SOMATOSENSORY SYSTEMS: Conscious and Non-Conscious Proprioception Kimberle Jacobs, Ph.D. Divisions of Somatosensory Systems The pathways that convey sensory modalities from the body to consciousness are divided into two groups: the anterolateral system (ALS) and the dorsal column system. These systems are named after the locations of their tracts in the spinal cord. These two systems provide a basis for the classical notion of a division of somatic sensation into protopathic (anterolateral) and epicritic (dorsal column) systems. The protopathic system has been thought to be a more primitive system subserving gross, diffuse recognition of stimuli (low spatial resolution) and functioning in an attention-provoking capacity with the activation of withdrawal, autonomic, and emotive reflexes. By contrast, the epicritic system has been considered to be more recently evolved, transmits information faster, and functions in the fine discrimination (high resolution) of sensory inputs, particularly in regard to the spatial and temporal features of a stimulus. The ALS consists of the lateral spinothalamic and anterior spinothalamic tracts. At the level of the pons, the ALS lies just lateral to the medial lemnsicus, and is called the spinal lemniscus. Another important difference between these two is the level at which the first synapse is made (between the first and second order neurons). For the older, cruder, slower system, the ALS, the synapse is made in the spinal cord. For the faster, more discrete, newer system, the Dorsal Columns, the synapse is made in the medulla. A major contributor to the overall speed of these pathways is the conduction speed of the first order fibers. Speed of conduction in first order fibers is due to the amount of myelination and diameter of the axon, with larger more heavily myelinated fibers being faster. Proprioception is subserved by the fastest fibers (Aα), which are associated with muscle spindles and golgi tendon organs. Fine Discrimination Touch is subserved by the next fastest fibers (Aβ), which are associated primarily with mechanoreceptors and muscle spindles. Fast pain, which can be described as sharp and pricking, is subserved by Aδ fibers, and associated with free nerve endings, and other noci -, thermo-, and mechanoreceptors. The Aδ fibers are small diameter, thinly myelinated fibers. Slow pain, which can be described as a burning type of pain, is subserved by C fibers, associated with free nerve endings, and other noci -, thermo-, and mechano-receptors. The C fibers are very small diameter, unmyelinated fibers. The larger, more heavily myelinated fibers conducting touch and proprioception, that will ascend in the dorsal funiculus before synapsing in the medulla, enter the spinal cord medial to the dorsal horn.
The thinner, lightly myelinated and unmyelinated axons enter the spinal cord more laterally, just over the gray matter of the dorsal horn, where they are about to synapse. Touch from the body - DORSAL COLUMN - MEDIAL LEMNISCUS SYSTEM Subserves fine discrimination touch, deep touch (pressure), proprioception, and vibration sensitivity. A. Receptors. Mechanoreceptors and proprioceptors: Meissner s Corpuscles, Pacinian Corpuscles, Merkel s discs, Ruffini end organs, muscle spindles, GTO) B. First-Order Neurons. Pseudounipolar neurons in the dorsal root ganglia. Central processes enter the spinal cord in the medial division of the dorsal root. At levels below mid-thoracic (T7 and below) the fibers enter the posterior funiculus and form the fasciculus gracilis, thus this conveys information about the lower body. In the upper thoracic and cervical segments (T6 and above) they occupy the fasciculus cuneatus. The posterior intermediate sulcus separates the gracile and cuneate fasciculi in the upper thoracic and cervical spinal cord regions. A lesion eliminating only the fasciculus cuneatus would therefore eliminate fine discrimination touch from the ipsilateral arms and upper body. C. Second-Order Neurons. The ascending primary afferent fibers terminate upon second-order neurons in the nucleus gracilis (lower body representation) and nucleus cuneatus (upper body representation) in the caudal medulla. Neurons in the gracile and cuneate nuclei give rise to axons known as internal arcuate fibers. These fibers course ventromedially, cross the midline (sensory decussation), and form the medial lemniscus on the contralateral side. Within the medial lemniscus, the lower body is represented more ventrally than the upper body. D. Third-Order Neurons. In the diencephalon, fibers of the medial lemniscus terminate in the ventral posterolateral (VPL) nucleus of the thalamus. Fibers from the VPL of the thalamus enter the posterior limb of the internal capsule and terminate in somatotopic register along the dorsomedial aspect of the postcentral gyrus in Brodmann s areas 3, 1, and 2. Brodmann s area 3 is Primary Somatosensory Cortex, in the post central gyrus. E. Fourth-Order Neurons. Cells within layer IV of the Primary Somatosensory Cortex are the fourth order neurons. Key Questions: a) Where are the cell bodies of origin for the Medial Lemniscus? Answer: Contralateral nucleus gracilius (lower body) and nucleus cuneatus (upper body).
