Integumentary System: Structure, Function, and Sensory Receptors

Integumentary System

Overview

The integumentary system is the body's largest organ system, consisting of the skin and its accessory structures such as hair, nails, and glands. It serves as the primary barrier between the internal environment and the outside world, providing protection, sensation, secretion, and temperature regulation.

• Functions: Protection, sensation, secretion, temperature regulation, and vitamin D synthesis.

• Components: Skin (epidermis and dermis), hair, nails, glands.

• Largest organ: The skin is the largest organ of the human body.

Skin Structure

Layers of the Skin

The skin is composed of two main layers: the epidermis and the dermis. Each layer has distinct structures and functions.

• Epidermis: Superficial layer, composed of keratinized stratified squamous epithelial tissue.

• Dermis: Deep layer, composed of connective tissue, blood vessels, nerves, and accessory structures.

Epidermis (Skin Layer)

The epidermis consists of several cell types and layers, each contributing to the skin's protective function.

• Keratinocytes: Most common cell type; produce keratin for waterproofing and protection.

• Melanocytes: Produce melanin pigment, which protects against UV radiation.

• Tactile (Merkel) Cells: Responsible for light touch reception.

• Dendritic (Langerhans) Cells: Specialized white blood cells that engulf and digest foreign particles.

Structure	Function	Notes
Stratum Corneum	Most superficial layer; many layers of dead cells	Continually shed
Stratum Lucidum	Few layers of dead keratinocytes	Only found in thick skin
Stratum Granulosum	Keratinocytes fill with keratin and begin to die
Stratum Spinosum	Many layers of cells; contains keratinocytes and dendritic cells
Stratum Basale	Single layer of cells; contains dividing keratinocytes and melanocytes	Deepest layer

Dermis (Skin Layer)

The dermis provides structural support and houses blood vessels, nerves, and accessory structures.

Structure	Function	Notes
Papillary Layer	Superficial dermis; loose areolar connective tissue	20% of dermis
Dermal Papillae	Finger-like projections; contact with epidermis	Form dermal ridges (fingerprints)
Reticular Layer	Deep dermis; dense irregular connective tissue	80% of dermis; provides strength and flexibility

Adipose Tissue

Adipose tissue is primarily composed of fat cells and provides insulation, energy storage, and protection.

Accessory Structures

Hair-Related Accessory Structures

Hair consists of a shaft and a root, with several associated structures that contribute to its growth and function.

Structure	Description/Function	Notes
Hair Root	Below skin surface; contains living keratinocytes
Hair Bulb	Knoblike base of hair root; encloses hair papilla
Hair Shaft	Made of dead keratinocytes; projects above skin surface
Hair Papilla	Contains capillaries that nourish dividing cells in matrix
Hair Follicle	Sheath that encloses the hair root
Arrector Pili Muscle	Bundle of smooth muscle fibers; contracts to pull hair upright	Causes goosebumps
Matrix	Location of keratinocyte generation

Gland-Related Accessory Structures

Glands in the skin secrete substances that aid in protection, lubrication, and temperature regulation.

Structure	Description/Function	Notes
Eccrine Sweat Gland	Located over entire body; secretes sweat containing water, salt, waste	Temperature regulation
Sweat Pore	Opening in skin surface for eccrine sweat gland
Apocrine Sweat Gland	Opens into hair follicles; found in axilla, genital areas	Secretes protein-rich sweat; sexual stimulation
Sebaceous Gland	Secretes sebum (oil) onto skin or hair follicles	Lubrication

Nail-Related Accessory Structures

Nails are protective coverings on the distal ends of fingers and toes, composed of keratinized cells.

Structure	Description/Function	Notes
Eponychium	Cuticle; most distal portion of proximal nail folds	Made of stratum corneum
Hyponychium	Under free edge of nail plate; attaches nail to bed	Known as the quick
Lunula	Crescent-shaped white area of nail plate	Sits above nail matrix
Nail Bed	Area directly under nail plate
Nail Fold	Folded skin that keeps nail in place
Nail Matrix	Proximal part of nail bed; growing region	Actively dividing cells
Nail Plate	Hard part of nail; dead cells
Nail Body	Exposed portion of nail plate
Nail Root	Part of nail plate under skin
Free edge of nail	Part that grows beyond hyponychium

Sensory Structures & Blood Vessels

Sensory Receptors

The skin contains various sensory receptors that detect touch, pressure, pain, temperature, and vibration.

