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.
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.
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.
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.
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 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.
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 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.
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.)
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.)
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.)
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.)