Brain (comparative anatomy)

Animals are multicellular organisms that obtain their nutrients by eating or ingesting other organisms, and many have locomotor abilities. Obtaining food and avoiding being eaten are behaviors enhanced by the ability of an animal to tell what is going on in its surroundings. Using information from sensory receptors and responding to changes in the environment are generally managed by a nervous system of some sort, usually with a center where processing occurs, a brain or brainlike structure. Invertebrate nervous systems are generally very primitive and may contain only a very rudimentary brainlike structure. Some animals, however, are so structurally simple that they have no neural processing center at all.

Invertebrates with and Without Brains

Sponges (phylum Porifera) are invertebrates with no brain or true nervous system in sedentary adults or free-swimming larvae, but some behaviors indicate the early stages of nervous system development. They sense stimuli through their cilia, and signals from this contact are transmitted throughout their body through synapses supported by genes encoding proteins. In response to the stimuli, sponges' bodies may contract, regulated by nitric oxide. Additionally, the genes that support the sense of touch in their cilia are the building blocks of a true nervous system.

Other primitive invertebrates such as hydra, jellyfish, corals, and sea anemones (phylum Cnidaria) have one or more nerve nets, but each class of cnidarians is highly diverse. For these radially symmetrical animals, food or danger can come from any direction in the water, and the meshlike nervous system can respond directly without a central control region. Some jellyfish also have a nerve ring that helps coordinate their movements, but no brain.

Bilaterally symmetrical invertebrates include the flatworms (phylum Platyhelminthes), roundworms (Nematodes), mollusks (Mollusca), segmented worms (Annelida), and insects and their relatives (Arthropoda). Most of these show cephalization, the presence of an anterior head containing the main processing center of the nervous system, specialized sensory receptors, and the mouth. Echinoderms (Echinodermata), such as starfish are bilaterally symmetrical as larvae, but develop radial symmetry as adults when they lack a head. Some mollusks are not symmetrical as adults, despite the bilateral symmetry of the larvae.

Among the flatworms, some have only nerve nets like those of cnidarians, while more complex planarians, tapeworms, and flukes generally have one or more pairs of ladderlike longitudinal nerve cords with ganglia at the head. These ganglia are clusters of cell bodies of neurons, the most primitive form of a brainlike structure. Nematodes or roundworms have a nerve ring and anterior ganglia organized around the anterior digestive tract, with nerve cords extending toward the head and tail from this center. Mollusks include clams and oysters (class Bivalva), snails and slugs (Gastropoda), and octopuses and squid (Cephalopoda). These animals have nervous systems that vary from simple and relatively uncephalized nerve rings and nerve cords to a more centralized system with at least four pairs of ganglia.

Octopuses, squid, and cuttlefish in the subclass Coleoidea have the most complex nervous systems of the mollusks and are the most intelligent invertebrates. The relatively large and lobed cephalopod brain contains many clustered or fused ganglia that manage sensory information from complex eyes and produce motor instructions for extremely rapid muscular responses. Giant nerve fibers in squid are the largest neurons known in any animal, up to one millimeter in diameter in a single cell, and can conduct rapid impulses that allow lightning-fast movements. Extensive studies of these neurons’ structure and function have provided scientific insights applicable to human neurons. Gastropods and cephalopods may show extremely complex behaviors, such as homing, territoriality, and learning. An octopus can have as many as thirty functional brain centers, some of which are memory banks used for experiential learning.

Annelids such as earthworms and leeches have paired cerebral ganglia near the mouth, connected by a solid ventral nerve cord to smaller paired ganglia in each body segment. Giant nerve fibers in the nerve cord allow rapid responses to escape from threats using reflex actions and patterned behavior. Earthworms can be taught to travel a maze by simple associative learning, in which repeated stimuli become linked to a specific behavior pattern, but this learning requires many repetitions and disappears within a few days if not reinforced.

Arthropods include spiders, scorpions, ticks, and mites (class Arachnida), lobsters, crabs, and shrimp (Crustacea), and insects (Insecta). The nervous system in arthropods is similar to that of annelids in its segmentation, but it is much more complex, and the anterior ganglia tend to be fused into a true brain. Many arthropods have giant neurons like those of some mollusks and annelids, capable of rapid nerve impulse transmission for efficient muscle control, and many have chemosensory abilities. The neurosecretory cells in the protocerebrum of anthropods' brains are highly developed. These cells manufacture and store the brain hormone called ecdysiotropin. Insects, especially ants and bees, are capable of complex learning and very intricate social behavior. Habituation allows individuals to learn to ignore repeated stimuli that do not produce harmful effects, and cockroaches and ants can learn to run mazes.

Sea urchins, sand dollars, sea stars (starfish), and sea cucumbers are echinoderms, in which the bilaterally symmetrical larvae develop a secondary radial or biradial symmetry as they mature. They are the only non-chordate deuterostome group to have a central nervous system. The nervous system consists of a nerve ring around the mouth connected to radial nerves extending down each of its arms and a nerve net. The nerve ring coordinates movements and bodily functions according to signals from the radial nerves. Their nervous system also aids their ability to regenerate limbs.

