Somatosensory Systems Part I—Somatosensory Pathways of Body




Learning Objectives



  1. Explain the location and function of the ascending somatosensory pathways for the body.



  2. Compare the types of sensory receptors and fibers involved in each pathway.



  3. Identify the location of the first-, second-, and third-order neurons for each path.



  4. Explain the location of fiber decussation for each path and its clinical significance resulting from damage to the spinal cord and brainstem.



  5. Predict the neurological deficits resulting from damage to a specific tract at various segmental levels.



  6. Compare the fiber type and path of the direct and indirect anterolateral paths.



  7. Explain the functional significance of the different points of termination for the direct and indirect anterolateral pathways.



  8. Describe the functional relationship in sensory receptor distribution for regions of the body, face, and fingers to the pictorial depiction in the sensory map of the cortex.



  9. Compare the similarities and differences between the pathways responsible for conscious proprioceptive processing and unconscious proprioceptive processing.




Overview of Ascending Somatosensory System




  • The present chapter describes the three pathways which comprise the ascending somatosensory system for the body: the anterolateral pathway, the dorsal column-medial lemniscus pathway, and the spinocerebellar system. A similar system, known as the trigeminal system, conveys somatosensory input from the head and face and is covered in Chapter 13.



Function of the Somatosensory System




  • The somatosensory system is responsible for conveying sensory input concerning the body’s external environment, its position, and its internal state from specific peripheral sensory receptors to the central nervous system (CNS). Specific types of mechanoreceptors, thermoreceptors, nociceptors, and chemoreceptors which are found in the skin, joints, muscles, and organs transmit sensory input through general somatic afferent (GSA) fibers and general visceral afferent (GVA) fibers.




    • GSA fibers carry sensory modalities such as touch, vibration, pressure, positional sense, pain, and temperature from the body (soma) and head, while GVA fibers convey sensations of pain, temperature, stretch, and distention from the viscera.



    • GSA and GVA fibers serve as the functional afferent component of spinal nerves. Some cranial nerves also carry these fibers. Specifically, the trigeminal nerve (CN V) carries GSA fibers, while the facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) carry both GSA and GVA fibers.



  • Based on the type of sensory modality transmitted, afferent fibers of the body enter the dorsal root of the spinal cord and ascend along specific sensory pathways to the cortex or cerebellum for conscious and unconscious processing.



Transmission of Conscious and Unconscious Sensations


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  • Although most sensory input is consciously perceived and involves neural processing in the cerebral cortex, some sensory information is unconsciously processed by the cerebellum or may be transmitted to other regions of the CNS as part of a reflexive response and autonomic control.



  • In general, GSA fibers that arise from exteroceptive and some proprioceptive receptors in the body transmit conscious sensations to the cerebral cortex through two principal anatomical pathways ():




    • Anterolateral system (ALS)



    • Dorsal column-medial lemniscus (DCML) pathway




      • The ALS consists of several associated tracts which convey sensations of pain, temperature, itch, and crude (nondiscriminative) touch from nonencapsulated nociceptors, mechanoreceptors, and thermoreceptors found throughout the body. The anterolateral pathway is located in the anterior and lateral funiculi and is important in the discriminative localization of painful stimuli.



      • The DCML pathway transmits tactile sensations involving fine, discriminative touch (two-point discrimination), pressure, and vibration, along with conscious proprioceptive input concerning the static position and movement of joints and limbs. Mechanosensory and proprioceptive input is mediated through several types of encapsulated and nonencapsulated mechanoreceptors. The pathway lies in the dorsal column of the spinal cord.



  • In addition to these conscious pathways, several ascending pathways transmit unconscious somatic afferent information.




    • The spinocerebellar system, which includes several paths, conveys unconscious proprioceptive input from encapsulated mechanoreceptors such as muscle spindles and Golgi tendon organs along GSA fibers to the cerebellum. The spinocerebellar pathway plays an important role in maintaining posture while standing or sitting and coordinating motor responses.



    • Several ascending tracts of the ALS, which are also known as indirect ALS paths, terminate in regions other than the primary somatosensory cortex and do not reach conscious perception. The indirect ALS pathways convey nociceptive stimulation and play a role in raising the level of awareness of painful stimuli. Components of this pathway are also involved in the affective aspects of processing of painful stimuli as well as modulating pain.



  • Unconscious visceral input which plays a role in mediating autonomic visceral reflexes is transmitted by GVA fibers from visceral receptors to various autonomic control centers throughout the CNS. Sensations associated with viscera are generally unconscious; however, under pathological circumstances, visceral input may produce conscious viscerosomatic sensations, such as pressure or pain which serves as a protective response.




