Evolution of the Brain

1. Evolution of the Brain • Ontogeny recapitulates phylogeny? • Ontogeny of brain is rapid. • Mirrors what we see in the evolutionary history of the vertebrates. • Begins as an expanded neural tube • Has Olfactory, Optic, and Otic lobes.

2. Note: Prosencephalon, Mesencephalon, and Rhombencephalon correspond to the Olfactory, Optic, and Otic portions of the brain.

4. Evolution of the Brain • Notice, there are several flexures in the brain. • The basic nerve tube (primordial brain) has the basic afferent and efferent components. • Components above the ‘tube’ generally are association centers. • Association neurons can be complex and operate on several levels.

5. Brain Architecture • Some re-organization in the brain • Reticular system • Basic motor control of locomotion in lower vertebrates. • Still present in mammals, but mediated by higher control centers. • Brain is bilaterally symmetric, with ‘wiring’ between halves.

7. Medulla oblongata • Lower vertebrates: essentially comparable to anterior section of spinal cord. • Contains cranial nerves III, IV, V, VI, VII, IX, and X. • In Higher vertebrates, the efferent and afferent columns are broken down into discrete nuclei. • Contains reflex circuits for sensory reception and responding effector organs of the head and gill region.

10. Cerebellum • Coordination and regulation of activities and the maintenance of posture. • Equilibrium and body orientation. • In mammals, plays an important role in locomotion. • Primary inputs are 1) acoustico-lateralis and 2) proprioceptors. • Skink, optic, and olfactory inputs.

11. Midbrain and Diencephalon • All vertebrates – center of nervous correlation and coordination. • Higher vertebrates – way station between lower brain areas and cerebral hemispheres. • Includes the tectum and tegmentum. • Floor of Diencephalon contains optic chiasma and pituitary gland (hypophysis cerebri) • Fish and amphibians – tectum is major brain center. • Reptiles and birds – also important. • Mammals – significantly reduced.

16. Cerebral Hemispheres • Initially, they were just loci of olfactory reception. • In early tetrapods, they were centers of sensory correlation. • In mammals, they are association centers.

17. Cerebral Hemispheres • Why begin as olfactory? • Olfaction was perhaps the most important sense in early vertebrates. • Olfaction is still crucial even to most mammals (think of dogs). • Olfaction is less important in primates, but still…..

18. Cerebral Hemispheres • In fish and amphibians, the cerebral hemispheres can be divided into • Paleopallium (olfactory lobe) • Archipallium (hippocampus) • Neopallium • Basal nuclei

19. Cerebral Hemispheres • Primitively, the cerebral hemisphere is merely an olfactory lobe. • Fibers go either to the tectum, or to the hypothalmus • Amphibians • Fibers to the basal nuclei (corpus striatum in mammals), the thalamus, and the tegmentum.

20. Cerebral Hemispheres • Gray matter moves to the surface to become the pallium (cloak). • The paleopallium is still primarily olfactory. • The archipallium is antecedent to the hippocampus of mammals. It is a correlation center in all tetrapods, and is related to emotional behaviors. • Note – modern teleosts do not use extensive olfaction, and not surprisingly, the basal nuclei and paleopallium in teleosts is displaced.

21. Cerebral Hemispheres • Reptiles • Birds • Also have reduced sense of smell • Minimal development of neopallium, but extensive basal nuclei (hence, bird brain). • Mammals • Monotremes and Marsupials • Eutherians