Sensory Physiology
The sensory system starts with a physical stimulus interacting with a sensory receptor, usually located in the skin in a region with less or no hair (glabrous skin). The sensory receptions have specific intrinsic properties that allow the transformation of a physical stimulus into an electrical nerve impulse. This nerve impulse travels to the CNS for integration and interpretation, thus becoming a perception.
Somatosensory Receptors
There are five somatosensory system modalities: vibration, proprioception, touch, pain, and temperature. There are three types of sensory receptors: mechanoreceptors, thermoreceptors, and nociceptors. A particular sensory stimulus is felt best within a region where the density of receptors is highest of that particular modality, called a receptive field.
- Mechanoreceptors: mediate vibration, touch, proprioception, and joint position
- Nociceptors: mediate pain
- Thermoreceptors: mediate temperature sense
Mechanoreceptors
Mechanoreceptors have two types of adaptation, meaning a decrease in firing potentials with time even when under constant stimuli
- Rapidly adapting: Pacinian and Meissner's corpuscles and most hair follicle receptors. These respond best to rapidly changing stimuli, e.g. swift movement from touch, or vibration.
- Slowly adapting: Merkels discs and Ruffini end organs. These respond best to continuous stimuli for example prolonged stretch or pressure.
These different receptors respond best to specific ranges of stimuli. For vibration for example, Pacinian corpuscles respond best to 60 - 400 Hz, Meissner's corpuscles to 20 - 50 Hz, and Merkel discs to 5 - 15 Hz. Humans are most sensitive to vibration at frequencies of 200 - 250 Hz.
Proprioception is sensed through muscle spindles, Golgi tendon organs, and joint capsule receptors.
Nociceptors
- Main article: Nociception
There are three types of nociceptors: mechanical, thermal, and polymodal.
- Mechanical: respond to painful tactile stimuli typically in the presence of tissue damage
- Thermal: respond to extreme temperature change (below 5 or above 45 degrees)
- Polymodal: respond to destructive mechanical, thermal or chemical stimuli.
Thermoreceptors
There are warm and cold receptors. Cold receptors usually start responding at around 34 degrees, but are most sensitive at 25 down to 5 degrees. Below 5 degrees nociceptors are activated. Warm receptors respond best at 45 degrees, above that level heat nociceptors respond.
Afferent Pathway
Each sensory receptor transmits its nerve impulse through specific types of nerve fibres.
| Sensory Modality | Fibre size | Nerve Fibres | Tract |
|---|---|---|---|
| Vibration, touch, and proprioception | "Large fibre" | myelinated (Aa or Ab) nerve fibres | Posterior/dorsal column |
| Temperature and pain | "Small fibre" | small-diameter, myelinated (Ad ) or unmyelinated (C) nerve fibres | Spinothalamic tract |
Clinical Testing
Vibration: Tested using a tuning fork, most accurate with a Rydel-Seiffer version which correlates with nerve conduction studies. See article on Vibration and Proprioception.
Proprioception: Tested with passive great toe movement. See article on Vibration and Proprioception.
Touch: A common method is the Semmes-Weinstein monofilaments, however it is controversial and may not correlate with nerve conduction studies. The examiner applies the filament to the skin until it bends and the patient states whether they can feel it. The clinical touch threshold is determined by which filament they can feel.
Pain: Most commonly tested using a sharp object like a safety pin or toothpick. "Neurotips" are another option. Also see article Pain Oriented Sensory Testing.
Temperature: Most classically tested using warm or cold flasks. One simpler option is to use a tuning fork for cold sense, and graded electric hand warmers for heat sense. Also see article Pain Oriented Sensory Testing which also covers temperature testing.
