Somatosensory Receptors (Mechanoreceptors)
Pathways Serving the Discriminative General Senses and Proprioception Pathways Serving Crude (Light) Touch and Movement Sensation
Proprioceptive Pathways of the Head Proprioceptive Pathways to the Cerebellum Functional Correlations
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become unaware of the object because the reacting sensors are rapidly adapting receptors.
Other receptors that continue to respond as long as the stimulus is applied are called slowly adapting receptors. Such receptors include nociceptors that are responsible for the warning that is the perception of pain.
Merkel disks respond to steady skin inden-tation from a tactile stimulus. Each has a large receptive field and is a slowly adapting recep-tor. Meissner corpuscles are associated with
the tactile sense called fluttering (felt as a gen-tle trembling of the skin). Each has a large receptive field and is a rapidly adapting recep-tor. Merkel disks and Meissner corpuscles have large fields and are rapidly adapting receptors. The Pacinian corpuscle is involved with vibratory sense (felt as a diffuse hum-ming sensation). Vibratory sense is poorly localized because Pacinian corpuscles have large receptive fields. In addition, they are rapid adapting receptors. Pacinian corpuscles
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Figure 10.1: Sensory nerve endings in the skin. Receptors located in the epidermis include free nerve endings associated with pain and thermal sense and Merkel’s corpuscles, activated by steady skin indentation—a form of touch. Receptors located in dermal papillae, at the junction of the der-mis and epiderder-mis, are Meissner’s corpuscles, which monitor touch, especially sensing fine spatial differences. The hair receptors (plexus around each hair follicle) subserve tactile sense and flutter.
Receptors located in the dermis are the pacinian corpuscles and corpuscles of Ruffini. Pacinian cor-puscles are involved with sensing deep pressure and vibration. The corcor-puscles of Ruffini and a vari-ant called the end bulbs of Krause subserve touch pressure and vibratory sense.
Figure 10.2: Sensory receptors of the skin. (A) Corpuscle of Ruffini, an encapsulated receptor, is supplied by a single myelinated axon that branches repeatedly to form diffuse unmyelinated ter-minals among bundles of collagen fibers in the core of the capsule. These terter-minals are presumably stimulated by the displacement of the collagen fibers among which they are intertwined. Modified Schwann cells are absent. (B) Free nerve endings in the epidermis where they lie between con-tiguous epithelial cells. (C) Pacinian corpuscle, an encapsulated receptor, is innervated by a single myelinated axon that extends as an unmyelinated ending through the center of the bulb. The flat-tened cells surrounding the axon in the core of the capsule are presumably modified Schwann cells.
(D) Meissner’s corpuscle, an encapsulated receptor, is innervated by a myelinated axon that forms an unmyelinated spiral ending amid flattened transversely oriented Schwann cells. (E) Merkel’s corpuscle is a modified epidermal cell located in the basal layer of the epidermis. It is innervated by a myelinated nerve fiber “synapsing” as a free nerve ending with a Merkel’s cell. (Adapted from Cormack, 1987).
are also located in the connective tissues of mesenteries, muscles, and interosseous mem-branes. Ruffini corpuscles are associated with the sense of touch pressure and vibratory sense. These corpuscles are of significance to the blind in “reading Braille” because they have small receptive fields and adapt rapidly, enabling them to resolve fine spatial differ-ences quickly. Each of these four receptor types has a low stimulus threshold and con-veys information to the CNS via A-beta nerve fibers of first-order neurons. Of the rapidly adapting receptors, the polymodal nociceptors of free nerve endings can act as somatosen-sory receptors (Chap. 9).
Receptors, especially muscle spindles and Golgi tendon organs (GTOs), are continuously monitoring the degree of muscle contraction and tension within the tendons (Chaps. 8 and 11). The resulting “unconscious propriocep-tion” is utilized in reflex arcs and by many pro-cessing centers, especially the cerebellum. It is now recognized that signals from spindles and GTOs are also integrated into the lemniscal system and contribute to the conscious appreci-ation of position and movement sense.
The major somatic modalities elicited by mechanoreceptors are (1) tactile sensations evoked by the application of mechanical stim-uli to the body surface and (2) proprioceptive sensations evoked by the mechanical displace-ments of muscles, ligadisplace-ments, and joints. Pro-prioception is the sense of balance, position, and movement.
