Acetylcholine is released from a presynaptic neuron into the synaptic cleft. Once in the synaptic gap, acetylcholine can: - Bind to presynaptic receptors: presynaptic activation or inhibition leads to automodulation of the presynaptic cholinergic neuron.
When welts are a sign of a medical problem, they are usually accompanied by other symptoms which provide clues to the problem. Physical trauma such as a blow can cause a welt, and welts can also be caused by allergic reactions, infections, injections, insect bites, and skin reactions, in very sensitive people. For example, in people with dermographism, even a gentle touch to the skin can cause a welt to emerge.
Causes. The welts that come with hives arise when certain cells release histamine and other chemicals into your bloodstream. Doctors often can't identify the reason for this skin reaction, or why it sometimes turns into a long-term problem (chronic hives). But the skin reaction may be triggered by: 1  Pain medications. 2  Insects or parasites. 3  Infection. 4  Scratching. 5  Heat or cold. 6  Stress. 7  Sunlight.