In the neural system,a chemical synapse shows a synaptic gap of about . . . . . . .

In the parasympathetic nervous system,which of the following is released?

Which of the following hormones act as neurotransmitters?

Alzheimer's disease in humans is associated with the deficiency of

True about electrical synapses:
$(I)$. Pre and postsynaptic neurons are in very close proximity.
$(II)$. Pre and postsynaptic neurons are separated by a synaptic cleft.
$(III)$. Impulse transmission is very fast.
$(IV)$. Electrical synapses are common in our system.
Select the correct option.

In $P$,the membranes of pre-synaptic and post-synaptic neurons are in very close proximity,whereas in $Q$,there is a space between the membranes of pre-synaptic and post-synaptic neurons.
$\quad\quad\quad P\quad\quad\quad Q$