Nervous system
The nervous system is a network of specialized nerve cells that conduct impulses from or to areas of the body to the brain and spinal cord and within the brain. It is composed of neurons and other specialized cells, like glial cells and neuroglia, that aid in the function of the neurons. Nerve cells are interconnected in complex arrangements and use electrochemical signals to transmit impulses between cells, they respond to a great variety of stimuli and form neural circuits that regulate an organisms perception and 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 grouped into both with gross anatomy, (which describes the parts that are large enough to be seen with the naked eye,) and microanatomy, (which describes the system at a cellular level.) At gross anatomy, the nervous system can be grouped in distinct organs, these being actually stations which the neural pathways cross through. Thus, with a didactical purpose, these organs, according to their ubication, can be divided in two parts: the central nervous system (CNS) and the peripheral nervous system (PNS).[2]
Central nervous system
The central nervous system (CNS) represents the largest part of the nervous system, including the brain and the spinal cord. The CNS is contained within the dorsal cavity, with the brain within the cranial cavity, and the spinal cord in the spinal cavity. The CNS is covered by the meninges. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae. The nervous system can be connected into many systems that can function together. The two systems are central nervous system (CNS) and the peripheral nervous system (PNS).
Peripheral nervous system
The PNS consists of all the other nervous structures that do not lie in the CNS. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) 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 effect glands, Interneurons connect neurons to other neurons with in 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
A less anatomical but much more functional division of the human nervous system is that classifying it according to the role that the different neural pathways play, regardless whether these cross through the CNS or the PNS:
The somatic nervous system is responsible for coordinating the body's movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.
Of digestion, it regulates from the esophagus to the stomach, small intestine and colon.
In turn, these pathways can be divided according to the direction in which they conduct stimuli:
- Afferent system by sensory neurons, which carry impulses from a receptor to the CNS
- Efferent system by motor neurons, which carry impulses from the CNS to an effector
- Relay system by relay neurons (also called interneurons), which transmit impulses between the sensory and motor neurones.
However, there are relay neurons in the CNS as well.
The junction between two neurones is called a synapse. There is a very narrow gap (about 20nm in width) between the neurons - the synaptic cleft, where an action potential is transmitted from one neuron to a neighboring one. They do this by relaying the message with the use of neurotransmitters which the next neuron then receives the electrical signal, known as a nerve impulse. The nerve impulse is determined by the neurotransmitter to then carry the message to its appropriate destination. These nerve impulses are a change in ion balance in the nerve cell, which the central nervous system can then interpret. The fact that the nervous system uses a mixture of electrical and chemical signals makes it incredibly fast, which is necessary to acknowledge the presence of danger. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the body would not tell the arm to move fast enough to escape dangerous burns. So the speed of the nervous system is a necessity for life.
| Physiological division | Somatic nervous system | Afferent system | ||
| Efferent system | ||||
| Autonomic nervous system | Sympathetic | Afferent system | ||
| Efferent system | ||||
| Parasympathetic | Afferent system | |||
| Efferent system | ||||
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
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.
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.
