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A variety of sensory receptors scattered throughout the body can become activated by exteroceptive, interoceptive, or proprioceptive input. Exteroceptive input relays sensory information about the body’s interaction with the external environment. Interoceptive input relays information about the body’s internal state, whereas proprioceptive input con- veys information about position sense from the body and its component parts. Each receptor is specialized to detect mechanical, chemical, nociceptive (L. nocere, “to injure,” “painful”), or thermal stimuli. Activation of a sensory recep- tor is converted into nerve impulses and this sensory input is then conveyed via the fibers of the cranial or spinal nerves to their respective relay nuclei in the central nervous system (CNS). The sensory information is then further processed as it
progresses, via the ascending sensory systems (pathways) , to the cerebral cortex or to the cerebellum. Sensory informa- tion is also relayed to other parts of the CNS where it may function to elicit a reflex response, or may be integrated into pattern-generating circuitry. The ascending sensory pathways are classified according to the functional components (modalities) they carry as well as by their anatomical localization. The two functional categor- ies are the general somatic afferent (GSA) system , which transmits sensory information such as touch, pressure, vibra- tion, pain, temperature, stretch, and position sense from somatic structures; and the general visceral afferent (GVA) system , which transmits sensory information such as pressure, pain, and other visceral sensation from visceral structures.
A 60-year-old woman com- plains of falls, imbalance, and numbness and tingling in her hands and legs. There is also some incoordination of hand use and she has difficulty manipu- lating small items such as buttons. She is unable to play the piano now since she cannot position her fingers correctly on the piano keys. She thinks the strength in her arms and legs is adequate. Symptoms started with very slight tingling sensa- tions, which she noticed about 5 years ago. The falls and difficulty walking have been present for about 2 months. Higher order cognitive functions are intact according to her husband. Her vision is
normal. On examination, the patient shows normal mental status. Strength seems essentially normal throughout. Sensation, particularly to vibration and joint posi- tion, is severely diminished in the distal upper and lower limbs (arms, legs, hands, and feet). Tendon reflexes are normal in the arms, but somewhat brisk in the legs at the knees and ankles. Gait is moder- ately ataxic and she has to reach out for support by touching the walls of the hallway at times. Fine movements of the fingers are performed poorly, even though finger and wrist strength seems normal.
C L I N I C A L C A S E
C H A P T E R 1 0
Ascending Sensory Pathways
CLINICAL CASE
SENSORY RECEPTORS
ANTEROLATERAL SYSTEM
TACTILE SENSATION AND PROPRIOCEPTION
SENSORY PATHWAYS TO THE CEREBELLUM
CLINICAL CONSIDERATIONS
MODULATION OF NOCICEPTION
NEUROPLASTICITY
SYNONYMS AND EPONYMS
FOLLOW-UP TO CLINICAL CASE
QUESTIONS TO PONDER
Anatomically, the ascending sensory systems consist of three distinct pathways: the anterolateral system (ALS), the dorsal column–medial lemniscal (DCML) pathway, and the somatosensory pathways to the cerebellum. The anterolateral system , which includes the spinothala- mic , spinoreticular , spinomesencephalic , spinotectal , and spinohypothalamic tracts , relays predominantly pain and temperature sensation, as well as nondiscriminative (crude or poorly localized) touch, pressure, and some propriocep- tive sensation (Table 10.1). The dorsal column–medial lemniscal pathway (which includes the fasciculus gracilis , fasciculus cuneatus , and medial lemniscus ) relays discriminative (fine) tactile sense, vibratory sense, and position sense (Table 10.1). The somatosensory pathways to the cerebellum , which include the anterior , posterior , and rostral spinocerebellar , as well as the cuneocerebellar tracts , relay primarily propri- oceptive (but also some pain and pressure) information (Table 10.1). The ascending sensory pathways are the main avenues by which information concerning the body’s interaction with the external environment, its internal condition, and the position and movement of its parts, reach the brain. One similarity shared by all three ascending sensory pathways from the body (not including the head or face) is that the first order neuron cell bodies reside in the dorsal root ganglia. It is interesting to note that conscious perception of sensory information from external stimuli is mediated by the spinothalamic and DCML pathways to the ventral posterior lateral nucleus of the thalamus , whereas sensations that do not reach consciousness are mediated by the spinoreticular , spinomesencephalic , spinotectal , spinohypothalamic , and the anterior , posterior , and rostral spinocerebellar , and cuneocerebellar tracts. These tracts terminate in the reticular formation, mesencephalon, hypothalamus and cerebellum, respectively. Sensory input may ultimately elicit a reflex or other motor response because of the functional integration of the
ascending (somatosensory) pathways, the cerebellum, and the somatosensory cortex, as well as the motor cortex and descending (motor) pathways. Furthermore, descending projections from the somatosensory cortex , as well as from the raphe nucleus magnus and the dorsolateral pontine reticular formation to the somatosensory relay nuclei of the brainstem and spinal cord, modulate the transmission of incoming sensory impulses to higher brain centers. This chapter includes a description of the sensory recep- tors and the ascending sensory pathways from the body, whereas the ascending sensory pathways from the head, transmitted mostly by the trigeminal system, are described in Chapter 15.
