Chapter 7 - Somatosensory Systems
The nervous system has many more sensory fibers and sensory pathways than motor fibers. This stems from the importance of properly understanding the environment prior to responding. These ascending tracts provide the bridge between the inputs from the environment and the organizing centers of the brain that provide the complexity of our responses. Somatosensory systems include the receptors and pathways for transmission of sensory information from the soma to the portions of the brain that need to integrate it and act upon it. While much of this is conscious, there are also ascending systems that convey unconscious information involved in coordination (proprioception) and brain stem reflexes.
There are several different modalities that fall under the broad topic of "exteroception". These sensations include well-localized touch 2-point discrimination), light touch, pain, temperature and vibration sense. These sensations can be tested clinically. Proprioception is the ability to detect the position of the body in space. This may be consciously perceived, such as with joint position sense, or it may be a sensation that is not perceived consciously, such as from muscles and ligaments.
We will first discuss the pathways for conscious perception before considering unconscious sensation. These pathways for conscious somatic sensation, at their simplest, require three neurons (and two relay sites) from the periphery all the way to termination the cerebral cortex. These steps are often described as first, second and third order neurons in the sensory pathway. The signal can be modified at each of the relay points (nuclei).
The pathways for detection of pain, temperature and very light (poorly localized) touch are distinct from those conveying well-localized touch, pressure, vibration and conscious proprioception. We will first consider the pathways for pain, temperature and light touch (see figure 10).
Thinly myelinated and unmyelinated nerve fibers (first order nerve fibers) convey pain, temperature and light touch modalities from the periphery. As with all sensations from the body, the neuronal cell bodies of these sensory axons are located in the dorsal root ganglion. The central processes from these small neurons enter the spinal cord in the lateral part of the dorsal root. As these lightly myelinated and unmyelinated nerve fibers enter the spinal cord, they give rise to collateral branches that ascend and descend several segments in a region near the cap of the dorsal horn of the spinal cord, called the tract (or zone) of Lissauer. These collateral branches enter the dorsal horn and synapse in the substantia gelatinosa, located in the dorsal part of the dorsal horn. This synapse occurs within several segments of entering the spinal cord.
Pain, temperature and light touch nerve fibers synapse on several cell types in the dorsal horn. Some of these are involved in local circuitry, including interneurons involved in local reflexes, particularly the withdrawal (flexor) reflex. They also synapse on neurons involved in sensory transmission. There are two groups of transmission neurons in the dorsal horn. There are marginal neurons that are located in the most dorsal part of the dorsal horn. Since these neurons respond almost exclusively to noxious inputs, they have been termed nociceptive specific neurons and appear to be most involved in signaling the presence and location of pain. There is a population of transmission neurons located more deeply in the dorsal horn, termed wide dynamic range neurons (WDR) because they respond with increasing frequency of discharge to more intense stimuli. These neurons have been studied intensively since they are involved in signaling the intensity of pain and also because their response characteristics can be altered by certain inputs in a process similar to "memory".
The substantia gelatinosa of the dorsal horn contains enkephalinergic interneurons and other inhibitory neurons that are involved in modulating pain. Descending pathways containing serotonin and norepinephrine are capable of activating some of these inhibitory local circuits, as can certain direct somatosensory inputs to the spinal cord (figure 11).
The axons from the transmission neurons (i.e., the second order neurons in the pathway) comprise the spinothalamic tract or "anterolateral system" that conveys pain and temperature sensation (figure 10). These axons decussate in the anterior white commissure of the spinal cord within several segments of their origin, although about ten percent of sensory fibers remain ipsilateral. The decussating axons enter the anterolateral portion of the white matter of the spinal cord where they ascend the spinal cord as the spinothalamic tract. Historically, this tract has been divided into a lateral and ventral spinothalmic tract, with the lateral being more for pain and the ventral for thermoreception. However, the distinction is not as clear as would be suggested by this nomenclature and most authors consider them together. The fibers that arise from the sacral spinal cord are closest to the surface of cord with sensory fibers from the upper portion of the spinal cord layered more towards the center of the cord. These fibers comprise the spinothalamic tract, which extends through the ventrolateral part of the tegmentum of the brain stem to terminate mainly in the ventral posterolateral nucleus of the thalamus. There are additional terminations in intralaminar and posterior thalamic nuclei.
