5.4 ANATOMY AND PHYSIOLOGY OF THE BRAIN Introduction

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1 5.4 ANATOMY AND PHYSIOLOGY OF THE BRAIN Introduction Action pulses generated at the distal end of sensory neurons propagate first to the cell body and then onward, conveyed by long axonal pathways. These ascend the spinal cord (dorsal root) until they reach the lower part of the central nervous system. Here the signals are relayed to other neurons, which in turn relay them onward. Three or four such relays take place before the signals reach particular loci in the cerebral cortex. Signal processing takes place at all levels, resulting in the state of awareness and conscious recognition of the various signals that characterize human physiology. The important integrative activity of the brain has been the subject of intense study, but its complexity has slowed the rate of progress. In this section a brief description is given of both the anatomy and the physiology of the brain Brain Anatomy The brain consists of neurons that are very closely interconnected via axons and dendrites. The neurons themselves are vastly outnumbered by glial cells. One neuron may receive stimuli through synapses from as many as 10 3 to 10 5 other neurons (Nunez, 1981). Embryologically the brain is formed when the front end of the central neural system has folded. The brain consists of five main parts, as described in Figure 5.5: 1. The cerebrum, including the two cerebral hemispheres 2. The interbrain (diencephalon) 3. The midbrain 4. The pons Varolii and cerebellum 5. The medulla oblongata data:text/html;charset=utf 8,%3Cdiv%20class%3D%22Text%22%20style%3D%22margin%3A%200px%200px%204pt%3B%20text align%3a%20justify% 1/5

2 Fig The anatomy of the brain. The entire human brain weighs about 1500 g (Williams and Warwick, 1989). In the brain the cerebrum is the largest part. The surface of the cerebrum is strongly folded. These folds are divided into two hemispheres which are separated by a deep fissure and connected by the corpus callosum. Existing within the brain are three ventricles containing cerebrospinal fluid. The hemispheres are divided into the following lobes:lobus frontalis, lobus parietalis, lobus occipitalis, and lobus temporalis. The surface area of the cerebrum is about 1600 cm², and its thickness is 3 mm. Six layers, or laminae, each consisting of different neuronal types and populations, can be observed in this surface layer. The higher cerebral functions, accurate sensations, and the voluntary motor control of muscles are located in this region. The interbrain or diencephalon is surrounded by the cerebrum and is located around the third ventricle. It includes the thalamus, which is a bridge connecting the sensory paths. The hypothalamus, which is located in the lower part of the interbrain, is important for the regulation of autonomic (involuntary) functions. Together with the hypophysis, it regulates hormonal secretions. The midbrain is a small part of the brain. The pons Varolii is an interconnection of neural tracts; the cerebellum controls fine movement. The medulla oblongata resembles the spinal cord to which it is immediately connected. Many reflex centers, such as the vasomotor center and the breathing center, are located in the medulla oblongata. In the cerebral cortex one may locate many different areas of specialized brain function (Penfield and Rasmussen, 1950; Kiloh, McComas, and Osselton, 1981). The higher brain functions occur in the frontal lobe, the visual center is located in the occipital lobe, and the sensory area and motor area are located on both sides of the central fissure. There are specific areas in the sensory and data:text/html;charset=utf 8,%3Cdiv%20class%3D%22Text%22%20style%3D%22margin%3A%200px%200px%204pt%3B%20text align%3a%20justify% 2/5

