are specialised cells that act like telegraph wires carrying messages in the form of electrochemical impulses throughout the body. These impulses travel very quickly, although not as quickly as an electric current would travel: it takes about one hundredth of a second for a pain in your little toe to register in your brain. This is quite remarkable, given that the impulse travels a complex path through many neurons and across the gaps (synapses) between neurons to reach its destination (Box 1: The human nervous system).

Brain functions

The brain performs a number of functions, many of which are related to the physical needs and actions of the body. For these functions, the brain can be thought of as the command centre of the human nervous system, much like the headquarters of a military unit. It receives information from its vast network of neurons throughout the body. Based on this information, it makes decisions and issues commands that stimulate muscles and give the body movement.

Other brain functions are more like those of a university than a military headquarters. These functions give us the ability to read, write, talk and think about issues more broad than where the next meal is coming from.

Structure of the brain – an overview

The brain is shaped like two fists standing side by side on a single wrist. The 'wrist' is the brainstem, connecting the brain to the spinal column, and the 'fists' constitute the left and right hemispheres of the largest part of the brain, the cerebrum. At the back of the brain, below the cerebrum, is the cerebellum: its main function is to synchronise the muscles of the body.

The cerebral cortex: Control centre

The cerebrum has an outer layer of grey matter arranged in folds. This layer, the cerebral cortex, is just a few millimetres thick but because of its numerous folds constitutes 40 per cent of the entire brain mass.

Different areas of the cerebral cortex play specific roles in human thought and activity. For example: the frontal lobes control behaviour, intellect and emotion; the speech area controls talking; specific sections of the motor area control voluntary muscles in different parts of the body, and so on.

In general, the right side of the brain controls movement in the left side of the body and the left side controls the right. However, there is some specialisation. For example, language is more a function of the left hemisphere and recognition of shapes is more a function of the right (Box 2: Functions of the left and right sides of the brain).

Humans have large brains

When body weights are taken into account, the brain is much larger in mammals than in other vertebrates and reaches its greatest size in monkeys, apes and humans.

The unusual size of the cerebral cortex in the human brain may partly explain its unique abilities. If the cerebral cortex of a frog is damaged or destroyed, there is no obvious change in the behaviour of the animal. A rat without a cerebral cortex can still move about. Human beings, though, are totally paralysed and unable to see, although internal functioning continues.

More to learn

It is not so long ago that the only way scientists could study the human brain was to dissect it after a person died. Now scientists and clinicians have access to several imaging techniques that open a window on the living, conscious brain. These techniques are powerful tools for research into normal brain function and for locating tumours or blocked blood vessels in the brain (Box 3: Brain imaging).

Australian research

Australian neuroscience boasts a Nobel laureate – the late Sir John Eccles. He was awarded the prize in 1963 for research that explained how impulses were transmitted between neurons. Eccles was also the first to record electrical signals from the interior of neurons within the central nervous system.

Australian researchers have continued to be leaders in the field of neuroscience, particularly in the area of neurotransmitters, the chemical messengers that convey impulses between neurons. Their studies have led to advances in the treatment of neurological diseases and may lead to the development of drugs that help improve memory (Box 4: Neurotransmitters and drugs).

Box 1. The human nervous system

The nerves of the body are organised into systems. The central nervous system consists of the brain and spinal cord. The peripheral nervous system is a vast network of nerves that extend to all parts of the body, linking with the spinal cord through 31 pairs of spinal nerves. The two systems function together, with nerves from the periphery entering and becoming part of the central nervous system, and vice versa.

There are three kinds of neuron in the peripheral nervous system: sensory, motor and autonomic. Sensory neurons are responsible for bringing information about changes inside and outside the body to the central nervous system. Sometimes the spinal cord can make decisions without any need to consult with the brain – the 'knee-jerk' reaction caused by a doctor tapping the tendon that connects the kneecap to the shin bone is a classic example of this. More complex information needs to be interpreted by the brain, which then issues instructions via motor neurons to skeletal muscles for appropriate action.

Internal organs, such as the heart, lungs, gut and glands, are not under conscious control. The neurons that serve these organs form the autonomic, or involuntary, nervous system. This system is a part of the peripheral nervous system.

Structure of neurons

Neurons vary so much in shape that it isn't possible to describe a 'typical' one, but they do have three major features in common. Each has a cell body containing a nucleus and an extension, the axon, which transmits nerve impulses to other cells. The third major feature of neurons are one or more (usually numerous) fine, branching extensions called dendrites. They receive nerve impulses from other cells.

Neurons are connected to other cells

If you step on something sharp, you normally withdraw your foot – and straighten your other leg to maintain your balance. Neurons in the affected foot must therefore be connected to neurons in both legs.

