What type of receptors embedded in the urinary bladder wall initiate the micturition reflex?

Answer is stretch receptors.

Stretch receptors respond to the distention of muscle wall of the bladder and are neurologically linked to the medulla. Stretch receptors in the bladder signal the parasympathetic nervous system to stimulate muscarine receptors, present in the detrusor muscle, to make him contract. This leads to the expulsion of urine.