How many activated rods are needed to activate a ganglion cell?

• A. 10
• B. 8
• C. 50
• D. 15

Answer: (A)

Rods converge their signals onto a ganglion cell through intermediate neurons, so the ganglion cell integrates input from multiple rods. This arrangement boosts sensitivity in low light, allowing vision with dim illumination. When about ten rods are activated, their summed excitatory inputs typically push the ganglion cell’s membrane potential past the firing threshold, triggering an action potential. So, roughly ten activated rods are enough to activate a ganglion cell, which is why this number is commonly cited. Smaller numbers may not reach threshold, while much larger numbers simply produce a stronger response but aren’t the typical threshold for activation.