The two types of tactile sensation are crude (light) touch and tactile discrimination. Crude touch is that felt by lightly stroking the skin with a wisp of hair or cotton. It can be tested by having an individual, with eyes closed, identify the location of a touch. Tactile discrimination or pressure touch is often called two-point dis-crimination, which is the ability to distinguish two separate loci where two points (e.g., a pin) are applied to the skin; spatial resolution is directly related to receptor density and is very fine at the fingertips. Tactile discrimination is also expressed as the ability to localize and to perceive the shape, size, and texture of an
object by palpation, otherwise known as stere-ognosis. Proprioception takes on various forms, including vibratory sense, static propriocep-tion, and dynamic proprioception. Perceiving vibrations when the stem of a vibrating tuning fork is placed on a joint or other body part can test vibratory sense. Static proprioception is expressed as the ability to sense the position of a body part from information received from that part (called position sense). Dynamic pro-prioception or kinesthetic sense is the ability to sense movement and balance.
PATHWAYS SERVING THE DISCRIMINATIVE GENERAL SENSES AND PROPRIOCEPTION DGS Pathway From the Body,
Limbs, and Back of Head
The primary role of the DGS pathway is to convey, in response to stimuli, neural informa-tion associated with kinesthetic sense and stere-ognosis. The latter is a complex sense that is based on such qualities as location, spatial form, and the sequence of inputs over time: The inte-gration of these qualities results in the percep-tion of form and shape of objects that are touched, felt, and held in hand. In addition, (1) the trigeminothalamic pathway responds to sim-ilar stimuli from the rest of the head, (2) the lat-eral cervical system (also called the spin cervical tract) responds to certain touch and DGS stim-uli, and (3) the anterior spinothalamic pathway is involved in mediating crude touch and move-ment sensations (see Figs. 10.3, 10.6, and 10.7).
The DGS pathway is serially organized as a basic sequence of three orders of neurons con-veying information to the cerebral cortex.
Information from the body, limbs, and back of the head (scalp posterior to the coronal plane through the ears) is conveyed from the periph-eral receptors over first-order neurons of the spinal nerves with cell bodies in the dorsal root ganglia (Features of Sensory Systems, Chap.
9). Their heavily myelinated fibers enter the spinal cord as the medial bundle of the dorsal roots (see Figs. 10.3 and 10.4) and branch into
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Figure 10.3: The discriminatory general sensory pathways originating in the spinal cord com-prise the posterior column–medial lemniscus pathway and the anterior spinothalamic tract. Note that the ventral posterolateral (VPL) thalamic nucleus projects to body regions of both SI and SII.
(1) collaterals, which terminate mainly in lam-inae III and IV of the posterior horn and (2) fibers that ascend in the ipsilateral fasciculi gracilis and cuneatus of the dorsal (posterior) column before terminating in the nuclei gra-cilis and cuneatus of the lower medulla. Some of the collaterals ending in the posterior horn synapse with interneurons involved with spinal reflex arcs (Chap, 8).
The ascending axons of the dorsal column–
medial lemniscus pathway exhibit a somato-topically organized lamination according to body area innervated (see Fig. 7.6). Fibers are added to the lateral aspect of the dorsal column (fasciculi gracilis and cuneatus) at each suc-cessively higher spinal cord level. The medial–
lateral lamination at upper cervical levels con-sists, in sequence, of fibers from sacral, lum-bar, thoracic, and cervical segments of the body. Fibers from the sacral, lumbar, and lower six thoracic levels compose the fascicu-lus gracilis of the posterior column and those of the upper six thoracic and all cervical levels (includes innervation of the back of head) form the fasciculus cuneatus. The fibers terminating in the nucleus gracilis originate from below T6 (including the lower extremity); those termi-nating in the nucleus cuneatus originate from above T6, including the upper extremities. The proprioceptive fibers from the lower extremity ascend in the dorsolateral fasciculus (located dorsally in the lateral column between the pos-terior gray horn and the pospos-terior spinocere-bellar tract; see Fig. 7.6) with the fibers of the lateral cervical system to the lateral cervical nucleus (see later). Following neural process-ing within the nucleus gracilis and nucleus cuneatus information is projected to the ven-tral posterolateral (VPL) nucleus of the thala-mus. The processing within the posterior column nuclei consists of both feedback inhi-bition and feed-forward inhiinhi-bition and, in addition, modulation by distal (reflected) inhi-bition from the cerebral cortex (Chap. 3; see Fig. 3.13). The axons of second-order neurons that emerge from the nuclei gracilis and cunea-tus arc anteriorly as the internal arcuate fibers, decussate in the lower medulla, ascend as the
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Figure 10.4: The ascending projections of the anterior spinothalamic tract. Receptors mediating crude touch and movement sensa-tions are innervated by fibers that terminate in the dorsal horn of the spinal cord. Axons aris-ing from cell bodies located in the dorsal horn decussate in the anterior white commissure and ascend in the anterior quadrant of the spinal cord as the anterior spinohalamic tract crude touch and movement sensation system (ASST);
collateral branches terminate in brainstem reticular nuclei B.R.nu). The main axons termi-nate in the ventral posterior inferior nucleus (VPI), ventral posterolateral nucleus (VPL), and central lateral (CL) nucleus of the intralaminar group of thalamic nuclei (Chap.
