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Human Physiology · Nervous system and exercise

Sensory Receptors and Somatosensation

The body feels touch, position, temperature and pain through specialised receptors and two main ascending pathways. This page explains how receptors encode stimuli, how signals reach the cortex, and how sensation can be assessed and trained.

11Sections
5Figures
3Tables
10Questions

Part 9 · Nervous System and Exercise · Chapter 40 of 47

Learning objectives

By the end of this page you should be able to:

  • Classify somatosensory receptors by stimulus and adaptation.
  • Explain transduction, receptor potentials and coding of intensity.
  • Trace the dorsal column and spinothalamic pathways.
  • Describe the somatosensory cortex and its body map.
  • Interpret common sensory tests.
  • Summarise evidence on tactile acuity in pain and proprioceptive training.

Why it matters

Sensation guides every movement. Patients with stroke, neuropathy, joint injury or chronic pain often have sensory deficits that limit recovery.

Accurate sensory testing localises lesions, and sensory training is part of rehabilitation.

Receptors

Figure 1 · Somatosensory receptors

Main receptor types for touch, proprioception, temperature and pain Four skin mechanoreceptors: Meissner corpuscles for light touch, Merkel discs for pressure, Pacinian corpuscles for vibration and Ruffini endings for stretch. Muscle spindles sense length, Golgi tendon organs sense tension, thermoreceptors sense warm and cold, and nociceptors sense tissue-damaging stimuli. Meissner corpusclelight touch, flutterrapidly adaptingMerkel discpressure, shape, edgesslowly adaptingPacinian corpusclevibrationvery rapidly adaptingRuffini endingskin stretchslowly adaptingMuscle spindlemuscle length andrate of changeGolgi tendon organmuscle tensionThermoreceptorswarm and coldNociceptorstissue-damaging stimulifree nerve endings
Each receptor is tuned to one kind of stimulus, its adequate stimulus. Joint receptors also contribute to position sense near the ends of range.

A stimulus deforms or excites the receptor membrane, opening ion channels and producing a graded receptor potential. If it reaches threshold, the afferent fibre fires action potentials.

Stronger stimuli produce higher firing rates and recruit more receptors. This is how intensity is coded.

Figure 2 · Slow and fast adaptation

Firing of slowly and rapidly adapting receptors during a sustained stimulus During a sustained stimulus, a slowly adapting receptor keeps firing, with its rate falling gradually. A rapidly adapting receptor fires a brief burst at the start and another at the end, signalling only change. StimulusSlowly adapting(Merkel, spindle)Rapidly adapting(Meissner, Pacinian)onoffkeeps firing while the stimulus lastssignals only change
Why you stop feeling your clothes. Rapidly adapting receptors report change and vibration; slowly adapting receptors report steady pressure and position.
FibreDiameter and myelinCarries
Aα (Ia, Ib)Largest, thick myelinMuscle spindle and tendon organ signals
AβLarge, myelinatedTouch, vibration, pressure
AδSmall, thinly myelinatedSharp pain, cold
CSmallest, unmyelinatedBurning pain, warmth, itch

Ascending pathways

Figure 3 · Two ascending pathways

Dorsal column and spinothalamic pathways crossing at different levels Fine touch, vibration and proprioception ascend on the same side in the dorsal columns and cross in the medulla. Pain and temperature fibres cross in the spinal cord within a segment or two and ascend in the opposite spinothalamic tract. Both relay in the thalamus to the sensory cortex. Cortex (postcentral gyrus)ThalamusMedullaSpinal cordmidlineSensory inputfrom right sideDorsal column: fine touch,vibration, proprioceptionSpinothalamic: pain,temperature, crude touchcrosses in medullacrosses in cord
Different crossing levels explain dissociated loss. A cord hemisection loses proprioception on the same side and pain on the opposite side below the lesion.

First-order neurons have cell bodies in the dorsal root ganglia. Second-order neurons cross the midline, and third-order neurons run from the thalamus to the postcentral gyrus.

For dermatome maps and tract anatomy, see the Human Anatomy pages.

The somatosensory cortex

The primary somatosensory cortex holds an ordered map of the opposite side of the body. Hands, lips and face take up a large share because they are densely innervated.

This map is plastic. Use, disuse, injury and pain can enlarge, shrink or blur body-part representations.

