Prof. J.P.N. Mishra
Functions of the Cerebral Cortex: few areas of the cerebral cortex in each hemisphere are engaged predominantly in one particular function. Differences between genders, and among individuals of both genders, are often common. Many cerebral functions have a typical location and is termed as the concept of cerebral localization. The fact that localization of function varies from person to person, and even at different times in an individual when the brain is damaged, is called cerebral plasticity. The function of each region of the cerebral cortex depends on the structures with which it communicates. It is important to remember that no part of the brain functions alone. Many structures of the central nervous system must function together for any one part of the brain to function normally.
Sensory Functions of the Cortex: Various areas of the cerebral cortex works jointly for normal functioning of the somatic senses as well as the so-called "special senses". The somatic senses include sensations of touch, pressure, temperature, body position (proprioception), and similar perceptions that do not require complex sensory organs. The special senses include vision, hearing, and other types of perception (Fig 1-16) that require complex sensory organs, for example, the eye and the ear.
The postcentral gyrus serves as a primary area for the general somatic senses. In other words, the cortex contains a sort of "somatic sensory map" of the body. Areas such as the face and hand have a proportionally larger number of sensory receptors, so their part of the somatic sensory map is larger. Likewise, information regarding vision is mapped in the visual cortex, and auditory information is mapped in the primary auditory area.
The cortex does more than just register separate and simple sensations. Information sent to the primary sensory areas is in turn relayed to the various sensory association areas, as well as to other parts of the brain. There, the sensory information is compared and evaluated. Eventually, the cortex integrates separate bits of information into whole perceptions. Suppose something has pricked your finger, obviously you will get pain. But at the same time you world also know the object or the reason of pricking your finger because you would perceive the total impression of many other sensations such as shape and size of the object and direction and position of your hand or feet, whatever it may be.
Motor Functions of the Cortex: Voluntary movement control mechanisms are extremely complex. In case of normal movements of the body parts many parts of the nervous system-including certain areas of the cerebral cortex have to function promptly. The precentral gyrus, constitutes the primary somatic motor area. A secondary motor area lies in the gyrus immediately anterior to the precentral gyrus. Neurons in the precentral gyrus are said to control individual muscles, especially those that produce movements of wrist, hand, finger, ankle, foot, and toe. Neurons in the premotor area just anterior to the precentral gyrus are thought to activate groups of muscles simultaneously.
Integrative Functions of Cerebral Cortex: Few very important Integrative functions controlled by cerebral cortex consist of all events that take place in the cerebrum between its sensory impulses and its motor impulses. Integrative functions of the cerebrum include consciousness and mental activities of all kinds. Consciousness, use of language, emotions, and memory are the integrative cerebral functions.
Consciousness : Consciousness is a state of awareness of one's self, environment, and other things. Very little is known about the neural mechanisms that produce consciousness. Consciousness depends on excitation of cortical neurons by impulses conducted to them by a network of neurons known as the reticular activating system. The reticular activating system consists of centers in the brainstem's reticular formation that receive impulses from the spinal cord and relay them to the thalamus and from the thalamus to all parts of the cerebral cortex. Both direct spinal reticular tracts and collateral fibres from the specialized sensory tracts relay impulses over the reticular activating system (ARS) to the cortex. Without continual excitation of cortical neurons by reticular activating impulses, an individual is unconscious and cannot be aroused. That means that ARC functions as the arousal or alerting system for the cerebral cortex, and its functioning is crucial for maintaining consciousness.
Certain variations in state of consciousness are taken as normal. Most often we experience different levels of wakefulness. At times, we are highly alert and attentive. At other times, we are relaxed and no attentive. All of us also experience different levels of sleep. Two of the best known stages are those called non-rapid eye movement (NREM) sleep or slow-wave sleep (SWS) and rapid eye movement (REM) sleep. Slow-wave sleep takes its name from the slow frequency, high-voltage brain waves that identify it. It is almost entirely a dreamless sleep. Rapid eye movement sleep, on the other hand, is associated with dreaming. In addition to the various normal states of consciousness, altered states of consciousness also occur under certain conditions. Anesthetic drugs produce an altered state of consciousness, namely, anaesthesia. Disease or injury of the brain may produce an altered state called coma. Meditation is a waking state but differs markedly in certain respects from the usual waking state. According to some, meditation is a "higher" or "expanded" level of consciousness. This higher consciousness is accompanied, almost paradoxically, by a high degree of both relaxation and alertness. With training in meditation techniques and practice, an individual can enter the meditative state at will and remain in it for an extended period of time.
