The brain

The brain is the integrative portion of the nervous system that serves to receive, process, and store sensory information and then plan and orchestrate the appropriate motor response. It is divided into several anatomically and functionally distinct regions (see Table 6.2). The forebrain consists of the cerebrum, basal ganglia, thalamus, and hypothalamus. The midbrain, along with the pons and the medulla of the hindbrain, composes the functional region referred to as the brainstem. The cerebellum is also considered a component of the hindbrain but is functionally distinct from the brainstem.

6.4.1 Cerebrum

The cerebrum is composed of two hemispheres, left and right, that are anatomically connected to ensure communication between them. Two types of tissue compose each hemisphere (see Figure 6.2):

The gray matter, which contains the cell bodies of neurons, is on the outer surface of the cerebrum and forms the cerebral cortex. The white matter, composed of the myelinated axons of neurons, is found underlying the cortex in the core of the cerebrum. These axons are bundled together according to function and organized into units referred to as tracts. The three types of tracts in the cerebrum are:

• Projection tracts

• Association tracts

• Commissural tracts

Table 6.2 Adult Brain Structures

Forebrain

Cerebrum

^ Cerebral cortex

Basal ganglia

Thalamus

Hypothalamus

^ Subcortical structures

(embedded within cerebrum)

Midbrain

Hindbrain

Pons

Medulla

Cerebellum

^ Brainstem

RIGHT CEREBRAL LEFT CEREBRAL HEMISPHERE HEMISPHERE

Figure 6.2 Frontal section of the brain. The cerebrum is composed of two types of tissue: internal white matter and external gray matter which forms the cerebral cortex. Embedded within the cerebral hemispheres are other masses of gray matter, basal ganglia, and thalamus. The ventricles are filled with cerebrospinal fluid (CSF).

Figure 6.2 Frontal section of the brain. The cerebrum is composed of two types of tissue: internal white matter and external gray matter which forms the cerebral cortex. Embedded within the cerebral hemispheres are other masses of gray matter, basal ganglia, and thalamus. The ventricles are filled with cerebrospinal fluid (CSF).

Projection tracts may be descending and carry motor nerve impulses from the cerebral cortex to lower regions of the brain or spinal cord or they may be ascending and carry sensory impulses from lower regions of the brain or spinal cord to the cortex. Association tracts transmit nerve impulses from one functional region of the cerebral cortex to another within the same hemisphere. Commissural tracts transmit impulses from one hemisphere to the other. The primary example of this type of tract is the corpus callosum, the thick band of tissue connecting the left and right hemispheres consisting of more than 100 million neurons. The communication provided by each of these types of tracts facilitates the integration, processing, and storage of information among various regions of the brain.

The cerebral cortex is not a smooth surface, but instead is highly folded and has a furrowed appearance (see Figure 6.3). A convolution formed by these folds is referred to as a gyrus (pl. gyri). Each gyrus is separated from another by a sulcus (pl. sulci), which is a shallow groove, or a fissure, which is a deeper groove. The functional importance of gyri, sulci, and fissures is that they significantly increase the surface area of the cerebral cortex, providing space for a greater number of neurons.

Frontal lobe

• voluntary motor

Precentral gyrus activity

Central sulcus

Precentral gyrus

Central sulcus

• higher i activities

Figure 6.3 Lateral view of the four lobes of the cerebral cortex.

activities

Figure 6.3 Lateral view of the four lobes of the cerebral cortex.

Both hemispheres of the cerebrum consist of four lobes, including:

• Frontal lobes

• Parietal lobes

• Occipital lobes

• Temporal lobes

Named for the bones of the cranium under which they lie, the lobes are conspicuously defined by prominent sulci of the cortex, which have a relatively constant position in human brains. Each lobe is specialized for different activities (see Figure 6.3). Located in the anterior portions of the hemispheres, the frontal lobes are responsible for voluntary motor activity, speaking ability, and higher intellectual activities. The parietal lobes, which are posterior to the frontal lobes, process and integrate sensory information. The occipital lobes, located in the posterior-most aspects of the cerebrum, process visual information, and the temporal lobes, located laterally, process auditory information.

6.4.2 Functional regions of the cerebral cortex

The cerebral cortex is organized into several functionally discrete areas (see Figure 6.4). However, it is important to remember that no single area functions in isolation. The activity in each area depends on neurons in other areas for incoming and outgoing messages.

