MINISTRY OF HEALTH OF UKRAINE
VINNYTSIA NATIONAL MEDICAL UNIVERSITY
NAMED AFTER M.I.PYROGOV
NEUROLOGY DEPARTMENT
MODULE - 1
Lessons # 2-3
Somatosensory System. Pathways.
Aids to the examination of the Somatosensory System.
- Basic questions:
2.1. Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits:
2.1.1. Receptor Organs.
2.1.2.Receptor types.
2.1.3. Receptors in the Skin.
2.1.4. Receptors in Deeper Regions of the Body.
2.2. Peripheral Nerve, Dorsal Root Ganglion, Posterior Root:
2.2.1. Peripheral nerve: Anatomy of the spinal roots and nerves.
2.2.2. Nerve plexus and posterior root.
2.2.3. Dorsal root ganglion.
2.2.4. Somatosensory Innervation by Nerve Roots and Peripheral Nerves.
2.3. Posterior Columns.
2.4. Anterior Spinothalamic Tract.
2.5. Lateral Spinothalamic Tract.
2.6. Central Components of the Somatosensory System:
2.7.1. Sensorimotor integration.
2.7.2. Differentiation of somatosensory stimuli by their origin and quality.
2.7. Testing for somatosensory deficits.
2.8. Somatosensory Deficits due to Lesions at Specific Sites along the Somatosensory Pathways
Somatosensory System
Peripheral Components of the SomatosensorySystem
Receptors
Receptors are specialized sensory organs that register physical and chemicalchanges in the external and internal environment of the organism and convert(transduce) them into the electrical impulses that are processed by thenervous system.
They are found at the peripheral end of afferent nerve fibers.Some receptors inform the body about changes in the nearby external environment(exteroceptors) or in the distant external environment (teleceptors, suchas the eye and ear). Proprioceptors, such as the labyrinth of the inner ear, conveyinformation about the position and movement of the head in space, tensionin muscles and tendons, the position of the joints, the force needed tocarry out a particular movement, and so on. Finally, processes within the bodyare reported on by enteroceptors, also called visceroceptors (including osmoceptors,chemoceptors, and baroceptors, among others). Each type of receptorresponds to a stimulus of the appropriate, specific kind, provided that the intensityof the stimulus is above threshold.
Most receptors in the skin are exteroceptors.
A second group of receptor organs lies deep to the skin, in the muscles, tendons,fasciae, and joints. In the muscles, for example, one finds musclespindles, which respond to stretching of the musculature. Other types of receptorsare found at the transition between muscles and tendons, in the fasciae, orin joint capsules (Fig 2.1)
Peripheral Nerve, Dorsal Root Ganglion, Posterior Root
The further “way stations” through which an afferent impulse must travel as it makes its way to the CNS are the peripheral nerve, the dorsal root ganglion, and the posterior nerve root, through which it enters the spinal cord.
Peripheral nerve. Action potentials arising in a receptor organ of one of the types described above are conducted centrally along an afferent fiber, which is the peripheral process of the first somatosensory neuron, whose cell body is located in a dorsal root ganglion.
Nerve plexus and posterior root. Once the peripheral nerve enters the spinal canal through the intervertebral foramen, the afferent and efferent fibers go their separate ways: the peripheral nerve divides into its two “sources,” the anterior and posterior spinal roots.
The anterior root contains the efferent nerve fibers exiting the spinal cord, while the posterior root contains the afferent fibers entering it.
A direct transition from the peripheral nerve to the spinal nerve roots is found, however, only in the thoracic region. At cervical and lumbosacral levels, nerve plexuses are interposed between the peripheral nerves and the spinal nerve roots (the cervical, brachial, lumbar, and sacral plexuses).
