Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein, notably alpha-keratin.
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
The diving reflex, also known as the diving response and mammalian diving reflex, is a set of physiological responses to immersion that overrides the basic homeostatic reflexes, and is found in all air-breathing vertebrates studied to date. It optimizes respiration by preferentially distributing oxygen stores to the heart and brain, enabling submersion for an extended time.
The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between hormones, neuropeptides, and immune cells. This complex interaction induces the hair follicle to produce different types of hair as seen on different parts of the body. For example, terminal hairs grow on the scalp and lanugo hairs are seen covering the bodies of fetuses in the uterus and in some newborn babies. The process of hair growth occurs in distinct sequential stages: anagen is the active growth phase, catagen is the regression of the hair follicle phase, telogen is the resting stage, exogen is the active shedding of hair phase and kenogen is the phase between the empty hair follicle and the growth of new hair.
A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number on the face and scalp, but also on all parts of the skin except the palms of the hands and soles of the feet. In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears. Surrounding the female nipple, areolar glands are specialized sebaceous glands for lubricating the nipple. Fordyce spots are benign, visible, sebaceous glands found usually on the lips, gums and inner cheeks, and genitals.
The fight-or-flight or the fight-flight-freeze-or-fawn is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It was first described by Walter Bradford Cannon. His theory states that animals react to threats with a general discharge of the sympathetic nervous system, preparing the animal for fighting or fleeing. More specifically, the adrenal medulla produces a hormonal cascade that results in the secretion of catecholamines, especially norepinephrine and epinephrine. The hormones estrogen, testosterone, and cortisol, as well as the neurotransmitters dopamine and serotonin, also affect how organisms react to stress. The hormone osteocalcin might also play a part.
Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid. A single blink is determined by the forceful closing of the eyelid or inactivation of the levator palpebrae superioris and the activation of the palpebral portion of the orbicularis oculi, not the full open and close. It is an essential function of the eye that helps spread tears across and remove irritants from the surface of the cornea and conjunctiva.
A skin condition, also known as cutaneous condition, is any medical condition that affects the integumentary system—the organ system that encloses the body and includes skin, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment.
The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. The reflex rapidly coordinates the contractions of all the flexor muscles and the relaxations of the extensors in that limb causing sudden withdrawal from the potentially damaging stimulus. Spinal reflexes are often monosynaptic and are mediated by a simple reflex arc. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.
Shivering is a bodily function in response to cold and extreme fear in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Skeletal muscles begin to shake in small movements, creating warmth by expending energy. Shivering can also be a response to fever, as a person may feel cold. During fever, the hypothalamic set point for temperature is raised. The increased set point causes the body temperature to rise (pyrexia), but also makes the patient feel cold until the new set point is reached. Severe chills with violent shivering are called rigors. Rigors occur because the patient's body is shivering in a physiological attempt to increase body temperature to the new set point.
Keratosis pilaris is a common, autosomal-dominant, genetic condition of the skin's hair follicles characterized by the appearance of possibly itchy, small, gooseflesh-like bumps, with varying degrees of reddening or inflammation. It most often appears on the outer sides of the upper arms, thighs, face, back, and buttocks; KP can also occur on the hands, and tops of legs, sides, or any body part except glabrous (hairless) skin. Often the lesions can appear on the face, which may be mistaken for acne or folliculitis.
The arrector pili muscles, also known as hair erector muscles, are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps (piloerection).
Muscle is a soft tissue, one of the four basic types of animal tissue. Muscle tissue gives skeletal muscles the ability to contract. Muscle is formed during embryonic development, in a process known as myogenesis. Muscle tissue contains special contractile proteins called actin and myosin which interact to cause movement. Among many other muscle proteins present are two regulatory proteins, troponin and tropomyosin.
A cold chill is described by David Huron as, "a pleasant tingling feeling, associated with the flexing of hair follicles resulting in goose bumps, accompanied by a cold sensation, and sometimes producing a shudder or shiver." Dimpled skin is often visible due to cold chills especially on the back of the neck or upper spine. Unlike shivering, however, it is not caused by temperature, but rather is an emotionally triggered response when one is deeply affected by things such as music, speech, or recollection. It is similar to autonomous sensory meridian response; both sensations consist of a pleasant tingling feeling that affects the skin on the back of the neck and spine.
Frisson, also known as aesthetic chills or psychogenic shivers, is a psychophysiological response to rewarding stimuli that often induces a pleasurable or otherwise positively-valenced affective state and transient paresthesia, sometimes along with piloerection and mydriasis . The sensation commonly occurs as a mildly to moderately pleasurable emotional response to music with skin tingling; piloerection and pupil dilation not necessarily occurring in all cases.
The axon reflex is the response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs. Reflexes are single reactions that respond to a stimulus making up the building blocks of the overall signaling in the body's nervous system. Neurons are the excitable cells that process and transmit these reflex signals through their axons, dendrites, and cell bodies. Axons directly facilitate intercellular communication projecting from the neuronal cell body to other neurons, local muscle tissue, glands and arterioles. In the axon reflex, signaling starts in the middle of the axon at the stimulation site and transmits signals directly to the effector organ skipping both an integration center and a chemical synapse present in the spinal cord reflex. The impulse is limited to a single bifurcated axon, or a neuron whose axon branches into two divisions and does not cause a general response to surrounding tissue.
Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.
Hackles are the erectile plumage or hair in the neck area of some birds and mammals.
Saccular acoustic sensitivity is a measurement of the ear's affectability to sound. The saccule's normal function is to keep the body balanced, but it is believed to have some hearing function for special frequencies and tones. Saccular acoustic sensitivity is considered to be simply an extension of the sense of hearing through the use of the saccule.
As in other mammals, thermoregulation in humans is an important aspect of homeostasis. In thermoregulation, body heat is generated mostly in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles. Humans have been able to adapt to a great diversity of climates, including hot humid and hot arid. High temperatures pose serious stress for the human body, placing it in great danger of injury or even death. For humans, adaptation to varying climatic conditions includes both physiological mechanisms resulting from evolution and behavioural mechanisms resulting from conscious cultural adaptations.