22). VPI and VPL thalamic nuclei project to primary (S1) and secondary (S2) somatic sen-sory cortex (Fig. 25.3). IV and V, spinal cord laminae IV and V, respectively; S1 (area 3), pri-mary somatic sensory cortex; S2, secondary somatic sensory area. (Adapted from Craig and Dostrosky.)
somatotopically organized medial lemniscus, and terminate in the VPL nucleus of the thala-mus. As it ascends, the medial lemniscus grad-ually shifts from a medial location in the medulla to a posterolateral location in the upper midbrain.
Thalamus and Somatic Sensory (Somato-sensory) Cortex. The VPL thalamic nucleus receives sensory input from the body via fibers of the medial lemniscus and spinothala-mic tract, which terminate in a somatotopic pattern. VPL is parceled functionally as fol-lows: The central core of the nucleus is responsive to stimuli from cutaneous recep-tors; the surrounding shell is responsive to stimuli from deep receptors (e.g., muscle spindles) (see Fig. 10.5). In addition, VPL receives reflected (distal) inhibitory influ-ences from somatosensory cortex. Axons of third-order neurons emerge from VPL, pass through the posterior limb of the internal cap-sule and corona radiata, and terminate in lam-ina IV of the somatosensory cortex of the parietal lobe, located in the postcentral gyrus and in the adjacent paracentral lobule on the medial surface of the hemisphere (Organiza-tion of Neocortex, Chap. 25). The somatic sensory cortex consists of the primary somatosensory cortex (SI; areas 1, 2, 3a, and 3b), secondary somatosensory cortex (SII) (both located in the postcentral gyrus), and the posterior parietal cortex (areas 5 and 7).
The somatotopic representations of the body surface are present in the cortex in an orderly manner, but with the cortical receptive field size of the two homunculi of the primary and secondary somatosensory cortices roughly pro-portional to the resolution of stimuli from regions of different densities of receptors (e.g., skin); one each for areas 1, 2, 3a, and 3b of SI and one for SII (see Fig. 25.4) The receptive field sizes are directly related to the density of receptors on the body. The back with a low density of receptors has a small cortical repre-sentation compared to the much larger cortical representation of the fingertips, which have a high density of receptors.
There are somatotopic projections (1) from the medial lemniscus and spinothalamic tracts to the VPL nucleus and (2) from the core and shell of VPL to somatosensory cortex (areas 1, 2, 3a, and 3b) (see Fig. 10. 5). (1) The core neurons receive cutaneous input from slow and rapidly adapting receptors involved with the discrimina-tion of texture. The third-order thalamic neurons of the core receiving inputs from these receptors have substantial projections that terminate in area 3b. Those thalamic neurons of the core receiving inputs from rapidly adapting receptors involved with sensing texture have sparse pro-jections that terminate in areas 1, 3b, and SII. (2) The shell neurons receive inputs from the deep tissue receptors monitoring muscle stretch, deep pressure, and joint sense. Those thalamic neu-rons of the shell receiving inputs from muscle spindle stretch receptors have substantial projec-tions that terminate in area 3a. Those thalamic neurons of the shell receiving stimuli from deep pressure and joint receptors involved with sens-ing size and shape of objects held in the hand have sparse projections that terminate in areas 3a, 2, and SII. In turn, neurons from areas 3a and 3b project to areas 1 and 2 and all four areas project to SII, which is involved in the discrimi-nation of shape, size, and texture. All five areas of SI and SII have connections with parietal lobe association areas 5 and 7 (Chap. 25).
