Joints and the disorders of muscles and bones · Biology… | StemPy Lab
Course 16.3·Lesson 3 of 3
Joints and the disorders of muscles and bones
A joint is a point of contact between bones, and it acts as the fulcrum of a lever when a muscle pulls. Joints are fibrous, cartilaginous or synovial, and the synovial joints are ball and socket, hinge, pivot, gliding and saddle. Disorders such as myasthenia gravis, tetany, muscular dystrophy, arthritis, osteoporosis and gout affect the muscles, the bones or the joints.
about 25 min1 simulation1 practice question
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Try the code and the simulations beside the lesson.
Where bones meet
Bend your knee, turn your head and open your hand. In each case, two bones meet and one moves over the other. A joint is a point of contact between bones, or between a bone and a cartilage.
A muscle pulls on a bone. The bone turns about the joint, which acts as a fulcrum, the fixed point of a lever. How much a joint can move depends on its structure. There are three kinds.
Eight small drawings of joints in two rows. Top row, the three kinds of joint. A fibrous joint: a zigzag suture between two flat skull bones, with no movement. A cartilaginous joint: a pad of cartilage between two vertebrae, with limited movement. A synovial joint: two bone ends in a capsule with a fluid-filled cavity between them, with much movement. Bottom row, five synovial joints, each with an example. Ball and socket: the shoulder. Hinge: the knee. Pivot: the atlas and the axis. Gliding: between carpals. Saddle: between the carpal and the metacarpal of the thumb.
Fibrous joints
In a fibrous joint, dense fibrous connective tissue joins the bones end to end. The joint allows no movement. The flat bones of the skull are joined in this way, by zigzag lines called sutures. They form the cranium.
Cartilaginous joints
In a cartilaginous joint, the bones are joined by cartilage. The joint between two neighbouring vertebrae is of this kind. It allows limited movement.
Synovial joints
A synovial joint has a fluid filled synovial cavity between the two bone ends. It allows considerable movement, and it helps in locomotion and other movements. There are five kinds.
Ball and socket: between the humerus and the pectoral girdle (the shoulder).
Hinge: the knee.
Pivot: between the atlas and the axis.
Gliding: between the carpals.
Saddle: between a carpal and the metacarpal of the thumb.
The joint between the femur and the pelvic girdle, the hip, is also a ball and socket joint.
A ligament joins a bone to a bone. A tendon joins a muscle to a bone. You met both in the chapter on animal tissues.
This part is not in the NEET syllabus, but A-Level and IB ask it. Muscles that move a joint work in pairs. When the biceps contracts, the elbow bends, and the triceps relaxes. When the triceps contracts, the elbow straightens, and the biceps relaxes. Such a pair is called an antagonistic pair. The forearm is a lever. The next simulation shows how large the pull of the biceps must be.
You should find that the biceps must pull with a force many times the weight of the load. The muscle is attached close to the elbow, so it works at a mechanical disadvantage.
Disorders of the muscular and the skeletal systems
Myasthenia gravis: an autoimmune disorder that affects the neuromuscular junction. It leads to fatigue, weakening and paralysis of skeletal muscle.
Muscular dystrophy: progressive degeneration of skeletal muscle, mostly because of a genetic disorder.
Tetany: rapid spasms, which are wild contractions of muscle, because of a low level of Ca2+ in the body fluid.
Arthritis: inflammation of the joints.
Osteoporosis: an age related disorder, with decreased bone mass and a greater chance of fractures. A decrease in the level of oestrogen is a common cause.
Gout: inflammation of the joints, because uric acid crystals collect in them.
An autoimmune disorder is one in which the body makes antibodies against its own cells. In myasthenia gravis, the neuromuscular junction is attacked, so the signal from the nerve does not reach the muscle as it should.
Try it in Python
The program looks up a joint in a dictionary and lists the synovial joints.
Practise with new numbers
Common mistakes
Calling the skull sutures cartilaginous. They are fibrous joints, with no movement.
Saying that a synovial joint has no cavity. The fluid filled synovial cavity is what marks it.
Mixing ligament and tendon. A ligament joins bone to bone, and a tendon joins muscle to bone.
Saying that tetany is caused by a high level of calcium. It is a low level of Ca2+.
Mixing arthritis and osteoporosis. Arthritis is the inflammation of joints. Osteoporosis is the loss of bone mass.
Saying that gout comes from calcium. It comes from uric acid crystals.
Remember these
Joints: fibrous (no movement, skull sutures), cartilaginous (limited, between vertebrae) and synovial (much movement).
Synovial: ball and socket (shoulder), hinge (knee), pivot (atlas and axis), gliding (carpals), saddle (thumb).