Disorders of Neuromuscular Transmission

Abstract

To understand better the disorders of the neuromuscular junction (NMJ) some basic knowledge of the anatomy and physiology of this region must first be mastered. The neuromuscular junction (Figure 3-1) consists of a motor nerve ending (presynaptic terminal) and a postsynaptic muscle fiber membrane separated by a space, the synaptic cleft. The motor nerve terminal is filled with packets called synaptic vesicles, which contain acetycholine (ACh), the chemical transmitter. Each vesicle contains a quantum of ACh, which is estimated to be between 103 and 104 ACh molecules (Hartzell et al. 1976). At rest, the muscle membrane is charged, being relatively more negative on the inside compared to the outside. This resting membrane potential, which measures approximately -80 mV, arises because:

• 1. the membrane at rest is more permeable to K⁺ than to Na⁺;

• 2. an active transport mechanism maintains the internal Na⁺ concentration at a low level;

• 3. the membrane is impermeable to organic anions within the cell.