SKELETAL SYSTEM
Sitting, standing, walking, picking up a pencil, and taking a breath all involve the skeletal system. Without the skeletal system to support our bodies, we would have no rigid framework to support the soft tissues of the body and no systems of levers so critical for movement. The skeletal system consists of bones and their associated connective tissues, including cartilage, tendons, and ligaments.
FUNCTIONS OF THE SKELETAL SYSTEM
- Bone is made up of several different tissues working together: bone or osseous tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue. It is complex and dynamic living tissue. It continually engages in a process called remodeling-building new bone tissue and breaking down old bone tissue.
- Support. Bone provides a rigid framework that supports the soft tissues of the body and maintains the body’s shape.
- Protection. Bones protect internal organs that are critical to survival.
- Assistance in Movement. Because skeletal muscles attach to bones, when muscles contract, they pull on bones. Together bones and muscles produce movement.
- Mineral homeostasis. Bone tissue stores several minerals, especially calcium and phosphorus. On-demand, the bone releases minerals into the blood to maintain critical mineral balances and to distribute the minerals to other parts of the body.
- Blood cell formation. Blood cells are produced in the marrow of many bones.
- Cartilage is somewhat rigid but more flexible than bone.
- Model for Bone Growth. Cartilage is abundant in the embryo and the fetus, where it provides a model from which most of the adult bones develop. Cartilage is a major site of skeletal growth in the embryo, fetus, and child.
- Smooth joint surfaces. In the adult, the surfaces of bones within movable joints are covered with cartilage, which provides a smooth cushion between adjacent bones.
- Support. Cartilage also provides a firm, yet flexible support within structures, such as the nose, external ears, ribs, and trachea.
- Tendons and ligaments form attachments. Tendons and ligaments are strong bands of fibrous connective tissue.
- Tendons. Attach muscle to bones.
- Ligaments. Attach bones to bones.
TYPES OF BONES
Long bone
The long bones are those that are longer than they are wide and grow primarily by elongation of the diaphysis, with an epiphysis at the ends of the growing bone.
The long bones include the femurs, tibias, and fibulas of the legs, the humeri, radii, and ulnas of the arms, metacarpals, and metatarsals of the hands and feet, and the phalanges of the fingers and toes.
Short bones
Short bones are somewhat cube-shaped because they are nearly equal in length and in width. They consist of spongy bone tissue except at the surface, where there is a thin layer of compact bone tissue.
Examples of short bones are wrist or carpal bones, and ankle or tarsal bones.
Flat bones
Are generally thin and composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue. Flat bones afford considerable protection and provide extensive areas for muscle attachment. Flat bones include the cranial bones, which protect the brain; the breastbone and ribs, which protect organs in the thorax; and the shoulder blades.
Irregular bones
The irregular bones are bones, which, from their peculiar form, cannot be grouped as long bone, short bone, flat bone, or sesamoid bone. Irregular bones serve various purposes in the body, such as protection of nervous tissue (such as the vertebrae protect the spinal cord), affording multiple anchor points for skeletal muscle attachment (as with the sacrum), and maintaining pharynx and trachea support, and tongue attachment (such as the hyoid bone). The irregular bones are the vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.
Sesamoid Bone
Sesamoid bones are typically found in locations where a tendon passes over a joint, such as the hand, knee, and foot. Functionally, they act to protect the tendon and to increase its mechanical effect. The presence of the sesamoid bone holds the tendon slightly farther away from the center of the joint and thus increases its moment arm.
Sesamoid bones also prevent the tendon from flattening into the joint as tension increases and therefore maintain a more consistent moment arm through a variety of possible tendon loads. This differs from menisci, which are made of cartilage and rather act to disperse the weight of the body on joints and reduce friction during movement.
Sesamoid bones can be found on joints throughout the body, including:
- In the knee – the patella
- In the hand – two sesamoid bones are located in distal portions of the first metacarpal bone. There is also commonly a sesamoid bone in distal portions of the second metacarpal bone. The pisiform of the wrist is a sesamoid bone as well.
- In the foot – the first metatarsal bone has two sesamoid bones at its connection to the big toe.
PARTS OF A BONE
- The diaphysis is the bone shaft or body- the long, cylindrical main portion of the bone.
- Epiphyses are the distal and proximal ends of a bone.
- Metaphyses are the regions in a mature bone where the diaphysis joins the epiphysis. In a growing bone, each metaphysis includes an epiphyseal plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length. When bone growth in length stops, the cartilage in the epiphyseal plate is replaced by bone, and the resulting bony structure is known as the epiphyseal line.
