Firing properties of muscle spindles supplying the intrinsic muscles of the foot in unloaded and free-standing humans

Abstract

Human posture and locomotion are dependent on the sensory apparatus - involving muscle spindles, cutaneous afferents and the vestibular system - that provides proprioception. In my previous work with my Bachelor of Medical Research, I investigated the relationship between galvanic vestibular stimulation and the sensitivity of muscle spindles of the long muscles of the leg. While that study showed no correlation between these systems it was limited by the lack of subject postural threat. In order to record from muscle spindles directly during unsupported free-standing, a new methodology for microneurographic recording from the posterior tibial nerve at the ankle was developed. For the first time, we have been able to identify the firing properties of muscle spindle endings in the small (intrinsic) muscles of the foot, as well as mechanoreceptors in the skin of the sole, while the participant is standing unsupported. This thesis presents this methodology along with the recordings made. In Study 1, the firing properties of 26 muscle spindles supplying the intrinsic muscles of the foot are described in unloaded conditions. Their responsiveness to stretch and related joint movements is shown to be similar to those in the short muscles in the hand and the long leg muscles. Only 27% were spontaneously active, of which there was no consistent resting firing rate or discharge variability. In Study 2, activity from 12 muscle spindles supplying the intrinsic foot muscles in unsupported free-standing conditions is described. In this group 50% were spontaneously firing and 67% had activity correlated with changes of centre of pressure recorded by a force plate, primarily (88%) along the anteroposterior axis. In Study 3, the activity of 28 multiunit cutaneous afferent recordings, as well as of 15 single-unit cutaneous afferents, supplying the sole of the foot in unsupported free standing is described. Activity of cutaneous afferents was found to be dependent on receptor type and location of receptive field. The data presented in this report is proof of this novel methodology's suitability for detailed study into the sensory sources in the foot contributing to maintaining the upright posture.

Date of Award	2021
Original language	English