Published on March 1, 2014
Notes: Zoology- VI Semester, University of Mumbai, India Comparative Anatomy of Chordates Nervous system in Vertebrates Part I: Development and differentiation of brain in Vertebrates 1. The entire nervous system arises from the ectoderm. 2. The ectodermal neural folds are developed which close dorsally to form neural tube. Development and differentiation of primary brain vesicles and their cavities: The vesicles of vertebrate brain: 1. Anterior part of neural tube is slightly swollen which initially differentiated in to three primary vesicles of brain namely first prosencephalon (forebrain), second mesencephalon (midbrain) and third rhombencephalon (hindbrain) containing cavities. 2. At later stages two secondary vesicles form in the area of the prosencephalon, 1a and 1b (see fig. below) developing into telencephalon and diencephalon respectively. 3. The diencephalon gives rise to sac like lateral outgrowths called the optic vesicles. The optic vesicles are pushed downward by two large processes growing forward from the anterior vesicle (the primitive cerebral hemispheres). These ultimately develop into the retina, and other nervous parts of the eye. 4. The other secondary vesicles are developed in the rhombencephalon namely 3a and 3b (see fig. below) developing into metencephalon and myelencephalon respectively. 5. Mesencephalon reduces to form a narrow commissure known as cerebral aqueduct (see fig. below). Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India The relationships between these early structures and the mature nervous system are summarized in table-1 below. Table-1: The Main Subdivisions of the Embryonic Central Nervous System and Mature Adult Forms Three-vesicle stage Five-vesicle stage Major mature derivatives Related cavity 1. Forebrain 1a. Telencephalon 1. Cerebral cortex, basal ganglia, Lateral ventricles (prosencephalon) (endbrain) hippocampal formation, amygdala, olfactory bulb 1b. Diencephalon 2. Thalamus, hypothalamus, Third ventricle subthalamus, epithalamus, retina, optic nerves and tracts 2. Midbrain 2. Mesencephalon 3. Midbrain Cerebral aqueduct (mesencephalon) (midbrain) 3. Hindbrain 3a. Metencephalon 4. Pons and cerebellum Fourth ventricle (rhombencephalon) (afterbrain) 3b. Myelencephalon 5. Medulla Fourth ventricle (medullary brain) 4. Caudal part of 4. Caudal part of 6. Spinal cord Central canal neural tube neural tube The cavities of vertebrate brain: 1. The cavities of brain are referred as ventricles. There are four ventricles in vertebrate brain namely 1. Olfactory ventricle. 2. Lateral ventricle. 3. Third ventricle. 4. Fourth ventricle 2. The olfactory lobes carry cavities known as olfactory ventricles arising from the anterior part of the cerebral hemispheres. These grow forward, and soon lose olfactory ventricles. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India 3. The telencephalic vesicles become the cerebral hemispheres, and their cavities become the paired lateral ventricles (lateral telocoeles) of the adult brain. 4. The third ventricle lies in the diencephalon (diocoele). Later in development the lateral walls of the diencephalon become greatly thickened to form the thalami, thus reducing the size and changing the shape of the diocoele, which is known in adult anatomy as the third brain ventricle. 5. The fourth ventricle lies in metencephalon. The anterior part of the fourth ventricle is known as metacoele and the posterior part as myelocoele. Flexures of brain: 1. As the result of unequal growth of these different parts of brain, three flexures are formed and the embryonic brain becomes bent on itself in a somewhat zigzag fashion; the two earliest flexures are concave ventrally and are associated with corresponding flexures of the whole head. 2. The first flexure appears in the region of the mid-brain, and is named the ventral cephalic flexure. By means of it the fore-brain is bent in a ventral direction around the anterior end of the notochord and fore-gut, with the result that the floor of the fore-brain comes to lie almost parallel with that of the hind-brain. This flexure causes the mid-brain to become, for a time, the most prominent part of the brain, since its dorsal surface corresponds with the convexity of the curve. 3. The second bend appears at the junction of the hind-brain and medulla spinalis. This is termed the cervical flexure, and increases from the third to the end of the fifth week, when the hind-brain forms nearly a right angle with the medulla spinalis; after the fifth week erection of the head takes place and the cervical flexure diminishes and disappears. 4. The third bend is named the pontine flexure, because it is found in the region of the future pons Varoli. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India 5. Both the cervical and the pontine flexures eventually straighten out, but the cephalic flexure remains prominent throughout development. Evolution of cerebral hemispheres & cerebellum with reference to shark, frog, lizard, pigeon & rabbit: 1. Telencephalon, called lamina terminalis and the roof of the cerebrum is called cortex or pallium. The telencephalon then appears to be composed of two divisions, the anterior of which is subsequently developed into the cerebral hemispheres, corpora striata, and the olfactory lobes. The hemispheres undergo enormous enlargement in their later development and extend dorsally and posteriorly as well as anteriorly, eventually covering the entire diencephalon and mesencephalon under their posterior lobes. 