Organization of the Nervous System II

As discussed later in this text, in early embryologic development, paired structures called somites are formed on the embryo. These somites differentiate into nonneural tissue (e.g., muscle, bone, and connective tissue). This somitic differentiation results in segmentally distributed zones called dermatomes. The dermatome region of each somite gives rise to a myotome, which is a muscle-forming body, and a skin plate, which is involved in formation of dermal tissue. The sensory component of each spinal nerve is distributed to a dermatome. Figure 3-3 shows the segmental distribution of cutaneous innervation, showing on the left side of the figure which area of skin is supplied by which spinal nerve. The peripheral nerve field illustration on the right refers to an area of skin that is supplied by a particular peripheral nerve in that spinal nerve’s distribution. Myotomes generally follow the same distribution as the dermatome, although there is more overlap of control. Box 3-1 lists the general myotome distribution for muscle innervation of upper and lower limbs by spinal nerves.

Unlike the spinal nerves, which attach to the cord at regular intervals, the cranial nerves are attached to the brain at irregular intervals. They do not all have dorsal (sensory) and ventral (motor) roots. Some have motor functions, some have sensory functions, and some have mixed functions. Their origin, distribution, brain and brainstem connections, functions, and evolution are complicated. (The cranial nerves are discussed in detail in Chapter 7.) They are traditionally designated by Roman numerals: cranial nerve I, olfactory; cranial nerve II, optic; cranial nerve III, oculomotor; cranial nerve IV, trochlear; cranial nerve V, trigeminal; cranial nerve VI, abducens; cranial nerve VII, facial; cranial nerve VIII, acoustic-vestibular; cranial nerve IX, glossopharyngeal; cranial nerve X, vagus; cranial nerve XI, spinal accessory; and cranial nerve XII, hypoglossal.

The cranial nerves from the brainstem are explicitly illustrated in Figure 3-2 and further outlined in Figure 3-4 regarding their location in the anterior (ventral) view and the anterolateral (ventrolateral) view of the brainstem.

The dura mater of the spinal cord is continuous with that of the brain through the opening in the skull called the foramen magnum. It consists of two layers that are closely united except where, in certain spots, they separate. In some parts of the dura mater of the brain they separate to form the venous sinuses (Fig. 3-5). In other parts of the brain, the inner layer also separates from the outer layer to reflect inward and form partitions, described as complex folds that divide the contents of the cranial cavity into different cerebral subdivisions. These infoldings or septa of the dura join with those formed in the opposite hemisphere to create three different double-layered partitions: the falx cerebri, the tentorium cerebelli, and the falx cerebelli (Fig. 3-6).

The falx cerebri develops first as two portions that become a single continuous structure later. It reflects downward between the hemispheres, attaching anteriorly and laterally to points on the skull and posteriorly to another partition, the tentorium cerebelli (also known simply as the tentorium). The falx cerebelli is located below the tentorium cerebelli on the middle of the occipital bone. This small dural infolding extends into the space between the cerebellar hemispheres, attaching to the occipital crest of the skull and the posterior portion of the tentorium. Part of it encloses the occipital venous sinuses. The tentorium cerebelli is the second largest of the dural folds. It attaches to bony walls of the skull in the temporal, occipital, and parietal regions. The tentorium cerebelli is the infolding found between the cerebral hemispheres above and the cerebellar hemispheres below. It is only found in mammals and birds, being absent in fish, amphibians, and reptiles.¹¹ There is another very small infolding of the dura, the diaphragma sella, which forms the roof of the sella turcica, the structure that encloses the pituitary gland. There is a tiny hole in the center of it, allowing the stalk (or infundibulum) of the pituitary to pass through. The anterior and posterior intercavernous sinuses are found in their respective edges of the diaphragma sella. These infoldings are illustrated in Figures 3-6 and 3-7 They are pictured as they appear on a magnetic resonance imaging scan.

When you consider the placement of these sturdy partitions being under and between the cerebral hemispheres as well as over and between the cerebellar hemispheres, it is fairly easy to understand how the dural infoldings brace the brain against rotary displacement.

