What is brain structure?

Introduction

About two weeks after conception, a fluid-filled cavity called the neural tube begins to form on the back of the human embryo. This neural tube will sink under the surface of the skin, and the two major structures of the central nervous system
(CNS) will begin to differentiate. The top part of the tube will enlarge and become the brain; the bottom part will become the spinal cord. The cavity will persist through development and become the fluid-filled central canal of the spinal cord and the four ventricles of the brain. The ventricles and the central canal contain cerebrospinal fluid, a clear plasmalike fluid that supports and cushions the brain and also provides nutritive and eliminative functions for the CNS. At birth, the average human brain weighs approximately twelve ounces (350 grams), a quarter of the size of the average adult brain, which is about three pounds (1,200 to 1,400 grams). Development of the brain in the first year is rapid, with the brain doubling in weight in the first six months.

The development of different brain areas depends on intrinsic and extrinsic factors. Internally, chemicals called neurotrophins promote the survival of neurons, the basic cells of the nervous system that are specialized to communicate electrochemically with one another, and help determine where and when those neurons will form connections and become diverse neurological structures. Externally, diverse experiences enhance the survival of neurons and play a major role in the degree of development of different neurological areas. Research has demonstrated that the greater the exposure a child receives to a particular experience, the greater the development of the neurological area involved in processing that type of stimulation. Although this phenomenon occurs throughout the life span, the greatest impact of environmental stimulation in restructuring and reorganizing the brain occurs in the earliest years of life.

Experience can alter the shape of the brain, but its basic architecture is determined before birth. The brain consists of three major subdivisions: the hindbrain (rhombencephalon, or “parallelogram-brain”), the midbrain (mesencephalon, or “midbrain”), and the forebrain (prosencephalon, or “forward brain”). The hindbrain is further subdivided into the myelencephalon (“marrow-brain”) and the metencephalon (“after-brain”), while the forebrain is divided into the diencephalon (“between-brain”) and the telencephalon (“end-brain”). To visualize roughly the locations of these brain areas in a person, one can hold an arm out, bend the elbow ninety degrees, and make a fist. If the forearm is the spinal cord, where the wrist enlarges into the base of the hand corresponds to the hindbrain, with the metencephalon farther up than the myelencephalon. The palm of the hand, enclosed by the fingers, would be the midbrain. The fingers would be analogous to the forebrain, with the topmost surface parts of the fingers being the telencephalon.

One can take the analogy a step further. If a fist is made with the fingers of the other hand and placed next to the fist previously made, each fist would represent the two cerebral hemispheres of the forebrain, with the skin of the fingers representing the forebrain’s cerebral cortex, the six layers of cells that cover the two hemispheres. Finally, the meninges cover the cortex like close-fitting gloves. The three layers of the meninges play a protective and nutritive role for the brain.

The more advanced the species, the greater the development of the forebrain, particularly the cortex. The emphasis here is placed on a neuroanatomical examination of the human brain, beginning with a look at the hindbrain and progressing to an investigation of the cerebral cortex. The terms “anterior” (toward the front) and “posterior” (toward the back) will be used frequently in describing the location of different brain structures. Additionally, the words “superior” (above) and “inferior” (below) will be used to describe vertical locations.

The Forebrain

Right above the midbrain, in the center of the brain, lies the thalamus, which is the center of sensory processing. All incoming sensory information except for the sense of smell goes to the thalamus first before it is sent on to the cerebral cortex and other areas of the brain. Anterior to and slightly below the thalamus is the hypothalamus. Hypothalamic activity is involved in numerous motivated behaviors such as eating, drinking, sexual activity, temperature regulation, and aggression, largely through its regulation of the pituitary gland, which is attached beneath the hypothalamus. The pituitary gland controls the release of hormones that circulate in the endocrine system.

