Course Icon


Chapter 30: All

SO Icon


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.

The Human Skeletal System

Human Skeleton

Notice how the human skeleton demonstrates bilateral symmetry: the left and right sides are mirror images of each other. The main areas are

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.


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:

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

Human Muscle Fibers

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.