While there is some evidence for crude pain perception in the thalamus, most pain perception and discrimination occurs at the level of the cerebral cortex. The third order neuron for transmission of pain, temperature and light touch to the cerebral cortex begins in the VPL of the thalamus. Axons from these thalamic neurons follow the thalamic radiations to terminate in the primary somatosensory cortex (Brodman's areas 3, 1, 2 of the postcentral gyrus) in a topographic manner (see Chapter 9).
Well-localized touch, pressure, vibration and joint position sense follows a different pathway (see figure 12). Large, myelinated sensory nerve fibers that conduct these modalities are located in the medial aspect of the dorsal root as it enters the spinal cord. Some of these sensory fibers terminate directly at the level of the spinal cord where they participate in reflex responses to touch and also are involved in inhibiting pain transmission through the substantia gelatinosa. However, the great majority of these large-diameter sensory fibers enter the dorsal column of the spinal cord.
These sensory axons ascend the length of the spinal cord to the level of the brain stem. Collectively, these fibers are called the dorsal columns since they comprise the entire region of the dorsal funiculus of the spinal cord. The afferent fibers from the lower half of the body (the fasciculus gracilis) are medial to those from the upper body (the fasciculus cuneatus) since fibers add to the dorsal columns from the lateral side. Note that the sensory fibers in the dorsal columns have not synapsed and are therefore are still the primary or "first order" sensory fibers.
The first order sensory fibers in the dorsal column tracts ascend to the level of the medulla. Here they synapse in the dorsal column nuclei, which include the nucleus gracilis and nucleus cuneatus located within the dorsal aspect of the caudal medulla. Projection neurons of the nucleui gracilis and cuneatus, that is, the second order neurons, give rise to axons that arch ventrally within the medulla (internal arcuate fibers). These axons decussate and reach the medial lemniscus. The medial lemniscus is a tract consisting of axons that connect the dorsal column nuclei with the ventral posterolateral nucleus of the thalamus.
The medial lemniscus has a topographic organization. The axons that arise from neurons in the nucleus gracilis, that is, axons that convey information from the legs, enter the ventral part of the medial lemniscus in the medulla with fibers from the nucleus cuneatus traversing more dorsal portions of the tract. As the medial lemniscus reaches the pons, it changes its orientation, such that the fibers from the legs are in the lateral portion of the tract. This shift in orientation is depicted in figure 12.
The medial lemniscus terminates in the VPL of the thalamus. The third order neuron in the VPL relays to the primary somatosensory cortex (postcentral gurus; cortical areas 3, 1, 2) in the topographic manner discussed above. The portions of the parietal lobe immediately posterior to the postcentral gyrus are responsible for synthesizing cortical information into a recognizable pattern. Damage to this somatosensory "association cortex" produces an inability to interpret a sensory signal, even though it can be detected (somatosensory agnosia).
There are other ascending somatosensory pathways. Spinoreticular fibers are mostly collateral branches of the spinothalamic tract. These fibers terminate in the reticular formation and participate in various reflex reactions to pain as well as alerting reactions. These responses can include autonomic reflex responses and some behavioral reactions to pain, for example. Additionally, relay from the reticular formation to more rostral aspects of the nervous system is an alternate route for pain to reach the cerebral cortex. The spinotectal tract that terminates in the superior colliculus contributes to reflex orientation of the head and eyes toward a somatic stimulus. Spinal projections to the periaqueductal gray can activate pain-suppression pathways.
In addition to these long ascending pathways, there are fibers that connect one segment of the spinal cord to neighboring portions of the cord. These fibers are located in the deepest part of the white matter, immediately adjacent to the gray matter. Therefore, they have the name of propriospinal fibers. These fibers participate in patterning intersegmental reflexes but also provide a route whereby crude sensory information can reach the higher centers of the brain.