Subcortical Structures

Numerous structures lie beneath the cerebral cortex in pairs, one in each hemisphere. Many of these structures are highly interconnected with one another and are therefore seen to be part of a system. Furthermore, most of the subcortical structures can be categorized as belonging to one of two major systems. Surrounding the thalamus is one system called the basal ganglia, which is most prominently involved in movements and muscle tone. The basal ganglia deteriorate in Parkinson’s and Huntington’s diseases, both disorders of motor activity. The three major structures of the basal ganglia are the caudate nucleus and putamen, which form the striatum, and the globus pallidus. The activities of the basal ganglia extend beyond motor control. The striatum, for instance, plays a significant role in the learning of habits as well as in obsessive-compulsive disorder, a disorder of excessive habits. In addition, disorders of memory, attention, and emotional expression (especially depression) frequently involve abnormal functioning of the basal ganglia.

The nucleus basalis, while not considered part of the basal ganglia, nevertheless is highly interconnected with those structures (and the hypothalamus) and receives direct input from them. Nucleus basalis activity is essential for attention and arousal.

The other major subcortical system is the limbic system, which was originally thought to be involved in motivated or emotional behaviors and little else. Later research, however, demonstrated that many of these structures are crucial for memory formation. The fact that people have heightened recall for emotionally significant events is likely a consequence of the limbic system’s strong involvement in both memory and motivation or emotion.

Two limbic structures are essential for memory formation. The hippocampus
plays the key role in making personal events and facts into long-term memories. For a person to remember information of this nature for more than thirty minutes, the hippocampus must be active. In people with Alzheimer’s disease, deterioration of the hippocampus is accompanied by memory loss. Brain damage involving the hippocampus is manifested by amnesias, indecisiveness, and confusion. The hippocampus takes several years to develop fully. This is thought to be a major reason that adults tend to remember very little from their first five years of life, a phenomenon called infantile amnesia.

The second limbic structure that is essential for learning and memory is the amygdala, which provides the hippocampus with information about the emotional context of events. It is also crucial for emotional perception, particularly in determining how threatening events are. When a person feels threatened, his or her amygdala will become very active. Early experiences in life can fine-tune how sensitive a person’s amygdala will be to potentially threatening events. A child raised in an abusive environment will likely develop an amygdala that is oversensitive, predisposing that person to interpret too many circumstances as threatening. Two additional limbic structures work with the amygdala in the perception and expression of threatening events, the septal nuclei and the cingulate gyrus. High activity in the former structure inclines one to interpret events as nonthreatening, while activity in the latter structure is linked to positive or negative emotional expressions such as worried, happy, or angry looks.

Other major structures of the limbic system include the olfactory bulbs and nuclei, the nucleus accumbens, and the mammillary bodies. The olfactory bulbs and nuclei are the primary structures for smell perception. Experiencing pleasure involves the nucleus accumbens, which is also often stimulated by anything that can become addictive. The mammillary bodies are involved in learning and memory.

Cortical Lobes

The most complex thinking abilities are primarily attributable to the thin layers that cover the two cerebral hemispheres, known as the cortex. It is this covering of the brain that makes for the greatest differences between the intellectual capabilities of humans and those of other animals. Both hemispheres are typically divided into four main lobes, the distinct cortical areas of specialized functioning. There are, however, many differences between people, not only in the relative size of different lobes but also in how much cerebral cortex is not directly attributable to any of the four lobes.

The occipital lobe is located at the back of the cerebral cortex. The most posterior tissue of this lobe is called the striate cortex, due to its distinctive striped appearance. The striate cortex is also called the primary visual cortex because it is where mostvisual information is eventually processed. Each of the layers of this cortical area is specialized to analyze different features of visual input. The synthesis of visual information and the interpretation of that result involve other lobes of the brain. The occipital lobe also plays the primary role in various aspects of spatial reasoning. Activities such as spatial orientation, map reading, or knowing what an object will look like if rotated a certain amount of degrees all depend on this lobe.