In very simple connections in the nervous system, a single string of neurons is arranged end to end, with the axon of one ending on a dendrite of the next. Usually the connections are more complicated than this: a single neuron may have as many as 20,000 connections to other neurons. Not all of the connections in the nervous system are between two neurons – they can also connect to muscles or glands.

Transmission of a nerve impulse

When activated, neurons transmit a wave of electrochemical change. This wave of change is called an impulse. The starting point of an impulse could be a sense organ such as the skin, an eye, an ear, the nose or the tongue, or it could be at a dendrite that has received a message from another neuron. When a neuron is stimulated, it transmits the impulse electrically along its axon. At the end of the axon the impulse travels across a tiny gap, called a synapse, to another neuron (or to a gland or a muscle) by means of special chemical messengers called neurotransmitters.

The neurotransmitters affect the next cell in one of two ways: they either 'excite' it, so that it will send the impulse to the next cell, or they 'inhibit' it. The neurotransmitter molecules either break down or are reabsorbed after they have delivered their 'message'.

The electrical transmission of a nerve impulse is basically the same in all instances. But at the junction between cells, the chemical transmission of the impulse provides the capacity for differentiating between messages.

Related sites

• Overview of the nervous system (Neurolab Online, NASA)

• Nerves (Wayne State University, USA)

• Neurons and the nervous system (Eastern Kentucky University, USA)

Box 2. Functions of the left and right sides of the brain

These studies have shown that the left side of the brain controls most analytic functions, speech and language; the right side controls artistic attributes and the ability to recognise patterns (such as how rooms and corridors, houses and streets, and hills and valleys are related in space). The two halves of the brain communicate with each other – nerve messages are sent between the across a thick band of nerve fibres called the corpus callosum.

Other studies have determined that two areas of the left hemisphere of the cerebral cortex govern speech and language. One area is responsible for vocabulary and grammar, while the other governs the physical mechanism of speech.

Related sites

• Consciousness and will in the brain (ABC radio's Ockham's Razor, 10 January 1999)

• Is it true that creativity resides in the right hemisphere of the brain? (Scientific American, Ask the experts)

• Brain plasticity (ABC radio's The Health Report, 26 January 1998)

Box 3. Brain imaging

Other, more sophisticated, imaging techniques were spawned by the computer revolution of the 1970s. In computerised axial tomography (CAT scans), the brain is X-rayed from a variety of angles. A computer combines the results of the different X-rays to produce a cross-sectional image.

In positron emission tomography (PET), a radioisotope that emits positrons (similar to electrons, but with a positive charge) is injected into the bloodstream. A scanner can then detect the location of the radioisotope in the body.

This technique can be used to determine regions of brain activity. Sugar, an energy source for cells, is 'labelled' with a radioisotope and injected into the blood stream. A PET scan will show those regions of the brain containing more radioisotopes – these will be the ones that used more sugar because they were more active.

Magnetic resonance imaging (MRI) is a technology that is often used to diagnose damage to tissue, including brain tissue. This technique forms images by detecting protons which respond to a magnetic field. MRI helps detect active areas of the brain by identifying the location of oxygen-rich blood. There are several advantages to MRI, including that it requires no injection of material into the body and no radioactive substances are used.

Diagnosing diseased brains

Not all imaging techniques pick up all neurological diseases, so the technology used for the search has to match the target.

• Alzheimer's disease, in which memory is affected by the degeneration of neurons in the temporal lobes of the cerebral cortex, shows up in PET scans but not in MRI.
• Brain tumours show up on MRI, but PET scans are needed to determine whether they are malignant.
• Epilepsy, which affects 1 per cent of the population, is caused when a large collection of neurons 'fire' at the same time causing a seizure. During seizures, severe epileptics experience a range of symptoms including involuntary movements, hallucinations, and emotional changes. They might also have feelings of fear, anger, paranoia and deja vu. Both PET scans and MRI can often detect epilepsy by registering reductions in activity in the parts of the brain that are affected by seizures.
• Parkinson's disease, the symptoms of which include involuntary tremors and rigidity, is caused by dysfunction of neurons in the middle part of the brain, and is diagnosed from PET scans.
• MRI is used to diagnose stroke. A stroke occurs when a blood vessel supplying a given part of the brain becomes blocked. The functioning of that portion of the brain affected is impaired: muscles controlled by that region, for instance, may no longer function. Depending on the region affected, a stroke may be fatal.

