A cataract is a clouding of the lens in the eye that affects vision. Most cataracts are related to aging. Cataracts are very common in older people. By age 80, more than half of all Americans either have a cataract or have had cataract surgery.
Acetylcholine. Acetylcholine (ACh) is the transmitter released by the preganglionic nerve terminals of both the sympathetic and the parasympathetic divisions (Fig 6.2). The synapse at those sites utilizes a nicotinic receptor similar in structure to the receptor at the neuromuscular junction.
Acetylcholine binds to receptors on the post-synaptic membrane and causes depolarization of the muscle fiber, which leads to its contraction. Typically, one action potential in the neuron releases enough neurotransmitter to cause one contraction in the muscle fiber. In muscle cells, the sarcolemma or plasma membrane extends transversely into the sarcoplasm to surround each myofibril, establishing the T-tubule system.