Somatosensation

Original Editor - Amanda Ager

Top Contributors - Amanda Ager, Kim Jackson, Manisha Shrestha, Lucinda hampton, Tolulope Adeniji, Aminat Abolade and Carina Therese Magtibay  

Introduction

The body functions and interacts with its surrounding environment through the simultaneous inputs of our five senses; gustation (taste), ocular (vision), olfaction (smell), vestibular (balance) and auditory (hearing), respectively. However, it is often forgotten that we also have a "sixth sense", understood to be our sense of somatosensation. Somatosensation is an overarching sense which includes the sub-modalities of:

• Thermoception (temperature);
• Nociception (pain);
• Equilibrioception (balance);
• Mechanoreception (vibration, discriminatory touch and pressure);
• Proprioception (positioning and movement).

Collectively, these senses allow us to participate in our activities of daily living (ADLs) by guiding our movements, all the while minimizing the possibility of sustaining an injury.

Somatosensation. Retrieved from: Ager, A.L., Borms, D., Deschepper, L., Dhooghe, R., Dijkhuis, J., Roy, J.S., &  Cools, A.Proprioception and shoulder pain: A Systematic Review. J Hand Ther. 2019 Aug 31. pii: S0894-1130(19)30094-8. doi: 10.1016/j.jht.2019.06.002. 

What is Somatosensation?

Somatosensation is a mixed sensory category, and is mediated, in part, by the somatosensory and posterior parietal cortices. They underlie the ability to identify tactile characteristics of our surroundings, create meaning about sensations, and formulate body actions related to the sensations. These various sensations contribute to the somatic aspects of the body scheme as a basis for interacting with our environments.

Body representations are reliant on multi-sensory cortical and sub‐cortical areas, including particularly premotor cortex, posterior parietal cortex and the putamen.

It is also important to note that the experience of sensing often involves a more complex integration of somatosensory inputs, and may be influenced by emotional and social contexts as well.

Word origin: somato– from soma (body) + sensation.

Receptors

Somatosensory receptors are distributed throughout the entire body, rather than being concentrated at specialized locations. Different parts of the body will also have different concentrations of each type of receptor. The various types of receptors are able to sense various kinds of stimuli such as pressure against the skin, limb position, distention of the bladder, and body temperature. If a stimulus becomes so strong that it may be harmful, the somatosensory system is also responsible for feeling pain (nociception).

Sensory receptors are classified into five categories:

1.     Mechanoreceptors: Detects mechanical changes or deformations in tissues.

2.     Thermoreceptors: Detects changes in temperature.

3.     Proprioceptors: Detects changes to limb position (joint position sense), movement detection (kinesthesia).

4.     Pain receptors (nociceptors): Neural processing of injurious stimuli in response to tissue damage.

5.     Chemoreceptors: Detects changes and is responsive to chemical stimuli.

A single stimulus usually activates many receptors, and each receptor is capable of encoding stimulus features such as intensity, duration and direction. It is the central nervous system (CNS) which interprets the activity of the different receptors involved in the sensation and uses these interpretations to generate coherent perceptions.

Methods for Measuring Somatosensation

As a clinician, it is important to note that there is no direct way of measuring somatosensation. Even though some of the tests mentioned below target a specific aspect (sub-category) of somatosensation, it is not practical to measure one sense in isolation. Often times various types of receptors are active to varying degrees with different stimuli. The follow are the most common categorizations for quantifying aspects of somatosensation.

Proprioception

Proprioception in of itself is a complicated sense to measure. Proprioception predominates as the most misused term within the sensorimotor system. It has been incorrectly used synonymously and interchangeably with kinesthesia, joint position sense, somatosensation, balance, and reflexive joint stability.

Proprioception has several sub-categories, including kinesthesia (sense of movement) and joint position sense (active or passive).

Some researchers and clinicians also include the senses of muscle tension, joint pressure, sense of force and sense of velocity under the term proprioception.

Some clinical tests include:

• Brief Kinesthesia Test (BKT): The person is seated in a standard height chair (19 inches) in front of a standard height table (29 inches) with vision occluded by a curtain. The person reproduces targeted reaching movements from a starting location to a target location on a test page after being guided by the evaluator. The distance from the response location to the target location is recorded in centimeters. Additional equipment needed to administer the BKT includes a visual shield and a tape measure.

• Active Joint Repositioning Test: Asking the person to actively reproduce a joint angle, using the contralateral limb, in the absence of visual feedback.
• Passive Joint Repositioning Test: Passively demonstrating an angle (3-5 seconds), and asking the person to identify the correct angle when the clinician passively reproduces the angle again.

Proprioceptive testing can also be tested with a isokinetic dynamometer, like the Biodex System.

Tactile Discrimination

Tactile discrimination:  Tactile discrimination (such as two-point discrimination) can affect stereognosis, the ability to identify objects based on touch alone. Discriminative touch is also subdivided into touch, pressure, flutter and vibration.

Discriminator

Tactile Discrimination Test

Self-reported pain

The most common method of self-reported pain is using a Verbal Rating Scale (VRS). The person is asked to rate their perceived level of pain, by asking "On a scale from zero to 10, how much pain are you experiencing?" Zero represents "no pain at all" and 10 represents "the worst pain of your life".

Clinical Implications

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