After reading this article you will learn about the role of central nervous system in behaviour of an individual.
The most elementary form of behaviour (reflex action) is coordinated and organised by the spinal cord which is a part of the central nervous system. Such behaviour is automatic and occurs without the awareness of the organism. But there are some other types of reflex actions or reflex responses exhibited by organisms which are acquired through experiences and are known as conditioned reflexes.
Conditioned reflexes, however, are coordinated and maintained by the cerebral cortex which is the highest centre of the nervous system. In fact, all complex actions which involve coordination and organisation of the activities of different parts of the body and which depend on learning are organised and maintained by the higher centres of the central nervous system. It also controls learning, memory, emotional responses, thinking and even complex physiological activities.
This takes us to a consideration of the structure of the central nervous system. It broadly includes the brain and the spinal cord. The brain, in turn, includes the cerebrum or the new brain, the cerebellum, the thalamus and the medulla oblongata or the brain-stem. The cerebrum is known as the new brain because biologically it is the last to evolve and at the human level it is the most important part of the brain.
The other parts are more or less well developed even at the level of the lower animals. Different parts of the central nervous system are responsible for different functions. But at the same time, they function in an integrated manner in complex human actions. Further, the different parts of the brain work in the form of coordinated functional systems rather than as individual parts.
We may briefly consider each one of these parts:
1. The Spinal Cord:
This refers to the long, thread-like collection of nerve fibres which runs along the full length of the body from the back of the brain through the entire spine. A cross-section of the spinal cord shows that its outer or peripheral regions are made of white fibres and the central regions of grey fibres. From the spinal cord nerves run to all parts of the body. These are called the spinal nerves and are of two types-receptors and effectors. The spinal cord is essentially concerned with the maintenance of reflex actions.
2. The Medulla or the Brain Stem:
It is that part of the central nervous system which connects the spinal cord and the spinal system with the higher regions of the body. The medulla essentially controls the process of the transmission of nerve impulses from the lower regions to the other parts of the brain. In addition, it also controls the activities of the autonomic nervous system which is concerned with the vegetative and emergency functions.
3. The Thalamus and the Hypothalamus:
These are centres located above the medulla and below the brain. Together they constitute the thalamic system.
The essential function of the thalamic system is that of relaying nerve messages to the higher and the lower centres. It is also actively involved in the functioning of the endocrine glands. In recent years there has been an increasing awareness of the crucial role of the thalamus and hypothalamus in emotional activities.
4. The Cerebellum:
The cerebellum sometimes also known as the small brain is another important centre of the brain. The cerebellum is mainly concerned with the maintenance of body equilibrium, postural sense etc. In addition, it is involved in a number of other activities as a coordinating agency. In lower organisms the cerebellum plays a much more crucial role than in the higher organisms.
5. The Cerebrum:
It is the highest and the most complex part of the nervous system. It is the highest organising, coordinating and maintaining agency. It is made up of two parts known as the cerebral hemispheres , the left hemisphere and the right hemisphere. The importance of having two hemispheres for behaviour has been brought out by a number of scientists. It has been shown that in some people the right hemisphere is more active and in others the left hemisphere.
This type of differences in dominance is presumed to account for difference in behaviour among individuals and also of cultures. Ralph Ornstein has stressed the importance of this point. The upper part of the cerebrum known as the cerebral cortex is not formed in a regular and smooth manner, but is folded.
The outer layer of the cerebrum or the cerebral cortex is grey in colour while the inner part is white. This is in contrast to the spinal cord which is white outside and greys inside. The cerebrum, in addition to being divided into two hemispheres, is also divided into different lobes.
The back portion known as the occipital lobe is primarily concerned with visual activity. The temporal lobe in the side is connected with activities like memory, speech, learning etc. The frontal lobe, towards the front, is connected with learning, thinking, consciousness and all complex forms of activity.
