Table 1.

Comparison of kinetic components and putative anatomical substrates at different ribbon synapses

Cell type and kinetic component	Rate constants		Anatomical parameters				References
	Exhaustion (1/s)	Recovery (1/s)	Pool size (vesicles)	Number of active zones	Vesicles/active zones	Putative substrate
Goldfish retinal bipolar cell
1st component	650	1.6	1100	~60	18	Vesicles docked at base of ribbon	1
2nd component	8	0.2	6000		100	Vesicles tethered to ribbons	2, 3
3rd component	0.1	—	12,000		200	Cytoplasmic vesicles	4, 5
Chick cochlear tall hair cell
1st component	>20	>5	1000	~25	40	Vesicles tethered to ribbons	6
2nd component	1.7	—	6000		240	Cytoplasmic vesicles	7
Mouse cochlear IHC
1st component	100	7	280	18–20, 25	?	Unknown	8
2nd component	No saturation	—	—		?	Unknown	8
Frog saccular hair cell
Unknown	—	—	600	~20	30	Vesicles docked at base of ribbon	9, 10
Unknown	—	—	6000		300	Vesicles tethered to ribbons	9, 10

Kinetic parameters and size of multiple functional pools from several different synaptic ribbon preparations are presented. To facilitate comparison of kinetic parameters of different functional vesicle pools from different preparations, all are expressed as rate constants defined as the inverse of the exponential time constant describing these exocytic components. For example, in the case of the mouse cochlear inner hair cell (Moser and Beutner 2000), an exocytic time constant of 10 ms is described by a rate constant of 100/s whereas a recovery time constant of 140 ms results in a rate constant of 7/s. In the case of the chick cochlear tall hair cell, the data are not described as exponential time courses. Therefore, the inverse of the minimal measured time to saturation provides a conservative estimate of the rate constant. For example, the fact that the 1st exocytic component is saturated after 50 ms suggests that the exponential time constant is 50 ms or less and thus the rate constant is 20/s or more. It is important to note that in contrast to the chick, frog, and goldfish ribbon synapse, quantitative ultrastructural data on the number of active zones in the mouse inner hair cell is unavailable. Moser and Beutner (2000) cited a personal communication from Charles Liberman of 25 active zones. Recent work by Rivas et al. (abstract 424, ARO Midwinter Meeting, 2004) reported on average 18–20 afferent endings contacting a mouse inner hair cell. However, multiple synaptic ribbons subserving a single afferent contact have been observed in the hair cells of neonatal mice (Sobkowicz et al. 1986) .

1—Mennerick and Matthews 1996.

2—von Gersdorff and Matthews 1994.

3—von Gersdorff et al. 1996.

4—von Gersdorff and Matthews 1997.

5—Heidelberger et al. 2002.

6—Reported here.

7—Eisen 2000.

8—Moser and Beutner 2000.

9—Lenzi et al. 1999.

10—Lenzi et al. 2002.