Mechanoreceptors

Mechanoreceptors are specialized sensory neurons that transduce the mechanical parameters of force or stretch of muscle fibers or tendons to spike frequency (muscle spindles, Golgi tendon organs, Mytilus anterior byssus retractor muscle stretch receptors). They also sense the bending of specialized hairs (trichoid hairs of arthropods, tricobothria, tricholiths) in a particular range of directions, or the angle of one limb segment with respect to the next segment (chordotonal organs). Other mechanore-ceptors respond to the fluid pressure directly surrounding the sensor (mammalian pacinian corpuscles), or the stretch of an elastic walled vessel in response to internal fluid pressure (mammalian baroreceptors).

Whether a mechanoreceptor neuron is said to respond to stretch, force, or pressure depends largely on the mechanical design of the supporting connective tissues to which the receptor is attached, or in which it is embedded. A mechanosen-sory neuron by itself is soft and has a great deal of compliance (little stiffness). So that the neuron will not be torn apart by relatively large forces and displacements, it is generally associated with tough, elastic tissue that is effectively in parallel with it and that protects it mechanically. For example, force sensing is generally done under relatively isometric conditions. That is, there is little compliance in the tendons that attach a muscle to its origin and insertion; the tendon behaves like a very stiff spring. The terminal arborizations of the force-sensing neurons attach to the collagen fibers of the tendon and thus experience little physical displacement themselves, enough, however, to produce a firing rate proportional to the total force on the tendon.

Another strategy is used to sense muscle length. A relatively stiff fiber is embedded in the muscle in parallel with its fibers, so that when the entire muscle is passively stretched to a new length, the embedded fiber undergoes a small elongation. It is this small elongation that is sensed by a stretch mechanoreceptor. The stretch mechanoreceptor stops firing when the muscle generates internal force and shortens. In the vertebrate spindle organ, one will see that the stiff fiber is a specialized muscle fiber that can be made to shorten to "take up the slack" when the entire muscle shortens, thus maintaining the sensitivity of the length receptor over a wide range of muscle length. Muscle length receptors generally generate a spike frequency signal that has a strong derivative component in it, as well as being sensitive to the stretch, AL. The terminal processes of mechanoreceptors responding to force, length, or pressure have specialized, strain-gated ion channels that admit Na+ (or some other cation), which depolarizes the neuron and causes the SGL to generate a spike output.

Below, some important vertebrate and invertebrate mechanoreceptors are described. Note that there are many more mechanoreceptors in nature than there is room to describe in this chapter. The examples chosen are interesting and cover a wide range of stimulus modalities.

2.3.1 Insect Trichoid Hairs

One of the most plentiful and simple mechanoreceptors is the ubiquitous trichoid hair mechanoreceptor found on all insects (and often on their larvae) in great numbers. Trichoid hairs are exosensors; they are deflected by direct touch, air currents, low-frequency sound, or a nearby static electrical charge of either polarity. Patches of trichoid hairs also act as external proprioceptors, advising the animal of the relative position of its head with its thorax, and the positions of its upper legs (coxae) with respect to the body (Schwartzkopff, 1964). Trichoid hairs are located all over insect bodies, including legs, abdomen, thorax, wings, head, and antennae. Orthopteran insects such as cockroaches have twin, spikelike appendages protruding from their anal region, called cerci (plural of cercus). Each cercus is covered with trichoid hairs, and in some species, other specialized mechanoreceptors (which may have evolved from hairs) that sense the direction of the gravity vector (see Section

2.6). Some of the trichoid hair sensillae on insect antennae also carry chemosensor cells and thus serve a dual exosensory role for the insect.

A trichoid hair has a specialized, elastic socket in the body surface cuticle that in the absence of external force maintains the hair erect from the body surface. One or two sensory neurons send thin, specialized, distal processes that attach to the movable base of the hair. When the hair bends in its socket, for whatever reason, the tips of the distal processes are stretched. This stretch induces a membrane depolarization, which spreads electrotonically to and over the soma of the sensory neuron, to the SGL on the axon where nerve spikes are initiated. The firing frequency is generally proportional to the rate of hair deflection plus its deflection. Always found in association with the monopolar sensory neuron(s) of a trichoid sensilla are two types of specialized epidermal cells: trichogen cells, which embryonically secrete the specialized cuticle of the hair, and tormogen cells, which secrete the cuticle forming the flexible socket. Figure 2.3-1 illustrates a schematic cross section through a typical (generic) insect mechanosensory hair. The diagram of another sensory hair system on the caterpillar of Vanessa urticae is shown in Figure 2.3-2.