The parietal lobe, in the centre is connected with body sensations and other forms of responses. These lobes in turn are interconnected by a series of nerve fibres and structures. Because of these interconnections the brain invariably functions as a coordinated entity in the form of systems connected with different functional activities.
The cerebral cortex is the most important part of the central nervous system. The nature of its functioning is yet to be understood completely. A number of attempts have been made to study its role in different activities. Such attempts have been known as studies in cortical localisation.
At one time it was believed that different specific parts of the cerebral cortex are connected with specific activities. But today we know that such a simplified concept is not correct. While certain parts of the cerebral cortex may be more involved in specific/activities, the cerebral cortex does not function in a compartmentalized manner. The issue of localisation of different functions in the cerebral cortex is discussed in greater detail below. This is followed by a discussion of two important functional systems of the nervous system.
These systems are functional because they involve the coordinated activity of different parts of the nervous system. The two systems which are considered here are the reticular activating system and the limbic system. It is, therefore, obvious that the central nervous system is primarily responsible for the behaviour of the individual. It acts in a direct manner and also in an indirect manner by controlling other systems. Its activities are complex and intricate.
While different parts of the central nervous system may be concerned to a greater or a lesser extent with different activities, nevertheless the brain functions as an organised whole. The more complex the level of activity, the greater is the involvement of the different parts of the brain. In fact, except in the case of the simple spinal reflexes, in almost all other activities the entire brain or for that matter the entire central nervous system is involved.
This phenomenon was revealed clearly by the brilliant experiments of Lashley who, based on his experiments, formulated and demonstrated two basic principles. The first, known as the principle of ‘mass action’, states that the brain functions as a total entity and not as specific units.
The functional effectiveness of the brain depends on the total amount of the brain which is healthy and normal. Lashley’s findings were based on experiments wherein he removed different amounts of the brain area and studied the behaviour of animals. It was shown that the larger the amount of the brain destroyed the greater was the impairment of behaviour.
The other principle formulated by Lashley known as the principle of ‘equipotentiality’ states that if a particular part of the brain was injured or destroyed, its functions could be taken over by other parts which are intact. This shows that if a particular activity is lost because of the injury or damage to a particular region of the brain, the organism can recover these functions with the other parts of the brain taking over the functions of the injured parts.
Lashley’s findings introduced a total change in the approach to the study of the functioning of the brain. The earlier assumption that the brain was a combination of different parts responsible for specific function independently gave place to the view that while specific parts of the brain may be responsible for specific functions, in the ultimate analysis the brain functions as a whole characterised by interdependence, inter-relationship and coordination among the parts, as a totality.
Lashley’s findings originally based on experimentation with rats, have been to a large extent, confirmed by evidence from human behaviour and the study of patients with brain injuries and of those who have been subjected to brain surgery. At the same time more detailed knowledge about localisation of specific functions has also become available through the work of neurologists and neurosurgeons like Penfield.
Localisation of Functions in the Cortex:
A number of findings have been arrived at regarding the functioning of the different parts of the cerebral cortex, even though the cortex generally functions as an integrated coordinating system. The cerebral cortex consists of two hemispheres.
Each hemisphere, in turn, consists of four lobes, the frontal, the parietal, the temporal and the occipital. We may now go briefly into some aspects of the specialisation of functions of these hemispheres and the four lobes. Such advances in knowledge were made possible because scientists were able to develop a number of ingenious techniques of studying cortical functioning.
Some of the methods used are extirpation, electrical stimulation, chemical stimulation, and observation of brain injured subjects. The method of extirpation is a surgical method by which different parts of the cortex are carefully removed and the impact of this on the behaviour of the organism is studied.
The method of electrical stimulation involves selective stimulation of well-defined areas of the cerebral cortex by passing different intensities of electrical current. The method of chemical stimulation achieves the same end by injecting different chemical substances.