Speech: As such language is the ability to speak and write words and the ability to understand spoken and written words. Certain areas in the frontal, parietal, and temporal lobes serve as speech centers-as crucial areas, that is, for language functions. The left cerebral hemisphere contains these areas in about 90% of the population; in the remaining 10%, either the right hemisphere or both hemispheres contain them.
Emotions: Emotions are the net outcome of 'Limbic System' in the form of both subjective experience and objective expression. The specialized structures of limbic system form a curving border around the corpus callosum, the structure that connects the two cerebral hemispheres. Most of the structures of the limbic system lie on the medial surface of cerebrum. They are the cingulated gyrus and the hippocampus (the extension of the hippocampal gyrus that protrudes into the floor of the inferior horn of the lateral ventricle). These limbic system structures have primary connections with various other parts of the brain, notably the thalamus, fornix, septal nucleus, amygdaloidal nucleus (the trip of the caudate nucleus, one of the cerebral nuclei), and the hypothalamus. Some physiologists, therefore, include these connected structures as parts of the limbic system.
The limbic system, commonly known as emotional brain, works in a way to make us experience many kinds of emotions-like anger, fear, sexual feelings, pleasure, and sorrow, etc. To bring about the normal expression of emotions, parts of the cerebral cortex other than the limbic system must also function. Considerable evidence are there to say that limbic activity without the modulating influence of the other cortical areas may bring on the attacks of abnormal, uncontrollable rage suffered periodically by some unfortunate individuals.
Memory: Memory is an important mental functions. The cerebral cortex is capable of storing and retrieving both short-term memory and long-term memory. Short-term memory involves the storage of information over a few seconds or minutes. Short-term memories can be somehow consolidated by the brain and stored as long-term memories that can be retrieved days-or even years-later.
It is an established fact that both short-term and long-term memory are functions of many parts of the cerebral cortex, especially of the temporal, parietal, and occipital lobes. Long-term memories are believed to consist of some kind of structural tracts-called engrams-in the cerebral cortex. Widely accepted today is the theory that an engram consists of some kind of permanent change in the synapses in a specific circuit of neurons. Repeated impulse conduction over a given neuronal circuit produces the synaptic change. What the change in still is a matter of speculation.
Few scientific studies indicate that the limbic system-the "emotional brain"-plays a key role in memory. When the hippocampus (a constituent part of the limbic system) is removed, the patient losses the ability to recall new information. Personal exhibits substances a relationship between emotion and memory.
Our brain receives many stimuli, but we are conscious of only a few of them. It has been estimated that of all the information that comes to our consciousness, only about 1 percent goes into long-term memory, and much of what goes into long-term memory is forgotten. It is fortunate that our brains select only a small percentage of our thoughts and lose much of what we stored; otherwise, our brain would be overwhelmed with information. It is a feature of memory that we can remember short lists easier than long ones. This is to say the obvious, but human memory does not record everything like an endless magnetic tape. It cannot remember or record long lists of details. Another feature of memory is that even when details are lost, the concept or main idea is retained. Then, interestingly, we can often explain the idea or concept-not like replaying a tape-but with our own selection of words and ways of explanation.
Exact mechanisms of memory is yet to be formulated. However, by integrating several clinical and experimental observations, scientists have developed few theories to understand the mechanisms, involved in the process of memory.
One theory of short-term memory states that memories may be caused by reverberating neuronal circuits-an incoming nerve impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from the second and third neurons synapse with the first, sending the impulse back through the circuit again and again. Once fired, the output signal may last from a few seconds to many hours, depending on the arrangement of neurons in the circuit. If this pattern is applied to short-term memory, an incoming thought-the phone number-continues in the brain even after the initial stimulus is gone. Thus, you can recall the thought only for as long as the reverberation continues.
It is also stated that the concept that short-term memory is somehow related to electrical and chemical events rather than structural changes in the brain. For example, several conditions may inhibit the electrical activity of the brain. These include anaesthesia, coma, electroconvulsive shock, and ischemia (reduced blood supply) of the brain. Although such states do interfere with the retention of recently acquired information, they do not usually interfere with long-term memory laid down prior to the interference with electrical activity.
