Inside the cell the normal levels of cyclic guanosine monophosphate (cGMP) keep the Na+ channel open, and thus in the resting state the cell is depolarised. The photon causes the retinal bound to the receptor protein to isomerise to trans-retinal. This causes the receptor to activate multiple G-proteins. This in turn causes the Ga-subunit of the protein to activate a phosphodiesterase (PDE6), which degrades cGMP, resulting in the closing of Na+ cyclic nucleotide-gated ion channels (CNGs). Thus the cell is hyperpolarised. The amount of neurotransmitter released is reduced in bright light and increases as light levels fall. The actual photopigment is bleached away in bright light and only replaced as a chemical process, so in a transition from bright light to darkness the eye can take up to thirty minutes to reach full sensitivity. When thus excited by light, the photoceptor sends a proportional response synaptically to bipolar cells which in turn signal the retinal ganglion cells. The photoreceptors are also cross-linked by horizontal cells and amacrine cells, which modify the synaptic signal before it reaches the ganglion cells, the neural signals being intermixed and combined. Of the retinas nerve cells, only the retinal ganglion cells and few amacrine cells create action potentials. In the retinal ganglion cells there are two types of response, depending on the receptive field of the cell. The receptive fields of retinal ganglion cells comprise a central, approximately circular area, where light has one effect on the firing of the cell, and an annular surround, where light has the opposite effect.
Triplegia is a medical condition characterized by the paralysis of three limbs. While there is no typical pattern of involvement, it is usually associated with paralysis of both legs and one arm—but can also involve both arms and one leg. Triplegia can sometimes be considered a combination of hemiplegia (paralysis of arm and leg of one side of the body) overlaying diplegia (paralysis of both legs), or as quadriplegia (paralysis of four limbs) with less involvement in one extremity.The condition is commonly associated with cerebral palsy, although conditions such as strokes can also lead to it. Triplegia has also been found to be due to an increase in intracranial pressure associated with hydrocephalus resulting from traumatic brain injury.A similar condition is triparesis, in which a person has paresis in three limbs, meaning that the limbs are very weak, but not completely paralyzed. In a case reported only due to its rarity, triplegia was reported following a surgical removal of the tonsils. An eight-year-old boy was sent to Willard Parker Hospital on August 12, 1929, and had been diagnosed with poliomyelitis. After an unrelated, and routine, tonsillectomy there was complete flaccid paralysis and loss of feeling in both the legs, right arm, and muscles in the trunk. == References ==