Introduction:

The nervous system is an organ system having a network of specialized cells termed as neurons which coordinate the actions of an animal and transmit signals among various parts of its body. In most of the animals the nervous system comprises of two portions, central and peripheral. The central nervous system of vertebrates (like humans) includes the spinal cord, brain and retina. The peripheral nervous system comprises of sensory neurons, clusters of neurons termed as ganglia and nerves joining them to each other and to the central nervous system. Such regions are all interconnected by means of the complex neural pathways. The enteric nervous system, a subsystem of the peripheral nervous system, contains the capacity, even when severed from the rest of the nervous system via its primary connection through the vague nerve, to function separately in regulating the gastrointestinal system.

Neurons send signals to the other cells as electro-chemical waves travelling all along thin fibers termed as axons that cause chemicals termed as neurotransmitters to be discharged at junctions termed as synapses. A cell which receives a synaptic signal might be excited, inhibited or else modulated. The sensory neurons are activated through physical stimuli impinging on them and send signals which inform the central nervous system of the state of the body and the external atmosphere. Motor neurons placed either in the central nervous system or in peripheral ganglia, join the nervous system to muscles or other effectors organs. Central neurons, that in vertebrates very much outnumber the other kinds, make all of their input and output connections by other neurons. The interactions of all such kinds of neurons form neural circuits which produce an organism's perception of the world and find out its behavior. All along with neurons, the nervous system includes other specialized cells termed as glial cells (or simply glia) that gives structural and metabolic support.

Structure of Nervous system:

The nervous system gets its name from nerves that are cylindrical bundles of fibers which emanate from the brain and central cord and branch repeatedly to innervate each and every part of the body. Nerves are long adequate to have been recognized through the ancient Egyptians, Greeks and Romans; however their internal structure was not understood till it became possible to observe them by employing a microscope.  A microscopic assessment exhibits that nerves comprise mainly of the axons of neurons, all along by a variety of membranes which wrap around them and segregate them into the fascicles. The neurons which give mount to nerves do not lie completely in the nerves themselves - their cell bodies reside in the brain, central cord and peripheral ganglia. 

1) Cells:

The nervous system is basically made up of two classes of cells: neurons and glial cells.

2) Neurons:

The nervous system is stated by the presence of a special kind of cell - the neuron (at times termed as 'neurone' or 'nerve cell').  Neurons can be differentiated from other cells in a number of manners; however their most basic property is that they communicate by other cells through synapses that are membrane-to-membrane junctions having molecular machinery which lets rapid transmission of signals, either chemical or electrical. In the nervous system of a single species like humans, hundreds of various kinds of neurons exist, having a broad variety of functions and morphologies. These comprise sensory neurons which transmute physical stimuli like light and sound into neural signals; and motor neurons which transmute neural signals into the activation of muscles or glands.

3) Glial cells:

Glial cells are non-neuronal cells which give nutrition and support, maintain homeostasis, form myelin, and contribute in signal transmission in the nervous system. In the human brain, it is anticipated that the net number of glia roughly equivalents the number of neurons; however the proportions differ in various brain regions. Among the most significant functions of glial cells are to support neurons and grasp them in place, to insulate neurons electrically, to supply nutrients to neurons, to annihilate pathogens and eradicate dead neurons and so on.

Functions of Nervous system:

At the most fundamental level, the main function of the nervous system is to send signals from one cell to others or from one portion of the body to others. There are numerous ways that a cell can send signals to the other cells. One is by discharging chemicals termed as hormones to the internal circulation, in such a way that they can diffuse to distant sites. In contrary to this 'broadcast' mode of signaling, the nervous system gives 'point-to-point' signals - neurons project their axons to particular target regions and make synaptic connections by specific target cells. Therefore, neural signaling is able of a much higher level of specificity than the hormonal signaling. It is as well much faster, that is, the fastest nerve signals travel at speeds which exceed 100 meters per second.

