The Low-Threshold Mechanoreceptors

On a mouse’s back, an awl/auchene hair follicle is fenced by the endings of three different kinds of touch neuron, and nearly every zigzag follicle by two. Each of those neurons sends its own signal to the spinal cord, at its own speed, with its own time course. This lesson asks what the classes are, how anyone found out which endings belong to which neurons, and how far the mouse map carries over to your own skin.

Touch and pain introduced the four classical receptors of the human hand by adaptation and receptive field, and sorted axons into Aβ, Aδ and C by speed with a latency problem. This lesson starts where that one stops: genetic access to each class, the end organs of hairy skin, and why one end organ can hold more than one afferent type.

The idea: an LTMR is defined by threshold, not by what it cannot feel

A low-threshold mechanoreceptor (LTMR) is a cutaneous sensory neuron activated by innocuous mechanical stimulation (Abraira and Ginty, 2013). The word is a statement about sensitivity, not a limit. In mice, Aβ field-LTMRs respond vigorously to a gentle stroke, and their firing kept increasing as indentation entered the noxious range (Bai and colleagues, 2015). An LTMR can fire to a pinch. What makes it an LTMR is that it also fires to a touch too light to hurt.

The labels RA (rapidly adapting) and SA (slowly adapting) are also test results. They describe the response to one stimulus, a held step indentation: RA afferents fire at onset and sometimes at offset, SA afferents keep firing (Handler and Ginty, 2021). Sensory coding treats adaptation in general. Here the point is narrower: a step response does not tell you everything a neuron reports during natural movement.

For decades, classes in hairy skin were defined by recording, and the ending behind each was inferred. What changed was genetic access. A Cre driver is a mouse line that expresses Cre recombinase from one gene’s regulatory DNA, so that a reporter marks cells that expressed that gene; CreER adds a tamoxifen switch so the experimenter picks the labelling window (tools for development covers how). A label is a handle, not a class, until physiology confirms it. Li and colleagues (2011) did that check: in their recordings, adult TrkB-expressing dorsal root ganglion neurons behaved as Aδ-LTMRs, and the cutaneous C-fibre neurons with C-LTMR properties stained TH-positive. Transcriptomic atlases, covered in the atlas lesson, now offer many more handles, and each still needs this step.

Six classes and their endings, mouse numbers attached

Two kinds of ending recur in hairy skin. A lanceolate ending is an axon branch that runs lengthwise along a hair follicle, parallel to the shaft, like one picket in a fence around the hair. A circumferential ending wraps around the follicle instead. A terminal Schwann cell is a non-neuronal cell whose processes interleave with lanceolate endings.

Class	End organ	Mouse conduction velocity	Response to a step	Best stimulus
Aβ RA1	Meissner corpuscle (glabrous); lanceolate endings at guard and awl/auchene hairs	13.6 ± 2.7 m/s, hairy skin (Li 2011)	Onset, sometimes offset	Skin motion; Meissner best near 40 to 60 Hz
Aβ RA2	Pacinian corpuscle; in mice mostly in periosteum of digits and limbs	No mouse value here	Onset and offset	Vibration, most sensitive near 100 to 300 Hz
Aβ SA1	Merkel cell-neurite complex; touch dome at guard hairs	No mouse value here	Sustained	Indentation, points and edges
Aβ field	Circumferential endings around many follicles	17.0 ± 2.0 m/s (Bai 2015)	Rapid near threshold, intermediate at higher force	Stroke over a wide area
Aδ-LTMR	Lanceolate endings at awl/auchene and zigzag hairs (TrkB)	5.8 ± 0.9 m/s (Li 2011)	Rapid	Tiny hair deflection, below 0.07 mN von Frey force
C-LTMR	Lanceolate endings at awl/auchene and zigzag hairs (TH)	0.58 ± 0.02 m/s (Li 2011)	Intermediate	Gentle deformation of hairy skin

Three readings keep you out of trouble. First, the Li velocities come from intracellular recordings in an ex vivo skin, nerve, ganglion and spinal cord preparation from 2 to 3 month old mice, with five C-LTMRs, 19 Aδ-LTMRs from seven animals and 18 Aβ RA-LTMRs from five animals. The often-quoted 15 to 100 m/s for Aβ is a review range pooled across species; the mouse hairy-skin Aβ RA mean sits below it. Second, RA1 is not a synonym for Meissner: in mouse hairy skin, the Aβ RA endings are lanceolate. Third, the frequency bands are bands of best sensitivity from reviews, not cutoffs.

