The rest of this chapter takes a regional approach to the spinal mechanosensory system. Progressing in sequence from the periphery to the cerebral cortex, the chapter examines the key components of the dorsal column–medial lemniscal system. Knowledge of the regional anatomy is important for understanding how injury to a discrete portion of the central nervous system affects different functional systems.
The Peripheral Axon Terminals of Dorsal Root Ganglion Neurons Contain the Somatic Sensory Receptors
The dorsal root ganglion neurons, named for the dorsal root ganglia in which their cell bodies are located, are pseudounipolar neurons (Figure 4–2A2). A single axon emerges from the cell body and bifurcates; one axonal branch is directed toward the periphery, where it innervates the skin or other tissues, and the other, directed centrally, synapses on central nervous system neurons. The peripheral and central axon branches of dorsal root ganglion neurons are often called primary sensory (or afferent) fibers.
The peripheral axon terminal is the receptive portion of the neuron. Here, stimulus energy is transduced into neural signals by membrane receptor-channel complexes that respond to a particular stimulus energy (eg, mecanical or thermal). Mechanoreceptors are activated when mechanical energy is conducted from the body surface, where stimulation occurs, to the membrane of the receptors, where stretch-activated channels are located. Mechanoreceptors for limb position sense are sensitive to muscle or tendon stretch as well as mechanical changes in the tissues around muscles and joints.
Mechanoreceptors have encapsulated axon terminals. Five major types of encapsulated sensory receptor neurons are located in the skin and underlying deep tissue that mediate mechanosensations: Ruffini's corpuscles, Merkel's receptors, Meissner's corpuscles, Pacinian corpuscles, and hair receptors (Figure 4–3A). Merkel's receptors and Meissner's corpuscles are located at the epidermis-dermis border. These receptors are sensitive to stimulation within a very small region of overlying skin; hence they have very small receptive fields. These receptors are important for fine tactile discrimination, such as reading Braille. Ruffini's and Pacinian corpuscles are located in the dermis. Ruffini's corpuscles are sensitive to skin stretch and are important in discriminating the shape of grasped objects. Pacinian corpuscles are the most sensitive mechanoreceptor, responding to skin displacement of as little as 500 nM. Merkel's receptors and Pacinian corpuscles are rapidly adapting, responding to changes in the stimulus, such as when it comes on or shuts off. Meissner's corpuscles and Ruffini's receptors are slowly adapting, firing action potentials for the duration of the stimulus. Hair receptors may be either slowly or rapidly adapting. Each primary sensory fiber has multiple terminal branches and, therefore, multiple receptive endings.
A. The morphology of peripheral somatic sensory receptors on hairy skin (left) and hairless, or glabrous, skin (right). B. The muscle spindle organ (top inset) is a stretch receptor located within the muscle. It receives an efferent innervation from the spinal cord that maintains receptor sensitivity during muscle contraction. Specialized motor neurons, termed gamma motor neurons, innervate muscle fibers (intrafusal fibers) within the receptor's capsule. The synapse between the gamma motor neuron and the intrafusal fiber is termed the gamma motor ending. C. The Golgi tendon organ, located within tendons, is most sensitive to active force generated by contracting muscle. (A, Adapted from Light AR, Perl ER. Peripheral sensory systems. In: Dyck P, Thomas PK, Lambert EH, Bruge R, eds. Peripheral Neuropathy, 3rd ed. Vol 1. Philadelphia, PA: W. B. Saunders; 1993. B, Adapted from Schmidt RF. Fundamentals of Neurophysiology, 3rd ed. Berlin, Heidelberg, New York: Springer; 1985.)
