Brains can be extremely complex. The cerebral cortex of the human brain contains roughly 15–33 billion neurons, perhaps more, depending on gender and age, linked with up to 10,000 synaptic connections each. Each cubic millimeter of cerebral cortex contains roughly one billion synapses. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells.
The brain controls the other organ systems of the body, either by activating muscles or by causing secretion of chemicals such as hormones and neurotransmitters. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness are possible without a brain: even single-celled organisms may be capable of extracting information from the environment and acting in response to it. Sponges, which lack a central nervous system, are capable of coordinated body contractions and even locomotion. In vertebrates, the spinal cord by itself contains neural circuitry capable of generating reflex responses as well as simple motor patterns such as swimming or walking.[6] However, sophisticated control of behavior on the basis of complex sensory input requires the information-integrating capabilities of a centralized brain.
Despite rapid scientific progress, much about how brains work remains a mystery. The operations of individual neurons and synapses are now understood in considerable detail, but the way they cooperate in ensembles of thousands or millions has been very difficult
Vertebrate brain regions
Neuroanatomists usually consider the brain to consist of six main regions: the telencephalon (cerebral hemispheres), diencephalon (thalamus and hypothalamus), mesencephalon (midbrain), cerebellum, pons, and medulla oblongata. Each of these areas in turn has a complex internal structure. Some areas, such as the cortex and cerebellum, consist of layers, folded or convoluted to fit within the available space. Other areas consist of clusters of many small nuclei. If fine distinctions are made on the basis of neural structure, chemistry, and connectivity, thousands of distinguishable areas can be identified within the vertebrate brain.
Some branches of vertebrate evolution have led to substantial changes in brain shape, especially in the forebrain. The brain of a shark shows the basic components in a straightforward way, but in teleost fishes (the great majority of modern species), the forebrain has become "everted", like a sock turned inside out. In birds, also, there are major changes in shape. One of the main structures in the avian forebrain, the dorsal ventricular ridge, was long thought to correspond to the basal ganglia of mammals, but is now thought to be more closely related to the neocortex.
Several brain areas have maintained their identities across the whole range of vertebrates, from hagfishes to humans. Here is a list of some of the most important areas, along with a very brief description of their functions as currently understood (but note that the functions of most of them are still disputed to some degree):
The medulla, along with the spinal cord, contains many small nuclei involved in a wide variety of sensory and motor functions.
The hypothalamus is a small region at the base of the forebrain, whose complexity and importance belies its size. It is composed of numerous small nuclei, each with distinct connections and distinct neurochemistry. The hypothalamus is the central control station for sleep/wake cycles, control of eating and drinking, control of hormone release, and many other critical biological functions.
Like the hypothalamus, the thalamus is a collection of nuclei with diverse functions. Some of them are involved in relaying information to and from the cerebral hemispheres. Others are involved in motivation. The subthalamic area (zona incerta) seems to contain action-generating systems for several types of "consummatory" behaviors, including eating, drinking, defecation, and copulation.
The cerebellum modulates the outputs of other brain systems to make them more precise. Removal of the cerebellum does not prevent an animal from doing anything in particular, but it makes actions hesitant and clumsy. This precision is not built-in, but learned by trial and error. Learning how to ride a bicycle is an example of a type of neural plasticity that may take place largely within the cerebellum.
The tectum, often called "optic tectum", allows actions to be directed toward points in space. In mammals it is called the "superior colliculus", and its best studied function is to direct eye movements. It also directs reaching movements, though. It gets strong visual inputs, but also inputs from other senses that are useful in directing actions, such as auditory input in owls, input from the thermosensitive pit organs in snakes, etc. In some fishes, such as lampreys, it is the largest part of the brain.
The pallium is a layer of gray matter that lies on the surface of the forebrain. In reptiles and mammals it is called cortex instead. The pallium is involved in multiple functions, including olfaction and spatial memory. In mammals, where it comes to dominate the brain, it subsumes functions from many subcortical areas.
The hippocampus, strictly speaking, is found only in mammals. However, the area it derives from, the medial pallium, has counterparts in all vertebrates. There is evidence that this part of the brain is involved in spatial memory and navigation in fishes, birds, reptiles, and mammals.