SENSORY RECEPTORS
Although sensory receptors vary according to their mor- phology, the velocity of con- duction, and the modality to which they respond, as well as to their location in the body, they generally all function in a similar fashion. The stimulus to which a specific receptor responds causes an alteration in the ionic permeability of the nerve endings, generating a receptor potential that results in the formation of action potentials. This transformation of the stimulus into an elec- trical signal is referred to as sensory transduction. Some receptors that respond quickly and maximally at the onset of the stimulus, but stop responding even if the stimulus continues, are known as rapidly adapting (phasic) receptors. These are essential in responding to changes but they ignore ongoing processes, such as when one wears a wristwatch and ignores the continuous pressure on the skin of the wrist. However, there are other receptors, slowly adapting (tonic) receptors , that continue to respond as long as the stimulus is present. Sensory receptors are classified according to the source of the stimulus or according to the modality to which they
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Although sensory receptors vary, they generally all function in a similar fashion
Anatomical system
Anterolateral (ALS)
Dorsal column–medial lemniscal (DCML)* Somatosensory to the cerebellum
*Indicates conscious level.
Functional component(s)
Pain, temperature, nondiscriminative (crude) touch, pressure, and some proprioceptive sensation
Discriminative (fine) touch, vibratory sense, position sense Primarily proprioceptive information (also some pain and pressure information)
Anatomical tracts
Spinothalamic* Spinoreticular Spinomesencephalic Spinotectal Spinohypothalamic Fasciculus gracilis Fasciculus cuneatus Anterior spinocerebellar Posterior spinocerebellar Rostral spinocerebellar Cuneocerebellar Table 10.1 = A general description of the anatomical and functional aspects of the ascending sensory pathways.
Nonencapsulated mechanoreceptors
Nonencapsulated mechanoreceptors are slowly adapting and include free nerve endings and tactile receptors
Free nerve endings (Fig. 10.1) are present in the epidermis, dermis, cornea, dental pulp, mucous membranes of the oral and nasal cavities and of the respiratory, gastrointestinal, and urinary tracts, muscles, tendons, ligaments, joint capsules, and bones. The peripheral nerve terminals of the free nerve endings lack Schwann cells and myelin sheaths. They are stimulated by touch, pressure, thermal, or painful stimuli.
Peritrichial nerve endings (Fig. 10.2) are specialized mem- bers of this category. They are large-diameter, myelinated, Aβ fibers that coil around a hair follicle below its associated sebaceous gland. This type of receptor is stimulated only when a hair is being bent. Tactile receptors (Fig. 10.3) consist of disc-shaped, periph- eral nerve endings of large-diameter, myelinated, Aβ fibers. Each disc-shaped terminal is associated with a specialized epithelial cell, the Merkel cell, located in the stratum basale of the epidermis. These receptors, frequently referred to as Merkel’s discs (Fig. 10.4), are present mostly in glabrous (hairless), and occasionally in hairy skin. Merkel’s discs
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Sensory receptors
Nociceptors Pain Temperature
Mechanoreceptors Free nerve endings
Merkel’s tactile discs
Meissner’s corpuscles
Pacinian corpuscles
Peritrichial nerve endings Ruffini’s organs
Muscle and tendon mechanoreceptors Nuclear bag fibers Nuclear chain fibers
Golgi tendon organs DCML, dorsal column–medial lemniscal.
Table 10.2 = Sensory receptors.
Mediate/ respond to
Tissue damage Extreme temperature
Touch, pressure
Discriminative touch, superficial pressure Two-point discriminative (fine) touch Deep pressure and vibratory sensation
Touch, hair movement (bending) Pressure on, or stretching of, skin
Detects onset of muscle stretch Muscle stretch in progress
Stretching of tendon
Endings
Branching free nerve (Aδ, C) endings (unmyelinated nonencapsulated)
Nonencapsulated
Nonencapsulated
Encapsulated
Encapsulated
Nonencapsulated
Encapsulated
Location
Epidermis, dermis, cornea, muscle, joint capsules
Epidermis, dermis, cornea, dental pulp, muscle, tendons, ligaments, joint capsules, bones, mucous membranes Basal epidermis
Papillae of dermis of hairless skin
Dermis, hypodermis, interosseous membranes, ligaments, external genitalia, joint capsules, peritoneum, pancreas Around hair follicle
Joint capsules, dermis, hypodermis
Skeletal muscle
Skeletal muscle
Skeletal muscle
Associated with
Aδ (group III) myelinated fibers, C (group IV) unmyelinated fibers Aδ, C fibers
Aβ (group II) myelinated fibers Aβ (group II) myelinated fibers
Aβ (group II) myelinated fibers
Aβ (group II) myelinated fibers Aβ (group II) myelinated fibers
Aα (group Ia) myelinated fibers also secondary afferents, muscle spindle afferents Aα (group Ib) myelinated fibers
Pathways
Anterolateral system
Anterolateral system
DCML pathway
DCML pathway
DCML pathway
DCML pathway
DCML pathway
DCML pathway and ascending sensory pathways to the cerebellum
Rate of adaptation
Slow
Slow
Slow
Rapid
Rapid
Rapid
Slow
Both slow and rapid
Slow
respond to discriminative touch stimuli that facilitate the distinguishing of texture, shape, and edges of objects.
