Because part of the fear response is that the mouth dries up, thus creating a swallowing response to help lubricate the throat.
I'm not scared now. I have been scared before though, have you? One of the most obvious examples that comes to mind for this is when you're either lying or exaggerating about something to someone else - you gulp when gazed at directly. I've found this in my own life to be true. So why exactly does this response happen?
Its probably more of a side effect of whats going on in the body. Fear triggers the release of stress hormones. One of the side effects is probably dry mouth. Also in a state of fear, you are going to burn up more of your bodies reserves of energy. One being water. Your going to sweat and loose water during a fight or flight situation. This is going to make you thirsty. Dry mouth is probably just one of the many effects of fear on the body.
Oh, cortisol (panic hormone) has the effect of causing dry mouth. Cortisol does other important stuff, and dry mouth is just an unpleasant side-effect of elevated cortisol levels. This effect is eventually reversed by the effects of acetylcholine, which causes the body to return to a state of rest.
To clear the throat in preparation for all the heavy breathing that may be required in a flight or fight scenario.
Because fear stimulates the "fight or flisht" response which is mediated by the sympathetic nervous system. This suppresses the parasympathetic nervour system which controls salivation, lactrimation, urination, and defication. Therefore, you have decreased salivation and a dry mouth.
sympathetic nervous system of the ANS is activated when scared/panic causing production of small amounts of thick mucin rich saliva. Strong stimulation of the sympathetic NS constricts the blood vessels supplying the salivary glands thus inhibiting saliva release leading causing dry mouth(xerostomia) . We thus gulp in attempt to lubricate mouth and throat.
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I agree. On the scariest moment of my life I had no time to gulp. Reacted exactly like this guy. http://www.youtube.com/watch?v=G1nDDg7ApfE