nervous-system

autonomic nervous system

Pathways of the autonomic nervous system. Nerve impulses begin in motor neurons in the brain or elipsis

Part of the nervous system that is not under conscious control and that regulates the internal organs. It includes the sympathetic, parasympathetic, and enteric nervous systems. The first, which connects the internal organs to the brain via spinal nerves, responds to stress by increasing heart rate and blood flow to the muscles and decreasing blood flow to the skin. The second comprises the cranial nerves and the lower spinal nerves, which increase digestive secretions and slow the heartbeat. Both have sensory fibres that send feedback on the condition of internal organs to the central nervous system, information that helps maintain homeostasis. The third division, embedded in the walls of the stomach and intestines, controls digestive movement and secretions.

Learn more about autonomic nervous system with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

nervous system

System of specialized cells (neurons, or nerve cells) that conduct stimuli from a sensory receptor through a neuron network to the site (e.g., a gland or muscle) where the response occurs. In humans, it consists of the central and peripheral nervous systems, the former consisting of the brain and spinal cord and the latter of the nerves, which carry impulses to and from the central nervous system. The cranial nerves handle head and neck sensory and motor activities, except the vagus nerve, which conducts signals to visceral organs. Each spinal nerve is attached to the spinal cord by a sensory and a motor root. These exit between the vertebrae and merge to form a large mixed nerve, which branches to supply a defined area of the body. Disorders include amyotrophic lateral sclerosis, chorea, epilepsy, myasthenia gravis, neural tube defect, parkinsonism, and poliomyelitis. Effects of disorders range from transient tics and minor personality changes to major personality disruptions, seizures, paralysis, and death.

Learn more about nervous system with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Central nervous system

In vertebrates, the central nervous system (CNS) is the part of the nervous system which is enclosed in the meninges. It contains the majority of the nervous system and consists of the brain (in vertebrates which have them), and the spinal cord. Together with the peripheral nervous system it has a fundamental role in the control of behavior. The CNS is contained within the dorsal cavity, with the brain in the cranial cavity and the spinal cord in the spinal cavity. The brain is protected by the skull, while the spinal cord is protected by the vertebrae.

Function

Since the strong theoretical influence of cybernetics in the fifties, the central nervous system is conceived as a system devoted to information processing, where an appropriate motor output is computed as a response to a sensory input. Yet, many threads of research suggest that motor activity exists well before the maturation of the sensory systems and then, that the senses only influence behaviour without dictating it.

Neuroanatomy

The telencephalon gives rise to the striatum (caudate nucleus and putamen), the hippocampus and the neocortex, its cavity becomes the lateral ventricles (first and second ventricles). The diencephalon give rise to the subthalamus, hypothalamus, thalamus and epithalamus, its cavity to the third ventricle. The mesencephalon gives rise to the tectum, pretectum, cerebral peduncle and its cavity develops into the mesencephalic duct or cerebral aqueduct. Finally, the rhombencephalon gives rise to the pons, the cerebellum and the medulla oblongata, its cavity becomes the fourth ventricle.

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

Evolution

Planarians, members of the phylum Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a central nervous system (CNS) and a peripheral nervous system (PNS). Their primitive brain, consisting of 2 fused anterior ganglia, and longitudinal nerve cords form the CNS and the laterally projecting nerves form the PNS. A molecular study found that more than 95% of the 116 genes involved in the nervous system of planarians, which includes those related to the planarian CNS, also exist in humans. Like planarians, vertebrates have a distinct CNS and PNS, though those of vertebreates display greater complexity.

The basic pattern of the CNS is highly conserved throughout the different species of vertebrates and during evolution. The major trend that can be observed is towards a progressive telencephalisation: while in the reptilian brain that region is only an appendix to the large olfactory bulb, it represents most of the volume of the mammalian CNS. In the human brain, the telencephalon covers most of the diencephalon and the mesencephalon. Indeed, the allometric study of brain size among different species shows a striking continuity from rats to whales, and allows us to complete the knowledge about the evolution of the CNS obtained through cranial endocasts.

Mammals – which appear in the fossil record after the first fishes, amphibians, and reptiles - are the only vertebrates to possess the evolutionarily recent, outermost part of the cerebral cortex known as the neocortex. The neocortex of monotremes (the duck-billed platypus and several species of spiny anteaters) as well as that of marsupials (such as kangaroos, koalas, opossums, wombats, Tasmanian devils, etc.) lack the convolutions - gyri and sulci - found in the neocortex of most placental mammals (eutherians). Within placental mammals, the size and complexity of the neocortex increased over time. The area of the neocortex of mice is only about 1/100 that of monkeys, and that of monkeys is only about 1/10 that of humans. In addition, rats lack convolutions in their neocortex (possibly also because they are small mammals), whereas the neocortex of cats has a moderate degree of convolutions, and that of humans exhibits quite extensive convolutions.
See also: Encephalization, Neocortex, Archicortex