Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Cutaneous, Deep, and Visceral Sensation
Neural Pathways

Based on their histological characteristics, the dorsal horns of the Spinal Cord are divided into laminae I–VI, with lamina I being the most superficial and lamina VI the deepest. Lamina II and part of lamina IV form the substantia gelatinosa—a lightly stained region near the apex of each dorsal horn. Three types of primary sensory fibers mediate cutaneous sensation: large myelinated Aβ fibers, which transmit impulses generated in response to mechanical stimuli; small myelinated Aβ fibers, a portion of which transmit impulses from cold receptors and nociceptors mediating fast pain (see below), while another portion conveys signals from mechanoreceptors; and small unmyelinated C fibers, which are primarily associated with pain and Temperature sensations. There are also a small number of C fibers that transmit impulses from mechanoreceptors. The distribution of these fibers in the dorsal columns and the various layers of the dorsal horn is illustrated in Fig. 7-1.

The main direct pathways for cutaneous sensation leading to the Cerebral Cortex are depicted in Fig. 7-2. Fibers that mediate Touch and Proprioception ascend via the dorsal columns to the Medulla Oblongata, where they synapse in The Nucleus gracilis and nucleus cuneatus. Second-order Neurons from these nuclei cross the midline and ascend within the medial lemniscus to the ventral posterior nucleus and its associated specific sensory Relay nuclei of the thalamus (see Chapter 11). This ascending system is frequently referred to as the dorsal Column-medial lemniscal system.

Other tactile fibers, together with those transmitting temperature and pain signals, terminate by synapsing on neurons of the dorsal column. The axons of these neurons cross the midline and enter the anterolateral quadrant of the spinal cord, forming the anterolateral ascending system. Other fibers course more dorsally. In general, the sensation of touch is mediated by the ventral spinothalamic tract, whereas pain and temperature are processed by the lateral tract, although there is no strict localization of function. Some fibers of the anterolateral system terminate in specific relay nuclei of the thalamus, others project to midline nuclei, and still others to the intralaminar nonspecific projection nuclei. There is a substantial influx of impulses from the anterolateral system to the reticular Formation of the Midbrain. Consequently, sensory impulses activate the reticular activating system, which in turn maintains cortical tone (see Chapter 11). Collaterals from the fibers comprising the dorsal columns pass through the dorsal horns. They can modulate incoming information to other cutaneous Sensory systems, including the pain system. The dorsal horn acts as a gating mechanism where impulses from sensory nerve fibers are converted into impulses of the ascending tracts; it is likely that the permeability of this gate depends on The Nature and pattern of the impulses reaching the substantia gelatinosa and adjacent areas. These gates are also influenced by descending impulses originating in the Brain. The relation of the gating mechanism to pain impulses is discussed below.

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Fig. 7-1. Schematic representation of the terminations of three types of primary afferent neurons in various layers of the dorsal horn of the spinal cord.

Fig. 7-2. Tactile, pain, and temperature pathways from the trunk and limbs. The anterolateral system (ventral and lateral spinothalamic tracts and related ascending pathways) also projects to the midbrain Reticular Formation and nonspecific thalamic nuclei.

The axons of the spinothalamic tracts from sacral and lumbar segments are displaced laterally by axons crossing the midline at progressively higher levels. Conversely, fibers of the dorsal column are displaced medially by fibers from higher segments (Fig. 7-3). Consequently, both ascending systems are laminated, with corresponding cervical, thoracic, lumbar, and sacral segments oriented mediolaterally within the anterolateral pathways, and sacral-to-cervical segments oriented mediolaterally within the dorsal columns. Due to this laminar Organization, extramedullary tumors originating outside the spinal cord initially compress the sacral and lumbar spinothalamic fibers, producing an early symptom of lost pain and Temperature Sensation in the sacral region. Tumors arising within the spinal cord initially cause sensory loss in higher segments.

Fig. 7-3. Cytology/practical/72.html">Cross section of the spinal cord showing the localization of ascending sensory pathways. Note the lamination within each pathway: S, sacral; L, lumbar; T, thoracic; C, cervical.