There is no subdural space between the dura mater and the next layer of the meninges. When there is subdural space identified, it means that tissue damage has occurred to the deepest layer, creating a space.

Because there is no normal subdural space, the next meningeal layer can be found immediately below the dura. This second layer is the arachnoid mater. This membrane bridges over the sulci or folds of the brain. In some areas it projects into the venous sinuses to form arachnoid villi. The arachnoid villi aggregate to form the arachnoid granulations. The arachnoid granulations are from where the cerebrospinal fluid diffuses into the bloodstream.

Beneath the arachnoid mater layer is a space called the subarachnoid space, which is filled with cerebrospinal fluid. All cerebral arteries and veins, as well as the cranial nerves, pass through this space. This is why you often hear of subarachnoid hemorrhage or bleed because damage to the brain has resulted in artery or vein tears and blood is released into this subarachnoid space.

The third meningeal layer, the pia mater, closely adheres to the surface of the brain, covering the gyri (ridges) and going down into the sulci. The pia mater also fuses with the ependyma (a cellular membrane lining the ventricles) to form the choroid plexus of the ventricles. Figure 3-8 illustrates the meningeal layers.

Figure 3-9 is an example of a midsagittal view of the brain showing the third and fourth ventricles in relation to the cerebral aqueduct and nearby structures.

The lateral ventricles are paired, one in each hemisphere. Each is a C-shaped cavity and can be divided into a body, located in the parietal lobe, and anterior, posterior, and inferior or temporal horns, extending into the frontal, occipital, and temporal lobes, respectively. The lateral ventricle is connected to the third ventricle by an opening called the intraventricular foramen, or the foramen of Munro. The choroid plexus of the lateral ventricle projects into the cavity on its medial aspect (see Fig. 3-9).

The third ventricle is a small slit between the thalami. It also is connected to the fourth ventricle through the cerebral aqueduct or the aqueduct of Sylvius. The choroid plexus is situated above the roof of the ventricle.

The fourth ventricle sits anterior to the cerebellum and posterior to the pons and the superior half of the medulla. It is continuous superiorly with the cerebral aqueduct and the central canal below. The fourth ventricle has a tent-shaped roof, two lateral walls, and a floor (see Fig. 3-9). It contains three small openings: the two lateral foramina of Luschka and the median foramen of Magendie. Through these openings the cerebrospinal fluid enters the subarachnoid space. The ventricular system serves as a pathway for the circulation of the cerebrospinal fluid. The choroid plexus of the ventricles appears to secrete cerebrospinal fluid actively, although some of the fluid may originate as tissue fluid formed in the brain substance.

The path of circulation of the CSF is illustrated in Figure 3-10. It flows from the lateral ventricles into the third ventricle, to the fourth ventricle, and into the subarachnoid space. It then travels to reach the inferior surface of the cerebrum and moves superiorly over the lateral aspect of each hemisphere. Some of it moves into the subarachnoid space around the spinal cord.

The CSF produced by the choroid plexus passes through the ventricular system to exit the fourth ventricle through the foramina of Luschka and Magendie. At this point, the CSF enters the subarachnoid space, which is continuous around the brain and spinal cord. The CSF in this subarachnoid space provides the buoyancy necessary to prevent the weight of the brain from crushing nerve roots and blood vessels against the internal surface of the skull. The weight of the brain, approximately 1400 g in air, is reduced to 45 g when it is suspended in CSF. Consequently, the tethers formed by delicate connective tissue strands traversing the subarachnoid space, the arachnoid trabeculae (see Fig. 3-8), are adequate to maintain the brain in a stable position.

Blockage of the CSF movement or a failure of the absorption mechanism results in the accumulation of fluid in the ventricles or around the brain tissue (Fig. 3-11). This results in hydrocephalus and is characterized by an increase in CSF volume enlargement of one or more of the ventricles, and usually an increase in CSF pressure.

