Article: Nervous system

The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. Prominent participants in a nervous system include neurons and nerves, which play roles in such coordination.

In animals without brains, the nervous system does not generate or conduct thoughts and emotions. Thus it is the system that animates "animals" (sponges are an exception).

Coelenterata

Members of infrakingdom coelenterata, such as jellyfish and hydra, have a simple nervous system called a nerve net. This is a set of neurons, connected by synapses or gap junctions, that communicate with one another. The nerve net is centered around the mouth, but there are no anatomical groupings of neurons. Some jellyfish have sensory neurons known as rhopalia which can perceive light or gravity.

Flatworms and roundworms

Planarian, a type of flatworm, have dual nerve cords running along the length of the body and merging at the tail. 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 darkness.

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 hermaprodites, 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 controls the mouthparts.

Many arthopods 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.

Mollusca

Most molluscs, such as snails and bivalves, have several groups of intercommunicating neurons called ganglia. The nervous system of the sea hare Aplysia has been extensively used in neuroscience experiments because of its simplicity and ability to learn simple associations.

The cephalopods, such as squid and octopuses, have relatively complex brains. These animals also have complex eyes. As in all invertebrates, the axons in cephalopods lack myelin, the insulator that allows fast saltatory conduction of action potentials in vertebrates. To achieve a high enough conduction velocity to control muscles in distal tentacles, axons in the cephalopods must have a very wide diameter in the larger species of cephalopods. For this reason, the squid giant axons were used by neuroscientists to work out the basic properties of the action potential.

Vertebrates

The nervous system of vertebrate animals are often divided into a central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all other nerves and neurons that do not lie within the CNS. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be PNS. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. For example, the human brain is part of the nervous system.

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.

The autonomic nervous system is then split into the sympathetic division, parasympathetic division, and enteric division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and genitourinary systems.

See also

• neural network
• neuroendocrinology
• neuroscience
• neurotoxin
• neural ensemble
• somatic sensation
• zootomy