B. Hair cells organized in a sensory epithelium (macula); two macula, one in utricle and one in saccule

C. Adequate Stimulus: linear acceleration and gravitational attraction

Note 1: Each hair cell unit is polarized; i.e. it increases firing rate when it bends in a particular direction and decreases firing rate when it bends in the opposite direction

Note 2: Because the hair cells are variously oriented, linear acceleration/gravity from all directions is sensed  (utricle and saccule contain a number of patches of hair cell groups, with each patch oriented in a unique direction)

Note 3: Hair cell units do not respond to constant velocity; otoliths and cilia will return to their rest position if velocity is continuously maintained

D. Role

1. sensation of acceleration and orientation in space

Note: orientation in space is derived from a combination of visual cues, proprioceptive input, cutaneous pressure, as well as utricle and saccule input

2. somatic reflex response to acceleration and gravity

Note:   utricle & saccule input very important when other cues, particularly vision, are absent or impaired