The brain weighs between 1100 and 2000 g. It contains an estimated 100 billion neurons. The average neuron has up to 10 000 synapses. Almost one-third of this immensely complex system is dedicated to the function of behavior.

Anatomy and behavioral considerations

Two types of cells make up the nervous system: neurons and neuroglial cells. Neurons are specialized to conduct bioelectrical messages, whereas the glial cells play a supportive role.

The neuron

The neuron is the structural and functional unit of the nervous system. It is made up of four distinctive regions: the soma (nerve cell body), the dendrites, the axon, and the synapse (Figures 3.1 and 3.4). The soma is the metabolic center of the cell and contains the cell nucleus. The nucleus is centrally located in the soma, and the cytoplasm immediately surrounding the nucleus is called the perikaryon. The cytoplasm of the axon is called the axoplasm.

Most neurons have several dendrites. Each neuron has a single axon (Figure 3.2). The axon arises from a specialized region of the cell body called the axon hillock (see Figure 3.1), which is specialized to facilitate the propagation of the all-or-none action potential.

Nissl substance (rough endoplasmic reticulum) and Golgi apparatus are restricted to the perikaryon and to the base of the dendrites. They synthesize proteins for use throughout the neuron. Three classes of proteins are produced. One of these classes produced in the perikaryon includes the neurotransmitters.

A hallmark of human brain function is cerebral lateralization and specialization. This specialization necessitates an efficient interhemispheric communication system. No other mammal possesses the degree of localization of function seen in the human. Only the human brain has the intellectual and computational capabilities necessary to study how neural systems both generate and respond to the intense information demands of the environment.

At first glance the anatomical brain appears largely symmetrical. More careful analysis reveals “typical counterclockwise hemispheric torque,” which is reflected in the fact that the left parieto-occipital region is wider and extends further posteriorly than the right. On the right side the frontal lobe is larger than the left and extends further anteriorly (Glicksohn and Myslobodsky, 1993). This difference is called petalia (Hadziselimovic and Cus, 1966). Fiber bundles interconnect the left and right sides. It must be assumed that these bundles play a role in the behavioral specializations that are reflected in the laterality of behavior.

The basal ganglia (basal nuclei) have been regarded traditionally as a motor control system. Lesions in the basal ganglia almost always result in movement disorders. Recently, these structures have been found to influence other emotionally related behaviors. Research on the behavioral influence of the basal ganglia was prompted by the repeated observation that emotional and cognitive dysfunctions frequently accompany movement disorders of basal ganglia origin. In some cases, psychiatric manifestations precede the onset of motor symptoms. With the advent of neuroimaging techniques, investigation into the anatomy and metabolic physiology of these structures in the awake behaving human has revealed intriguing behavioral relationships.

Motor activity is controlled by the intricate interaction of three major systems: the cerebral cortex, the cerebellum, and the basal ganglia. The few milliseconds that intervene between thought and action are crucial for our adjustment in modern society. Understanding the structures that influence those few milliseconds, such as the basal ganglia, will help unravel some of the mysteries of human behavior. It is interesting that the main input to the basal ganglia comes from the cerebral cortex and that its output returns (via the thalamus) to the frontal cortex (motor, premotor, and prefrontal cortex). The frontal cortex, including the prefrontal areas, thus mediates and plays a significant role in the various functions of the basal ganglia.

Originally the basal ganglia were described as a group of cerebral (telencephalic) nuclei. These classical basal ganglia included the caudate nucleus, putamen, globus pallidus, claustrum, and amygdala.

Emil Kraepelin (1919) suggested that abnormalities in the frontal lobe were responsible for problems in reasoning and that damage to the temporal lobe resulted in delusions and hallucinations in patients with dementia praecox (schizophrenia). The classical findings of Kluver and Bucy in the 1930s clearly and strongly linked the temporal lobes to behavior. Their work provided the basis from which the concept of the limbic system has developed.

The temporal lobe can be divided into two regions: lateral and ventromedial. The lateral region supports cognitive functions associated with several sensory systems. It is recognized as neocortex and is the focus of this chapter. The ventromedial region of the temporal lobe contains major portions of the limbic system and thus contributes significantly to emotional tone. The ventromedial, limbic temporal lobe is discussed in Chapter 11.

