Final answer:
A third-degree burn is usually painless at first due to the damage to nerve endings, while a fourth-degree burn, which is more severe, also tends to be painless initially for the same reason. First and second-degree burns are painful due to the intact nerve endings.
Step-by-step explanation:
Among the degrees of burns, a third-degree burn is typically painless at first. This type of burn extends through the entire dermis, destroying the tissue and affecting the nerve endings, which means the sensory function is compromised. A fourth-degree burn is even more severe and likewise can be painless initially because it affects not only the full thickness of the skin but also the underlying muscle and bone, resulting in damaged nerve endings.
On the other hand, a first-degree burn affects only the epidermis and can be quite painful and swollen. A second-degree burn also causes pain because it involves swelling and blistering that incorporate not just the epidermis but also part of the dermis. Both these degrees of burns have intact nerve endings, which cause the sensation of pain.