Final answer:
The neurotransmitter released by parasympathetic neurons that reduces heart rate and increases salivation is acetylcholine (ACh). ACh has an inhibitory effect on the heart by binding to muscarinic receptors and a stimulating effect on salivary glands, contrasting with norepinephrine from the sympathetic system which increases heart rate.
Step-by-step explanation:
The neurotransmitter released by parasympathetic neurons in the autonomic nervous system that causes reduced frequency of cardiac muscle contraction and an increase in salivary secretions is acetylcholine (ACh). Parasympathetic stimulation, which can be triggered via the vagus nerve, releases ACh at neuromuscular junctions. Upon release, ACh binds to muscarinic receptors causing cardiac cells to hyperpolarize, and consequently reduces heart rate, as the cells reach the threshold for depolarization more slowly. This action is similar to pressing down on a car's brake pedal to slow down the vehicle. In addition to affecting the heart rate, ACh also prompts increased salivary gland secretions. Therefore, ACh's role as a neurotransmitter enables the parasympathetic system to slow down the heart and stimulate digestion processes.
In contrast, the sympathetic nervous system releases norepinephrine (NE) at the neuromuscular junction, which leads to an increased heart rate by binding to adrenergic receptors on cardiac cells, accelerating their depolarization. It's important to understand these differing actions in the sympathetic and parasympathetic systems as they work oppositely but in harmony to maintain homeostasis in the body. Furthermore, in parasympathetic sites other than the heart, ACh still serves as the primary neurotransmitter released by the postganglionic fibers and binds to muscarinic receptors on their target organs to exert its effects.