When the head suddenly begins to rotate in any direction (called angular acceleration), the endolymph in the semicircular ducts, because of its inertia, tends to remain stationary while the semicircular ducts turn. This mechanism causes relative fluid flow in the ducts in the direction opposite to head rotation.
Figure 1 shows a typical discharge signal from a single hair cell in the crista ampullaris when an animal is rotated for 40 seconds, demonstrating that (1) even when the cupula is in its resting position, the hair cell emits a tonic discharge of about 100 impulses per second; (2) when the animal begins to rotate, the hairs bend to one side and the rate of discharge increases greatly; and (3) with continued rotation, the excess discharge of the hair cell gradually subsides back to the resting level during the next few seconds.
Fig1. Response of a hair cell when a semicircular canal is stimulated first by the onset of head rotation and then by stopping rotation.
The reason for this adaptation of the receptor is that within the first few seconds of rotation, back resistance to the flow of fluid in the semicircular duct and past the bent cupula causes the endolymph to begin rotating as rapidly as the semicircular canal itself; then, in another 5 to 20 seconds, the cupula slowly returns to its resting position in the middle of the ampulla because of its own elastic recoil.
When the rotation suddenly stops, exactly opposite effects take place: The endolymph continues to rotate while the semicircular duct stops. This time, the cupula bends in the opposite direction, causing the hair cell to stop discharging entirely. After another few seconds, the endolymph stops moving and the cupula gradually returns to its resting position, thus allowing hair cell discharge to return to its normal tonic level, as shown to the right in Figure 1. Thus, the semicircular duct transmits a signal of one polarity when the head begins to rotate and of opposite polarity when it stops rotating.
“Predictive” Function of the Semicircular Duct System in the Maintenance of Equilibrium. Because the semicircular ducts do not detect that the body is off balance in the forward direction, in the side direction, or in the backward direction, one might ask: What is the function of the semicircular ducts in the maintenance of equilibrium? All they detect is that the person’s head is beginning or stopping to rotate in one direction or another. Therefore, the function of the semicircular ducts is not to maintain static equilibrium or to maintain equilibrium during steady directional or rotational movements. Yet, loss of function of the semicircular ducts does cause a person to have poor equilibrium when attempting to perform rapid, intricate changing body movements.
The function of the semicircular ducts can be explained by the following illustration: If a person is running forward rapidly and then suddenly begins to turn to one side, he or she will fall off balance a fraction of a second later unless appropriate corrections are made ahead of time. However, the maculae of the utricle and saccule cannot detect that the person is off balance until after the loss of balance has occurred. The semicircular ducts, however, will have already detected that the person is turning, and this information can easily apprise the central nervous system of the fact that the person will fall off balance within the next fraction of a second or so unless some anticipatory correction is made.
In other words, the semicircular duct mechanism predicts that disequilibrium is going to occur and thereby causes the equilibrium centers to make appropriate anticipatory preventive adjustments, which helps the person maintain balance before the situation can be corrected.
Removal of the flocculonodular lobes of the cerebellum prevents normal detection of semicircular duct signals but has less effect on detecting macular signals. It is especially interesting that the cerebellum serves as a “predictive” organ for most rapid movements of the body, as well as for those having to do with equilibrium. These other functions of the cerebellum are discussed in Chapter 57.
