MCAT Biology Review
Chapter 11: The Musculoskeletal System
11.2 The Skeletal System
There are two types of skeletons: exoskeletons and endoskeletons. Exoskeletons encase whole organisms and are usually found in arthropods, such as crustaceans and insects. Vertebrates, including humans, have endoskeletons. Endoskeletons are internal, but are not able to protect the soft tissue structures as well as exoskeletons. However, exoskeletons must be shed and regrown to accommodate growth. Endoskeletons are much better able to accommodate the growth of a larger organism.
The components of our skeletal system are divided into axial and appendicular skeletons. The axial skeleton consists of the skull, vertebral column, ribcage, and hyoid bone (a small bone in the anterior neck used for swallowing); it provides the basic central framework for the body. Theappendicular skeleton consists of the bones of the limbs (humerus, radius and ulna, carpals, metacarpals, and phalanges in the upper limb; and femur, tibia and fibula, tarsals, metatarsals, and phalanges in the lower limb), the pectoral girdle (scapula and clavicle), and pelvis. Both skeleton types are covered by other structures (muscle, connective tissue, and vasculature). The structure of the skeleton is shown in Figure 11.7, with many of the bones labeled.
Figure 11.7. Anatomy of the Human Skeleton
The skeleton is created from two major components: bone and cartilage.
An adult human has 206 bones. Over 100 of these are in the hands and feet.
Bone is a connective tissue derived from embryonic mesoderm. Bone is much harder than cartilage, but is relatively lightweight.
Macroscopic Bone Structure
The structure of bone can be seen in Figure 11.8.
Figure 11.8. Anatomy of a Long Bone (Humerus)
Bone’s characteristic strength comes specifically from compact bone. It lives up to its name, as it is both dense and strong. The other type of bone structure is spongy or cancellous bone. The lattice structure of spongy bone is visible under microscopy and consists of bony spicules (points) known as trabeculae. The cavities between trabeculae are filled with bone marrow, which may be either red or yellow. Red marrow is filled with hematopoietic stem cells, which are responsible for generation of all the cells in our blood; yellow marrow is composed primarily of fat and is relatively inactive.
Bones in the appendicular skeleton are typically long bones, which are characterized by cylindrical shafts called diaphyses that swell at each end to form metaphyses, and that terminate in epiphyses. The outermost portions of bone are composed of compact bone, whereas the internal core is made of spongy bone. Long bone diaphyses and metaphyses are full of bone marrow. The epiphyses, on the other hand, use their spongy cores for more effective dispersion of force and pressure at the joints. At the internal edge of the epiphysis is an epiphyseal (growth) plate, which is a cartilaginous structure and the site of longitudinal growth. Prior to adulthood, the epiphyseal plate is filled with mitotic cells that contribute to growth; during puberty, these epiphyseal plates close and vertical growth is halted. Finally, a fibrous sheath called the periosteum surrounds the long bone to protect it as well as serve as a site for muscle attachment. Some periosteum cells are capable of differentiating into bone-forming cells; a healthy periosteum is necessary for bone growth and repair.
Structures in the musculoskeletal system are held together with dense connective tissue. Tendons attach muscle to bone and ligaments hold bones together at joints.
The root lig– refers to a connection between two things. Think of DNA ligase, discussed in Chapter 6 of MCAT Biochemistry Review. Think of ligands in complex ions, discussed in Chapter 9 of MCAT General Chemistry Review. In this case, ligaments attach bones to each other to stabilize joints.
Microscopic Bone Structure
The strength of compact bone comes from the bone matrix, which has both organic and inorganic components. The organic components include collagen, glycoproteins, and other peptides. The inorganic components include calcium, phosphate, and hydroxide ions, which harden together to form hydroxyapatite crystals (Ca10(PO4)6(OH)2). Minerals such as sodium, magnesium, and potassium are also stored in bone.
Strong bones require uniform distribution of organic and inorganic materials. The bony matrix is ordered into structural units known as osteons or Haversian systems, as shown in Figure 11.9. Each of these osteons contains concentric circles of bony matrix called lamellae surrounding a central microscopic channel. Longitudinal channels (those with an axis parallel to the bone) are known as Haversian canals, while transverse channels (those with an axis perpendicular to the bone) are known as Volkmann’s canals. These canals contain the blood vessels, nerve fibers, and lymph vessels that maintain the health of the bone. Between the lamellar rings are small spaces called lacunae, which house mature bone cells known as osteocytes. The lacunae are interconnected by tiny channels called canaliculi that allow for the exchange of nutrients and wastes between osteocytes and the Haversian and Volkmann’s canals.
Figure 11.9. Bone Matrix Cross-sectional and longitudinal views highlighting Haversian systems.
Bone appears to be rigid and static, but it is actually quite dynamic. It is both vascular and innervated, which is why it hurts so much to break a bone. In addition, bone remains in a vigorous equilibrium between construction and destruction, known as bone remodeling.
