Mechanoreceptors (mechanical-energy sensors)

A mechanoreceptor is a sensor designed to detect mechanic energy, either kinetic energy (energy of movement,
e.g. touch, sound, vibration, changing pressure) or potential energy (e.g. gravity). The fundamental unit of
cuticular sensors in insects are
sensilla (singular sensillum). Most hairs on an insect are sensilla, such as the
touch sensor illustrated above. Sensilla may be grouped into arrays or closely integrated into sensory organs.

Insects have a tough outer cuticle, which poses problems for sensing external stimuli. In insects, stimulating
mechanical forces reach the sensory cells only where the cuticle is deformable. It is a fundamental property of all
nerve cells to fire when their membrane is stretched. This is thought to open-up protein channels allowing ion
exchange across the membrane (see Morris, 1990). Therefore, wherever a region of cuticle can be distorted,
underlying nerve cells can be stimulated. Therefore, there exists a variety of deformable cuticular structures
which stimulate specific mechanoreceptive dendrites. The ease with which the cuticular parts can be deformed or
deflected determines the sensitivity of the sensory unit. Mechanoreceptors can be further divided into 1)
proprioceptors, which monitor the position, movement and tension of body parts, 2) thigmo- or touch receptors,
3) auditory receptors and receptors measuring vibrations, and receptors which measure the movement of air,  
and 4) gravity receptors.

Touch receptors

For a review of the general morphology, ultrastructure and function of touch receptors see Keil and Steinbrecht
(1984). These are hairs, usually pointed (trichoid sensilla) and which are deflected directly by the stimulus. The
sockets need to be flexible to permit this deflection, and are composed of a thick, elastic fibrillar membrane called
the joint, articular or
socket membrane. The tip of the mechanosensory dendrite is inserted into the underside
of this socket membrane. This dendrite is actually a modified cilium projecting from the sensory cell body. A
cross-section through its tip, and associated supporting structures is shown below. The dendrite is covered by a
cuticular sheath, the tip of which is attached to the hair base. This sheath is connected to the socket membrane.
The cuticular sheath is attached to the dendrite by fine filaments (Keil, 1978) (the fibres in the diagram below).
The tip of the dendrite is richly packed with microtubules or fibrils, which are often surrounded by an
electron-dense matrix (sometimes containing 10 nm filaments) which interconnects the microtubules (MT). This is
the characteristic
tubular body, which may be circular or flattened in cross-section (Thurm, 1981,82). The
electron-dense matrix may be homogeneous,  layered, or reduced to intertubular bridges. There may be from 30
microtubules, as in the tsetse fly (Rice et al., 1973), to up to 1000, as in cockroach cercal filiform hairs (Nicklaus
et al., 1967). The distinct peripheral layer of MT ( which maybe the only layer present) is connected to the
dendritic membrane by fine bridge structures (Gaffal and Hansen, 1972), so-called
cones (MICs)
(Thurm, 1981,82). Thus, a whole series of structures connect the hair base to the tubules and
membrane of the dendrite; but how are these structures involved in stimulus transduction?
labelled diagram of a trichoid sensillum mechanoreceptor
unlabelled diagram of trichoid sensillum
An insect mechanosensitive trichoid
- in other words a senor for
mechanical movement, in this case
touch. When the hair (
seta) is
deflected, the stimulus is amplified in
sensory dendrite of a sensory
nerve cell beneath the surface of the
cuticle. This sensory nerve cell is a
modified epithelial cell with a single
immotile cilium which is modified as a
sensor and called a dendrite.

BM; basement membrane; BSC: the
cell body of the bipolar sensory cell;
EC, epithelial cell;  ID, inner segment
of sensory cell dendrite, inner
dendrite; OD: outer segment of
sensory cell dendrite, outer dendrite;
RLS, receptor lymph space.

Accessory cells. The sensory cell is
accompanied by several accessory,
supporting or sheath cells: the
tormogen cell secretes the socket,
trichogen cell secretes the seta,
and the
thecogen cell (or sheath
cell) secretes the
scolopale sheath.
glial cell also wraps around the
axon, forming an insulating

The dendrite is about 400 nm in

Click images to view full size.
Mechanical sensors in insects
A tongue of cuticle projects from the hair base in Calliphora macrochaetae (macrochaeta = large trichoid sensilla)
and compresses the tubular body when the hair is deflected in a particular direction (Keil, 1978). It is not unusual
for the geometry of the socket to maximise deflection or sensitivity to deflection in one plane only, allowing the
direction of the stimulus to be determined.