The principal receptor for proprioception is the muscle spindle receptor, which is located within the muscle belly. It measures muscle stretch (Figure 4–3B). This structure is innervated by multiple sensory fibers with different properties. The muscle spindle is more complicated than the other encapsulated mechanoreceptors because it also contains tiny muscle fibers, controlled by the central nervous system, that regulate the receptor neuron's sensitivity. There is another deep mechanoreceptor, the Golgi tendon receptor, which is entwined within the collagen fibers of tendon and is sensitive to the force generated by contracting muscle. It may have a role in an individual's sense of how much effort it takes to produce a particular motor act. The muscle spindle and Golgi tendon receptors also play key roles in the reflex control of muscle. The joints are innervated by mechanoreceptors, but they play more of a role in sensing joint pressure and the extremes of joint motion than proprioception.
The capsule covering Pacinian, Ruffini's, and Meissner's corpuscles and non-neural structures associated with muscle spindle and Golgi tendon receptors do not participate directly in stimulus transduction. Rather, they modify the mechanoreceptor's response to a stimulus. For example, Pacinian corpuscles are normally rapidly adapting but become slowly adapting when the capsule is dissected away. Merkel's receptors are different in their organization. The peripheral terminals of the sensory fiber contacts Merkel's cells, located in the skin. Merkel's cells appear to form a synapse-like apposition with the fiber terminal, suggesting that mechanosensory transduction is accomplished by the Merkel's cell, which synaptically activates the sensory fiber.
The protective senses have their own specialized receptors. Nociceptors respond to noxious stimuli and mediate pain, whereas itch-sensitive neurons, or pruritic receptor, respond to histamine. Receptor neurons sensitive to cold or warmth are termed thermoreceptors. The morphology of these three classes of receptor neurons is simple; they are bare nerve endings (Figure 4–3A). The viscera are also innervated. These receptors will be discussed in Chapter 6.
The modality sensitivity of a receptor neuron also determines the diameter of its axon and the patterns of connections it makes in the central nervous system. Most mechanoreceptors have a large-diameter axon covered by a thick myelin sheath. The larger the diameter of the axon, the faster it conducts action potentials. The mechanoreceptors are the fastest conducting sensory receptor neurons in the somatic sensory system. The dorsal column–medial lemniscal system receives sensory input principally from these fast conducting mechanoreceptors with large-diameter axons. By contrast, dorsal root ganglion neurons that are sensitive to noxious stimuli, temperature, and itch have small-diameter axons that are either thinly myelinated or unmyelinated. Table 4–1 lists the functional categories of primary sensory fibers, including the two fiber nomenclatures based on axonal diameter: A-α (group 1), A-β (group 2), A-δ (group 3), and C (group 4).
Dermatomes Have a Segmental Organization
The central branches of dorsal root ganglion neurons collect into the dorsal roots (Figure 4–2A2). The spinal cord has a rostrocaudal segmental organization, which forms early during development. Mesodermal tissue breaks up into 38 to 40 pairs of repeating units, called somites (Figure 4–4A). These somites—from which the muscles, bones, and other structures of the neck, limbs, and trunk develop—have a rostrocaudal organization. There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal somites. Importantly, for each of these somites, there is a corresponding vertebra and spinal cord segment, with associated dorsal and ventral roots. Each spinal cord segment (Figure 4–4B) provides the sensory and motor innervation of the skin and muscle of the body part derived from its associated somite. Thus, each segment contains repeated elements of somatic sensory and motor circuits that are present in adjacent rostral and caudal segments. In the mature spinal cord, segmentation is apparent as the series of dorsal and ventral roots emerging from its surface. The cervical segments (Figure 4–4C) innervate the skin and muscles of the back of the head, neck, and arms. The thoracic segments innervate the trunk, and the lumbar and sacral segments innervate the legs and perineal region. (Most of the coccygeal segments disappear later in development.) The segments providing the sensory and motor innervation of the upper and lower extremities are enlarged to accommodate more dorsal horn neurons, needed for the greater sensitivity of the extremities, and more motor neurons, for finer control; the cervical (C5-T1) and lumbosacral (L1-S2) enlargements (Figures 4–4C and 4–5).