2. Posterior part of forebrain, the diencephalon, consisting of the thalamus and hypothalamus, representing the anterior vesicle. The median evagination in the roof of the diencephalon develops into epiphysis. 3. The mesencephalon becomes specialized as the optic lobes, visual centers associated with the optic nerves. The dorsal and lateral walls of the mesencephalon later increase rapidly in thickness and become the optic lobes (copora quadrigemina) of the adult brain. . It serves as the main pathway of the fiber tracts which connect the cerebral hemispheres with the posterior part of the brain and the spinal cord. 4. The hindbrain became divided into anterior metencephalon and posterior myelencephalon. The metencephalon shows ventrally and laterally an extensive ingrowth of fiber tracts giving rise to the pons and to the cerebellar peduncles of the adult metencephalon. The roof of the metencephalon undergoes extensive enlargement and becomes the cerebellum of the adult brain. The ventro-lateral wall of the cerebrum becomes thick and called corpus striatum. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India The ventral and lateral walls of the myelencephalon become the floor and side-walls of the medulla of the adult brain. The medulla becomes specialized as a control center for some autonomic and somatic pathways concerned with vital functions (such as breathing, blood pressure, and heartbeat). The dorsal wall of myelencephalon is thin and receives rich blood vessels thus forming posterior choroid plexus. The pons is above the medulla and also acts as a connecting tract. The cerebellum enlarged and became a structure concerned with balance, equilibrium, and muscular coordination. Part II: Comparative anatomy of brain in vertebrates: Brain of all vertebrates, from fish to man, is built in accordance with the same architectural plan. However, form of brain differs in different vertebrates in accordance with the habits and behavior of the animals. 1. Elasmobranches: Olfactory lobes are correspondingly large. Optic lobes and pallium are relatively moderate in size. Saccus vasculosus, thin-walled vascular sensory organ, is attached to pituitary and connected with cerebellum. Pineal apparatus is well-developed. Cerebellum is especially large due to active swimming habit. Ruffle-like restiform bodies are present. 2. Amphibians: Olfactory lobes are smaller than optic lobes. Corpus striatum receives greater number of sensory fibres. Cerebral hemispheres are more developed than in fishes. Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India Less-developed cerebellum. Medulla is also small. Pineal body is small. 3. Reptilians: Telencephalon becomes the largest region of brain. Olfactory lobes are larger than in amphibians. A pair of auditory lobes is found posterior to optic lobes. The third ventricle is reduced to a narrow cerebral aqueduct. Cerebellum is somewhat pear-shaped and larger than in amphibians. Modern reptiles show a great development in the basal parts of the forebrain. There are large numbers of nervous connections between the thalamus and the hemispheres. The latter are larger than the optic lobes, showing the increased importance of the former. The walls of the thalamus are very thick and many of the optic nerve paths end there as well as many in the midbrain. 4. Birds: Brain is proportionately larger than that of a reptile. Olfactory lobes are small. Two cerebral hemispheres are larger, smooth and projected posteriorly over the diencephalon to meet the cerebellum. Pallium is thin but corpus striatum is greatly enlarged. Optic lobes are conspicuously developed. The cerebellum is greatly enlarged with several superficial folds. 5. Mammals: Brain is proportionately larger than in other vertebrates. Cerebral hemispheres of Prototheria, Metatheria & Eutheria are smaller & smooth, larger & smooth, greatly enlarged & divided into lobes respectively. The two hemispheres are jointed internally by transverse band-like fibres called corpus callosum. Olfactory lobes are relatively small but well defined. Four almost solid optic lobes are present. The mammalian brain is completely dominated by the cerebral hemispheres. The roof has developed enormously and spread out forming the cerebral cortex which in humans is thrown into a number of elaborate folds and almost covers the rest of the brain. The cortex is made up millions of cells. The more folded the surface, the more cells it can contain. These cells make up the gray matter. Their axons, which make up the tracts or pathways in the brain, form the white Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
Notes: Zoology- VI Semester, University of Mumbai, India matter underneath the cortex. The white matter of the spinal cord is also made up of nerve axons, surrounding the central gray matter. Conclusion: Discussing the evolution of brain in these vertebrate groups, it is clear that they are originated from a common ancestral stock. Their general plan of brain supports this. Prepared by Mr. S. D. Rathod Associate Professor Department of Zoology B. N. Bandodkar College of Science, Thane Prof. S. D. Rathod, B. N. Bandodkar College of Science, Thane -400605
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