CORTICAL AREA

FUNCTION

Sensory Input

Relayed from afferent neuronal receptors i

Primary Sensory Areas

Initial cortical processing of sensory input i

Unimodal Association Areas

Further processing of information from a single sensory modality i

Multimodal Sensory Association Areas

Highest level of processing, integration, and interpretation of diverse sensory input for planning purposeful action i

Multimodal Motor Association Areas

Neuronal programming of movements according to cortical and subcortical input i

Primary Motor Cortex

Transmission of impulses to somatic efferent motor neurons in spinal cord to initiate voluntary contraction of skeletal muscle

Figure 6.4 Potential route of transmission of electrical impulses through association pathways of the cerebral cortex.

The somatosensory cortex is located in the postcentral gyrus, which is the most anterior region of the parietal lobes (see Figure 6.3). This region contains the terminations of ascending pathways that transmit nerve impulses concerning temperature, touch, pressure, pain, and proprioception. The latter is the awareness of posture, movement, changes in equilibrium, and the position of one's body parts, particularly in reference to surrounding objects. As such, the somatosensory cortex is the site for initial cerebral processing of these types of inputs.

Each section of this region of cortex receives sensory input from a specific area of the body in a highly organized and sequential manner. Interestingly, the size of the region of the cortex devoted to different areas of the body is quite disproportionate. For example, the trunk of the body and the legs are not densely innervated with sensory neurons. As a result, axonal terminations of pathways originating in these body parts are limited in number and take up only a small portion of the somatosensory cortex. Conversely, the face, tongue, and hands are very densely innervated with sensory neurons. Therefore, terminations of pathways originating in these body parts are numerous and represented in a much larger portion of the somatosensory cortex. In other words, the proportion of cortex devoted to a given body part is determined by the degree of sensory perception associated with that body part. The somatosensory cortex not only localizes the source of sensory input but it also perceives the intensity of the stimulus.

These ascending sensory pathways cross from one side of the CNS to the other so that sensory input from the left side of the body is transmitted to the somatosensory cortex of the right cerebral hemisphere and visa versa. Therefore, damage to this region of cortex in a given hemisphere results in sensory deficits such as numbness and tingling in the opposite side of the body.

In addition to the somatosensory cortex, special senses areas in the cerebral cortex are involved with the primary or initial processing of a specific type of stimulus. The primary visual cortex (sight) is located in the occipital lobes; the primary auditory cortex (hearing) and the primary olfactory cortex (smell) are located in the temporal lobes; and the primary gustatory or taste cortex is located at the base of the somatosensory cortex in the parietal lobes. Each of these primary areas is surrounded by a "higher order" sensory area or a unimodal association area that further integrates information from a single sensory modality and provides more complex aspects of the input. For example, the primary visual cortex is the first site of processing of visual information. Association tracts originating in this area then project to the surrounding unimodal association area for higher-level processing of this visual input.

The posterior parietal cortex is located posterior to the somatosensory cortex and serves as its unimodal association area. In addition to further processing of somatosensory input, information from the somatosensory cortex is integrated with visual inputs in this region. Association tracts from both the somatosensory cortex and the visual cortex terminate here. This activity is important for planning complex movements and for hand (prop-rioception)-eye (visual) coordination.

The unimodal association areas in turn project to multimodal sensory association areas that integrate information about more than one sensory modality. The highest level of cognitive brain function takes place in these areas. These areas process, integrate, and interpret sensory information and then link these data to the planning of movement and goal-directed action.

The prefrontal multimodal association area is located in the most anterior region of the frontal lobe. It is involved primarily with motor integration, including memory and planning of motor activity; long-term planning and judgment; personality traits; and behavior. Consistent with this notion, lesions to this association area result in profound cognitive deficits, impaired motor activity, and changes in personality and social behavior. These patients do not respond to environmental stimuli in a way similar to normal individuals. They tend to achieve less in life — a behavior that suggests their ability to plan and organize everyday activities is impaired. Interestingly, however, their general intelligence, perception, and long-term memory are rather intact.

The posterior multimodal association area is located at the junction of the parietal, temporal, and occipital lobes. It pools and integrates somatic, auditory, and visual stimuli for complex perceptual processing. As such, this area is involved primarily with visuospatial localization, language, and attention. Lesions here interfere with awareness of one's body position and of the space in which it moves as well as the ability to integrate and make sense of elements of a visual scene. In other words, these patients have normal visual acuity but cannot focus on an object of interest.

