19.1 Overview of Bone and Cartilage
Together, the bones and muscles make up the musculoskeletal system.
Organization of Tissues in the Skeleton
A bone is classified by its shape. A bone is enclosed by a tough, fibrous, connective tissue covering called the periosteum. A joint is where a long bone contacts another bone and it is covered by a layer of cartilage.
Structure of Bone and Associated Tissues
The primary connective tissues of the skeleton are bone, cartilage, and dense fibrous connective tissue. All connective tissues contain cells separated by a matrix that contains fibers.
Bone tissue is strong because the matrix contains mineral salts, notably calcium phosphate. Compact bone is highly organized and composed of tubular units, by comparison spongy bone has an unorganized appearance. Bones contain yellow bone marrow, which contains a large amount of fat. The spaces of spongy bone are often filled with red bone marrow, a specialized tissue that produces all types of blood cells.
Cartilage is not as strong as bone, but it is more flexible because the matrix is gel-like and contains many collagenous and elastic fibers. There are three types of cartilage, all of which are associated with bones: hyaline cartilage, fibrocartilage, and elastic cartilage.
Dense Fibrous Connective Tissue
Ligaments bind bone to bone while tendons connect muscle to bone at joints, both of which are dense fibrous connective tissue.
Bone Growth and Remodeling
Bones are composed of living tissues.
Bone Development and Growth
The bones of the human skeleton, except those of the skull, first appear during embryonic development as hyaline cartilage. The cartilaginous structures are then gradually replaced by bone, a process called endochondral ossification.
Remodeling of Bones
In the adult, bone is continually being broken down and built up again, a process called remodeling.
19.2 Bones of the Skeleton
The functions of the skeleton pertain to particular bones and include: supporting the body, protecting soft body parts, producing blood cells, storing minerals and fat, and permitting flexible body movement.
Classification of the Bones
The approximately 206 bones of the skeleton are classified as either axial (in the midline of the body) or appendicular (the limbs along with their girdles).
The Axial Skeleton
The axial skeleton consists of the skull, hyoid bone, vertebral column, rib cage, and ossicles.
The skull is formed by the cranium and the facial bones.
The cranium protects the brain. It is composed of eight bones fitted tightly together.
The Facial Bones
The most prominent facial bones are the mandible, the maxillae, the zygomatic bones, and the nasal bones.
The Hyoid Bone
The hyoid bone is the only bone in the body that does not articulate with another bone; it is attached to processes of the temporal bones by muscles and ligaments and to the larynx by a membrane.
The Vertebral Column
The vertebral column consists of 33 vertebrae, normally it has four curvatures that provide resilience and strength for an upright posture.
Types of Vertebrae
The various vertebrae are named according to their location in the vertebral column.
Between the vertebrae are intervertebral disks composed of fibrocartilage that act as a kind of padding.
The Rib Cage
The rib cage is composed of the thoracic vertebrae, the ribs and their associated cartilages, and the sternum; it is part of the axial skeleton.
There are twelve pairs of ribs, and all twelve connect directly to the thoracic vertebrae in the back. The upper seven pairs of ribs connect directly to the sternum by means of costal cartilages.
The sternum, or breastbone, helps protect the heart and lungs.
The Appendicular Skeleton
The appendicular skeleton consists of the bones within the pectoral and pelvic girdles and their attached limbs.
The Pectoral Girdle and Upper Limbs
Each pectoral girdle consists of a scapula (shoulder blade) and a clavicle (collarbone). The upper limb consists of the humerus in the arm and the radius and ulna in the forearm. The joint between the scapula and the humerus allows the arm to move in almost any direction. The hand has many bones, increasing its flexibility. The wrist and palm contain carpal and metacarpal bones, respectively. The bones of the digits are called phalanges.
The Pelvic Girdle and Lower Limb
The pelvic girdle consists of two heavy, large coxal bones (hipbones). The pelvis is a basin composed of the pelvic girdle, sacrum, and coccyx. The femur (thighbone) is the longest and strongest bone in the body. The kneecap is called the patella and the lower leg is formed from the fibula and the tibia. The ankle and foot contain tarsal and metatarsal bones, respectively. The toes are made of phalanges.
Bones are jointed at the joints, which are classified as fibrous, cartilaginous, or synovial based on their structure and their ability to move.
