Sensation

Original Editors - Naomi O'Reilly Top Contributors -

Introduction[edit | edit source]

Humans can perceive various types of sensations, and with this information, our motor movement is determined. We become aware of the world by way of sensation. Sensations can also be protective to the body, by registering eg environmental cold or warm, and painful needle prick. All the daily activities carry associations with sensations.

SensoryProcessing.png

Broadly, these sensations can classify into two categories.

  1. General sensations which include touch, pain, temperature, proprioception, and pressure.
  2. Special Senses: Vision, hearing, taste, and smell which convey sensations to the brain through cranial nerves[1].

The peripheral nervous system (PNS) consists of sensory receptors that extend from the central nervous system (CNS) to communicate with other parts of the body. These receptors respond to changes and stimuli in the environment. Sense organs (made up of sensory receptors and other cells ) operate the senses of vision, hearing, equilibrium, smell, and taste[2].

Sensory system examination is necessary if the patient has a chronic disease, which may result in neuropathy, or by any spinal injury or accident followed by complaints of decreased sensation.[1]

Special Senses[edit | edit source]

  1. Senses.jpeg
    Sight: ability of the eye(s) to focus and detect images of visible light on photoreceptors in the retina that generate electrical nerve impulses for varying colors, hues, and brightness.
  • 2 types of photoreceptors: rods and cones.
    • Rods are very sensitive to light, but do not distinguish colors.
    • Cones distinguish colors, but are less sensitive to dim light. The inability to see is called blindness.

Vision plays an important role in balance and movement, therefore any deficits may have a huge impact on functional activities.

2. Hearing: the sense of sound perception. Mechanoreceptors in the inner ear turn vibration motion into electrical nerve pulses. The vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear that detect the mechanical motion of the fibers.

  • Sound can also be detected as vibrations conducted through the body by tactition. The inability to hear is called deafness or hearing impairment.
  • The sense of hearing is of paramount importance in spoken communication, and hearing loss can lead to problems with communication, decreased awareness of environmental clues and even social isolation. See eg Hearing in the Older Person
Smell.jpg

3.Taste: refers to the ability to detect substances such as food, certain minerals, poisons, etc. The sense of taste is often confused with the concept of flavor, which is a combination of taste and smell perception. Flavor depends on odor, texture, and temperature as well as on taste.

  • Humans receive tastes through sensory organs called taste buds, or gustatory calyculi, concentrated on the upper surface of the tongue. Five basic tastes exist: sweet, bitter, sour, salty, and umami. The inability to taste is called ageusia.

4.Smell: the olfactory system is the sensory system used for the sense of smell (olfaction). This sense is mediated by specialized sensory cells of the nasal cavity.  In humans, olfaction occurs when odorant molecules bind to specific sites on the olfactory receptors in the nasal cavity. These receptors are used to detect the presence of smell. They come together at a structure (the glomerulus) that transmits signals to the olfactory cortex in the brain. The inability to smell is called anosmia.

General Sensation or Somatosensory Senses[edit | edit source]

  • Somatosensation.jpg
    Touch: is a perception resulting from the activation of neural receptors in the skin, including hair follicles, tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure (firm, brushing, sustained, etc.).
    • The loss or impairment of the ability to feel anything touched is called tactile anesthesia.
    • Paresthesia is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary.
  • Pain or nociception (physiological pain): Signals nerve and other tissue damage.
  • Balance or equilibrioception: Allows the sensing of body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance.
  • Body awareness or proprioception: Provides the parietal cortex of the brain with information on the relative positions of the parts of the body.
  • Sense of time or chronoception: Refers to how the passage of time is perceived and experienced but is not associated with a specific sensory system. According to psychologists and neuroscientists, however, human brains have a system governing the perception of time.
  • Temperature sensing or thermoception: The sensation of heat and the absence of heat (cold).

Interoceptive senses: if also considered, sensation can be expanded to include stretch (as in muscles or organs like the lungs), oxygen and carbon dioxide sensing, pH sensing, and more.[2]

Sense to Perception[edit | edit source]

Optical illusion disc with man pumping water-corrected.gif

Sensation and perception are two distinct stages of processing during human sensing. Sensation is a function of the low level, biochemical, and neurological mechanisms that allow the receptor cells of a sensory organ to detect an environmental stimulus. Image R: An optical illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality

  • Stimuli from the environment (distal stimuli) are transformed into neural signals, which are then interpreted by the brain through a process called transduction. Transduction can be likened to a bridge connecting sensation to perception.

