JAndrew/Mechanoreceptors.pptx

Entomology is study of insects. Insects have receptors in their body. Here we investigate the mechanoreceptors in insects.

1. Mechanoreceptors THE SENSE ORGANS

2. Insect mechanoreceptors Insect mechanoreceptors can be found almost anywhere on the surface of an insect’s body.   They may act as  tactile  receptors, detecting movement of objects in the environment, or they may provide  proprioceptive  cues (sensory input about the position or orientation of the body and its appendages).   These receptors are innervated by one or more sensory neurons that fire in response to stretching, bending, compression, vibration, or other mechanical disturbance .   Some mechanoreceptors produce a  phasic response   when stimulated — that is, they fire once when activated and again when deactivated.   Other receptors generate a  tonic response , firing repeatedly as long as a stimulus persists.   Neural processing centers in the brain or segmental ganglia interpret the combinations of tonic and phasic signals sent from nearby receptors.

3. Mechanoreceptors in insects: 1. cuticular structures with bipolar neurons. A. hair-like projections from the cuticle with a basal socket B. dome-like campaniform sensilla. Both types have similar arrangements of neurons and sheath cells called a porous sensilla Mechanosensory hairs The wall of the hair consists of exocuticle with an outer layer of epicuticle. the hairs taper from base to tip, and they are often known as trichoid sensilla Shorter, peg-like multimodal (chemo- and mechanosensory) hairs with a single pore at the tip are referred to as basiconic Mechanosensory hairs on the cerci of crickets and cockroaches, for example, are very long relative to their diameter and do not taper; they are called t richobothria . Some insects possess club-shaped hairs that can signal body orientation relative to the gravitational field.

4. Hair sensillum. Diagram showing the principal features of a mechanosensitive sensillum .

5. Externally a thin cuticular joint membrane is continuous with the general body cuticle and that of the hair. Beneath this, bridging a gap between the body cuticle and the hair, are rings of suspension fibers. inside, is the fibrous socket septum. Socket septum runs across the outer part of the receptor lymph cavity to the dendrite sheath. suspensory structures restore the hair to its original position following imposed movement.

6. Campaniform sensillum

7. Campaniform sensillum: campaniform sensillum : areas of thin cuticle, domed; usually oval in shape, with a diameter of 5–30 micron. The dome of thin cuticle consists of an outer homogeneous layer like exocuticl e a middle layer of transparent spongy cuticle an i nner lamellar or fibrous layer dome is connected to the surrounding cuticle by a joint membrane. The dendrite sheath, enclosing a single dendrite, is inserted into the center of the dome, and a molting pore may be visible on the outside. Campaniform sensilla are situated in areas of the cuticle that are subject to stress commonly they are usually found close to the joints . Orthopteroid insects have a single campaniform sensillum at the base of each of the tibial spines. Campaniform sensilla also occur on the mouthparts, on the basal segments of the antennae, on veins close to the wing, base and on the ovipositor. large numbers are present on the haltere of Diptera.

8. SENSORY CELLS Air movement detectors.: Some insects have hairs that are specialized for the detection of air movements, sometimes including sounds . Grasshoppers and some Lepidoptera have groups of small trichoid mechanoreceptors on specific regions at the front and top of the head Pressure receivers. Most aquatic insects are buoyant because of the air they carry beneath the water surface when they submerge, but Aphelocheirus (Hemiptera: Heteroptera), living on the bottom of streams and using a plastron for gas exchange , is not buoyant. The plastron only functions efficiently in water with a high oxygen content, such as normally occurs in relatively shallow water, so some mechanism of depth perception is an advantage to Aphelocheirus On the ventral surface of the second abdominal segment of Aphelocheirus is a shallow depression

9. containing hydrofuge hairs. These hairs are inclined at an angle of about 30 to the surface of the cuticle and mechanosensory hairs are dispersed among them. The volume of air trapped by these groups of hairs changes with pressure as the insect moves up and down in the water column, bending the sensilla. The insect responds to an increase in pressure by swimming up, but there is no behavioral response to a decrease in pressure. Mushroom-shaped pressure receptors associated with external openings of the tracheal system in Nepa (Hemiptera) detect body tilt.

