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Nervous System

Objective :

  • To learn the procedure for collection of different parts of nervous system
  • To learn the structure of the nervous system of rat

Theory :

Macroscopic Structure :

            The nervous system is divided into various functional domains. The dorsal portions of the brain and spinal cord coordinate incoming impulses, while the ventral regions control effector actions. Major activities include motion, sensation, association (“higher thought”), and control of homeostasis. Somatic components regulate the response to the external environment, while the visceral elements control the internal milieu. The motor system consists of a somatic motor domain, which conveys impulses to voluntary (skeletal) muscles, and a visceral motor portion, which supplies involuntary (cardiac and smooth) muscles. In like manner, the sensory system is composed of a somatic sensory part, which receives signals from the skin and body wall, and a visceral sensory domain that serves the internal organs. Associative domains integrate signals between lower and higher brain centers.

            To a large extent, functional domains are correlated with specific neuroanatomic sites. Within these regions, neurons with similar functions are arranged in stereotypical (“somatotopic”) patterns that are often, but not always, similar among species. In the mammalian forcebrain, the primary somatic centers are typically found in the dorsolateral cerebral cortex, with the motor are located rostral to the sensory area. The spinal cord exhibits a similar functional segregation, with input from the somatic and visceral sensory domains entering the dorsal gray matter, while signals from the somatic motor domain originate in the ventral gray matter. The visceral sensory and motor centers are located in nuclei of the mid, brain, brain stem, and intermediate column of gray matter in the spinal cord. The autonomic nervous system is primarily an elaboration of the visceral motor domain. The primary associative center is located in the prefrontal cortex, although white matter cross-connections within and between the hemispheres, and to subcortical centers, provide extensive additional associative capacities. Homeostatic activities usually are regulated at the unconscious level in the many nuclei of the limbic system (espcieally the hypothalamus), midbrain, and brain stem.

            A defined neuroanatomic structure may, in fact, have multiple subdivisions, each of which may serve a unique function. For example, the apparently homogenous neuronal populations seen in the corpus striatum and substantia nigra on routinely stained brain sections can be demonstrated to consist of anatomically and chemically distinct compartments using special stains. It is also true that distinct neuroanatomic structures may serve the same function. For instance, the internal division of the globus pallidus (in the diencephalons) and the pars reticulate of the substantia nigra (mid-brain) form a single functional unit. Thus, functional domains are more related to specific signal pathways (architectonic units) than to anatomical landmarks.

            The mammalian species of toxicologic importance may be divided into rodent (including mouse, rat), carnivores (cat and dog), and primate (monkeys and humans) classes, on the basis of neuroanatomic and functional criteria in the CNS. The most important differentiating features are the degree of brain fissuration, the size of certain spinal tracts, and the prominence of various cortical regions. The arrangement of sulci and gyri varies with both age and species, and within a species it varies between the right and left sides, presumably as a consequence of hemisphere-specific functional needs. In the spinal cord, many white matter tracts, including the dorsal funiculus (the main conduit for transmission of most tactile and proprioceptive sensory impulses) and corticospinal pathways (which mediate cortical control of voluntary motor functions), are largest in primates, of intermediate size in carnivores, and smallest in rodents. In the cerebral cortex, main functional domains have comparable locations in all mammalian species. However, in the carnivore, the primary sensory and motor areas and olfactory cortex account for 80% of the cortex, while these same zones in the primate represent less than 20% of the tissue. The most telling interspecies difference in cortical function is the ratio of paleocortex (which is largely controlled by the limbic system) to that of neocortex (which participates in associative and cognitive activities). Rodent cortex consists almost entirely of paleocortex, while the neocortex account for a large proportion of the primate brain.

            The chicken, which is occasionally used in neurotoxicology, has a unique neuroanatomical organization. Major anatomic differences relative to the mammalian pattern include the prominent jelly-filled opening (sinus rhomboidalis) in the dorsal midline of the lumbosacral spinal cord, the reduced size of the proprioceptive pathway (the dorsal funiculus and its rostral extension, the medial lemniscus), the greatly enlarged cerebellospinal tract (located just medial to the spinocerebellar tract in the lateral funiculus), and the absence of the corticospinal tract in the bird. These latter two findings reflect the markedly reduced thickness, the relatively unlamented nature of the avian cerebral cortex, and extensive subcortical control of avian motor functions by the enormous basal ganglia and the cerebellum.

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