Cerebral Cortex

The cerebrum consists of an outer cerebral cortex, composed of 2 to 4 mm of gray matter and underlying white matter. The cerebral cortex is characterized by numerous folds and grooves called convolutions. The elevated folds of the convolutions are called gyri, and the depressed grooves are the sulci. Each cerebral hemisphere is subdivided by deep sulci, or fissures, into five lobes, four of which are visible from the surface (fig. 8.6). These lobes are the frontal, parietal, temporal, and occipital, which are visible from the surface, and the deep insula, which is covered by portions of the frontal, parietal, and temporal lobes (table 8.1).

The frontal lobe is the anterior portion of each cerebral hemisphere. A deep fissure, called the central sulcus, separates the frontal lobe from the parietal lobe. The precentral gyrus (figs. 8.5 and 8.6), involved in motor control, is located in the frontal lobe, just in front of the central sulcus. The neuron cell bodies located here are called upper motor neurons, because of their role in muscle regulation (chapter 12). The postcentral gyrus, which is located just behind the central sulcus in the parietal lobe, is the primary area of the cortex responsible for the perception of somatesthetic sensation—sensation arising from cutaneous, muscle, tendon, and joint receptors. This neural pathway is described in chapter 10.

The precentral (motor) and postcentral (sensory) gyri have been mapped in conscious patients undergoing brain surgery. Electrical stimulation of specific areas of the precentral gyrus causes specific movements, and stimulation of different areas of the postcentral gyrus evokes sensations in specific parts of the body. Typical maps of these regions (fig. 8.7) show an upside-down picture of the body, with the superior regions of cortex devoted to the toes and the inferior regions devoted to the head.

A striking feature of these maps is that the areas of cortex responsible for different parts of the body do not correspond to the size of the body parts being served. Instead, the body regions with the highest densities of receptors are represented by the largest areas of the sensory cortex, and the body regions with the greatest number of motor innervations are represented by the largest areas of motor cortex. The hands and face, therefore, which have a high density of sensory receptors and motor innervation, are served by larger areas of the precentral and postcentral gyri than is the rest of the body.

The temporal lobe contains auditory centers that receive sensory fibers from the cochlea of each ear. This lobe is also involved in the interpretation and association of auditory and visual information. The occipital lobe is the primary area responsible for vision and for the coordination of eye movements. The functions of the temporal and occipital lobes will be considered in more detail in chapter 10, in conjunction with the physiology of hearing and vision.

The insula is implicated in memory encoding and in the integration of sensory information (principally pain) with visceral responses. In particular, the insula seems to be involved in coordinating the cardiovascular responses to stress.

Motor areas involved with the control of voluntary muscles

Central sulcus

Frontal lobe

Motor speech area (Broca's area)

Lateral sulcus

Sensory areas involved with cutaneous and other senses

Parietal lobe

Motor areas involved with the control of voluntary muscles

Central sulcus

Frontal lobe

Sensory areas involved with cutaneous and other senses

Parietal lobe

Lateral sulcus

Auditory area

Interpretation of sensory experiences, memory of visual and auditory patterns

Occipital lobe

Temporal lobe

Brain stem

Auditory area

Interpretation of sensory experiences, memory of visual and auditory patterns

Occipital lobe

Combining visual images, visual recognition of objects

Temporal lobe

Brain stem

■ Figure 8.6 The lobes of the left cerebral hemisphere. This diagram shows the principal motor and sensory areas of the cerebral cortex.

Frontal Voluntary motor control of skeletal muscles; personality; higher intellectual processes (e.g., concentration, planning, and decision making); verbal communication Parietal Somatesthetic interpretation (e.g., cutaneous and muscular sensations); understanding speech and formulating words to express thoughts and emotions; interpretation of textures and shapes

Temporal Interpretation of auditory sensations; storage (memory) of auditory and visual experiences Occipital Integration of movements in focusing the eye; correlation of visual images with previous visual experiences and other sensory stimuli; conscious perception of vision Insula Memory; sensory (principally pain) and visceral integration

Frontal Voluntary motor control of skeletal muscles; personality; higher intellectual processes (e.g., concentration, planning, and decision making); verbal communication Parietal Somatesthetic interpretation (e.g., cutaneous and muscular sensations); understanding speech and formulating words to express thoughts and emotions; interpretation of textures and shapes

Temporal Interpretation of auditory sensations; storage (memory) of auditory and visual experiences Occipital Integration of movements in focusing the eye; correlation of visual images with previous visual experiences and other sensory stimuli; conscious perception of vision Insula Memory; sensory (principally pain) and visceral integration

People with Alzheimer's disease have (1) a loss of neurons; (2) an accumulation of intracellular proteins forming neurofibrillar tangles; and (3) an accumulation of extracellular protein deposits called amyloid plaques. The major constituent of the plaques is a polypeptide called amyloid P-peptide (AP). AP is formed by cleavage of a precursor protein by an enzyme called secretase. One isoform of the enzyme, Y-secretase, is activated by presenilin proteins, which are defective in some people with an inherited type of Alzheimer's. The structure of another isoform of the enzyme, P-secretase, has recently been characterized. Scientists hope that this will help them to develop a drug that will block secretase action and perhaps thereby slow the progression of Alzheimer's disease.

Visualizing the Brain

Several relatively new imaging techniques permit the brains of living people to be observed in detail for medical and research purposes. The first of these to be developed was x-ray computed tomography (CT). CT involves complex computer manipulation of data obtained from x-ray absorption by tissues of different densities. Using this technique, soft tissues such as the brain can be observed at different depths.

