All systems in human body are very much close to each other and interdependent. For example, digestive system and circulatory system are interdependent because the distribution of digested food becomes possible only through blood circulation on the one hand and circulatory system cannot feed its vessels without digestive system at other. Similarly, respiratory system in the absence circulatory system becomes meaningless. Hence it is clear that all these systems are interlinked with each other. The co-ordination is of two types: structural and chemical. The major part of structural coordination is possible only through nervous system and chemical co-ordination is effected by endocrine system which deals with endocrine glands secreting hormones into the blood stream. Of course, chemical coordination plays a minor role in comparison to nervous system.
You know that animals live in their surrounding environment and they change their activities in relation to the environment. This is called external response. Similarly, the cells found deep inside the animal body respond to something which happens far away from them. This is called internal response. So for effective coordination of all the parts and of the animal with its environment, three basic conditions must be fulfilled such as: there must be a coordinator, information must pass from the body part to the coordinator and from the environment to the coordinator, and information must pass from the coordinator to the body parts. All these activities are performed by the nervous system which acts as an effective coordinator.
Functions of Nervous system
The general function is integration which means intimate association of different acts, so that they cooperate towards a common end. The nervous system functions in the following ways.
1. It relates the animal to its environment by means of the sense organs (e.g. skin, eye, ear) which are sensitive to specific changes in the environment.
2. It functions to integrate activities within an animal. In other words, this system receives information from all parts of the body and responds to the change and reacts accordingly.
3. This system keeps a record of previous activities for certain duration.
Division of the nervous system
For the sake of convenience, nervous system is divided into
1. Central nervous system
2. Peripheral nervous system.
Central nervous system comprises the brain and the spinal cord. This system coordinates and controls the activities of the animal.
The peripheral nervous system is again divided into two subsystems such as.
1. Peripheral nervous system proper which includes cranial and spinal nerves composed of somatic sensory and somatic motor fibers and
2. Automatic nervous system which includes sympathetic and parasympathetic nerves composed chiefly of visceral sensory and visceral motor fibers. The peripheral nervous system, as a whole, forms the connecting link between organs and tissues and the central nervous system.
Functional components of the nervous system
Stimulus: Living organisms show sensitivity or irritability to any change in the internal or external environment. Any change, whether physical or chemical is called a stimulus. The behaviour or survival of the organism is based on the stimulus response reaction. There are three main types of structures related to stimuli and responses. They are receptors, neurons and effectors.
Receptors: Receptors may be neurons themselves or they may be organs that are specialized for detecting stimuli. In higher animals the organs of special senses are eye, nose, skin and ear.
Neurons: These are structural and functional units of the nervous system. The detailed structure of the neuron is given below. Neurons are often very long and specialized for transmitting nerve impulses usually in one direction only.
Nerve impulse: A nerve impulse is a wave of changes traveling along the cell membrane involving movement of the ions and chemical reactions. Usually it begins at one end of the neuron and travels rapidly to the other. For example, in some neurons of man the nerve impulse moves with a speed of approximately 100 meters per second.
Types of neurons: Functionally, three types of neuron are found, namely sensory nuron, motor neuron and associate neuron.
1. Sensory neurons: These neurons carry impulses from receptors to the central nervous system i.e., receive impulses from sensory cells or receptors through single elongated fiber or dendron and transmit them to the next cell in the network in its axon.
2. Motor neurons: They carry impulses from the central nervous system to effectors. In other words, they conduct impulses from the brain or spinal cord along an axon to a muscle fiber or a gland. When it reaches muscle fiber, the axon terminates in a branched structure called a motor end plate lying within the sheath of muscle fiber, impulses are transmitted through this structure to the fiber causing it to contact.
3. Association neurons: These are also called multipolar neurons or inter neurons which occur in the brain and spinal cord. The brain and spinal cord are built of many millions of association neurons. They possess many dendrites but no associated long fibers. They have many processes connecting them with other neurons. They have a most important part to play in nerve function i.e., the integration of sensory and motor neuron functions.
Effectors: They are structures that respond when they are stimulated by nerve impulses. In complex higher animals the principal effectors are muscles and glands. Muscles respond by contracting themselves and glands respond by secreting substances.
Synapse: Generally several neurons are involved in nay behavioral activity. One neuron carries an impulse along its length. When the impulse reaches the end of the first neuron, it stimulates the next neuron. The gap between the end of one neuron and the beginning of another is called the synapse. In the region of synapse, the branched ending of axon is applied to the dendrites or cell body of another neuron. Two neurons are separated by a microscopic space without any direct cytoplasmic connections. It is believed that the impulses are conducted across the synapse by a chemical mechanism.
Nerves: Most of the nerves contain motor or sensory fibers. Mostly their cell bodies and synapse lie in the central nervous system. According to their functions, nerves are of following types:
1- Sensory – Carrying impulses from receptors to the central nervous system.
2- Motor – Carrying impulses from central nervous system to the effector organs.
3- Mixed – They perform both sensory and motor functions and possess both types of nerve fibers.
All the activities of the body are divided into somatic and visceral. Somatic includes skin and muscles which take part. Visceral action includes internal receptors and effectors of the body. These visceral acts are without our awareness, bringing messages from the heart, liver, digestive tube, blood vessels, kidney and other internal organs to the brain. This helps in co-ordination and all normal functioning of digestion, respiration, circulation and excretion etc. So there are four types of nerve fibers found in the nervous system:
1. Somatic sensory: These are nerves which carry impulses from receptors such as skin, eyes, nose, and muscles to the brain and spinal cord.
