Anatomy and Physiology of the Hair, Skin, and Scalp
This domain tests the structures you touch every day: the follicle and its parts, the three skin layers, the growth cycle, and the bones, muscles, and nerves of the head and face.
Hair structure and growth
Each hair grows from a follicle; at its base the bulb surrounds the papilla, which supplies blood and nerves. Sebaceous glands lubricate the hair, and the arrector pili muscle makes it stand up. The shaft has three layers: cuticle (overlapping protective scales), cortex (bulk, melanin, disulfide bonds), and medulla (core, sometimes absent in fine hair). Types are lanugo (prenatal), vellus (short, fine, unpigmented), and terminal (long, pigmented, medullated). The cycle runs anagen (active growth, years), catagen (transition, weeks), telogen (resting, months), then exogen (shedding); roughly 90 percent of scalp hair is in anagen and 50 to 100 hairs shed daily.
Bonds
Disulfide bonds are broken by chemical services; hydrogen bonds by water and heat; salt bonds by pH change. Peptide bonds are not broken by normal salon services.
Skin and scalp
Epidermis (stratum corneum, granulosum, spinosum, basale), dermis (papillary and reticular), subcutaneous layer. Functions: protection, sensation, heat regulation, excretion, secretion, absorption. The scalp hosts sebaceous glands and normal flora; distinguish ordinary dandruff from seborrheic dermatitis and from fungal tinea capitis.
Head and face
Skull bones include frontal, parietal, occipital, temporal, zygomatic, maxilla, and mandible. Muscles include the epicranius (frontalis and occipitalis linked by the galea aponeurotica), temporalis, masseter, and platysma. The trigeminal (fifth) nerve is sensory to the face through ophthalmic, maxillary, and mandibular branches; the facial (seventh) nerve is motor.
Worked example
A relaxer straightens hair because the reducer breaks disulfide bonds; the neutralizer's oxidation rebuilds them in the new shape. Skip the neutralizer and the bonds never re-form, so the hair breaks.
Traps
Calling the papilla the shaft; swapping trigeminal and facial nerve roles; claiming cutting makes hair grow faster.