BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.3. Evolution of the Nervous System

Studies of the evolutionary METABOLISM/13.html">History of the animal kingdom demonstrate a gradual complication of The Nervous System in the series ranging from Coelenterates to mammals.

Class="center">17.3.1. Coelenterates

The Emergence of multicellular Organization in coelenterates (Cnidarians) led to the spatial Separation of signal perception from the response to it, i.e., the receptor from the effector. However, THE ORIGIN OF multicellularity was accompanied by The formation of specialized Nerve Cells connecting the receptor and effector. The nervous system of primitive cnidarians, such as the hydra (Hydra), is represented by a nerve net, or plexus, consisting of a single layer of Neurons. Numerous short processes of neurons connect with each other, forming a meshwork that permeates the entire body of the animal. Impulses propagate through it in all directions, with a fraction of the impulses fading at each synapse; these fading impulses are utilized to activate synapses so that subsequent impulses can pass through them (a process termed facilitation). This is known as decrementing (decremental) conduction. The transmission of nerve signals in such organisms proceeds slowly due to the Abundance of synapses along their path and is spatially limited, since impulses gradually fade with distance from the site of stimulation. Such a system provides localized responses, such as tentacle movement, but is of little use to the Organism as a whole unless the stimulation is intense or prolonged. In most cnidarians, such as scyphozoans and sea anemones, In addition to the nerve net, There is a system of bipolar neurons forming so-called through-conduction pathways, along which impulses propagate rapidly over considerable distances without noticeable fading. This system allows the organism to mount a fairly rapid generalized response to threatening stimuli (e.g., withdrawing all tentacles) and can be regarded as the first step toward the integration of neurons into nerves, as observed in more highly organized animals.

17.3.2. Annelids

In annelids (typified by the earthworm), the integration of neurons has led to the formation of a nervous system consisting of a single longitudinal cord running the entire length of the body—the ventral nerve cord. This cord consists of segmentally arranged paired ganglia connected by nerve tracts. Segmental nerves branch out from each ganglion to the Organs and Tissues of that segment (see Fig. 18.26).

As a consequence of the unidirectional mode of locomotion in these animals, a distinct HEAD evolved. This Structure AIDS in locating and ingesting food, and since it is the first to encounter new environmental elements, it concentrates the primary Sensory Organs that perceive external information. The increased influx of information from these organs into the nervous system resulted in the enlargement of the anterior nerve ganglia, i.e., the formation of a primitive type of "Brain." This concentration of feeding, sensory, and neural structures in the head region is termed cephalization, or simply the Evolution of the head. It should be noted that this term refers to The Development of all features that distinguish the head from the rest of the body, rather than just the Development of the brain. The latter case is referred to as encephalization.

The nervous system of annelids is structured According to the plan typical of all other invertebrates. The expanded anterior section of the nerve cord forms a pair of cerebral ganglia (brain ganglia) located above the Pharynx and connected to the ventral nerve cord by circumoesophageal connectives.

17.3.3. Arthropods

In arthropods, such as insects, the organizational principle of the nervous system is practically identical to that of annelids, except that their cerebral ganglia lie dorsal to the gut. These ganglia are analogous to the vertebrate brain, but do not play the same dominant role as the latter. For instance, the removal of the head in an invertebrate has little effect on its movements, whereas in vertebrates all movements are initiated and controlled by the brain. Apparently, in invertebrates the cerebral ganglia act merely as Relay centers between receptors and effectors, and their role in the integration and coordination of Functions is restricted to a few neuroendocrine influences affecting, for example, the timing of reproduction in certain annelids or molting in arthropods.