The CSF is important in medical diagnostic procedures. The pressure of the fluid can be measured; if it is abnormally high, intracranial tumor, hemorrhage, hydrocephalus, meningitis, or encephalitis may be suspected. Chemical and cell studies may be made on CSF that is drawn out of the nervous system through a procedure called a lumbar puncture or spinal tap. This route also may be used to inject drugs to combat infection or induce anesthesia. Circulation of the CSF is illustrated in Figure 3-10.

If a patient sent to the speech-language pathologist has a severe language disorder and some paralysis of the right arm and leg, these symptoms suggest that the brain lesion causing this motor deficit is probably in the left cerebral hemisphere (Fig. 3-15). The severe language disturbance accompanying the right limb paralysis serves as a confirming sign of left-sided brain lesion because in the majority of the population language dominance is located in the left hemisphere. Why the nervous system provides contralateral motor control of the limbs is not completely known, but the fact illustrates that knowledge of principles of neurologic organization can be used to posit the location of causative lesions seen in neurology and speech pathology.

An example of bilateral damage effects of motor speech lesions may be seen in spastic dysarthria. When damage to the upper motor neurons occurs, spasticity results. With spasticity, affected muscles (in this case, those dealing with articulation, respiration, and phonation) show an increased resistance to passive movements or movement manipulation. The more rapidly the individual tries to move the articulators (or even an upper or lower extremity, as in cerebral palsy) the greater is the resistance to the movement. In spastic dysarthria, bilateral damage to the upper motor neurons usually is caused by stroke, head trauma, or a degenerative disease (see Chapter 8). The bilateral damage is within both direct activation pathways (corticobulbar and corticospinal tracts) and indirect pathways (extrapyramidal tract).

The right and left hemispheres may be designated as nonverbal and verbal, and the anterior and posterior portions may be characterized as motor and sensory areas. The central sulcus divides the cerebral hemispheres into anterior and posterior regions. Figure 3-18 provides a lateral view of the cerebral hemisphere, showing lobes and the sensory and motor cortices. In human beings, approximately half of the volume of the cerebral cortex is taken up by the frontal cortex. The frontal lobe contains the primary motor cortex, the premotor cortex, and Broca’s area, the primary motor speech association area. In the anterior portion of the frontal lobes are the prefrontal areas, which are generally concerned with behavioral control of both cognitive and emotional functions.

Castro et al.⁵ described lesions in the prefrontal cortex that may produce an “internal agnosia or the inability to communicate with one’s limbic system as though suffering from an impairment of the ‘sixth sense’ that distinguishes appropriate from inappropriate behaviors and right from wrong.” Mental shifts become difficult, and perseveration and rigidity are observed, as are a lack of self-awareness and a tendency toward concreteness. In brief, the frontal lobe appears to excel in the control, integration, and regulation of emotional and cognitive behavior. Cortical areas of the left hemisphere that mediate the processing of language are shown in Figure 3-19. Lesions in the inferior frontal lobe may result in Broca’s, or expressive, aphasia, whereas damage to the angular gyrus, the supramarginal gyrus, and the superior temporal gyrus may result in Wernicke’s, or receptive, aphasia.

In contrast, the posterior cortex appears to dominate the control, integration, and regulation of sensory behavior. The defects arising from the posterior cortex are related to the specific sensory association areas implicated by a lesion.

The occipital lobe, as previously noted, contains the primary visual cortex and visual association areas. Deficits in the primary cortex result in blind spots in the visual field, and total destruction of the cortex produces complete blindness. Visual imperception and agnosias (see Chapter 5) are associated with the visual association areas.

The left parietal lobe is associated with constructional disturbances and visuospatial defects. Disorders of recognition, called agnosias, are common. The inferior parietal lobe is concerned with language association tasks, and lesions there cause defects in reading and writing. Contralateral neglect is associated with hemispheric lesions of the nondominant parietal association cortex. Such patients usually neglect stimuli on their left, as illustrated in Figure 3-20. In extreme cases the patient does not recognize the left side of his or her own body, a condition termed asomatognosia. An example is when a patient has a left neglect and ignores the left side when getting dressed or grooming.

