Original: Like other opioids, e.g. oxycodone (OxyContin, Percocet, Percodan), hydromorphone (Dilaudid, Palladone), and diacetylmorphine (Heroin), morphine acts directly on the central nervous system (CNS) to relieve pain.
Simplified: Like other opioids, e.g. diacetylmorphine (heroin), morphine acts directly on the central nervous system (CNS) to relieve pain, and at synapses of the nucleus accumbens in particular.