    • The conscious perception of painful visceral sensations is often detected in specific regions of the body which differ from the actual source of the visceral stimulus. These painful sensations, known as referred pain, are discussed in more detail in Chapter .



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Fig. 12.1 Transmission of conscious and unconscious sensations.



Overview of ascending somatosensory tracts: conscious and unconscious processing






















































Overview of ascending somatosensory tracts: conscious and unconscious processing

Conscious Somatosensory Pathways


Name of Path


Location in Spinal Cord


Receptor


Function of Tract


Termination


Direct anterolateral pathway


(aka


lateral


spinothalamic tract)



(neospinothalamic tract)


Anterolateral funiculus


Primarily lateral funiculus


Nociceptor; thermoreceptors


Lightly myelinated Aδ fibers (Type III)


Mediates sharp localized, pain; cool temperature; itch,


crude (light) (nondiscriminative) touch


Conscious cortical processing; immediate awareness of pain


Contralateral primary somatosensory cortex via VPL of thalamus


Dorsal column-medial lemniscus


Posterior funiculus


Tactile mechanoreceptors:


Meissner, Merkel, Pacinian, Ruffini


Proprioceptors:


muscle spindles; Golgi tendon organ


Aα (Type Ia, Ib) Type II (Aβ)


Mediates tactile two-point discrimination; conscious proprioception regarding limb positions


Conscious cortical processing


Contralateral


primary somatosensory cortex via VPL of thalamus


Unconscious Somatosensory Pathways


Name of Path


Location in Spinal Cord


Receptor


Function of Tract


Termination


Spinocerebellar


tracts


Lateral funiculus


Proprioceptors


Muscle spindles Golgi tendon organ


Aα (Type Ia, Ib) Type II (Aβ)


Mediates unconscious proprioception necessary for coordination of motor movements; postural maintenance and reflex motor responses


Unconscious processing in ipsilateral


cerebellum


Indirect


anterolateral pathways


(aka


anterior spinothalamic, spinomesencephalic,spinoreticular etc.—paleospinothalamic and archispinothalamic)


Anterolateral funiculus


Primarily anterior funiculus


Polymodal nociceptors


Unmyelinated C fibers


Mediates arousal and the affective/emotional component of pain


Dull, diffused, poorly localized pain and crude touch


Unconscious processing


Termination of input in regions other than primary somatosensory cortex


Abbreviation: VPL, ventral posterolateral.


Note: **Some textbooks consider the anterolateral system or spinothalamic tract to be divided into a lateral and an anterior spinothalamic tracts. The lateral spinothalamic tract is associated with fast pain, whereas anterior spinothalamic tract is associated with slow pain that is poorly localized due to points of termination.



Common Features of the Conscious Somatosensory Pathways


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Comparison of conscious pathways




































































Comparison of conscious pathways

Structure and Location


Dorsal Column-Medial Lemniscus (DCML)


Anterolateral Spinothalamic Tract


Functional sensory modality


Discriminative fine touch;


Vibration


Pressure


Positional sense (static)


Kinesthetic sense (movement)


Nondiscriminative (crude) touch


Temperature (hot, cold)


Sharp, fast, acute, pricking pain (Aδ fibers)


Slow, dull, aching, chronic pain (C fibers)


Type of peripheral sensory receptor


Encapsulated, low-threshold mechanoreceptors (LTM)


Meissner, Pacinian, Ruffini (Type II/Aβ), muscle spindle (Type Ia/Aα and II fibers), Golgi tendon organ (Type Ib/Aα)


Nonencapsulated (free nerve endings)


Afferent fiber type


Myelinated Aβ (Type II); Aα (Type Ia, Ib)


Myelinated Aδ; unmyelinated C fibers


First-order neuron/location


Large, pseudounipolar neuron


Dorsal root ganglion (DRG)


Small, pseudounipolar neuron


Dorsal root ganglion (DRG)


Point of entry to spinal cord


Medial part of dorsal root


Lateral part of dorsal root


Name of primary afferent fiber tracts and location in CNS


Dorsal column of spinal cord


Fasciculus gracilis (T6 below)


Fasciculus cuneatus (T6 above)


Ascends ipsilateral from spinal cord to medulla


Posterolateral tract of spinal cord (tract of Lissauer)


Ascend descends 1, 2-3 and segments ipsilateral in spinal cord before synapsing on second-order neurons


