Muscle, nervous tissue, and the skin
The last two tissue types make movement and communication possible. Skin brings several tissues together into the body's largest organ.
Skeletal muscle tissue
Skeletal muscle cells (fibres) are long, cylindrical, and have many nuclei and visible stripes (striations). They are under voluntary control and move the skeleton. Each fibre is a single cell that can be several centimetres long.
Cardiac and smooth muscle
Cardiac muscle, found only in the heart, is striated and branched, with one or two nuclei per cell; cells are joined by intercalated discs that pass signals quickly so the heart contracts as a unit. Smooth muscle has spindle-shaped cells without striations; it lines blood vessels and hollow organs and contracts involuntarily.
Nervous tissue
Nervous tissue has neurons, which generate and conduct electrical signals, and supporting cells called neuroglia (glial cells), which outnumber neurons. Glia insulate axons, nourish neurons, and defend against infection.
Skin structure
Skin has two layers. The epidermis is stratified squamous epithelium; new cells form at its base and move up, filling with the tough protein keratin and dying before they flake off, about four weeks later. The dermis below is dense irregular connective tissue with blood vessels, nerves, hair follicles, and glands. Beneath lies the hypodermis, mostly fat.
Skin functions
Skin protects against microbes, chemicals, and water loss; regulates temperature through sweat and blood flow; senses touch, pressure, pain, and temperature; and makes vitamin D when exposed to ultraviolet light. Melanin, made by melanocytes, absorbs ultraviolet radiation and protects DNA.
Hair, nails, and glands
Hair and nails are made of keratin. Sebaceous glands release oil (sebum) into hair follicles, keeping skin and hair supple. Sweat glands release watery sweat for cooling; another type in the armpits and groin releases a thicker secretion that bacteria break down, producing body odour.
Burns and skin cancer
Burns are classified by depth: first-degree affect the epidermis (like sunburn); second-degree reach into the dermis and blister; third-degree destroy the full thickness and may need skin grafts. Skin cancer is the most common cancer; melanoma is the most dangerous. The ABCDE rule (asymmetry, border, colour, diameter, evolving) helps spot warning signs in moles.
Worked example
A person gets a deep paper cut that bleeds. Bleeding shows the cut reached the dermis, because the epidermis has no blood vessels. The epidermis regenerates over the cut, while deeper damage is repaired by fibroblasts laying down collagen, sometimes leaving a small scar.
Key takeaways
Skeletal muscle is striated and voluntary; cardiac muscle is striated, branched, and joined by intercalated discs; smooth muscle is unstriated and involuntary. Nervous tissue has neurons and supporting glia. Skin has an epidermis of keratinised epithelium over a vascular dermis. It protects, regulates temperature, senses, and makes vitamin D. Burns are graded by depth.
Flashcard
Striated, voluntary, multinucleate
Check your understanding
Which muscle type has intercalated discs?