A similar structural and functional organiza-tion is expressed in the ventral posterior medial (VPM) thalamic nucleus, which is the nucleus receiving somatosensory input from the head primarily from the trigeminal nerve (see Figs. 10.6, 14.5, and 23.3). The VPM nucleus receives input from the trigeminothalamic pathways and projects to the head region of the somatosensory cortex.
The Paths From Receptors to Columns of the Cortex. The sensory pathways involved with sensation are composed of sequences of neurons forming paths transmitting labeled line codes and pattern codes. The lemniscal and trigeminal pathways consist primarily of pro-jections extending from the somatic receptors in the body to functional columns (slabs) in the
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Figure 10.5: Schema illustrating the projections from the ventral posterior lateral (VPL) and ventral posterior medial thalamic (VPM) nuclei to somatosensory cortex. VPL receives input from the medial lemniscus and spinothalamic tracts, and VPM receives input from the trigeminothala-mic tracts. These nuclei are organized into a central core consisting of two zones, each responsive to cutaneous stimuli, and an outer shell responsive to deep stimuli. Neurons of the central core proj-ect to cortical areas 3b and 1 (cutaneous). Neurons of the outer shell projproj-ect to areas 3a (muscle spindles) and 2 (deep receptors). These projections are somatotopic.
The somatosensory cortex of the parietal lobe consists of three major subdivisions: primary somatosensory cortex (SI of areas 3, 1, and 2), secondary somatosensory cortex (SII of areas 3, 1, and 2), and posterior parietal cortex (areas 5 and 7). Neurons in areas 3a and 3b project to areas 1 and 2. Neurons of SI (areas 3a, 3b, 1, and 2) project to the secondary sensory cortex (SII). Neurons from SI and SII and some thalamic neurons project to area 5, and the latter to area 7. (Adapted from Carpenter and Sutin, 1983.)
postcentral gyrus of the parietal lobe (Chap.
25). Each receptor exhibits specificity, in that it responds to specific stimulus energy. Each line conveys a specific stimulus quality (e.g., posi-tion sense) and processes the message in each nucleus of the pathway before arriving for more processing in a cortical column. The stimulus feature encoded by a receptor in the body is faithfully reproduced by the signal received by that line in the cortex. For example, slowly adapting receptors are coupled to slowly adapting neurons of the thalamus that are sequentially coupled with slowly adapting neu-rons in a column of areas 3a and 3b of the somatosensory cortex. It is of significance that all six layers in each cortical column represent the same modality. Thus, many lines transmit-ting different features of each sensation are paths where parallel processing of the stimulus features occurs. It is in the highest centers in the cortex that the features are integrated into a sensation. The paths are not redundant because they accent different features. Parallel process-ing of stimulus features in several lines has a significant role in the generation of the variety and subtleties associated with perceptions.
Trigeminothalamic Pathway From the Facial Region
The discriminative general senses from the facial region (head anterior to a coronal plane through the ears) are served via neurons of the trigeminal nerve, which enter the brainstem through the lateral midpons. Most fibers termi-nate in the principal sensory trigeminal nucleus. Other fibers bifurcate into collaterals, which branch and terminate in the principal sensory trigeminal nucleus and/or descend for a short distance in the spinal tract of n.V (spinal trigeminal tract) and terminate in the pars oralis of the spinal nucleus of n.V (spinal trigeminal nucleus). The principal trigeminal nucleus is the cranial equivalent of the nuclei gracilis and cuneatus. In all of these nuclei are located the cell bodies of the second-order neurons of the discriminative general senses (see Fig. 10.6).
From cell bodies of neurons of the second order, located in the principal sensory
trigemi-nal nucleus and rostral portion of the spitrigemi-nal trigeminal nucleus, axons decussate in the pon-tine tegmentum and ascend as the trigemino-thalamic tract (anterior trigeminal tract) before terminating in the ventral posteromedial thala-mic nucleus (see Fig. 10.6). Some axons of second-order neurons of the principal sensory trigeminal nucleus ascend uncrossed as the posterior trigeminothalamic tract (posterior trigeminal tract) to the same thalamic nucleus.
