Final answer:
The main factor contributing to the increased excitability of small motor neurons is the neurotransmitter release process, which is initiated when Ca2+ enters the axon terminals and interacts with vesicles, leading to exocytosis into the synaptic cleft.
Step-by-step explanation:
The main factor contributing to increased excitability of small motor neurons and the consequential nature of the size principle is related to the action potential initiation and its propagation along the neuron. When an action potential reaches the axon terminals, voltage-gated Ca2+ channels open, allowing Ca2+ to enter the synaptic end bulb. This increase in Ca2+ concentration facilitates the fusion of neurotransmitter vesicles with the presynaptic membrane, enabling neurotransmitter release through exocytosis into the synaptic cleft. This process affects the excitability of the motor neurons, with small motor neurons being more easily excited due to their relatively higher resistance and lower threshold for activation than larger neurons.
At the neuromuscular junction, the released acetylcholine binds to receptors on the sarcolemma, causing it to depolarize and initiate muscle contraction. This interaction demonstrates the critical role of neurotransmitter release in neuron excitability and the operation of the size principle, which states that small motor neurons are recruited first during muscle contraction and larger motor neurons are recruited as the force of contraction increases.