Breadcrumb Papers Afferent islands are larger than mu-opioid receptor patch in striatum of rat pups. Afferent islands are larger than mu-opioid receptor patch in striatum of rat pups. Breadcrumb Papers Afferent islands are larger than mu-opioid receptor patch in striatum of rat pups. Afferent islands are larger than mu-opioid receptor patch in striatum of rat pups.