b) Where does the Medial Lemniscus terminate? Answer: Ipsilateral VPL of the thalamus. c) Where does the crossing occur for this pathway? Answer: In the brainstem (lower medulla). d) What symptoms are associated with a lesion of the medial lemniscus? Answer: Loss of Fine Discrimination Touch in the contralateral Body. Figure 1: Dorsal Columns Pathway subserving fine discrimination touch. Figure from Fix,JD, 2000, High-Yield Neuroanatomy, Philadelphia, Lippincott Williams & Wilkins.
Clinical Syndromes 1. Tabes Dorsalis Degeneration of myelinated, large diameter axons in the dorsal columns. This occurs in late stages of Syphilis. Because the lesion is bilateral, the deficits are bilateral. The symptoms are bilateral loss of Fine Discrimination Touch, while pain and temperature sensations remain intact (unaltered). 2. Brown Sequard Syndrome incomplete spinal cord lesion characterized by a clinical picture reflecting hemisection of the spinal cord, often in the cervical cord region. It is rare for the entire hemisection of the cord to be affected, but does occur in some cases. More often incomplete hemisection is found. The size of the lesion will affect what symptoms are observed. A small dorsal lesion will destroy the dorsal columns, resulting in loss of fine discrimination touch ipsilaterally at the level of the lesion and below. A larger lesion that includes lateral funiculus, will affect the lateral spinothalamic tract, resulting in loss of contralateral pain and temperature sensation from the level of the lesion and below. In addition for complete hemisections, there will be a focal region of ipsilateral loss of pain and temperature, due to destruction of Lissauer s tract, incoming fibers, and the fibers crossing the midline. If the lesion includes destruction of the ventral horn, there will be motor effects of ipsilateral spasticity and weakness. Trigeminal Pathways specifically, fine discrimination touch from the head and face. A. Receptors. Free nerve endings, Merkel s discs, hair receptors and receptors with encapsulated endings. B. First-Order Neurons. Pseudounipolar cell bodies in the trigeminal ganglion, located outside the CNS, similar to dorsal root ganglion neurons. Peripheral processes (component of axon that extends from the sensory receptor to the ganglion) come from 3 branches of the trigeminal nerve, the Opthalamic, Maxillary, and Mandibular. The central portion of axons transmitting fine discrimination touch information will be distributed to the Principal (also called the Main, also called the Chief) Trigeminal Nucleus. C. Second Order Neurons. Second order neurons are located in the Trigeminal Nucleus (Principal component). Principal Sensory Nucleus of V (also called the Main or Chief Trigeminal Nucleus) located mid pons. This nucleus and subsequent pathway is similar to the dorsal column system. The axons of the second order neurons project to the thalamus through 2 paths. Most
axons from the Main Sensory Nucleus of V cross the midline and then ascend as the Ventral Trigeminothalamic tract (axons) to ultimately synapse in the VPM nucleus of the Thalamus. Some axons project ipsilaterally as the Dorsal Trigeminothalamic Tract, ultimately synapsing in the ventral posterior medial (VPM) nucleus of the ipsilateral Thalamus. NOTE: Remember that below the level of the pons, the ventral trigeminal thalamic tract will contain only pain and temperature information (from the contralateral head and face). The axons of the Principal Trigeminal Nucleus Neurons will be added to this tract at the level of the pons. Therefore at the level of the pons and above, the ventral trigeminal tract will contain information about fine discrimination touch as well as pain and temperature, all from the contralateral head and face. D. Third-Order Neurons. The dorsal and ventral Trigeminothalamic tracts terminate on neurons in the ventral posteromedial (VPM) nucleus of the thalamus. The axons of neurons in VPM project through the posterior limb of the internal capsule to terminate in the ventrolateral portion of the postcentral gyrus of the cerebral cortex, which is primary somatosensory cortex, and can be identified as Brodmann s areas 3, l, and 2. E. Fourth-Order Neurons. Layer IV Neocortical neurons of Primary Somatosensory Cortex (Bordmann s areas 3, 1, and 2). Key Questions: a) Where are the cell bodies of origin for the Ventral Trigeminothalamic tract? Answer: Contralateral Trigeminal Nucleus (Main and Spinal Components). b) Where does the Ventral Trigeminothalamic tract terminate? Answer: Ipsilateral Ventral Posterior Medial Nucleus of the Thalamus. c) What symptoms would result from lesion of the Ventral Trigeminothalamic tract? Answer: Loss of pain, temperature and touch senses in the contralateral face and head. Cerebellar Tracts - Nonconscious Proprioception from the Body The cerebellum is a coordinator of movement, but does not initiate movement. It receives information about limb position, joint angles, muscle length and tension from the same receptor and primary afferent types that subserve conscious proprioception in the dorsal column system. The general