The temporal lobe on the left is concerned with hearing and related functions. It contains the primary auditory and auditory association areas. Auditory memory storage and complex auditory perception are among the functions of the temporal lobe. An area known as the speech zone surrounds the sylvian fissure and appears to contain the major components of the language mechanism. Damage in the speech zone produces Wernicke’s aphasia (see Fig. 3-19).

With a clinical knowledge of primary sensory and related association areas and behavioral correlates to these areas, the speech-language pathologist is able to infer the approximate location of a lesion from the patient’s behavioral symptoms and to recognize the well-known speech-language syndromes associated with cortical dysfunction. The general clinical principle is that specific cortical deficits can be associated with specific behavioral syndromes. A major exception to this is difficulty with word-finding or naming which is typically a non-localizable symptom.

The CT scan yields a three-dimensional representation of the brain (Fig. 3-21), unlike the conventional radiograph, which provides a two-dimensional projection of a three-dimensional object. On a radiograph, the body appears on x-ray films as overlapping structures that are sometimes difficult to distinguish. The CT scanner uses an x-ray beam that is passed through the brain from one side of the head, and the radiation not absorbed by the intervening tissue is absorbed by a series of detectors revolving around the subject’s head. The data from the radiation detectors allow a calculation of the density of tissue in a particular slice of brain. A computer then reconstructs a two-dimensional cross-sectional picture of the brain observed by the camera. Several cross sections may be printed corresponding to different planes through the head. Contrast substances are sometimes injected in the patient to increase the density of damaged tissue. This enhancement technique allows clearer visualization and more accurate diagnosis. Spiral CT, in which the x-ray tube rotates continuously around the patients while the table moves, was developed in the early 1990s along with a computational method that would eliminate the motion artifact this introduced. Spiral CT allowed much more flexible and rapid study. Because CT studies are quicker and less expensive to perform than MRI, the technology is more widely available in general medical settings. CT is frequently used in the emergency department and other acute settings. It is more sensitive to skull fracture and can detect the presence of foreign bodies, such as glass and metal, that are radiopaque. It can reveal the presence of blood in the linings and parenchyma of the brain and, therefore, is the first study of choice in a vascular event of unknown etiology so that hemorrhage could be ruled out first. Because MRI uses a magnetic field, CT is the study that would be chosen for patients with pacemakers, defibrillators, and other such devices.

MRI, probably the most widely used diagnostic imaging technique in neurology, generates cross-sectional images by using radio waves, a strong magnetic field, and gradient coils to detect the distribution of water molecules in living tissue (Figs. 3-22 and 3-23). The technique allows accurate assessment of brain tissue densities, and an excellent pictorial image can be generated by the computer. The explanation of the technology is beyond the scope of this text but it is helpful to know what “tissue weighting,” which you will see as T1, T2, or proton density (PDW), does for an image. These parameters determine the contrast between tissues, allowing certain tissues to be more easily seen in an image. A T1-weighted image highlights tissues such as fat (like white matter) and proteins, whereas CSF will appear dark. A T2-weighted image makes CSF lighter and fat appears darker, which may make this weighting more useful for identifying pathologies. If the contrast between these T1 tissues and CSF is not what is of interest, a PDW will be used, which will highlight differences in the proton densities of the two tissues with the denser tissue emphasized in the image and gray matter appearing brighter than white matter.¹²

Generally, MRI is more sensitive to abnormalities than CT. However, it is significantly more expensive to generate the image. Damasio and Damasio⁷ pointed out that the analysis of CT and MRI images is sometimes difficult in that the number of brain slices provided for viewing may vary from institution to institution and from patient to patient. The number of slices may even vary in the same patient as scanning devices are improved over time.