The neural signals are transmitted to the brain and processed. The resulting mental recreation of the distal stimulus is the percept. The sound stimulating a person’s auditory receptors is the proximal stimulus, and the brain’s interpretation of this as eg the loud explosion is the percept.

  • All perception involves signals in the nervous system that result from physical stimulation of the sense organs. eg vision involves light striking the retinas of the eyes, smell is mediated by odor molecules, and hearing involves sound waves.

Perception is not the passive receipt of these signals, rather a process of: organisation; identification; interpretation.

  • Although the senses were traditionally viewed as passive receptors, the study of illusions and ambiguous images, has demonstrated that the brain’s perceptual systems actively influence sensory systems in an attempt to construct useful representations of our environment[2].

Somatosensory Pathways[edit | edit source]

Sensory Receptor Nerve Fibre Sensory Pathway
Light Touch

Meissner's Corpuscle

Merkel's Discs

Aβ Fibers Spinothalamic
Pressure

Pacinian Corposcles

Aβ Fibers
Pain / Pin Prick

Free Nerve Endings

Aδ Fibers (Nociceptors of Neospinothalamic Tract)
C Fibers (Nociceptors of Paleospinothalamic Tract)
Temperature

Thermoreceptors

Aδ Fibers (Cold Receptors)
C Fibers (Warmth Receptors)
Vibration

Meissner's Corpuscle

Pacinian Corposcles

Aβ Fibers Dorsal Column
Proprioception

Muscle Spindle

Ruffini Endings

Aβ Fibers
Touch Localization

Meissner's Corpuscle

Merkel's Discs

Aβ Fibers
Stereognosis Hemispheric
Graphesthesia


Sensory Examination[edit | edit source]

The evaluation of sensation is highly dependent on the ability and desire of the patient to cooperate.

  • Sensation belongs to the patient (i.e., is subjective) and the examiner must therefore depend almost entirely on their

One of the principle goals of the sensory exam is to identify meaningful patterns of sensory loss.

  • Basic testing should sample the major functional subdivisions of the sensory systems.
  • The patient's eyes should be closed throughout the sensory examination.

Exam in this order

  1. Superficial (Exteroceptive) sensation
  2. Proprioceptive(deep) sensation
  3. Combined cortical sensations.
  • If the superficial sensation is impaired then some impairment is also seen in deep and combined sensations.
  • Sensory tests are done from the distal to the proximal direction.[3]
Superficial Sensation Deep Sensation Combined Cortical Sensation
  1. Pain Perception
  2. Temperature Awareness
  3. Touch Awareness
  4. Pressure Perception
  1. Kinesthesia Awareness
  2. Vibration Perception
  1. Stereognosis Perception
  2. Tactile Localization
  3. Two-Point Discrimination
  4. Double Simultaneous Stimulation
  5. Graphesthesia
  6. Recognition of Texture
  7. Barognosis

Pain Perception

It is also known as sharp/dull discrimination. To test this sensation, the sharp and dull end of any objects like a safety pin, a reshaped paperclip, or neurological pin is used. The sharp and dull end is randomly applied perpendicular to the skin, should not be applied too close to each other or in a too rapid manner to avoid the summation of impulses. The patient is asked verbally to indicate sharp/dull when a stimulus is felt. All areas of the body should be tested. After testing the instrument should be sterilized or disposed.

Temperature Awareness

Two test tubes with stoppers are required for this examination; one should be filled with the cold water (between 5°C to 10°C) and warm water( 40°C to 45°C). It should be taken care that the temperature should remain within this range for accuracy. The test tubes are randomly placed in contact with the skin area to be tested. All skin surfaces should be tested. The patient is asked to respond hot and cold after each stimulus application.

[4]

Touch Awareness

A piece of cotton, camel-hair brush, or tissue is used to perceive the tactile touch input. Light touch or stroke is applied in the area to be tested. The patient is asked to indicate where he/she recognizes that a stimulus has been applied .