The hindbrain and spinal cord are segmented structures. In the caudal brain stem the segments are called rhombomeres; in the spinal cord they are called body somites. Four occipital somites form structures of the head. These are shown in the caudal medulla (A). A. The position of the developing nervous system in the embryo is illustrated as well as the segmental organization of rhombomeres and somites. The cranial nerves that contain the axons of brain stem motor neurons are also shown. From rostral to caudal the following cranial nerves are illustrated: IV, V VI, VII, IX, X, and XII. The two mesencephalic segments and the segment between the metacephalon and mesencephalon are not shown. B. Drawing of a single spinal cord segment from the mature nervous system. C. Lateral view of the mature spinal cord in the vertebral canal. Note that the spinal nerves exit from the vertebral canal through intervertebral foramina. (A, Adapted from Lumsden A. The cellular basis of segmentation in the developing hindbrain. Trends Neurosci. 1990;13:329–335.)
The dermatomes of the body have a segmental organization. The inset illustrates dermatomal overlap. The brain and spinal cord are visible on the dorsal view (right). Note that the spinal cord ends at L1 segment. This is where cerebrospinal fluid can be withdrawn by lumbar tap (Figure 3-18B).
The area of skin innervated by the axons in a single dorsal root is termed a dermatome. Since the roots have a segmental organization, so too do the dermatomes. Dermatomes of adjacent dorsal roots overlap extensively with those of their neighbors (Figure 4–5, inset). This is because the primary sensory fibers have extensive rostro-caudal branches in the spinal cord. This explains the common clinical observation that, when a physician probes sensory capacity after injury to a single dorsal root, typically no anesthetic area is observed, although patients with such damage sometimes experience tingling or even a diminished sensory capacity. Single dorsal root injury commonly produces radicular pain, which is localized to the dermatome of the injured root. By comparing the location of radicular pain or other sensory disturbances with a dermatomal map, such as in Figure 4–5, the clinician can localize the site and extent of damage.
The Spinal Cord Gray Matter Has a Dorsoventral Sensory-Motor Organization
Very early during development, the dorsal and ventral halves of the spinal cord gray matter become committed to mediating somatic sensory and motor functions. The dorsal half becomes the dorsal horn, which mediates sensory functions; many dorsal horn neurons project to the brain stem or diencephalon; others are interneurons. The ventral half becomes the ventral horn, which mediates motor functions. Motor neurons are located in the ventral horn; they project their axons to the periphery via the ventral roots (Figure 4–6). Because the spinal cord has a longitudinal organization, the dorsal and ventral horns form columns of neurons that run rostrocaudally. Between the dorsal and ventral horns is an overlapping region (intermediate zone; Figure 4–6B) that will be considered further in Chapters 5 and 10. The spinal gray matter has a laminar organization (I-X; Figure 4–6A); this is important for pain and motor function and also will be considered in Chapters 5 and 10.
Organization of spinal cord segment. A. Terminations and spinal projections of a large-diameter fiber. Note that small-diameter fibers also terminate in other laminae. B. Myelin-stained section through the cervical spinal cord.
Dorsal root ganglion neurons that are sensitive to mechanical stimuli, on the one hand, and pain, temperature, and itch, on the other, synapse in different parts of the dorsal horn. We will see in later chapters that somatic motor neurons controlling striated muscle are located in different parts of the ventral horn than are neurons controlling visceral structures.
Mechanoreceptor Axons Terminate in Deeper Portions of the Spinal Gray Matter and in the Medulla
The central branch of a dorsal root ganglion neuron enters the spinal cord at its dorsolateral margin (Figure 4–6A). Once inside the spinal cord, dorsal root ganglion axons branch extensively. Mechanosensory fibers, which have a large diameter, enter the spinal cord medial to Lissauer's tract (Figure 4-6B), a region containing mostly unmyelinated and thinly myelinated fibers for pain and temperature senses (see Chapter 5). The axons skirt over the cap of the gray matter to enter the dorsal column (Figure 4–6A), where they give off an ascending branch into the dorsal column and numerous segmental branches into the gray matter. The segmental branches terminate in the deeper layers of the dorsal horn and in the ventral horn (Figure 4–6A) and play complex roles in limb and trunk reflexes. Whereas all mechanoreceptor classes have branches that terminate within the dorsal horn, the muscle spindle receptors are the only mechanoreceptors to terminate within the motor nuclei (Figure 4–6A). The muscle spindle receptor mediates the monosynaptic stretch (eg, knee jerk) reflex (see Figure 2–5A).