Encapsulated mechanoreceptors
Encapsulated mechanoreceptors include Meissner’s corpus- cles, pacinian corpuscles, and Ruffini’s end organs.
Meissner’s corpuscles are present in the dermal papillae of glabrous skin of the lips, forearm, palm, and sole, and in the connective tissue papillae of the tongue
Meissner’s corpuscles (Fig. 10.5) are present in the dermal papillae of glabrous skin of the lips, forearm, palm, and sole, as well as in the connective tissue papillae of the tongue. These corpuscles consist of the peripheral terminals of Aβ fibers, which are encapsulated by a peanut-shaped structural device consisting of a stack of concentric Schwann cells surrounded by a connective tissue capsule. They are rapidly adapting and are sensitive to two-point tactile (fine) discrim- ination, and are thus of great importance to the visually impaired by permitting them to be able to read Braille.
Pacinian corpuscles are the largest of the mechanoreceptors
Pacinian corpuscles (Fig. 10.6), the largest of the mechanoreceptors, are rapidly adapting and resemble an onion in cross-section. Each Pacinian corpuscle consists of Aβ-fiber terminals encapsulated by layers of modified fibroblasts that are enclosed in a connective tissue capsule. Pacinian corpuscles are located in the dermis, hypodermis, interosseous membranes, ligaments, external genitalia, joint capsules, and peritoneum, as well as in the pancreas. They are more rapidly adapting than Meissner’s corpuscles and are believed to respond to pressure and vibratory stimuli, including tickling sensations.
Ruffini’s end organs are located in the joint capsules, dermis, and underlying hypodermis of hairy skin
Ruffini’s end organs (corpuscles of Ruffini) (Fig. 10.7) are located in joint capsules, the dermis, and the underlying hypodermis of hairy skin. The unmyelinated peripheral terminals of Aβ myelinated fibers are slowly adapting. They intertwine around the core of collagen fibers, which is sur- rounded by a lamellated cellular capsule. Ruffini’s end organs respond to stretching of the collagen bundles in the skin or joint capsules and may provide proprioceptive information. Muscle spindles and Golgi tendon organs (GTOs) are also encapsulated mechanoreceptors, but, due to their spe- cialized function, they are discussed separately.
Muscle spindles and Golgi tendon organs
The mucle spindles and GTOs detect sensory input from the skeletal muscle and transmit it to the spinal cord where it plays an important role in reflex activity and motor control involving the cerebel- lum. In addition, sensory input from these muscle receptors is also relayed to the cerebral cortex by way of the DCML pathway, which mediates information concerning posture, position sense, as well as movement and orientation of the body and its parts.
Muscle spindles
Structure and function
Skeletal muscle consists of extrafusal and intrafusal fibers
Extrafusal fibers are ordinary skeletal muscle cells constitut- ing the majority of gross muscle, and their stimulation results in muscle contraction. Muscle spindles , composed of small bundles of encapsulated intrafusal fibers , are dispersed throughout gross muscle. These are dynamic stretch receptors that continuously check for changes in muscle length. Each muscle spindle is composed of two to 12 intrafusal fibers enclosed in a slender capsule, which in turn is
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Figure 10.1 = Free nerve endings in the skin. The free nerve endings terminating in the epidermis lose their myelin sheath. Many free nerve endings have unmyelinated axons.
Two types of proprioceptors, the neuromuscular (muscle) spindles and the GTOs (neurotendinous spindles), are associated with skeletal muscle only
Figure 10.5 = Meissner’s corpuscles are located in dermal papillae of the skin.
Figure 10.6 = Pacinian corpuscles are located in the dermis of the skin.
Figure 10.7 = A corpuscle of Ruffini.
Figure 10.4 = Merkel’s discs (corpuscles) terminate on the basal surface of the epidermis.
surrounded by an outer fusiform connective tissue capsule whose tapered ends are attached to the connective tissue sheath surrounding the extrafusal muscle fibers (Fig. 10.8). The compartment between the inner and outer capsules con- tains a glycosaminoglycan-rich viscous fluid. There are two types of intrafusal fibers based on their morphological characteristics: nuclear bag fibers and nuclear chain fibers. Both nuclear bag and nuclear chain fibers pos- sess a central, noncontractile region housing multiple nuclei, and a skeletal muscle (myofibril-containing) contractile por- tion at each end of the central region. The nuclear bag fibers are larger, and their multiple nuclei are clustered in the “bag- like” dilated central region of the fiber. The nuclear chain fibers are smaller and consist of multiple nuclei arranged sequentially, as in a “chain” of pearls, in the central region of the fiber.