Anatomy of the spinal roots and nerves. In total, there are 31 pairs ofspinal nerves; each spinal nerve is formed by the junction of an anterior and aposterior nerve root within the spinal canal. The numbering of the spinalnerves is based on that of the vertebral bodies. Even though there areonly seven cervical vertebrae, there are eight pairs of cervical nerves, becausethe highest spinal nerve exits (or enters) the spinal canal just above the firstcervical vertebra. Thus, this nerve, the first cervical nerve (C1), exits the spinalcanal between the occipital bone and the first cervical vertebra (atlas); the remainingcervical nerves, down to C7, exit above the correspondingly numberedvertebra; and C8 exits between the seventh (lowest) cervical vertebra and thefirst thoracic vertebra. At thoracic, lumbar, and sacral levels, each spinal nerveexits (or enters) the spinal canal below the correspondingly numbered vertebra.There are, therefore, just as many pairs of nerves in each of these regions asthere are vertebrae (12 thoracic, 5 lumbar, and 5 sacral). Lastly, thereis a single pair of coccygeal nerves (or, occasionally, more than one pair).
Dorsal root ganglion. The dorsal root ganglion is macroscopically visible as aswelling of the dorsal root,immediately proximal to its junction with the ventralroot (Fig. 2.4). The neurons of the dorsal root ganglion are pseudounipolar, i.e.,they possess a single process that divides into two processes a short distancefrom the cell, in a T-shaped configuration. One of these two processes travels tothe receptor organs of the periphery, giving off numerous collateral branchesalong theway, so that a single ganglion cell receives input from multiple receptororgans. The other process (the central process) travels byway of the posteriorroot into the spinal cord, where it eithermakes synaptic contact with the secondsensory neuron immediately, or else ascends toward the brainstem. There are no synapses within the dorsal root ganglion itself.
The fibers of individual nerve roots are redistributed into multiple peripheral nerves by way of the plexuses, and each nerve contains fibers from multiple adjacent radicular segments. The fibers of each radicular segment regroup in the periphery, however (Fig. 2.6), to innervate a particular segmental area of the skin (dermatome). Each dermatome corresponds to a single radicular segment, which, in turn, corresponds to a single “spinal cord segment.”
When a peripheral nerve is injured, the fibers within it, derived from multiple nerve roots, can no longer rejoin in the periphery with fibers derived from the same nerve roots but belonging to other peripheral nerves—in other words, the fibers in the injured nerve can no longer reach their assigned dermatomes. The sensory deficit thus has a different distribution from that of the dermatomal deficit seen after a radicular injury (Fig. 2.8).
Central Components of the Somatosensory System
Posterior Columns. We can feel the position of our limbs and sense the degree of muscle tension inthem.We can feel the weight of the body resting on our soles (i.e., we “feel theground under our feet”). We can also perceive motion in the joints. Thus, atleast some proprioceptive impulses must reach consciousness. Such impulsesare derived from receptors in muscles, tendons, fasciae, joint capsules, and connective tissue (Vater-Paciniand Golgi-Mazzonicorpuscles), as well as cutaneousreceptors. The afferent fibers conveying them are the distal processesof pseudounipolar neurons in the spinal ganglia. The central processes of thesecells, in turn, ascend the spinal cord and terminate in the posterior column nucleiof the lower medulla.
Central continuation of posterior column pathways. In the posterior funiculus of the spinal cord, the afferent fibers derived from the lower limbs occupy the most medial position. The afferent fibers from the upper limbs join the cord at cervical levels and lie more laterally, so that the posterior funiculus here consists of two columns (on either side): the medial fasciculus gracilis (column of Goll), and the lateral fasciculus cuneatus (column of Burdach). The fibers in these columns terminate in the correspondingly named nuclei in the lower medulla, i.e., the nucleus gracilis and the nucleus cuneatus, respectively. These posterior column nuclei contain the second neurons, which project their axons to the thalamus (bulbothalamic tract). All of the bulbothalamic fibers cross the midline to the other side as they ascend, forming the so-called medial lemniscus (Figs. 2.16b and 2.17). These fibers traverse the medulla, pons, and midbrain and terminate in the ventral posterolateral nucleus of the thalamus (VPL, Fig. 6.4). Here they make synaptic contact with the third neurons, which, in turn, give off the thalamocortical tract; this tract ascends by way of the internal capsule (posterior to the pyramidal tract) and through the corona radiata to the primary somatosensory cortex in the postcentral gyrus. The somatotopic organization of the posterior column pathway is preserved all the way up from the spinal cord to the cerebral cortex (Fig. 2.19). The somatotopic projection on the postcentral gyrus resembles a person standing on his head—an inverted “homunculus” (Fig. 9.19).