The ascending branch of a dorsal root ganglion neuron is the principal one for perception, and it relays information to the dorsal column nuclei. Whereas the majority of axons in the dorsal column are the central branches of mechanoreceptors, a small number of dorsal horn neurons project their axons into the dorsal columns, comprising approximately 10%-15% of the axons in the path. Surprisingly, these are important for visceral pain (Chapter 5). The branching patterns of the pain, temperature, and itch fibers, which have a small-diameter axon, are different from that of the mechanosensory fibers, terminating within the more dorsal portion of the dorsal horn (see Figure 5–3).
The Ascending Branches of Mechanoreceptive Sensory Fibers Travel in Dorsal Columns
Each dorsal column transmits somatic sensory information from the ipsilateral side of the body to the ipsilateral medulla. Axons from each dermatome lie within thin sheets that are parallel to the midline. Axons innervating the most caudal dermatomes are located close to the midline. Axons from progressively more rostral dermatomes are added on laterally. Axons transmitting information from the lower limb ascend in the most medial portion of the dorsal column, termed the gracile fascicle (Figure 4–7A). Axons from the lower trunk ascend lateral to those from the lower limb, but still within the gracile fascicle. Within the cuneate fascicle, axons from the upper trunk, upper limb, neck, and occiput ascend. The cuneate fascicle begins approximately at the level of the sixth thoracic segment. The gracile and cuneate fascicles are separated by the dorsal intermediate septum, and the dorsal columns of the two halves of the spinal cord are separated by the dorsal median septum (Figure 4–7A). Spinal cord injury can interrupt the dorsal column axons, resulting in a mechanosensory loss below the level of the injury. This is discussed in Chapter 5, where we will learn that a spinal cord injury typically produces a complex pattern of ipsilateral mechanosensory and contralateral pain impairment (see Box 5–1).
Somatotopic organization of the dorsal columns. A. Somatotopic arrangement of incoming axons. Dorsal spinal landmarks are shown on the left. B. The somatotopic organization of the dorsal columns can be demonstrated by examining spinal cord sections from a patient who sustained damage to the lumbar spinal cord. B1. Four levels through the spinal cord are shown, rostrocaudally from top to bottom: a section rostral to the cervical enlargement, a section through the cervical enlargement, and two thoracic sections. B2. The course taken by the central branches of the dorsal root fibers as they enter the spinal cord and ascend in the dorsal columns. The dashed line depicts the course of a degenerated axon transected by the crush. The anterolateral system is for pain and temperature sense; this will be considered in Chapter 5.
The dermatomal organization of the dorsal columns can be examined in postmortem tissue from individuals who sustained spinal cord trauma. The sections shown in Figure 4–7B1 were taken from a person whose lumbar spinal cord was crushed in a traumatic spinal injury. The sections are stained for myelin. Axons that have degenerated have lost their myelin sheath and are not stained. In the caudal thoracic spinal cord (Figure 4–7B1, bottom section), close to the crushed region, nearly all of the axons in both gracile fascicles have degenerated. At more rostral levels, the degenerated region becomes confined medially as new contingents of healthy axons continue to enter the spinal cord lateral to the degenerated axons from the lumbar cord. The pattern by which axons enter and ascend in the dorsal columns is shown schematically in Figure 4–7B2. This injury also affects pain and temperature pathways (Figure 4–7; anterolateral system), which is considered in Chapter 5.