Each intrafusal fiber of a muscle spindle receives sensory innervation via the peripheral processes of pseudounipolar sensory neurons
Each intrafusal fiber of a muscle spindle receives sensory innervation via the peripheral processes of pseudounipolar sensory neurons whose cell bodies are housed in dorsal root ganglia, or in the sensory ganglia of the cranial nerves (and in the case of the trigeminal nerve, within its mesencephalic nucleus). Since the large-diameter Aα fibers spiral around the noncontractile region of the intrafusal fibers, they are known as annulospiral or primary endings. These endings become activated at the beginning of muscle stretch or tension. In addition to the annulospiral endings, the intrafusal fibers, mainly the nuclear chain fibers, also receive smaller diameter, Aβ peripheral processes of pseudounipolar neurons. These nerve fibers terminate on both sides of the annulospiral end- ing, are referred to as secondary or flower spray endings , and are activated during the time that the stretch is in progress (Fig. 10.8).
In addition to sensory innervation, intrafusal fibers also receive motor innervation via gamma motoneurons that innervate the contractile portions of the intrafusal fibers, causing them to contract
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Figure 10.8 = Right: an extrafusal skeletal muscle fiber. Left: a neuromuscular spindle containing the two types of intrafusal fibers—the nuclear bag fiber with multiple nuclei in the dilated central region and a nuclear chain fiber with a row of nuclei in its central region.
During muscle contraction, as the muscle shortens, tension is produced in the tendons anchoring that muscle to bone, compressing the nerve fiber terminals interposed among the inelastic intrafusal collagen fibers. This compression acti- vates the sensory terminals in the GTOs, which transmit this sensory information to the CNS, providing proprioceptive information concerning muscle activity and preventing the placement of excessive forces on the muscle and tendon. In contrast, the noncontractile portions of the muscle spindles are not stretched, and are consequently undisturbed. The contractile regions of the muscle spindles, however, undergo corresponding contraction that enables them to detect a future change in muscle length (resulting from stretch or contraction). During slight stretching of a relaxed muscle, the muscle spindles are stimulated whereas the GTOs remain undis- turbed and quiescent. During further stretching of the muscle, which produces tension on the tendons, both the muscle spindles and the GTOs are stimulated. Thus GTOs monitor and check the amount of tension exerted on the muscle (regardless of whether it is tension generated by muscle stretch or contraction), whereas muscle spindles check muscle fiber length and rate of change of muscle length (during muscle stretch or contraction).
ANTEROLATERAL SYSTEM
The anterolateral system (ALS) transmits nociceptive, thermal, and nondiscrimina- tory (crude) touch informa- tion to higher brain centers (see Table 10.2), generally by a sequence of three neurons and interneurons (Fig. 10.10). The neuron sequence consists of:
1 A first order neuron (pseudounipolar neuron) whose cell body is located in a dorsal root ganglion. It transmits sen- sory information from peripheral structures to the dorsal (posterior) horn of the spinal cord. 2 A second order neuron whose cell body is located within the dorsal horn of the spinal cord, and whose axon usu- ally decussates and ascends:
- in the direct pathway of the ALS (spinothalamic tract) to synapse in the contralateral thalamus, and sending some collaterals to the reticular formation;
- in the indirect pathway of the ALS (spinoreticular tract) to synapse in the reticular formation, and send- ing some collaterals to the thalamus; or
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The ALS transmits nociceptive, thermal, and nondiscriminatory touch information to higher brain centers, generally by a sequence of three neurons and interneurons
Third order neuron
First order neuron
First order neuron
Second order neuron (spinothalamic tract)
Second order neuron (spinothalamic tract)
Cerebral cortex
Thalamus
RF
RF
RF
VPL VPI Intralaminar nuclei
Dorsal root ganglion
Cerebellum (^) Dorsal root ganglion
Interneuron Muscle (reflexes)
Aδ
Figure 10.10 = The direct pathway of the anterolateral system. Note the first order neuron in the dorsal root ganglion, the second order neuron in the dorsal horn of the spinal cord, and the third order neuron in the thalamus. The second order neuron sends collaterals to the reticular formation (RF). VPI, ventral posterior inferior; VPL, ventral posterior lateral.
- as spinomesencephalic , spinotectal , or spinohypo- thalamic fibers to synapse in several brainstem nuclei.
3 A third order neuron whose cell body is located in the thalamus, and whose axon ascends ipsilaterally to ter- minate in the somatosensory cortex.
In some cases, the first order neuron may synapse with an interneuron that resides entirely within the dorsal horn, and whose axon synapses with the second order neuron.