There are several pathways for unconscious proprioception. These transmit information necessary for the maintenance of normal muscle tone and posture as well as for coordination. Much of this information arises from muscle stretch and tension receptors and also from spinal interneurons that participate in reflexes and regulate motor output. While this is important information if movements are to be performed smoothly, there is very little conscious awareness of these sensations. The pathways for unconscious proprioception primarily terminate in the ipsilateral cerebellum, in two topographic maps in the spinocerebellum (anterior lobe and paramedian lobule). Some of these pathways, such as the dorsal spinocerebellar tract and the cuneocerebellar tract, require only 2 neurons, while some pathways are polysynaptic. Many of the polysynaptic pathways synapse on neurons of brain stem nuclei that, in turn, project to the cerebellum (so-called, "precerebellar" nuclei).
The dorsal spinocerebellar tract is the best studied of these tracts (figure 13). Muscle stretch and tension nerve fibers (i.e., afferent fibers from muscle spindles and Golgi tendon organs), terminate in the nucleus dorsalis (of Clarke). This is a collection of neurons located in the central gray matter of the spinal cord between the C8 and L3 levels. Muscle proprioceptive fibers from all muscles that are innervated below the level of the neck terminate within this nucleus. Since the nucleus dorsalis does not exist below the L3 level. Muscle stretch and tension information entering the cord with the sacral and lower lumbar nerve roots enters the dorsal columns and ascends the spinal cord to reach the upper lumbar region before synapsing. Therefore, damage to the lower portions of the dorsal columns, or to the large muscle stretch and tension afferent nerve fibers can produce loss of coordination of the legs that has the appearance of cerebellar damage.
Projection neurons within the nucleus dorsalis give rise to second order sensory fibers that ascend the ipsilateral dorsal part of the lateral funiculus of the spinal cord. At the level of the medulla these fibers enter the cerebellum through the inferior peduncle. They terminate in a topographic fashion within the spinocerebellum and provide the cerebellum with muscle stretch and tension information.
Since the nucleus dorsalis does not extend into the cervical spinal cord above C8, the dorsal spinocerebellar tract does not convey information from the upper limb. Therefore, there is another pathway for muscle stretch and tension information from the arms, the cuneocerebellar tract (figure 13). Afferent nerve fibers arising from muscle spindles and Golgi tendon organs in this limb join the fasciculus cuneatus and ascend the cervical spinal cord to reach the medulla. These fibers terminate in the lateral (external) cuneate nucleus. The second order neurons in this pathway enter the ipsilateral inferior cerebellar peduncle, terminating topographically in the spinocerebellum (see Chapter 8).
The ventral spinocerebellar tract arises from neurons in the intermediate gray matter of the thoracic and lumbar spinal cord. These spinal interneurons participate in reflexes and regulate activity of the motor neurons in the cord. Axon collateral branches from these interneurons decussate in the anterior white commissure and ascend in the ventral part of the lateral funiculus as the ventral spinocerebellar tract. This tract is adjacent to the spinothalamic tract. These axons can be followed through the brain stem to the level of the rostral pons, where they turn dorsally and join the superior cerebellar peduncle. They enter the cerebellum and, for the most part, recross to terminate ipsilaterally. These fibers are important since they convey the level of activity in spinal cord interneurons. Since these interneurons are part of the pathway for most voluntary movements, it is important that the cerebellum have access to this information in order to participate in coordination of activity.
There are several other direct and indirect spinocerebellar pathways. These pathways provide necessary information regarding the current status of reflex pathways, as well as muscle tone, length and tension. This is all critical information to permit the cerebellum to coordinate motor activity.
There are several additional ascending spinal pathways with which you should have some passing familiarity. The spinocervical tract is an important pain pathway in some species, although it's role in humans is less well understood. This tract relays in the lateral cervical nucleus and projects to the VPL of the thalamus. The spinohypothalamic and spinoamygdalar tract provides sensory input to areas of the nervous system involved in controlling autonomic, endocrine and emotional responses.
It is important to know the location of the principal sensory pathways in the spinal cord. These are depicted in figure 14.