In the Brainstem, fibers conveying sensation from the HEAD join the lemniscal and anterolateral systems. Pain and temperature impulses are integrated in the spinal trigeminal nucleus, whereas tactile and proprioceptive impulses are primarily relayed through the main sensory and mesencephalic nuclei of the Trigeminal nerve.

Cortical Representation

Maps of the sensory areas of the cortex are constructed based on animal experiments and neurosurgical Procedures in humans. Modern non-Invasive Methods for mapping the human cerebral cortex rely on positron emission tomography (PET) and functional Magnetic Resonance imaging (MRI). These techniques, described in Chapter 32, have formed the basis for major breakthroughs not only in sensory physiology but across all aspects of research into normal human cortical Functions.

From specialized sensory nuclei of the thalamus, neurons carrying sensory information project in a highly specific manner to two somatosensory areas of the cortex: primary somatosensory cortex (SI) in the postcentral gyrus, and secondary somatosensory cortex (SII) in the lateral sulcus. Additionally, SI projects to SII. SI corresponds to Brodmann's areas 1, 2, and 3. Brodmann was a histologist who meticulously divided the cerebral cortex into numerous areas based on their histological architecture.

Thalamic fibers within SI are organized such that body parts are represented in an orderly sequence along the postcentral gyrus—the feet at the apex and the head at the Base of the gyrus

(Fig. 7-4). Within the postcentral gyrus, not only is there a precisely focused topography of fibers from various body parts, but the area of the cortex devoted to receiving impulses from a specific body region is proportional to the density of receptors in that part. The relative sizes of these cortical areas are illustrated in Fig. 7-5, where the Proportions of the homunculus are distorted to reflect the allocation of cortical real estate. The sensory cortical areas receiving input from the trunk and back are relatively small, whereas those Processing impulses from the hand and PARTS OF THE Mouth associated with speech are exceptionally large.

Studies of the sensory areas demonstrate the discrete Nature of the extremely precise localization of peripheral body parts in the cerebral cortex, providing further Evidence for the validity of The Doctrine of specific nerve energies (see Chapter 5). Stimulation of various Regions of the postcentral gyrus elicits sensations referred to corresponding parts of the body. Typically, this evokes sensations of numbness, tingling, or movement; however, with sufficiently fine microelectrodes, one can elicit relatively distinct sensations of touch, warmth, cold, or pain. Cells in the postcentral gyrus are arranged in vertical columns, much like those in the visual cortex (see Chapter 8). All cells within a given column are activated by ascending impulses from a specific part of the body and respond to a single modality of sensation.

SII is located in the upper bank of the lateral sulcus (Sylvian fissure), which separates the temporal lobe from the frontal and parietal lobes; the head representation lies at the lower end of the postcentral gyrus, and the feet extend toward the depth of the lateral sulcus. Body parts are represented here less comprehensively and in less detail than in the postcentral gyrus.

Fig. 7-4. Correspondence of cerebral cortical fields to somatic sensation, along with cortical projection areas for other sensory modalities in the human brain. Numbers refer to Brodmann's cortical areas. The primary auditory area is located deep within the lateral sulcus on the crown of the superior temporal gyrus and is not normally visible.

Fig. 7-5. Sensory homunculus laid across a coronal section through the postcentral gyrus (reproduced with permission from Penfield W, Rasmussen G: The Cerebral Cortex of Man. Macmillan, 1950).

Cortical Plasticity

It is now well established that the numerous neural connections described above are neither innate nor immutable and can be relatively rapidly altered, at least in sensory areas, through experience involving the active use of a given region. For example, if a finger is amputated in a monkey, the cortical representation of the adjacent fingers expands into the zone previously dedicated to the amputated digit. Conversely, if the cortical area corresponding to a specific finger is excised, the somatosensory map of that finger shifts to adjacent cortex. Extensive, long-term deafferentation of limbs leads to even more dramatic shifts in cortical somatosensory representation—for instance, when a limb cortical area comes to respond to touch on the face. Furthermore, if a monkey is trained to discriminate fine tactile differences between vibrational frequencies on a single digit of one hand, the cortical territory representing that finger expands. Such plastic processes occur during both development and adulthood. For instance, the receptive fields of individual neurons in area 3 of the sensory cortex normally reside on a single digit (Fig. 7-6, left). However, if an adult monkey is continually exposed to stimuli that activate only the distal pads of digits 1, 2, and 3, the receptive fields of individual neurons expand to encompass the distal pads of all three fingers (see Fig. 7-6). The explanation for these shifts apparently lies in the fact that the cortical connections of sensory units possess a high degree of convergence and divergence, with connections weakening through disuse and strengthening through use. In rats, this process involves the basal Forebrain: cortical responses from this region are enhanced when auditory stimuli are paired with basal forebrain stimulation. The size of the auditory sensory cortex becomes larger than when stimuli are presented in the absence of basal forebrain activation.