- Articular cartilage is a thin layer of hyaline cartilage covering the epiphysis where the bone forms an articulation with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints. Because articular cartilage lacks a perichondrium, repair of damage is limited.
- The periosteum is a tough sheath of dense irregular connective tissue that surrounds the bone surface wherever it is not covered by articular cartilage. The periosteum contains bone-forming cells that enable bone to grow in diameter or thickness but not in length. It also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons.
- Medullary Cavity or Marrow Cavity is the space within the diaphysis that contains fatty yellow bone marrow in adults.
- Endosteum is a thin membrane that lines the medullary cavity. It contains a single layer of bone-forming cells and a small amount of connective tissue.
TYPES OF BONE CELLS
Osteogenic Cells
These are unspecialized stem cells derived from mesenchyme, the tissue from which all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting daughter cells develop into osteoblasts. Osteogenic cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within the bone that contain blood vessels.
Osteoblasts
These are bone-building cells. They synthesized and secrete collagen fibers and other organic components needed to build the matrix of bone tissue, and they initiate calcification. As osteoblasts surround themselves with matrix, they become trapped in their secretions and become osteocytes.
Osteocytes
These are mature bone cells, the main cells in the bone tissue, and maintain their daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division.
Osteoclasts
These are huge cells derived from the fusion of as many as 50 monocytes and are concentrated in the endosteum. On the side of the cell that faces the bone surface, the osteoclasts plasma membrane is deeply folded into a ruffled border. Here the cells release powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix. This breakdown of bone matrix, termed resorption, is part of the normal development, growth, maintenance, and repair of bone.
Joints
A joint, or articulation, is the place where two bones come together. There are three types of joints classified by the amount of movement they allow: immovable, slightly movable, and freely movable. The joints are the places of union between skeletal elements that are more or less moveable. Joints are commonly defined as being between bones, but joints also occur between bones and cartilages, between cartilages, and between bones and teeth. The articular system joins the skeleton, allows and/or restrains movement, and allows growth of the skeleton until the end of puberty.
Classification of joints by range of movement
- Synarthroses – immoveable joints
- Amphiarthroses – “mixed” joints of limited movement
- Diarthroses – moveable joints
Immovable joints (synarthroses)
In this type of joint, the bones are in very close contact and are separated only by a thin layer of fibrous connective tissue. An example of synarthrosis is the suture in the skull between skull bones.
Classification of joints by structure
- Fibrous joints – joints composed of dense collagenous or elastic connective tissue
- Cartilaginous joints – joints composed of hyaline cartilage or fibrocartilage
- Bony unions – a fusion between two bones
- Synovial joints – joints containing a synovial cavity filled with synovial fluid
Slightly movable joints (amphiarthroses)
This type of joint is characterized by bones that are connected by hyaline cartilage (fibrocartilage). The ribs that connect to the sternum are an example of an amphiarthrosis joint.
Freely movable joints (diarthrosis)
Synovial (diarthrosis): Synovial joints are by far the most common classification of joints within the human body. They are highly moveable and all have a synovial capsule (collagenous structure) surrounding the entire joint, a synovial membrane (the inner layer of the capsule) which secretes synovial fluid (a lubricating liquid), and cartilage known as hyaline cartilage which pads the ends of the articulating bones. There are 6 types of synovial joints which are classified by the shape of the joint and the movement available.
Six types of diarthroses joints
| Joint Type | Movement at joint | Examples | Structure |
| Hinge | A convex projection on one bone fits into a concave depression in another permitting only flexion and extension as in the elbow joints.Flexion/Extension |  Elbow/Knee | Hinge joint |
| Pivot | Rounded or conical surfaces of one bone fit into a ring of one or tendon allowing rotation. An example is the joint between the axis and atlas in the neck. Rotation of one bone around another |  Top of the neck(atlas and axis bones) | Pivot Joint |
| Ball and Socket | The ball-shaped end of one-bone fits into a cup shaped socket on the other bone allowing the widest range of motion including rotation. Examples include the shoulder and hip.Flexion/Extension/Adduction/Abduction/Internal & External Rotation |  Shoulder/Hip | Ball and socket joint |
| Saddle | This type of joint occurs when the touching surfaces of two bones have both concave and convex regions with the shapes of the two bones complementing one other and allowing a wide range of movement. The only saddle joint in the body is in the thumb.Flexion/Extension/Adduction/Abduction/Circumduction |  CMC joint of the thumb | Saddle joint |
| Condyloid | Oval shaped condyle fits into elliptical cavity of another allowing angular motion but not rotation. This occurs between the metacarpals (bones in the palm of the hand) and phalanges (fingers) and between the metatarsals (foot bones excluding heel) and phalanges (toes).Flexion/Extension/Adduction/Abduction/Circumduction |  Wrist/MCP & MTP joints | Condyloid joint |
| Gliding | Flat or slightly flat surfaces move against each other allowing sliding or twisting without any circular movement. This happens in the carpals in the wrist and the tarsals in the ankle.Gliding movements |  Intercarpal joints | Gliding joint |
Big toe
The largest and innermost toe of the human foot.
