Lab 5 - Somatosensory, Viscerosensory and Spinocerebellar Pathways

Quiz

In the next two sections identify the structures in each image. The following review of the ascending pathways covered in this section may be helpful before proceeding to the quiz.

Spinothalamic Pathways

The spinothalamic pathways are chiefly concerned with pain and temperature sensations. They also mediate tactile sensations (called simple or crude touch) that, in the absence of the posterior funiculi, are difficult to localize or identify as to their quality, i.e., as rough or smooth, vibrating, moving stimuli. The spinothalamic pathway can be subdivided into a "fast" conducting neospinothalamic pathway and a "slow" conducting paleospinothalamic pathway. The neospinothalamic pathway is involved in conveying the "sharp/cutting" pain elicited at the time tissue is damaged. The paleospinothalamic pathway is involved in conveying the "dull/burning" pain that accompanies the later inflammatory reaction in the damaged tissue. Temperature and simple touch information are probably carried in a subdivision of the paleospinothalamic pathway. The 1° afferents of the spinothalamic pathways send A­delta and C fibers to the periphery where they form free nerve endings in skin, muscle/tendon, joint capsules and viscera. The central processes of these neurons (pseudounipolar cells of the posterior root ganglia) enter the spinal cord in the lateral division of the posterior root. The axons branch and send fibers to the gray matter at the segment of entry and in the tract of Lissauer. The 1° afferents of the spinothalamic systems may end in the segment of entry or one or two segments up on 2° afferents in the nucleus posteromarginalis ­ the neospinothalamics or on 2° afferents in the substantia gelatinosa ­ the paleospinothalamics.

The axons of the nucleus posteromarginalis (neospinothalamic 2° afferents) cross in the anterior white commissure to collect in the spinothalamic tract within the contralateral anterior and lateral funiculi. The axons of the substantia gelatinosa (2° paleospinothalamic afferents) travel a short distance to terminate in or near the nucleus proprius on 3° afferents. The axons of some of the 3° nucleus proprius afferents remain uncrossed or cross in the anterior white commissure to collect bilaterally in the spinothalamic tracts of the anterior and lateral funiculi. These spinothalamic tracts are often referred to collectively as the anterolateral spinothalamic tract. Along their course to the thalamus, many of the paleospinothalamic afferents and some of the neospinothalamic afferents leave the spinothalamic tracts to terminate in the reticular formation or periaqueductal gray. The fibers terminating in the reticular formation are sometimes referred to as spinoreticular fibers and those ending in the periaqueductal gray as spinomesencephalic fibers. The remaining spinothalamic tract fibers either ascend up to the ventral posterolateral (VPL) nucleus of the thalamus to terminate on 3° (neospinothalamic) afferents, or to the intralaminar nuclei of the thalamus to terminate on 4° (paleospinothalamic) afferents. The neospinothalamic fibers do not terminate on the same group of VPL neurons that receive medial lemniscal fibers; i.e., pain, temperature, simple touch information is kept separate from the discriminative touch and proprioceptive information. Although the spinothalamic VPL neurons send their axons to the primary somatosensory cortex (i.e., the postcentral gyrus of the parietal lobe), destruction of this cortical area does not seem as detrimental to the appreciation of painful stimuli as it is to the appreciation of other somatic sensations. The projections of the intralaminar nuclei to more diffuse areas of the cerebral cortex are believed to play a role in a more poorly localized sense of pain.

Trigeminal Sensory Systems

The somatic innervation of the face area (i.e., the skin, muscles and joints of the face, the mucous membranes of the oral, nasal and ear cavities, the teeth, jaw, and the dura) is provided by four cranial nerves: The trigeminal nerve innervates most of the face area, while the facial, glossopharyngeal and vagal nerves provide somatic innervation to the oral and ear cavities and the skin covering the ear. The cell bodies of the 1° afferents are located in the ganglia of these nerves and their central processes enter the brain stem in the cranial nerve roots. Once in the brain stem, the fibers branch and send collaterals to cranial motor nuclei (reflex responses), to the reticular formation, and into the spinal trigeminal tract or to the main sensory trigeminal nucleus. The 1° afferents ending in the main trigeminal nucleus are homologues of the 1° afferents of the medial lemniscal pathway: They convey discriminative touch and proprioceptive information. The 1° afferents descending in the spinal trigeminal tract are the homologues of the 1° afferents of the spinothalamic tracts: They convey pain, thermal and simple touch information. The 2° main sensory trigeminal nucleus axons decussate and collect in the contralateral ventral trigeminothalamic tract. The 1° afferents in the spinal trigeminal tract leave the tract as it descends towards the spinal cord to terminate on 2° neurons in the spinal trigeminal nucleus. The axons of the spinal trigeminal nucleus also decussate and collect in the contralateral ventral trigeminothalamic tract. The innervation of the jaw joints and muscles is unique in having the cell bodies of the 1° afferents located within the brain stem in the mesencephalic trigeminal nucleus. Most of the fibers of the ventral trigeminothalamic tract ascend to and terminate in the ventral posteromedial (VPM) nucleus of the thalamus (main sensory trigeminal afferents and discriminative components of spinal trigeminal afferents) and also in the intralaminar nuclei of the thalamus (pain and non-discriminative components of spinal trigeminal afferents). The axons of the VPM 3° afferents ascend in the posterior limb of the internal capsule to the primary somatosensory cortex (the postcentral gyrus) of the parietal lobe. The axons of the intralaminar nuclei travel to more diffuse areas of the cerebral cortex.

