A collection of specialized cells (neurons) in the head that regulates behavior as well as sensory and motor functions. The brain is the enlarged anterior portion of the central nervous system in most bilaterally symmetrical animals, controlling and coordinating mental and physical actions. In vertebrates, this organ of soft nervous tissue, composed of neurons (nerve cells), is enclosed in the skull (Fig. 1). The neurons grow long threadlike structures—an axon and a dendritic tree—from their cell bodies, which provide them with a rapid communication network throughout the body. The axon uses pulses to transmit a signal to thousands of other neurons or to muscle or gland cells. The dendritic tree uses waves of electric current to integrate the pulses from thousands of other neurons. Groups of neurons form ganglia in chains along both sides of the body axis from “head to tail.” The largest of these paired groups, the brain, is in the head, where the distance receptors (nose, eyes, and ears) are located. These receptors respond to smells, sights, and sounds coming so far from the brain collective that the collective has time to receive the inputs, interpret them as signals, plan an action before being overtaken by circumstance, and act while monitoring and correcting its action. These are the minimal functions of a brain. The power of a brain lies not in its size, but in the complexity of the connections among its functional parts. See also: Central nervous system; Nervous system (invertebrate); Nervous system (vertebrate); Neuron
The three main parts of the brain in vertebrates are the cerebrum, the cerebellum, and the brainstem that connects them with each other and with the spinal cord (Fig. 2). The two cerebral hemispheres are separated by a midline fissure that is bridged by a massive bundle of axons running in both directions; this bundle is called the corpus callosum. Each hemisphere has a core of groups of neurons (the basal ganglia), an outer shell of neurons in layers (the cerebral cortex), and massive bundles of axons for communication within the cerebrum and with the rest of the brain. These bundles are called white matter because of the waxy myelin sheaths surrounding the axons. See also: Spinal cord
The basal ganglia of the brain comprises three main groups: the thalamus, the striatum, and the hypothalamus. The thalamus receives axons from all sensory systems and transmits information to the cortex. It also receives feedback from cortical neurons during sensory processing. The striatum, comprising bundles of axons cutting through the groups of neurons, also has two-way communication with the cortex and assists in the organization of body movement. The hypothalamus receives orders from the cortex and organizes the chemical systems that support body movement. One output channel is hormonal and controls the pituitary gland (hypophysis), which in turn controls the endocrine system. The other channel is neural, comprising axons coursing through the brainstem and spinal cord to the motor neurons of the autonomic nervous system, which regulates the heart, blood vessels, lungs, gastrointestinal tract, sex organs, and skin. The autonomic and endocrine systems are largely self-regulating, but they are subject to control by the cortex through the hypothalamus. See also: Autonomic nervous system; Endocrine system (vertebrate); Neurobiology
The cortex of the brain is also called gray matter because it contains the axons, cell bodies, and dendrites of neurons, but there is very little myelin. An index of the capacity of a brain is cortical surface area. In higher mammals, the cortical surface increases more rapidly than the volume during fetal development; as a result, the surface folds, taking the form of wrinkles, that is, convexities (gyri) and fissures (sulci) that vary in their details from one brain to another. However, they are sufficiently reliable to serve as landmarks on the cerebral hemisphere that it can be subdivided into lobes.
Four lobes make up the shell of each hemisphere (Fig. 3), namely, the frontal, parietal, temporal, and occipital lobes. Each lobe contains a motor or sensory map, which is an orderly arrangement of cortical neurons associated with muscles and sensory receptors on the body surface. The central sulcus delimits the frontal and parietal lobes. The precentral gyrus contains the motor cortex, whose neurons transmit signals to motor neurons in the brainstem and spinal cord, which control the muscles in the feet, legs, trunk, arms, face, and tongue of the opposite side of the body, in that order from medial to lateral position between Broca's area and the tongue area of the motor cortex. The number of neurons for each section is determined by the fineness of control; it is not determined by the size of the muscle (for example, the lips and tongue have larger areas than the trunk). Within the postcentral gyrus is the primary somatosensory cortex. Sensory receptors in the skin, muscles, and joints send messages to the somatosensory cortical cells through relays in the spinal cord and the thalamus to a map of the opposite side of the body in parallel to the map in the motor cortex. The lateral fissure separates the temporal lobe from the parietal and frontal lobes. The cortex on the inferior border of the fissure receives input relayed through the thalamus from the ears to the primary auditory cortex. The occipital lobe receives thalamic input from the eyes and functions as the primary visual cortex.
