The only neurons in the retina that produce all-or-none action potentials are ganglion cells and amacrine cells. The photoreceptors, bipolar cells, and horizontal cells instead produce only graded depolarizations or hyperpolarizations, analogous to EPSPs and IPSPs.
The transduction of light energy into nerve impulses follows a cause-and-effect sequence that is the inverse of the usual way in which sensory stimuli are detected. This is because, in the dark, the photoreceptors release an inhibitory neurotransmitter that hyperpo-larizes the bipolar neurons. Thus inhibited, the bipolar neurons do not release excitatory neurotransmitter to the ganglion cells. Light inhibits the photoreceptors from releasing their inhibitory neuro-transmitter and by this means stimulates the bipolar cells, and thus the ganglion cells that transmit action potentials to the brain.
A rod or cone contains many Na+ channels in the plasma membrane of its outer segment (see fig. 10.38), and in the dark, many of these channels are open. As a consequence, Na+ continuously diffuses into the outer segment and across the narrow
In Dark cGMP
In Light cGMP
Decline in cGMP closes Na+ channel
Dark current stops
Na+/K+ pumps continue
Loss of cations causes rod to become hyperpolarized, inhibiting its release of neurotransmitter
■ Figure 10.39 The effect of light on the retinal cells. In the dark (a), a continuous dark current causes the rod to release inhibitory neurotransmitter. In the light (b), the conversion of cyclic GMP (cGMP) to GMP causes Na+ channels in the outer segment of the rod to close (see text for details). This prevents the dark current, thereby hyperpolarizing the rod. When the rod is hyperpolarized, it no longer secretes inhibitory neurotransmitter. This allows the bipolar cell to become activated, releasing excitatory neurotransmitter to the ganglion cell, which can then generate action potentials in its axon (part of the optic nerve).
Release of inhibitory neurotransmitter continuous in the dark
No inhibitory neurotransmitter
Bipolar cell does not stimulate ganglion cell
Release of excitatory neurotransmitter stimulates ganglion cell
stalk to the inner segment. This small flow of Na+ that occurs in the absence of light stimulation is called the dark current, and it causes the membrane of a photoreceptor to be somewhat depolarized in the dark. The Na+ channels in the outer segment rapidly close in response to light, reducing the dark current and causing the photoreceptor to hyperpolarize.
It has been discovered that cyclic GMP (cGMP) is required to keep the Na+ channels open, and that the channels will close if the cGMP is converted into GMP. Light causes this conversion and consequent closing of the Na+ channels. When a photopigment absorbs light, H-c/s-retinene is converted into its isomer, all-trans-retinene (fig. 10.38) and dissociates from the opsin, causing the opsin protein to change shape. Each opsin is associated with over a hundred regulatory G-proteins (see chapter 7) known as transducins, and the change in the opsin induced by light causes the alpha subunits of the G-proteins to dissociate. These G-protein subunits then bind to and activate hundreds of molecules of the enzyme phosphodiesterase. This enzyme converts cGMP to GMP, thus closing the Na+ channels at a rate of about 1,000 per second and inhibiting the dark current (fig. 10.38). The absorption of a single photon of light can block the entry of more than a million Na+, thereby causing the photoreceptor to hyperpolarize and release less inhibitory neuro-transmitter. Freed from inhibition, the bipolar cells activate ganglion cells, and the ganglion cells transmit action potentials to the brain so that light can be perceived (fig. 10.39).
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