Questions: Photoreceptors and Phototransduction: Converting Light to Neural Signals

Question 1

A rod photoreceptor in complete darkness holds a membrane potential near −40 mV and continuously releases glutamate. When bright light strikes it, what happens?

• A. The membrane potential rises toward 0 mV (depolarization) and glutamate release increases
• B. The membrane potential falls toward −70 mV (hyperpolarization) and glutamate release decreases
• C. The membrane potential falls toward −70 mV (hyperpolarization) but glutamate release increases to signal the stimulus
• D. The membrane potential stays near −40 mV; only the frequency of glutamate vesicle release changes

Answer: (B)

Light activates rhodopsin → transducin → phosphodiesterase → cGMP hydrolyzed → cGMP-gated channels close → dark current stops → hyperpolarization. Less depolarization means less calcium-dependent glutamate release at the synaptic terminal. This 'inverted' signaling (less activity with more light) is counterintuitive but is the actual mechanism. Option A describes a conventional neuron's response, not a photoreceptor's.

Question 2

Why is the dark current design — maintaining a tonically active resting state that light then suppresses — advantageous over a simpler design where light would directly depolarize photoreceptors?

• A. It reduces metabolic cost because cGMP-gated channels are closed most of the time in normal daylight
• B. The tonic baseline enables signaling both increases and decreases in illumination, and the enzymatic cascade provides amplification sufficient for single-photon detection
• C. It prevents photoreceptor saturation at extremely high light intensities by limiting the maximum depolarization
• D. It ensures that the photoreceptor never becomes refractory, allowing sustained responses to steady light

Answer: (B)

Operating from a tonically active baseline allows graded bidirectional responses: brighter light hyperpolarizes more (less glutamate), dimmer light depolarizes slightly (more glutamate). Additionally, the enzymatic cascade — one rhodopsin activating hundreds of transducins, each activating a PDE that destroys thousands of cGMP molecules — provides enormous signal gain, enabling rods to detect a single photon. A direct depolarizing design couldn't achieve this amplification.

Question 3

A single activated rhodopsin molecule can trigger closure of hundreds of cGMP-gated ion channels through amplification by the G-protein transducin and phosphodiesterase.

• T. True
• F. False

Answer: True

This is the amplification cascade: one photon isomerizes one retinal → activates one rhodopsin → activates ~500 transducin molecules → each activates a PDE → each PDE hydrolyzes ~1000 cGMP/second → many cGMP-gated channels close. This cascade amplification is why rods can detect single photons. Without this gain, thermal noise in individual molecules would swamp the signal.

Question 4

Photoreceptors release more glutamate in bright light than in darkness.

• T. True
• F. False

Answer: False

The opposite is true. In darkness, the dark current (inward flow of Na+ and Ca²+ through cGMP-gated channels) partially depolarizes photoreceptors to about −40 mV, causing continuous glutamate release. Light closes these channels, hyperpolarizes the cell, and reduces glutamate release. Downstream neurons (bipolar cells) are wired to interpret a decrease in glutamate as a light signal, not an increase.

Question 5

Explain why photoreceptors hyperpolarize in response to light rather than depolarizing like most neurons, and describe one functional advantage of this inverted signaling design.

Model answer: In darkness, high cGMP keeps cation channels open, partially depolarizing the cell (the dark current). Light activates a cascade that hydrolyzes cGMP, closing the channels and stopping the inward current — hyperpolarization. One advantage: the tonically active baseline enables graded bidirectional responses (more light → more hyperpolarization → less glutamate; less light → partial depolarization → more glutamate). Another advantage: enzymatic amplification in the cascade enables single-photon detection sensitivity.

This counterintuitive design exists because it solves two problems simultaneously. First, signal direction: by starting from a depolarized baseline, photoreceptors can signal both light increments (more hyperpolarization) and light decrements (less hyperpolarization). Second, sensitivity: the multi-step enzymatic cascade between photon absorption and channel closure provides massive signal amplification that would be impossible with a direct ligand-gated channel. The metabolic cost of maintaining the dark current is the price paid for this extraordinary sensitivity.