Date of Award

2000

Degree Name

Biomedical Sciences

College

Joan C. Edwards School of Medicine

Type of Degree

Ph.D.

Document Type

Dissertation

First Advisor

Todd L. Green

Second Advisor

William B Rhoten

Third Advisor

Ruu-Tong Wang

Fourth Advisor

Monica Valentovic

Fifth Advisor

Sasha N. Zill

Sixth Advisor

Leonard J. Deutsch

Abstract

The ability to detect changes in forces is important for effective use of a leg in posture and locomotion. This thesis examines how forces are detected in the legs of cockroaches by tibial campaniform sensilla. Campaniform sensilla are mechanoreceptors that encode forces through ovoid cuticular caps embedded in the exoskeleton. The tibial sensilla are unique in that they consist of two subgroups with mutually perpendicular cap orientations.

We characterized the responses of the tibial receptors in restrained preparations by applying forces to the leg at controlled magnitudes and rates. The tibial sensilla, as a group, were sensitive to increasing and decreasing forces. Discharges of individual afferents depended upon the direction of force application and the orientation of the receptor’s cuticular cap. Tonic discharges of the sensilla signaled the magnitude of force while the phasic activity accurately encoded the rate of force application. Sensitivities to changing rates of force were strictly preserved in the presence of a wide range of static loads. These discharges could be utilized to adapt posture and walking when animals stand upon or traverse irregular terrains. Discharges to decreasing forces indicate leg unloading during walking and could rapidly signal force decreases during slipping or loss of ground support.

We also tested the response properties of tibial sensilla in freely standing animals. The substrate upon which the animal stood was displaced horizontally using ramp and hold stimuli at varied rates. The receptors showed short latency responses that were initiated in the early period of platform movement. The activity of individual sensilla depended upon the direction of displacement and the orientation of their cuticular cap.

Receptors of different cap orientations responded to different directions of displacement, although the range of directional sensitivities was relatively broad. Afferent responses were extremely sensitive to the rate of platform movement. These results support the hypothesis that discharges of the receptors result from forces that are transmitted to the tibia when the foot initially moves with the substrate. These studies confirm the findings in restrained preparations and suggest that tibial sensilla are tuned to monitor changing forces that could occur during posture and locomotion.

Subject(s)

Cockroaches – Anatomy.

Locomotion – Research.

Posture – Research.

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