Axons of third-order neurons arising in the ventral posterior medial thalamic nucleus pass through the posterior limb of the internal cap-sule and corona radiata before terminating in the head area of the postcentral gyrus. Follow-ing processFollow-ing within this gyrus, connections via association fibers are made with areas 5 and 7 of the parietal lobe (see Fig. 10. 5 and Chap. 25).
This trigeminal pathway is structurally and functionally the same as the dorsal column–
medial lemniscal pathway. (1) The axonal pro-jections from sensory receptors to columns in the postcentral gyrus are similar. (2) These con-nections are maintained and sharpened by the same processing circuits. (3) The projections from the ventral posterior medial nucleus are similar to those of the ventral posterior lateral nucleus. The projection fibers terminate in areas 1, 2, 3a, and 3b of the postcentral gyrus.
Subsequent connections with SII and areas 5 and 7 of the parietal lobe are equivalent.
PATHWAYS SERVING CRUDE (LIGHT) TOUCH AND MOVEMENT SENSATION Pathway From the Body, Limbs, and Back of Head
Crude touch and movement sensation from the body and the back of the head (C2 der-matome) is conveyed from peripheral receptors via first-order neurons with cell bodies in the dorsal root ganglia of the peripheral nerves to the posterolateral tract of Lissauer where the fibers bifurcate (see Fig. 7.6). In addition to those that terminate at the level of entry, some
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Figure 10.6: The discriminatory general sensory pathways originating in the brainstem are the anterior and posterior trigeminothalamic tracts. Note that the VPM thalamic nucleus projects to head regions of both SI and SII. The jaw reflex, illustrated on the left side of the figure, comprises (1) afferent fibers with cell bodies in the mesencephalic nucleus of n.V and (2) efferent (lower motoneurons) fibers with cell bodies in the motor nucleus of n.V. For details of structures in the brainstem, refer to Figs. 13.7, 13.10, 13.11, and 13.14.
ascend and descend several spinal levels before terminating on interneurons of the posterior horn. Processing occurs within the interneu-ronal circuits of the posterior horn.
The axons of neurons of the second order, with cell bodies presumably in laminae VI and VII, decussate through the anterior white com-missure, ascend as the anterior spinothalamic tract which is somatotopically organized, and terminate in the ventral posterolateral nucleus of the thalamus. The anterior and lateral spinothalamic tracts together are referred to as the anterolateral tract or system (Chap. 9).
Third-order neurons in VPL emit axons that pass through the posterior limb of the internal capsule and the corona radiata before terminat-ing in the postcentral gyrus. After neural pro-cessing in the gyrus, pyramidal neurons of the cortex project to the parietal association cortex.
Crude touch is also conveyed via the posterior column–medial lemniscus pathway and the spinocervicothalamic pathway (see Fig. 10.7).
Pathways From the Facial Region
From receptors in the facial region (anterior to coronal plane through the ears), light touch fibers convey impulses via the three divisions of the trigeminal nerves (ophthalmic, maxil-lary, and mandibular) and to a small extent via cranial nerves VII, IX, and X. The cell bodies of first-order fibers are located in the trigeminal ganglion, geniculate ganglion, and superior ganglia of nerves IX and X. Upon entering the brainstem, some of these fibers terminate in the principal trigeminal nucleus and others descend in the spinal tract of n.V and terminate in the caudal part of the spinal trigeminal nucleus known as nucleus caudalis. Second-order neurons from these nuclei have axons that decussate and join the ascending trigeminothalamic tract and terminate in the ventral posteromedial nucleus of the thalamus.
From this thalamic nucleus, axons pass through the internal capsule before terminating somato-topically as a homunculus in both the primary and secondary somatosensory cortex. In turn, SI and SII project to the parietal association cortex, where more processing occurs.
Lateral Cervical System
(Spinocervicothalamic Pathway)
The lateral cervical system mediates touch, proprioception, vibratory sense, and to a small degree, noxious stimuli. This system is a fast-conducting four-neuron pathway (see Fig.
10.7).
The first-order neurons, with cell bodies in the dorsal root ganglia, have axons that termi-nate in laminae III, IV, and V of the dorsal horn.
Second-order neurons from these laminae emit axons that ascend without decussating in the dorsolateral fasciculus of the lateral column to the lateral cervical nucleus (see Fig. 7.6). Third-order fibers from this nucleus, located in the upper two cervical spinal segments and the lower medulla, decussate in the lower medulla, ascend in the contralateral medial lemniscus, and terminate in the ventral posterior lateral nucleus of the thalamus. The fourth-order neu-rons project from VPL to the somatosensory cortex (SI and SII).
