Changes

[[image: Section of hair follicle.jpg|thumb|250px|right|Section of skin showing a hair follicle. From Gray's Anatomy]]

[[image: Section of hair follicle.jpg|thumb|250px|right|Section of skin showing a hair follicle. From Gray's Anatomy]]

The '''''arrector pili''''' muscle is attached to the proximal end of the follicle and the dermis close to the point at which the hair is externalised.  Involuntary contraction of this muscle in cold ambient temperatures causes erection of the hairs to trap warm air against the skin, thus providing insulation.  This can also be induced by the ‘fight or flight’ mechanism of the '''sympathetic nervous system'''.

The '''''arrector pili''''' muscle is attached to the proximal end of the follicle and the dermis close to the point at which the hair is externalised.  Involuntary contraction of this muscle in cold ambient temperatures causes erection of the hairs to trap warm air against the skin, thus providing [[Thermoregulation in Skin - Anatomy & Physiology|insulation]].  This can also be induced by the ‘fight or flight’ mechanism of the '''sympathetic nervous system'''.

Hairs have a finite life span and are shed and replaced at varying frequencies, depending on species.  The most noticeable is seasonal shedding and growth.  An increase in ambient temperature causes a slowing of the growth of existing hairs.  This is termed the '''catagen phase'''.  Atrophy of the papillae and hair matrix occurs and growth ceases in the '''telogen phase'''.  The follicle shortens, pushing the hair further from the surface.  When growth resumes, the follicle retreats deeper into the epidermis, leaving the old hair unattached and thus it falls out.  '''Anagen phase''' is where a new matrix produced and another hair grows in the place of the old one.

Hairs have a finite life span and are shed and replaced at varying frequencies, depending on species.  The most noticeable is seasonal shedding and growth.  An increase in ambient temperature causes a slowing of the growth of existing hairs.  This is termed the '''catagen phase'''.  Atrophy of the papillae and hair matrix occurs and growth ceases in the '''telogen phase'''.  The follicle shortens, pushing the hair further from the surface.  When growth resumes, the follicle retreats deeper into the epidermis, leaving the old hair unattached and thus it falls out.  '''Anagen phase''' is where a new matrix produced and another hair grows in the place of the old one.