Lateral Spinothalamic Tract. The free nerve endings of the skin are the peripheral receptors for noxious and thermal stimuli.
These endings constitute the end organs that are, in turn, the peripheral processes of pseudounipolar neurons in the spinal ganglia. The central processes pass in the lateral portion of the posterior roots into the spinal cord and then divide longitudinally into short collaterals that terminate within one or two segments in the substantia gelatinosa, making synaptic contact with funicular neurons (second neurons) whose processes form the lateralspinothalamic tract (Fig. 2.16d). These processes cross the midline in the anterior spinal commissure before ascending in the contralateral lateral funiculus to the thalamus. Like the posterior columns, the lateral spinothalamic tract is somatotopically organized; here, however, the fibers from the lower limb lie laterally, while those from the trunk and upper limb lie more medially (Fig. 2.20).
The fibers mediating pain and temperature sensation lie so close to each other that they cannot be anatomically separated. Lesions of the lateral spinothalamic tract thus impair both sensory modalities, though not always to the same degree.
Central continuation of the lateral spinothalamic tract. The fibers of the lateral spinothalamic tract travel up through the brainstem together with those of the medial lemniscus in the spinal lemniscus, which terminates in the ventral posterolateral nucleus of the thalamus (VPL). The third neurons in the VPL project via the thalamocortical tract to the postcentral gyrus in the parietal lobe (Fig. 2.19). Pain and temperature are perceived in a rough manner in the thalamus, but finer distinctions are not made until the impulses reach the cerebral cortex.
Anterior Spinothalamic Tract. The impulses arise in cutaneous receptors (peritrichial nerve endings, tactile corpuscles) and are conducted along a moderately thickly myelinated peripheral fiber to the pseudounipolar dorsal root ganglion cells, and thence by way of the posterior root into the spinal cord. Inside the cord, the central processes of the dorsal root ganglion cells travel in the posterior columns some segments upward, while collaterals travel 1 or 2 segments downward, making synaptic contact onto cells at various segmental levels in the gray matter of the posterior horn (Fig. 2.16c). These cells (the second neurons) then give rise to the anterior spinothalamic tract, whose fibers cross in the anterior spinal commissure, ascend in the contralateral anterolateral funiculus, and terminate in the ventral posterolateral nucleus of the thalamus, together with the fibers of the lateral spinothalamic tract and the medial lemniscus (Fig. 2.17). The third neurons in this thalamic nucleus then project their axons to the postcentral gyrus in the thalamocortical tract.
Central Processing of Somatosensory Information. Fig. 2.17 traces all of the sensory pathways discussed above, in schematicallysimplified form and in spatial relation to one another, as they ascend fromthe posterior roots to their ultimate targets in the brain. The sensory third neuronsin the thalamus send their axons through the posterior limb of the internalcapsule (posterior to the pyramidal tract) to the primary somatosensorycortex, which is located in the postcentral gyrus (Brodmann cytoarchitecturalareas 3a, 3b, 2, and 1). The third neurons that terminate here mediate superficialsensation, touch, pressure, pain, temperature, and (partly) proprioception(Fig. 2.19).
Sensorimotor integration. In fact, not all of the sensory afferent fibers from thethalamus terminate in the somatosensory cortex; some terminate in the primarymotor cortex of the precentral gyrus. Thus, the sensory and motor corticalfields overlap to some extent, so that the precentral and postcentral gyri aresometimes together designated the sensorimotor area. The integration of functionoccurring here enables incoming sensory information to be immediatelyconverted to outgoing motor impulses in sensorimotor regulatory circuits,about which we will have more to say later. The descending pyramidal fibersemerging from these circuits generally terminate directly—without any interveninginterneurons—on motor neurons in the anterior horn. Finally, eventhough their functions overlap, it should be remembered that the precentralgyrus remains almost entirely a motor area, and the postcentral gyrus remainsalmost entirely a (somato)sensory area.