Figure 4 · Tactile acuity varies across the body

Two-point discrimination thresholds in different body regions The smallest distance at which two points are felt as two is about 3 millimetres at the fingertip, 5 at the lips, 10 at the palm, 20 at the sole, 35 at the forearm and 40 at the back. Acuity is reduced in several chronic pain conditions. Fingertipabout 3 mmLipsabout 5 mmPalmabout 10 mmSoleabout 20 mmForearmabout 35 mmBackabout 40 mmTypical two-point discrimination thresholds (smaller = finer acuity)Acuity is reduced in arthritis, chronic low back pain and CRPS [1]
Acuity follows receptor density and cortical area. Reduced acuity in chronic pain may reflect changes in the body map [1]. Typical textbook values.

Catley and colleagues pooled 16 studies comparing people with chronic non-neuropathic pain against controls [1]. Tactile acuity was reduced in arthritis, complex regional pain syndrome and chronic low back pain [1].

The evidence carries a high risk of bias, but it supports testing acuity in chronic pain [1].

Proprioception

Position and movement sense come mainly from muscle spindles, helped by skin stretch, joint receptors and tendon organs. Injury, ageing, neuropathy and stroke all reduce it.

Figure 5 · Proprioception can be trained

Average improvements with proprioceptive training Proprioceptive training improved outcomes by an average of 52 per cent; muscle vibration above 30 hertz for longer durations gave improvements of up to 60 per cent; joint position and reaching training improved joint position sense by 48 per cent on average. Proprioceptive training, all outcomes52% averageMuscle vibration above 30 Hzup to 60%Joint position and reaching training48% averageImprovement from baseline; 51 studies; joint position sense improved up to 109% [2]
Active and passive movement, with and without vision, tended to work best. Evidence quality is mixed [2].

Aman and colleagues reviewed 51 studies of proprioceptive training [2]. Training improved outcomes by an average of 52%, with stroke the most studied condition [2].

Testing sensation

ModalitySimple testPathway
Light touchCotton woolMixed
PainNeurotip, sharp versus bluntSpinothalamic
TemperatureWarm and cold tubesSpinothalamic
Vibration128 Hz tuning fork on bony pointsDorsal column
Joint positionSmall movements of a digit, eyes closedDorsal column
Tactile acuityTwo-point discriminationDorsal column and cortex

Practical skill

Test with eyes closed, compare sides, start distally and work proximally, and map the edge of any loss. Demonstrate on a normal area first.

Clinical relevance for the physiotherapist

  • Localise. Dissociated loss points to cord or brainstem lesions.
  • Neuropathy. Lost vibration and position sense raise fall and ulcer risk.
  • Chronic pain. Test tactile acuity; it may be reduced [1].
  • Train. Include proprioceptive training after injury and stroke [2].
  • Safety. Protect insensate skin from heat and pressure.

Assessment and application

SituationInterpretationAction
Stocking loss of vibration in a diabetic patientLarge-fibre neuropathyFoot care; balance training
Chronic back pain with poor two-point discriminationPossible altered body map [1]Consider sensory discrimination training
Poor joint position sense after strokeProprioceptive deficitActive and passive training with and without vision [2]
Pain loss on one side, position loss on the otherCord hemisection patternUrgent medical review if new

Common mistakes

  • Testing with the patient watching.
  • Testing only light touch.
  • Ignoring sensory loss in chronic pain [1].
  • Assuming proprioception cannot improve [2].
  • Applying heat to numb skin.

Exam preparation

Long answer

Classify sensory receptors. Describe the pathways for touch and pain.

  • Classification and adaptation (Figures 1, 2).
  • Fibre types (table).
  • Dorsal column and spinothalamic pathways (Figure 3).
  • Sensory cortex and body map (Figure 4).
  • Clinical applications [1, 2].

Short notes

  • Adaptation of receptors.
  • Two-point discrimination [1].
  • Brown-Séquard pattern.
  • Proprioceptive training [2].

Viva and practical

  • “Which receptor senses vibration?” — Pacinian corpuscle.
  • “Where do pain fibres cross?” — in the spinal cord.
  • “Can proprioception be trained?” — yes, about 52% average improvement [2].
  • OSCE: test vibration and joint position sense in the feet.

Current evidence

Exam and traditional teaching

Sensory pathways are fixed; sensation is tested only to localise lesions.

