Which of these is a neurotransmitter?

A) Proteins:
Proteins are generally too large to diffuse freely across the cell membrane. They require specific transport mechanisms, such as endocytosis or transport proteins, to move in and out of cells. The hydrophilic nature of most proteins further complicates their passage across the lipid bilayer.

B) Enzymes:
Like proteins, enzymes are large molecules that do not freely diffuse across cell membranes. Enzymes, being proteins, also need specialized transport mechanisms or need to be secreted or endocytosed to enter or exit cells.

C) Hormones:
Many hormones, particularly lipid-soluble ones like steroid hormones, can diffuse freely across the cell membrane. These hormones pass through the lipid bilayer due to their hydrophobic nature and bind to intracellular receptors, initiating cellular responses. However, water-soluble hormones (e.g., insulin) typically do not diffuse freely but interact with receptors on the cell surface.

D) Electrolytes:
Electrolytes (such as sodium, potassium, calcium, and chloride ions) are charged particles that cannot pass freely through the lipid bilayer due to the hydrophobic interior of the cell membrane. They require specific ion channels or transporters to move in and out of the cell. Diffusion of electrolytes is facilitated through these channels, but it is not a simple diffusion process as seen with small, uncharged molecules.

A) Pupil constriction: Pupil constriction (miosis) is primarily mediated by the parasympathetic nervous system through muscarinic receptors, not nicotinic receptors. Nicotinic receptors, when stimulated, affect skeletal muscle contraction, not the size of the pupils.

B) Muscle contraction: Nicotinic receptors are located on skeletal muscle cells and are responsible for mediating muscle contraction when activated. When a drug stimulates nicotinic receptors, the expected outcome is muscle contraction, as these receptors play a critical role in neuromuscular transmission.

C) Increase GI motility: While nicotinic receptors are involved in muscle contraction, they are not the primary mediators of gastrointestinal motility. Muscarinic receptors, which are part of the parasympathetic nervous system, primarily control GI motility. Stimulating muscarinic receptors would lead to increased motility, but not nicotinic receptors.

D) Decrease in heart rate: A decrease in heart rate is typically associated with the stimulation of muscarinic receptors, particularly those that influence the parasympathetic nervous system (vagal tone). Nicotinic receptor activation does not directly influence heart rate; in fact, their stimulation is more related to skeletal muscle contraction.