The next technique to be developed was positron-emission tomography (PET). In this technique, radioisotopes that emit positrons are injected into the bloodstream. Positrons are like electrons but carry a positive charge. The collision of a positron and an electron results in their mutual annihilation and the emission of gamma rays, which can be detected and used to pinpoint brain cells that are most active. Scientists have used PET to study brain metabolism, drug distribution in the brain, and changes in blood flow as a result of brain activity.

A newer technique for visualizing the living brain is magnetic resonance imaging (MRI). This technique is based on the concept that protons (H+) respond to a magnetic field. The magnetic field is used to align the protons, which emit a detectable radio-wave signal when appropriately stimulated. With this technique, excellent images can be obtained (figs. 8.8 and 8.9) without subjecting the person to any known danger. Scientists are now using MRI together with other techniques to study the function of the brain (see fig. 8.8) in a technique called functional magnetic resonance imaging (fMRI). Various techniques for visualizing the functioning brain are summarized in table 8.2.

Electroencephalogram

The synaptic potentials (discussed in chapter 7) produced at the cell bodies and dendrites of the cerebral cortex create electrical currents that can be measured by electrodes placed on the scalp. A

The Central Nervous System

Central sulcus

The Central Nervous System

Central sulcus

Motor area

Sensory area

Motor area

Sensory area

■ Figure 8.7 Motor and sensory areas of the cerebral cortex. (a) Motor areas that control skeletal muscles and (b) sensory areas that receive somatesthetic sensations.

■ Figure 8.8 An MRI image of the brain reveals the sensory cortex. The integration of MRI and EEG information shows the location on the sensory cortex that corresponds to each of the digits of the hand.

Fox: Human Physiology, Eighth Edition

8. The Central Nervous System

Text

© The McGraw-H Companies, 2003

Chapter Eight record of these electrical currents is called an electroencephalogram, or EEG. Deviations from normal EEG patterns can be used clinically to diagnose epilepsy and other abnormal states, and the absence of an EEG can be used to signify brain death.

There are normally four types of EEG patterns (fig. 8.10). Alpha waves are best recorded from the parietal and occipital regions while a person is awake and relaxed but with the eyes closed. These waves are rhythmic oscillations of 10 to 12 cycles/second. The alpha rhythm of a child under the age of 8 occurs at a slightly lower frequency of 4 to 7 cycles/second.

Beta waves are strongest from the frontal lobes, especially the area near the precentral gyrus. These waves are produced by visual stimuli and mental activity. Because they respond to stim

■ Figure 8.9 An MRI scan of a normal brain. In this coronal view of the brain, the lateral and third ventricles can be clearly seen. The arrow points to a part of the hippocampus.

From W. T. Carpenter and R. W. Buchanan, "Medical Progress: Schizophrenia" in New England Journal of Medicine, 330:685, 1994, fig IA. Copyright © 1994 Massachusetts Medical Society. All rights reserved.

■ Figure 8.9 An MRI scan of a normal brain. In this coronal view of the brain, the lateral and third ventricles can be clearly seen. The arrow points to a part of the hippocampus.

From W. T. Carpenter and R. W. Buchanan, "Medical Progress: Schizophrenia" in New England Journal of Medicine, 330:685, 1994, fig IA. Copyright © 1994 Massachusetts Medical Society. All rights reserved.

uli from receptors and are superimposed on the continuous activity patterns, they constitute evoked activity. Beta waves occur at a frequency of 13 to 25 cycles per second.

Theta waves are emitted from the temporal and occipital lobes. They have a frequency of 5 to 8 cycles/second and are common in newborn infants. The recording of theta waves in adults generally indicates severe emotional stress and can be a forewarning of a nervous breakdown.

Delta waves are seemingly emitted in a general pattern from the cerebral cortex. These waves have a frequency of 1 to 5 cycles/second and are common during sleep and in an awake

Alpha

Theta

Delta

1 sec

■ Figure 8.I0 Different types of waves in an electroencephalogram (EEG). Notice that the delta waves (bottom) have the highest amplitude and lowest frequency.

Table 8.2

Techniques for Visualizing Brain Function

Abbreviation

Technique Name

Principle Behind Technique

EEG

Electroencephalogram

Neuronal activity is measured as maps with scalp electrodes.

fMRI MEG

Functional magnetic resonance imaging Magnetoencephalogram

Increased neuronal activity increases cerebral blood flow and oxygen consumption in local areas. This is detected by effects of changes in blood oxyhemoglobin/deoxyhemoglobin ratios. Neuronal magnetic activity is measured using magnetic coils and mathematical plots.

PET

Positron emission tomography

Increased neuronal activity increases cerebral blood flow and metabolite consumption in local areas. This is measured using radioactively labeled deoxyglucose.

SPECT

Single photon emission computed tomography

Increased neuronal activity increases cerebral blood flow. This is measured using emitters of single photons, such as technetium.

Source: Burkhart Bromm "Brain images of pain." News in Physiological Sciences 16 (Feb. 2001): 244-249.

Source: Burkhart Bromm "Brain images of pain." News in Physiological Sciences 16 (Feb. 2001): 244-249.

The Central Nervous System 197

infant. The presence of delta waves in an awake adult indicates brain damage.

Two different types of EEG patterns are seen during sleep, corresponding to the two phases of sleep: rapid eye movement (REM) sleep, when dreams occur, and non-REM, or resting, sleep. During non-REM sleep the EEG displays large, slow delta waves (high amplitude, low-frequency waves). Superimposed on these are sleep spindles, which are waxing and waning bursts of 7 to 14 cycles per second that last for 1 to 3-second periods. During REM sleep, when the eyes move about rapidly, the EEG waves are similar to that of wakefulness. That is, they are lower in amplitude and display high-frequency oscillations.

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