In one breath
Touch, proprioception, vibration, two-point discrimination and stereognosis each have their own receptors, but all of them reach the brain by the dorsal column pathway, and crude touch has a second route in the anterior spinothalamic tract. Touch uses rapidly adapting (Meissner’s, Pacinian) and slowly adapting (Merkel, Ruffini) receptors carried by Aβ fibers; vibration is a composite of touch and deep pressure picked up mainly by Pacinian corpuscles. Vibration and proprioception fail together in dorsal column disease, while two-point discrimination, tactile localization and stereognosis are cortical sensations, severely impaired when the cortex is damaged.
Builds on: Ascending tracts: dorsal column and anterolateral system · Sensory receptors and their classification · Leads to: Abnormalities of dorsal column sensations · Somatosensory cortex
Touch
Receptors
- Touch can be felt even from skin that has no specialized receptors, so free nerve endings can serve touch.
- The specialized touch receptors differ in how fast they adapt:
| Adaptation | Receptors |
|---|---|
| Rapid | Meissner’s corpuscles, Pacinian corpuscles |
| Slow | Merkel discs, Ruffini endings |
- They are stimulated by mechanical pressure.
- Touch receptors are closely tied to a Na⁺ channel, BNC1. It belongs to the degenerins, a protein family that makes neurons degenerate when it is over-expressed.
Distribution of receptors
- Densest in the hands, above all the fingertips, and in the lips.
- Fewer in the proximal limbs and the trunk, and very few on the back.
- They crowd around hair follicles, so moving a hair is a strong touch stimulus.
- They are also found in the skin and subcutaneous tissue of hairless areas.
Pathway
- Touch travels in Aβ fibers, which ascend by two routes:
| Touch | Route | Quality |
|---|---|---|
| Fine | Dorsal column pathway | Well localized, finely discriminated |
| Crude | Anterior spinothalamic tract | Poorly localized, poorly discriminated |
Proprioception and kinesthesia
Definitions
- Proprioception: the afferent input from mechanoreceptors in muscles, tendons and joints that signals how joints and body parts are moving and where they are in space.
- Kinesthesia: the sense of body parts moving relative to one another.
- Together the two terms cover the sense of position and movement. They leave out the sense of body position and orientation that comes from the eyes and the vestibular receptors.
Receptors
- Slowly adapting spray endings in joints and muscles, which resemble Golgi tendon organs
- Pacinian corpuscles in the synovium and ligaments
- Possibly the touch receptors in the skin over a joint
- Worth knowing, though not in your pages: standard texts also count the muscle spindles and Golgi tendon organs themselves among the proprioceptors (see Muscle spindle and Golgi tendon organ).
Pathway
- Proprioception climbs the dorsal column.
- From the nucleus gracilis and nucleus cuneatus, a large proprioceptive projection goes on to the cerebellum.
- So dorsal column disease causes ataxia: it cuts off this input to the cerebellum.
Vibration
- Felt as trembling or buzzing when a mechanical stimulus oscillates rapidly, between 2 and 400 Hz. The ability to feel it is pallesthesia.
- At the low end of this range the feeling is flutter; at the high end it is vibration.
- Tested by placing a vibrating tuning fork on the skin, best over a bony prominence.
- A composite sense: touch plus rapid changes in deep pressure.
Receptors and pathway
- Receptors: rapidly adapting touch receptors, above all Pacinian corpuscles.
- Pathway: the dorsal columns.
Clinical importance
- Vibration sense and proprioception go together: disease that dulls one dulls the other.
- In cortical lesions both are affected early and severely.
- Loss of vibration sense is an early sign of dorsal column degeneration, which is seen in:
- chronic diabetes mellitus
- vitamin deficiency, especially of vitamin B12 (pernicious anemia)
- the early stage of tabes dorsalis
- Worth knowing, though not in your pages: your book calls this a “depression of the threshold”. What is depressed is the sense: the threshold actually rises, so the fork must vibrate harder before it is felt. The fork used at the bedside is usually the 128 Hz one.
Two-point discrimination
- Definition: the smallest distance at which two points, touched at the same moment, are felt as two separate points.
- The two-point threshold is a measure of tactile acuity.
Receptors
- Fine-touch receptors, but telling the two points apart needs an intact cortex.
- The threshold varies from part to part. It is smallest where touch receptors are densest, and it also depends on how much sensory cortex the part is given.
| Site | Two-point threshold |
|---|---|
| Fingertips | As little as 3 mm |
| Back | 65 mm or more |
Pathway and clinical significance
- Pathway: the dorsal column.
- It is abolished when the cortex is damaged.
- The cortex also assembles two-point discrimination, tactile localization and stereognosis, so cortical lesions impair them severely. That is why they are called the cortical sensations.
Stereognosis
- Definition: recognizing a familiar object by handling it with the eyes closed.
- Receptors: touch and pressure receptors.
- Pathway: the dorsal column.
Clinical significance
- Loss of it is astereognosis, also called tactile agnosia. The other agnosias follow the same pattern:
| Agnosia | Cannot identify |
|---|---|
| Tactile (astereognosis) | An object by touch |
| Visual | An object by sight |
| Auditory | Sounds or words |
| Positional | Where a limb is |
- Stereognosis is one of the main cortical sensations, so its loss is an early sign of cortical disease, even while touch and pressure are still intact.
- It is also impaired in diseases of the dorsal column.
Cortical loss versus pathway loss
In a cortical lesion, simple touch and pressure may be normal while stereognosis, two-point discrimination and localization fail. In dorsal column disease the input itself is cut, so the basic sensations are lost as well. Always test simple touch before calling a failed stereognosis test “cortical”.
Exam-answer skeleton: "Vibration sense" (short note)
- Definition: the feeling of a mechanical stimulus oscillating at 2–400 Hz; the ability is pallesthesia, and its loss pallanesthesia.
- Nature: a composite of touch and rapid changes in deep pressure; low frequencies feel like flutter.
- Receptors: rapidly adapting touch receptors, mainly Pacinian corpuscles.
- Pathway: dorsal column → nucleus gracilis and cuneatus → crosses in the medulla → medial lemniscus → VPL → somatosensory cortex.
- Testing: a vibrating tuning fork on a bony prominence.
- Clinical: runs parallel to proprioception; lost early in dorsal column degeneration (diabetes, B12 deficiency, early tabes dorsalis) and impaired early in cortical lesions.
Asked in exams
- Vibration sense: Jan 2011, 2 marks