Hair Insects

FIGURE 2.3-1 Schematic cross section through a typical insect hair plate sensillum. A single mechanosensory cell sends a thin process into the hollow core of the movable hair. The tormogen cell secretes the elastic cuticle that supports the hair and allows it to bend. The trichogen cell secretes the specialized cuticle that forms the hair. Deflection of the hair strains the distal process of the sensory cell and leads to depolarization and spikes. (Not shown are the basement membrane cells, glial cells around the sensory cell, and supporting tissues.)

FIGURE 2.3-1 Schematic cross section through a typical insect hair plate sensillum. A single mechanosensory cell sends a thin process into the hollow core of the movable hair. The tormogen cell secretes the elastic cuticle that supports the hair and allows it to bend. The trichogen cell secretes the specialized cuticle that forms the hair. Deflection of the hair strains the distal process of the sensory cell and leads to depolarization and spikes. (Not shown are the basement membrane cells, glial cells around the sensory cell, and supporting tissues.)

Tactile Hair Insects

FIGURE 2.3-2 Schematic of cross section through the base of a tactile hair from the caterpillar of Vanessa urticae. Parts: a, base of hair; b, articular membrane; c, sensitive process with scolops; d, sense cell; e, sheath cell (neurilemma); f, cuticula; g, trichogen cell; h, basal membrane; i, tormogen cell, k, vacuole; l, hypodermis. (From Schwartzkopff, J., in The Physiology of Insecta, Vol. 1, M. Rockstein, Ed., Academic Press, New York, 1964, © Academic Press. With permission from Academic Press.)

FIGURE 2.3-2 Schematic of cross section through the base of a tactile hair from the caterpillar of Vanessa urticae. Parts: a, base of hair; b, articular membrane; c, sensitive process with scolops; d, sense cell; e, sheath cell (neurilemma); f, cuticula; g, trichogen cell; h, basal membrane; i, tormogen cell, k, vacuole; l, hypodermis. (From Schwartzkopff, J., in The Physiology of Insecta, Vol. 1, M. Rockstein, Ed., Academic Press, New York, 1964, © Academic Press. With permission from Academic Press.)

In addition to body part proprioception, trichoid sensillae found on the heads of flying insects provide the creature with information about aerodynamic speed and air direction, and thus can probably sense flight instability such as yaw. Because an insect hair is an electret, i.e., it has permanent bound charges on its surface, the near approach of any other charged or conducting object will attract or repel the hair, producing a sensory output. This property can be of use to insects like cockroaches in avoiding predators in the dark.

2.3.2 Insect Campaniform Sensilla

Like trichoid hair sensors, campaniform sensilla (CS) are widely distributed over the bodies of terrestrial insects and chelicercates (scorpions). CS are proprioceptors, however, sending information to the insect CNS about mechanical strains present in the surrounding cuticular exoskeleton. They are found in groups in various locations on an insect's body where it is important to monitor the forces acting on certain body parts. For example, they are found in two groups on the subcosta of each of the locust's forewings, and in one group on each subcosta of the hind wings. When the CS are destroyed or blocked electrically, reflex control of wing twisting (required for normal flight) and body orientation during flight are abnormal. The CS of the hind wings are necessary for the regulation of forewing twisting during constant-lift, and those of the forewing for the maintenance of stability of the body about its three axes (Finlayson, 1968).

A more static application of CS is on the various segments of the legs of all insects. On the trochanter of the cockroach leg there are about 70 CS, located in three ventral and one dorsal group. The CS sense strain in the leg cuticle; hairs and other interoceptors monitor the positions of the parts of a leg. Finlayson (1968) states, "Probably the major function of the campaniform sensillum is to register stresses produced (a) by the weight of the insect's body on the limb, (b) by the resistance of the cuticle to the actions of the muscles and (c) by external agencies that tend to alter the normal spatial relationship between different regions of the legs, at rest or in motion, and the body." Since most insects lack specific gravity receptors, it is apparent that the CS figure in that important role, albeit indirectly. DiCaprio et al. (1998) examined the nonlinear transfer function of CS in the cockroach tibia using the white noise method (see Section 8.8).

Another insect organ that is well populated with CS is the haltere (see Section 2.7). Halteres are organs evolved from the hind wings of dipteran insects that serve as vibrating gyroscopes. It can be shown that any departure from level flight (i.e., roll, pitch, or yaw) will produce torques on the bases of the halteres at twice the vibration frequency. It is known that the bases of each haltere are well endowed with sensors including hairs, chordotonal organs, and CS (Bullock and Horridge, 1965). It is likely that the CS measure the gyro torques on the halteres and send this information to the fly's flight control center.