The limbic multimodal association area is partially located in each of the temporal, parietal and frontal lobes. It is concerned with emotional expression and memory storage. Although these functions appear to be unrelated, it is important to note that the emotional impact of an event is a major determinant of whether the event is remembered. Once again, it is important to remember that, although each of these multimodal association areas has its own characteristic function, all are highly interconnected and work together toward an end result.

The multimodal sensory association areas then project to the multimodal motor association areas located in the frontal lobes, including the premotor cortex and the supplementary motor cortex. Neurons here are active during preparation for movement. These regions receive input from the basal ganglia, cerebellum, somatosensory cortex, and posterior multimodal association cortex (all of which provide information about the ongoing movement) as well as the prefrontal multimodal association area. As such, these areas are important in programming complex sequences of movements and in orienting the body and limbs toward a specific target. Lesions in these multimodal motor association areas interfere with coordination and performance of complex integrated movements.

Following the development of the motor program, neurons originating in the multimodal motor association areas transmit impulses by way of association tracts to neurons of the primary motor cortex. The primary motor cortex is located in the precentral gyrus, which is the most posterior region of the frontal lobe adjacent to the multimodal motor association areas (see Figure 6.3); this area initiates voluntary contractions of specific skeletal muscles. Neurons whose cell bodies reside here transmit impulses by way of descending projection tracts to the spinal cord, where they innervate the alpha motor neurons (which innervate skeletal muscles).

As with the somatosensory cortex, neurons here are highly organized, with each section of the cortex innervating specific body parts in a sequential manner. Also like the somatosensory cortex, the size of the region of the primary motor cortex devoted to different parts of the body is quite disproportionate. Large portions of the primary motor cortex innervate the muscles of the hands, which perform complex movements, as well as muscles responsible for speech and eating. On the other hand, little cortex is devoted to motor pathways terminating in the trunk of the body or the lower extremities, which are not capable of complex movements. Therefore, the distortions in cortical representation parallel the importance of a particular part of the body in terms of complexity of motor skills. A third similarity between the primary motor cortex and the somatosensory cortex is that the projection tracts cross from one side of the CNS to the other; therefore, activity of motor neurons in the left cerebral hemisphere causes muscle contraction on the right side of the body and vice versa. Because the commands for muscle contraction originate in the primary motor cortex, lesions in this region of cortex in a given hemisphere will result in paralysis in the opposite side of the body.

The exchange of information among individuals is largely limited to species with advanced nervous systems and is found predominantly in birds and mammals. In humans, communication takes place primarily through language or the use of spoken or written words to convey a message. The processing of language requires a large network of interacting brain areas, both cortical and subcortical. However, the two predominant cortical areas are Wernicke's area and Broca's area. In approximately 96% of people, these cortical areas for language skills are found only in the left hemisphere. Even languages such as American sign language that rely on visuomotor abilities instead of auditory speech abilities depend primarily on the left hemisphere.

Sensory input to the language areas comes from the auditory cortex (hearing) or the visual cortex (reading). This input goes first to Wernicke's area, located in the left cerebral cortex near the junction of the parietal, temporal, and occipital lobes. This area is involved with language comprehension and is important for understanding spoken and written messages. It is also responsible for formulating coherent patterns of speech. In other words, this area enables an individual to attach meaning to words and to choose the appropriate words to convey his thoughts. Impulses are then transmitted to Broca's area, which is located in the left frontal lobe in close association with the motor areas of the cortex that control the muscles necessary for articulation. Broca's area is therefore responsible for the mechanical aspects of speaking.

A patient with a lesion in Wernicke's area is unable to understand any spoken or visual information. Furthermore, the patient's speech, while fluent, is unintelligible because of frequent errors in the choice of words. This condition is known as receptive aphasia. On the other hand, a patient with a lesion in Broca's area is able to understand spoken and written language but is unable to express his response in a normal manner. Speech in this patient is nonfluent and requires great effort because he cannot establish the proper motor command to articulate the desired words. This condition is known as expressive aphasia.

6.4.3 Basal ganglia

The basal ganglia consist of four nuclei or masses of gray matter embedded within the white matter of each cerebral hemisphere (see Figure 6.2). As with the cerebral cortex, this gray matter consists of functional aggregations of neuronal cell bodies. An important function of the basal ganglia is their contribution to the control of voluntary movement. The axons of neurons originating in the primary motor cortex travel through descending projection tracts to the spinal cord where they stimulate motor neurons to cause skeletal muscle contraction. At the same time, by way of divergence, these neurons transmit impulses to the basal ganglia. It is these impulses that form the primary source of input to these structures. In turn, the basal ganglia send impulses to the brainstem, which also transmits to motor neurons in the spinal cord as well as the thalamus, which transmits back to the motor areas of the cerebral cortex.

