Nervous System of Human Body

The nervous system (???????? ???????) is a highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. Invertebrates consist of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The nervous system is one of the most important systems of the body, Siddha Spirituality of Swami Hardas Life System wishes our readers to know about in detail.

Nervous system Structure (???????? ??????? ??????)

The nervous system derives its name from nerves, which are cylindrical bundles of fibers, that emanate from the brain and spinal cord and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope.

Nervous system Cells (???????? ??????? ??????)

The nervous system contains two main categories or types of cells:

• Neurons cells, and
• Glial cells

Neurons (?????????)

The nervous system is defined by the presence of a special type of cell—the neuron. Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical. Many types of neurons possess an axon, a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts; axons typically extend throughout the body in bundles called nerves.

Glial cells (?????? ??????)

Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.

Among the most important functions of glial cells are to support neurons and hold them in place:

• To supply nutrients to neurons
• For insulating neurons electrically
• To destroy pathogens and remove dead neurons
• To providing guidance cues directing the axons of neurons to their targets

A very important type of glial cell generates layers of a fatty substance called myelin that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently. Recent findings indicate that glial cells, such as microglia and astrocytes, serve as important resident immune cells within the central nervous system.

Anatomy in vertebrates (??????? ???? ??? ????)

The nervous system of vertebrates is divided into:

• The central nervous system (CNS), and
• The peripheral nervous system (PNS)

Central nervous system (???????? ?????? ?????)

The (CNS) is the major division and consists of the brain and the spinal cord. The spinal canal contains the spinal cord, while the cranial cavity contains the brain. The CNS is enclosed and protected by the meninges, a three-layered system of membranes, including a tough, leathery outer layer called the dura mater. The brain is also protected by the skull, and the spinal cord by the vertebrae.

Peripheral nervous system (?????? ???????? ?????)

The peripheral nervous system (PNS) is a collective term for the nervous system structures that do not lie within the CNS. The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into somatic and visceral parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles.

Comparative anatomy and evolution (????????? ??????? ???? ?? ?????)

Neural precursors in sponges (????? ??? ???????? ???????)

Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. However, the function of this structure is currently unclear. 

Radiata (???????)

Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In comb jellies, radiata is concentrated near the mouth. The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only has two primordial cell layers, endoderm, and ectoderm. 

Bilateria (???????????)

The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Ediacaran period, 550–600 million years ago.

Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk, it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain.

Worms (?????)

Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth.

Arthropods (??????????)

Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly. Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the supraesophageal ganglion.

In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands, and certain muscles.

Identified neurons (????????? ?? ?????)

A neuron is called identified if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties.

Nervous system Function (???????? ????? ???????)

At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells. One is by releasing chemicals called hormones into the internal circulation so that they can diffuse to distant sites.

In contrast to this “broadcast” mode of signaling, the nervous system provides “point-to-point” signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.

At a more integrative level, the primary function of the nervous system is to control the body. It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response.

Neurons and synapses (????????? ?? ???????????)

Most neurons send signals via their axons, although some types are capable of dendrite-to-dendrite communication. Neural signals propagate along an axon in the form of electrochemical waves called action potentials, which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells.

Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common and much more diverse in function. At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of the molecular machinery that carries out the signaling process.

Neural circuits and systems (???????? ?????? ?? ??????????)

The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation, and timing, and there are seen to be countless types of information processing possible.

A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior.

Reflexes and other stimulus-response circuits (????? ?? ???? ????????-??????????? ??????)

The simplest type of neural circuit is a reflex arc, which begins with sensory input and ends with a motor output, passing through a sequence of neurons connected in series. This can be shown in the “withdrawal reflex” causing a hand to jerk back after a hot stove is touched. The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change the electrical field across the membrane.

There the axon makes excitatory synaptic contacts with other cells, some of which project to the same region of the spinal cord, others projecting into the brain. One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles.

The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away.

Intrinsic pattern generation (?????? ?????? ?????)

Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence.

When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos. A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator.

Mirror neurons (????? ??????)

A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron “mirrors” the behavior of the other, as though the observer were itself acting.

Such neurons have been directly observed in primate species. Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory cortex, and the inferior parietal cortex.

The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception/action coupling. 

Nervous system Development (???????? ??????? ?? ?????)

Invertebrates, landmarks of embryonic neural development include :

• Birth and differentiation of neurons from stem cell precursors
• Migration of immature neurons from their birthplaces in the embryo to their final positions
• An outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners
• Generation of synapses between these axons and their postsynaptic partners
• Lifelong changes in synapses are thought to underlie learning and memory

All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one end called the animal pole and the other the vegetal pole.

Nervous system Diseases (???????? ??????? ?? ???)

Nervous system diseases, also known as the nervous system or neurological disorders, refers to a small class of medical conditions affecting the nervous system. This category encompasses over 600 different conditions, including:

• Genetic disorders
• Infections
• Cancer
• Seizure disorders e.g. epilepsy
• Conditions with a cardiovascular origin e.g. stroke
• Congenital and developmental disorders e.g. spina bifida
• Degenerative disorders e.g. Multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, and Amyotrophic lateral sclerosis

Reference:

• https://en.wikipedia.org/wiki/Nervous_system
• https://en.wikipedia.org/wiki/Nervous_system_disease