An _____ is a chemical substance that mimics or enhances the effects of a neurotransmitter.

• A. excitatory neurotransmitter
• B. inhibitory neurotransmitter
• C. antagonist
• D. agonist

Answer: (D)

Verified step by step guidance

1. Understand the role of neurotransmitters: Neurotransmitters are chemical messengers that transmit signals across a chemical synapse, such as between a neuron and a muscle cell or another neuron.
2. Differentiate between types of neurotransmitters: Excitatory neurotransmitters promote the generation of an electrical signal in the receiving neuron, while inhibitory neurotransmitters prevent it.
3. Define an agonist: An agonist is a chemical that binds to a receptor and activates the receptor to produce a biological response. It mimics or enhances the effects of a neurotransmitter.
4. Contrast with an antagonist: An antagonist is a substance that binds to a receptor but does not activate it, thereby blocking or dampening a biological response.
5. Apply the concept: In the context of the problem, identify that an agonist is the chemical substance that mimics or enhances the effects of a neurotransmitter, making it the correct answer.