Final answer:
A neurotransmitter is the chemical agent responsible for transmitting messages between nerve cells across the synapse, allowing for rapid and coordinated responses.
Step-by-step explanation:
The chemical agent that relays messages from one nerve cell to the next is called a neurotransmitter. Neurotransmitters are crucial for neuronal communication, facilitating the transfer of signals across the synapse between adjacent neurons. When an electrical impulse reaches the end of a neuron’s axon, neurotransmitters are released into the synaptic gap and bind to receptors on the dendrites of the adjoining neuron, triggering a response. This rapid signaling mechanism is essential for coordinated and immediate responses, such as reflex actions.