Table 2.

Number of Ca²⁺ channel at hair cell synapses. This table summarizes part of the available information obtained in diverse preparations. The analysis is complicated by the low open probability of hair cell L-type Ca²⁺ channels in the absence of the dihydropyridine agonist of L-type Ca²⁺ channels, BayK8644. The numbers provided are rough approximations relying on several assumptions (e.g., extrasynaptic Ca²⁺ channel density) and should be taken with caution (LF low frequency, HF high frequency)

Species/organ	Hair cell	Total number of channels	Channels per active zone (total/maximal open)	References
Frog sacculus		~1,800	~90/~20	Roberts et al. 1990a, assuming an exclusively synaptic Ca2+ channel localization
Turtle basilar papilla		~2,240 (HF) ~400 (LF)	~40 ~20	Wu et al. 1995b; Sneary 1988, assuming an exclusively synaptic Ca2+ channel localization
Mammalian cochlea	Inner hair cells	~1,700	~80/~30	Brandt et al. 2005,a assuming an extrasynaptic channel density of 1 channel/μm2
Avian basilar papilla	Tall hair cells	~350 (HF) ~200 (LF)	~15 ~23	Martinez-Dunst et al. 1997b, assuming an exclusively synaptic Ca2+ channel localization
	Short hair cells	~100

^aEstimates obtained by non-stationary fluctuation analysis on Ca²⁺ tail currents estimates

^bApproximations provided by the cited reference based on whole-cell Ca²⁺ or Ba²⁺ current measurements and single channel currents