Viscerosensory Systems

The internal organs of the body (i.e., the viscera), the blood vessels, and the membranes lining the body wall and in the body cavities are also richly innervated by free nerve endings. Most of the stimuli, which activate the afferents forming these free nerve endings, are never consciously perceived. They function to initiate and regulate visceromotor functions on an unconscious level. For example, normally we are unaware of our blood gas levels, heart rate, and blood pressure, although these are monitored continually by the visceral afferents. These afferents provide the autonomic system with the information necessary to regulate respiratory and cardiovascular mechanisms to maintain the required blood gas levels, heart rate and blood pressure. The viscerosensory pathways, we define here, involve those parts of the visceral afferent system which ultimately terminate in the cerebral cortex and which when stimulated result in a conscious sensoryperception. Gut and bladder distensions are normally perceived. The viscerosensory afferents may convey information about the distention and inflammation of tissues that result in a general sense of discomfort and pain. The viscerosensory system involves two pathways, a spinal pathway with 1° afferents in the posterior root ganglia of the sacral, upper lumbar and thoracic segments and a cranial pathway with 1° afferents in the cranial (inferior) ganglia of the glossopharyngeal and vagus nerves. The 1° afferents are pseudounipolar cells that give rise to A­fibers or C fibers. These fibers travel in the posterior or cranial root and in spinal somatic, sympathetic, or parasympathetic nerves to terminate on the target organs. The central processes of the posterior roots are believed to terminate in the substantia gelatinosa of the spinal cord, while those of the glossopharyngeal and vagus nerves terminate in the nucleus solitarius of the medulla. Within the spinal cord, the 2° substantia gelatinosa neurons send their axons a short distance to the nucleus proprius where they terminate on 3° neurons. It is believed that many of the 2° and/or 3° neurons are also innervated by somatic afferents, and thus aremultimodal in function. The axons of the 3° spinal viscerosensory afferents appear to ascend the neuraxis bilaterally in or near the spinothalamic tract and in the fasciculus proprius. Many of these fibers terminate in the brainstem reticular formation and midbrain periaqueductal gray. The remainder terminates in the intralaminar nuclei of the thalamus. The cranial viscerosensory component has the cell bodies of its 2° afferents located in the more caudal regions of the nucleus solitarius. Like the spinal component, most of the axons of these afferents end in the brainstem reticular formation and few reach the intralaminar nuclei of the thalamus. The thalamic afferents send their fibers in the posterior limb of the internal capsule and project diffusely to the to the cortex. A cortical primary receiving area for visceral sensation has been identified in the insular cortex near the insular gustatory receiving area.

Spinocerebellar Pathways

Three spinocerebellar pathways are examples of afferent pathways that are not sensory in function: They terminate in the cerebellum (and not in the cerebral cortex) and when stimulated do not elicit a perceived sensation. Damage to these pathways does not produce significant sensory deficits but does produce deficits in motor function. The two major spinocerebellar pathways are the posterior spinocerebellar pathway and the cuneocerebellar pathway. The 1° afferents are the posterior root ganglion cells that innervate muscle and joint receptors (primarily) and skin receptors (A­Ia, A­Ib, A­II and A­ß fibers). Branches of these afferents ascend in the gracile fasciculus of the sacral and lower lumbar spinal cord to the 2° afferents in the dorsal nucleus of Clarke which is located only at spinal cord levels L3 ­ C8. The 1° afferents from the upper lumbar and thoracic spinal cord pass directly to the dorsal nucleus of Clarke in their segment of root entry. These 1° afferents (i.e., from coccygeal to C8) terminate in the dorsal nucleus of Clarke, whose axons pass to the ipsilateral lateral funiculus to form the posterior spinocerebellar tract. This tract ascends the spinal cord, enters the inferior cerebellar peduncle in the medulla and terminates in the cerebellum. The 1° afferents from the cervical spinal segments pass into thecuneate fasciculus and also ascend the spinal cord without decussating. These afferents terminate on 2° afferents in thelateral cuneate nucleus of the medulla. The axons of these 2° afferents enter the inferior cerebellar peduncle to travel to and terminate in the cerebellum.

A third, minor, pathway is the anterior spinocerebellar pathway. It conveys more complex information from the leg to the cerebellum: Golgi tendon organs (predominantly), muscle spindle afferents, and cutaneous receptors send afferents to the 2° afferents, the spinal border cells, which are only found in the anterior horn of cord levels L5 to T12. These 2° afferents also synapse with spinal interneurons and descending tract fibers. The 2° anterior spinocerebellar afferents cross in the spinal cord and ascend the cord in the anterior funiculus as the anterior spinocerebellar tract. The anterior spinocerebellar tract ascends to the rostral pons where it joins the superior cerebellar peduncle to descend to the cerebellum. Within the cerebellum, the anterior spinocerebellar tract fibers cross again and terminate in the cerebellum ipsilateral to the body area represented.