Chapter 2: Somatosensory Systems
Patrick Dougherty, Ph.D., Department of Anesthesiology and Pain Medicine, MD Anderson Cancer Center (content provided by Chieyeko Tsuchitani, Ph.D.) Reviewed and revised 07 Oct 2020
The somatosensory systems inform us about objects in our external environment through touch (i.e., physical contact with skin) and about the position and movement of our body parts (proprioception) through the stimulation of muscle and joints. The somatosensory systems also monitor the temperature of the body, external objects and environment, and provide information about painful, itchy and tickling stimuli. The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. For example, the posterior column-medial lemniscal pathway carries discriminative touch and proprioceptive information from the body, and the main sensory trigeminal pathway carries this information from the face. Whereas, the spinothalamic pathways carry crude touch, pain and temperature information from the body, and the spinal trigeminal pathway carries this information from the face.
This first series of chapters on somatosensory systems concentrates on the somatosensory systems that provide accurate information about the location and temporal features of stimuli and about sharp pain, tactile stimuli and the position and movement of body parts. This chapter describes somatosensory stimuli, the sensations produced when they are applied, and the cutaneous, muscle, and joint receptors that are responsible for initiating the perceived somatic sensations. Subsequent chapters describe the pathways processing other pain, temperature, crude touch and visceral sensations.
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2.1 Somatic Stimuli
Modality Specificity in the Somatosensory System. The somatosensory systems process information about, and represent, several modalities of somatic sensation (i.e., pain, temperature, touch, proprioception). Each of these modalities can be divided into sub-modalities, as shown in Table 1 (e.g., pain into sharp, pricking, cutting pain; dull, burning pain; and deep aching pain). Discriminative touch is also subdivided into touch, pressure, flutter and vibration. Each of these sensations (i.e., sub-modalities) is represented by neurons that exhibit modality specificity. That is, when a somatosensory neuron is stimulated naturally (e.g., by skin warming) or artificially (e.g., by electrical stimulation of the neuron), the sensation perceived is specific to the information normally processed by the neuron (i.e., warm skin). Consequently, a “warm” somatosensory neuron will not respond to cooling of the skin or to a touch stimulus that does not “warm” the skin. The somatosensory receptor and its central connections determine the modality specificity of the neurons forming a somatosensory pathway.
Table I The Sensory Modalities Represented by the Somatosensory Systems
|Modality||Sub Modality||Sub-Sub Modality||Somatosensory Pathway (Body)||Somatosensory Pathway (Face)|
|Pain||sharp cutting pain||Neospinothalamic||Spinal Trigeminal|
|dull burning pain||Paleospinothalamic|
|deep aching pain||Archispinothalamic|
|Touch||itch/tickle & crude touch||Paleospinothalamic|
|discriminative touch||touch||Medial Lemniscal||Main Sensory Trigeminal|
|Proprioception||Position: Static Forces||muscle length|
|Movement: Dynamic Forces||muscle length|
Tactile Stimuli. Tactile stimuli are external forces in physical contact with the skin that give rise to the sensations of touch, pressure, flutter, or vibration. We normally think of touch as involving minimal force on-or-by an object that produces very little distortion of the skin. In contrast, pressure involves a greater force that displaces the skin and underlying tissue. Time varying tactile stimuli produce more complex sensations such as object movement or object flutter (20 to 50 Hz) or vibration (100 to 300 Hz). An initial clinical examination of discriminative touch often involves testing the vibratory sense by applying a 128 Hz tuning fork over a bony prominence.
Proprioceptive Stimuli.1 Proprioceptive stimuli are internal forces that are generated by the position or movement of a body part. Static forces on the joints, muscles and tendons, which maintain limb position against the force of gravity, indicate the position of a limb. The movement of a limb is indicated by dynamic changes in the forces applied to muscles, tendons and joints. An initial clinical examination of proprioception often involves testing the position sense by having the patient, with eyes closed, touch one finger with another after the target finger has been moved.
Proprioception is critical for maintaining posture and balance. Somatosensory proprioceptive cues are combined with vestibular proprioceptive cues and visual cues to control motor responses to changes in body/head position. During a clinical examination, the Romberg test requires the patient to maintain balance while standing with feet together and eyes closed. It tests whether the proprioceptive components are working properly when the visual cues are missing and proprioceptive cues are the major sources of information.
Sharp Cutting Pain Stimuli. Painful (nociceptive) stimuli are tissue-damaging sources of energy that may be external or internal to the body surface. Sharp, cutting pain is the sensation elicited on initial contact with the painful stimulus. The sensation of dull, burning pain may follow as a consequence of tissue inflammation. An initial clinical examination of the pain sense often involves testing sharp, cutting pain sensitivity by asking the patient, who has her/his eyes closed, what they feel when pricked with a pin. Pain mechanisms and pathways are described in detail in later chapters.
2.2 Introduction to Peripheral Organization of Somatosensory Systems
Peripheral Somatosensory Neurons. The cell bodies of the first-order (1°) somatosensory afferent neurons2 are located in posterior root or cranial root ganglia (i.e., are part of the peripheral nervous system, Figure 2.1). The 1° afferents are pseudounipolar cells. The cell body gives rise to a single process that divides to form a peripheral axon and a central axon. The peripheral axon travels to and ends in the skin, muscle, tendon or joint and the central axon travels to and ends in the central nervous system.
Somatosensory Receptor Organ. The receptors of most sensory systems are located in specialized sensory receptor organs (e.g., the photoreceptors in the eye and the auditory and vestibular hair cells in the inner ear) or within a restricted part of the body (e.g., the taste buds in the moy2kcenter.org and the olfactory receptors in the olfactory mucosa of the nose). For the tactile component of the somatosensory system, the skin covering the entire body, head and face functions as the touch receptor organ, whereas joint tissues, muscles and tendons act as the proprioception receptor organs. These sensory receptor organs “house” the somatosensory receptors and deliver the somatosensory stimuli to the receptors.
Sensory Receptors. Specialized sensory receptor cells (e.g., the photoreceptors of the eye) are located in specialized receptor organs, produce receptor potentials, contain synaptic specializations, and release neural transmitters (Figure 2.2). Specialized sensory receptors may be modified neurons (e.g., the photoreceptors and olfactory receptors) or modified epithelial cells (e.g., taste receptors and the auditory and vestibular hair cells).