The basal ganglia are a group of interconnected structures in the forebrain, of which our understanding has increased enormously over the last few years. The primary function of the basal ganglia seems to be action selection. They send inhibitory signals to all parts of the brain that can generate actions, and in the right circumstances can release the inhibition, so that the action-generating systems are able to execute their actions. Rewards and punishments exert their most important neural effects within the basal ganglia.
The olfactory bulb is a special structure that processes olfactory sensory signals, and sends its output to the olfactory part of the pallium. It is a major brain component in many vertebrates, but much reduced in primates.
Mammals
The cerebral cortex is the part of the brain that most strongly distinguishes mammals from other vertebrates, primates from other mammals, and humans from other primates. The hindbrain and midbrain of mammals are generally similar to those of other vertebrates, but dramatic differences appear in the forebrain, which is not only greatly enlarged, but also altered in structure. In non-mammalian vertebrates, the surface of the cerebrum is lined with a comparatively simple layered structure called the pallium. In mammals, the pallium evolves into a complex 6-layered structure called neocortex or isocortex. In primates, the neocortex is greatly enlarged, especially the part called the frontal lobes. In humans, this enlargement of the frontal lobes is taken to an extreme, and other parts of the cortex also become quite large and complex. The hippocampus of mammals also has a distinctive structure.
Unfortunately, the evolutionary history of these mammalian features, especially the 6-layered cortex, is difficult to trace.[34] This is largely because of a missing link problem. The ancestors of mammals, called synapsids, split off from the ancestors of modern reptiles and birds about 350 million years ago. However, the most recent branching that has left living results within the mammals was the split between monotremes (the platypus and echidna), marsupials (opossum, kangaroo, etc.) and placentals (most living mammals), which took place about 120 million years ago. The brains of monotremes and marsupials are distinctive from those of placentals in some ways, but they have fully mammalian cortical and hippocampal structures. Thus, these structures must have evolved between 350 and 120 million years ago, a period that has left no evidence except fossils, which do not preserve tissue as soft as brain.
Primates
The brains of humans and other primates contain the same structures as the brains of other mammals, but are considerably larger in proportion to body size. Most of the enlargement comes from a massive expansion of the cortex, focusing especially on the parts serving vision and forethought. The visual processing network of primates is very complex, including at least 30 distinguishable areas, with a bewildering web of interconnections. Taking all of these together, visual processing makes use of more than half of the primate neocortex. The other part of the brain that is greatly enlarged is the prefrontal cortex, whose functions are difficult to summarize succinctly, but relate to planning, working memory, motivation, attention, and executive control.
For basic information about brains
The human brain is the center of the human nervous system. Enclosed in the cranium, the human brain has the same general structure as that of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Most of the spatial expansion comes from the cerebral cortex, a convoluted layer of neural tissue which covers the surface of the forebrain. Especially expanded are the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision, the occipital lobe, is also greatly enlarged in human beings.
Brain evolution, from the earliest shrew-like mammals through primates to hominids, is marked by a steady increase in encephalization, or the ratio of brain to body size. The human brain has been estimated to contain 80 or 90 billion (~85 109) non-neuronal cells (glial cells) and an approximately equal number of (~86 109) neurons, of which about 10 billion (1010) are cortical pyramidal cells. These cells pass signals to each other via as many as 1000 trillion (1015, 1 quadrillion) synaptic connections. Due to evolution and synaptic pruning, however, the modern human brain has been shrinking over the past 28,000 years.
The brain monitors and regulates the body's actions and reactions. It continuously receives sensory information, and rapidly analyzes these data and then responds accordingly by controlling bodily actions and functions. The brainstem controls breathing, heart rate, and other autonomic processes that are independent of conscious brain functions. The neocortex is the center of higher-order thinking, learning, and memory. The cerebellum is responsible for the body's balance, posture, and the coordination of movement.
Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the human brain is susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a wide variety of chemicals that can act as neurotoxins. Infection of the brain, though serious, is rare due to the biological barriers which protect it. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, multiple sclerosis, and Alzheimer's disease. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought to be associated with brain dysfunctions, although the nature of such brain anomalies is not well understood.