It is now accepted that dysfunction of the dorsolateral region of the temporal lobe may be associated with several psychopathological states. Temporal lobe lesions due to a variety of neurological insults can lead a patient to present with signs and symptoms that are more consistent with a psychiatric diagnosis than with a traditional neurological one.

Anatomy and behavioral considerations

The temporal lobe lies ventral to the lateral fissure (of Sylvius) and the parietal lobe. It is rostral to the occipital lobe (Figure 5.1). The superior temporal sulcus and the lateral fissure lie above the superior temporal gyrus and are particularly deep.

Occipital lobe

Anatomy and behavioral considerations

The occipital lobe is clearly demarcated from the parietal lobe on the medial surface by the parieto–occipital sulcus and by the anterior limb of the calcarine fissure (Figure 4.1). The short section of parieto–occipital sulcus on the dorsolateral surface is used as an anchor for an imaginary line that extends ventrally to the preoccipital notch (see Figure 5.1). This imaginary line is the border between the occipital and parietal as well as the temporal lobe on the lateral cortical surface. The border between the occipital and temporal lobes on the ventral surface is less distinct (see Figure 5.2). Some authors include all of the inferior temporal and fusiform gyri (medial occipitotemporal gyrus) with the temporal lobe; others assign the caudal portions of these gyri to the occipital lobe.

The cortex of the occipital lobe consists of Brodmann's areas (BA) 17, 18, and 19 (Figures 2.2, 2.3, 4.1, 4.2). Brodmann's area 17 is the primary visual cortex (striate cortex) and occupies a large portion of the medial aspect of the occipital lobe. Much of the primary visual cortex lies within the calcarine fissure which extends approximately 2.5 cm deep into the occipital lobe. A portion of BA 17 curves around the posterior surface of the brain onto the lateral surface of the occipital lobe. Brodmann's areas 18 and 19 are recognized as secondary and tertiary visual areas, respectively. Together, BA 18 and 19 represent the visual association area.

The last ten years has witnessed an explosion in the understanding of the neurochemical and neurophysiological processes that underlie behavior. Our understanding of the pathophysiology of many psychiatric disorders has increased as well. Clinicians are now faced with the overwhelming challenge of the need to keep up with the flood of basic neuroscientific knowledge that appears monthly in scientific journals, as well as the need to assimilate it with an ever-increasing number of reports in the clinical journals that identify structural and biochemical abnormalities associated with clinical disorders. The gap that has always existed between the basic science of neuroanatomy and clinical behavioral science seems to be widening at an increasing rate.

Although the current level of knowledge of behavior and psychopathology does not necessitate a detailed understanding of all neuroanatomy, a basic level of some neuroanatomical knowledge is necessary. Familiarity with those brain regions that are heavily implicated in both normal and abnormal behavior will help the clinician assimilate new knowledge as the field evolves. As the clinician becomes more aware of the structure and function of the behaviorally sensitive regions of the brain, the concept that brain abnormalities can produce the symptomatology that is seen in the clinic becomes progressively more understandable.

Currently available neuroanatomy books are written with the neurologist in mind. Emphasis is placed on the neuroanatomy that is examined during a standard neurological exam.

The brain is that portion of the central nervous system (CNS) that lies within the skull. Three major subdivisions are recognized: the brainstem, the cerebellum, and the cerebrum. The cerebrum includes both the cerebral hemispheres and the diencephalon.

Brainstem

The brainstem is the rostral continuation of the spinal cord. The foramen magnum, the hole at the base of the skull, marks the junction of the spinal cord and brainstem. The brainstem consists of three subdivisions: the medulla, the pons, and the midbrain (Figure 2.1).

Medulla

The caudal limit of the medulla lies at the foramen magnum. The central canal of the spinal cord expands in the region of the medulla to form the fourth ventricle (IV in Figure 1.2). Cranial nerves associated with the medulla are the hypoglossal, spinal accessory, vagus, and the glossopharyngeal.