Outstanding contributions in the field of cortical research came through advances in the field of neurosurgery like lobectomy and leucotomy. Neurosurgery today has made tremendous advances. A very important aid in this context was the discovery of the EEG. The EEG stands for electroencephalograph. This instrument makes it possible to obtain a graph or picture of the working of the cerebral cortex under different conditions. The arrival of the EEG made it possible to study the patterns of cortical activity even in normal people.
Cerebral Lateralization:
It has been shown that the two hemispheres, the right and left, specialise in different functions. Further, the left hemisphere controls the functioning of the right side of the body and vice versa. In most people the left hemisphere has been found to dominate in functions related to language, thinking and other analytic symbolic activities, thus, showing, for example, the predominant right-handedness in all human populations.
On the other hand, the right hemisphere shows greater operative efficiency in global undifferentiated perception and emotional behaviour. This functional differentiation between the two hemispheres is not, however, absolute. The evidence for this type of functional differentiation has come through the works of Sperry and Hoppe. Reference has already been made to the views of Ornstein, classifying people and cultures into left-hemisphere dominant and right-hemisphere dominant.
Localisation of Functions in the Different Lobes:
Considerably more evidence is available on the localisation of different functions in the different lobes of the cerebral cortex than on the issue of cortical lateralization.
We may briefly examine this:
1. Frontal Lobe:
It is situated in the front of central fissure. In fact it is the front part of our brain. When compared to lower organisms, the frontal lobe is much more developed in man and has often been said to make for all the difference between man and animal.
Some of the functions attributed to this lobe are:
(a) It initiates volitional or voluntary movements. A peculiar feature here is that the left frontal lobe is concerned with the functioning of the right side of the body and vice-versa. The frontal lobe organises, regulates and controls voluntary movements in all complex activities like locomotion, eating, speaking, eye movements etc.
(b) The frontal lobe governs all higher intellectual processes like judgement, reasoning, abstract thinking, attention, concentration and other related activities.
(c) It integrates, elaborates and initiates psychomotor activity.
(d) It inhibits and controls autonomic and emotional responses.
Injury to the frontal lobe results in impairment of speech, intellectual functioning, emotional control, and in short, the total personality.
2. Parietal Lobe:
It is located behind the central fissure on the left and right sides of the brain. As in the case of the frontal lobe the parietal lobe on one side is concerned with the bodily activities of the opposite side.
Its main functions are as follows:
(a) It discriminates, appreciates, localizes, integrates and elaborates sensory stimuli arising from the body.
(b) It integrates visual and auditory stimuli with body stimuli and orientation.
(c) It coordinates spatial orientation, organisation of spatial perception etc.
Injuries to the parietal lobe have been found to result in spatial disorientation, right-left disorientation and visual field defects.
3. Temporal Lobe:
This lobe is located behind and below the Sylvain fissure. The temporal lobe on each side is again connected to the opposite side of the body.
The functions of this lobe are as follows:
(a) It stores up symbolic sounds or words in the memory.
(b) It differentiates and interprets sounds as meaningful words.
(c) It forms and stores visual and verbal memory.
(d) It directs functioning of gustatory and olfactory organs.
(e) It controls the storage and revival of memory.
Injury or damage to the temporal lobe has been found to result in loss of memory, loss of temporal sequence in audition and speech and even psychological blindness.
4. Occipital Lobe:
This lobe is located in the posterior tip of the cerebrum.
It is primarily concerned with visual function. Its main functions are:
(a) Reception of visual impulses from the eye.
(b) Perception and interpretation of such visual impulses.
(c) Visual recall, visual orientation and association.
Injuries to the occipital lobe cause impaired visual discrimination, loss of ability to recognise colour and form, illusions and hallucinations. In the above paragraphs an attempt has been made to present some major findings regarding the specialised functions of the different lobes of the cerebral cortex. It is once again evident from what has been said above that though different functions are located in the different lobes, complex human behaviour involves integrated and coordinated functioning of all the lobes.