Biology
Chapter 30: All
Lecture
Motion and Support: The Muscle and Skeletal Systems
WebLecture Topics
The Skeletal System
Types of Body Support Systems
Three different types of skeletal systems provide support against gravity and protection for the organisms that use them.
- Animals with hydroskeletons use tubes filled with water for both support and locomotion. Hydra, earthworms, and echinoderms control the level of fluids in their tubular systems by pumping ions into to extend an extremity like a tentacle or arm, then drain the two in order to retract the extremity. If the extremity has suckers are hairs that can fasten to a surface, the organism can use an extend-fasten-contract process to pull itself along.
- A number of phyla including insects use exoskeletons for protection, Barriers to water loss, and support. The exoskeleton sections are made of chitin,a leathery starch-based polymer, and sclerotin, a protein matrix that provides more strength but less flexibility. In molluscs, and crustacean shells, the chitin is impregnated with calcium carbonate and becomes rigid but much stronger.
Exoskeletons are usually composed of sheets that cover the more essential organ areas, but not necessarily the entire organism. Pure chitin strands connect muscles to the exoskeleton segments and allow the organism to move the segment for locomotion. Because exoskeletons are rigid, they limit the growth of the organism. New exoskeletons develop beneath the existing exoskeleton in a soft form. the organism sheds the hard outer exoskeleton and expands within the softer inner shell to grow while it hardens. During this period, the animal is vulnerable to attack by predators, to infection, and water loss. Because of its size limitations, a species with an exoskeleton is likely to be small compared to vertebrates that share its habitat.
- Vertebrates have endoskeletons. These provide land animals (and fish) with support against gravitational forces and allow for sustained and rapid movement. Individual bones are held together with ligaments and attached to muscles with tendons. Because of the mass of bone material required, few endoskeletons have plate-like components for protection, with skull protection for the brain being one exception.
The Human Skeletal System
Human Skeleton courtesy Wikipedia Commons
Notice how the human skeleton demonstrates bilateral symmetry: the left and right sides are mirror images of each other. The main areas are
- the skull, a set of plates protecting the brain: cranium, jawbone
- the shoulders, rib-cage, and backbone, protecting the internal organs of the upper torso: clavicle, scapula, sternum, ribs, cervical vertebrae, thoracic vertebrae
- the arms and hands of the upper extremities: humerus, ulna, radius, carpals, metacarpals, phalanges
- the hipbones and spine providing support for the upper body balanced on the legs: pelvic girdle, lumbar vertebrae
- the leg bones and feet of the lower extremities: femur, patella, tibia, fibula, tarsals, metatarsals, phalanges
Bones are considered connective tissue. Small bones in the hands and feet provide flexibility for fine motor control. Larger bones provide support against gravity, but also function as part of the blood system (also connective tissue): they are the source of the platelets and erythrocytes (red blood cells or RBCs) of the circulation system, and the white blood cells (leukocytes, T-cells, and B-cells) of the immune system.
Joints
When a human baby is born, these plates are not yet fused together, but after a few years of growth, the joints close together. These become fused joints or immovable joints. The main types of movable joints are:
- ball-and-socket joints: shoulder joint, hip joint
- pivot or rotation joints: elbow, wrist, ankles
- hinge joints: knee
Some classification systems break joint types down into further divisions, depending on the type of mechanism involved. In most cases, muscles on each side of the joint operate in antagonist pairs to move the bones back and forth: if one muscle contracts, the bone moves forward; when the other muscle contracts, the bone moves backwards.
Over-extending bone movements may damage the tendons joining bone to muscle (strain), or the ligaments joining bones to bones (sprain). Stretching muscles before exercise can reduce the risk of tears during exercises in tendon or ligament tissue.
Muscles and Movement
Muscle cell courtesy Wikipedia Commons [OpenStax College]
Muscle tissue is made of long, fibrous muscle cells which contain filaments called myofibrils. Each myofibril filament contains a sequence of sarcomere segments bounded by Z-zones. Long chains of double-stranded actin and many-stranded myosin proteins are attached to the Z-zone boundaries, with each myosin strand presenting a "head" that contains binding sites matching locations on the actin.
When a motor neuron fires, it releases neurotransmitters that cross down T-tubules at the Z-zones penetrating the sacromere structure. The presence of the neurotransmitters triggers the myofibrils to release Ca+ ions into the channels between the actin and myosin strands. These Ca+ ions bind to troptomyosin molecules attached to the actin strands in the myofibrils and detach these molecules. The actin strands are now free to attach to the myosin heads and contract, pulling the myosin fibers and contracting the sarcomere segment. ATP supplies energy to break the myosin-actin bond, at which point the two strands become independent again and allow the myosin sarcome segment to relax to its normal length.
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