The Dorsal Column Nuclei Are Somatotopically Organized
Dorsal column axons synapse on neurons in the dorsal column nuclei (Figure 4–8D), the first major relay in the ascending pathway for touch and limb position senses. These and other somatic sensory relay nuclei have local circuits that enhance sensitivity so that when adjacent portions of the skin are touched, the person can discern the difference. Axons of the gracile fascicle synapse in the gracile nucleus, which is located close to the midline, whereas those from the cuneate fascicle synapse in the cuneate nucleus.
Course of medial lemniscus through brain stem. A. Myelin-stained transverse section through the midbrain. B. Pons. C. Mid-medulla. The pattern of arterial perfusion of the rostral medulla is shown at this level. D. Caudal medulla. Myelin-stained transverse section through the dorsal column nuclei. Trajectories of internal arcuate fibers from the gracile and cuneate nuclei are shown. The inset shows the approximate plane of section.
Throughout the somatic sensory systems a systematic relationship exists between the position of axons in tracts and neurons in nuclei and cortex. This organization is termed somatotopy. Beginning with the sequential ordering of the dorsal roots (Figure 4–5) and the dermatomal organization of the dorsal columns, the organizational plan adheres to a simple rule: Adjacent body parts are represented in adjacent sites in the central nervous system. This is the somatotopic organization, an arrangement that ensures that local neighborhood relations in the periphery are preserved in the central nervous system. In the dorsal column nuclei, there is a coherent map of the body surface. Similar principles apply to the topographic organization of the peripheral receptive sheet in the visual system (retinotopy) and in the auditory system (tonotopy).
The Decussation of the Dorsal Column–Medial Lemniscal System Is in the Caudal Medulla
From the dorsal column nuclei, the axons of the second-order neurons sweep ventrally through the medulla, where they are called the internal arcuate fibers, and decussate (Figure 4–8D). Immediately after crossing the midline, the fibers ascend to the thalamus in the medial lemniscus. Axons from the gracile nucleus decussate ventral to axons from the cuneate nucleus and ascend in the ventral part of the medial lemniscus, compared with axons from the cuneate nucleus. Because of this pattern, the somatotopic organization of the medial lemniscus in the medulla resembles a person standing upright. In the pons, the medial lemniscus is located more dorsally than in the medulla and is oriented from medial to lateral (Figure 4–8B); in the midbrain, the medial lemniscus is located more laterally (Figure 4–8A). Axons in the medial lemniscus ascend uninterrupted through the brain stem and synapse in the thalamus.
The caudal brain stem receives blood from perforating branches of the vertebral-basilar, or posterior, circulation (see Figure 3–3B). Occlusion of small (unnamed) branches of the vertebral artery can damage axons of the medial lemniscus (Figure 4-8C). As a consequence, touch and limb position senses are disrupted. Vertebral artery infarction produces mechanosensory deficits on the contralateral side of the body, because the internal arcuate fibers decussate at a more caudal level in the medulla (Figure 4–8D). This type of infarction also destroys axons of the corticospinal tract in the pyramid.
Mechanosensory Information Is Processed in the Ventral Posterior Nucleus
The thalamus (Figure 4–9) is a nodal point for the transmission of sensory information to the cerebral cortex. Indeed, with the exception of olfaction, information from all sensory systems is processed in the thalamus and then relayed to the cerebral cortex. The dorsal column–medial lemniscal system is no exception. The various aspects of mechanical sensations are processed in the ventral posterior nucleus (Figure 4–9A). The ventral posterior nucleus has a lateral division, the ventral posterior lateral nucleus (Figures 4–9 and 4–10A), which receives input from the medial lemniscus and projects to the primary somatic sensory cortex (Figure 4–9B). The ventral posterior nucleus also has a medial division, the ventral posterior medial nucleus (Figures 4–9 and 4–10A), which mediates aspects of somatic sensations from the face and perioral structures (Chapter 6). The ventral posterior nucleus is important in discriminative aspects of the mechanical sensations, such as being able to precisely localize the stimulation site on the body. The MRI in Figure 4–10B reveals the thalamus, medial to the posterior limb of the internal capsule, but has insufficient resolution to reveal the component nuclei.