A CS has one ciliary-based sensory cell located under the cuticle of the insect exoskeleton. The sensory cell sends a modified ciliary microtubule distal process through a conical opening in the cuticle to make contact with the cap, which is held in a craterlike socket on the outside of the exoskeleton. Scanning electron micrographs show the cap to be ellipsoidal, seen from the top. The ciliary distal process of the sensory neuron is covered with a thick tubule that makes contact with the cap. Thus, shear forces in the cuticle that distort the cap (presumably more on one axis because of its noncircularity), presumably impart a microdisplacement to the ciliary microtubule distal process, leading to depolarization and spikes on the axon. Figure 2.2-3 illustrates a schematic cross section through a CS. The crater diameter is about 20 to 25 |im. Similar to insect hair cells, there are supporting cells surrounding the sensory cell under the cuticle.

Tactile Hair Insects
FIGURE 2.3-3 Highly schematic cross section through a campaniform sensillum. The "crater" that holds the cap plate is about 20 |m in diameter. Insects use campaniform sensilla to measure shear forces in their exoskeletons.

2.3.3 Muscle Length Receptors

One of the more interesting mechanoreceptors known to physiologists and to biomedical engineers is the vertebrate muscle spindle. This organ serves to advise an animal's motor control system of the length of an individual muscle and the rate of change of its length (rather than the force acting on the muscle). The spindle is particularly interesting because it is an enteroreceptor that operates under feedback control that preserves its sensitivity regardless of muscle length.

Figure 2.3-4 illustrates a spindle schematically. The entire spindle is 4 to 10 mm in length, and contains from 3 to 12 specialized intrafusal muscle fibers. There are three types of intrafusal muscle fibers: (1) a dynamic nuclear bag fiber; (2) a static nuclear bag fiber; (3) one or more nuclear chain fibers (see Kandel et al., 1991, Ch. 37). The contractile states of these muscle fibers are controlled by two motor nerves: a dynamic gamma motor axon (yd) (innervates the dynamic nuclear bag fiber), and a static gamma motor axon (ys) (innervates the static nuclear bag fiber and the nuclear chain fibers). (y refers to the diameter classification of the motor fibers; they are often called fusimotor fibers, as well.) The centers of the intrafusal muscle fibers of the spindle are noncontractile, elastic connective tissue. Thus, the y motor innervation is applied to both ends of the intrafusal fibers.

FIGURE 2.3-4 Schematic of a muscle spindle of about 4 mm in length. It is nestled between the main bundles of fibers of the extrafusal muscle (EFM) and connected to them by fascia. Note the two y-motoneutrons that stimulate the intrafusal muscle fibers (IMF) of the spindle, whose output nerves are the type Ia and II afferents shown. See text for more details. (From J.R. LaCourse,48 with permission.)

Wrapped around the centers of each fiber are the dendritic endings of the sensory neurons. The sensory endings of the single, primary, type Ia, afferent fiber (annulospiral endings) make contact with all three types of intrafusal muscle fibers. However, the sensory endings of the type II, secondary fiber (flower spray endings) make contact mostly with the centers of the nuclear chain fibers and also the center of the static nuclear bag fiber. Surrounding the y motor end plates and the sensory endings of the type Ia and II spindle afferent nerves is a protective, fluid-filled capsule. The whole spindle is attached by connective tissue in parallel with the normal extrafusal fibers of a muscle. When the muscle is stretched (lengthens), so is the spindle to a lesser degree. The stretching of the intrafusal fibers stimulates the sensory nerves to spike. When the extrafusal muscle shortens (contracts actively), the spindle fibers go slack and the spike activity on the type Ia and II sensory nerves goes to zero. However, a spindle normally operates under closed-loop conditions so that the CNS activates the y fusimotor fibers

Muscle Spindle

FIGURE 2.3-4 Schematic of a muscle spindle of about 4 mm in length. It is nestled between the main bundles of fibers of the extrafusal muscle (EFM) and connected to them by fascia. Note the two y-motoneutrons that stimulate the intrafusal muscle fibers (IMF) of the spindle, whose output nerves are the type Ia and II afferents shown. See text for more details. (From J.R. LaCourse,48 with permission.)

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Responses

  • Mari
    Where are mechanoreceptors in insects found?
    8 years ago
  • fiyori
    Is there mechanoreceptor in insect leg hair?
    8 years ago

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