Scientific Researched related to long-term memory mostly focus on anatomical or biochemical changes at synapses that might enhance facilitation at synapses. Anatomical changes occur in neurons when they are either stimulated or made in active. These include an increase in the number of presynaptic terminals, enlargement of synaptic end bulbs, and in increase in the branching patterns and conductance of dendrites. There may also be an increase in the number of neuroglia. Moreover, neurons grow new synaptic end bulbs with increasing age, presumably as a result of increased utilization. These changes, which are correlated with faster learning, suggest enhancement of facilitation at synapses. Such changes do not occur when neurons are inactive.
There is also interest in the possible involvement of nucleic acids in long-term memory. The molecules DNA and RNA store information, and these molecules, especially DNA, tend to persist for the lifetime of the cell. Studies have shown an increase in the RNA content of activated neurons. Conversely, there is some evidence that shows that long-term memory will not occur to any significant extent when RNA formation is inhibited.
A recent hypothesis concerning memory involves a series of chemical reactions in the brain that permanently alter connections between neurons in the brain. This alteration may be the basis for memory. Simply stated, when a nerve impulse reaches a synapse, a neurotransmitter is released which can fit into a receptor on the postsynaptic neuron. This interaction opens calcium channels in the membrane of the postsynaptic neuron, causing the influx of the ions. Inside the postsynaptic neuron, calcium ions activate an enzyme called calpain that breaks down the cytoskeleton, changing the shape of the postsynaptic neuron and somehow making it more sensitive to subsequent transmission of information and thus contributing to the formation of a memory. Such synapses might form the first link of a memory trace.
Another recent hypothesis involves neurons that have two different ways of transmitting information. Such neurons contain receptors called NMDA receptors, named after the chemical N-methyl D-aspartate that is used to detect them. These receptors permit calcium influx into neurons. All other receptors involved in neuronal firing respond to either neurotransmitters or a change in voltage. NMDA receptors are unique in that they must first be stimulated by a voltage change and then by a neurotransmitter (glutamic acid). Such neurons have a special mode of signal transmission that is activated only if the cell receives the signals in a row-the first signal cocks the gun; the second signal fires it. This unique type of signal transmission provides the neurons with a different way to process information related to memory formation (Saladin, 2004).
Special features of Cerebral Hemispheres: The right and left hemispheres of the cerebrum are specialized to execute different functions. Left hemisphere specializes in language functions-it does the talking, so to speak. The left hemisphere also appears to dominate the control of certain kinds of hand movements, notably skilled and gesturing movements. Most people use their right hands for performing skilled movements, and the left side of the cerebrum controls the muscles on the right side that execute these movements. Right hemisphere of the cerebrum specializes in certain other functions the one being of auditory stimuli. Right hemisphere perceives no speech sounds such as melodies, coughing, crying, and laughing better than the left hemisphere. The right hemisphere may also function better at tactual perception and for perceiving and visualizing spatial relationships. Despite the specializations of each cerebral hemisphere, both sides of a normal person's brain communicate with each other via the corpus caliosum to accomplish the many complex functions of the brain (Tortora at. el., 2006).
Basal Nuclei: The basal nuclei constitute several masses of gray matter that lie deep within the white matter of the cerebrum. The precise function of the basal nuclei is not known, but they are a part of the limbic system discussed next. Also they must have some control over voluntary muscle action because when they are diseased, Parkinson disease may develop and there may be spastic movement of limbs.
Limbic System: The limbic system involves portions of both the unconscious and conscious brain. It lies just beneath the cerebral cortex and contains neural pathways that connect portions of the frontal lobes, the temporal lobes, the thalamus, and the hypothalamus.
Stimulation of different areas of the limbic system causes the subject to experience rage, pain, pleasure, or sorrow. By causing pleasant or unpleasant feelings about experiences, the limbic system apparently guides the individual into behavior that is likely to increase the chance of survival.
The limbic system is also involved in learning and memory. Learning requires memory, and memory is stored in the sensory regions of the cerebrum, but just what permits memory development is not definitely known. The involvement of the limbic system in memory explains why emotionally charged events result in our most vivid memories. The fact that the limbic system communicates with the sensory areas for touch, smell, vision, and so forth accounts for the ability of any particular sensory stimulus to awaken a complex memory (Saladin, 2004).