One end organ can hold more than one afferent type. The awl/auchene follicle is the clearest case, with three classes interdigitated around it, and Li and Ginty (2014) showed that a single terminal Schwann cell can host endings of more than one LTMR type. The 2021 review by Handler and Ginty adds that mouse Meissner corpuscles receive two afferent populations, some of them more slowly adapting, a result from Neubarth and colleagues (2020) that the corpuscle lesson takes up. Handler and colleagues (2023) located tagged Piezo2 in the sensory axon membrane of lanceolate, Meissner and Pacinian endings (Piezo channels covers the channel). Merkel cells get their own lesson, and field-LTMRs and C-LTMRs return in hairy skin and affective touch.

One follicle, several neurons: the Li 2011 map

Mouse trunk skin has three main hair types: long guard hairs, awl/auchene hairs, and the short, bent zigzag hairs. Li and colleagues labelled the C, Aδ and Aβ RA classes genetically and confirmed their identity by recording. No mouse line for SA1 neurons existed yet, so they marked SA1 endings through the Merkel cells they contact, using anti-Troma1 staining. They then imaged fixed whole mounts of trunk skin from 3 to 10 week old mice to see which follicles each label surrounded. The combinations were:

• Guard: Aβ RA lanceolate endings, plus Merkel-associated Aβ SA1 endings in the touch dome beside the hair.
• Awl/auchene: Aβ RA, Aδ and C lanceolate endings, interdigitated.
• Zigzag: Aδ and C lanceolate endings, in virtually all follicles examined.

“Stereotyped” keeps its exceptions: the paper reports very few Aβ RA associations with zigzag follicles. The three hair types occurred in an approximate 1:23:76 ratio (guard to awl/auchene to zigzag), and recurring clusters of them led the authors to describe peripheral LTMR units. In separate experiments, cholera toxin B (CTB), a tracer taken up by axons, was injected into small patches of back skin at postnatal day 14 to 25. The central endings of afferents serving one patch lined up in columns in the dorsal horn. That alignment is anatomy; how the columns combine their inputs is the subject of dorsal horn touch.

Worked problem, part (a): endings versus neurons. Take an idealized patch of exactly 100 mouse trunk follicles in Li’s ratio: 1 guard, 23 awl/auchene, 76 zigzag. Use only the dominant map above, count each class once per follicle it innervates, and leave out field-LTMRs and the rare exceptions.

1. Aβ RA: guard plus awl/auchene = 1 + 23 = 24 associations.
2. Aβ SA1: guard only = 1.
3. Aδ-LTMR: awl/auchene plus zigzag = 23 + 76 = 99.
4. C-LTMR: awl/auchene plus zigzag = 23 + 76 = 99.
5. Total: 24 + 1 + 99 + 99 = 223.
6. Check by follicle: (1 × 2) + (23 × 3) + (76 × 2) = 2 + 69 + 152 = 223.

associations = sum over follicle types of (number of follicles × classes per follicle)

Is 223 a neuron count? No. One afferent contacts many follicles; a single mouse field-LTMR arbor reached 20 to 180 follicles (Bai and colleagues, 2015). And 1:23:76 is a ratio of hairs, not of neurons. The calculation tells you how often each class appears at a hair, which is what a deflection of a random hair would engage.

Direction from development: polarized Aδ endings and BDNF

Aδ-LTMRs are the most sensitive hair afferents in the mouse. Rutlin and colleagues (2014) asked whether they care which way a hair bends. In adult mouse dorsal trunk skin, recorded intracellularly in an ex vivo preparation, genetically identified Aδ-LTMRs fired, on average, more than three times as many spikes to caudal-to-rostral deflection (hair tip moving toward the head) as to an equal deflection the other way.

The anatomy matched the tuning. Aδ-LTMR lanceolate endings were enriched on the caudal side of each follicle. Keep two variables apart: the endings sit on the tail side, and the preferred movement is toward the head. A weathervane analogy helps: where the vane is mounted is not the direction it points. Reporter histology showed that BDNF in the follicle epithelium sat on the caudal side, next to the TrkB-positive Aδ endings. BDNF signalling through TrkB is covered in Trk and p75.

Then the test. Deleting Bdnf from epithelium (K5Cre;Bdnf conditional mutants) disrupted the caudal polarity of Aδ endings and randomized the neurons’ preferred direction relative to the body axis. Aδ-LTMR survival and general maturation were retained. Because the deletion acted through development, the result shows that epithelial BDNF is required to organize the endings, not that BDNF is released during touch. How a caudally placed ending converts into a preference for rostral deflection is a mechanical question that this experiment does not answer.

Worked problem, part (b): a direction index. Suppose, as illustration, an Aδ-LTMR fires 12 spikes to a caudal-to-rostral deflection and 3 spikes to an equal rostral-to-caudal one.

direction index = (preferred − opposite) / (preferred + opposite)

1. Ratio: 12 / 3 = 4, in the range of the paper’s report that average responses to caudal-to-rostral deflection were over 3-fold larger than to the opposite direction.
2. Index: (12 − 3) / (12 + 3) = 9 / 15 = 0.60.
3. Check: an index of 0 means no preference; 1 means no response to the opposite direction. A ratio of 3 gives (3 − 1) / (3 + 1) = 0.50, so 0.60 is a plausible illustrative value; the paper reports an average over tested spots, not a per-neuron floor.

