100Nervous system

Nervous System

The nervous system is a network of specialized cells that communicate information about an organism'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 glial cells,(plural form 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 processes and conducts signals back 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 regulate an organism's 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.

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).

Central Nervous System[]

Main article: Central nervous system

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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[]

Main article: 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 cell bodies of afferent PNS nerves lie in the dorsal root ganglia.


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


Main article: Neuron

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[]

Main article: Glial cell

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.[1]

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[]

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.


Main article: Neural development in humans

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.


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.


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.



  1. Azevedo FA, Carvalho LR, Grinberg LT, Farfel JM, Ferretti RE, Leite RE, Jacob Filho W, Lent R, Herculano-Houzel S. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol. 513(5):532-41. Template:PMID