At a much integrative level, the main function of the nervous system is to regulate the body.  It does this through extracting information from the environment by employing sensory receptors, sending signals which encode this information to the central nervous system, processing the information to find out a suitable response and sending output signals to the glands or muscles to activate the response.

The evolution of a complex nervous system has made it probable for a variety of animal species to have advanced perception capabilities like vision, fast coordination of organ systems, complex social interactions, and integrated processing of the concurrent signals.

Neurons and synapses:

Neurons obtain information from the sensory organs, send information to the motor organs, or share information by other neurons. The procedure of communicating information is much similar, whether it is to the other neuron or to a gland or muscle cell. Though, by far the biggest number of neuronal connections is with other neurons. The transmission of information is achieved in two manners:

i) Electrically: The neuron is directly adjacent to the other neurons. Small holes in each and every cell's membrane, termed as gap junctions, are juxtaposed so that as the action potential reaches the end of the axon (that is, at the terminal boutons), the depolarization carry on across the membrane directly to the postsynaptic neuron.

ii) Chemically: There is a space (that is, the synaptic cleft) among the axon terminus and the adjacent neuron. As the action potential reaches the end of axon, a chemical is discharged which travels across the synaptic cleft to the subsequent neuron to modify its electric potential.

The synapse is basically the junction at which neurons trade information. This is not a physical component of a cell however instead a name for the connection among two cells: the presynaptic cell (that is, giving the signal) and the postsynaptic cell (that is, receiving the signal). There are two kinds of possible reactions at the synapse: a chemical reaction and an electrical reaction. Throughout a chemical reaction, a chemical termed as a neurotransmitter is discharged from one cell to other. In an electrical reaction, the electrical charge of one cell is affected by the charge an adjacent cell.

Neural circuits and system:

The fundamental neuronal function of sending signals to other cells comprises a capability for neurons to exchange signals with one other. Networks made by interconnected groups of neurons are able of a broad variety of functions, comprising feature detection, pattern generation and timing. However, it is difficult to assign limits to the kinds of information processing which can be taken out by neural networks. Warren McCulloch and Walter Pitts showed in the year 1943 that even networks made from a greatly simplified mathematical abstraction of the neuron are able of universal calculation.

Reflexes and other stimulus-response circuits:

The simplest kind of neural circuit is a reflex arc that starts with a sensory input and ends by a motor output, passing via a series of neurons in between. For illustration, consider the 'withdrawal reflex' causing the hand to jerk back after a hot stove is touched. The circuit starts by sensory receptors in the skin that are activated through injurious levels of heat:  a special kind of molecular structure embedded in the membrane causes heat to produce an electrical field across the membrane. When the electrical potential change is large adequate, it evokes an action potential that is transmitted all along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts by other cells, a few of which project to the similar area of the spinal cord, others projecting to the brain. One goal is a set of spinal interneuron which project to motor neurons controlling the arm muscles. The interneuron's excite the motor neurons, and when the excitation is strong adequate, some of the motor neurons produce action potentials that travel down their axons to the point where they form excitatory synaptic contacts by means of muscle cells. The excitatory signals induce contraction of the muscle cells that causes the joint angles in the arm to modify, pulling the arm away. In actuality, this straightforward schema is subject to many complications. 

Intrinsic pattern generation:

However stimulus-response methods are the simplest to understand, the nervous system is as well able of controlling the body in ways which don't need an external stimulus, by means of internally generated rhythms of activity. Due to the variety of voltage-sensitive ion channels which can be embedded in the membrane of a neuron, numerous kinds of neurons are able, even in the isolation, of producing rhythmic series of action potentials or rhythmic alternations among high-rate bursting and serenity. If neurons which are intrinsically rhythmic are joined to each other through excitatory or inhibitory synapses, the resultant networks are able of a broad variety of dynamical behaviors, comprising attractor dynamics, periodicity and even chaos.