[5]

Pressure Perception

The therapist's fingertip or a double-tipped cotton swab is used to apply a firm pressure on the skin surface. This test can also be administered using the thumb and finger to squeeze the Achilles tendon. The patient is asked to indicate when an applied stimulus is recognized.

Kinesthesia Awareness

Awareness of movement is known as kinesthesia. The Therapist passively moves a joint through a relatively small range of motion and the patient is asked to describe the direction of movement. The patient can also respond by simultaneously duplicating the movement with the opposite extremity.

Proprioception Awareness

Proprioception includes position sense and awareness of joint at rest. The joint is moved through a range of motion and held in static position by the therapist, the patient is asked to describe the position either verbally or by demonstrating on another limb.

Vibration Perception

The perception of a vibratory stimulus is tested by placing the base of the vibrating tuning fork on the bony prominence( sternum, elbow, ankle). Generally, the tuning fork should be of 128Hz. If there is impairment patient will be unable to distinguish between a vibrating and nonvibrating tuning fork. Therefore, there should be a random application of vibrating and nonvibrating stimuli.

[6]

Stereognosis Perception

Tactile object recognition is determined in this test. A familiar object of different shape and size are required like keys, coins, combs, safety pins, pencils). A single object is placed in a hand and the patient manipulates it to identify the object and say it verbally. For speech impairment patients sensory testing shield can be used.

[7]

Tactile Localization

The test checks the ability to localize touch sensation on the skin.This test is not performed in isolated manner rather it is done in combination with pressure perception or touch awareness.

Two-Point Discrimination

It determines the ability to perceive two points applied to the skin simultaneously. Aesthesiometer or the circular two-point discriminator are the devices to test. The two tips of the instrument are applied to the skin simultaneously with the tip spread apart. With each successive application, the two tips are gradually brought closer together until the stimuli are perceived as one. The small est distance between the stimuli that is still perceived as two distinct points is measured.

Double Simultaneous Stimulation(DSS)

DSS examines the ability to perceive a simultaneous touch stimulus on opposite sides of the body; proximally and distally on a single extremity; or proximally and distally on one side of the body.

Graphesthesia(Traced Figure Identification)

The ability to recognize letters, numbers, or designs traced on the skin is examined using fingertip or the eraser end of the pencil. the patient is asked verbally the figures drawn on the skin.

Recognition of Texture

The test examine the ability to differentiate among various textures like cotton, wool, or silk.

Barognosis( Recognition of weight)

For the test different weights are used. the therapist may choose to place a seres of different weights in the same hand one at a time, place a different weight in each hand simultaneously.

[8]

The sense of hearing is of paramount importance in spoken communication, and hearing loss can lead to porblems with communication, decreased awareness of environmental clues and even social isolation.
Hearing in the Older Person

References[edit | edit source]

  1. 1.0 1.1 Gadhvi M, Waseem M. Physiology, Sensory System.2019 Available from: https://www.ncbi.nlm.nih.gov/books/NBK547656/(accessed 18.3.2021)
  2. 2.0 2.1 2.2 Lumen learning Sensation Available from: https://courses.lumenlearning.com/boundless-ap/chapter/sensation/ (accessed 19.3.2021)
  3. O'Sullivan SB, Schmitz TJ. Physical rehabilitation fifth edition. FA Davis.
  4. Clinical Examinations Videos. Temperature perception test - Spinothalamic tracts test. Available from:https://www.youtube.com/watch?v=7it5E9OBl2k [last accessed 21/12/2020]
  5. Clinical Examinations Videos. Neurological sensory examination- Light touch and pain(pin prick test). Available from: https://www.youtube.com/watch?v=XVOVpq-41BY [last accessed 21/12/2020]
  6. Clinical Examinations Videos. Neurological peripheral vibration test. Available from:https://www.youtube.com/watch?v=iEfyHSm2fCA [last accessed 21/12/2020]
  7. Onlinemedicalvideo. Normal Sensory Exam ; Stereognosis. Available from:https://www.youtube.com/watch?v=6fyiGUAg2GY [last accessed 21/12/2020]
  8. Clinical Examinations Videos . Two point discrimination - Graphesthesia - Stereognosis. Available from:https://www.youtube.com/watch?v=xhygnHkC-9Y [last accessed 21/12/2020]