When stimulated, an electrical voltage spreads from the tubular body, down the dendrite to the cell body. This is
called the
generator potential. If this voltage exceeds a minimum threshold then action potentials (spikes of
electrical activity) are initiated at the base of the axon and these travel rapidly down the axon to the central
nervous system. The sensor is an
energy transducer - converting mechanical energy into bio-electrical energy.

So how does the compression of the tubular body bring about a generator potential? Moran and Varela (1971)
suggest that, when compressed, the MTs release bound enzymes, but this is not a very popular theory at present.
It is thought that the membrane becomes stretched over the microtubules. This is thought to stretch-open protein

ion channels
in the membrane, allowing ion exchange to occur (see review by Morris, 1990). These ion channels
are, therefore,
stretch-activated ion channels. Electrically charged ions move in or out of the dendrite tip,
producing an electrical voltage. This voltage activates other ion channels which are
voltage-gated ion
further down the dendrite - initiating a chain-reaction as the generator potential spreads down the
dendrite. If the voltage inside the dendrite increase, relative to the fluid outside, the dendrite, or its membrane, is
said to become
depolarised. This is the typical case for sensors, since depolarisation activates, whereas the
opposite drop in voltage, or hyperpolarisation inhibits. (These terms make sense since in resting cells the voltage
across the membrane is negative inside the cell - cells are polarised, meaning that they store electric charge
across the cell membrane which acts as a
capacitor.) Therefore, it seems likely that compression of the tubular
body somehow stretches the sensory cell membrane (Thurm, 1981,82).

As an informative aside, let us consider the situation in typical motile cilia. It appears that movement of a
non-motile cilium can affect local membrane depolarisation (Gray and Pumphrey, 1958). This is thought to occur
at part of the basal body, known as the ciliary necklace, which is connected to the cell membrane by protein 'feet'.

In the insect tubular body, however, depolarisation may occur where the MICs contact the dendritic membrane, as
proposed by Thurm (1982). Thurm (1982) notes three observations which support this: 1) MICs are only found in
the region of maximum mechanical impact. 2) The short receptor latencies (0.1 ms, the time between stimulation
and initiation of a generator potential) suggests that the membrane ion-channels are very close to the
stimulus-receiving structures, and 3) the MICs would serve to concentrate the low mechanical energy input  
(about 10 E-19 J) onto a small area (10 E-16 J / 1000 MICs), and will greatly increase sensitivity. To summarise, it
seems likely that, when the tubular body is compressed, the dendrite membrane is stretched against the
projecting MICs, locally intensifying the stretch in the membrane, stretching open ion channels and  causing
membrane depolarisation and initiating a generator potential.
cross-section through tubular body
Scolopidia and Johnston's Organ - sensors with motile cilia?

In all the ciliary receptors described, so far, the cilium is non-motile and does not beat. Although these immotile cilia
possess microtubules, they lack the usual arrangement of 9 outer doublets and two central singlets seen in motile
cilia. However, in another type of proprioceptor, or more specifically tension receptor, the cilium is thought to move
actively in response to stimulus, transmitting the stimulus to the cell body (Moran
et al., 1977). This is the
scolopidium type of receptor. Scolopidia are thought to be derived from campaniform sensilla by becoming
elongated and deeply sunk within the body (see Wigglesworth, 1972). However, their different mode of
transduction would dispute this. The distal process of the sensory cell is inserted into a pliable region of cuticle,
such as an intersegmental membrane, either directly or via an intervening ligament cell. Stretching of this pliable
cuticle, due to movements of the body segments stimulates the scolopidium. The ligament attaches to the
scolopale sheath, which fits very loosely over the cilium, which would give it plenty of space to undulate.

The second antennal segment in insects, the pedicel, always contains a group of scolopidia, radially arranged, with
their proximal ends attached to the pedicel wall, and their distal ends attached to the intersegmental membrane
between the pedicel and first flagellar segment. This arrangement is known as '
Johnston's organ' (Eggers, 1923)
and responds to (passive) movements of the flagellum relative to the pedicel. The antenna also contains a number
of independent scolopidia, but these become less important as Johnston's organ becomes better developed (see
Wigglesworth, 1972). In the honey-bee, Johnston's organ is used to regulate flight-speed (Heran, 1959), while in
the male mosquito it is an auditory organ (Roth, 1948), but generally it is proprioceptive, detecting changes in the
position of the antenna. During flight or sound detection, the antennae vibrate in response to moving air and
sound waves respectively.