Organization of the somatic sensory thalamocortical projections. A. The ventral posterior nucleus has a somatotopic organization: Neurons receiving input from the leg and arm are located in the lateral division of the nucleus (ventral posterior lateral nucleus, VPL; darker shading), whereas neurons receiving input from the face are located in the medial division (ventral posterior medial nucleus, VPM; lighter shading). Axons from the ventral posterior nucleus ascend to the primary somatic sensory cortex in the internal capsule. B. A schematic slice through the postcentral gyrus, showing the somatotopic organization of the primary somatic sensory cortex. The territory receiving input from the ventral posterior lateral nucleus is shaded darker then the territory receiving input from the ventral posterior medial nucleus.
Myelin-stained transverse sections through the ventral posterior nucleus (A) and corresponding MRI (B). The boxed region over the MRI corresponds to the myelin-stained section in part A. The shape of the thalamus and brain stem can be discerned, but not the component nuclei. The inset shows the approximate planes of section.
The Primary Somatic Sensory Cortex Has a Somatotopic Organization
Mechanoreceptive sensory information is processed primarily by three cortical areas: (1) the primary somatic sensory cortex, (2) the secondary somatic sensory cortex, and (3) the posterior parietal cortex. (The motor cortical areas also receive mechanoreceptive information, but this information is important in controlling movements.) Located in the postcentral gyrus of the parietal lobe (Figure 4–10), the primary somatic sensory cortex is the principal region of the parietal lobe to which the ventral posterior nucleus projects. Axons from this nucleus travel to the cerebral cortex through the posterior limb of the internal capsule (Figures 4–9A and 4–10; see also Figure 2–14). The primary somatic sensory cortex receives somatotopically organized inputs from the ventral posterior lateral and medial nuclei (Figure 4–9B). This thalamocortical projection forms the basis of a body map on the postcentral gyrus, the sensory homunculus, originally described in the human by the Canadian neurosurgeon Wilder Penfield. Local circuit connections, both excitatory and inhibitory, use this information to construct the representations of the various body parts on the sensory map. Remarkably, the representations of different body parts do not have the same proportions as the body itself (Figure 4–9B). Rather, the portions of the body used in common discriminative tactile tasks, such as the fingers, have a disproportionately greater representation on the map than areas that are not as important for touch, such as the elbow. It was once thought that these differences were fixed, established genetically to determine the discriminative capacity of different body parts. We now know that the body map of the brain is not static but is also dynamically controlled by the pattern of use of different body parts in touch exploration.
The Primary Somatic Sensory Cortex Has a Columnar Organization
The cerebral cortex is a laminated structure; most regions have at least six cell layers (Figure 4–11). The thalamus projects primarily to layer IV (and the adjoining portion of layer III), and this incoming information is distributed to neurons in more superficial and deeper layers. Most of the excitatory connections within a local area of cortex remain somewhat confined to a vertical slice of cortex, termed a cortical column (Figure 4–11). The cortical column constitutes a functional unit. Neurons within a column in the primary somatic sensory cortex, spanning all cortical layers, receive input from the same peripheral location on the body and from the same class, or classes, of mechanoreceptor. Other cortical regions have a columnar organization. For example, in the primary auditory cortex, neurons within a column are sensitive to the same frequency of sound, and in the motor cortex, neurons in a column participate in controlling movement of the same joint, or sets of joints.
Three-dimensional schematic of a portion of the postcentral gyrus (A). The cortex comprises six layers (B) where neuronal cell bodies and their processes are located. Neurons whose cell bodies are located in layers II and III project to other cortical areas, those in layer V project their axons to subcortical regions, and those in layer VI project to the thalamus. Neurons in layer IV receive thalamic input and transmit information to neurons in other cortical layers.
