Comparative Account of Brain in Vertebrates

Date: 15.01.21 UNIT-IV Comparative account of brain in vertebrates: Introduction: Brains of all vertebrates from fish to man is built in accordance with the same basic architectural plan. However, the form of brain differs in different vertebrates in accordance with the habits and behaviour of the animals. a) Cephalochordata: In amphioxus, the brain is made up of an anterior prosencephalon or cerebral vesicle with a single enlarged ventricle. Midbrain and hindbrain are absent, in some cases absence of forebrain is also found. b) Cyclostomes: Brain is very primitive. Sub-divisions are not prominent but in some cases olfactory lobes are large and developed. The cavities of cerebral hemispheres are rudimentary. From the dorsal aspect of the brain, cephalon has a median eye area developed. Encephalic lobes are not even differentiated. Medulla oblongata is well developed. c) Fishes: 1) Elasmobranchs: In elasmobranchs, olfactory lobes are large, attach to the cerebrum by short olfactory tracts. It is also known as peduncles. Optic lobes are relatively moderate in size. (ii) Elasmobranch: The brain cavity is large and extends onto optic lobes. Pituitary and pineal apparatus is well developed. (iii) Osteichthyes: Very specialized than elasmobranchs. In perch, olfactory lobes and cerebral hemispheres are smaller, while optic lobes and cerebellum are larger. Some bony fishes have a large diencephalon. In bottom feeders, the anterolateral sides of medulla show enlarged bulged sides, which are known as vagal lobes. (iv) Amphibians: A cranial, olfactory lobes and larger optic lobes are present. Floor of cerebrum receives greater number of sensory fibers which is known as corpus striatum or paleostriatum. Two cerebral hemispheres show greater development with more complex activities which affect locomotion, hibernation, breeding, etc. Optic lobes are probably the dominant over the other lobes. The walls of the midbrain are thickened and then reduce to a narrow passage-like structure called aqueduct. Poor development of cerebellum shows decrease in muscular activities. Medulla is small and a small pineal body is present. (v) Reptiles: Reptilian brain shows advancement in size and proportion over that of amphibians because of complete terrestrial mode of life. In all amniotes, the midbrain is the center of brain activity and supplies for the first time their greatest nerve center to the cerebrum. Telencephalon increases to become the largest region of the brain. Seven long olfactory lobes are connected to cerebral hemisphere. A pineal nerve is present which goes to the olfactory bulbs. A pair of auditory lobes are found posterior to optic lobes. Cerebellum is larger than in amphibians. (vi) Birds: Avian brain is proportionately larger than that of a reptile. But the avian brain is broad and rounded in comparison to reptile. Olfactory lobes are extremely small and degenerate for birds' poor sense of smell and secure cere of birds. Two cerebral hemispheres are larger and project posteriorly over the diencephalon. 3rd ventricle of brain is also narrow. The diencephalon has small dorsal pineal body and an anterior choroid plexus. Optic diencephalon has small dorsal pineal body and an anterior choroid plexus. Optic lobes are exceptionally large enough. The cerebellum is greatly enlarged and superficially folded known as flocculi, which evolved for muscular coordination, flight and perching. In the cerebellum, the inner mass of white matter presents a branching pattern called arbor vitae. (vi) Mammals: The brain reaches its highest development in mammals. Cerebral hemispheres of prototheria are smaller and smooth like that of reptiles, but in metatheria they are large and smooth. In higher mammals like Eutheria, cerebral hemispheres are greatly enlarged and divided into lobes with thick cerebral cortex of grey matter. The two hemispheres are fused externally by transverse nerve fibres called corpus callosum but it is not found in other vertebrates or even in prototheria and metatheria. Olfactory lobes are small and covered by the hemispheres. Diencephalon and hindbrain are also covered by cerebral hemisphere. There are four optic lobes called corpora quadrigemina found on the roof of midbrain. Cerebellum is large, convoluted, and developed into a number of lobes (central lobe or vermis, two lateral lobes or floccular lobes). The medulla oblongata lies posteriorly and is much thickened. It has a dorsal posterior choroid plexus. The medulla evolves for the regulation of digestion, respiration and circulation. The end SPINAL CORD: Date: 17.07.21 11:00 am Spinal cord or nerve cord is from the neural tube behind the brain. Thus, it is the posterior continuation of medulla oblongata. It runs posteriorly from the foramen magnum of occipital bone of skull and through the neural canal of vertebral canal upto lower end of the 1st lumbar at right 3rd lumbar vertebrae in case of mammals. However, it extends to the posterior end of the tail in fishes, while in amphibians, reptiles, and birds, it extends the full length of vertebral column. In cyclostomes, fishes, and in some limbless amphibians, the nerve cord has a uniform diameter but in tetrapods, it has two enlargements called as cervical and lumbar enlargements.