10. Plastron respiration Plastron respiration Some insects have specialized structures holding a permanent thin film of air on the outside of the body in such a way that an extensive air–water interface is always present for gaseous exchange. This film of gas is called a plastron and the tracheae open into it so that oxygen can pass directly to the tissues. In adult insects the plastron is held by a very close hair pile in which the hairs resist wetting because of their hydrofuge properties and their orientation . In the adult Aphelocheirus , the plastron covers the ventral and part of the dorsal surface of the body. The hairs which hold the air are 5–6 m m high and about 0.2 m m in diameter . They are packed very close together . They are able to withstand a pressure of about 400 kN m 2 before they collapse. Hence, this is an extremely stable plastron that will only be displaced by water at excessive depths. The spiracles open into a series of radiating canals in the cuticle and these connect with the plastron via small pores. These canals are lined with hairs so that the entry of water into the tracheal system is prevented.

11. Proprioceptors Trichoid sensilla . Groups or rows of small hairs often occur at joints between leg segments (see Fig. 8.7a), on the basal antennal segments, on the cervical sclerites, at the wing base and elsewhere. The hairs are positioned so that movements of one part of the cuticle with respect to another cause the hairs to bend (Fig. 23.6). Individual hairs may have preferred directions of response. Often the hairs in a hair plate are graded in size,

12. subcuticular structures with bipolar neurons, known as chordotonal organs internal multipolar neurons which function as stretch or tension receptors. 3.

13. CHORDOTONAL ORGANS Chordotonal, or scolopophorous , organs are subcuticular receptors that act most commonly as joint proprioceptors or as hearing organs. They consist of single units or, more commonly, groups of similar units, called scolopidia , which are attached to the cuticle at one or both ends. In many insects, the scolopidia are clustered into distinct groups , called scoloparia , that are identifiable morphologically and functionally (Fig. 23.7). Each scolopidium consists of three cells: a sensory neuron, an enveloping or scolopale cell and an attachment (cap) cell

14. CHORDOTONAL ORGAN:SCOLOPIDIUM

15. SUBGENUAL ORGAN IN ANT

16. Johnston’s organ Johnston’s organ is a chordotonal organ in the antennal pedicel with its distal insertion in the articulation between the pedicel and flagellum. It occurs in all adult insects and, in a simplified form, in many larvae. It consists of a single mass or several groups of scolopidia which respond to movements of the flagellum with respect to the pedicel.

17. Tympanal organ of a grasshopper . (a) Diagrammatic horizontal section through the metathorax and first abdominal segments showing the positions of the tympanal membranes. Notice that the air sacs form a continuum across the body. (b) Diagram showing the attachment of the scolopidia to the inside of the tympanum . The cuticle of the air sac which normally covers Muller’s organ and the tympanum has been removed. The orientations of scolopidia in different parts of the chordotonal organ are indicated by arrows. Letters show the positions of cells, some of whose sensitivities are shown in Fig. 23.16a. The styliform body, elevated process and folded body are cuticular structures continuous with the tympanum ( Locusta ) (after Gray, 1960). (c) Thickness of the tympanum. Lines mark contours of thickness and numbers show the thickness in microns. Hatched areas are regions of well-sclerotized cuticle( Schistocerca ) (after Stephen and Bennet-Clark, 1982).

18. Tibial chordotonal organ of bush cricket: The complex of chordotonal organs exposed when the dorsal (anterior) cuticle of the tibia is removed ( Decticus )

19. Stretch and tension receptors Stretch and tension receptors differ from other insect mechanoreceptors in consisting of a multipolar neuron with free nerve endings, while all the others contain one or more bipolar neurons . They have a variety of forms. Sometimes they are an integral part of an oriented structure such as a muscle fiber or a strand of connective tissue, but in other cases they have no specific orientation or associated structures. The term “stretch receptor ” is sometimes used rather loosely to include all multipolar receptors, but can be misleading as some respond to isometric tension rather than degree of stretch.

20. 2.subcuticular structures with bipolar neurons, known as chordotonal organs 3. internal multipolar neurons which function as stretch or tension receptors.