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Skeletal System: Bones, Joints, Cartilage, Ligaments, Bursae

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Skeletal System: Bones, Joints, Cartilage, Ligaments, Bursae

Our musculoskeletal system is made up of muscles, tendons, ligaments, bones, cartilage, joints and bursae. Our muscles work with the nervous system to contract when stimulated with impulses (messages through the receptor arc) from motor nerves. The muscles are attached to the bones with ligaments. Our skeletal system is made mostly of bones and cartilage. Bones attach to bones with cartilage or ligaments. Bursae are small fluid filled at friction points near joints to protect ligaments and tendons from rubbing against bare bones.

The Skeleton

Labeled Skeleton System
Click to see Larger Image of Skeletal System

Our skeleton forms a strong, solid internal framework of bones for our body, yet our bones only make up about 14% of our total body weight. Bones get their elasticity from tough elastic ropelike fibers of collagen. The core of some small bones is called marrow, it is soft and jellylike. The hard outside of bones is reinforces by strong rods called osteons.Bones have special cells called osteoblasts that make new bone and osteoclasts that break up the old bone. Bones grow by getting longer on the ends called the epiphyseal plate or growth plate. Bones are made rigid by hard deposits of minerals like phosphate and calcium. The bones of the skeleton support our skin, give our body shape, protect and support our organs and make it possible for us to move by acting as single and double levers. Bones do not move on their own; muscles move our bones by pulling on them. Muscles cannot push against the bone, so muscles come in pairs, one muscle pulls the bone one way and the paired muscle pulls the bone back the other way. We have a total of 233 bones. Some bones come in pairs that are almost identical in size and shape — the bones in the left arm are the same as the bones in the right arm. There are also single bones in the median plane of our body — the vertebrae in our back and neck. However, since our bones are constantly being rebuilt as we get older, both their structure and form can change. Our bones can be rigidly connected to each other or joined by rubbery cartilage, or flexibly linked by muscular or ligamentous joints. An adult skeleton has 206 bones, although some people have extra bones in their spine (backbone). A baby’s skeleton has 300 bones or more. As the baby grows, some of the bones fuse such as the bones in the skull and the pelvis. Most girls and women have smaller skeletons than boys and men of the same age. There are two main parts of the skeleton—the axial skeleton and the appendicular skeleton.

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The Axial Skeleton

The axial skeleton (trunk) is made up of the 80 bones in our upper body. Bones of the axial skeleton include:

Our arms and shoulders hang from the axial skeleton.

The Appendicular Skeleton

There are 126 bones in the arms, shoulders, hips, and legs. The appendicular skeleton is made up of our limbs or appendages—two arms and two legs—our pelvis and right and left shoulders. Our arms hang from our shoulders and legs attached to our hips.

Bones of the Upper Appendage (Arm)
Bones of the Lower Appendage (Leg)
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The Joints

Joints—also called articulations—are formed where the surfaces of two or more bones meet and articulate with each other. There are about 400 joints in the human body. Joints allow both movement and flexibility. Joints are classified by how much movement they allow—function—or what they are made of—structure. Joints are usually classified structurally by the tissue that connects them. The tissue could be cartilage, fibrous tissue, synovial fluid, or some combination of the three. Functionally, joints can be classified by the degree of movement possible, the number of bones involved, and the complexity of the joint. Most body joints allow us to move, and some only allow movement in certain ways. Fixed or immovable joints allow no movement. A dislocated joint happens when the bones of the joint are forced out-of-place, usually while playing sports but can also happen with accidents. There are 3 major functional joints and 3 major types of structural joints.

Types of Functional Joints
Types of Structural Joints
Types of Synovial Joints
Synovial Joints of the Skeletal System
Synovial Joints of the Skeletal System


Based on the type of movement the joint allows and its structure, synovial joints can be put into several categories.

Joint Range of Motion

Range-of-motion means how far and in what direction a joint can move. All joints have a normal range of motion–that is when they are healthy and normal they should be able to move a certain distance and direction. Range of motion is measured in angles using a goniometer. A joint has a limited range of motion when it cannot move to it’s full range. Limited motion can be cause be injury, a mechanical problem or a disease process. When you have a physical exam, your range of motion is checked to see if you have full or limited range of motion. Surgery can also cause limit the range of motion in a joint. These are degrees of normal ranges of motion:

Joints of the Upper Appendage (Arm)
Joints of the Lower Appendage (Leg)

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