19.3 Skeletal Muscles
Humans have three types of muscle tissue: smooth, cardiac, and skeletal. Skeletal muscle makes up the greatest percentage of muscle tissue in the body.
Skeletal Muscles Work in Pairs
The contraction of skeletal muscles causes the bones at a joint to move. The origin of a muscle is on the stationary bone, and the insertion of a muscle is on the bone that moves. Most muscles have antagonists that bring about movement in the opposite direction.
Major Skeletal Muscles
There are approximately 650 skeletal muscles in the human body. The major ones are shown in Figure 19.12.
Skeleton muscles are named based on size, shape, location, direction of muscles fibers, number of attachments, and action.
19.4 Mechanism of Muscle Fiber Contraction
Skeletal muscle tissue has alternating light and dark bands due to the arrangement of myofilaments in the muscle fiber.
A muscle fiber is a cell containing the usual cellular components, but special names have been assigned to some of these components.
Myofibrils and Sarcomeres
Myofibrils are cylindrical in shape and run the length of the muscle fiber. The striations of skeletal muscle fibers are formed by the placement of myofilaments within units of myofibrils called sarcomeres. A sarcomere contains two types of protein myofilaments: myosin and actin.
A thick filament is composed several hundred molecules of myosin, each of which is shaped like a golf club.
A thin filament consists of two intertwining strands of actin.
When a sarcomere shortens, the actin filaments slide past the myosin filaments and approach one another, this relative movement is called the sliding filament model of muscle contraction.
Skeletal Muscle Contraction
Muscle fibers are stimulated to contract by motor neurons whose axons are in nerves. Sarcomere contraction causes myofibril contraction, which in turn results in the contraction of a muscle fiber and, eventually, a whole muscle.
The Molecular Mechanism of Contraction
Two other proteins, tropomyosin and troponin, are associated with actin filaments. When calcium ions are released form the sarcoplasmic reticulum, they combine with troponin, and this causes the tropomyosin threads to shift their position, revealing ATP binding sites.
Energy for Muscle Contraction
ATP produced previous to strenuous exercise lasts a few seconds, and then muscles acquire new ATP in three different ways.
Creatine Phosphate Breakdown
Creatine phosphate is a high-energy compound built up when a muscle is resting, it can regenerate ATP.
Cellular respiration usually provides most of a muscle’s ATP.
Fermentation supplies ATP without consuming oxygen. During fermentation, glucose is broken down to lactate, the accumulation of which makes the cytoplasm of muscle fibers more acidic.
When a muscle uses creatine phosphate or fermentation to supply its energy needs, it incurs an oxygen debt. Oxygen debt is obvious when a person continues to breathe heavily after exercising.
19.5 Whole Muscle Contraction
In the Laboratory
A muscle fiber contracts completely or not at all, whereas a whole muscle shows degrees of contraction. A myogram is a visual pattern representing the mechanical force of a signal contraction (muscle twitch) in an isolated muscle. It is customarily divided into three stages: the latent period, the contraction period, and the relaxation period. Maximal sustained contraction is called tetanus.
In the Body
In the body, nerves cause muscles to contract. A nerve fiber along with all the muscle fibers it innervates is called a motor unit. A motor unit obeys the all-or-none law. Even when muscles appear to be at rest, they exhibit muscle tone, in which some of their fibers are always contracting.
Athletics and Muscle Contraction
Athletes and the general public are interested in staying fit by exercising.
Exercise and Size of Muscles
Muscles that are not used or that are used for only very weak contractions decrease in size, or atrophy.
Slow-Twitch and Fast-Twitch Muscle Fibers
Slow-twitch fibers have a steadier tug and more endurance, and tend to be aerobic. Fast-twitch fibers tend to be anaerobic and seem designed for strength.
19.6 Disorders of the Musculoskeletal System
Disorders of the Skeleton and Joints
Bones are susceptible to being broken, or fractured, the most common cause of which is trauma. Osteoporosis is a condition in which bone loses mass and mineral content. There are many different types of arthritis, or inflammation of the joints. Rheumatoid arthritis is considered an autoimmune disease, in which the body’s immune system attacks the joints as well as other tissues.
Disorders of the Muscles
Muscle cramps and twitches don’t usually rise to the level of a disorder. Fibromyalgia is a disorder that causes chronic pain in the muscles and ligaments. Muscular dystrophy refers to a group of genetic diseases that affect the muscles.