Final answer:
The transmitter causing twitch contractions in an isolated chick biventer cervicus nerve-muscle preparation is acetylcholine, which acts through the activation of nicotinic receptors.
Step-by-step explanation:
An isolated chick biventer cervicus nerve-muscle preparation (cbc) is set up for electrical stimulation which produces 'twitch' contraction responses. The transmitter causing these twitches is acetylcholine which is acting via activation of nicotinic receptors. Therefore, the correct answer is A) Acetylcholine; nicotinic.
Upon electrical stimulation, a normally functioning motor nerve releases acetylcholine, which diffuses across the synaptic cleft and binds to receptors on the muscle, initiating a muscle contraction. Specifically, at the neuromuscular junction of skeletal muscles, acetylcholine activates nicotinic receptors, which are ligand-gated ion channels that result in the depolarization of the muscle fiber leading to contraction.
All ganglionic neurons of the autonomic system, including those in both sympathetic and parasympathetic ganglia, are activated by acetylcholine released from preganglionic fibers. The nicotinic receptors on these neurons facilitate the depolarization needed for autonomic responses. In comparison, postganglionic parasympathetic fibers release acetylcholine onto muscarinic receptors, influencing a variety of 'rest-and-digest' physiological responses.