PROPRIOCEPTIVE PATHWAYS OF THE HEAD
Mesencephalic Nucleus of the Trigeminal Nerve; Jaw Jerk Reflex
Information from proprioceptive endings (e.g., muscle spindles) in the extraocular mus-cles and musmus-cles of mastication and facial expression are conveyed to the CNS by Ia nerve fibers of cranial nerves III to VII. These Ia fibers have their cell bodies in the mesencephalic nucleus of the trigeminal nerve. This nucleus is unique, in that, it is the only nucleus of primary sensory neurons located in the CNS and is actu-ally composed of unipolar dorsal root (trigemi-nal) ganglion cell bodies (see Fig. 14.2).
The jaw jerk is a two-neuron reflex, similar to the knee jerk reflex (see Fig. 10.6), which involves the temporalis, masseter, and internal pterygoid muscles. Tapping the chin of the slightly opened mouth with a reflex hammer evokes this reflex. The afferent limb of the
reflex arc is composed of neurons with cell bodies of the mesencephalic nucleus. These neurons convey influences from the muscle spindles directly via collateral fibers to lower
motoneurons in the motor nucleus of the trigeminal nerve. These lower motoneurons form the efferent limb innervating the muscles of mastication.
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Figure 10.7: Ascending tracts from the spinal cord, including the anterior and posterior spin-ocerebellar tracts, cunespin-ocerebellar tract, and spinocervicothalamic pathway.
PROPRIOCEPTIVE PATHWAYS TO THE CEREBELLUM
The cerebellum plays an essential role in body movement and maintenance of equilib-rium. Thanks to the cerebellum, voluntary mus-cles are coordinated in their contraction and relaxation so as to permit smooth movement.
For this, the cerebellum requires a continuous supply of unconscious information from mus-cles, tendons, and joints to which receptors throughout the body and limbs contribute. The pathways for this input are outlined here (see Fig. 10.7); the main discussion of the cerebel-lum is in Chapter 17.
Information that arrives from muscle spin-dles (Ia fibers) and Golgi tendon organs (Ib fibers) is primarily unconscious and proprio-ceptive in nature, but, in addition, there are inputs from exteroceptors for crude touch, pressure, and pain. There are direct and indirect pathways from these receptors. The direct path-ways convey input without an intervening synapse from the spinal neurons to the cerebel-lum. They are (1) the posterior spinocerebellar tracts for information from the lower limbs and lower half of the body and (2) the cuneocere-bellar tracts for information from the upper limbs and upper half of the body (see Fig.
10.7). The indirect pathways are (1) the spin-ocervicocerebellar pathway, with a synaptic relay in the lateral reticular nucleus of the medulla (see Fig. 13.9), and (2) the spinoolivo-cerebellar pathway, with a synaptic relay in the inferior olivary nucleus of the medulla (see Fig.
13.10).
Two complete somatotopic representations can be traced on the cerebellar cortex, one homunculus on the anterior lobe and the other (in halves) on the posterior lobe (see Fig. 18.2).
Posterior Spinocerebellar Tract
First-order neurons that convey impulses from peripheral receptors into the spinal cord terminate in the dorsal nucleus (Clarke’s nucleus), found in lamina VII. Second-order fibers arise from this nucleus, located at levels
T1 through L2, and ascend uncrossed as the posterior spinocerebellar tract. The fibers enter the cerebellum by way of the inferior cerebel-lar peduncle, one of three fiber bundles on each side giving access to the cerebellum. The pos-terior spinocerebellar tract is primarily con-cerned with conveying information from muscles and joints in the lower limbs.
Cuneocerebellar Tract
First-order neurons ascend in the ipsilateral fasciculus cuneatus of the spinal cord and ter-minate in the accessory cuneate nucleus (equiv-alent to the dorsal nucleus of Clarke), which is located lateral to the cuneate nucleus (see Fig.
13.9). Second-order fibers, identified now as the cuneocerebellar tract, enter the cerebellum through the inferior cerebellar peduncle and terminate in portions of the anterior and poste-rior lobes dedicated to the upper extremities.
This pathway is the rostral equivalent of the posterior spinocerebellar tract.
