Nervous system
The Human Nervous System. Red is CNS and blue is PNS.
Latin	systema nervosum

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The nervous system is a network of specialized cells that coordinate the actions of an animal and send signals from one part of its body to another. These cells send signals either as electrochemical waves traveling along thin fibers called axons, or as chemicals released onto other cells. The nervous system is composed of neurons and other specialized cells called glial cells (plural form glia).

In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain and spinal cord. The neurons of the central nervous system are interconnected in complex arrangements and transmit electrochemical signals from one to another. The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system. Sensory neurons are activated by inputs impinging on them from outside or inside the body, and send signals that inform the central nervous system of ongoing events. Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect neurons to muscles or other effector organs. The interaction of the different neurons form neural circuits that regulate an organism's perception of the world and its body and behavior.

Nervous systems are found in most multicellular animals, but vary greatly in complexity.^[1] Sponges have no nervous system, although they have homologs of many genes that play crucial roles in nervous system function, and are capable of several whole-body responses, including a primitive form of locomotion. Radiata, including jellyfish, have a nervous system consisting of a simple nerve net. Bilaterian animals, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, spinal cord, and peripheral nerves.

Structure

The nervous system is defined by the presence of a special type of cell, the neuron. Neurons are distinctive 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.

All animals more advanced than sponges have a nervous system. However, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Cambrian period, over 500 million years ago.

Vertebrates

Diagram showing the major divisions of the vertebrate nervous system.

The nervous system of vertebrate animals is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

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 of the axons, which are commonly called nerves, are considered to be PNS.

The autonomic nervous system, which mediates involuntary behaviors such as heartbeat and breathing, consists of two parts: the sympathetic nervous system and the parasympathetic nervous system.

The cell bodies of afferent PNS nerves lie in the dorsal root ganglia.

Invertebrates

Neural precursors in sponges

Sponges have no cells connected to each other by synaptic junctions, that is, no neurons. They do, however, have homologs of many genes that play key roles in synaptic function. In particular, recent studies have shown that they possess a group of proteins that cluster together to form a structure resembling a postsynaptic density—the signal-receiving part of a synapse.^[2] The function of this structure is currrently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves, which mediate some simple actions such as whole-body contraction.^[3]

Radiata

Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.^[4]

The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, endoderm and ectoderm. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.

Bilateria

Nervous system of a bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front

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 Cambrian period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".

The basic architecture of the bilaterian nervous system is seen most clearly during embryonic development. All bilaterian animals at an early stage of development form a gastrula, which is a disk with three layers of cells, an inner layer called the endoderm, which gives rise to the lining of most internal organs; a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer layer called the ectoderm, which gives rise to the skin and nervous system.^[5] In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the CNS, the outer portion the PNS. As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest, running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation.

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]

Arthropods

Internal anatomy of a spider, showing the nervous system in blue

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.

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 total human brain, the number of glia is estimated to be roughly the same as neurons.^[6]

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.

Function

Simplified schema of basic nervous system function: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal cord and then out to motor neurons

At the most basic level, the function of the nervous system is to control the body. One of the most important ways it does this is 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 finally sending signals via the peripheral nervous system to muscles or glands, in order to activate the response.

The nervous system enables basic motor skills and sensing. The five classical senses (touch, taste, sight, smell, and hearing) are powered by the nervous system as are others such as equilibrioception (the sensing of gravity), nociception (the sensing of pain), and proprioception (the sensing of relative limb location and motion, as when touching the nose with closed eyes). Inhibition of these senses would retard basic motor skills.

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 gets damaged. This leads to paralysis or other permanent damage. Physical lesions or genetic abnormalities of the brain can also lead to major harm.

Physiological division

A less anatomical but much more functional way of dividing 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 20 nm 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 for the reaction time to be negligible in day to day situations. 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.

Plasticity

To be written

Development

Neural development in most species has many similarities with neural development in humans.

As shown in a 2008 study, one factor common to all bilateral organisms (including humans) is a family of secreted signaling molecules called neurotrophins which regulate the growth and survival of neurons^[7]. Zhu et al. identified DNT1, the first neurotrophin found in flies. DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in Drosophila. Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation.

In vertebrates, 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.

Pathology

The nervous system is susceptible to malfunction in a wide variety of ways, as a result of genetic defects, physical damage due to trauma or poison, infection, or simply aging. The medical specialty of Neurology studies the causes of nervous system malfunction, and looks for interventions that can alleviate it.

The central nervous system is protected by major physical and chemical barriers. Physically, the brain and spinal cord are surrounded by tough meningeal membranes, and enclosed in the bones of the skull and spinal vertebrae, which combine to form a strong physical shield. Chemically, the brain and spinal cord are isolated by the so-called blood-brain barrier, which prevents most types of chemicals from moving from the bloodstream into the interior of the CNS. These protections make the CNS less susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to have more serious consequences.

Although peripheral nerves tend to lie deep under the skin except in a few places such as the elbow joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Damage to nerves can also be caused by swelling or bruises at places where a nerve passes through a tight bony channel, as happens in carpal tunnel syndrome. If a peripheral nerve is completely transected, it will often regenerate, but for long nerves this process may take months to complete. In addition to physical damage, peripheral neuropathy may be caused by many other medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory conditions such as Guillain-Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or shingles, or poisoning by toxins such as heavy metals. Many cases have no cause that can be identified, and are referred to as idiopathic. It is also possible for peripheral nerves to lose function temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop in temperature, or chemical interactions with local anesthetic drugs such as lidocaine.

Physical damage to the spinal cord may result in loss of sensation or movement. If an injury to the spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the spine are actually destroyed, the loss of function is usually permanent. Experimental studies have shown that spinal nerve fibers attempt to regrow in the same way as peripheral nerve fibers, but in the spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves.

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 two systems: the central nervous system (CNS) and the peripheral nervous system (PNS).^[8]

References

External links

• Neuroscience for Kids
• The Human Brain Project Homepage
• Kimball's Biology Pages, CNS
• Kimball's Biology Pages, PNS