Central nervous system (display 90)

The brain or encephalon is a complex organ that acts as the body’s control centre. Along with the spinal cord, it makes up the central nervous system which sends, receives and processes sensorial information and controls voluntary movements.

All vertebrates, apart from hagfish, have the same cerebral divisions, attesting to a shared evolutionary origin.

The forebrain or prosencephalon has two subdivisions known as the telencephalon and the diencephalon. The telencephalon, which is mainly composed of the two cerebral hemispheres, is responsible for the processing of sensory information, memory and reasoning. The diencephalon is made up of the thalamus, hypothalamus and epithalamus. It relays sensory information and connects the components of the endocrine system to the nervous system.

The midbrain or mesencephalon is situated between the forebrain and hindbrain. It is composed of the cerebral peduncles and the tegmentum. It controls movements and facilitates the processing of auditory and visual information.

The hindbrain or rhombencephalon is composed of the pons (annular protuberance), the cerebellum and the medulla oblongata. It contributes to the control of autonomic functions, the maintenance of balance, the coordination of movements and the relay of sensory information. 

Protected by the vertebral column, the spinal cord is connected to the medulla oblongata. It receives sensory information from the body and transfers it to the brain which, in turn, decodes and processes the message sending back an appropriate response.

The nervous system of all animals is made up of neurons and glial cells.

Neurons are composed of a cell body, of dendrites that receive external nervous impulses, and of an axon, of varying length, which ensures the transmission of the impulses. 

Glial cells provide structural and functional support to the neurons. Some, such as the oligodendrocytes, are responsible for the formation of the myelin sheath that surrounds the axons.

Within the brain and spinal cord are light-coloured zones, known as white matter, which contain myelinated axons, and darker zones, known as grey matter, composed of cell bodies, dendrites and axons without myelin. 

The central nervous system of fish is considered as a base model, common to all vertebrates. The encephalon only occupies part of the cranial cavity and can vary considerably in size from species to species. In fish, growth of the body and brain slows down over time but never stops. Lampreys and hagfish, which are not fish, have a more archaic brain. 

The telencephalon is principally made up of the olfactory bulbs, centres of smell. These are particularly well-developed in certain groups such as eels.

The mesencephalon is an important centre of sensory integration which receives almost all of the optical nerves and the nerve fibres coming from the auditory receptors, the lateral line (a sensory system that allows fish to perceive perturbations in the water) and the electroreceptors. 

The rhombencephalon unites the medulla oblongata and the cerebellum. The latter is absent in hagfish and is poorly developed in lampreys. The medulla oblongata encloses the sensory nuclei which receives messages from various receptors. Connections passing through the cerebellum allow the control of motor, visual, auditory, spatial and electrical activities. The degree of development of the cerebellum changes according to the animal's needs in its environment, and is the result of a long evolutionary process. In the Torpedinidae family, for example, the large volume of the cerebellum attests to increased specialization in the processing of electrical impulses.