What the evidence adds

Tactile acuity is reduced in several chronic pain conditions [1].

Proprioceptive training produces meaningful improvements [2].

What the student should understand

Sensation is an adaptable system. Assess it carefully and train it like strength.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. A systematic review found tactile acuity was diminished in which conditions?
  1. (A) Arthritis, CRPS and chronic low back pain
  2. (B) Burning mouth syndrome only
  3. (C) No chronic pain condition
  4. (D) Only acute injury

Answer: (A) Acuity was reduced in arthritis, CRPS and chronic low back pain [1].

Q2. In the same review, tactile acuity in burning mouth syndrome was:
  1. (A) Markedly reduced
  2. (B) Not diminished
  3. (C) Improved
  4. (D) Not studied

Answer: (B) Burning mouth syndrome did not show reduced acuity [1].

Q3. Reduced tactile acuity in chronic pain is thought to reflect:
  1. (A) Skin thickening
  2. (B) Altered cortical representation of the painful part
  3. (C) Loss of all receptors
  4. (D) Vision problems

Answer: (B) It has been suggested to reflect altered cortical representation [1].

Q4. The strength of evidence for tactile acuity changes in chronic pain is:
  1. (A) Very high
  2. (B) Weakened by inconsistent results and high risk of bias
  3. (C) Absent
  4. (D) Perfect

Answer: (B) All included studies had high risk of bias [1].

Q5. Proprioceptive training improved outcomes on average by about:
  1. (A) 5%
  2. (B) 52%
  3. (C) 200%
  4. (D) 0%

Answer: (B) The average improvement across outcomes was 52% [2].

Q6. Muscle vibration above what frequency, applied for longer durations, gave improvements up to 60%?
  1. (A) 1 Hz
  2. (B) 10 Hz
  3. (C) 30 Hz
  4. (D) 1000 Hz

Answer: (C) Vibration above 30 Hz for minutes rather than seconds [2].

Q7. Joint position and target reaching training improved joint position sense by up to:
  1. (A) 10%
  2. (B) 48%
  3. (C) 109%
  4. (D) 500%

Answer: (C) Improvements were up to 109%, average 48% [2].

Q8. Which training approaches tended to be most beneficial?
  1. (A) Passive only, eyes closed
  2. (B) Both passive and active movements, with and without visual feedback
  3. (C) Rest
  4. (D) Stretching only

Answer: (B) Combined passive and active training with and without vision tended to work best [2].

Q9. Which disease was most studied in proprioceptive training research?
  1. (A) Cortical stroke
  2. (B) Asthma
  3. (C) Diabetes
  4. (D) Glaucoma

Answer: (A) Cortical stroke was the most studied condition [2].

Q10. A practical test that links to cortical body maps in chronic pain is:
  1. (A) Two-point discrimination
  2. (B) Grip dynamometry
  3. (C) Spirometry
  4. (D) Blood pressure

Answer: (A) Tactile acuity is measured with two-point discrimination [1].

Quick review

Key takeaways

  • Receptors are tuned to one stimulus and adapt slowly or rapidly.
  • Intensity is coded by firing rate and recruitment.
  • Dorsal columns cross in the medulla; spinothalamic fibres cross in the cord.
  • The sensory cortex holds a plastic body map.
  • Tactile acuity is reduced in arthritis, CRPS and chronic back pain [1].
  • Proprioceptive training improves outcomes about 52% on average [2].

Further reading

SourceWhy it earns its place
Catley MJ et al. — J Pain, 2014Tactile acuity in chronic pain [1]
Aman JE et al. — Front Hum Neurosci, 2015Proprioceptive training [2]
Kandel, Principles of Neural ScienceSomatosensory system

References

  1. Catley MJ, O'Connell NE, Berryman C, et al. Is tactile acuity altered in people with chronic pain? A systematic review and meta-analysis. Journal of Pain. 2014;15(10):985–1000. doi:10.1016/j.jpain.2014.06.009 PMID 24983492 Systematic review and meta-analysis
  2. Aman JE, Elangovan N, Yeh IL, et al. The effectiveness of proprioceptive training for improving motor function: a systematic review. Frontiers in Human Neuroscience. 2015;8:1075. doi:10.3389/fnhum.2014.01075 PMID 25674059 Systematic review

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Reviewed by the Physiotherapist India Team. · Human Physiology contents