In thinking of the nervous system one typically describes that found in VERTEBRATES, which includes the CENTRAL NERVOUS SYSTEM (BRAIN and SPINAL CORD, and associated cranial and SPINAL NERVES); and the PERIPHERAL NERVOUS SYSTEM, which includes the SOMATIC NERVOUS SYSTEM and the AUTONOMIC NERVOUS SYSTEM (which is divisible into the SYMPATHETIC NERVOUS SYSTEM, PARASYMPATHETIC NERVOUS SYSTEM and the ENTERIC NERVOUS SYSTEM). Vertebrate nervous systems may differ in detail (see for example RODENT VS. PRIMATE PREFRONTAL CORTEX) but all work to this common plan. But of course, the INVERTEBRATES also have nervous systems. The simplest form of nervous system is the NERVE NET, a system of neurons (see NEURON) that branch throughout an organisms’ body: cnidarians (hydras) have these. Slightly more complex are NERVE RINGS, which are centralized within a body, with RADIAL NERVES extending away from this: echinoderms—such as the starfishes—have these. Many invertebrates have bodies that are bilaterally symmetrical and have nervous systems that reflect this. NERVE CORDS—thick bundles of neurons—are frequently present, either singly, running along the middle of the animal (as is the case with, for example, leeches) or with two nerve cords, one on either side of the body (as is the case with flatworms such as Planaria). More complex invertebrate nervous systems show the development of GANGLIA (singular, GANGLION), which are clusters of neurons. In some species there may be multiple ganglia, often systematically reflecting different body segments. From ganglia, rudimentary brains develop.
Just as in mammalian NEURODEVELOPMENT, the brain develops as an extension of the NEURAL TUBE and is enclosed with the CRANIUM, ganglia that are gathered in the head end of an invertebrate body are sometimes referred to as brains, or head ganglia. The degree to which this occurs is referred to by the term CEPHALIZATION: the flatworms are the most primitive group of invertebrates to show any degree of cephalization, while the most encephalized invertebrates are the cephalopods (cuttlefish, squid and octopus—the term cephalopod is from Greek, kephale: head, and podos: foot—cephalopods have a highly cephalized head, and the feet have been specialized into tentacles radiating from that).
Note well that there are some difficulties with terminology: CEPHALIZATION is used to indicate the degree to which an animal has developed a head, with specialized sensory receptors, feeding organs and, perhaps most importantly, a concentration of neural tissue (either a head ganglion or a brain). The term ENCEPHALIZATION has been used in three ways. (i) It is a simple index of the degree to which the CEREBRAL CORTEX has developed. (ii) Encephalization has been used to describe the degree to which more recently evolved tissue—the cerebral cortex and particularly the NEOCORTEX—has taken over functions that were previously regulated by structures further down the NEURAXIS. It remains uncertain to what degree functions are ‘taken over’ as opposed to ‘made more sophisticated’ (that is, do older and newer structures combine with each other or do newer structures comprehensively take over the functions of older ones?). (iii) The word encephalization features in the ENCEPHALIZATION QUOTIENT (EQ) which is the ratio of log brain mass to log body mass. The encephalization quotient assumes that animals that are in some sense more intelligent (see INTELLIGENCE) have brains that are relatively larger in proportion to their body size compared to rather less intelligent animals. It is a measure that was introduced in the 1970s by H.J.Jerison, and was initially used in assessment of the likely intelligence of extinct animals, principally, of course, dinosaurs (see PALAEONEUROLOGY; DINOSAUR BEHAVIOR). It is a relatively crude measure: later authors interested in determining the relative intelligence of different species have taken measures such as a NEOCORTEX RATIO, the ratio of neocortical mass to the total brain mass. Similar measures could be taken for any identifiable brain structure.
