Arrector pili muscles: a smooth muscle and under physiological or emotional stress such as cold or fright, autonomic nerve endings stimulate the arrector pili to contract and pull the hair perpendicular to the skin surface. Hair root plexuses: surrounding each hair follicle are dendrites of neurons called Hair Root Plexuses that are sensitive to touch. Nerve impulses are generated if their hair shafts are moved.
2. Hair Growth
Each hair follicle goes through a growth cycle, consisting of a growth stage, a regression stage, and a resting stage. Growth stage: AKA anagen stage. During this stage, cells of the hair matrix divide. As new cells from the hair matrix are added to the base of the hair root, existing cells of the hair root is pushed upward, and the hair grows longer. As cells of the hair are being pushed upward, they become keratinized and die. Regression stage: AKA catagen stage. After the growth phase and cells of the matrix stop dividing, the hair follicle atrophies (shrinks) and the hair stops growing. Resting stage: AKA telogen stage. After the regression stage, the hair enters the resting stage. Following the resting stage, a new growth stage begins. The old hair root falls out or is pushed out and new hair grows in its place. The duration of growth and resting stages varies with the type & location of hair and the individual. Factors such as hormones, climate, nutrition & grooming will influence the cycle of hair growth. Horses continually shed their hair. They go through 2 main sets of shedding in North America: a spring & fall shedding season. This means that the anagen phase is genetically set. The catagen phase typically lasts a few weeks followed by the telogen phase.
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