The mechanism by which the human eye can detect a single photon of light provides a marvelous example of the chemical processes underlying neuronal function. It involves both trapping of photons and the transducer effect, whereby the energy of light is converted into a chemical form, which is then ultimately transmuted into an action potential by a retinal ganglion neuron. A number of the intermediates are as yet not precisely known, but the underlying hypothesis is that the receptor protein, rhodopsin, is coupled to the G-protein. There are several sequence homologies of rhodopsin with the adrenergic β-receptor and with the muscarinic ACh receptor. The main steps take place in the following order (Fig. 39.4):
- Cis-retinal is converted to trans-retinal,
- Rhodopsin becomes activated,
- The level of cGMP decreases,
- Na+ entry is blocked,
- The rod cell hyperpolarizes,
- There is release of glutamate (or aspartate),
- An action potential depolarizes the adjacent bipolar cell,
- This depolarizes the associated ganglion neuron, to send an action potential out of the eye.
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| Figure 39.4 Neurochemistry of synaptic transmission in the mechanism of vision. The figure shows the consequences of photon activation of rhodopsin via G-protein coupling in a rod cell. Phosphodiesterase is activated and hydrolyzes the second messenger, cGMP, thereby blocking the entry of sodium and causing hyperpolarization of the cell. Currently, the steps through which neurotransmission subsequently proceeds to produce the final ganglion neuron action potential are not known in detail. Compare G-protein-coupled receptor (Fig. 38.3). |
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| Twenty-four hours after eating home-preserved vegetables, a healthy young woman experienced progressive onset of blurred vision, severe vomiting, dysphagia, and advancing limb weakness starting in the shoulders. Her doctor admitted her to hospital, and electrophysiological studies confirmed the clinical diagnosis of botulism. Trivalent antiserum, made from inactivated toxin is administered immediately and, with the help of assisted ventilation, the patient recovers within a few weeks. |
| Comment. The vegetables contained the exotoxin of the anerobe, Clostridium botulinum, which had not been destroyed during the preservation process. The toxin hydrolyzes the presynaptic proteins involved in the release of neurotransmitter, and thus the blockade is similar to the functional lesion in Lambert-Eaton myasthenic syndrome; however, in botulism the blockade can be lethal, especially at the level of the phrenic nerve which is essential for appropriate respiratory lung movement. |
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