3 motor cortex whose elements correspond to certain parts of the body. The size of each such area is proportional to the required accuracy of sensory or motor control. These regions are described in Figure 5.6. Typically, the sensory areas represented by the lips and the hands are large, and the areas represented by the midbody and eyes are small. The visual center is located in a different part of the brain. The motor area, the area represented by the hands and the speaking organs, is large. Fig The division of sensory (left) and motor (right) functions in the cerebral cortex. (From Penfield and Rasmussen, 1950.) Brain Function Most of the information from the sensory organs is communicated through the spinal cord to the brain. There are special tracts in both spinal cord and brain for various modalities. For example, touch receptors in the trunk synapse with interneurons in the dorsal horn of the spinal cord. These interneurons (sometimes referred to as second sensory neurons) then usually cross to the other side of the spinal cord and ascend the white matter of the cord to the brain in the lateral spinothalamic tract. In the brain they synapse again with a second group of interneurons (or third sensory neuron) in the thalamus. The third sensory neurons connect to higher centers in the cerebral cortex. In the area of vision, afferent fibers from the photoreceptors carry signals to the brain stem through the optic nerve and optic tract to synapse in the lateral geniculate body (a part of the thalamus). From here axons pass to the occipital lobe of the cerebral cortex. In addition, branches of the axons of the optic tract synapse with neurons in the zone between thalamus and midbrain which is the pretectal nucleus and superior colliculus. These, in turn, synapse with preganglionic parasympathetic neurons whose axons follow the oculomotor nerve to the ciliary ganglion (located just behind the eyeball). The reflex loop is closed by postganglionic fibers which pass along ciliary nerves to the iris muscles (controlling pupil aperture) and to muscles controlling the lens curvature (adjusting its refractive or focusing qualities). Other reflexes concerned with head and/or eye movements may also be initiated. Motor signals to muscles of the trunk and periphery from higher motor centers of the cerebral cortex first travel along upper motor neurons to the medulla oblongata. From here most of the axons of the upper motor neurons cross to the other side of the central nervous system and descend the spinal cord in the lateral corticospinal tract; the remainder travel down the cord in the anterior corticospinal tract. The upper motor neurons eventually synapse with lower motor neurons in the data:text/html;charset=utf 8,%3Cdiv%20class%3D%22Text%22%20style%3D%22margin%3A%200px%200px%204pt%3B%20text align%3a%20justify% 3/5

4 ventral horn of the spinal cord; the lower motor neurons complete the path to the target muscles. Most reflex motor movements involve complex neural integration and coordinate signals to the muscles involved in order to achieve a smooth performance. Effective integration of sensory information requires that this information be collected at a single center. In the cerebral cortex, one can indeed locate specific areas identified with specific sensory inputs (Penfield and Rasmussen, 1950; Kiloh, McComas, and Osselton, 1981). While the afferent signals convey information regarding stimulus strength, recognition of the modality depends on pinpointing the anatomical classification of the afferent pathways. (This can be demonstrated by interchanging the afferent fibers from, say, auditory and tactile receptors, in which case sound inputs are perceived as of tactile origin and vice versa.) The higher brain functions take place in the frontal lobe, the visual center is in the occipital lobe, the sensory area and motor area are located on both sides of the central fissure. As described above, there is an area in the sensory cortex whose elements correspond to each part of the body. In a similar way, a part of the brain contains centers for generating command (efferent) signals for control of the body's musculature. Here, too, one finds projections from specific cortical areas to specific parts of the body. 5.5 CRANIAL NERVES In the central nervous system there are 12 cranial nerves. They leave directly from the cranium rather than the spinal cord. They are listed in Table 5.2 along with their functions. The following cranial nerves have special importance: theolfactory (I) and optic (II) nerves, which carry sensory information from the nose and eye; and the auditory vestibular (VIII) nerve, which carries information from the ear and the balance organ. Sensory information from the skin of the face and head is carried by the trigeminal (V) nerve. Eye movements are controlled by three cranial nerves (III, IV, and VI). The vagus nerve (X) controls heart function and internal organs as well as blood vessels. Table 5.2. The cranial nerves Number Name Sensory/ Motor Functions Origin or terminus in the brain I olfactory s smell cerebral hemispheres (ventral part) II optic s vision thalamus III oculomotor m eye movement midbrain IV trochlear m eye movement midbrain V trigeminal m masticatory movements midbrain and pons s sensitivity of face and tongue medulla VI abducens m eye movements medulla VII facial m facial movement medulla VIII auditory s hearing medulla vestibular s balance IX glossopharyngeal s,m tongue and pharynx medulla X vagus s,m heart, blood vessels, viscera medulla XI spinal accessory m neck muscles and viscera medulla XII hypoglossal m medulla data:text/html;charset=utf 8,%3Cdiv%20class%3D%22Text%22%20style%3D%22margin%3A%200px%200px%204pt%3B%20text align%3a%20justify% 4/5