Anterior Spinocerebellar Tract (see Fig. 10.7) This tract originates from cells, called spinal border cells, in the lumbosacral cord on the periphery of the anterior horn and other cells in the posterior horn and intermediate gray.
Second-order fibers decussate in the spinal cord and ascend through the brainstem as the anterior spinocerebellar tract, which enters the cerebellum along the dorsal margin of the supe-rior cerebellar peduncle. Most fibers terminate somatotopically in the vermis of the anterior lobe on the contralateral side in that part of the homuncular area representing the lower extremity and trunk. Some recross within the cerebellum to terminate on the same side as origin, whereas others terminate bilaterally (see Fig. 18.2).
Rostrospinocerebellar Tract
This tract is presumed to arise from cells in the intermediate gray zone of the cervical enlargement. It ascends as an uncrossed tract whose fibers pass through both the inferior and superior cerebellar peduncles before terminat-ing in the area of the homunculus representterminat-ing
the upper extremity in the anterior lobe of the cerebellum.
The pattern of termination of the posterior spinocerebellar and cuneocerebellar inputs to the cerebellum is somatotopic to form separate homunculi rostrally in the anterior lobe and caudally in the posterior lobe (see Fig. 18.2).
The anterior spinocerebellar and rostrospino-cerebellar tracts were previously thought to convey somatic sensory information from the lower and upper limbs, respectively. It is now thought that they relay feedback signals to the cerebellum, monitoring the amount and quality of activity in the descending motor pathways, rather than conveying information from the periphery (Chap. 17).
Indirect Pathways
The spinoreticular fibers of the anterolateral pathway include a population originating at all spinal levels and terminating in the lateral cer-vical nucleus and other small nuclei in the medulla. This spinocervicocerebellar pathway is completed by neurons arising from these nuclei and terminating in the cerebellum. In this way, the cerebellum receives exteroceptive input.
The spinoolivary tract, activated by cuta-neous and proprioceptive afferent fibers of the spinal nerves, originates from cell bodies located at all spinal levels (see Fig. 7.6). The tract terminates in the inferior olivary nuclei of the medulla (see Fig. 13.10). Olivocerebel-lar fibers cross the midline and enter the cerebellum through the inferior cerebellar peduncle.
FUNCTIONAL CORRELATIONS The general sensory pathways conveying pain and temperature, tactile sensibility, and discriminative senses have, with a few excep-tions, similar features. The neurons of the first-order innervate receptors in the periphery and terminate within nuclei (or laminae) in the ipsi-lateral half of the spinal cord or brainstem. The cell bodies of these neurons are located in
gan-glia (with no synapses within them) just out-side the CNS: dorsal root ganglia, trigeminal ganglion, geniculate ganglion, and superior ganglia of cranial nerves IX and X. The neu-rons of the second order have cell bodies in a nucleus on the ipsilateral side and give rise to axons that decussate to the contralateral side and ascend as tracts, which terminate in the thalamus (ventral posterior nucleus and poste-rior thalamic region). The neurons of the third order project from the thalamus to the postcen-tral gyrus (primary somatic area) and adjacent to the secondary somatic area (see Fig. 25.3).
Note that the spinothalamic fibers (neurons of the second order) decussate at all levels of the spinal cord, with each fiber crossing at a spinal level near the location of its cell body, whereas all second-order neurons of the posterior column–medial lemniscal pathway have axons that decussate at a common level in the lower medulla, where they are known as internal arcuate fibers.
Crude touch can be conveyed via two path-ways: (1) the anterior spinothalamic tract (and its cranial equivalent, the anterior trigeminal tract) and (2) the posterior column–medial lem-niscal pathway (and its cranial equivalent, the anterior and posterior trigeminal tracts).
Loss of tactile sensibility is known as tactile anesthesia. Diminution is tactile hypesthesia and an exaggeration, which is often unpleasant, is tactile hyperesthesia. The latter can be accom-panied by paresthesias, the sensations of numbness, tingling, prickling, and feeling of discomfiture.
Impairment of the Posterior Column-Medial Lemniscus Pathway
Interruption of the posterior column-medial lemniscus pathway causes disturbances in the appreciation of certain sensations and in the regulation and control of movements.
The alterations in the appreciation of the discriminative general senses include the following:
1. Diminution, not loss, of crude touch. This modality is partially retained because the
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