Diffusion-weighted imaging (DWI) is another way of enhancing the usefulness of MRI in some cases. DWI allows the imaging of molecular motion or diffusion of water protons within tissue. DWI which highlights reduced diffusion has been found to be very useful to rapidly identify acute cerebral ischemia or loss of blood flow to an area. MRI study using DWI can be positive for reduced diffusion in these cases as early as 30 minutes after onset.¹² Figure 3-24 shows the usefulness of DWI in identifying axonal shearing which has been typically difficult to verify in traumatic injury.

Another advance in imaging to better see the white and gray matter in the brain is through voxel-based morphometry (VBM). This is primarily a research tool in which statistical methods originally applied to positron emission tomography (PET) scans are used on MRI scans. A voxel is the basic unit of computed tomography measurement, and the term comes from a combination of the words volume and pixel. It is essentially an imaged “slab” of tissue that has been divided into small volume elements called voxels with each voxel having an x, y, and z dimension on which statistical and other manipulations can be studied.¹ VBM has been used to study changes in gray matter volume in normal development, aging, and disease.¹⁶ It also has been employed in anatomic studies of speech and language disorders.¹⁵ Comparing the images of MRI, functional MRI (fMRI), and VBM has made it possible for advances in VBM for future research and clinical assessment.

Functional MRI (fMRI) uses an MRI scanner to measure regional blood flow, cerebral blood volume, and the change in blood oxygenation. fMRI study results in inferences about the location of brain activity. The most common fMRI study is a BOLD study, which stands for blood oxygenation level–dependent contrast. This study tracks the hemodynamic response to neural activity. This means that changes in blood flow and blood oxygenation are traced. fMRI has begun to be a popular technology for research by SLPs in collaboration with neurology and radiology. Brain activity in such areas as acquired language disorders, autism, and other communication disorders have been investigated with BOLD MRI.6,8,10

Positron emission tomograpy (PET) is a visual technique in which the subject is given a radioactively labeled form of glucose, which is metabolized by the brain. The radioactivity is later recorded by a special detector. Unlike CT and MRI scans, a PET scan measures metabolic activity in different brain areas. More active areas metabolize more glucose, focusing more radioactivity in these areas. Thus regional three-dimensional quantification of glucose and oxygen metabolism or blood flow in the human brain is achieved. This technique is advantageous in that glucose metabolism is a more direct measure of the function of neural tissue than is cerebral blood flow, particularly in patients whose regulatory vascular mechanisms are affected by cerebral injury or disease. PET scan studies have been used to research higher mental functions during different cognitive and language tasks and appear to offer an excellent tool for the study of language in the human brain. This technology is expensive because it requires a cyclotron or atomic accelerator. Because of the color limitations of this text, an example PET scan cannot be displayed, but many can be found through a search of web images. One interesting example is found from the Berkeley Labs site (www.lbl.gov).

Single-photon emission computed tomography (SPECT) uses the mechanism of CT scan reconstruction, but instead of detecting x-rays, the instrument detects single photons emitted from an external tracer. Radioactive compounds that emit gamma rays are injected into the subject. As these biochemicals reach the brain, emissions are picked up that are converted into patterns of metabolism or blood flow in three-dimensional cross sections of the brain. SPECT has somewhat better temporal resolution than PET but its spatial resolution is less than PET or MRI and it is more invasive. The equipment is less expensive because a cyclotron is not required and it therefore may be used at small medical centers.

A promising recent functional imaging technique that has been put to use both clinically and in research protocols is that of optical imaging, which uses the technology called near infrared spectroscopy (NIRS)¹⁴ to assess response to brain activation. It is similar to fMRI in its objective, but unlike fMRI, which measures changes in deoxygenated hemoglobin, NIRS can monitor changes in deoxygenated and oxygenated hemoglobin plus changes in localized blood volume. Optical imaging is based on measuring the light absorption of different tissue properties through use of various infrared sources and detectors. It is noninvasive and can be done during motor, sensory, and cognitive tasks. Zeller et al.¹⁷ used optical imaging in a visual-spatial task with patients with Alzheimer’s disease and found decreased activation in the parietal lobe compared with control subjects.