Pons

The pons lies above (rostral to) the medulla (see Figure 2.1). The bulk of the medulla is continuous with the pontine tegmentum. The tegmentum consists of nuclei and tracts that lie between the basilar pons and the floor of the fourth ventricle (IV in Figure 1.2; see Figure 10.2). The basilar pons consists of tracts along with nuclei that are associated with the cerebellum. The fourth ventricle narrows at the rostral end of the pons to connect with the cerebral aqueduct of the midbrain (see Figures 1.2, 10.2–10.4). Cranial nerves associated with the pons are the statoacoustic (previously known as the auditory), facial, abducens, and trigeminal.

The brainstem is the connection between the spinal cord, the cerebellum, and the cerebrum. Only recently has it been implicated in behavior. The brainstem is anatomically comprised of three areas: (1) the medulla, (2) the pons, and (3) the midbrain (see Figure 2.1). The medulla, the inferior segment of the brainstem, represents a conical, expanded continuation of the upper cervical spinal cord. The pons lies between the medulla and the midbrain. The midbrain is the smallest and least differentiated division of the brainstem. The nuclei of cranial nerves III through XII are located in the brainstem along with long sensory and motor tracts that pass between the brain and spinal cord. Several regions of the brainstem, however, seem to be significantly involved in behavior. These behaviorally active regions include: (1) the reticular formation, (2) the parabrachial nucleus, (3) the raphe nuclei, (4) the periaqueductal gray, (5) the nucleus locus ceruleus, (6) the lateral tegmental nucleus, and (7) the ventral tegmental area (VTA). The VTA is considered to be one of the basal ganglia (see Chapter 7).

Anatomy and behavioral considerations

Reticular formation

The reticular formation is one of the oldest portions of the brain and represents the core of the brainstem. It is composed of complex collections of cells that form both diffuse cellular aggregations and more defined nuclei.

The ascending reticular activating system (ARAS) is a physiological concept. It is represented anatomically by the central core region of the brainstem (Figure 10.1), including the raphe nuclei (Figure 10.2).

Human behavior is a direct reflection of the anatomy and physiology of the central nervous system (CNS). The goal of the behavioral neuroscientist is to uncover the neuroanatomical substrates of behavior. Complex mental processes are represented in the brain by their elementary components. Elaborate mental functions consist of subfunctions and are constructed from both serial and parallel interconnections of several brain regions. An introduction to the nervous system covers general terminology and the ventricular system.

Major subdivisions

The nervous system is divided anatomically into the central nervous system (CNS) and the peripheral nervous system (PNS).

• The CNS is made up of the brain and spinal cord.

• The PNS consists of the cranial nerves and spinal nerves.

Physiologically, the nervous system can be divided into somatic and visceral (autonomic) divisions.

• The somatic nervous system deals with the contraction of striated muscle and the sensations of the skin (pain, touch, temperature), the innervation of muscles and joint capsules (proprioception), and the reception of sensations remote to the body by way of special senses. The somatic nervous system senses and controls our interaction with the environment external to the body.

• The autonomic nervous system controls the tone of the smooth muscles and the secretions of glands. It senses and controls the condition of the internal environment.

Common terms

The neuraxis is the long axis of the brain and spinal cord (Figure 1.1). A cross section (transverse section) is a section taken at right angles to the neuraxis.

Hypothalamus

The hypothalamus is the region of the mammalian brain that is most important in the coordination of behaviors essential for the maintenance and continuation of the species. Although the hypothalamus occupies only about 0.15% of the volume of the human brain, it plays a major role in the regulation and release of hormones from the pituitary gland, maintenance of body temperature, and organization of goal-seeking behaviors such as feeding, drinking, mating, and aggression. It is the primary center for the control of autonomic function. It is also the region of the brain that is essential for behavioral adjustments to changes in the internal or external environment (Figure 8.1). The hypothalamus is a very old structure with striking similarity between humans and lower animals. It is made up of a number of nuclei and scattered cell groups. Some hypothalamic cell groups control specific functions (e.g., blood pressure, heart rate, etc.) through the coordinated action of short intrahypothalamic connections. Other nuclei operate by projections to structures outside the confines of the hypothalamus.

