Vertebrate brain

Vertebrate brain (evolution)

A highly complex organ consisting of sensory and motor systems that constitutes part of the nervous system. Virtually all of the brain systems that are found in mammals occur in birds, reptiles, and amphibians, as well as in fishes and sharks. These systems have become more complex and sophisticated as the adaptive requirements of the animals changed. Occasionally, new sensory systems arose, most likely as specializations of existing systems; however, some of these disappeared as animals left the aquatic world. Some sensory systems arose and declined several times in different lineages. In spite of the many changes in brain structure, some of them quite dramatic, the evolution of the nervous system, from the earliest vertebrates through to those of today, has been relatively conservative. See Nervous system (invertebrate), Nervous system (vertebrate)

Central nervous system

The nervous system consists of two main divisions: the central nervous system, which is made up of the brain and the spinal cord, and the peripheral nervous system. The peripheral nervous system consists of the nerves that bring information from the outside world via the sensory systems, and the nerves that carry information from the body's interior to the spinal cord and brain. These nerves also convey commands from the brain and spinal cord to the external muscles that move the skeleton, as well as to various internal organs and glands. See Central nervous system

The brain consists of a variety of systems, some of which are sensory and deal with the acquisition of information from the internal and external environments. Other systems are motor and are involved with the movement of the skeletal muscles; the muscles of the internal organs, such as the heart and the digestive and respiratory systems; and the secretions of certain glands, such as the salivary and tear glands. The bulk of the brain, however, is composed of systems that are integrative and organize, coordinate, and direct the activities of the sensory and motor systems. These integrative systems regulate such processes as sleep and wakefulness, attention, the coordination of various muscle groups, emotion, social behavior, learning, memory, thinking, planning, and other aspects of mental life. Social behavior itself is highly complex and includes such interactions between individuals as courtship, mating, parental care, and the organization and structure of groups of individuals. See Brain, Motor systems

Development

The nervous system develops in the embryo as a hollow tube. The remnants of this hollow tube in the adult are known as the brain ventricles and are filled with the cerebrospinal fluid. Two types of overall brain organization are found among vertebrates based on the relationship of the neurons to these ventricles. In brains with laminar organization, the neurons have not migrated very far from the layer immediately surrounding the hollow ventricular core of the brain. This type of organization is typical of amphibia and is also found in the brains of some sharks, fishes (especially those in which their skeletons are partly or mostly made of cartilage rather than bone), and lampreys. In contrast, in the brains of those vertebrates with the elaborated type of organization, the neurons have migrated from the zone around the ventricles to occupy nearly all of the interior of the brain. This type of organization is typical of reptiles, mammals, birds, fishes (with fully bony skeletons), as well as skates, rays, some sharks, and hagfishes.

In general, brain size or weight varies in proportion to the size of the body. In some species, however, brain size or weight is greater than would be expected for that body weight, for example, in humans, chimpanzees, and porpoises. Birds have brains that are comparable to those of mammals of equivalent body weight. In fact, crows have brain weights that would be expected of a small primate of equivalent body weight. In contrast to birds and mammals, amphibians, reptiles, and bony fishes have relatively small brains for their body weights, as do jawless fishes.

Subdivisions of brain

The brain is divided into a hindbrain, a midbrain, and a forebrain.

Hindbrain

The hindbrain is a region that contains nerve endings that receive information from the outside world and from the body interior; these are known as sensory cranial nerves. The neuron groups upon which they terminate are known as sensory cranial nuclei. Also found in the hindbrain are motor nerves that control internal and skeletal muscles and glands, which are called motor cranial nerves; the neuron groups from which they originate are known as motor cranial nerve nuclei.

Many animals possess senses that humans do not possess. One such is the lateral line sense, which derives from receptors located in the lateral line organ which can easily be seen on most bony fishes as a thin, horizontal line running the length of the body from behind the gill opening to the tail. Other lateral line organs can be found on the head and jaws. These organs contain mechanoreceptors that respond to low-frequency pressure waves that might be produced by other fishes nearby or the bow wave of a fast-swimming predator about to strike. Lateral line systems and a special region of the hindbrain dedicated to lateral line sense are found in fishes and sharks, jawless fishes, and bony fishes of various sorts.