Pain pathways from the body
First order neurons (sensory receptors)
Receptors that transmit nociceptive information consist of high-threshold free nerve endings ramifying near the external surface and internal environment of the organism
Receptors that transmit nociceptive information consist of high-threshold free nerve endings ramifying near the external surface and internal environment of the organism. These are dendritic arborizations of small, pseudounipolar,
first order neurons (Fig. 10.11) whose somata are housed in a dorsal root ganglion. The peripheral processes of these pseudounipolar neurons consist of two main types of fiber (Fig. 10.12):
1 Thinly myelinated A δδ (fast-conducting) fibers , which relay sharp, short-term, well-localized pain (such as that resulting from a pinprick). These fibers transmit sensa- tions that do not elicit an affective component associated with the experience. 2 Unmyelinated C (slow-conducting) fibers , which relay dull, persistent, poorly localized pain (such as that result- ing from excessive stretching of a tendon). These fibers transmit sensations that elicit an affective response.
The central processes of these pseudounipolar neurons enter the spinal cord at the dorsal root entry zone, via the lat- eral division of the dorsal roots of the spinal nerves, and upon entry collectively form the dorsolateral fasciculus (tract of Lissauer) , which is present at all spinal cord levels. These central processes bifurcate into short ascending and descend- ing branches. These branches either ascend or descend one to three spinal cord levels within this tract, to terminate in their
ASCENDING SENSORY PATHWAYS === 147
Secondary somatosensory cortex
Spinothalamic tract
Spinothalamic tract
Spinothalamic tract
Primary somatosensory cortex
Arm area
Trunk area
Thigh area
Parietal lobe
Medulla
Spinal cord T
Spinal cord L
Foot
Leg
VPL
VPI
Intralaminar nuclei
Hypothalamus
Reticular formation
Neuron in dorsal root ganglion Receptor endings for pain and temperature stimuli
Lissauer's tract
Ascending and descending fibers in Lissauer's tract
Substantia gelatinosa
Ventral white Figure 10.11 = The ascending sensory pathway that commissure transmits nondiscriminative (crude) touch, pain, and temperature sensations from the body. (Modified from Gilman, S, Winans Newman, S (1992) Essentials of Clinical Neuroanatomy and Neurophysiology. FA Davis, Philadelphia; fig. 19.)
target laminae of the dorsal horn, where they synapse with second order neurons (or with interneurons). Therefore, although stimulation of the peripheral endings of fibers car- ried by one spinal nerve may enter the spinal cord at a specific spinal level, collaterals of the ascending and descending branches spread the signal to neighboring spinal levels above and below the level of entry. These collaterals play an import- ant function in intersegmental reflexes.
Second order neurons
The cell bodies of the second order neurons transmitting nociception reside in the dorsal horn of the spinal cord
The cell bodies of the second order neurons transmitting nociception reside in the dorsal horn of the spinal cord (Fig. 10.12). Recent findings indicate that the axons of these second order neurons course in either the direct (spinothala- mic) or indirect (spinoreticular) pathways of the ALS, or as three sets of fibers (the remaining components of the ALS): the spinomesencephalic , spinotectal , or spinohypothalamic fibers. Approximately 15% of nociceptive fibers project dir- ectly to the thalamus whereas 85% project to the thalamus via a relay in the reticular formation.
Direct pathway of the anterolateral system
The spinothalamic tract transmits not only nociceptive input, but also thermal and nondiscriminative touch input to the contralateral ventral posterior lateral nucleus of the thalamus
Type A δδ fibers of first order neurons synapse primarily with second order neurons in lamina I (posteromarginal nucleus, or zone) and lamina V (reticular nucleus) of the spinal cord gray matter. However, many first order neurons synapse with spinal cord interneurons that are associated with reflex motor activity. The axons of the second order neurons flow across the midline to the contralateral side of the spinal cord in the anterior white commissure, forming the spinothalamic tract (Fig. 10.13). The spinothalamic tract transmits not only nociceptive input, but also thermal and nondiscriminative (crude) touch input to the contralateral ventral posterior lateral nucleus of the thalamus. It also sends some projections to the ventral posterior inferior , and the intralaminar nuclei of the thala- mus. Although the spinothalamic tract ends at the thalamus, as it ascends through the brainstem it also sends collaterals to the reticular formation. Since the spinothalamic tract (direct pathway of the ALS: spinal cord → thalamus) is phylogenet- ically a newer pathway, it is referred to as the neospino- thalamic pathway. Only about 15% of the nociceptive fibers from the spinal cord, ascending in the ALS and carrying nociceptive informa- tion, terminate directly in the thalamus via the spinothalamic tract. Although referred to as the “spinothalamic tract,” it actu- ally consists of two anatomically distinct tracts: the lateral
spinothalamic tract (located in the lateral funiculus) and the very small anterior spinothalamic tract (located in the ante- rior funiculus). Earlier studies indicated that the lateral spinothalamic tract transmitted only nociceptive and thermal input, whereas the anterior spinothalamic tract transmitted only nondiscriminative (crude) touch. Recent studies how- ever, support the finding that both the anterior and lateral spinothalamic tracts (as well as the other component fibers of the ALS: spinoreticular, spinomesencephalic, spinotectal, and spinohypothalamic), transmit nociceptive , thermal , and non- discriminative (crude) tactile signals to higher brain centers.
