The vestibular apparatus is about the size of a pea, located in the inner ear. Within this small volume are sensory receptors, which are stimulated by angular accelerations as low as 0.05°/s2 (0.9mrad/s2) and linear acceleration of less than 0.01 G (0.1 m/s2).
In form and function the vestibular apparatus may be divided into two distinct parts – the semicircular canals (SCC) and the otolith organ. The semicircular canals are responsible for detecting angular acceleration and the otolith organ detects the linear acceleration. The basic sensory unit of the vestibular system is the hair cell. Both the SCC and the otolith organ use the movement of the hair cells to detect magnitude and direction of acceleration.
The Semicircular Canals. The semicircular canals contain the receptors responding specifically to angular accelerations with the help of three ducts (anterior/superior, lateral/horizontal, and posterior) located at 90° to each other (Figure above refers). Each semicircular duct has a swelling, the ampulla, where the sensory cells are congregated in a ridge, the crista. These cells have many hair-like projections arising from them, and are covered by a gelatinous structure, the cupula. The cupula fills across-section of the ampulla, and can be considered as a watertight swing-door, which is deflected by movement of the endolymph within the membranous duct. The activity of the sensory cells is determined by the bending of the hairline processes, which in turn is dependant on the position and movement of the ‘swing-door cupula’. Movement of the fluid inside the canal results in deflection of the cupula, which is sensed as angular acceleration.
The Otolith Organs. The otolith organs are the specialised receptors of linear accelerations and gravity. An otolith organ has two endolymph filled sacs, the utricle and saccule. This lies below the semicircular ducts and contain plate-like congregations of sensory cells – the maculae. These cells, like those found in the ampulla, have many hair-like projections which, when deflected, alter the activity of the sensory cells. In both the utricular and saccular maculae, the cells are grouped together in an irregular saucer-shaped area and are covered with a gelatinous layer, the outer surface of which is invested with small calcium carbonate crystals.
There are two otolith organs on each side of the head: those of the utricle lie in an approximately horizontal plane when the head is in a normal upright position, and those of the saccule lie in the vertical plane. Any linear acceleration causes displacement of the gel matrix, which results in deformation of the hair cells. This process is amplified due to the presence of calcium carbonate crystals. Deformation of the hair cells is interpreted as acceleration. The otolith organ is constantly subjected to the acceleration due to gravity. It thus has the tendency to interpret the resultant of any sustained gravitational force as gravity or ‘vertically down’.
Limitations of Vestibular System: Mulder’s Constant
The lowest reported threshold of perception of rotation is 0.035°/sec2, but can only be obtained with continuous acceleration after 20 to 40 seconds. A more useful way of describing perceptual thresholds is Mulder’s constant. This constant is an acceleration-time product and is found to be 2°/sec. Thus, for example, an angular acceleration of 5°/sec2 for half a second (5 x 0.5 = 2.5°/sec) is perceptible while 10°/sec2 acceleration for one tenth of a second (10 x 0.1 = 1°/sec) is not.
Otolith thresholds have been found to be on the order of 1.5° change in the direction of applied +Gz force or 0.001to 0.03 +Gz changes in the magnitude of G. As for angular acceleration, the acceleration-time product concept is useful for talking about the perception of linear acceleration. The perceptual threshold is thought to be 0.3 – 0.04 m/sec.
Stabilization of image in motion – The Vestibular-Visual Reflex Responses
The function of the vestibular-visual reflex is to stabilize the position of the eye, relative to an object fixed in space, when the head moves. Thus when the head is suddenly turned, the eyes reflexly move in the opposite direction to that of the head in order to stabilize the image of the outside world on the retina. This is vital in orientation since despite of movements of the head, the object in focus remains stable for recognition or reference, a cue vital in aviation too.
And now, we can turn to discuss Spatial Disorientation…
1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.
3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.