Electroreception is another way of dealing with a murky environment. Scientists have described two types of electroreception: active and passive. The receptors are also located in the lateral line canals and sometimes on the skin. Animals with passive electroreception, such as sharks and rays, platypuses, and echidna, can detect the presence of the very weak electric fields that are generated around a living body, which they then follow to capture their prey. Animals with active electroreception generate stronger electric fields around themselves using specialized electric organs. By detecting changes in these electric fields, they can derive a picture of their environment. Electrosensory cranial nerves terminate in a region of the hindbrain known as the electrosensory area. A second group of active electrosensory fishes are capable of generating electric fields so powerful they can stun a prey or an enemy. Among these are the electric eel, the electric catfish, and an electric shark (the torpedo). These animals also use their low-level electric fields to detect objects and creatures in the environment.

Not only did the hindbrain change in response to sensory evolution, but it also underwent major motor transformations; for example, motor-neuron groups involved in swallowing, chewing, and salivating evolved as a consequence of the transition to land and the loss of the water column to carry food from the opening of the mouth into the throat.

The hindbrain also contains two important coordinating or integrating systems: the cerebellum and the reticular formation. The functions of the cerebellum are varied; they include the integration of a sense of balance with aspects of movement and motor learning and motor memory, as well as playing an important role in electrosensory reception.

The reticular formation coordinates the functions of various muscle groups. For example, the actions of the jaws and tongue must be coordinated so that an animal does not eat its own tongue while eating its meal. It also coordinates the motor-neuron groups that control the air column that enters and leaves the mouth and throat, which produces the various vocalizations of land animals, including speech. The reticular formation also is involved in sleep, wakefulness, and attention.

Midbrain

The midbrain contains the motor cranial nerves that move the eyes. It also contains neuron groups that are organized to form maps of visual space, auditory space, and the body. These maps are coordinated with each other such that a sudden, unexpected sound will cause the head and eyes to move to the precise region of space from which the sound originated. In those animals that make extensive use of sound localization, such as owls and bats, the map areas of the midbrain are very highly developed. In addition, certain snakes, such as rattlesnakes and boa constrictors, have infrared detectors on the snout or under the eyes that can sense the minute heat from a small animal's body at a distance of 1 m (3 ft) or more. The midbrains of these animals also have infrared maps that are in register with the auditory, visual, and body maps to permit the animal to correlate all the necessary information to make a successful strike on prey in virtually total darkness.

Forebrain

The forebrain is a very complex region that consists of the thalamus, the hypothalamus, the epithalamus, and the cerebrum or telencephalon. In addition, the forebrain contains the limbic system, which has components in all regions of the forebrain as well as continuing into the midbrain. The thalamus processes and regulates a large quantity of the information that enters and emanates from the forebrain. As the cerebrum increases in size and complexity in land animals, the thalamus increases accordingly. The hypothalamus regulates autonomic functions as well as behaviors such as feeding, drinking, courtship and reproduction, parental, territoriality, and emotional, which it controls in conjunction with the limbic system. The hypothalamus also regulates the endocrine system. The size and complexity of the hypothalamus, relative to the rest of the brain, is greatest in fishes and sharks; it declines considerably in proportion to the rest of the brain in land animals. The epithalamus contains the pineal gland, which is involved in various biological rhythms that depend on daylight, including seasonal changes. In some animals, such as certain reptiles, the pineal takes on the form of an eye, located on the top of the head and known as the parietal eye. This eye has a lens and a primitive retina that capture light and transmit information, such as the amount of daylight, to the hypothalamus. The epithalamus, like the hypothalamus, is relatively smaller in the brains of land animals.

The greatest evolutionary expansion of the forebrain is seen in the cerebrum. The cerebrum consists of an outer layer, the pallium, and a series of deep structures, known as the subpallium. The subpallium is composed of the corpus striatum, the amygdala, and the hippocampus. The outer layer of the cerebrum in mammals is known as the cerebral cortex. Considerable debate surrounds the evolutionary relationship between the cerebral cortex of mammals and the pallium and subpallium of nonmammals. Most specialists, however, seem to agree that the mammalian cortex arose from the pallium and certain regions of the subpallium. The cerebrum is relatively small in animals with laminar brains and larger in those with complex brains. Scientists have only begun to catalog the many complicated behavioral functions of the cerebrum. Among them appear to be memory, thinking and reasoning, and planning. With the advent of life on the land, the cerebrum underwent an extreme degree of elaboration in reptiles and birds and especially in mammals. See Endocrine system (vertebrate)

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