Two cell types are largely responsible for building and maintaining strong bones: osteoblasts and osteoclasts. Osteoblasts build bone, whereas osteoclasts, polynucleated resident macrophages of bone, resorb it. These processes together contribute to the constant turnover of bone, as shown in Figure 11.10. During bone formation, essential ingredients such as calcium and phosphate are obtained from the blood. During bone resorption, these ions are released back into the bloodstream. Bone remodeling occurs in response to stress, and bone actually remodels in such as a way as to accommodate the repetitive stresses faced by the body. Endocrine hormones may also affect bone metabolism. Parathyroid hormone, a peptide hormone released by the parathyroid glands in response to low blood calcium, promotes resorption of bone, increasing the concentration of calcium and phosphate in the blood. Vitamin D, which is activated by parathyroid hormone, also promotes the resorption of bone. This may seem counterintuitive at first—isn’t vitamin D used to promote bone strength? Indeed, the resorption of bone in response to vitamin D actually encourages the growth of new, stronger bone, thus overcompensating for the effect of resorbing bone in the first place. Finally, calcitonin, a peptide hormone released by the parafollicular cells of the thyroid in response to high blood calcium, promotes bone formation, lowering blood calcium levels.
Figure 11.10. Bone Remodeling
Osteoblasts build bone. Osteoclasts chew bone.
Osteoporosis is the most common bone disease in the United States. It is thought to be the result of increased osteoclast resorption and some concomitant slowing of bone formation, both of which lead to loss of bone mass. Estrogen is believed to help prevent osteoporosis by stimulating osteoblast activity.
Cartilage is softer and more flexible than bone. Cartilage consists of a firm but elastic matrix called chondrin that is secreted by cells called chondrocytes. Fetal skeletons are mostly made up of cartilage. This is advantageous because fetuses must grow and develop in a confined environment and then must traverse the birth canal. Adults have cartilage only in body parts that need a little extra flexibility or cushioning (external ear, nose, walls of the larynx and trachea, intervertebral discs, and joints). Cartilage also differs from bone in that it is relatively avascular (without blood and lymphatic vessels) and is not innervated.
Most of the bones of the body are created by the hardening of cartilage into bone. This process is known as endochondral ossification and is responsible for the formation of most of the long bones of the body. Bones may also be formed through intramembranous ossification, in which undifferentiated embryonic connective tissue (mesenchymal tissue) is transformed into, and replaced by, bone. This occurs in bones of the skull.
JOINTS AND MOVEMENT
Like bone and cartilage, joints are also made of connective tissue and come in two major varieties: immovable and movable. Immovable joints consist of bones that are fused together to form sutures or similar fibrous joints. These joints are found primarily in the head, where they anchor bones of the skull together.
Movable joints, structures of which are in Figure 11.11, include joints (like the elbow or knee), ball-and-socket joints (like the shoulder or hip), and others. They permit bones to shift relative to one another. Movable joints are strengthened by ligaments, which are pieces of fibrous tissue that connect bones to one another, and consist of a synovial capsule, which encloses the actual joint cavity (articular cavity). A layer of soft tissue called the synovium secretes synovial fluid, which lubricates the movement of structures in the joint space. The articular cartilagecontributes to the joint by coating the articular surfaces of the bones so that impact is restricted to the lubricated joint cartilage, rather than to the bones.
Figure 11.11. Structures in a Movable Joint
Degradation of articular cartilage (cartilage in joints) can lead to medical issues like osteoarthritis. Osteoarthritis (or “arthritis” in the lay population) is painful because a lack of cartilage in joints leads to bones rubbing directly on one another.
When a muscle is attached to two bones, its contraction will cause one of the bones to move. The end of the muscle with a larger attachment to bone (usually the proximal connection) is called the origin. The end with the smaller attachment to bone (usually the distal connection) is called theinsertion. Often, our muscles work in antagonistic pairs; one relaxes while the other contracts. Such is the case in the arm, where the biceps brachii and triceps brachii work antagonistically, as shown in Figure 11.12. When the biceps contracts and the triceps relaxes, the elbow is flexed; when the triceps contracts and the biceps relaxes, the elbow is extended. Muscles can also be synergistic—working together to accomplish the same function.
Figure 11.12. Antagonistic Muscle Pairs The biceps brachii and triceps brachii are an example of a muscle pair that works antagonistically; the contraction of one causes the other to elongate.
Muscles may also be classified by the types of movements they coordinate. A flexor muscle decreases the angle across a joint (like the biceps brachii); an extensor increases or straightens this angle (like the triceps brachii). An abductor moves a part of the body away from the midline (like the deltoid); an adductor moves a part of the body toward the midline (like the pectoralis major). Medial and lateral rotation describe motions that occur in limbs, rotating their axis toward or away from the midline, respectively.
MCAT Concept Check 11.2:
Before you move on, assess your understanding of the material with these questions.
1. What is the difference between compact and spongy bone?
· Compact bone:
· Spongy bone:
2. What are the three structural parts of a bone? Which part contributes most to linear growth?
3. What chemical forms most of the inorganic component of bone?
4. What are the functions of osteoblasts, osteoclasts, and chondrocytes?
5. What provides the lubrication for movable joints? What tissue produces the lubrication?