Anatomy in vertebrates:

The vertebrate animal's nervous system (comprising humans) is categorized into the central nervous system (CNS) and peripheral nervous system (PNS); aneurone nervous system.

The central nervous system (CNS) is the biggest part and comprises the brain and spinal cord. The spinal cavity includes the spinal cord whereas the head includes the brain. The CNS is enclosed and protected through meninges, a three-layered system of membranes, comprising a tough, leathery outer layer termed as the dura mater. The brain is as well protected via the skull and the spinal cord via the vertebrae.

The peripheral nervous system (PNS) is a collective word for the nervous system structures which don't lie in the CNS. The big majority of the axon bundles termed as nerves are considered to fit into the PNS, even when the cell bodies of the neurons to which they fit into reside in the spinal cord or brain. The PNS is splitted into somatic and visceral parts.

The autonomic nervous system (ANS) itself comprises of two portions: the sympathetic nervous system and the parasympathetic nervous system. A few authors as well comprise sensory neurons whose cell bodies lie in the periphery (that is, for senses like hearing) as part of the PNS; others, though, omit them. 

Comparative anatomy and evolution:

1) Neural precursors in sponges:

Sponges contain no cells joined to each other via synaptic junctions, that is, no neurons, and thus no nervous system. They do, though, have homolog of numerous genes which play key roles in synaptic function. Latest studies have shown that sponge cells deduce a group of proteins which cluster altogether to form a structure look like a postsynaptic density (that is, the signal-receiving portion of a synapse).

2) Radiate:

Comb jellies, Jellyfish and related animals have diffuse nerve nets instead of a central nervous system. In most of the jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated close to the mouth. The nerve nets comprise of sensory neurons which pick up chemical, tactile and visual signals, motor neurons which can activate contractions of the body wall and intermediate neurons which detect patterns of activity in the sensory neurons and send signals to the groups of motor neurons as an outcome.

3) Bilateria:

The huge majority of existing animals are bilaterians, meaning animals having left and right sides which are approximate mirror images of one other. All bilateria are thought to encompass descended from a general wormlike ancestor who appeared in the Cambrian period, 550 to 600 million years ago. The basic bilaterian body form is a tube having a hollow gut cavity running from mouth to anus, and a nerve cord having an enlargement (ganglion) for each body segment, by a particularly big ganglion at the front, termed as the brain. Even mammals, comprising humans, represent the segmented bilaterian body plan at the level of the nervous system.

4) Worms:

Worms are the simplest bilaterian animals and reveal the fundamental structure of the bilaterian nervous system in the most straightforward manner. As an instance, earthworms encompass dual nerve cords running all along the length of the body and merging at the tail and the mouth. Such nerve cords are joined by transverse nerves such as the rungs of a ladder. Such transverse nerves assist coordinate the two sides of the animal. The two ganglia at the head end function identical to a simple brain.

5) Arthropods:

Arthropods, like crustaceans and insects, encompass a nervous system made up of a sequence of ganglia, joined through a ventral nerve cord made up of two parallel connectives running all along the length of the belly. Usually, each body segment consists of one ganglion on each side, although a few ganglia are fused to form the brain and other big ganglia.

6) Identified neurons:

A neuron is termed as identified if it consists of properties which differentiate it from every other neuron in the similar animal - properties like location, gene expression pattern, neurotransmitter and connectivity and when each and every individual organism belonging to the similar species consists of one and only one neuron by the similar set of properties. 

In the vertebrate nervous systems very few neurons are 'identified' in this sense - in humans, there is supposition to be none - however in simpler nervous systems, some or all neurons might be therefore unique. In the roundworm C. Elegans, whose nervous system is the most methodically explained of any animals, each and every neuron in the body is exclusively identifiable by the similar location and the similar connections in each and every individual worm. One remarkable effect of this fact is that the form of the C. Elegans nervous system is entirely specified by the genome, devoid of experience-dependent plasticity.

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