one end of each semicircular duct is a crista, a ridge of hair cells across the axis of the
b. When a jet takes off, a passenger tends to remain in place at first and can
feel the resulting pressure of the seat against his back. Also, when the jet is no longer
accelerating, the passenger can feel that the pressure of the seat against his back has
returned to normal.
c. Likewise, in the appropriate semicircular duct, the endolymph ("passeng-
er") tends to remain in place early during an acceleration. Because the duct ("seat")
itself is moving with the body ("jet"), the hairs of the crista are affected by the change in
movement. Later, when acceleration stops, the effect upon the hairs of the crista is also
d. However, the cristae of the semicircular ducts detect rotation of the head
(angular acceleration and angular velocity). Linear acceleration, as with our example of
the passenger and the jet, is detected primarily by the maculae, discussed above.
13-18. RESULTING INPUTS FOR THE SPECIAL SENSE OF EQUILIBRIUM
The combined inputs from the maculae of the sacs and the cristae of the
semicircular ducts provide continuous, instantaneous information about the specific
location and posture of the head in relationship to the center of gravity of the earth.
These inputs are transmitted by the vestibular neurons to the hindbrainstem.
13-19. INPUTS FOR THE GENERAL BODY SENSE
In addition to the inputs from the membranous labyrinth, various other inputs
are used to continuously monitor the second-to-second posture of the human body.
a. We have already examined the proprioceptive sense, which monitors the
condition of the muscles of the body.
b. Various other receptors are associated with the joint capsules, the
integument, etc. They indicate the precise degree of bending present in the body.
c. A very important body sense is vision. Even when other inputs are lacking,
if an individual can see his feet, he may still be able to stand and move.
13-20. POSTURAL REFLEXES
To automatically control the posture, the human nervous system has a number
of special reflexes. These reflexes are coordinated through the cerebellum.
The head and neck tonic reflexes orient the upper torso in relationship to