Indirect pathway of the anterolateral system
Type C fibers of first order neurons terminate on interneu- rons in laminae II (substantia gelatinosa) and III of the dorsal horn. Axons of these interneurons synapse with second order neurons in laminae V–VIII. Many of the axons of these second order neurons ascend ipsilaterally, however a small number of axons sweep to the opposite side of the spinal cord in the anterior white commissure. These axons form the more prominent ipsilateral and smaller contralateral spinoreticu- lar tracts. The spinoreticular tracts transmit nociceptive , thermal , and nondiscriminatory (crude) touch signals from the spinal cord to the thalamus indirectly, by forming multi- ple synapses in the reticular formation prior to their thalamic projections. Since the spinoreticular tract (indirect pathway of the ALS: spinal cord → reticular formation → thalamus) is phylogenetically an older pathway, it is referred to as the paleospinothalamic pathway (Fig. 10.14).
ASCENDING SENSORY PATHWAYS === 149
Somatosensory information
Somatosensory cortex (Brodmann's areas 3, 1, and 2)
VPL nucleus of the thalamus
Fasciculi gracilis and cuneatus
Two-point discrimination Tactile sense (discriminative (fine) touch) Vibratory sense Kinesthetic sense Muscle tension Joint position sense
Somatosensory cortex Prefrontal cortex Anterior cingulate gyrus Anterior insular cortex Striatum, S-II
VPL, VPI, intralaminar nuclei of the thalamus
Spinothalamic tract
Pain Temperature Nondiscriminative (crude) touch Deep pressure
Figure 10.13 = Somatosensory information to consciousness. VPI, ventral posterior inferior; VPL ventral posterior lateral.
The spinoreticular tract is a bilateral (primarily uncrossed) tract that conveys sensory information to the brainstem reticular formation, the region responsible for producing arousal and wakefulness, thus alerting the organism follow- ing an injury. Impulses from the reticular formation are then relayed bilaterally to the intralaminar nuclei of the thala- mus , via reticulothalamic fibers. Since these nuclei lack somatotopic organization, there is only an indistinct localiza- tion of sensory signals carried by this pathway. The reticular formation and its continuation into the diencephalon, the intralaminar nuclei of the thalamus, are components of the reticular-activating system (RAS). The RAS functions in activating the organism’s entire nervous system, so as to elicit responses that will enable it to evade painful stimuli. In addition, there are some second order neurons from the dorsal horn that bypass the reticular formation and relay sen- sory input from C fibers directly to the intralaminar nuclei of the thalamus.
Other component fibers of the anterolateral system
In addition to the spinothalamic and spinoreticular tracts, the ALS also contains spinomesencephalic, spinotectal, and spinohypothalamic fibers
The spinomesencephalic fibers terminate in the periaque- ductal gray matter and the midbrain raphe nuclei, both of which are believed to give rise to fibers that modulate noci- ceptive transmission and are thus collectively referred to as the “descending pain-inhibiting system” (see discussion later). Furthermore, some spinomesencephalic fibers ter- minate in the parabrachial nucleus, which sends fibers to the amygdala—a component of the limbic system associated with the processing of emotions. Via their connections to the limbic system, the spinomesencephalic fibers play a role in the emotional component of pain. The spinotectal fibers terminate mainly in the deep layers of the superior colliculus. The superior colliculi have the reflex function of turning the upper body, head, and eyes in the direction of a painful stimulus (Fig. 10.14). The spinohypothalamic fibers ascend to the hypoth- alamus where they synapse with neurons that give rise to the hypothalamospinal tract. This pathway is associated with the autonomic and reflex responses (i.e., endocrine and cardiovascular) to nociception (Fig. 10.14). Approximately 85% of the nociceptive fibers from the spinal cord ascending in the ALS, terminate in the brainstem reticular formation. From there, the information eventually reaches the thalamus via multiple additional synapses that
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Superior colliculus
Spinohypothalamic tract
Spinoreticular tract
Spinoreticular tract ending in reticular formation
Spinotectal tract
Midbrain
Pons
Inferior cerebellar peduncle
Medulla oblongata Inferior olivary nucleus
Figure 10.14 = Spinotectal, spinoreticular, and spinohypothalamic tracts of the indirect pathway of the anterolateral system. Note that the spinomesencephalic tract is not shown.
Brodmann’s areas 3a, 3b, 1, 2 (Fig. 10.15A). The secondary somatosensory cortex (S-II) consists of Brodmann’s area 43, located on the superior bank of the lateral fissure, at the inferior extent of the primary motor and sensory areas. Axons of the thalamic third order neurons terminate in somatotopically corresponding regions of the primary somatosensory cortex. Regions of the head are represented in the inferior half of the postcentral gyrus near the lateral fissure, whereas those of the upper limb and the trunk are represented in its superior half. The lower limb is represented in the medial surface of the postcentral gyrus, and the per- ineum in the paracentral lobule. The body areas with the largest cortical area representation are the head and upper limb, reflecting the great discriminative capability that struc- tures in these regions possess (Fig. 10.16). In summary, nociceptive signals relayed from the spinal cord directly to the ventral posterior lateral, the ventral post- erior inferior, and the intralaminar nuclei of the thalamus via the spinothalamic tract (neospinothalamic, direct pathway of the ALS) are transmitted to the somatosensory cortex (both to S-I and S-II). The postcentral gyrus is the site where pro- cessing of pain localization, intensity, quality, and sensory integration takes place at the conscious level. The primary somatosensory cortex sends projections to the secondary somatosensory cortex, which is believed to have an import- ant function in the memory of sensory input.
