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
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)
Neuroglia or Glial cells or Neuroglial cells or Glial
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
2. Multipolar Neurons
3. Bipolar Neurons
Function of nervous system
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.
ii. Memory –
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.