Brain of vertebrates

Brain of vertebrates

1. By- Sanju Sah St. Xavier’s college, Maitighar , Kathmandu Department of Microbiology Comparative Anatomy of Vertebrate Brain

2. Development of Brain The anterior end of embryonic ectodermal neural tube is thickened known as ‘Encephalon’/embryonic brain. Encephalon undergoes differential growth and two constrictions appear dividing it into a linear series of three primary cerebral vesicles: Prosencephalon (fore brain), Mesencephalon (mid brain) and Rhombencephalon (hind brain).

3. Development of Brain Prosencephalon : Prosencephalon is divided into anterior Telencephalon and posterior diencephalon . Telencephalon grows and forms paired cerebral hemispheres. From anteroventral part of telencephalon paired olfactory lobes ( Rhinencephalon ) grow anteriorly. Cerebral hemispheres enclose lateral ventricles (1 st and 2 nd )

4. Diencephalon has upper epithalamus , middle thalamus and lower hypothalamus . Dorsally , diencephalon gives out unpaired parietal body and pineal body. Ventrally hypothalamus forms infundibulum that fuses with pituitary gland. Diencephalon encloses third ventricle .

5. Development of Brain Mesencephalon: Mesencephalon has thick roof forming two optic lobes (4 in mammals) and two longitudinal bands of nerve fibres called cerebral peduncles or crura cerebri .

6. Rhombencephalon : - Anterior dorsal part forms metencephalon or cerebellum . - Remaining part forms myelencephalon or medulla oblongata . In higher vertebrates (mammals) the floor of metencephalon thickens due to the tracts of many nerve fibres and forms distinct pons varolii . Rhombencephalon encloses 4 th ventricle .

7. Development of Brain Generalized Pattern of Vertebrate Brain Foramen of Monro Iter or Cerebral aqueduct

8. Development of Brain Divisions Subdivisions Parts Cavity PROSENCEPHALON 1. Telencephalon Rhinencephalon Rhinocoel Cerebral Hemispheres Lateral ventricles (1 st and 2 nd ) 2. Diencephalon Epithalamus III ventricle Thalamus Hypothalamus MESENCEPHALON - Optic lobes & Crura Cerebri RHOMBENCEPHALON Metencephalon Cerebellum IV ventricle 2. Myelencephalon Medulla Oblongata

9. Neural Plate Neural Tube

10. Comparative Account of Vertebrate Brain Cyclostomes: Brain is very primitive without well marked subdivisions. Olfactory lobes are distinct . Cerebral hemispheres are small. Lateral ventricles are rudimentary. Parietal and pineal bodies are well developed. Two optic lobes are imperfectly differentiated. Cerebellum is small transverse dorsal band - sluggish animals. Medulla oblongata is very well developed.

11. b . Fishes Elasmobranchs / Chondrichthyes Olfactory organs are enormous, so olfactory lobes of brain are proportionately large, attached to cerebrum by short olfactory peduncle. Cerebrum is undivided. Diencephalon is narrow with well developed dorsal pineal body. Mid brain is very large with two oval optic lobes; centre for nervous coordination. Cerebellum is especially large due to active swimming habit. To assist cerebellum in maintaining equilibrium, Restiform bodies are present in antero -lateral angles of medulla oblongata.

12. ii.Teleosts / Osteichthyes : Olfactory lobes are large without olfactory peduncles. Cerebral hemispheres and diencephalon are smaller. Optic lobes and cerebellum are larger than elasmobranch . The antero -lateral sides of medulla oblongata shows unusual bulging or vagal lobes for taste.

13. c. Amphibians: Forebrain is large. Olfactory lobes are smaller but cerebral hemispheres are larger in accordance with more complex activities of locomotion, hibernation, breeding etc. Cerebral hemispheres are separated by a median longitudinal fissure . Diencephalon is short with vestigial pineal body. Mid brain is small but well developed forming two large optic lobes (corpora digemina). Optic lobes are the probable dominant coordinating centre in amphibian brain. Cerebellum is poorly developed with just a transverse band on dorsal side. Medulla oblongata is short.

14. d. Reptilians: Reptilian brain shows advancement in size and proportions over that of amphibians because of complete terrestrial mode of life. Cerebral hemispheres are larger enclosing lateral ventricles. Two long olfactory lobes are connected to cerebral hemispheres which are larger than that of amphibians. Diencephalon is small and rounded. Mid brain has large optic lobes, may be the important coordinating centre. Cerebellum is small, simple and some what pear shaped; larger than that of amphibians.

15. e. Birds Brain of birds is relatively larger but short, broad and rounded in shape. Olfactory bulbs are small and degenerate due to poor sense of smell. Cerebral hemispheres are very large and smooth; responsible for intelligent behavior and to control reflex behavior. Diencephalon is covered by cerebrum and cerebellum; bears a small pineal body. Mid brain is highly developed with large rounded optic lobes. Optic lobes are very well developed correlated with keen sight and are laterally placed. Cerebellum is large with several superficial folds ( flocculi ) extending forwards. It is greatly enlarged due to many activities involving muscular coordination and equilibrium such as flight and perching.

16. f. Mammals: Brain is proportionately larger. Cerebral hemispheres are immense, projecting forward above olfactory lobes and behind above the diencephalon and mid brain. In Prototheria : Cerebral hemispheres are small and smooth. In Metatheria : They are larger but smooth. In Eutheria : they are greatly enlarged with convoluted surface. The elevations of convolutions are called ‘ Gyri ’ and the depressions are called ‘ Sulci ’. The convolution/folding increases surface area of cerebral cortex resulting in greater intelligence. Two cerebral hemispheres are separated by median longitudinal fissure and are joined internally by transverse band of fibres – Corpus callosum .

17. Mammals (Contd.) Olfactory lobes are relatively smaller. Diencephalon and mid brain are also covered by cerebrum. Mid brain bears 4 optic lobes (corpora quadrigemina ). Cerebellum is also very large, highly folded bearing ‘folia’. Pons varolii (present only in mammals) bears crossing fibres connecting opposite sides of cerebrum and cerebellum. Medulla oblongata lies ventrally and is much thickened. It bears involuntary centres controlling respiration, heart rate, diameter of blood vessels etc.

18. Cranial Nerves Anamniotes : 10 pairs Amniotes: 12 pairs