The information available on the transduction mechanism of scolopidia suggests that the cilia actually undergo an
active cycle of bending (similar to that for motile cilia) in response to a stimulus (Moran
et al., 1977). This bending
is transmitted to the ciliary necklace at the basal complex of the cilium (where it enters the cell body or proximal
dendrite extension of the cell body) which displaces membrane (probably via protein 'feet' which join the ciliary
necklace to the plasmalemma), causing local ionic permeability changes, initiating a generator potential. In Moran
et al.'s study of Melanoplus legs, the scolopidial cilia have a "9x2 + 2" arrangement, with dynein arms. That is, they
possess 9 outer pairs of microtubules, or doublets, linked by dynein arms and two central microtubules. This is the
arrangement found in motile cilia. Cilia lacking the central doublets are either immotile or bend very slowly (Alberts
et al., 1983). This evidence combined may indicate that the cilia can undergo a cycle of contraction when
stimulated mechanically (as do normal motile cilia) but are incapable of sustaining movement of their own accord.
Certainly the presence of dynein arms would indicate some motile activity. The purpose of such a mechanism is
presumably to amplify the initial stimulus signal, enabling the sensation of slight vibrations of the antenna.
labelled diagram of campaniform sensillum
Above: a transverse section (cross-section) through the tip of the sensory dendrite containing
the tubular body. In addition to the scolopale sheath which covers the whole outer dendrite
(and which is secreted by the thecogen cell) the tip is also enclosed in an outer cuticular
sheath, which may be partial or complete, and which appears continuous with the socket or
articular membrane (as shown in the diagram).
The sensory cell is bipolar - with a dendrite at one pole and an axon at the other. The
dendrite carries signals towards the cell body, the axon away from the cell body and
towards the central nervous system. The
cell body is the rounded region of the
sensory cell containing the nucleus. The signals are electrical in nature (strictly
electrochemical).The whole structure rests, along with the rest of the epithelium, on
basement membrane - a supporting skeleton of extracellular matrix materials.

It is often difficult to label a receptor clearly as a touch receptor or a vibration receptor, because in something as
small as most insects, the touch receptors can probably also detect airborne and substrate vibrations. As already
mentioned, Johnston's organ is known to be used to measure sound and airspeed, as well as antennal movements
generally, in some insects. However, some scolopidia are highly modified to detect sounds, especially in larger
insects. Probably one of the best characterised is the tympanal organ in grasshoppers and locusts (Doolan and
Young, 1981). This consists of a cuticular plate in the surface of the cuticle, which is free to vibrate in response to
air pressure changes. These movements are detected by scolopidia underlying the tympanum and inserted into it.
Trichobothria are easily movable hairs connected to the body cuticle by a thin cuticular lamella. The sockets of
these hairs often have a complex arrangement, to maximise sensitivity. Filiform hairs on the cerci of Acheta are
modified trichoid sensilla, with a hair base which limits deflection in one exact plane only. They have been shown to
be sensitive to air streams and low frequency sounds (see Keil and Steinbrecht, 1984).

Gravity receptors

The so-called clavate hairs found on the cerci of Acheta, have been attributed to gravity perception (Bischof, 1975).
The hair shaft is transformed into a fluid-filled club. The hair base is enclosed in a wide cup which limits deflection.
These hairs are really modified trichoid sensilla, and possess characteristic tubular bodies. In some cases, modified
campaniform organs detect gravity. A deflectable cup is surrounded by campaniform sensilla which detect its
deflections (Dumpert & Gnatzy, 1977).

Soft-tissue Receptors

So far we have considered sensilla that respond to distortions of the cuticle. However, a second class of
mechanoreceptor is found in insects which consists of naked branched dendrites, naked in that they are not
associated with specialised structures that transduce and amplify the structure, though they are sheathed by
supporting glial cells. These cells are multipolar, since they have many dendrites. In soft-skinned larvae, such as
caterpillars, many of these sensory cells, and also similarly naked neurones that are bipolar, form a subepithelial
nerve plexus - that is an intricate nerve network beneath the skin. Such an arrangement is well adapted to sense the
complex distortions of the flexible body wall and act as proprioceptors and pressure sensors. They may also occur in
the soft articular membranes of the joints of insects with rigid cuticles. Similar sensory cells innervate muscle fibres
and are modified to act as stretch receptors that measure muscle movement.

The Diversity and Evolution of Mechanosensors

This article only covers the basic types of insect mechanoreceptors, but they occur in a remarkable range of
modifications and uses. It is interesting to consider the evolution of these systems. Many single-celled eukaryotes,
such as many protozoa, have one or more cilia or flagella that act as engines to move the cell in locomotion. (See
cell motility). It is often said that insects have no cilia. This is not strictly true, though it could be said they have no
motile cilia (with the possible exception of scolopidia). Certainly they do not use cilia for transport or locomotion, as
say many free-living flatworms do (using ciliary tracts for gliding locomotion) or as mammals do (using cilia to
transport a mucus sheet from the airways). This modification in insects is understandable - a tough cuticle is
secreted over the body, covering any cilia that would be present. However, like many animals, the epithelium that
covers insect bodies does contain ciliated cells, but those cells are modified as sensors. These specialised epithelial
cells are sensory nerve cells, with axon 'wires' to carry signals to the central nervous system. There is considerable
evidence that nervous systems evolved from epithelial cells covering the animal body. This makes sense, since this
outermost layer of cells comes into contact with the outside world. Some of these cells, however, later migrated
deeper into the body, forming the rest of the nervous system (as they still do in the developing embryo).