In humans, the association cortex surrounds the primary sensory and motor areas that make up a small fraction of each lobe. The occipital lobe has many specialized areas for recognizing visual patterns of color, motion, and texture. The parietal cortex has areas that support perception of the body and its surrounding personal space. Its operation is manifested by the phenomenon of phantom limb, in which the perception of a missing limb persists for an amputee. Conversely, individuals with damage to these areas suffer from sensory neglect because parts of the body may no longer exist for them. The temporal cortex contains areas that provide recognition of faces and of rhythmic patterns, including those of speech, dance, and music. The frontal cortex provides the neural capabilities for constructing patterns of motor behavior and social behavior. It was the rapid enlargement of the frontal and temporal lobes in human evolution over the past 500,000 years that supported the transcendence of humans over other species. This is where the capacity to create works of art and also to anticipate pain and death is located. Insight and foresight are both lost with bilateral frontal lobe damage, leading to reduced experience of anxiety, asocial behavior, and a disregard of consequences of actions. See also: Human brain evolution; Perception; Role of the frontal lobe in violence; Vertebrate brain (evolution)
A small part of the frontal lobe output goes directly to motor neurons in the brainstem and spinal cord for fine control of motor activities (for example, search movements by the eyes, head, and fingers), but most goes either to the striatum from which it is relayed to the thalamus and then back to the cortex, or to the brainstem from which it is sent to the cerebellum and then through the thalamus back to the cortex. In the cerebellum, the cortical messages are integrated with sensory input predominantly from the muscles, tendons, and joints, but also from the eyes and inner ears (for balance) to provide split-second timing for rapid and complex movements. The cerebellum also has a cortex and a core of nuclei to relay input and output. Their connections, along with those in the cerebral cortex, are subject to modification with learning in the formation of a working memory (the basis for learned skills). See also: Memory; Motor systems
The cerebellum and striatum do not set goals, initiate movements, store temporal sequences of sensory input, or provide orientation to the spatial environment. These functions are performed by parts deep in the brain that constitute another loop, the limbic system. Its main site of entry is the entorhinal cortex, which receives input from all of the sensory cortices, including the olfactory system. The input from all the sensory cortices is combined and sent to the hippocampus, where it is integrated over time. Hippocampal output returns to the entorhinal cortex, which distributes the integrated sensory information to all of the sensory cortices, updates them, and prepares them to receive new sensory input. This new information also reaches the hypothalamus and part of the striatum (the amygdaloid nucleus) for regulating emotional behavior. Bilateral damage to the temporal lobe, including the hippocampus, results in the loss of short-term memory. Damage to the amygdaloid nucleus can cause serious emotional impairment. The Papez circuit is formed by transmission from the hippocampus to the hypothalamus by the fornix, and then to the thalamus, parietal lobe, and entorhinal cortex. The limbic system generates and issues goal-directed motor commands, with corollary discharge to the sensory systems that prepares them for the changes in sensory input caused by motor activity (for example, when one speaks and hears oneself, as distinct from another).
Each hemisphere has its own limbic, Papez, cortico-thalamic, cortico-striatal, and cortico-cerebellar loops, together with sensory and motor connections. When isolated by surgically severing the callosum, each hemisphere functions independently, as though two conscious persons occupied the same skull, but with differing levels of skills in abstract reasoning and language. The right brain (spatial)–left brain (linguistic) cognitive differences are largely due to preeminent development of the speech areas in the left hemisphere in most right- and left-handed persons. Injury to Broca's area (located in the frontal lobe) and Wernicke's area (located in the temporal lobe) [Fig. 3] leads to loss of the ability, respectively, to speak (motor aphasia) or to understand speech (sensory aphasia), appearing as a loss of declarative memory for facts and words. Studies of blood flow show that brain activity during intellectual pursuits is scattered broadly over the four lobes in both hemispheres. See also: Aphasia; Cognition; Hemispheric laterality