Looking down on the top of the brain, a deep groove called the central sulcus can be seen roughly in the middle of the brain. Between the central sulcus and the occipital lobe is the parietal lobe. The parietal lobe’s predominant function is the processing of the bodily sensations: taste, touch, temperature, pain, and kinesthesia (feedback from muscles and joints). A parietal band of tissue called the postcentral gyrus that is adjacent to the central sulcus (posterior and runs parallel to it) contains the somatosensory cortex in which the surface of the body is represented upside down in a maplike fashion. Each location along this cortical area corresponds to sensations from a different body part. Furthermore, the left side of the body is represented on the right hemisphere and vice versa. Damage to the right parietal cortex usually leads to sensory neglect of the left side of the body—the person ignores sensory input from that side. However, damage to the left parietal cortex causes no or little sensory neglect of the right side of the body.

The parietal lobe is involved with some aspects of distance sensation. The posterior parietal lobe plays a role in the visual location of objects and the bringing together of different types of sensory information, such as coordinating sight and sound when a person looks at someone who just called his or her name. Some aspects of the learning of language also engage the operation of the parietal cortex.

On the sides of each hemisphere, next to the temples of the head, reside the temporal lobes. The lobes closest to the ears are the primary sites of the interpretation of sounds. This task is accomplished in the primary auditory cortex, which is tucked into a groove in each temporal lobe called the lateral sulcus. Low-frequency sounds are analyzed on the outer part of this sulcus; higher-pitched sounds are represented deeper inside this groove. Closely linked with auditory perception are two other major functions of the temporal lobe: language and music comprehension. Posterior areas, particularly Wernicke’s area, play key roles in word understanding and retrieval. More medial areas are involved in different aspects of music perception, especially the planum temporale.

The temporal cortex is the primary site of two important visual functions. Recognition of visual objects is dependent on inferior temporal areas. These areas of the brain are very active during visual hallucinations. One area in this location, the fusiform gyrus, is very active during the perception of faces and complex visual stimuli. A superior temporal area near the conjunction of the parietal and occipital lobes is essential for reading and writing.

The temporal lobe is in close proximity to, and shares strong connections with, the limbic system. Thus, it is not surprising that the temporal lobe plays a significant role in memory and emotions. Damage to the temporal cortex leads to major deficits in the ability to learn and in maintaining a normal emotional balance.

The largest cerebral lobe, comprising one-third of the cerebral cortex, is the frontal lobe. It is involved in the greatest variety of neurological functions. The frontal lobe consists of several anatomically distinct and functionally distinguishable areas that can be grouped into three main regions. Starting at the central sulcus, which divides the parietal and frontal lobes, and moving toward the anterior limits of the brain, one finds, in order, the precentral cortex, the premotor cortex, and the prefrontal cortex. Each of these areas is responsible for different types of activities.

In 1870, German physicians Gustav Fritsch and Eduard Hitzig were the first to stimulate the brain electrically. They found that stimulating different regions of the precentral cortex resulted in different parts of the body moving. Subsequent research identified a “motor map” that represents the body in a fashion similar to the adjacent and posteriorly located somatosensory map of the parietal lobe. The precentral cortex, therefore, can be considered the primary area for the execution of movements.

The premotor cortex is more responsible for planning the operations of the precentral cortex. In other words, the premotor cortex generates the plan to pick up a pencil, while the precentral cortex directs the arm to do so. Thinking about picking up the pencil but not actually doing so involves more activity in the premotor cortex than in the precentral cortex. An inferior premotor area essential for speaking was discovered in 1861 by Paul Broca and has since been named for him. Broca’s area, usually found only in the left hemisphere, is responsible for coordinating the various operations necessary for the production of speech.

The prefrontal cortex is the part of the brain most responsible for a variety of complex thinking activities, foremost among them being decision making and abstract reasoning. Damage to the prefrontal cortex often leads to an impaired ability to make decisions, rendering the person lethargic and greatly lacking in spontaneous behavior. Numerous aspects of abstract reasoning, such as planning, organizing, keeping time, and thinking hypothetically, are also greatly disturbed by injuries to the prefrontal cortex.

