Human nervous system is mainly divided into 3 divisions, which are as follows: 1. Central Nervous System 2. Peripheral Nervous System 3. Autonomic Nervous System.
The nervous system is mainly divided into central nervous system, peripheral nervous system and autonomic nervous system.
But some scientists have classified them into two divisions in which the ANS is included under peripheral nervous system category.
1. Central Nervous System:
Central nervous system consists of brain and spinal cord.
It is the highest and the most important nerve centre. It is situated in the skull cavity. It comprises of the cerebrum, cerebellum, thalamus, hypothalamus, pons and medulla oblongata.
It is the largest part of the brain filling the whole of the upper part of the skull. It is divided into left and right hemispheres by a longitudinal fissure. This is covered by a layer of gray matter called cerebral cortex and the inner part by white matter.
Below the central portion of the falx, the cerebral hemispheres are connected by a substantial bridge of white matter called Corpus callosum. The most prominent feature on the surface of each hemisphere is numerous folds called gyri, which greatly increase the surface area of the cerebral cortex.
It is estimated that if this folded surface is spread out, it can occupy about 670 square inches. In lower animals the cerebrum has no folds, whereas the folds begin to appear increasingly in higher animals.
The mental ability of an individual will be proportionate the folds of his cerebrum. It contains nerve centres which control the greater part of our behaviour.
All the important psychological processes like memory, imagination, thinking, reasoning, senses are all controlled by cortex. Figure. 2.3 shows the cross-section of brain and its parts.
Each cerebral hemisphere is divided into four lobes, named for the skull bones overlying them. The frontal lobe is important in voluntary motor functions, motivation, aggression, mood and olfactory reception. The parietal lobe is the centre for the reception and evaluation of most sensory information such as touch, pain, temperature, balances and taste. The frontal and parietal lobes are separated by an intervening groove called ‘central sulcus’ or ‘fissure of Rolando’.
The temporal lobe situated at the lateral sides receives and evaluates olfactory (smell) and auditory (hearing) input and plays an important role in memory. Its anterior and inferior portions are called as the ‘psychic cortex’, and they are associated with functions such as abstract thought and judgement.
This lobe is separated from the rest of the cerebrum by a lateral fissure called ‘lateral sulcus’ or ‘fissure of Sylvius’. Finally the occipital lobe, which functions in the reception and integration of visual sensory input is situated at the back portion of the skull.
As stated, in peripheral nervous system the nerves perform two types of functions – sensory and motor. The sensory nerves or the afferent nerves which carry the information from sense organs to the cortex and in turn, motor or efferent nerves carry the instruction from the brain to the muscles of organs in the body.
The sensory nerve tracts which carry the impulses project to specific regions of the cerebral cortex called Primary Sensory areas where sensations are perceived. Cortical areas immediately adjacent to the primary sensory centres are called ‘Association area or Association cortex’.
These portions of the cortex are involved in the process of recognition. For example, the optic nerve fibres from the retina of the eye reach the visual cortex where the image is perceived. These nerve fibres then pass from the visual cortex to the visual association cortex or area where the present visual information is compared with past visual experiences (seen before).
On the basis of this comparison, the visual association cortex decides whether the image is of a known person or not.
The other areas of association cortex include the auditory association area, adjacent to the primary auditory cortex and the somatic sensory association cortex adjacent to the primary somatic sensory cortex.
Earlier it was thought that association areas just connect sensory and motor pathways. Currently it is believed that the association cortex performs the function of processing and integration of sensory information leading to meaningful behaviour.
The primary motor cortex is located in the posterior portion of the frontal lobe, directly anterior to the central sulcus. Efferent nerves located in this area control voluntary movements of skeletal muscles.
These are connected in an upside down system – where the upper portion of the brain controls the lower portion of the body and the head by lower part of cortex. The motivation and the foresight to plan and initiate movements occur in the anterior portion of the frontal lobes called prefrontal area.
This is a region of association cortex that is well developed only in primates, especially in humans. It is involved in motivation and regulation of emotional behaviour and mood. The motor speech area called Broca’s area (named after Dr. Paul Broca who recognised this area) situated in left hemisphere in majority of people and rarely in right hemisphere also, performs its function in the same manner like – sensory input —> association areas —> motor output.