Anatomical and behavioral considerations

The hypothalamus lies on either side of the walls of the third ventricle below the level of the hypothalamic sulcus (Figure 8.2, see Figures 9.2, 9.3, and 14.6). It is bounded in front (rostrally) by the lamina terminalis and optic chiasm, laterally by the optic tracts, and behind by the mamillary bodies. Some hypothalamic nuclei are continuous across the floor of the third ventricle.

The elusive functions of the frontal lobe continue to fascinate the neuroscientist and the neuropsychologist. The frontal lobe is impressively developed in humans and makes up more than one-third of the entire cortical area (Damasio and Anderson, 1993). It controls actions of our body through its motor areas. It also appears to be responsible for shaping our attitudes and organizing our repertoire of behaviors through the actions of the prefrontal areas. Functions that are hallmarks of human behavior, such as intentionality, self-regulation, and self-awareness, are thought to be under the executive control of the frontal lobe.

An ongoing controversy among the prefrontal cortex researchers is whether this region contains regions with discrete functions subservient to an overall executive module that provides an integrated output of the system, or whether the entire prefrontal region is involved in this integrative function. The latter hypothesis requires the neural modules of the prefrontal regions to be highly dynamic. Evidence for both theories exists and the truth is likely to have elements of both schemas where both specialization and versatility contribute to the proper functioning of this most fascinating of brain regions. Different competing, and not necessarily mutually exclusive, theories will be introduced in this chapter.

Anatomical subdivisions

The frontal lobe lies rostral (anterior) to the central sulcus and is made up of three anatomically distinct regions: the dorsolateral aspect, the medial aspect, and the orbital (inferior) aspect.

The thalamus functions as the principal relay station for sensory information destined for the cerebral cortex. It is made up of a number of nuclei, which can be grouped into relay nuclei and diffuse-projection nuclei (Table 9.1). Each relay nucleus is associated with a single sensory modality or motor system and projects to a specific region of the cerebral cortex with which it has reciprocal connections. The diffuse-projection group of nuclei have more widespread connections with the cortex and also interact with other thalamic nuclei. It is believed that this group of nuclei is involved with regulating the level of arousal of the brain. The limbic thalamus consists of a number of the thalamic nuclei that project to the limbic cortex and includes both relay and diffuse-projection nuclei.

Anatomy and behavioral considerations

The thalamus consists of a symmetrical pair of ovoid structures located above (dorsal to) the hypothalamus. The left thalamus and right thalamus are separated medially by the third ventricle and bounded laterally by the posterior limb of the internal capsule (Figures 9.1–9.3). The massa intermedia (interthalamic adhesion) is a bridge of cells that spans the third ventricle and joins the left with the right thalamus. The thalamus is bounded in front (anteriorly) by the head of the caudate nucleus and the genu of the internal capsule and behind (posteriorly) by the midbrain. The subthalamic nucleus (subthalamus) lies immediately below (ventral to) the thalamus and is sandwiched between the thalamus, internal capsule, and pretectal area (Figures 9.2 and 9.3).

Several brain regions have emerged relatively recently as major contributors to the human psychological system. A good example is the cingulate cortex. As early as the 1930s, it was observed that cats become mute and akinetic after the occurrence of lesions of the cingulate cortex. However, it was not until the 1950s that the cingulate gyrus was examined more carefully. Together with the parahippocampal gyrus, which lies below it, the cingulate gyrus (Figure 12.1) forms a large arcuate convolution that surrounds the rostral brainstem and constitutes what Broca referred to as the grande lobe limbique (see Figure 13.1).

Anatomy and behavioral considerations

The cingulate cortex lies deep within the longitudinal cerebral fissure and spans the corpus callosum like a great arc (see Figure 12.1). It is separated from the frontal and parietal cortex above by the cingulate sulcus, within which much of the cingulate cortex lies. Two major subdivisions of the cingulate cortex and a connecting bundle can be identified:

• The anterior cingulate cortex (ACC) forms a large region around the genu of the corpus callosum. The majority of the ACC consists of BA 24 and lies just beneath the supplementary motor area. The smaller paragenu area of the ACC wraps around the front and underside of the genu of the corpus callosum and corresponds largely with BA 25 (see Figures 2.3 and 12.1). The terms infralimbic, subcallosal and anterior limbic have been applied to all or parts of the paragenu area located below the genu (IL and PG, Figure 12.1). […]