Related sites

• Brain imaging (ABC radio's Health Report, 15 June 1998)

• Brain briefings: Brain imaging (Society of Neuroscience, USA)

• Brain tumor basics (American Brain Tumor Association)

• The three Moirai – Parkinson's and Huntington's diseases and Tourette's syndrome (ABC radio's Ockham's Razor, 25 May 1997)

Box 4. Neurotransmitters and drugs

What neurotransmitters do

Neurotransmitters are central to memory, learning, mood, behaviour, sleep, pain perception and sexual urge. They operate at the junctions between neurons, allowing communication between cells. When a nerve impulse arrives at the end of an axon, neurotransmitters are released, diffusing across a tiny gap to the next neuron. Here they bind to receptors – proteins on the surface of the cell – as a key fits into a lock. On delivery of their 'messages' these chemical couriers are destroyed or reabsorbed by the nerve endings in which they were produced.

Different neurotransmitters operate at different parts of the nervous system, and have different effects. Some promote the transmission of impulses while others inhibit it.

Involuntary nervous system neurotransmitters

Australian researchers played a major role in investigations into the neurotransmitters of the involuntary (or autonomic) nervous system which controls the gastrointestinal, cardiovascular, respiratory, excretory and endocrine system. The existing theory held that only two neurotransmitters, acetylcholine and nor-adrenalin, were involved in the control of internal organs. Max Bennett of Sydney University detected nerves that did not release either of these substances. Since there must be a chemical signal to relay the nerve impulse between adjacent neurons, this discovery started a race to identify the other transmitters involved.

More neurotransmitters are being found

Scientists have so far found hundreds of neurotransmitters, and the list is still growing. Neurotransmitters have an important role in the normal functioning of an individual. Research on neurotransmitters has brought greater understanding of some psychological diseases and this has led to more successful treatments. For example, we now know that manic depressive syndrome is a result of an imbalance in neurotransmitters, and we can correct the imbalance with drugs.

Specific neurotransmitters and their effects

The neurotransmitter serotonin plays a major role in emotions and judgement, and also sleep. Depression, suicidal behaviour, anxiety, impulsive behaviour and even eating disorders have been linked to serotonin imbalances. Recent research in Finland has suggested that murderers have very low levels of serotonin. Serotonin re-uptake inhibitors – a class of drugs including the well-known anti-depressant, Prozac – act by preventing the reabsorption of serotonin by the nerve endings. Illicit drugs including cannabis, Ecstasy and lysergic acid (LSD) also act on serotonin levels, producing feelings of euphoria.

The amino acids glutamate and GABA (gamma-aminobutyric acid) are the brain's most widespread neurotransmitters. They are involved in most facets of brain function, ranging from memory to sleep. They are also implicated in anxiety, and are the targets of drugs such as Valium and Mogadon.

David Curtis (John Curtin School of Medical Research) and his colleagues were the first to establish that these amino acids were neurotransmitters in the mammalian spinal cord and brain. They showed that GABA (and another amino acid, glycine) were the brain's major inhibitory transmitters whereas glutamate was the major excitatory transmitter. This work has had profound implications for the understanding and treatment of neurological disorders such as epilepsy and certain forms of spasticity.

Sydney University neuropharmacologist Graham Johnston has discovered a new class of GABA receptors involved in memory. He is designing a drug to stimulate the receptors, making them more responsive to GABA molecules. It is hoped that the drug will benefit Alzheimer's sufferers.

Meanwhile, Fred Mendelsohn, of the Howard Florey Institute of Medical Research in Melbourne, has discovered another brain chemical involved in memory. The chemical, a short chain of amino acids called a peptide, is either a neurotransmitter or a modulator, a substance that interacts with a neurotransmitter.

Imbalances in another neurotransmitter, dopamine, are implicated in Parkinson's disease. Dopamine's normal function is in regulating mood and movement. It is also involved in memory and schizophrenia.

Endorphins are neurotransmitters that relieve pain and induce euphoria. Athletes and gym junkies get a 'fix' of endorphins from excessive exercise. In the 1970s American scientists studying opium addiction discovered that morphine molecules lock into specific receptors in the brain. Endorphins, the brain's own morphine-like molecules, lock into these same sites.

Brain chemicals

One of the most recent finds is of a brain chemical aptly named anandamide after 'ananda', the Sanskrit word for bliss. Anandamide has a similar effect to tetrahydrocannabinol (THC), the active chemical in cannabis. THC locks into anandamide receptors in brain cells.

Scientists have recently discovered yet another natural brain chemical, nociceptin, which reduces anxiety. Mice injected with nociceptin become fearless, overcoming their terror of bright lights and open spaces.

Related sites

• Neurotransmitters and neuroactive peptides (Neuroscience for kids – University of Washington, USA)

• How do nerve cells communicate? (Society for Neuroscience, USA)

• Anandamide (Frostburg State University, Maryland, USA)

• Brain briefings: The opiate receptor (Society for Neuroscience, USA)

Activities

• Public Broadcasting Service (USA)
  • Probe the brain – students learn about the brain's motor cortex. Click on 'Probe the brain activity' for an on-screen demonstration showing which parts of the motor cortex control different parts of the body.
    (Note: The activity requires Shockwave.)

