The somatosensory system is one of the critical components of the human anatomy that has various functions. This system is primarily responsible for the function of touch that creates conscious awareness for our surrounding environment. The skin is the main organ responsible for touch stimulation. It has a series of sensory receptors distributed unevenly to assist in detecting thermal, mechanical, and chemical sensation. Receptors can be categorized under four main groups, pain receptors, thermoreceptors, mechanoreceptors and proprioceptors, each functioning distinctly to fulfil a distinct role. The somatosensory pathway is responsible for transmitting information to the brain, where tactile signals are processed. Affective touch is also vital in psychological awareness of our bodies both during infancy and adulthood. The cognitive perception of the shape and size of our body is directly affected by touch senses.
Touch Sensorimotor System
The touch sensorimotor system is significant to the development and functioning of human beings. Touch is a sensory system whose primary organ is the skin. As the largest organ, the skin has a series of nerve endings that have specific responses to different aspects of environmental stimuli such as temperature, pain, and itch. The combination of receptors and nerve endings is referred to as the somatosensory system. This system includes four primary receptors: pain receptors, thermoreceptors, mechanoreceptors and proprioceptors (Jacobs, 2018). The somatosensory system has a complex physiological and psychological organization that helps determine the brain's response to external changes in the environment, including creating a cognitive perception of the body.
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Physiological Underpinnings
Pain receptors are responsible for detecting stimuli that cause tissue damage in the body. Over three million pain receptors exist within the body's organs, mainly the muscle, skin and bones. These receptors can detect chemical, mechanical and thermal changes in stimuli that cause damage, such as burns and cuts. Chemical stimuli can be poison from stings, while mechanical stimuli can be cuts from objects. Pain receptors have a significant role or protecting the body from harm by warning the person to avoid harmful stimuli.
Thermoreceptors are found in the skin and can be categorized as either cold or hot receptors. Cold receptors sense changes in skin temperatures of below 95 degrees Fahrenheit. Cold receptors peak at 77 degrees Fahrenheit and cease stimulation at 41 degrees Fahrenheit. On the other hand, hot receptors are stimulated when skin temperature is at 86 degrees Fahrenheit and peak at 113 degrees Fahrenheit. Beyond these temperatures, the pain receptors are stimulated to warn the body of potential damage.
Mechanoreceptors are responsible for sensing vibration and pressure stimuli. The four types of receptors that perform these functions are Meissner's corpuscles, Pacinian corpuscles, Merkle's disks and Ruffini corpuscles (Jacobs, 2018). The combination of Meissner's corpuscles and Merkel's disk is predominantly present in fingertips, palms and other non-hairy parts of the skin. This combination optimizes texture since Meissner's corpuscles, and Merkle's disk is slowly and rapidly adapting receptors, respectively.
Pacinian and Ruffini's corpuscles are primarily present in the dermis' muscles, tendons, and deep areas. They are responsible for sensing limb movements and vibrations along with tendons. It is this pair of receptors that assists in performing physical actions. Proprioceptors are primarily responsible for the detection of muscular tension within the body. Proprioceptors are located within the muscles, joints and tendons, and their vital purpose is assisting in the dynamic planning of movements that require multiple organs.
Structure
The touch sensorimotor system structure has three categories of neurons that include the primary, secondary and tertiary. The primary neuron exists in the periphery of the pathway and includes receptors that detect thermal and texture stimuli (Hayward, 2018). The primary neuron is located within the spinal ganglion of the cranial ganglia. The secondary neuron has ascending axons that connect both sides of the brainstem or spinal cord where it is located. Termination of these neurons occurs in the cerebellum or the thalamus. The tertiary neuron exists in senses of posture, pain, and touch. It has its cellular body in the cerebellum in detecting posture and the thalamus through to the parietal lobe in touch sensory.
The mechanoreceptors exist in the periphery and detect texture which is then transmitted as an impulse by neurons to the central nervous system. The afferent neurons that detect and transmit these sensory stimuli are of different structure and sizes. The properties of a neuron are determined by the sensory stimuli that it detects. The texture is detected by myelinated, thick and fast neurons, while pain is detected by unmyelinated, thin and slow neurons (Jacobs, 2018). There also exist ascending pathways in the spinal code that are connected to the brain. The postcentral gyrus is the primary target in the brain that receives signals from various sensory input receptors. The sensory homunculus is located in this area and acts as a sensory map. The cortical homunculus is a depiction of the human body's distribution showing the sensory alignment in the brain.
