Skin, Bones and Muscles

Bump your shin and you will notice pain, pressure and a bruise forming, all within seconds. That single moment involves three systems working together at once: skin sensing the impact, bone absorbing it, and muscle already tensing in response. This lesson looks at how skin, bones and muscles each do a distinct job, and why a doctor examines them as one connected system.

Bump your shin and you will notice pain, pressure and a bruise forming, all within seconds. That single moment involves three systems working together at once: skin sensing the impact, bone absorbing it, and muscle already tensing in response. This lesson looks at how skin, bones and muscles each do a distinct job, and why a doctor examines them as one connected system. Skin: an active organ, not just a covering The skin is often thought of as a passive wrapping, but it is one of the body's most active organs. Its outer layer forms a barrier against infection, injury and fluid loss. Just beneath it, deeper layers contain blood vessels, nerves, sweat glands and hair follicles. This is why skin can sense touch, pressure, pain and temperature, and why it plays a real role in regulating body temperature through sweating and blood flow. Bones: living structure, not dead scaffolding Bones are living tissue, not the static scaffolding they are sometimes pictured as. They protect vital organs, the skull around the brain, the ribcage around the heart and lungs, the spine around the spinal cord, and they act as anchor points for muscles to pull against. Bone tissue constantly remodels itself, repairing minor damage and responding to the stresses placed on it, and it stores minerals such as calcium and phosphate that the rest of the body draws on when needed. Joints and the tissues that hold them together Where two bones meet is a joint, and different joints trade off movement against stability. The shoulder allows a huge range of motion. Other joints allow much less, in exchange for a more stable connection. Ligaments connect bone to bone and prevent a joint moving too far. Cartilage cushions the ends of bones where they meet. Tendons connect muscle to bone. Together, these tissues are what let a joint move smoothly rather than grinding or slipping. Muscles: how movement actually happens Skeletal muscles are attached to bone by tendons, and movement happens when a muscle contracts and pulls on the bone it is attached to, rotating it around a joint. But skeletal muscle, the type you can consciously control, is only one of three types. Smooth muscle lines structures like the gut and blood vessels, working automatically without conscious control. Cardiac muscle, found only in the heart, has its own distinct properties that let it contract rhythmically for an entire lifetime. Why this system depends on several others None of this works in isolation. Nerves tell muscles exactly when to contract. The circulation delivers the oxygen and nutrients muscle and bone both need to function and repair. Hormones regulate bone strength and growth. And the immune system is what drives healing whenever any of these tissues is injured. This is why a doctor examining a joint or a muscle rarely stops at that one structure. What a doctor checks when this system is affected A doctor examining the musculoskeletal system usually works through a consistent sequence, regardless of which joint or muscle is involved. Look. The doctor looks for swelling, redness, muscle wasting, or an obvious deformity, comparing one side of the body with the other. Feel. Gently pressing over the joint or muscle checks for tenderness, warmth, or an effusion, extra fluid within a joint. Move. Moving the joint through its normal range, both actively (the patient moves it) and passively (the doctor moves it), reveals whether stiffness, pain or weakness limits its normal range. Special tests. Specific manoeuvres test particular ligaments, tendons or nerves, and imaging such as an X ray or scan can show the bone or joint structure directly when the examination alone does not give a clear answer. When this system is affected When something is wrong with skin, bone, joint or muscle, people may notice pain, swelling, stiffness, reduced movement, or a visible change such as a rash, deformity or muscle wasting. Because these three tissues depend on each other and on the nervous, circulatory, hormonal and immune systems, a symptom in one place does not always mean the problem started there. These symptoms are explored further in the lessons "Joint Pain and Swelling", "Common Rashes: What to Look For" and "Muscle Aches and Weakness".

Skin protects and senses, bones support and store minerals, joints trade off movement against stability, and muscles create movement by pulling on bone, all coordinated by the nervous, circulatory, hormonal and immune systems.