In contrast, nociceptive signals relayed from the spinal cord to the reticular formation via the spinoreticular tracts (paleospinothalamic, indirect pathway of the ALS) are then transmitted to:
- the intralaminar nuclei of the thalamus (the cranial extension of the reticular formation into the thalamus), which in turn project to the primary somesthetic cortex;
- the hypothalamus ; and
- the limbic system.
The projections through the reticular formation function in the arousal of the organism in response to nociceptive input, whereas the projections to the hypothalamus and limbic system have an important function in the autonomic, reflex, and emotional (suffering) responses to a painful experience.
Projections to the cingulate and insular cortices
Cerebral imaging studies such as electroencephalography (EEG), functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG), and positron emission tomography (PET) have demonstrated that nociceptive signals are not only processed at the primary and secondary
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Foot
Toes
Intra-abdominal
Pharynx
Tongue
Jaw
Gums Teeth
Lower lip
Lips
Upper lip
Face
Nose
Eye
Thumb
Fingers
ForearmHand
Elbow Arm
Trunk NeckHead Hip Leg
Genitals
Figure 10.16 = Coronal section through the primary somatosensory cortex (postcentral gyrus), showing the sensory homunculus. Note that the amount of cerebral cortex representing each body part is proportional to the extent of its motor innervation.
somatosensory cortices, but also in the anterior cingulate cortex , anterior insular cortex , and even the supplemental motor area of the motor cortex. The anterior cingulate and anterior insular cortices are connected with the limbic cortex, which plays a role in the emotional aspect of pain.
Visceral pain
A striking characteristic of the brain is that although it receives and processes nociceptive information, the brain itself has no sensation of pain. During brain surgery, the patient is often awake and has no pain sensation from the brain tissue itself. The structures that have to be anesthetized during brain surgery are the dura mater, the bones of the skull, and the extra- cranial soft tissues. Moreover, although the internal (visceral) organs themselves have no pain receptors, pain receptors are present embedded in the walls of the arteries serving these organs. Visceral pain is characterized as diffuse and poorly local- ized, and is often “referred to” and felt in another somatic structure distant or near the source of visceral pain. Nociceptive signals from the viscera generally follow the same pathway as signals arising from somatic structures. General visceral afferent nociceptive information from vis- ceral structures of the trunk is carried mostly by type C , A δδ, or A ββ fibers (Fig. 10.17). The peripheral terminals of these fibers are associated with Pacinian corpuscles that respond to excessive stretching of the intestinal wall, a lesion in the wall of the gastrointestinal tract, or to smooth muscle spasm. The cell bodies of these sensory (pseudounipolar), first order neurons are housed in the dorsal root ganglia, and their central processes carry the information, via the dorsolateral fasciculus (tract of Lissauer) , to the dorsal horn and lateral gray matter of the spinal cord. Here, these central processes synapse with second order neurons as well as with neurons associated with reflex activities. The axons of the second order neurons join the anterolat- eral system to relay nociceptive signals from visceral struc- tures to the reticular formation and the thalamus. Fibers from the reticular formation project to the intralaminar nuclei of the thalamus , which in turn project to the cerebral cortex and the hypothalamus. Recall that the intralaminar nuclei of the thalamus lack somatotopic organization, result- ing in only an indistinct localization of sensory signals carried by this pathway. Visceral pain signals relayed to the primary somatosen- sory cortex may be associated with referred pain to a somatic structure. In addition to projections to the somatosensory cortex, recent studies indicate that nociceptive signals are also relayed to the anterior cingulate and anterior insular cortices , two cortical areas implicated in the processing of visceral pain.