Up is rostral (toward the head) in both pictures. In each dial the thin line is the neuron’s preferred direction; the bold arrow’s length is its direction index. The number under a dial is that neuron’s body-axis index, DI × cos θ. The ringed dial is the neuron drawn on the follicle.

Spikes to preferred deflection, P: 12 Spikes to equal opposite deflection, O: 3

Genotype

Control K5Cre;Bdnf epithelial deletion

Resample

Ratio P / O

Direction index, DI = (P − O) / (P + O)

Mean body-axis index, expected

Mean body-axis index, observed (12 sketch neurons)

In this sketch: spike counts are illustrative, not Rutlin’s data. Mutant angles are drawn uniformly at random, which is why the expected mean is 0. Two assumptions are mine, not findings: the body-axis index is cosine tuning, DI × cos θ, and every neuron has the same DI, which each mutant neuron keeps while only its angle is randomized (the lesson does not say whether single-neuron tuning strength survived). The deletion shows a developmental role for epithelial BDNF, not BDNF release during touch. The rule to keep is DI = (P − O) / (P + O): 0 is no preference, 1 is no response to the opposite direction.

Mouse and human: what microneurography can and cannot match

In microneurography, a tungsten microelectrode inserted through the skin into a peripheral nerve records single afferents in a conscious person. Johansson and Vallbo (1979) recorded 334 low-threshold units from the glabrous skin of the hand through the median nerve and sorted them into RA, PC, SA I and SA II. Their classes rest on firing and receptive fields; no ending was seen.

That is why the fourth class is a mismatch. Human SA II physiology is secure. The 2021 review keeps the SA2-Ruffini pairing controversial and notes that SA2-like responses have been recorded in mice without a corresponding Ruffini ending being identified. So “mice lack SA2” is wrong, and “SA2 is the Ruffini afferent” is unproven in both species. The experiment that would settle it in mice is the one Li used for hair: genetic access to SA2-like afferents, recording to confirm the response, and reconstruction of the ending.

Hairy skin brings the opposite problem. Vallbo and colleagues (1999) recorded unmyelinated afferents in human forearm hairy skin that respond to gentle deformation, the C-tactile (CT) afferents, physiologically similar to mouse C-LTMRs. Watkins and colleagues (2021) then reported three CT afferents in human glabrous hand skin, at roughly one per 80 myelinated afferents encountered. That ratio is a sampling rate, not a density. The awl/auchene and zigzag map cannot be transplanted onto human hair, and the human CT ending is unknown.

Human recordings also warn against one-class, one-feature thinking. Dione and colleagues (2021) changed surface friction with an ultrasonic device while recording from eight afferents in five women aged 21 to 28; frictional transients evoked time-locked spikes across the sampled classes. In mice, C-LTMRs are not only about pleasant touch either: optogenetic activation evoked wet dog shakes, and adult ablation cut oil-droplet-evoked shakes by about half (Zhang and colleagues, 2024).

Design problem

Rutlin and colleagues found caudally enriched Aδ endings, caudal BDNF in the follicle epithelium, and a preference for rostral deflection. Design the experiment that tests whether BDNF sets the direction tuning, and state what it could and could not conclude.

One way to do it

Delete Bdnf only where the cue is made. A germline Bdnf knockout would affect many neurons and the animal’s health, so use an epithelial driver (K5Cre with a floxed Bdnf allele) and compare mutants with littermate controls.

Label Aδ-LTMRs genetically. Use a label that does not itself recombine Bdnf. A TrkB-GFP knock-in does this, and Rutlin and colleagues recorded from TrkB-GFP neurons in K5Cre;Bdnf-flox mutants; a tamoxifen-induced TrkB-CreER line would also excise the floxed Bdnf allele in the labelled neurons. TrkB is expressed by other neurons too, so confirm identity by conduction velocity and threshold.

Measure two outcomes in the same line: ending polarity (fraction of Aδ endings on the caudal side of each follicle, in whole mounts) and tuning (spikes to equal rostral and caudal deflections, as a direction index, in ex vivo recordings).

Control for survival. Count labelled DRG neurons in mutants and controls; Rutlin found survival and maturation retained, so lost tuning is not lost neurons.

Conclude a developmental organizing role for epithelial BDNF in mouse trunk skin. The design does not test acute BDNF release during touch (that would need an adult, inducible deletion), and it says nothing about human hair afferents.