Intracellular recording from a muscle fiber at a normal neuromuscular junction shows small, spontaneous depolarizations of about 0.4 mV occurring randomly at the end plate region. These miniature end plate potentials represent:

• A Spontaneous exocytosis of the contents of a single synaptic vesicle ✓
• B Leakage of individual acetylcholine molecules across the presynaptic membrane
• C Simultaneous discharge of all motor units innervating the fiber
• D Summation of hundreds of quanta released by a nerve impulse

Explanation

Acetylcholine is packaged in synaptic vesicles, each holding roughly 5,000 to 10,000 molecules, and this packet constitutes one quantum. Occasional vesicles fuse spontaneously with the presynaptic membrane even without a nerve impulse, and the resulting focal depolarization is the miniature end plate potential. It is far too small to reach threshold, unlike the end plate potential of a nerve impulse, which is the summed effect of many simultaneous quanta and exceeds threshold by a large safety margin.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.