Differentiation of somatosensory stimuli by their origin and quality. It has alreadybeen mentioned that somatosensory representation in the cerebral cortexis spatially segregated in somatotopic fashion: the inverted sensoryhomunculus has been encountered in Fig 2.19 and will be seen again inFig9.19. But somatosensory representation in the cortex isalso spatially segregated by modality: pain, temperature, and the other modalitiesare represented by distinct areas of the cortex.
Stereognosis. The recognition by touch of an object laid in the hand (stereognosis)is mediated not just by the primary sensory cortex, but also by associationareas in the parietal lobe, in which the individual sensory features of theobject, such as its size, shape, consistency, temperature, sharpness/dullness,softness/hardness, etc., can be integrated and compared with memories of earliertactile experiences.
TESTING FOR SOMATOSENSORY DEFICITS
Testing for Pain
Have the patient discriminate between the point (“sharp”) and head (“dull”) of a pin. You have to be careful to avoid simply tapping into the sense of touch. One should avoid using the same pin with different patients, as there is evidence that certain viruses can be transmitted in this fashion.
Testing for Proprioception
Have the patient attempt to localize his or her limb in space following passive movement by the examiner (with patients eyes closed) or indicate the state of flexion or extension of one’s limb. However, perhaps the easiest and certainly one of the most sensitive and specific tests of proprioception is to passively extend or flex a digit (e.g., the great toe or a finger) while asking the patient to indicate the direction in which it is being moved. One also may check the integrity of the posterior columns by asking the patient to stand erect with the feet together. If the patient shows considerably more difficulty maintaining balance with the eyes closed than opened, this suggests posterior column compromise (Romberg’s sign). If comparable difficulties are noted regardless of whether the eyes are open or closed, cerebellar disease should be suspected.
Testing for Stereognosis
Here we might ask the patient to differentiate shapes, textures, or similarly configured smallobjects (e.g., a paper clip versus a safety pin) by touch. The patient also may be asked toidentify numbers written on the fingertip or palm of the hand (graphesthesia), to maketwo-point discriminations, or to localize stimuli applied to various parts of the face, limbs,or torso.
Testing for Vibration
This procedure basically calls for the application of a tuning fork (256 cps) to bony prominencesof the distal upper and lower extremities. The examiner must perform trials withand without the tuning fork vibrating to assure reliability and comprehension of the test.The patient is instructed to indicate whether the tuning fork is vibrating when touchingthe limbs, and if it is when the vibration appears to stop. A vibratory sensory level can bedetermined by starting distally and working one’s way proximately up the limb. The latterprocedure would be important if a neurologist expects a peripheral neuropathy.
Testing for Temperature
The patient is asked to discriminate between objects of different temperatures. The examinershould ensure that the temperatures are readily discriminable, but neither is extreme. Testtubes filled with warm and cool water make reasonable testing devices.In all cases, the examiner always should compare performances on the right versus theleft side of the body. One should be alert to the possibility of cortical neglect as well as oldcentral or peripheral injuries or disease processes that might have an effect on peripheralprocesses, such as peripheral neuropathy secondary to diabetes or chronic alcohol abuse.
LESIONS AFFECTING THE ASCENDING ANDDESCENDING TRACTS
Posterior column lesions. The posterior columns mainly transmit impulsesarising in the proprioceptors and cutaneous receptors. If they are dysfunctional,the individual can no longer feel the position of his or her limbs; norcan he or she recognize an object laid in the hand by the sense of touch alone oridentify a number or letter drawn by the examiner’s finger in the palm of thehand. Spatial discrimination between two stimuli delivered simultaneously atdifferent sites on the body is no longer possible. As the sense of pressure is alsodisturbed, the floor is no longer securely felt under the feet; as a result, bothstance and gait are impaired (gait ataxia), particularly in the dark or with theeyes closed. These signs of posterior column disease are most pronouncedwhen the posterior columns themselves are affected, but they can also be seenin lesions of the posterior column nuclei, the medial lemniscus, the thalamus,and the postcentral gyrus.