Function
The touch sensorimotor system is essential for the proper operation of the body's performance. The signals from the skin are utilized to feel and handle objects with significant ease and coordination. In cases where the sense of vision is absent, the sense of touch can accurately identify surrounding objects. Sense of touch also helps the brain detect the physical properties of objects such as temperature, texture, and weight. The body is also warned of physical danger through the sense of touch and can avoid harm caused by burns and injury. In general, touch is an essential sensorimotor system in exploring the physical environment alongside the vision system.
The three areas of the somatosensory system, namely the periphery, the spinal cord and the brain, perform different functions. The periphery has sensory receptors that detect different stimuli. Mechanical stimuli on the skin's surface are detected through keratinocytes of the epidermis layer of the skin. The changes in the environment's aspect can cause harm to the body if these changes reach extreme levels. For example, extremely hot temperatures can cause burns, and cold temperatures can cause shock on vital systems of the body. Extreme pressure on the body by external objects can cause damage to organ tissues.
The periphery detects these changes in the environment and transmits the signal to the somatosensory pathway to avert the risk of harm or injury. Afferent neurons of the somatosensory pathway send signals that the periphery has detected to the somatosensory cortex for processing. The postcentral gyrus functions to create perception from the transmitted signals relating to movements of body parts, pain and itch. This perception contributes to creating a conscious experience that determines human behaviour. Numbness may be a sign of a malfunctioning touch sensorimotor system.
Psychological Underpinnings
The sense of touch is critical in the positive psychological implication and the body's precautionary response to harm. It has been proved that infants' positive development is contributed by touch (Narvaez et al., 2019). Empathic responses are evoked by the somatosensory cortex when we experience the perception of pain in others. Touch may also increase oxytocin levels in the brain and is often a beneficial therapeutic experience in emotionally distressed people (Crucianelli & Filippetti, 2018). This sensory information from physical touch detected by receptors evokes emotional reactions. Encoding this information is different from other information like pain and has similar neurological responses to emotions from sound and vision senses. Physical touch may elicit activity in the anterior cingulate cortex causing increased adrenaline and feelings of pleasantness.
Cognitive-Perceptual
The neuron activity in the brain is known to be responsible for creating the perception of touch. Spatial processing of touch signals relies on the perceived location of the touch and visual representation of the body in the brain (Badde & Heed, 2016). When there is an absence of the concurrence between the two information, the body's perception is readjusted to fit the shape of the object causing the touch. Perception regarding touch is flexible, as is demonstrated by Pinocchio illusion where someone touches their nose with their eyes closed and strong vibrations acted on the triceps.
The brain creates an illusion of an outward movement of the arm and elongation of the nose. This illusion illustrates how touch input is significantly responsible for dynamic cognitive perception. There exists no sensory input that directly communicates the shape of the body to the brain. Therefore, touch becomes a vital sensorimotor system that determines perception. Tactile perception is complex to predict from the primary input since receptors are not evenly distributed on the skin. This uneven balance of receptors influences the perception of objects' size as they appear bigger in dense areas (Shao et al., 2016).
In conclusion, the somatosensory system has different mechanical, physiological, and psychological properties that effectively detect changes in the body's external environment. As illustrated, the touch senses are essential as a protective mechanism for the body by detecting and alerting the brain of pain and extreme temperatures. Different receptors are responsible for this detection function, and the sensorimotor pathway transports the signals to the postcentral gyrus. Touch also has psychological properties of eliciting an emotional response and contributing to infant development.
References
Badde, S., & Heed, T. (2016). Towards explaining spatial touch perception: Weighted integration of multiple location codes. Cognitive Neuropsychology , 33 (1-2), 26–47. https://doi.org/10.1080/02643294.2016.1168791
Crucianelli, L., & Filippetti, M. L. (2018). Developmental Perspectives on Interpersonal Affective Touch. Topoi , 39 (3), 575–586. https://doi.org/10.1007/s11245-018-9565-1
Hayward, V. (2018). A Brief Overview of the Human Somatosensory System. Springer Series on Touch and Haptic Systems , 29–48. https://doi.org/10.1007/978-3-319-58316-7_3
Jacobs, K. M. (2018). Somatosensory System. Encyclopedia of Clinical Neuropsychology , 3218–3223. https://doi.org/10.1007/978-3-319-57111-9_359
Narvaez, D., Wang, L., Cheng, A., Gleason, T. R., Woodbury, R., Kurth, A., & Lefever, J. B. (2019). The importance of early life touch for psychosocial and moral development. Psicologia: Reflexão e Crítica , 32 (1). https://doi.org/10.1186/s41155-019-0129-0
Shao, Y., Hayward, V., & Visell, Y. (2016). Spatial patterns of cutaneous vibration during whole-hand haptic interactions. Proceedings of the National Academy of Sciences , 113 (15), 4188–4193. https://doi.org/10.1073/pnas.1520866113
