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The nervous system is a network of specialized cells that communicate information about an animal's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glia, that aid in the function of the neurons. The nervous system is divided broadly into two categories; the peripheral nervous system and the central nervous system. Neurons generate and conduct impulses between and within the two systems. The peripheral nervous system is composed of sensory neurons and the neurons that connect them to the nerve cord, spinal cord and brain, which make up the central nervous system. In response to stimuli, sensory neurons generate and propagate signals to the central nervous system which then process and conduct back signals to the muscles and glands. The neurons of the nervous systems of animals are interconnected in complex arrangements and use electrochemical signals and neurotransmitters to transmit impulses from one neuron to the next. The interaction of the different neurons form neural circuits that regulate an organisms perception of the world and what is going on with its body, thus regulating its behavior. Nervous systems are found in many multicellular animals but differ greatly in complexity between species.^[1]

Nervous system in humans

The human Nervous system can be described both by gross anatomy, (which describes the parts that are large enough to be seen with the naked eye,) and by microanatomy, (which describes the system at a cellular level.) In gross anatomy, the nervous system can be divided into distinct organs, these being stations through which the neural pathways cross. These organs can be divided into two systems: the central nervous system (CNS) and the peripheral nervous system (PNS).^[2]

Central nervous system

The central nervous system (CNS) is the largest part of the nervous system, and includes the brain and spinal cord. The spinal cavity holds and protects the spinal cord, while the head contains and protects the brain. The CNS is covered by the meninges, a three layered protective coat. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae.

Peripheral nervous system

The PNS is a regional term for the collective nervous structures that do not lie in the CNS. The bodies of the nerve cells lie in the CNS, either in the brain or the spinal cord, and the longer of the cellular processes of these cells, known as axons, extend through the limbs and the flesh of the torso. The large majority the axons which are commonly called nerves, are considered to be PNS.

Microanatomy

The nervous system is, on a small scale, primarily made up of neurons. However, glial cells also play a major role.

Neurons

Neurons are electrically excitable cells in the nervous system that process and transmit information. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. A number of different types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the spinal cord and brain, motor neurons receive signals from the brain and spinal cord and cause muscle contractions and affect glands, Interneurons connect neurons to other neurons within the brain and spinal cord.

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, glia are estimated to outnumber neurons by about 10 to 1.^[3]

Glial cells provide support and protection for neurons. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons.

Physiological division

Diagram showing the major divisions of the nervous system.

A less anatomical but much more functional way of dividing of the human nervous system is classification according to the role that the different neural pathways play, regardless of whether or not they cross through the CNS/PNS:

The somatic nervous system is responsible for coordinating voluntary body movements (i.e. activities that are under conscious control).

The autonomic nervous system is responsible for coordinating involuntary functions, such as breathing and digestion.

In turn, these divisions of the nervous system can be further divided according to the direction in which they conduct nerve impulses:

Afferent system by sensory neurons, which carries impulses from a somatic receptor to the CNS
Efferent system by motor neurons, which carries impulses from the CNS to an effector
Relay system by interneurons (also called "relay neurons"), which transmit impulses between the sensory and motor neurons (both in the CNS and PNS).

The junction between two neurons is called a synapse. There is a very narrow gap (about 20nm in width) between the neurons called the synaptic cleft. This is where an action potential (the "message" being carried by the neurons, also known as the nerve impulse) is transmitted from one neuron to the next. This is achieved by relaying the message across the synaptic cleft using neurotransmitters, which diffuse across the gap. The neurotransmitters then bind to receptor sites on the neighboring (postsynaptic) neuron, which in turn produces its own electrical/nerve impulse. This impulse is sent to the next synapse, and the cycle repeats itself.

Nerve impulses are a change in ion balance between the inside and outside of a neuron. Because the nervous system uses a combination of electrical and chemical signals, it is incredibly fast. Although the chemical aspect of signaling is much slower than the electrical aspect, a nerve impulse is still fast enough that it is perceived as being instantaneous. Speed is a necessary characteristic in order for an organism to quickly identify the presence of danger, and thus avoid injury/death. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the nervous system would not be able to signal the arm to move fast enough to escape dangerous burns. Thus, the speed of the nervous system is evolutionarily valuable, and is in fact a necessity for life.

Development

Some landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.

Importance

The evolution of a complex nervous system makes it possible for various animal species to have advanced perception abilities like sight, complex social interactions, rapid coordination of other organ systems, and integrated processing of many concurrent signals. In humans, the advanced development of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other outcomes of human society that would not be possible without our brains.

Many people have lost basic motor skills and other skills because of spinal cord injuries. If this portion is damaged, the biggest nerve and the most important one get damaged. This leads to paralysis or other permanent damage. Physical lesions or genetic abnormalities of the brain can also lead to major harm.

Abilities

The nervous system is able to make basic motor skills and other skills possible. The basic 5 senses of texture, taste, sight, smell, and hearing are powered by the nervous system. If disabled, basic motor skills may be lost.

Non-humans

Vertebrates

The nervous system of all vertebrate animals, is often divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord.

Worms

Planaria, a type of flatworm, have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two large ganglia at the head end function similar to a simple brain. Photoreceptors on the animal's eyespots provide sensory information on light and dark.

The nervous system of the roundworm Caenorhabditis elegans has been mapped out to the cellular level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons [1]

Arthropoda

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 [2]. Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia [3].

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.

Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain.

Development

Neural development in most species have many similarities neural development in humans.

References

^ "Nervous System". Columbia Encyclopedia. Columbia University Press. {{cite encyclopedia}}: |access-date= requires |url= (help); Check date values in: |accessdate= (help)
^ Maton, Anthea (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. pp. 132–144. ISBN 0-13-981176-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ sfn.org Society for Neuroscience, 2000

External links

[1] "Nervous System". Columbia Encyclopedia. Columbia University Press. {{cite encyclopedia}}: |access-date= requires |url= (help); Check date values in: |accessdate= (help)

[2] Maton, Anthea (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. pp. 132–144. ISBN 0-13-981176-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[sfn-3] sfn.org Society for Neuroscience, 2000

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Central nervous system	Brain	Prosencephalon	Telencephalon	Rhinencephalon, Amygdala, Hippocampus, Neocortex, Lateral ventricles
			Diencephalon	Epithalamus, Thalamus, Hypothalamus, Subthalamus, Pituitary gland, Pineal gland, Third ventricle
		Brain stem	Mesencephalon	Tectum, Cerebral peduncle, Pretectum, Mesencephalic duct
			Rhombencephalon	Metencephalon	Pons, Cerebellum,
				Myelencephalon	Medulla oblongata
	Spinal cord

v t e Sensory receptors
Touch	Mechanoreceptor Vibration Lamellar corpuscle Light touch Tactile corpuscle Pressure Merkel nerve ending Stretch Bulbous corpuscle
Pain	Free nerve ending Nociceptors
Temperature	Thermoreceptors
Proprioception	Golgi organ Muscle spindle Intrafusal muscle fiber Nuclear chain fiber Nuclear bag fiber
Other	Hair cells Baroreceptor