Chapter 15 Nervous System Organization and Biological Clocks
The Organization and Evolution of Nervous Systems
Animals have evolved nervous systems with varying degrees of centralization and complexity. There are homologies between the nervous systems of different animal groups.
Most phyla of animals have bilateral symmetry and have evolved central nervous systems (CNSs) that centralize control functions. Sensory neurons convey information into the CNS, and motor neurons convey outward commands to effectors. CNSs usually have some degree of cephalization (concentration of neural structures into a clear anterior brain).
Arthropods have a ganglionic nervous system, one major form of nervous system organization. The arthropod CNS is a ventral ladderlike chain of segmental paired ganglia joined by connectives. A vertebrate CNS, in contrast, is a continuous column of cells and axons.
The Vertebrate Nervous System: A Guide to the General Organizational Features of Nervous Systems
The CNS of vertebrates consists of the brain and spinal cord. Cranial and spinal nerves emanate from the CNS to form the PNS. The brain is divided into a forebrain, midbrain, and hindbrain; the forebrain is enlarged in birds and especially in mammals.
Vertebrate brain functions are somewhat localized. However, brain functions are also somewhat distributed, involving circuits rather than centers.
Many vertebrate brain regions preserve the orderly spatial arrangements of the corresponding external world, for example, as somatotopic maps of body sensory input and motor output.
Brains change with development, experience, and learning and memory. Understanding the structural and synaptic bases of these changes is a major challenge to investigators.
The PNS of vertebrates has a somatic division that controls skeletal muscle and an autonomic division that controls effectors associated with internal organs. The autonomic nervous system is divided into sympathetic and parasympathetic divisions, which usually have opposite physiological effects, and the enteric division, which controls gut contraction and other aspects of digestive tract physiology.
A circadian rhythm has a period of about a day. It is an example of an endogenous rhythm, one that does not require sensory information for timing.
A circadian rhythm of an animal will drift, or free-run, in constant light or darkness, when there are no sensory timing cues. A light–dark cycle entrains the circadian rhythm to exactly 24 h.
A biological clock is the physiological basis of an animal’s ability to time an endogenous rhythm. Biological clocks exert rhythmically changing control, modulating the outputs of the nervous and endocrine systems to prepare an animal for daily changes and seasonal changes. In mammals, the suprachiasmatic nucleus (SCN) of the brain is the principal biological clock for circadian rhythms.
Animals may possess other timing mechanisms for shorter rhythmic periods (such as circatidal rhythms) or longer periods (such as circannual rhythms) than those of circadian rhythms.