The human brain is an organ of great mystery. In addition to being unbelievably complex, it is very difficult to investigate experimentally. We now know that different areas of the brain have particular functions, but how did we find out? Some of the earliest clues came from accidents in which people suffered damage to a particular part of the brain.
Consider the tale of Phineas Gage, a US railroad worker. He was a popular and reliable man, polite and responsible, and he had been made a foreman. In September 1848, he was jamming a stick of dynamite into a hole using a tamping iron. A spark from the iron ignited the dynamite, and the metal rod came out of the hole a lot faster than it went in.
The rod entered Gage’s face below his left cheekbone, passing through the eyeball and through the top of his skull before landing several metres away. Gage fell back in a heap, as you would expect, but remarkably he did not die. He did not even pass out. He was driven by oxcart to a local physician, John Harlow. As the doctor stuck his fingers into the holes in Gage's head until the tips met, Gage asked when he would be ready to return to work. Within a couple of months he had recovered physically, but was no longer himself. Instead of being a gentle, honest, conscientious worker, Gage became ‘a foul-mouthed and ill-mannered liar’.
He lived on as something of a celebrity for another 13 years before dying from an epileptic fit. On hearing of the death, Harlow managed to get Gage's body donated to medical research. He believed that the changes in Gage's behaviour had been caused by the damage to the frontal lobes of the brain.
‘The equilibrium… between his intellectual faculties and animal propensities seems to have been destroyed’, Harlow wrote.
Some 130 years on, scientists were able to use computer modelling to trace the damage done to Gage’s brain. The metal rod missed the areas associated with language and motor function, but destroyed the ventromedial region on the left side of his frontal lobe. This is what made Gage so antisocial. People who have had tumours in this area of the brain have undergone the same sort of transformation.
WHAT IS A NERVOUS SYSTEM?
All organisms are sensitive to changes in their surroundings. Even the simplest single-celled organisms can detect and respond to stimuli. Some bacteria, for example, move towards areas of high oxygen concentration using their whip-like flagella, and an amoeba can ‘swim’ away from areas of high salt concentration.
In these examples, the single cell must act as both receptor and effector; the stimulus is detected and the response brought about in the same place. Receptors detect stimuli and effectors bring about a response. They are connected by nerve cells or neurones.
In multicellular organisms, different cells become specialized for different roles. Even in the simplest animals such as the cnidarians (hydra, jellyfish and sea anemones) there are neurones connecting receptors and effectors. These animals have a nerve net made up of interconnected neurones but there is nothing that looks remotely like a brain.
Though, not the most sophisticated of creatures, the jellyfish is one of the simplest organisms that is actually classed as an animal. Despite the fact that it has very few specialized sense organs and no head or brain to speak of, it does have a network of nerve cells (a nerve net) that allows responses and co-ordinated swimming movements. And at the other end of the invertebrate scale from the jellyfish is the Octopus. It is the most intelligent invertebrate. It has well-developed eyes and a brain that allows it to solve problems. An octopus can be made to work for its dinner, by giving it shrimps inside a Perspex box with a catch. The animal quickly learns to open the catch. In one recorded incident, an octopus climbed out of its tank, walked across a dry table and climbed into a tank that contained a crab – its favored prey. Having been out to dinner, the animal then went home again.
IF YOU WANT TO GET A HEAD, GET A BRAIN
There is a clear and predictable pattern in animal evolution; nervous systems become more complex and sophisticated. In the next ‘step up’ from the jellyfish stage, animals began to move in a particular direction, sense organs became concentrated around the mouth and mouth-parts at the anterior (front) end, and a distinct head developed. Behind the sense organs a mass of nerves developed to process all the incoming information. This process of ‘head development’ is called cephalisation.
In many invertebrates such as annelid worms and arthropods, the mass of neurons that develops behind the head is called a ganglion, and there can be many more ganglia along the length of the body. These ganglia are little more than nerve exchanges, where a particular stimulus leads to what is generally a fixed response. There is little capacity for learning or memory, though it would be wrong to think that the behaviour of these organisms is completely inflexible.
In the course of evolution, the ganglia became less numerous and congregated towards the head end. As these ganglia became more elaborate, and gained more control over the whole organism, they became what we call a brain. The annelid worms, molluscs and insects all have ganglia rather than a brain. It is well-documented ‘fact’ that if you cut off the head of some insects, e.g. cockroaches, they will continue to live until they starve to death. This doesn’t work with vertebrates – a physiological fact that the French demonstrated with great effect when they dismissed their monarchy in 1789.