It is also interesting to study the development of sense organs in insects, since they show the whole spectrum from
isolated sensory cells (single sensilla) to arrays of sensilla, such as those trichoid sensilla grouped together to
detect movement at a joint, to integrated organs containing many sensors together in a single functional unit, such
as in Johnston's organ.

Finally, insect thermohygroreceptors are apparently specially modified mechanoreceptors, as we shall see in a
future article.


To be added soon ...
labelled diagram of scolopidium
unlabelled diagram of scolopidium
Section through a pair of scolopidia in
the antenna of
Aleochara bilineata.
There are a ring of 27-28 of these
sensors, arranged in pairs.. Each
inserts into the articular mnembrane at
the junction between the pedicel and
the first flagella sub-segment (F1, the
first falgellomere). Eachs ensory cell is
of the usual bipolar type, with a cell
body and axon emerging further down
(not shown). A glial cell provides the
axon with a sheath.
Not all scolopidia have the arrangement shown above. In some the long cilium may attach directly to an attachment
cell, no ligament being present. There may also be a swelling just beneath the tip of the cilium (not visible in this
trichoid proprioceptive sensilla
Above: trichoid sensilla functioning as proprioceptive sensors on the antenna of Aleochara
. Left: a probable group of proprioceptors at the scape/head junction, positioned to detect
movements of the antennal base (scape) relative to the head. Note that more of these hairs will be
deflected (in the direction of the arrow) the further the antenna bends relative to the head. Bar = 4
micrometres. Right: a group of three trichoid sensilla at the junction of the first antennal segment
(scape) and the second antennal segment (pedicel) positioned to detect bending of these two
segments relative to one-another. Deflection of these hairs by bending at the joint has been
observed. Rings of similar hairs occur at the junctions of the other antennal segments.
trichoid proprioceptive sensilla

Perhaps the simplest type of insect mechanoreceptor is that of a non-ciliated sensory cell with many branched
dendrites, underlying a region of flexible cuticle, such as an intersegmental membrane. This type of
proprioceptive cell also occurs in many of the soft tissues, such as skeletal and gut muscles (e.g. Weevers,
1966a,b,c; Finlayson, 1968); because of its simplicity it is hard to detect with standard TEM (transmission
electron microscopy) methods. (See section below).

Trichoid sensilla may act as external proprioceptors, rather than touch receptors. This can be achieved by the
strategic positioning of one or more trichoid hairs next to the moving parts of a joint. Movement of the joint will
deflect the hairs, which have identical structure (and mode of transduction?) as those trichoid sensilla acting as
touch receptors (Finlayson, 1968)

A very common type of proprioceptor in insects is the
campaniform sensillum. It consists of a small dome
-shaped region of flexible cuticle, and is found at joints on legs, wings, and often the pedicel-flagella joint of the
antenna, and on the halteres of flies (see Wigglesworth, 1972).

An extensive study has been done on cockroach campaniform sensilla, by Moran and Varela (1971) and Moran
et al. (1971, 76). Underneath the cuticular dome is the ciliary-dendritic process of a sensory cell, which contains
a tubular body, similar to that in trichoid sensilla, in its flattened paddle-shaped tip. The tubular body presses the
membrane of the sensory tip tightly against the borders of a slot in a cuticular cap, which fits into the dome,
maintaining membrane tension. When the joint, upon which the campaniform sensilla is situated, flexes, it causes
the inward (i.e. downward) displacement of the dome. Trigonometrical calculations show that when the cap moves
inwards, the sides (between them) move inwards three times as far, pinching the tubular body three times as
hard as the downwards movement alone. Thus, the campaniform arrangement amplifies the stimulus three-fold,
increasing the sensitivity (gain) of the sensillum. In other respects, the transduction mechanism is assumed to be
the same as that for trichoid mechanoreceptors described earlier.
senory hairs on the antenna of the beetle Aleochara
Left: the small pointed hairs are
mechanosensitive trichoid sensilla which
respond to touch, but also probably
vibrations, air movements and hence
possibly sound. The large pointed hair
is also a trichoid sensilla but has a dual
function: working as both a touch and
taste sensor. The small blunt conical
pegs are olfactory sensors (see