The activity of the basal ganglia tends to be inhibitory. The thalamus positively reinforces motor activity in the cerebral cortex. Impulses from the basal ganglia modulate this effect. Through their inputs to the brainstem and, ultimately the motor neurons in the spinal cord, the basal ganglia inhibit muscle tone (recall that the degree of skeletal muscle contraction and tone is determined by the summation of excitatory and inhibitory inputs to the motor neurons). They also contribute to the coordination of slow sustained contractions, especially those related to posture and body support. Motor disturbances associated with the basal ganglia include tremor and other involuntary movements; changes in posture and muscle tone; and slowness of movement without paralysis. Thus, disorders of the basal ganglia may result in diminished movement (Parkinson's disease) or excessive movement (Huntington's disease).

6.4.4 Thalamus

The thalamus is located between the cerebrum and the brainstem. Lying along the midline of the brain, it consists of two oval-shaped masses of gray matter, one in each cerebral hemisphere (see Figure 6.2). The thalamus is often described as a relay station because ascending tracts transmitting upward from the spinal cord, as well as sensory tracts from the eyes and the ears, extending ultimately to the cerebral cortex, pass through it. All sensory fiber tracts (except olfactory tracts) transmitting impulses to the cerebral cortex first synapse with neurons in the thalamus.

The thalamus acts as a filter for information to the cortex by preventing or enhancing the passage of specific information depending upon its significance to the individual. In fact, more than 99% of all sensory information transmitted toward the brain is discarded because it is considered irrelevant and unimportant. This selection activity is accomplished largely at the level of the thalamus. As mentioned previously in the discussion of the basal ganglia, the thalamus also plays a role in regulation of skeletal muscle contraction by positively reinforcing voluntary motor activity initiated by the cerebral cortex.

6.4.5 Hypothalamus

As its name suggests, the hypothalamus lies beneath the thalamus and above the pituitary gland. Although it is quite small, accounting for only about 4 g of the total 1400 g of the adult human brain, it plays a vital role in maintenance of homeostasis in the body. It is composed of numerous cell groups and fiber pathways, each with a specific function.

The hypothalamus plays a particularly important role in regulating the autonomic nervous system, which innervates cardiac muscle, smooth muscle, and glands. Many of these effects involve ascending or descending pathways of the cerebral cortex passing through the hypothalamus. Endocrine activity is also regulated by the hypothalamus by way of its control over pituitary gland secretion. Recent studies have demonstrated that the hypothalamus serves to integrate autonomic nervous system responses and endocrine function with behavior, especially behavior associated with basic homeostatic requirements. The hypothalamus provides this integrative function by regulating the following:

• Blood pressure and electrolyte composition by regulating mechanisms involved with urine output, thirst, salt appetite, maintenance of plasma osmolarity, and vascular smooth muscle tone

• Body temperature by regulating metabolic thermogenesis (e.g., shivering) and behaviors that cause an individual to seek a warmer or cooler environment

• Energy metabolism by regulating food intake, digestion, and metabolic rate

• Reproduction by way of hormonal control of sexual activity, pregnancy, and lactation

• Responses to stress by altering blood flow to skeletal muscles and other tissues as well as enhancing secretion of hormones from the adrenal cortex (glucocorticoids) whose metabolic activities enable the body to physically cope with stress

The hypothalamus regulates these physiological parameters by a three-step process involving negative feedback mechanisms (Chapter 1). First, the hypothalamus has access to and monitors sensory information from the entire body. Next, it compares this information to various biological set points that have been established for optimal cellular function. Finally, if a deviation from set point for a given parameter is detected, the hypothalamus elicits a variety of autonomic, endocrine, and behavioral responses to return the parameter to its set point and reestablish homeostasis. For example, blood glucose levels are monitored by the hypothalamus; when blood glucose is low (<50 mg glucose/100 ml blood), it mediates the sensation of hunger to drive the individual to ingest food.

6.4.6 Brainstem

The functional region known as the brainstem consists of the midbrain, and the pons and medulla of the hindbrain. It is continuous with the spinal cord and serves as an important connection between the brain and spinal cord because all sensory and motor pathways pass through it. The brainstem consists of numerous neuronal clusters or centers, each of which controls vital, life-supporting processes.