Ankle
The bones which constitute the ankle are the two long bones of the lower leg (tibia and fibula), which articulate with a short anklebone called the talus. This is a ‘uniaxial’, or hinge, joint, which allows flexion and extension movements. In the case of the ankle, these movements are called dorsiflexion (sole of the foot up) and plantarflexion (foot down) respectively. Plantarflexion is achieved by the calf muscles (gastrocnemius and soleus), which form a large strong tendon (Achilles tendon) which inserts into the bone of the heel (calcaneum).
The ankle joint acts like a hinge. But it’s much more than a simple hinge joint. The ankle is actually made up of several important structures. The unique design of the ankle makes it a very stable joint. This joint has to be stable in order to withstand 1.5 times your body weight when you walk and up to eight times your body weight when you run.
Knee
The knee joint is functionally a hinge joint, which principally allows movements of the lower leg forwards (extension) and backward (flexion), although a limited degree of rotation is also possible towards the end of the extension. The extension is achieved by a group of four large muscles at the front of the thigh (quadriceps), whilst muscles at the back of the thigh (hamstrings) produce flexion.
The lower end of the femur articulates, through two condyles, with the top of the tibia, which is shaped rather like a plateau. In addition to the cartilage covering the surfaces of these bone-ends, there is another piece of cartilage (meniscus) separating them on each side. These can be torn by rotational injuries, particularly in football and rugby players, a condition commonly referred to as torn cartilage.
Hip
The hip joint is an example of a ‘ball and socket’ (multiaxial) type of joint, with the top (head) of the long bone of the leg (femur) being the ‘ball’ and the socket being a depression in the bone of the pelvis known as the acetabulum.
This arrangement permits movements in three planes — forwards and backward (extension/flexion); inwards and outwards (adduction/abduction); and inward twist and outward twist (internal and external rotation). The combination of these movements also gives rise to ‘circumduction’, a circular movement of the whole leg, which describes a ‘cone’ with the foot at the base and the hip at the apex. The joint is spanned by powerful muscles, which are required not only for postural control and movement but also to confer stability at the hip.
Wrist and Hand
The joint between the end of the forearm and the hand. Movements occur in two planes — flexion/extension and adduction/abduction (inward/outward). This is a relatively complex joint as it is an articulation between the lower end of the long bones of the forearm (radius and ulna) and the eight small bones of the hand (carpal bones). These carpal bones are connected to one another by ligaments so that they form an arch, concave towards the palm, with its ends connected by a fibrous tissue band. Through this ‘tunnel’ run long tendons which control the fingers and, more importantly, the median nerve which carries the nerve supply to some muscles of the hand and to the skin of some of the fingers.
Elbow
The elbow is the region surrounding the elbow-join—the ginglymus or hinges joint in the middle of the arm. Three bones form the elbow joint: the humerus of the upper arm, and the paired radius and ulna of the forearm. An example of a hinge joint (uniaxial) with movement essentially limited to flexion and extension.
The condyles at the lower end of the humerus in the upper arm articulate with the heads of both the radius and the ulna in the lower arm. Twisting movements of the lower arm (pronation and supination) are possible because the top end (the head) of the radius can rotate against the lower end of the humerus. Flexion of the elbow is achieved by the action of the biceps muscle, which shortens and bulges, a muscle often shown to advantage in the classic pose of the bodybuilder.
Shoulder
The flexible ball-and-socket joint formed by the junction of the humerus and the scapula. This joint is cushioned by cartilage that covers the face of the glenoid socket and the head of the humerus.
The joint is stabilized by a ring of fibrous cartilage (the labrum) around the glenoid socket. Ligaments connect the bones of the shoulder, and tendons join these bones to surrounding muscles. The biceps tendon attaches the biceps muscle to the shoulder and helps stabilize the joint. Four short muscles that originate on the scapula pass around the shoulder, where their tendons fuse together to form the rotator cuff.