rule is that the cerebellum receives information from the ipsilateral side of the body. Four pathways carry proprioceptive information from the body to spinal cord and ultimately the cerebellum. A fifth pathway carries proprioceptive information from the head to the cerebellum. A. Dorsal (Posterior) Spinocerebellar Tract First-order neurons (include groups Ia and II fibers from neuromuscular spindles, group Ib fibers from Golgi tendon organs, touch receptors, and pressure receptors) are pseudounipolar neurons in the dorsal root ganglion and convey coded information from the receptors directly to the second-order neurons in the nucleus dorsalis of Clarke (located in lamina VII of segments T1 to L2 only). The second-order axons from this nucleus ascend ipsilaterally as the uncrossed dorsal spinocerebellar tract. This tract ascends through the posterolateral aspect of the lateral funiculus of the spinal cord, enters the inferior cerebellar peduncle, and terminates in the vermis of the ipsilateral anterior and posterior lobes of the cerebellum. This pathway has a role in the coordination of individual muscles of the lower trunk and lower extremity during postural adjustments and movements. B. Cuneocerebellar Tract This is the upper body equivalent to the dorsal spinocerebellar tract. First-order neurons are located in the dorsal root ganglia. Central processes enter the spinal cord, ascend in the ipsilateral fasciculus cuneatus, and terminate upon second-order neurons located in the accessory cuneate nucleus (also called the lateral cuneate nucleus). This nucleus, located in the lower medulla just lateral to the cuneate nucleus, is the rostral equivalent to the nucleus dorsalis of Clarke. Axons of accessory cuneate neurons course ipsilaterally as the cuneocerebellar tract through the inferior cerebellar peduncle and terminate in the ipsilateral cerebellar cortex. This pathway is involved in the coordination of individual muscles in the upper trunk and upper extremity. C. Ventral (Anterior) and Rostral Spinocerebellar Tracts Afferents from the lower extremity have first-order neurons in the dorsal root ganglia and enter the spinal cord via the dorsal roots terminating upon second-order neurons in the dorsal horn of the spinal cord (Rexed s laminae VI and VII) in the lumbosacral (L3-S1) segments. The axons of the second-order neurons cross the midline in the anterior white commissure of the spinal cord and ascend as the ventral spinocerebellar tract located in the anterolateral aspect of the lateral funiculus. Some fibers of this tract are uncrossed. The tract courses through the lateral brainstem and along the dorsal surface of the superior cerebellar peduncle before terminating in the vermis of the anterior lobe of the cerebellum. This pathway has a role
in the general coordination of muscles of the lower part of the body during movement (e.g., walking). The counterpart of this pathway for the innervation of the upper extremity is the rostral spinocerebellar tract, which originates from neurons in lamina VII in the cervical enlargement (C4-C8). Figure 2: Spinocerebellar pathways. The Dorsal and Ventral Spinocerebellar tracts carry information from the lower body. The Rostral and Cuneocerebellar tracts carry information from the upper body. Note information from the face is carried in separate trigeminal pathways to the cerebellum. Figure from Haines,DE, 1997, Fundamental Neuroscience, New York, NY, Churchill Livingstone. D. Trigeminocerebellar Fibers Central processes of neurons in the trigeminal ganglion that innervate jaw muscle receptors, periodontal afferents, and the temperomandibular joint course in the spinal trigeminal tract and terminate in the pars interpolaris subdivision of the spinal nucleus of V. It resembles cytoarchitectonically the accessory cuneate nucleus and, like the latter, many of its cells project to the cerebellar cortex via the inferior cerebellar peduncle. Neurons in the
mesencephalic nucleus of V also project to the cerebellum through the superior cerebellar peduncle Rostral SpinoCerebellar Tract Lateral Cuneate Nucleus CuneoCerebellar Tract Dorsal SpinoCerebellar Tract Figure 3: Location of Spinocerebellar pathways within the medulla. These pathways convey nonconscious proprioception from the body to the cerebellum. Ventral SpinoCerebellar Tract Figure 4: Pathways that convey nonconscious proprioception from the head to the ipsilateral cerebellum.