Vertebrates generally have a more sophisticated nervous system than invertebrates, though there is some overlap: the octopus could be said to be more intelligent than the average fish. The brain of a fish is little more than a swelling at the front end of the spinal cord. Like all vertebrate brains, it is divided into three areas, the forebrain, midbrain and hindbrain. In fish, these are associated with the senses of smell, sight and balance, respectively.
THE HUMAN NERVOUS SYSTEM: GENERAL ORGANISATION
The human nervous system can be divided into two parts: the central nervous system (CNS) and the peripheral nervous system. The CNS consists of the spinal cord, which is protected by the vertebral column and the brain, which is enclosed within the bony shell of the cranium. The peripheral nervous system brings information from the sense organs into the CNS, and then relays information out to the structures that bring about responses: the muscles and glands. The peripheral nervous system is divided into the somatic and autonomic nervous systems. The somatic system is responsible mainly for controlling voluntary actions, such as walking and talking, while the autonomic system keeps the involuntary body functions such as heartbeat, blood pressure and breathing ticking over. The autonomic system is further divided into the sympathetic system and the parasympathetic system.
What is grey matter? Different areas of nerve tissue in the central nervous system are different colours. Grey matter contains nerve cell bodies; their nuclei are responsible for the grey colour. White matter consists largely of myelinated fibres (their fatty sheaths are creamy-white).
THE SPINAL CORD
The spinal cord starts at the base of the brain and ends at the first lumbar vertebra (this is roughly at waist level). The spinal cord is enclosed within the vertebral column and has a diameter of about 5 millimetres. Further protection is provided by three layers of tough membranes called spinal meninges, and by the cerebrospinal fluid that cushions the cord, acting as a shock-absorber. Each vertebra has an opening on its right and left sides to let spinal nerves pass through. These nerves extend into the body, forming the peripheral nervous system. Meningitis is a potentially fatal disease characterized by inflammation of the meninges – the protective membranes that surround the brain and spine.
In the model illustrating the basic structure of the spinal cord. The delicate nerve cord itself runs down the middle of a channel in the centre of the vertebra. Between each bony vertebral disc is a shock-absorbing intervertebral disc made of cartilage. When the outer layer of the disc breaks or ruptures, part of the cartilage can push against the nerve cord, causing pain, numbness and – in severe cases partial paralysis.
THE REFLEX ARC
A reflex arc is the simplest example of co-ordination, so is a good place to start a study of how the nervous system works. The key feature of a reflex is that a particular stimulus leads to a fixed response –this is very rapid and can’t be controlled because it does not pass through the conscious parts of the brain. An important feature of reflex arcs is that they contain as few synapses as possible. This speeds up the response and in many cases – such as the blinking reflex or the reaction to heat and pain – avoids danger or minimizes damage. Other reflex arcs are mainly to do with posture.
Let me remind you that nerve nets allow an animal to respond in a limited way. This is similar to reflexes because one stimulus invariably leads to the same response. In higher animals reflexes bypass the brain in order to speed up the response time. In the cnidaria there is no brain to bypass.
The knee-jerk reflex
The knee-jerk reflex is a classic example of a reflex arc. This is a postural reflex, one of the many mechanisms we have to maintain our position without having to constantly think about fine adjustments.
The knee-jerk reflex is initiated by the stretch receptor in the patellar tendon – a tap on this tendon just below the knee has the same effect as the knee bending. Nerve impulses pass up the sensory neuron and into the spine. Here the sensory neurons synapse with motor neurons and the nerve pulses pass straight out of the spine in the motor nerve, where they pass to the thigh muscle (the quadriceps). Contraction of the quadriceps straightens the leg. Impulses will also pass from the sensory nerves up the spine to the brain, but we are conscious of the stimulus only after the response has been initiated.
Most reflexes contain more synapses than the knee-jerk example. Blinking when a foreign object enters the eye and the withdrawal reflex, or pulling your hand away from a hot pan, both involve a circuit containing sensory receptors, sensory neurons, spinal relay neurons (interneurons), motor neurons and effector muscles. The principle is the same: the heat of the pan stimulates receptors, and impulses pass along sensory neurons towards the spinal cord. Here, instead of making synapses with motor neurons, they pass their signals on to relay neurons. These connect with motor neurons that cause muscles to contract, moving your hand away from the pan.
You know that you have touched the hot pan because some impulses travel to the brain, but the movement that is part of the reflex is involuntary, not under your conscious control. It is more difficult to persuade people that the swearing that accompanies such an event is also involuntary.
I will be pausing here till next time when I will be discussing extensively about the brain and its different areas.
Thanks for reading.