2. Somatic motor: These are nerves which carry impulses from brain and spinal cord to the effector organs, mainly muscles.
3. Visceral sensory: These are nerves which carry stimuli from the visceral receptors, i.e., situated in the wall of alimentary canal, to brain and spinal cord.
4. Visceral motor: These nerves carry impulses from the brain and spinal cord to the effector organs such as involuntary muscles and glands of alimentary canal and other visceral organs.
Nervous System of Man
The nervous system is a complex system through which the whole body is worked together and controlled. The nervous system of man can be divided mainly into two distinct parts: the somatic nervous system and the autonomic nervous system. Again the somatic nervous system can be subdivided into central nervous system and peripheral nervous system.
- The central nervous system includes brain and spinal cord
- The peripheral nervous system deals with peripheral nerves such as cranial nerves and spinal nerves.
- The autonomic nervous system controls the activities of viscera, glands and blood vessels etc. and includes a pair of ganglionated nerve cord and associated plexuses. It is of two types namely Sympathetic nervous system and Parasympathetic nervous system.
The Central nervous system
The central nervous system is the coordinating center of the whole neural mechanism. It comprises the brain and spinal cord. The brain achieves its highest development in man amongst all animals and it controls various activities of the man.
The brain is situated in the cranial cavity of the skull but the spinal cord arises from the brain and runs inside the neural canal or vertebral canal. The brains as well as the spinal cord are covered by three connective tissue membranes which are collectively called meninges or meningeal membranes.
Meninges: The meninges are formed of three membranes namely pia mater, middle arachnoid layer and outer dura mater.
Pia mater: It is a thin membrane and forms the innermost layer. It is richly supplied with blood vessel, and is closely applied to the surface of the brain and spinal cord. The pia mater is fused with brain at two places to form choroid plexuses which project into cavities of the brain. It is separted from the arachnoid membrane by cerebo spinal fluid.
Arachnoid layer: it is the middle layer and present in between pia mater and dura mater. It contains a network of fibers and separated from dura mater by subdural cavity.
Dura mater: It is the outermost, tough, dense, fibrous and highly vescular membrane which forms the inner lining of the cranial cavity and vertebral column.
Function of Cerebrospinal fluid:
- It affords some protection to the central nervous system against mechanical injury.
- It serves to maintain a constant pressure inside the cranium in spite of fluctuations in the volume and pressure of blood in the cranial vessels.
- The exchange of material between cerebrospinal fluid and neurons helps in their nutrition and excretion.
The Peripheral nervous system
The peripheral nervous system deals with the nerves which arise from the brain and spinal cord and conduct impulses to different parts of the body. The nerves which arise from the different parts of the brain are called cranial nerves and nerves from the spinal cord are called spinal nerves.
1. Cranial nerves: There are twelve pairs of cranial nerves in man which arise from different parts of the brain. Nerves which have their origin from the dorsal roots are sensory and those having their origin from the ventral roots are motor. The mixed nerves arise from dorsal roots having ganglia and consist of both sensory and motor fibers.
Each cranial nerve is numbered with a Roman numerical according to the order in which it takes its superficial origin. Each is given a name generally according to its distribution.
2. Spinal nerves: There are 31 pairs of spinal nerves which originate from the spinal cord of man. Out of which 8 pairs are cervical, 12 pairs thoracic, 5 pairs lumbar, 5 pairs sacral and one pair coccygeal nerves. These nerves run between spinal cord and the peripheral tissues and are of mixed types.
Each spinal nerve originates from the spinal cord by two roots, a dorsal afferent and a ventral efferent. Each dorsal root is characterized by an ovoid enlargement, the dorsal root ganglion which conveys impulses from the periphery, into the spinal cord. Dorsal root originates from the dorsal horn of gray matter and ventral root originates from ventral horn of gray matter of the spinal cord. Somatic motor fibers are present in the ventral roots of all the spinal nerves. The dorsal root and ventral root after joining with each other form a short spinal nerve proper which divide usually after its emergence from the intervertebral foramen into three branches namely ramus dorsalis, ramus ventralis and ramus communicans. Another small branch called meningeal ramus arises from the spinal nerve and passes back through intervertebral foramen after its origin to supply meninges and blood vessels of spinal cord.
Autonomic nervous system
Autonomic nervous system controls the internal activities of visceral organs by a system of ganglia and nerves. It is a visceral motor system arising from the ventral part of the spinal cord. It is partly independent and not under voluntary control, though it is involuntarily controlled by the central nervous system.
The autonomic nervous system is defined as “a system of nuclei and fiber tracts in the brain and spinal cord, visceral afferent and visceral efferent, preganglionic and postganglionic neurons and of autonomic ganglia and plexus which control the activities of visceral organs such as heart, blood vessels, intestine, stomach, uterus, urinary bladder, lungs, sweet glands, salivary glands, liver, pancreas, certain endocrine glands and other organs”.
The efferent neurons of this nervous system are called preganglionic and post gaanglionic neurons. The preganglionic neurons originate from central nervous system and their fibers enter certain of the cranial and spinal nerves to terminate in autonomic ganglia.