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.
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.
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
Although it is convenient to understand and subdivide somatosensory senses and pathways for theoretical purposes, it is important to keep in mind that most somatosensory stimuli act simultaneously and concurrently. Quantifying different aspects of somatosensation continues to be a challenge for researchers and clinicians alike.
It is also 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 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, in the absence of visual feedback.
Proprioceptive testing can also be performed with a isokinetic dynamometer, like the Biodex System.
Tactile discrimination can affect stereognosis, the ability to identify objects based on touch alone. Discriminative touch is also subdivided into touch, pressure, flutter and vibration.
- Two-point discrimination (Touch Test) with a Discriminator (moving or static testing)
Sense of temperature
Usually, if the sensation of pain has been tested (or quantified) and is normal, there is no need to test temperature sensation. To test cold sensation, apply the cool prongs of your tuning fork to the skin. To test warm sensation, use a glass tube or other container filled with warm water.
For an objective method of measuring temperature, you can performing testing with test tubes of water and thermometers.
Sense of Temperature:
Expected findings - The person will should be able to discriminate between the hot and cold stimuli with 100% accuracy.
Equipment - Hot (115 to 120 °F) and cold (40 °F) metal probe thermometers or clean vials/test tubes filled with hot (115 to 120 °F) or cold (40 °F) water.
- Explain the procedure to the patient with his/her eyes open. For example, “I am going to touch various parts of your arms (or other body part) with these test tubes filled with hot and cold water, and when you feel the tube, tell me if it feels hot or cold.”
- Demonstrate the procedure with the patient’s eyes open until the patient understands the procedure to be performed. It is prudent to test in another area of the body, which you are not interested in testing (avoid over-stimulation in the part of the body of interest).
- The patient closes his/her eyes, or vision is otherwise occluded.
- Place the hot or cold test tube against the patient’s skin, making certain not to vary the surface area of contact between the test tube and skin or the pressure of contact.
- Have the patient identify each stimulus as hot or cold. Make certain the appropriate temperatures of the stimulus objects are maintained throughout testing.
- Vary the time between stimuli to avoid developing a rhythmic pattern.
- Repeat throughout areas being tested.
- Document your findings.
The most common method of quantifying patient-reported pain is using a Verbal Rating Scale (VRS). The patient is asked to rate their perceived level of pain. They are asked "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".
It is important to be aware of possible somatosensation deficits with your patients, as difficulties in tactile and proprioceptive discrimination can limit a person's spontaneous hand use and the ability to manipulate and grip objects, affecting quality of life and perhaps more importantly, safety.
Somatosensory impairments can be both acute (short-term) and chronic (longer-term). Injuries or lesions to areas of the brain involved with somatosensation, and/or damage to sensory receptors or the nerve pathways can cause somatosensory impairments. Conditions such as cerebral palsy, nerve injury or laceration, stroke, and nervous system disorders for instance spinal cord and brain injury, can result in somatosensory impairments.
Symptoms of somatosensory impairments may include, but are not limited to:
- Disrupted movement coordination
- Decreased sense of movement (kinesthesia, proprioception)
- Numbness to affected areas
- Pins and needles / tingling (or other similar neurological descriptions)
- Pain (affecting daily activities such as walking)
- Reduced sensation in one or more areas of the body, (tactile, pressure, temperature, vibration etc.).
As a clinician, it is important to understand that signs and symptoms can differ greatly according to the severity of the injury or condition.
Your treatment plan should be tailored to the specific sensory impairements of your patient. A multidisciplinary approach is often indicated as well as specific treatments from Occupational Therapy. As a Physiotherapist, you may be able to affect change in the following areas of rehabilitation:
- Motor control exercises
- Proprioception exercises (movement training, joint repositioning tasks)
- Balance training
- Sensitization exercises (temperature, vibration, textures etc.)
- Desensitization exercises (if the affected areas are over-stimulated)
- Gait (re)training
- Practicing both fine motor skills and gross motor skills
- Functional activities which target specific systems (for example: activities that stimulate the proprioceptive system include deep pressure, hugging, and climbing).
- Activities that stimulate the vestibular system which involve movement (such as swinging, jumping, and spinning).
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