Vertebrate Brain has undergone evolution which resulted in the formation of hindbrain, midbrain and forebrain. Central canal of the spinal cord extends in the brain and expands into channels called ventricles. The ventricles are filled with cerebrospinal fluid.
Hindbrain:
It is continuous with spinal cord and includes medulla oblongata, cerebellum and pons.
Medulla oblongata:
It is enlargement where the spinal cord enters the brain. It contains reflex centres for breathing, swallowing, cardio vascular function and gastric secretion. Medulla oblongata is well developed in all jawed vertebrates reflecting its ability to control visceral functions and to serve as a screen for information that leaves or enters the brain.
Cerebellum:
It is an outgrowth of medulla oblongata. It coordinates motor activity associated with limb movement maintaining posture, and spatial orientation. Cerebellum in cartilaginous fishes has distinct anterior and posterior lobes. In Teleosts the cerebellum is large in active swimmers and small in inactive fishes.
Amphibians often have rudimentary cerebellum, reflecting their simple locomotor pattern.
In other Tetrapads cerebellum is laterally expanded. These expanded lateral lobes provide locomotor control of muscles of the appendages.
The cerebellum is much larger in Birds and Mammals, a reflection of complex locomotor patterns and a common evolutionary history of limb development and phylogeny as terrestrial vertebrates.
Pons:
Pons is a bridge of transverse nerve tracts from the cerebrum of the forebrain of both sides of the cerebellum. It also contains tracts that connect the forebrain and spinal cord in all vertebrates.
Midbrain:
Midbrain was originally centre for coordinating reflex response to visual input. As the brain evolved it look on added functions relating to tactile touch and auditory hearing input but it did not change in size. The roof of midbrain called optic rectum is thickened region of gray matter that integrates visual and auditory signals.
Forebrain:
Vertebrate forebrain has changed great deal during vertebrate evolution. Forebrain has two main parts, diencephalons and telencephalon.
Diencephalon: It lies just in front of midbrain and contains pineal gland, pituitary gland, hypothalamus and thalamus. Thalamus relays all sensory information to higher brain centres. Hypothalamus lies below the thalamus and regulates many functions such as body temperature, sexual drive, carbohydrate metabolism, hunger and thirst. Pineal gland controls body rhythms. Pituitary is major endocrine gland.
In fishes and amphibians diencephalons possesses sensory information. In reptiles and birds most important part of brain is corpus striatum which plays a role in their complex behaviour patterns. As the diencephalons slowly expanded during evolution and handled more and more sensory functions. The telencephalon (first part of forebrain) expanded rapidly in both size and complexity.
Cerebrum:
External to corpus striatum is cerebrum by which large groove divides into right and left cerebral hemispheres. The parts of brain related to sensory and motor integration hanged greatly as vertebrates became more agile. Many functions shifted from optic tectum to the expanding cerebral hemispheres. In mammals outermost part of cerebrum called cerebral cortex increased in size and complexity. This layer folds back on itself to remarkable extent, suggesting that the evolution of mammalian cerebrum output the enlargement of the skull bones housing it. Different parts of the cerebrum have specific functions. For example cerebral cortex contains primary sensory areas and primary motor areas. Other areas of the cortex are involved in the perception of visual or auditory signals from the environment. In humans this includes the ability to use language both written and spoken.
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