Plasticity of this type is observed not only with input from cutaneous receptors but also from other sensory modalities. For example, in cats with small retinal lesions, the cortical region corresponding to the scotoma begins to respond to light stimuli presented to other retinal areas. The refinement of adult retinal projections to the visual cortex is another manifestation of this plasticity. In extreme cases, experimentally rerouting visual information into the auditory cortex during development results in The formation of functional visual receptive fields within the auditory system.

Similar plastic changes also occur in humans, mirroring those described above in experimental animals. For instance, in some upper-limb amputees, touching the face elicits sensations projected onto the missing hand. PET scans have further revealed cross-modal plasticity between different sensory modalities. For example, tactile and auditory stimuli increase metabolic activity in the visual cortex of blind individuals. Conversely, deaf individuals react faster and more accurately to visual motion stimuli in the peripheral visual field compared to normal-Hearing controls. Plasticity is also a hallmark of the motor cortex (see Chapter 12). These Examples underscore the remarkable flexibility and adaptability of the human brain.

Effects of Cerebral Cortex Lesions

Ablation of somatosensory area I in animals results in deficits in position sense and The ability to discriminate size and shape, whereas lesions in somatosensory area II impair tactile learning and discrimination. Removal of SI leads to a deficit in Sensory Information Processing within SII, whereas the removal of SII has little effect on processing within SI. Evidently, areas SI and SII process sensory information sequentially rather than in parallel. Area SI also projects to the posterior parietal cortex (see Fig. 7-4), and damage to this association area causes a complex set of deficits in the spatial orientation of the contralateral side of the body (see Chapter 16).

Fig. 7-6. The Role of input signals in shaping receptive fields of cortical neurons. Left: Normal receptive field of a single cortical neuron on the lateral surface of a monkey's middle finger. Middle: Following monkey training on tasks requiring stimulation of the distal segments of the first, second, and third fingers. Right: Modified receptive field of a cortical neuron resulting from training (after Wang et al., modified from Sur M: Maps of time and space. Nature 1995;378:13).

It is worth noting that in both experimental animals and humans, cortical damage does not abolish bodily sensations altogether. Proprioception and tactile sensations are the most vulnerable to cortical lesions. Temperature sensations are less vulnerable, and Pain Sensation is only mildly affected. Thus, perception remains possible even in the absence of the cortex. During recovery, pain sensation returns first, followed by temperature sense, and finally proprioception and light touch.

Principles of Sensory Physiology

The fundamental General Principles governing the physiology of sensory systems are detailed in Chapter 5. Each sensory organ is specialized to transduce a specific form of energy into action potentials in sensory nerves. Every sensory modality has a dedicated pathway to the brain, and the resulting sensation—as well as the part of the body where it is localized—is determined by the specific region of the brain that is activated.

Differences in the intensity of a given sensation are encoded in two ways: by Changes in the firing frequency of action potentials in sensory nerves and by changes in the number of activated receptors. Increasing the intensity of stimulation of a sensory organ has little to no effect on the quality of the sensation.

Another principle governing cutaneous Sensory Organs is the point-to-point representation principle. Detailed, millimeter-by-millimeter mapping of the Skin using a fine Hair reveals that touch sensations originate exclusively from specific points overlying tactile receptors; they cannot be elicited from the areas lying between these points. Similarly, sensations of pain and temperature are elicited by skin stimulation only at those specific points where the corresponding receptor structures for these stimuli are located.