Evolutionary Development of the Vertebrate Brain

The location of most animals’ brains, or brainlike organs, at the anterior or superior end of the body, is important since it places the brain at the leading end of the moving animal or at its highest point. Many sensory receptors are located in the head, and information from the eyes, ears, and nose can be rapidly received and processed if the processing center is in the same region.

In vertebrates, the nervous system is much more advanced than the primitive systems of invertebrates. The vertebrate brain is an anterior enlargement of the dorsal hollow nerve cord that develops above the notochord in all chordates. This swelling of the nerve cord allows the development of a large collection of neurons that receive, process, and store information, and determine what the organism’s response to that information will be. The central nervous system consists of the brain at the anterior end of the nerve cord and the spinal cord behind it, encased in a skull and vertebral column of bone or cartilage. The rest of the vertebrate nervous system is called the peripheral nervous system, with nerve fibers bundled into nerves. Clusters of the cell bodies of neurons in the central nervous system are called nuclei, while the same kind of clusters in the peripheral nervous system are called ganglia.

The components of the vertebrate embryonic brain are divided into three areas or primary vesicles, known as the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). As development occurs, the three primary vesicles form five secondary vesicles that continue to develop into mature brain structures. The forebrain becomes subdivided into the telencephalon, which matures into the cerebrum, and the diencephalon, which contains the thalamus and hypothalamus. The midbrain does not undergo further developmental separation. The hindbrain develops into the metencephalon, which will form the pons and cerebellum, and the myelencephalon, which becomes the medulla oblongata, that is connected to the spinal cord. The lower or posterior part of the brain is called the brain stem, consisting of the medulla oblongata, pons, and midbrain, which manages the most primitive functions required for life. Higher brain functions reside in the cerebrum, particularly in the outer cortex of gray matter on its surface. The cerebellum coordinates skeletal muscle or motor activities, while the diencephalon processes and sends sensory information to the cerebrum and cerebellum, as well as being the center of autonomic or visceral motor control.

The different classes of vertebrates are grouped into subphylum Vertebrata within phylum Chordata, with the main classes including cartilaginous fish (Chondrichthyes), bony fish (Osteichthyes), amphibians (Amphibia), reptiles (Reptilia), birds (Aves), and mammals (Mammalia). In general, the brains of fish and amphibians are relatively primitive compared to those of other vertebrates. Amphibians have elongated brains that are similar in structure to the brains of mammals, only more simple, while fish have brains that are divided into the telencephalon, diencephalon, mesencephalon, and hindbrain. Olfactory lobes for processing sensations of smell and taste are located in the cerebrum, which is relatively large in amphibians. Optic lobes for vision in the diencephalon are large compared to other parts of the brain, and responses are generally reflexive. Many fish and amphibian species, however, have evolved to learn to use tools, map an area, create social relationships, and more. Additionally, some newts and axolotls can regenerate some parts of their central nervous system.

Animals that lay eggs with shells, called amniotes and including reptiles and birds, are adapted to the rigorous requirements of life on land and have larger, more complex brains than fish and amphibians. The amniote brain has a larger telencephalon and is able to process and store more information about the land environment, which is much more likely to vary than a watery environment. In addition to having a larger telencephalon, the brain contains more gray matter that is closer to the brain surface in amniotes than in fish or amphibians.

Mammals and some reptiles have much, or all, of the surface of the cerebrum covered in gray matter, which forms a structure called the cerebral cortex. The evolutionarily newer portion of this cortex is called the neocortex, while the older part is called the paleocortex. The paleocortex is the control center for drive-related behaviors, such as activities associated with feeding (licking, chewing, swallowing), sexual behavior, and primitive emotions (anger, fear). The limbic system occupies the paleocortex, which is sometimes called the reptilian brain because it is the highest brain area present in reptiles and governs nearly all their behaviors. The neocortex is a “higher” control area that is well developed even in primitive mammals, but it is most completely expressed and covers the entire cerebral surface in humans. In cetaceans (whales and dolphins) and primates, the neocortex is the center of higher learning, logical thinking, and the storage of many memories. The activities of the neocortex can override the more primitive responses of the paleocortex under most conditions, but when the higher brain areas are inactive, as in alcoholic intoxication in humans or when removed surgically in experimental animals, the lower areas reassert themselves and take control, often causing inappropriate behaviors.

Mammalian brains have convolutions on the surface of the cerebrum and cerebellum, with the neural cortex following and covering every “hill” and “valley” of the convolutions. This provides a much greater surface area occupied by gray matter, especially in humans, the species in which the convolutions and cerebral cortex are most extensive. Below the gray matter surface is white matter, myelinated neuron fibers that carry information from one area of gray matter to another. Deep in this white matter are basal nuclei, gray matter centers that help regulate subconscious and involuntary control of body functions.

