What property of the neuromuscular junction makes its transmission 'obligatory,' distinguishing it from typical neuronal synapses?
AThe motor neuron releases more types of neurotransmitters than neurons in the CNS
BThe end-plate potential is reliably suprathreshold, virtually always triggering a muscle action potential
CThe NMJ uses electrical rather than chemical transmission, eliminating synaptic delay
DAcetylcholinesterase is absent at the NMJ, allowing ACh to act indefinitely
Unlike CNS synapses — where many inputs must summate to reach threshold — a single motor neuron action potential releases enough ACh to generate an end-plate potential (EPP) that far exceeds threshold. This one-to-one fidelity is what makes NMJ transmission obligatory. Acetylcholinesterase is actually present and essential; its absence (as with organophosphates) causes pathological overstimulation.
Question 2 True / False
The end-plate potential (EPP) at the neuromuscular junction is the same as the action potential that propagates along the muscle fiber membrane.
TTrue
FFalse
Answer: False
The EPP is a large, localized, graded depolarization produced by ACh opening nicotinic receptors at the motor end plate. It is not self-propagating. The EPP depolarizes the adjacent muscle membrane past threshold, triggering a separate, all-or-nothing action potential that then propagates along the entire muscle fiber. Confusing graded potentials with action potentials is one of the most common errors in neuromuscular physiology.
Question 3 Short Answer
What is a motor unit, and why does having smaller motor units in the fingers enable finer motor control than the larger motor units in the back muscles?
Think about your answer, then reveal below.
Model answer: A motor unit is one motor neuron plus all the muscle fibers it innervates. Smaller motor units (fewer fibers per neuron) allow smaller force increments — the CNS can recruit individual units to finely grade force. Larger motor units produce coarser, larger force jumps, making fine control difficult.
Force gradation in skeletal muscle is achieved by recruiting more motor units (spatial summation) and increasing firing rate (temporal summation). When each unit controls only a few fibers, the force increments are small and precise. When a unit controls hundreds of fibers, each recruitment step is a large, coarse jump — adequate for postural muscles but ill-suited for precision tasks.