Temperature pathways from the body
Temperature sensory input is transmitted to the CNS via unmyelinated C fibers that are activated by warm stim- uli, or by lightly myelinated Aδ fibers that are activated by cold stimuli (see Fig. 10.12; Table 10.2). All fibers enter the spinal cord at the dorsal root entry zone in the lateral division of the dorsal root of the spinal nerves. These fibers accompany nociceptive fibers and, upon
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A striking characteristic of the brain is that although it receives and processes nociceptive information, the brain itself has no sensation of pain
collect to form part of
and terminate in
terminate in terminate in
synapse with
join
fibers decussate in some fibers decussate in
synapse with
enter spinal cord, and course in the
Receptors ( free nerve endings, Pacinian corpuscles ) Peripheral processes of pseudounipolar neurons in thoracic, abdominal, or pelvic structures
Cell bodies of type C , A δ, A β pseudounipolar neurons ( first order neurons ) in dorsal root ganglia
Central processes of pseudounipolar neurons
Lateral division of dorsal root of spinal nerves
Dorsolateral fasciculus (tract of Lissauer) as ascending and descending branches
Dorsal horn Lateral gray matter
Second order neurons
Spinothalamic tract Spinoreticular tract
Anterior white commissure Anterior white commissure, others ascend ipsilaterally VPL nucleus of the thalamus
Reticular formation (some fibers end here)
synapse with
their fibers course in multiple synapses
Third order neurons
Posterior limb of the internal capsule
Corona radiata
Postcentral gyrus (processing of referred pain to a somatic structure)
Intralaminar nuclei of the thalamus
Hypothalamus and widespread areas of the cerebral cortex (including the insula and cingulate cortex ) (processing of autonomic, reflexive, and emotional aspects of pain)
Neurons involved in various reflexes
Figure 10.17 = The ascending sensory pathway relaying pain sensation from the viscera. VPL ventral posterior lateral.
Temperature sensory input is transmitted to the CNS via unmyelinated C fibers that are activated by warm stimuli, or by lightly myelinated Aδ fibers that are activated by cold stimuli
- free nerve endings responding to touch, pressure, and proprioception in the skin, muscles, and joint capsules;
- tactile (Merkel’s) discs responding to touch and pressure in the skin;
- peritrichial endings stimulated by touch of the hair follicles;
- Meissner’s corpuscles activated by touch of the skin; and
- Pacinian corpuscles stimulated by touch, pressure, vibra- tion, and proprioception in the deep layers of the skin, and in visceral structures.
These first order pseudounipolar neurons , whose cell bodies are located in the dorsal root ganglia, send peripheral processes to somatic or visceral structures. These peripheral processes are medium-size type A ββ and large-size type A αα fibers. Upon being stimulated, the peripheral processes transmit the sensory information to the spinal cord by way of the central processes of the pseudounipolar neurons, which enter the spinal cord at the dorsal root entry zone via the medial division of the dorsal roots of the spinal nerves. Upon entry into the posterior funiculus of the spinal cord, the afferent fibers bifurcate into long ascending and short descending fibers.
Bifurcating fibers
The long ascending and short descending fibers give rise to collateral branches that may synapse with several distinct cell groups of the dorsal horn interneurons and with ventral horn motoneurons. These fibers collectively form the dorsal column pathways , either the fasciculus gracilis or the fasci-
culus cuneatus , depending on the level of the spinal cord in which they enter.
Below level T The central processes that enter the spinal cord below level T6 include the lower thoracic, lumbar, and sacral levels that bring information from the lower limb and lower half of the trunk
The central processes that enter the spinal cord below level T include the lower thoracic, lumbar, and sacral levels. They bring information from the lower limb and lower half of the trunk. The central processes enter the ipsilateral fasciculus gracilis (L. gracilis, “slender”) and ascend to the medulla to terminate in the ipsilateral nucleus gracilis. It should be recalled that the fasciculus gracilis is present in the entire length of the spinal cord.
Level T6 and above The central processes that enter the spinal cord at level T6 and above bring information from the upper thoracic and cervical levels, that is from the upper half of the trunk and upper limb
The central processes that enter the spinal cord at level T6 and above bring information from the upper thoracic and cervical levels, that is from the upper half of the trunk and upper limb. These central processes enter the ipsilateral fasciculus cun- eatus (L. cuneus, “wedge”) and ascend to the medulla to synapse with second order neurons in the ipsilateral nucleus cuneatus. It should be noted that the fasciculus cuneatus is present only at the upper six thoracic and at all cervical spinal cord levels.
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Table 10.3 = Continued.
Location of pathway in spinal cord
Lateral funiculus
Anterior funiculus
Posterior funiculus
Medulla
Termination of third order neuron
Postcentral gyrus Cingulate gyrus Prefrontal cortex Postcentral gyrus
Postcentral gyrus
Conscious/ subconscious
Conscious
Conscious
Conscious
Location of cell body of third order neuron
VPL nucleus of the thalamus Intralaminar nuclei VPL nucleus of the thalamus
VPL nucleus of the thalamus
Pathway (second order neuron) termination
VPL, VPI, and intralaminar nuclei of the thalamus VPL nucleus of the thalamus
VPL nucleus of the thalamus
Function(s)
Relays pain and thermal sensation from the body
Relays nondiscriminative (crude) touch sensation from the body
Relays two-point discriminative (fine) touch tactile sensation, vibratory sense, proprioceptive sense from muscles and joints of the body
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Thalamus (VPL nucleus)
Primary somatosensory cortex Third order neuron (^) In internal capsule and corona radiata
Medial lemniscus Cuneate nucleus Gracile nucleus
Fasciculus cuneatus
Fasciculus gracilis
Primary afferent neuron in dorsal root ganglion
Posterior column
Figure 10.18 = The dorsal column–medial lemniscal pathway relaying discriminative (fine) touch and vibratory sense from the body to the somatosensory cortex. VPL, ventral posterior lateral.