The gray matter of the cerebrum in birds is nearly all in the deep basal nuclei, which are relatively much larger than they are in mammals, and in an overlying gray area specific to birds called the hyperstriatum. The avian brain lacks a neocortex entirely, with no equivalent of the cerebral cortex present. The area of the basal nuclei, called the corpus striatum, is apparently the center for complex behavior patterns, while the hyperstriatum manages learning and memory.

Human brains show lateralization—one side of the cerebrum (left) controls language production and interpretation, while the other side (right) controls spatial awareness and artistic creativity. Historically, this lateralization was believed to not exist in other vertebrates, but further research has revealed lateralization functions in many animals. In some birds, memories of song patterns and migratory homing directions are located in gray matter areas on specific sides of the brain. Using conditional reflexes, the right hemisphere of the brains in cats, rats, and mice was responsible for spatial analysis, while the left controlled motor skills and time analysis. Other animals that present with brain lateralization include horses, domesticated pigs, chickens, dogs, and primates.

Because the vertebrate central nervous system develops from a dorsal hollow nerve cord, the anterior end of its hollow, fluid-filled central canal enlarges into four ventricles or spaces. These are the first and second or lateral ventricles of the cerebral hemispheres, the third ventricle within the diencephalon, and the fourth ventricle associated with the pons, medulla oblongata, and cerebellum. The midbrain retains a simple canal called the cerebral or mesencephalic aqueduct that connects the third and fourth ventricle spaces.

The fluid that fills the canal and ventricle spaces is cerebrospinal fluid (CSF), produced by the filtration of fluids from the blood at specialized capillary beds called choroid plexuses within the ventricles. Besides filling the hollow spaces of the central nervous system, CSF also washes over the surfaces of the brain and spinal cord in an area below the arachnoid layer, one of the central nervous system’s coverings or meninges. It provides protection against traumatic injury, delivers nutrients, removes wastes, and helps regulate neurochemicals for the central nervous system.

The Primate Brain

Primates, the order of animals that includes monkeys, apes, and humans, contains species that show a higher level of brain development than most other mammals. Primate brains, especially in humans, are among the largest in the animal kingdom compared to the animal's body size. For many decades, scientists believed brain size was associated with the animal's body size, but later research revealed that brain and body size have a non-linear, curved relationship. This discovery explains why very large animals sometimes have smaller brains than expected. The primate brain retains in its structure the earlier forms and functions that have developed in lower vertebrates over evolutionary time, such as the brainstem and limbic system, but higher areas give new and more complex possibilities for learning and behavior.

Because humans are upright, bipedal walkers, the human brain is at the top of the spinal cord rather than somewhat in front of it as in other primates. The human brain weighs only about three pounds or about 2 percent of the weight of a 150-pound individual, but that is still larger relative to body size than the brains of other primates, even chimpanzees. The cerebrum makes up about 87 percent of the volume of the brain, and the cerebellum occupies most of the remaining volume. The diencephalon and brainstem in primates are relatively smaller than in most other mammals compared to the entire brain size. The cerebral cortex in humans contains only six layers of cell bodies in the gray matter on the surface of the cerebrum. Many axons extending down from these cell bodies into the underlying white matter cross-connect the neurons that receive stimuli, process information, determine responses, and store memories. Specific areas of this cerebral cortex determine the body’s voluntary muscle actions, or receive and analyze sensory information from the skin, muscles, and joints. Other cortical areas process incoming information about smell, taste, vision, and hearing, and compare those sensations to previous memories or store them as new memories.

The most “human” aspect of the brain is the prefrontal cortex of the cerebrum, where logical analysis, predictions of the results of specific actions, and social interactions occur, although even in monkeys and apes, the front of the brain manages social awareness and behavior. Since the primate brains of apes and monkeys are so similar to those of humans, many studies of brain function have involved experimentation on these animals, humans’ closest relatives. Other mammals, such as mice, rats, cats, and dogs, have also been subjects of brain studies related to their specific behavior and how the human brain works in its various components. Since neurons are very similar, whether they come from sea slugs, squid, or mammals, experimentation with these animals has produced insight into how all brains and nervous systems work.

Principal Terms

Brainstem: Lowest or most posterior portion of the vertebrate brain, including midbrain, pons, and medulla oblongata; controls “housekeeping” functions such as breathing and heartbeat

Cell Body: The central portion of a neuron, containing the nucleus, where most processing and integration of information occur

Cerebellum: Second largest part of the brain, manages fine muscle control and muscle memories

Cerebrum: Largest part of most vertebrate brains, with areas that control vocalizations, vision, hearing, smell, and taste, as well as voluntary skeletal muscle movements

Cortex: Thin layer of gray matter that covers surfaces of the cerebrum and cerebellum

Ganglia: Clustered cell bodies of neurons that may form a brainlike center in lower animals

Gray Matter: Region of the brain or spinal cord that contains cell bodies of neurons, where information processing and storage occur

White Matter: Region of neural tissue that contains axons of neurons that carry electrical nerve impulses from one processing center to another

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