The nervous system of all animals is composed of highly specialised cells called neurons which can detect , receive and transmit different kinds of stimuli .
The nervous system organization is very simple in invertebrates . For example, in Hydra it is composed of a network of neurons. The nervous system is better organised in insects, where a brain is present along with a number of ganglia and neural tissue . The vertebrates have a more developed neural system.
Nervous system in humans
The nervous system, in association with the endocrine system, serves as the primary control center of the body working below the level of consciousness. For example- the hypothalamus of the brain is where the body’s ‘thermostat’ is found . The hypothalamus also stimulates the pituitary gland to release various hormones that control metabolism and development of the body. The nervous system in humans regulates various systems such as respiratory (controls pace and depth of breathing), cardiovascular system ( controls heart rate and blood pressure), endocrine organs (causes secretion of ADH and oxytocin), the digestive system (regulates the digestive tract movement and secretion) and the urinary system ( it helps adjust renal blood pressure and also controls voiding the bladder) . It also involved in our sexual behaviour and functions .
Nervous system in humans is divided into three parts :
- The central nervous system (CNS)
- The peripheral nervous system(PNS)
- Autonomic nervous system
Central neural system (CNS)
The CNS includes the brain and the spinal cord and is the site of information processing and control. The brain is the body’s ‘control center’.The CNS has various centers located within it that carry out the sensory,motor and integration of data. These centers can be sub-divided into lower centers (including the spinal cord and brain stem) and higher centers communicating with the brain via effectors.
Peripheral nervous system
The PNS comprises of all the nerves of the body associated with the CNS (cranial nerve and spinal nerve) . It contains sensory receptors which help in processing changes in the internal and external environment . The nerves fibres of the PNS are of two types:
- a. Afferent fibres
- b. Efferent fibres
The afferent nerve fibres transmit impulses from tissue/organs to the CNS and the efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.
The PNS is divided into two sub-divided called somatic nervous system and automatic nervous system. The somatic nervous system relays impulses from the CNS to skeleton muscles . The somatic has valuntary control of skin,bones,joints and skeleton muscle . While the automatic nervous system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body . The autonomic has invoulantry control of internal organs,blood vessels, smooth and cardiac muscles. The automatic nervous system is further classified into sympathetic nervous system and parasympathetic nervous system .
Autonomic nervous system
The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.
Nervous Tissue ( Neuron)
Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialised nerve cells called neurons . Neurons are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibres (neurons) bound together by connective tissue. A sheath of dense connective tissue, the epineurium surrounds the nerve . Blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibres. There are three main types of neurons, which are classified according their function : Those that conduct impulses from the sensory organs to the central nervous system ( brain and spinal cord ) are called sensory (or afferent) neurons, those that conduct impulses from the central nervous system to the effector organs ( such as muscles and glands) are called motor ( or efferent ) neurons. Interneurons (also known as connector neurons or association neurons) are those that connect sensory neurons to motor neurons.
Neuroglia or Glial cells or Neuroglial cells or Glial
Neuroglia ( neuro – nerve ; glia- glue ) make up about half I volume of the CNS . Their name derives from the idea of early histologists that they were the ‘glue’ that held nervous tissue together. Neuroglia are the packing and supporting cells found in the brain and spinal cord . Generally, they are smaller than neurons and 5 to 50 times more numerous. In case of injury or disease, neuroglia multiply to fill in the spaces formerly occupied by neurons . Brain tumors derived from glia, called gliomas, tend to be highly malignant and grow rapidly. Of the six types of neuroglia , four – astrocytes , oligodendrocytes , microglia and ependymal cells are found only in the CNS . The remaining two types – Schwann cells and satellite cells are present in the PNS .
Structure of Neuron
A neuron is a microscopic structure composed of three major parts , namely, cell body , dendrites and axon . The cell body contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules. A motor neuron has many processes (cytoplasmic extension), called dendrites, which enter a large , grey cell body at one end. A single process, the axon, leaves at the other end, extending towards the dendrites of the next neuron or to form a motor endplate in a muscle. Dendrites are usually short and divided while the axons are very long neuron and does not branched freely. The impulses are transmitted through the motor neuron in one direction, i.e., into the cell body by the dendrites and away from the cell body by the axon. The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules, called Nissl, bodies are found in the cytoplasm of the cell body. Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon. The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells. The myelin sheath together with the neurilemma is also known as the medullary sheath. This medullary sheath is interrupted at intervals by the nodes of Ranvier. Axons and dendrites in the central nervous system are typically only about a micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10-25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the squids gaints axon, an ideal experimental preparation because of its relatively immense size (0.5-1 millimeter thick, several centimetres long).
Nerve cells are connected to each other at a junction known as synapse, where the terminal branches of an axon and the dendrites of another neuron lie in close proximity to each other but never make direct contact.
Classification of Neurons
On the basis of their structure, neurons can also be classified into three main types :
Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar Neurons . It usually found in the embryonic stage.
2. Multipolar Neurons
Motor neurons, which have numerous cell processes (an axon and many dendrites) are often referred to as multipolar Neuron. Interneurons are also multipolar. It found in cerebral cortex .
3. Bipolar Neurons
Bipolar neurons are spindle-shaped, which a dendrite at one end and an axon at the other .Ans example can be found in the light- sensitive retina of the eye. It found in the retina of eye.
Function of nervous system
The nervous system has three main functions-
i. Control and Coordination
Nervous system controls and coordination the working of all parts of the body so that it functions as an integrated unit. This is achieved by three overlapping process : sensory input, integration of data and motor output.
- Sensory input means conduction of external information gathered by the sensory receptors (neurons, gila and synapses) .The nervous system is composed of excitable nerve cells and synapses connecting the cells to one another,to centers throughout the body or to other neurons. These neurons operate on excitation or inhibition and although nerve cells can vary in size and location their communication with one another determines their function. These nerves conduct impulses from sensory receptors to the brain and spinal cord.
- Integration involves analysis and interpretation of the incoming information to produce sensations, such as vision, pain,smell, etc. And initiation of proper instructions for signals. Integration generally occurs in brain and spinal cord.
- Motor output means relaying the signals from integrating centre (CNS) to the effectors, such as muscles and glands which give a suitable response to the stimuli acting on the receptors. In other words after the brain has processed the information, impulses are then conducted from the brain and spinal cord to muscles and glands which is called motor output.
Nervous system stores the impression of previous stimuli and retrieve (recalls) these impressions to guide the animal in future. These impressions are referred to as the experiences, or memory.
Nervous system also receives information about the changes within the body and coordinates the activities of the viscera in the light of these changes . This helps in the maintenance of homeostasis in the body’s internal environment.
Important keyword points
- Nerve fibre – Axon or dendron of a nerve cell covered with one or two sheaths is termed a nerve fibre. On the basis of structure , nerve fibre are two types – medullated (myelinated) and non- medullated ( non- myelinated).
- Nerve – Nerve are the bundles of nerve fibres in the peripheral nervous system.
- Neurology– Study of structure, functions and diseases of the nervous system is called neurology.
- Nerve impulses- The nervous system is composed of neurons ( nerve cells) which exercise control by sending electrical signals called nerve impulses .
- Schwann cells – In the peripheral nervous system, Schwann cells wrap around the axons of neurons. They cover the axon with concentric layers of insulting plasma membrane.
- Neurotoxin – A poison destructive to nervous tissue.
- Neuralgia- pain arising or carried along a nerve .
- Neuritis- Inflammation of nerve.
- Smallest nerve – Pathetic (trochlear)