This is also called hind brain. It regulates the muscles and their movements. It is mainly responsible for co-ordination of the muscular movements and maintenance of the body equilibrium.
It acts like a telephone exchange or a relay station. All sensory impulses pass through this centre before they reach higher centre of the brain.
Situated below the thalamus and is mainly responsible for the control of emotional behaviour, metabolism and sexual activities.
This is a way station for tracts running between the cerebrum and the cerebellum.
This is situated at the base of the skull. It connects the brain and spinal cord. All nerve impulses to and from brain pass through this. It controls the vital functions like heart beat, blood pressure, blood circulation and respiration.
It also controls the reflexes like papillary reflex, sneezing and winking reflexes. If the upper portion of the medulla is damaged, the connection of cerebrum with rest of the body is broken, if lower portion is damaged, the vital functions are affected and person may die immediately.
This is the continuation of the medulla oblongata and runs through the canal of the vertebral column. Both sensory and motor nerves pass through this, to and from the brain.
This is also the centre for reflex actions (reflex action- is receiving the impressions from different parts of the body through sensory nerves and sending back instruction through motor nerve for suitable action on its own authority without consulting the brain, this structure is called Reflex Arc). If any damage takes place to spinal cord the area below that part will get paralysed.
2. Peripheral Nervous System:
This system consists of all the neuron cell bodies and processes located outside the brain and spinal cord. This system can be classified into 2 parts: 12 pairs of cranial nerves and 31 pairs of spinal nerves.
The cranial nerves are divided into 3 types:
(a) Afferent or sensory nerves which carry the information from inside and on the surface of the individual and relay the same to the central nervous system where it is evaluated,
(b) Efferent or motor nerves relay the information from central nervous system to muscles and glands in various parts of the body to regulate activity of those structures,
(c) Mixed nerves perform both afferent and efferent functions. Though there is difference in functions, structure of all the nerves remain the same.
The spinal nerves arise along the spinal cord. All of them are mixed nerves. Some of them are also sympathetic and parasympathetic, influencing the behaviour.
3. Autonomic Nervous System:
The autonomic nervous system (ANS) plays a very important role in controlling the muscular and glandular functions which are directly related with our behaviour. This system controls the emotional activities of a person.
If this system is affected, the individual’s experiencing of emotions will also be disturbed. The ANS not only controls the emotional behaviour, but also helps to manage the biochemical balances and metabolic activities. This system is responsible for all the physiological and psychological changes during emotions.
The ANS is composed of sympathetic and parasympathetic divisions. Increased activity of sympathetic divisions generally prepares the individual for physical activity, whereas the parasympathetic stimulation generally activates vegetative (growth) functions such as digestion, normally associated with body at rest.
The sympathetic division of ANS prepares a person for physical activity by increasing heart rate and blood pressure, by dilating the respiration passages and by stimulating perspiration. It also stimulates the release of glucose from liver for energy, at the same time inhibits digestive activities and diverts all energy to the concerned part of the body during emergency.
In other words, it prepares the person for “fight or flight”, because it prepares the individual either to stand and face the threat or escape from that situation as quickly as possible.
The parasympathetic division stimulates the vegetative activities such as digestion, defecation and urination, at the same time slows down heart rate and respiration. It also causes the pupil of the eye to constrict and the lens to thicken. In this way increased activity of sympathetic division generally prepares the individual for physical activity, whereas parasympathetic division generally activates vegetative function.
Most organs that receive autonomic motor neurons are innervated by both sympathetic and parasympathetic divisions. However, the sweat glands and blood vessels are innervated exclusively by sympathetic neurons. But smooth muscles associated with lens of the eye are innervated by parasympathetic neurons.
In most cases the influence of two divisions of ANS is opposite on the organs that receive both innervations. For example, sympathetic division increases the heart rate whereas the parasympathetic division decreases the heart rate.
Though ANS is also a part of central nervous system, the control of brain is less on the functions of this system, hence it is called ANS. But this system is controlled by mid-brain and hypothalamus. It has direct relationship with many muscles and glands of the body and thereby controls both internal and external behaviour.