5 13 Electroencephalography 13.1 INTRODUCTION The first recording of the electric field of the human brain was made by the German psychiatrist Hans Berger in 1924 in Jena. He gave this recording the name electroencephalogram (EEG). (Berger, 1929).(From 1929 to 1938 he published 20 scientific papers on the EEG under the same title "Über das Elektroenkephalogram des Menschen".) 1. spontaneous activity, 2. evoked potentials, and 3. bioelectric events produced by single neurons. Spontaneous activity is measured on the scalp or on the brain and is called the electroencephalogram. The amplitude of the EEG is about 100 µv when measured on the scalp, and about 1 2 mv when measured on the surface of the brain. The bandwidth of this signal is from under 1 Hz to about 50 Hz, as demonstrated in Figure As the phrase "spontaneous activity" implies, this activity goes on continuously in the living individual. Evoked potentials are those components of the EEG that arise in response to a stimulus (which may be electric, auditory, visual, etc.) Such signals are usually below the noise level and thus not readily distinguished, and one must use a train of stimuli and signal averaging to improve the signalto noise ratio. Single neuron behavior can be examined through the use of microelectrodes which impale the cells of interest. Through studies of the single cell, one hopes to build models of cell networks that will reflect actual tissue properties THE BRAIN AS A BIOELECTRIC GENERATOR PRECONDITIONS: SOURCE: Distribution of impressed current source elements i (volume source) CONDUCTOR: Finite, inhomogeneous The number of nerve cells in the brain has been estimated to be on the order of Cortical neurons are strongly interconnected. Here the surface of a single neuron may be covered with 1, ,000 synapses (Nunez, 1981). The electric behavior of the neuron corresponds to the description of excitable cells introduced in the earlier chapters. The resting voltage is around 70 mv, and the peak of the action potential is positive. The amplitude of the nerve impulse is about 100 mv; it lasts about 1 ms. The bioelectric impressed current density i associated with neuronal activation produces an electric field, which can be measured on the surface of the head or directly on the brain tissue. The electric field was described by Equation 7.10 for a finite inhomogeneous model. This equation is repeated here: data:text/html;charset=utf 8,%3Cp%20class%3D%22ChapterNumber%22%20style%3D%22margin%3A%201pt%200px%3B%20text align%3a%20cente 1/4

6 (13.01) While for most excitable tissue the basis for the impressed current density i is the propagating action potential, for the EEG it appears to arise from the action of a chemical transmitter on postsynaptic cortical neurons. The action causes localized depolarization that is, an excitatory postsynaptic potential (EPSP) or hyperpolarization that is, an inhibitory postsynaptic potential (IPSP). The result in either case is a spatially distributed discontinuity in the function σφ (i.e., σ o Φ o σ i Φ i ) which, as pointed out in Equation 8.28, evaluates a double layer source in the membranes of all cells. This will be zero for resting cells; however, when a cell is active by any of the aforementioned processes (in which case Φ o Φ i = V m varies over a cell surface), a nonzero primary source will result. For distant field points the double layer can be summed up vectorially, yielding a net dipole for each active cell. Since neural tissue is generally composed of a very large number of small, densely packed cells, the discussion in Section 8.5 applies, leading to the identification of a continuous volume source distribution i which appears in Equations 7.6 and Although in principle the EEG can be found from the evaluation of Equation 7.10, the complexity of brain structure and its electrophysiological behavior have thus far precluded the evaluation of the source function i. Consequently, the quantitative study of the EEG differs from that of the ECG or EMG, in which it is possible to evaluate the source function. Under these conditions the quantitative EEG is based on a statistical treatment, whereas the clinical EEG is largely empirical.. Fig Frequency spectrum of normal EEG EEG LEAD SYSTEMS data:text/html;charset=utf 8,%3Cp%20class%3D%22ChapterNumber%22%20style%3D%22margin%3A%201pt%200px%3B%20text align%3a%20cente 2/4

