Anatomy, Physiology, and Histology of Hair, Skin, and Nails
Anatomy, Physiology, and Histology of Hair, Skin, and Nails
This domain tests the structures you work on and the systems that support them: hair layers and growth cycles, skin layers and glands, and nail anatomy.
Core rules
- Hair has three layers: the cuticle (outer overlapping scales), cortex (middle, holds melanin and gives strength and elasticity), and medulla (central core, often absent in fine hair).
- The follicle holds the hair root; the bulb sits at the base over the papilla, which supplies blood and nutrients. The arrector pili muscle raises the hair.
- Growth cycles: anagen (active, 2-7 years), catagen (transition, weeks), telogen (resting, about 3-4 months), then shedding.
- Skin layers: epidermis (outer, including the stratum corneum), dermis (nerves, glands, vessels, hair follicles), and subcutaneous layer (fat).
- Nails: the nail plate grows from the matrix; the lunula is the visible part of the matrix; the nail bed lies beneath the plate; the cuticle seals the base.
- Facial nerves: the trigeminal (V) carries sensation; the facial (VII) controls the muscles of expression.
Worked example
A client reports numbness on one cheek after a long dental procedure. Reasoning: numbness is sensory, so it points to the trigeminal nerve, not the facial nerve. You avoid shaving or waxing that area until sensation returns and document the observation.
Traps
- Melanin is in the cortex, so removing the cuticle does not change natural color.
- The lunula is matrix, not a separate bone-like structure.
- Confusing motor (facial) and sensory (trigeminal) nerves is a common exam error.
- Hair does not grow after death; skin retracts.