The vertebrate system
The nervous system of vertebrates has two main divisions: the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, which in humans includes 12 pairs of cranial nerves, 31 pairs of spinal nerves, and the autonomic, or involuntary, nervous system.
Anatomic structures such as the nervous system are described according to their position. In four-legged animals the upper (back) surface is called dorsal and the lower (belly) surface ventral. The terms anterior, cranial, cephalic, and rostral refer to the head end of the body, posterior and caudal to the tail end. In humans, since they stand erect, the situation is more complicated: dorsal becomes equivalent to posterior, and ventral is the same as anterior; cranial is often called superior, and caudal inferior. Objects near the middle plane of the body are medial and those farther away are lateral. Proximal refers to structures nearest the central bulk of a structure and distal to ones away from it. In referring to another structure, if it is located on the same side of the body, it is known as ipsilateral; if it is on the opposite side, it is contralateral.
Neurons are often gathered into localized masses. In the peripheral nervous system these accumulations are called ganglia; in the central nervous system they are called nuclei. Portions of the central nervous system in which unmyelinated neurons and neuroglia predominate are called gray matter; areas in which myelinated neurons dominate are called white matter. Efferent, or motor, nerve fibres carry impulses away from the central nervous system; afferent, or sensory, fibres carry impulses toward the central nervous system. Visceral fibres innervate the viscera such as the heart and intestines, and somatic fibres innervate the body-wall structures such as skin and muscle. In the central nervous system the nerve fibres are organized in bundles called tracts, or fasciculi. Ascending tracts carry impulses along the spinal cord toward the brain, and descending tracts carry them from the brain or higher regions in the spinal cord to lower regions. The tracts are often named according to their origin and termination; for example, the corticospinal tract consists of fibres running from the cerebral cortex in the brain to the spinal cord.
The primitive condition
The vertebrates constitute an advanced subdivision of the phylum Chordata. All chordates at some time in their life have a rodlike bar called the notochord running the length of the body. Lower chordates (acorn worms, tunicates, and amphioxus), which lack a vertebral column, illustrate the most primitive features of the chordate nervous system. In these animals the nerve cord is a rather uniform-appearing dorsally placed tube with a hollow cavity, which corresponds roughly to the spinal cord of the vertebrates, suggesting that the spinal cord is the most primitive component of the central nervous system.
In amphioxus and in lower vertebrates such as lampreys, the sensory fibres and motor fibres leave the cord in dorsal and ventral roots to supply the adjacent body segments called myotomes. The dorsal and ventral roots remain separate nerves and arise at alternate positions along the cord. In lower fishes there is still alternation of dorsal and ventral roots, but the roots unite in a single spinal nerve. In higher vertebrates the two roots unite in a single spinal nerve and leave the cord at the same level, one above the other. Each spinal nerve supplies a single myotome. When appendages (fins, wings, arms, and legs) develop from several myotomes, the nerves continue to supply their original segments, and branches of the spinal nerves become interwoven to form plexuses.
The brain of vertebrates developed by the accumulation of nerve cells at the cephalic end of the nerve cord. At first this diffuse collection of nerve cells regulated the reflex activity of spinal motor neurons. These cells are comparable to the reticular formation occupying the brainstem of higher vertebrates. The brainstem, thus, is the oldest portion of the brain.
Encephalization
Early in the evolution of vertebrates, a special sensory system became associated with each major part of the brain: the olfactory organs with the forebrain, the eye with the midbrain, and the ear and related organs with the hindbrain. Each of the three sections, furthermore, developed dorsal outgrowths of gray matter forming, respectively, the cerebrum, the midbrain roof, or tectum, and the cerebellum. With these developments the three-part brainstem was then transformed into a brain of five regions: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. The addition of these nerve centres to the primitive brainstem allowed greater coordination and association between the sensory and motor fibres.
In tracing the development of the parts of the brain in the different vertebrate classes, some general features are apparent. There is a correlation between the size of a particular part of the brain and its importance in the functions of an animal. Some neural structures (e.g., the olfactory bulb) have considerable size and importance in more-primitive animals but are less conspicuous in most recent animals. Progressing from primitive to recent animals, there is a gradual cephalic shift of function from the lower brainstem to the higher cerebral cortex.
The hindbrain is comparable to an enlarged, anterior section of the spinal cord. In the gray matter are dorsal sensory and ventral motor columns similar to those present in the cord. The longitudinal continuity of these columns is preserved in the earlier vertebrates, but in more recent vertebrates the columns break up into discrete nuclei that serve some of the cranial nerves. The hindbrain exerts partial control over the spinal motor neurons through the reticular formation. Fish and tailed amphibians, in addition, have a pair of giant cells called the cells of Mauthner, which exert some control over the local spinal-cord reflexes responsible for the rhythmic swimming undulations and the flip-tail escape response characteristic of these animals.
The hindbrain is the area of reception of one of the main sensory systems, the acoustico-lateralis system, which consists of the ear (hearing and equilibrium) and the lateral-line organs (vibration and pressure). The latter, situated in rows along the head and body, are retained in fish but disappear in the land vertebrates.
The cerebellum originated as a specialized part of the acoustico-lateralis area. The oldest part of the cerebellum—the archicerebellum—is concerned with equilibrium and connected with the inner ear and the lateral-line system. The anterior lobe of the cerebellum represents the paleocerebellum, an area that regulates equilibrium and muscle tone. It constitutes the main mass of the cerebellum in fish, reptiles, and birds. In mammals the development of the cerebral cortex and its connections with the cerebellum are correlated with the appearance of the large cerebellar hemispheres. This new part of the cerebellum, or neocerebellum, coordinates skilled movements initiated at cortical levels. In mammals a great mass of fibres connects the brainstem to the cerebellum. This region forms the pons, which, together with the cerebellum, constitutes the metencephalon. The caudal part of the hindbrain remains as the medulla oblongata (myelencephalon).
The midbrain (mesencephalon) and the diencephalon constitute the anterior portion of the brainstem. Sensory and motor nuclei for cranial nerves extend from the hindbrain to the midbrain. The roof of the midbrain, or tectum, developed as the primary visual centre. The optic lobes, especially prominent in fish and birds, are a part of this area. In fish and amphibians the tectum is the major centre of the nervous system and wields the greatest influence on body activity. While this area is still significant in reptiles and birds, it is supplanted in importance by the cerebral hemispheres. In mammals most of the optic sensations are relayed to the cerebral cortex. With development of the cerebral cortex, the thalamus becomes less significant as an association area and more important as a relay centre for sensory impulses. Centres for visceral sensations and visceral motor responses become established in the hypothalamus.