7 The internationally standardized system is usually employed to record the spontaneous EEG. In this system 21 electrodes are located on the surface of the scalp, as shown in Figure 13.2A and B. The positions are determined as follows: Reference points are nasion, which is the delve at the top of the nose, level with the eyes; and inion, which is the bony lump at the base of the skull on the midline at the back of the head. From these points, the skull perimeters are measured in the transverse and median planes. Electrode locations are determined by dividing these perimeters into 10% and 20% intervals. Three other electrodes are placed on each side equidistant from the neighboring points, as shown in Figure 13.2B (Jasper, 1958; Cooper, Osselton, and Shaw, 1969). In addition to the 21 electrodes of the international system, intermediate 10% electrode positions are also used. The locations and nomenclature of these electrodes are standardized by the American Electroencephalographic Society (Sharbrough et al., 1991; see Figure 13.2C). In this recommendation, four electrodes have different names compared to the system; these are T 7, T 8, P 7, and P 8. These electrodes are drawn black with white text in the figure. Besides the international system, many other electrode systems exist for recording electric potentials on the scalp. The Queen Square system of electrode placement has been proposed as a standard in recording the pattern of evoked potentials in clinical testings (Blumhardt et al., 1977). Bipolar or unipolar electrodes can be used in the EEG measurement. In the first method the potential difference between a pair of electrodes is measured. In the latter method the potential of each electrode is compared either to a neutral electrode or to the average of all electrodes (see Figure 13.3). The most recent guidelines for EEG recording are published in (Gilmore, 1994). data:text/html;charset=utf 8,%3Cp%20class%3D%22ChapterNumber%22%20style%3D%22margin%3A%201pt%200px%3B%20text align%3a%20cente 3/4

8 Fig The international system seen from (A) left and (B) above the head. A = Ear lobe, C = central, Pg = nasopharyngeal, P = parietal, F = frontal, Fp = frontal polar, O = occipital. (C) Location and nomenclature of the intermediate 10% electrodes, as standardized by the American Electroencephalographic Society. (Redrawn from Sharbrough, 1991.). Fig (A) Bipolar and (B) unipolar measurements. Note that the waveform of the EEG depends on the measurement location. data:text/html;charset=utf 8,%3Cp%20class%3D%22ChapterNumber%22%20style%3D%22margin%3A%201pt%200px%3B%20text align%3a%20cente 4/4

9 13.5 THE BEHAVIOR OF THE EEG SIGNAL From the EEG signal it is possible to differentiate alpha (α), beta (β), delta (δ), and theta (Θ) waves as well as spikes associated with epilepsy. An example of each waveform is given in Figure The alpha waves have the frequency spectrum of 8 13 Hz and can be measured from the occipital region in an awake person when the eyes are closed. The frequency band of the beta waves is Hz; these are detectable over the parietal and frontal lobes. The delta waves have the frequency range of Hz and are detectable in infants and sleeping adults. The theta waves have the frequency range of 4 8 Hz and are obtained from children and sleeping adults.. Fig Some examples of EEG waves THE BASIC PRINCIPLES OF EEG DIAGNOSIS The EEG signal is closely related to the level of consciousness of the person. As the activity increases, the EEG shifts to higher dominating frequency and lower amplitude. When the eyes are closed, the alpha waves begin to dominate the EEG. When the person falls asleep, the dominant EEG frequency decreases. In a certain phase of sleep, rapid eye movement called (REM) sleep, the person dreams and has active movements of the eyes, which can be seen as a characteristic EEG signal. In deep sleep, the EEG has large and slow deflections called delta waves. No cerebral activity can be detected from a patient with complete cerebral death. Examples of the above mentioned waveforms are given in Figure data:text/html;charset=utf 8,%3Ca%20name%3D%2205%22%3E%3Ch1%20style%3D%22margin%3A%2020pt%200px%2010pt%3B%20text align%3a% 1/2

10 Fig EEG activity is dependent on the level of consciousness. data:text/html;charset=utf 8,%3Ca%20name%3D%2205%22%3E%3Ch1%20style%3D%22margin%3A%2020pt%200px%2010pt%3B%20text align%3a% 2/2