Correct Answer:
B. Synapses
The correct answer is Neurotransmitters. This question tests your understanding of the analogy between the two major communication systems in the body: the endocrine system and the nervous system. Both systems rely on chemical messengers to transmit signals, but they use different types of molecules and pathways.
Breaking Down the Analogy
- Hormones : Endocrine System :: Neurotransmitters : Nervous System
- Hormones are chemical substances secreted by endocrine glands (like the pituitary, thyroid, or adrenal glands) into the bloodstream. They travel through the circulatory system to reach target cells or organs, where they trigger specific physiological responses. They are the primary chemical messengers of the endocrine system.
- Neurotransmitters are chemical messengers released from the axon terminals of neurons into the synaptic cleft. They cross the synapse to bind to receptors on the post‑synaptic neuron, muscle, or gland, thereby transmitting signals across neural junctions. They are the primary chemical messengers of the nervous system.
Why the Other Options Are Incorrect
- Nerves: Nerves are bundles of axons (nerve fibers) that transmit electrical impulses. They are structural components, not chemical messengers. The analogy requires a chemical messenger, so this is not a match.
- Synapses: Synapses are the junctions between two neurons, or between a neuron and a target cell. They are the sites where neurotransmitters are released, but they are not the messengers themselves.
- Action potentials: These are electrical signals (rapid changes in membrane potential) that travel along the axon. They trigger the release of neurotransmitters, but they are not the chemical messengers that carry the signal across the synapse.
Thus, the correct analogy is: Hormones are to the endocrine system as neurotransmitters are to the nervous system, because both are the respective chemical messengers of their systems.