Research with patients who have prefrontal disturbances has demonstrated the important role of this neurological area in personality and social behavior. Patients with posterior prefrontal damage exhibit many symptoms of depression, such as apathy, restlessness, irritability, lack of drive, and lack of ambition. Anterior abnormalities, particularly in an inferior prefrontal region called the orbitofrontal area, result in numerous symptoms of psychopathy, including lack of restraint, impulsiveness, egocentricity, lack of responsibility for one’s actions, and indifference to others’ opinions and rights.

The prefrontal cortex also contributes to the emotional value of decisions, smell perception, working memory (the current ability to use memory), and the capacity to concentrate or shift attention. Children correctly diagnosed with attention-deficit hyperactivity disorder (ADHD) often have prefrontal abnormalities.

Hemispheric Differences

The two cerebral hemispheres are connected by a large band of fibers called the corpus callosum and several small connections called commissures. In the early 1940s, American surgeon William van Wagenen, to stop epileptic seizures from crossing from one hemisphere to the other, performed the first procedure of cutting the two hundred million fibers of the corpus callosum. The results were mixed, however, and it was not until the 1960s that two other American surgeons, Joe Bogen and Philip Vogel, decided to try the operation again, this time also including some cutting of commissure fibers. The results reduced or stopped the seizures in most patients. However, extensive testing by American psychobiologist Roger Sperry and his colleagues demonstrated unique behavioral changes in the patients, called split-brain syndrome. Research with split-brain syndrome and less invasive imaging techniques of the brain, such as computed tomography (CT) and positron-emission tomography (PET) scans, has demonstrated many anatomical and functional differences between the left and right hemispheres.

The degree of differences between the two cerebral hemispheres varies greatly depending on a number of factors. Men develop greater lateralization—larger differences between the hemispheres—and develop the differences sooner. Those with a dominant right hand have greater lateralization than left- or mixed-handers. Therefore, when there is talk of “left brain” versus “right brain,” it is important to keep in mind that a greater degree of difference exists in right-handed men. A minority of people, usually left-handers, show little differences between the left and right hemispheres.

The right hemisphere tends to be larger and heavier than the left hemisphere, with the greatest difference in the frontal lobe. Conversely, several other neurological areas have been found to be larger in the left hemisphere, including the occipital lobe, the planum temporale, Wernicke’s area, and the Sylvian fissure. One gender difference in hemispheric operation is that the left-hemisphere amygdala is more active in women and the right-hemisphere amygdala is more active in men.

The left-brain/right-brain functional dichotomy has been the subject of much popular literature. Although there are many differences in operation between the two hemispheres, it is important to realize that many of the differences are subtle, and in many regards both hemispheres are involved in a given psychological function, only to different degrees. The most striking difference between the two hemispheres is that the right hemisphere is responsible for sensory and motor functions of the left side of the body and the left hemisphere controls those same functions for the body’s right side. This contralateral control is found to a lesser degree for hearing and, due to the optic chiasm, not at all for vision.

In the domain of sound and communication, the left hemisphere plays a greater role in speech production, language comprehension, phonetic and semantic analysis, visual word recognition, grammar, verbal learning, lyric recitation, musical performance, and rhythm keeping. A greater right-hemisphere contribution is found in interpreting nonlanguage sounds, reading Braille, using emotional tone in language, understanding humor and sarcasm, expressing and interpreting nonverbal communication (facial and bodily expressions), and perceiving music. Categorical decisions, the understanding of metaphors, and the figurative aspects of language involve both hemispheres.

Regarding other domains, the right hemisphere plays a greater role in mathematical operations, but the left hemisphere is essential for remembering numerical facts and the reading and writing of numbers. Visually, the right hemisphere contributes more to mental rotation, facial perception, figure-ground distinctions, map reading, and pattern perception. Detail perception draws more on left-hemisphere resources. The right hemisphere is linked more with negative emotions, such as fear, anger, pain, and sadness, while positive affect is associated more with the left hemisphere. There are exceptions to this, however; schizophrenia, anxiety, and panic attacks have been found to be related more to increases in left-hemisphere activity.

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