Virtual Medical Centre

Human Brain

• What is the brain?
• The Hemispheres of the Brain which are divided into:
  • The Frontal Lobe
  • The Parietal Lobe
  • The Temporal Lobe
  • The Occipital Lobe
  • The Limbic Lobe and
  • The Basal Ganglia.

Purpose of the Brain

The brain can be considered the most complicated computer in the world. Saying this though, doesn't quite do justice to just how complicated and amazing the brain is. In one sense the brain does act as a computer - it receives information from inputs such as the eyes and ears, and interprets this information. It also uses this information as well as other, more mysterious processes, to control the movement of the various parts of the body. The way in which it does this is as fascinating as it is complicated, with the brain being clearly divided into different areas, each having a slightly different function. As each part of the brain has a slightly different function, the loss of these areas through traumatic damage or a stroke can lead to a wide variety of outcomes. Functions such as vision, movement, hearing and the other senses are actually understood in a bit of detail, however there is another side to the brain that is far more unknown, and people have still not been able to work out the intricacies of it. Emotions, thoughts, memories and what drives someone to do something, things which some people may say are the very essence of what it is to be human, are still complete mysteries. Parts of the brain seem to be associated with this, but how it works is still unknown. The brain is made up of two types of 'matter', the grey matter and white matter. The grey matter is the thinking part of the brain - the brain cells. In the brain they lie over they surface as well as in several 'islands' called nuclei that are deeper. The surface coating of brain cells is known as the cortex. The other type of matter is white matter, which is composed of all the nerves of the brain - the connectors that allow the brain cells to communicate with each other. It is white because all of these nerves are coated with a substance called myelin that is quite fatty. So the brain is a coating of 'grey' cells around the outside that send their signalling 'white' nerves towards the centre. It should be noted that in life, the brain is actually pinkish.

Hemispheres

The cerebral hemispheres are the largest sections of the brain, and are the most obvious. Most of the visible brain is the hemispheres. Like the hemispheres of the world, the brain is divided into two. Unlike the world, the brain is divided into a left and right hemisphere, and these are almost completely separate. The two hemispheres are connected by a very thick area nerves (that can be thought of like signalling cables) called the corpus callosum. Without this connection, the two halves of the brain would be unable to communicate with each other. The surface of the brain is covered will all sorts of ridges and dips. These ridges are called 'gyri' and the dips are called 'sulci'. The purpose of this complex surface structure is because the surface of the brain is where all the actual brain cells reside; so the more surface that the brain has, the more complex calculations it is able to do. These gyri and sulci give the brain a far bigger surface area than if it was flat, much like crumpling a piece of paper lets it fit in a smaller area. The hemispheres are divided into other areas based on the function of the area.

Frontal Lobe

The frontal lobe is the largest of all the lobes and is composed of the area anterior to a deep sulcus (the 'dips' on the brain surface) called the central sulcus that runs horizontally across the brain. The frontal lobe is all the brain in front of this sulcus all the way to the front of the brain. Laterally, it stops at what is called the 'lateral sulcus' because it runs along the side of the brain. Below this sulcus is the temporal lobe. The frontal lobe is further divided into other, smaller sub-areas. In front of the central sulcus is a gyrus (the 'bumps') called the precentral gyrus, and in front of it are areas known as the superior, middle and inferior frontal gyri that are arranged as horizontal tiers. The inferior frontal gyrus can be further subdivided into three areas, the pars orbitalis (which lies over the eye), the pars triangularis (because it is triangular in shape) and the pars opercularis (which literally means 'lid' because it covers another part of the brain). The frontal lobe has several different functions, each assigned to a different general functional area:

1. The precentral gyrus is the 'primary motor cortex', which literally means the part of the brain that controls movement. The cells here each correspond to a part of the body, and control its movement. More details of this will be discussed later on.
2. The premotor and supplementary motor areas are located in front of the motor cortex. This area is responsible for the initiation of voluntary movements.
3. In the so-called 'dominant' hemisphere, there is an area of the brain known as Broca's area. This is found in the pars triangularis and opercularis of the frontal lobe. It is responsible for the motor control of language, with other parts of the brain sending it signals telling it what sounds need to be formed, and Broca's area will work out the mouth movements that are required. These signals will then be sent to the motor cortex so that it can actually move the muscles.
4. The prefrontal cortex makes up the rest of the frontal lobe and is responsible for some of the more mysterious human characteristics such as personality, insight and foresight.

