Final answer:
The postganglionic axon carries signals from the sympathetic chain back to the spinal nerve, using norepinephrine to slow down processes such as the heartbeat.
Step-by-step explanation:
The component that leads from the sympathetic chain back to the spinal nerve and is postganglionic in nature is the postganglionic axon. This axon carries slower, because it utilizes norepinephrine as a neurotransmitter to communicate with the end effector organs upon exiting the sympathetic ganglia. However, in the specific case of sweat glands and blood vessels in skeletal muscle, acetylcholine (ACh) is released onto muscarinic receptors instead.