Second-order neuron/ synapse location


Dorsal column nuclei—lower medulla


Multipolar projection neurons




  • Nucleus gracilis—medially located



  • Nucleus cuneatus—laterally located


Gray matter of dorsal horn


Multipolar projection neurons




  • Rexed lamina I—receive primarily Aδ



  • Rexed laminae II and V—receive primarily C fibers


Point of decussation


Internal arcuate


Lower medulla


Collective crossing of all fibers


Anterior white commissure of spinal cord


Cross at each level of synapse on secondary neurons


Secondary fiber tracts/location


Forms medial lemniscus tract


Ascends contralateral from lower medulla to thalamus


Forms anterolateral spinothalamic tract (part of spinal lemniscus in anterior and lateral funiculus)


Ascends from spinal cord entry to contralateral thalamus


Third-order neuron/


Synapse location


Ventral posterolateral (VPL) nucleus of thalamus—relay nuclei


Ventral posterolateral (VPL) nucleus of thalamus—relay nuclei


Primary termination in cortex


Primary somatosensory cortex—postcentral gyrus


Brodmann’s areas 3a, 3b, 2, and 1


Primary somatosensory cortex—postcentral gyrus


Brodmann’s areas 3a, 3b, 2, and 1


Reflexive response


Monosynaptic muscle stretch reflex


Golgi tendon reflex


Polysynaptic withdrawal (pain) reflex


Result of lesion or damage to tract


*depends on lesion location


Impairment of proprioception, kinesthesia, and tactile discrimination


Result: Sensory ataxia (uncoordinated movements—more pronounced with eyes closed)


Deficits in stereognosis (astereognosis)—difficulty recognizing shape and texture


Lesion in spinal cord: Ipsilateral loss due to injury occurring prior to decussation


Lesion in medulla: Contralateral loss due to injury occurring after decussation


Diminished crude touch, loss of temperature, itch, tickle sensations


Result: Loss of pain (analgesia) and temperature


Lesion in spinal cord or higher:


Contralateral loss due to lesion occurring after fiber decussation


Abbrevviation: CNS, central nervous system.




  • The anterolateral and dorsal column-medial lemniscus pathways follow distinct anatomical routes; however, both ascending pathways share several common attributes:




    • Both pathways consist of bundles of axonal tracts that interconnect a chain of three neurons, sequentially arranged as a relay circuit.



    • The neurons are referred to as first-, second–, and third-order neurons ().



    • The sensory neuron cell body is known as the primary afferent neuron or first-order neuron. The first-order neuron for the somatosensory system of the body resides in the dorsal root (spinal) ganglia (DRG). For the region of the head, it resides in cranial sensory ganglia (see Chapters 13 and 14).




      • Morphologically, primary afferent neurons are pseudounipolar neurons with a single peripheral process that terminates as peripheral sensory receptors or on peripheral targets, and a single, centrally projecting process that transmits sensory input toward the CNS. The central process enters the dorsal root of the spinal cord to synapse on second-order neurons.



      • Both the ALS and DCML somatosensory pathways exhibit a somatotopic arrangement of nerve fibers from the point of entry into the spinal cord to the point of termination in the somatosensory cortex ( , ). The point-to-point connection within the pathway creates a somatotopic map that allows for the specific localization and identification of the stimulus modality.



    • The cell bodies of second-order neurons reside in the gray matter on the same (ipsilateral) side as the primary afferent neuron; however, the location of the second-order neuron varies according to the type of sensory receptor neuron and sensory modality transmitted.




      • Primary afferent fibers that transmit conscious proprioceptive input and discriminative touch synapse on second-order mechanoreceptive and proprioceptive neurons in the medulla, while those conveying pain, temperature, or crude touch synapse on nociceptive neurons in the Rexed lamina of the dorsal horn of the spinal cord.



    • Axons of second-order neurons cross the midline (decussate) at specific locations and then ascend on the contralateral (opposite) side to synapse on third-order neurons found in the thalamus.




      • The point of decussation differs between the two paths and has important clinical relevance for determining the point of injury: the DCML decussates in the medulla, whereas the fibers of the anterolateral pathway decussate in the spinal cord.



    • Nerve fibers from third-order neurons remain on the contralateral side and project from the ventral posterolateral (VPL) nuclei of the thalamus to the primary somatosensory cortex so that information entering the CNS on one side is consciously processed by the cerebral cortex of the opposite side.




      • The thalamus serves as a sensory relay station and all sensory information processed in the somatosensory cortex must first synapse in the thalamus before projecting to the cortex.



      • Somatosensory fibers from the thalamus pass through the posterior limb of the internal capsule to terminate in the primary somatosensory cortex.



    • The primary somatosensory cortex (S1) is found within the postcentral gyrus of the parietal lobe and represents the cortical region responsible for conscious sensory perception. The region of the primary somatosensory cortex is subdivided into four regions known as Brodmann’s areas 3a, 3b, 1, and 2 ().