• NASA (USA)
  • Perception and action projects for classrooms – demonstrates perceptual illusions and behavioural adaptation.

• Access Excellence (USA)
  • Signal transmission in the nervous system – students time how long it takes for a hand squeeze to travel around a circle of people.

• Newton's Apple (USA)
  • Brain mapping
  • Brain – students build a model of a neuron.
  • Memory – students exercise observation and memory skills.

• Science upd8, UK
• Love on the brain – students investigate which areas of the brain are active when you are in love. The same areas are deactivated during depression and sadness.

Further reading

• Our tactile brain (by Mark Rowe)
• Extrovert or introvert? (by Yvonne Tran, Ashley Craig and Paul McIsaac)
• Splitting perceptions (by John Bradshaw)
• Sulfate's role in autism (by Daniel Markovich)
• Sweet key to learning (by Ruani Fernando)
• Does your brain rule your heart? (by Clive May)
• Treating brain disease and injury with adult neural stem cells (by Natalie Bull)
• When and where does schizophrenia affect the brain? (by Christos Pantelis)
• Banking on a cure for motor neuron disease (by Surindar Cheema)
• Clues to alcohol-seeking behaviour (by Andrew Lawrence)
• Brainstorming new epilepsy treatments (by Steven Petrou)

Useful sites

• The brain is the boss (The Nemours Foundation, USA)
  http://kidshealth.org/kid/body/brain_noSW.html

• Parts of the brain (American Brain Tumour Association, USA)
  http://www.abta.org/siteFiles/SitePages/E3C6ACECEEB3873E1722853CA6259C30.pdf

Australian Broadcasting Corporation

• Split brains and other heady tales (ABC Radio National, 21 June 2008)
  A discussion of the left and right hemispheres of the brain.
  http://www.abc.net.au/rn/allinthemind/stories/2008/2276587.htm

• Rebels and the cause – the adolescent brain (All in the Mind, 13 November 2004)
  Imaging technology is revealing that certain parts of the brain don't fully mature until well in to our twenties.
  http://www.abc.net.au/rn/science/mind/stories/s1240968.htm

NeurOn (Neurolab Online)

The NeurOn project is part of the NASA Neurolab mission which is studying neurological and behavioural changes in space. Click on 'Background information' for an overview of the nervous system and a glossary of terms. Click on 'Teachers' lounge' for links to activities.
http://quest.arc.nasa.gov/neuron

How your brain works (How Stuff Works, USA)

Examines the structures of the brain and what each structure does.
http://www.howstuffworks.com/brain.htm

Brain basics: know your brain (National Institute of Neurological Disorders and Stroke, USA)

This introduction to the brain includes the architecture of the brain, how signals are sent through nerve cells and an overview of some key neurotransmitters.
http://www.ninds.nih.gov/health_and_medical/pubs/brain_basics_know_your_brain.htm

Brain briefings (Society for Neuroscience, USA)

A series of short articles on neuroscience discoveries. The articles are organised under the following headings: Brain and nervous system disorders, Nervous system repair, The senses, Sleep, Technology, Development, Drugs, Emotions, Brain mechanisms.
http://www.sfn.org/briefings

Seeing, hearing and smelling the world (Howard Hughes Medical Institute, USA)

Covers the senses and nervous system.
http://www.hhmi.org/senses/

Glossary

electrode. An electrical conductor. Electrochemical reactions occur on the surface of an electrode.

An electrode can be used to deliver electricity to the body or to receive electricity from it. Delivering electricity to the body is used to stimulate; receiving electricity from the body can be used to detect and record signals. In either case the term refers to the contact formed by the stimulating or recording device within the body.

grey matter. The tissue of the nervous system that appears greyish because of the relatively high proportion of nerve cell nuclei that occur there. This is in contrast to white matter which consists mainly of axons. It appears whitish because of the insulating lipid-protein sheath around axons. Photomicrographs of grey matter and white matter can be found at a site from the Department of Anatomy and Cell Biology at the University of Kansas, USA.

neurotransmitter.A chemical substance, given off by the ends of the axon of a nerve cell or nerve fibre which allows a message to be passed between different links in the chain. It is the arrival of the electrical impulse at the end of the nerve fibre that causes the release of a neurotransmitter into the small gap (called the synapse) between nerve cells. The neurotransmitter travels across the synapse and excites or inhibits the next nerve cell in the chain.

radioisotope. A form of an element that spontaneously disintegrates into other substances and emits small particles (radiation or radioactivity). The presence and movement of the radioisotope in the body can be detected by monitoring the emission of the small particles.