The medulla contains control centers for subconscious, involuntary functions, such as cardiovascular activity, respiration, swallowing, and vomiting. The primary function of the pons is to serve as a relay for the transfer of information between the cerebrum and the cerebellum. Along with the medulla, it also contributes to the control of breathing. The midbrain controls eye movement and relays signals for auditory and visual reflexes. It also provides linkages between components of the motor system including the cerebellum, basal ganglia, and cerebrum.

In addition, the brainstem contains a diffuse network of neurons known as the reticular formation. This network is best known for its role in cortical alertness, ability to direct attention, and sleep. It is also involved with coordination of orofacial motor activities, in particular those involved with eating and the generation of emotional facial expressions. Other functions include coordination of eating and breathing, blood pressure regulation, and response to pain.

6.4.7 Cerebellum

The cerebellum (Latin, little brain) is part of the hindbrain and is attached to the dorsal surface of the upper region of the brainstem. Although it constitutes only 10% of the total volume of the brain, it contains more than half of all its neurons. Its surface consists of a thin cortex of gray matter with extensive folding, a core of white matter, and three pairs of nuclei embedded within it.

The specialized function of the cerebellum is to coordinate movement by evaluating differences between intended movement and actual movement. It carries out this activity while a movement is in progress as well as during repetitions of the same movement. Three important aspects of the cerebellum's organization enable it to carry out this function. First, it receives extensive sensory input from somatic receptors in the periphery of the body and from receptors in the inner ear providing information regarding equilibrium and balance. Second, output from the cerebellum is transmitted to premotor and motor systems of the cerebral cortex and the brainstem — systems that control spinal interneurons and motor neurons. Finally, circuits within the cerebellum exhibit significant plasticity, which is necessary for motor adaptation and learning.

The cerebellum consists of three functionally distinct parts:

• Vestibulocerebellum

• Spinocerebellum

• Cerebrocerebellum

The vestibulocerebellum receives sensory input regarding motion of the head and its position relative to gravity as well as visual input. Outputs control axial muscles (primarily head and neck) and limb extensors, assuring balance while standing still and during movement. Outputs also control eye movements and coordinate movement of the head and eyes. Lesions here affect an individual's balance. The ability to use the incoming sensory information to control eye movements when the head is rotating and movements of the limbs and body during standing and walking is also impaired.

The spinocerebellum influences muscle tone and coordinates skilled voluntary movements. It receives sensory input from interneurons in the spinal cord transmitting somatic information, in particular from muscle and joint proprioceptors providing data regarding body movements and positions that are actually taking place. It also receives input from the cortical motor areas providing information regarding intended or desired movement. The spinocerebellum then compares these inputs. If the actual status of a body part differs from the intended status, the spinocerebellum transmits impulses back to the motor areas of the brain to make appropriate adjustments in activation of the associated skeletal muscles.

The cerebrocerebellum is involved with the planning, programming, and initiation of voluntary activity. It also participates in procedural memories or motor learning. This region of the cerebellum receives input from and provides output to the cortical motor areas directly. Lesions of the cerebro-cerebellum cause delays in initiating movements and irregularities in the timing of multistep movements.

Disorders of the human cerebellum result in three types of abnormalities. The first is hypotonia or reduced muscle tone. Another includes abnormalities in the execution of voluntary movements or ataxia (defective muscular coordination). The third type of muscular malfunction is intention tremors. These tremors differ from the resting tremors of Parkinson's disease in that they occur during a movement and are most pronounced at the end of the movement when the patient attempts to terminate it.

Pharmacy application: centrally acting drugs

Combinations of centrally acting drugs are frequently used to achieve a desired therapeutic effect, particularly when the agents used have different mechanisms of action. For example, a patient with Parkinson's disease may be treated with one drug that blocks the effects of the neurotransmitter, acetylcholine, and a second drug that enhances the activity of another neurotransmitter, dopamine. However, potentially detrimental effects may occur when the agents used have additive effects. The effect of a CNS stimulant or depressant is additive with the effects of all other categories of stimulant and depressant drugs. For example, the combination of benzodiazepines (diazepam, Valium®) or barbiturates (pentobarbital, Nembutal®) with ethanol is not only additive, it may be fatal. Each of these drugs has a depressant effect on the respiratory center in the brainstem, so high doses may cause breathing to stop. The effect of a CNS drug is also additive with the physiological state of the patient. For example, anesthetics and antianxiety drugs are less effective in a hyperexcitable patient compared to a normal patient.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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