they are difficulties, obstacles or impediments, something that prevents progress and causes some deviation from the expected conclusion. In the long run however, difficulties… are not insurmountable (MORE)
. stumÂ·bled , stumÂ·bling , stumÂ·bles .
v. intr. 1. a. To miss one's step in walking or running; trip and almost fall. b. To proceed unsteadily …or falteringly; flounder. See Synonyms at blunder. c. To act or speak falteringly or clumsily. 2. To make a mistake; blunder. 3. To fall into evil ways; to err. 4. To come upon accidentally or unexpectedly (MORE)
from answers.com When a nerve impulse reaches an axon ending, voltage-gated calcium channels in the axonal membrane open and calcium, which is extremely low inside the cell…, enters the nerve ending. The increase in calcium-ion concentration causes hundreds of synaptic vesicles to fuse with the cell membrane and expel acetylcholine into the synaptic cleft (exocytosis). The acetylcholine released at a neuromuscular junction binds reversibly to acetylcholine receptors in the muscle end-plate membrane, a postsynaptic membrane that is separated from the nerve ending by a very short distance. The receptor is a cation channel which opens when two acetylcholine molecules are bound, allowing a sodium current to enter the muscle cell and depolarize the membrane. The resulting impulse indirectly causes the muscle to contract. (MORE)
It really depends on what form of writing you are talking about. Ifyour talking in past tense then it is 'stumbled' as in "He stumbledbackwards." But if you are talking in pre…sent tense then you woulduse 'stumble' as in "I stumble backwards." I hope that helped :) (MORE)