Parietal Lobe

The parietal lobe extends posteriorly from the central sulcus to the parieto-occipital sulcus. Inferiorly it is bounded by the lateral sulcus, below which is the temporal lobe. The parietal lobe is further subdivided into the postcentral gyrus, which lies behind the central sulcus, and the superior and inferior parietal lobules. The functions of the parietal lobe are loosely divided into three general groups:

1. The postcentral gyrus is much like the precentral gyrus, in that each part of the gyrus corresponds to an area of the body; however rather than being a motor area, the postcentral gyrus processes sensory information such as touch and information about where the body is and which direction it is moving.
2. The inferior parietal lobule of the dominant hemisphere, together with some areas of the temporal lobe, are involved with the comprehension of language.
3. The remaining areas of the parietal lobe deal with very complex areas of human behaviour that are not very well understood including spatial orientation and perception.

Temporal Lobe

The region below the lateral sulcus is the temporal lobe, which extends posteriorly to the line connecting the top of the parietooccipital sulcus and the preoccipital notch. The temporal lobe can be further subdivided into superior, middle and inferior temporal gyri. Beneath the temporal lobe is a small area of cortex known as the insula. This area can be revealed by lifting the overlying areas of the frontal, parietal or temporal lobes. The temporal lobe's major functions include:

1. The superior part of the temporal lobe is home to the primary auditory cortex. This deals with the information coming in from the nerves of the ear and interprets the signal.
2. In the superior temporal gyrus of the dominant hemisphere is an area of the brain known as Wernicke's area. This receives information from the auditory cortex and tries to work out if any of the sounds heard are words. If they are, then it also has to work out what these words mean. Thus it is vital in the interpretation and comprehension of language.
3. Much of the temporal lobe is involved with higher-order interpretation and processing of visual information. This includes working out what certain shapes are, and what they are.
4. The medial parts of the temporal lobe are also important in learning and memory, but this is one area of neurological functioning that is still beyond current medical understanding.

Occipital Lobe

The occipital lobe lies most posteriorly in the brain, with its anterior, lateral and medial boundaries defined by the borders of the parietal and temporal lobes, which lie against it. The configuration of the gyri varies from person to person. It is almost entirely devoted to primary visual processing, but some areas deal more with the more complex interpretation of vision.

Limbic Lobe

The limbic lobe is an area of cortex that appears to lie around the junction between the cerebral hemispheres and the brain stem. It lies between the corpus callosum (which is the big connector between the two hemispheres) and the frontal, parietal and occipital lobes. The limbic lobe is very heavily connected to lots of other areas of the brain. It is important in letting us feel emotions, have drive to accomplish and do things, and is also very important in memory. Again, exactly how this happens is beyond current understanding.

Basal Ganglia

Within the cerebral hemispheres are areas of massed grey matter (brain cells) called the cerebral nuclei. The basal ganglia consist of several nuclei within each hemisphere, embedded in central white matter. Some of these structures include:

• The caudate nucleus, which while previously thought to have a role in motor function, appears to be involved with learning and memory.
• The lenticular nucleus is thought to be responsible for integrating information about movement and sensation and how they effect each other, as damage to these structures often accompanies movement disorders such Parkinson's disease, Huntington's disease and Tourette syndrome.
• The amygdala is another nucleus contained within the cerebral hemispheres and it, along with the hippocampus, are the major parts of the limbic system. The limbic system is vital for establishing emotion and behavioural drive, linking conscious intellectual functions of the cortex with unconscious functions, as well as facilitating memory storage and retrieval.