Final answer:
Merkel discs are unencapsulated, slowly-adapting mechanoreceptors in the skin that respond to light touch, especially in areas with high tactile acuity like the fingertips and lips, making them receptor cells in the skin.
Step-by-step explanation:
Merkel discs, also known as Merkel's disks, are one of the four primary tactile mechanoreceptors located in the human skin. Specifically, Merkel discs are unencapsulated, slowly-adapting mechanoreceptors found in the upper layers of the skin near the base of the epidermis. They respond to light touch, such as the sensation required for tasks like typing on a keyboard, and are especially densely distributed in areas like the fingertips and lips. As they are able to provide fine sensory detail, including the shape and texture of objects, they play a critical role in our sense of touch.
In contrast, other types of mechanoreceptors include Meissner's corpuscles, which respond to light touch and low-frequency vibration, Ruffini endings, which detect skin stretch and warmth, and Pacinian corpuscles, which are sensitive to transient pressure and high-frequency vibration. Krause end bulbs, another type of mechanoreceptor, respond to cold and are found only in specialized regions. Therefore, Merkel discs are best described as receptor cells in the skin, which makes the correct answer to the student's question option 1) Receptor cells in the skin.