Structure	Function	Notes
Free Nerve Endings	Pain, temperature, touch receptors
Sensory Nerve	Detects changes in environment; sends info to CNS
Tactile (Meissner's) Corpuscle	Light touch reception
Lamellar (Pacinian) Corpuscle	Pressure & vibration
Blood vessels (artery & veins)	Carry blood to and from tissues

Types of Sensory Receptors

Function (Description)	Notes
Thermoreceptor	Detects changes in temperature
Nociceptor	Detects pain/tissue damage
Mechanoreceptor	Detects changes in pressure, stretch, vibration
Referred pain	Pain felt in one area although source is elsewhere
Two-point threshold	Minimal distance where two points can be felt
Adaptation	Sensory event where repeated stimulation leads to cessation of sensation
Sensory homunculus	Map of cortex showing relative areas devoted to sensations

Thick vs. Thin Skin

Comparison of Structures

Thick and thin skin differ in the presence and thickness of certain layers and accessory structures.

Structure	Thick Skin	Thin Skin
Thickness of stratum corneum	a lot	not a lot
Stratum lucidum present?	yes	no
Hair follicles present?	no	yes
Sebaceous glands present?	no	yes

Lab Activities & Sensory Testing

Location of Hot and Cold Receptors

Hot and cold receptors are distributed unevenly across the skin. Hot receptors are less numerous than cold receptors.

• Hot receptors: Thermoreceptors; less numerous.

• Cold receptors: Thermoreceptors; more numerous.

Adaptation of Thermal Receptors

Thermal receptors adapt to constant stimuli, resulting in decreased sensation over time.

• Adaptation: Sensory event where repeated stimulation leads to cessation of sensation.

• Example: Hands placed in cold water become less sensitive over time.

Two-Point Discrimination Test

This test measures the minimal distance at which two points of contact can be felt as separate. It is used to assess tactile sensitivity in different regions of the skin.

Location	Two-point threshold (mm)
Palm	9
Back of hand	22
Fingertip	5
Back of neck	7

• Mechanoreceptors: Sense the points in the test.

• Touch receptors: Most dense in fingertips.

Referred Pain

Referred pain occurs when pain is perceived at a location other than the site of the stimulus, often due to shared neural pathways.

• Example: Ulnar nerve stimulation causes sensation in the arm.

Key Terms & Definitions

• Keratin: Protective protein found in skin, hair, and nails.

• Melanin: Pigment produced by melanocytes; protects against UV radiation.

• Stratum: Layer of tissue, especially in the skin.

• Dermal papillae: Projections in the dermis that form fingerprints.

• Sebum: Oily secretion from sebaceous glands.

• Mechanoreceptor: Sensory receptor for pressure, stretch, or vibration.

• Thermoreceptor: Sensory receptor for temperature changes.

• Nociceptor: Sensory receptor for pain.

Formulas & Equations

• Two-point threshold: $\text{Two-point threshold} = \text{Minimum distance (mm) where two points are felt as separate}$

Summary Table: Skin Layers and Functions

Layer	Main Function	Key Cells
Stratum Corneum	Protection, waterproofing	Dead keratinocytes
Stratum Lucidum	Extra protection (thick skin)	Dead keratinocytes
Stratum Granulosum	Keratin formation	Keratinocytes
Stratum Spinosum	Strength, flexibility	Keratinocytes, dendritic cells
Stratum Basale	Cell division, pigment production	Keratinocytes, melanocytes

Additional info:

• Thick skin is found on the palms of the hands and soles of the feet; thin skin covers the rest of the body.

• Fingerprints are formed by dermal papillae in the papillary layer of the dermis.

• Adaptation is a general property of sensory receptors, allowing the body to ignore constant stimuli.