    • Thalamocortical projections are somatotopically arranged so that neurons receiving input from the body are in the lateral portion of the ventral posterior nucleus (VPL) of the thalamus, while fibers of the head are in the medial part of the ventral nucleus (VPM).



    • The somatotopic organization of thalamocortical fibers establishes within the primary somatosensory cortex a sensory map of the body and head that reflects a specific point-for-point location of sensory input from specific regions of the body. In the sensory map, the somatotopic arrangement of input from the foot, leg, upper trunk, and face is medial to lateral ().




      • The sensory map, also known as sensory homunculus, depicts a distorted representation of each body region. The amount presented in the cortical region is proportional to the receptor density. The oral cavity, face, and fingertips have a greater receptor density and so the corresponding cortical region appears larger than other regions.



Ascending tracts of the spinal cord




















































Ascending tracts of the spinal cord

Tract


Location


Function


Neurons


1


Anterior spinothalamic tract


Anterior funiculus


Pathway for crude touch and pressure sensation


1st afferent neurons located in spinal ganglia; contain 2nd neurons and cross in the anterior commissure


2


Lateral spinothalamic tract


Anterior and lateral funiculus


Pathway for pain, temperature, tickle, itch, and sexual sensation


3


Anterior spinocerebellar tract


Lateral funiculus


Pathway for unconscious coordination of motor activities (unconscious proprioception, automatic processes, e.g., jogging, riding a bike) to the cerebellum


Projection (2nd) neurons receive proprioceptive signals from 1st afferent fibers originating at the 1st neurons of spinal ganglia


4


Posterior spinocerebellar tract


5


Fasciculus cuneatus


Posterior funiculus


Pathway for position senses (conscious proprioception) and fine cutaneous sensation (touch, vibration, fine pressure sense, two-point discrimination)


Conveys information from upper limb (not present below T3)


Cell bodies of 1st neuron located in spinal ganglion; pass uncrossed to the dorsal column nuclei


6


Fasciculus gracilis


Conveys information from lower limb


Source: Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy. Second Edition. © Thieme 2017. Illustrations by Markus Voll and Karl Wesker.



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Fig. 12.2 (a, b) Overview of sensorimotor integration. (a) Schematic illustration of the somatosensory pathway of incoming impulses from primary (first-order) afferent neurons that form a relay circuit with secondary in the brainstem and tertiary afferent (sensory) neurons. (b) The tertiary neurons for most somatosensory pathways reside in the thalamus and terminate on neurons in the somatosensory cortex. Cortical processing is necessary for conscious perception. An interneuron links this with an upper motor neuron in the motor cortex which then descends through the white matter to a lower motor neuron in the spinal cord. The lower motor neuron projects to effector organs. Most descending motor tracts bypass the thalamus. (a: Modified with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)



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Fig. 12.3 The location of the ascending somatosensory tracts. Transverse section through the spinal cord. The anterolateral spinothalamic (ALS) tract and dorsal column-medial lemniscus (DCML) systems exhibit a somatotopic arrangement of nerve fibers from the point of entry to the point of termination. The ALS transmits pain, temperature, and crude (nondiscriminative light) touch, whereas the DCML transmits fine (discriminative) touch and conscious proprioception. The spinocerebellar tracts carry unconscious proprioception to the cerebellum. An example of the somatotopic arrangement of the input into the fasciculi gracilis and the cuneatus of the dorsal column (posterior column) is illustrated. (Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)



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Fig. 12.4 (a) Primary somatosensory cortex and parietal association cortex are shown; left lateral view. (a) The primary somatosensory cortex (S1) lies within the postcentral gyrus of the parietal lobe. (b) The four Brodmann’s areas within the cortex that receive somatosensory input are numbered in the sectional view. The parietal association cortex receives information from both sides of the body, whereas the primary somatosensory cortex receives input from the contralateral head and body. The perioral region is the exception and is represented bilaterally. (Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)



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Fig. 12.5 The somatotopic representation of the body and face in the primary somatosensory cortex (anterior view of right postcentral gyrus). Somatosensory information originating from each body region projects to a specific cortical area of the postcentral gyrus and creates a topographical map depicted as the sensory homunculus. Within this sensory map, the foot, leg, upper trunk, and face exhibit a medial to a lateral arrangement. The cortical body regions are not proportionate to their actual size but in proportion to the receptor density. Note the axons of the sensory neurons ascending from the thalamus travel side by side with the axons forming the pyramidal tract (red) in the posterior limb of the internal capsule. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

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Sep 13, 2022 | Posted by in NEUROLOGY | Comments Off on Somatosensory Systems Part I—Somatosensory Pathways of Body
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