Efferent projections arise from the primary somatic sensory cortex (Figure 4–11). As discussed in Chapter 2, pyramidal neurons in different layers project to different targets. Corticocortical association neurons, located in layers II and III, project to other cortical areas on the same side, including higher-order somatic sensory cortical areas (see next section) for further processing of sensory information, and the primary motor cortex for movement control. Callosal neurons, also located in layers II and III, project their axons to the contralateral somatic sensory cortex through the corpus callosum. One function of these callosal connections may be to join the representations of each half of the body in the primary somatic sensory cortex of each hemisphere. Descending projection neurons, located in layers V and VI, send their axons primarily to the thalamus, brain stem, and spinal cord—where somatic sensory information is processed—to act as gatekeepers that regulate the quantity of mechanosensory information that ascends through the central nervous system.
Based on its lamination pattern, the primary somatic sensory cortex consists of four cytoarchitectonic divisions, or Brodmann's areas (see Figure 2–19), numbered 1, 2, 3a, and 3b (Figure 4–12). As in other cortical areas, regions of the primary somatic sensory cortex with a different cytoarchitecture have different functions. Area 3a processes information from mechanoreceptors located in deep structures, such as the muscles and joints, and plays an important role in limb position sense. Areas 3b and 1 process information from mechanoreceptors of the skin, and are important in texture discrimination. Area 2 receives information from both deep structures and the skin and is important in discrimination of the shape of grasped objects.
A. The locations of the primary and higher order somatic sensory areas are indicated on a lateral view of the cerebral cortex. The light green region corresponds to the areas beneath the surface, in the insular cortex and the parietal and temporal operculum. B. A schematic section cut perpendicular to the mediolateral axis of the postcentral gyrus. (Adapted from Marshall WH, Woolsey CN, Bard P. Observations on cortical somatic sensory mechanisms of cat and monkey. J Neurophysiol. 1941;4:1-24.)
Higher-Order Somatic Sensory Cortical Areas Are Located in the Parietal Lobe, Parietal Operculum, and Insular Cortex
Projections from the primary sensory cortical area distribute the information to multiple cortical regions, although these other areas may also receive direct thalamic inputs. These upstream areas appear to be devoted to processing a specific aspect of the sensory experience. Although sequential pathways from one region to the next can be identified, the primary and higher-order sensory areas are also extensively interconnected and the operations of any one set of connections are dependent on the operations of others. The higher-order sensory areas typically project to cortical regions that receive inputs from the multiple sensory modalities and are termed association areas. One such multimodal convergent zone is the large expanse of cortex at the junction of the parietal, temporal, and occipital lobes.
There are three major projection streams from primary somatic sensory cortex: ventral, dorsal, and rostral. The ventral and dorsal projections comprise the "what" and "where" pathways, respectively. The "what" pathway targets the secondary somatic sensory cortex, which is located on the parietal operculum and insular cortex (Figure 4–12A). Similar to the primary area, the secondary somatic sensory cortex is somatotopically organized. This part of the cortex begins a sequence of somatic sensory projections to insular cortical areas and the temporal lobe that are important for recognizing objects by touch alone, without vision, such as distinguishing one coin from another in your pocket.
The "where" pathway targets the posterior parietal cortex (Figure 4–12A), which includes Brodmann's area 5, sometimes termed the tertiary somatic sensory cortex, and area 7. In addition to the awareness of object location, the projection to the posterior parietal cortex plays two other major functions. First, these areas play an important role in perception of body image. A lesion of this region in the nondominant hemisphere (typically the right hemisphere) produces a complex sensory syndrome in which the individual neglects the contralateral half of the body. For example, a patient may fail to dress one side of her body or comb half of her hair. Second, portions of the posterior parietal cortex receive visual and auditory inputs as well as somatic sensory information. These areas are involved in integrating somatic sensory, visual, and auditory information for perception and attention.
The "where" pathway, together with the rostral projection, targets the motor areas of the frontal lobe, especially the motor cortex. This projection is important for using mechanoreceptive sensory information to guide reaching movements. The motor cortex is essential for production and control of voluntary movements. The "where" pathway is also the "how" pathway for action.