Lesson 2 BIOL 30143 Anatomy PDF

UNIT II THE MUSCULOSKELETAL SYSTEM Lesson 3 Skeletal System INTRODUCTION This chapter will give you a wider perspective on the functions and importance of skeletal system by examining...

UNIT II THE MUSCULOSKELETAL SYSTEM Lesson 3 Skeletal System INTRODUCTION This chapter will give you a wider perspective on the functions and importance of skeletal system by examining the mechanisms involved with the growth, remodeling, and repair of the skeleton. The bones of the skeleton are more than just racks from which muscles hang. They have a variety of vital functions. In addition to supporting the weight of the body, bones work with muscles to maintain body position and to produce controlled, precise movements. Without the skeleton to pull against, contracting muscle fibers could not make us sit, stand, walk, or run. The skeleton may appear to be nothing more than a dry, nonliving framework for the body, but it is far from it. The 206 bones in the adult human body are actually dynamic living tissue. Bone constantly breaks down and rebuilds itself, not just during the growth phases of childhood, but throughout the life span. Bone is filled with blood vessels, nerves, and living cells; in addition, its interaction with other body systems is necessary not only for movement, but also for life itself. LEARNING OBJECTIVES 1. Describe the primary functions of the skeletal system. 2. Classify bones according to shape and internal organization, giving examples of each type, and explain the functional significance of each of the major types of bone markings. 3. Identify the cell types in bone and list their major functions. 4. Compare the structures and functions of compact bone and spongy bone. 40  5. Discuss the effects of exercise, hormones, and nutrition on bone development and on the skeletal system. 6. Describe the types of fractures and explain how fractures heal. Summarize the effects of the aging process on the skeletal system. 7. Identify the bones of the axial and appendicular skeleton, and specify their functions; and 8. Identify the bones of the upper and lower limbs, their functions, and their superficial features. A. Primary Functions Your skeletal system includes the bones of the skeleton and, the cartilages, ligaments, and other connective tissues that stabilize or interconnect the bones. This system has five primary functions: a. Support. The skeletal system provides structural support for the entire body. Individual bones or groups of bones provide a framework for the attachment of soft tissues and organs. b. Storage of Minerals and Lipids. Minerals are inorganic ions that contribute to the osmotic concentration of body fluids. Minerals also take part in various physiological processes, and several are important as enzyme cofactors. Calcium is the most abundant mineral in the human body. The calcium salts of bone are a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. In addition, the bones of the skeleton store energy as lipids in areas filled with yellow bone marrow. c. Blood Cell Production. Red blood cells, white blood cells, and other blood elements are produced in red bone marrow, which fills the internal cavities of many bones. We will describe blood cell formation when we examine the cardiovascular and lymphatic systems. d. Protection. Skeletal structures surround many soft tissues and organs. The ribs protect the heart and lungs, the skull encloses the brain, the vertebrae shield the spinal cord, and the pelvis cradles digestive and reproductive organs. e. Leverage. Many bones function as levers that can change the magnitude and direction of the forces generated by skeletal muscles. The movements produced range from the precise motion of a fingertip to changes in the position of the entire body. 41 B. Bone Shapes Figure 16: Bone Shapes C. Bone Markings The surfaces of each bone in your body have characteristic features. Elevations or projections form where tendons and ligaments attach, and where adjacent bones articulate (that is, at joints). Depressions, grooves, and tunnels in bone are sites where blood vessels or nerves lie alongside or penetrate the bone. Detailed examination of these bone markings, or surface features, can yield an abundance of anatomical information. For example, anthropologists, criminologists, and pathologists can often determine the size, age, sex, and general appearance of an individual on the basis of incomplete skeletal remains. 42 Figure 17: Bone Markings 43 D. Bone Structure Figure 18 introduces the anatomy of the femur, the long bone of the thigh. This representative long bone has an extended tubular shaft, or diaphysis. At each end is an expanded area known as the epiphysis. The diaphysis is connected to each epiphysis at a narrow zone known as the metaphysis. The wall of the diaphysis consists of a layer of compact bone. Compact bone, or dense bone, is relatively solid. It forms a sturdy protective layer that surrounds a central space called the medullary cavity (medulla, innermost part), or marrow cavity. Figure 18: The structure of femur in longitudinal section E. Bone Cells Bone contains four types of cells: osteocytes, osteoblasts, osteogenic cells, and osteoclasts. a. Osteocytes are mature bone cells that make up most of the cell population. Osteoblasts (produce new bone matrix in a process called ossification, or osteogenesis. b. Osteoblasts make and release the proteins and other organic components of the matrix. c. Bone contains small numbers of mesenchymal cells called osteogenic cells or osteoprogenitor cells. These squamous stem cells divide to produce daughter cells that differentiate into osteoblasts. d. Osteoclasts are cells that absorb and remove bone matrix. They are large cells with 50 or more nuclei. 44  Figure 19: Bone Cells F. Compact Bones and Spongy Bones Compact bone functions to protect, support, and resist stress. It is thickest where stresses arrive from a limited range of directions. Spongy bone provides some support and stores marrow. Spongy bone is found where bones are not heavily stressed or where stresses arrive from many directions. The trabeculae are oriented along stress lines and extensively cross-braced. G. Bone Development The growth of the skeleton determines the size and proportions of your body. The bony skeleton begins to form about six weeks after fertilization, when the embryo is approximately 12 mm (0.5 in.) long. (At this stage, the existing skeletal elements are made of cartilage.) During subsequent development, the bones undergo a tremendous increase in size. Bone growth continues through adolescence, and portions of the skeleton generally do not stop growing until about age 25. Ossification or osteogenesis refers specifically to the formation of bone. H. Bone Fracture Despite its mineral strength, bone can crack or even break if subjected to extreme loads, sudden impacts, or stresses from unusual directions. The damage is called a fracture. Most fractures heal even after severe damage, if the blood supply and the cellular components of the endosteum and periosteum survive. 45  Figure 20: Bone Fractures I. Axial Skeleton The axial skeleton forms the longitudinal axis of the body. The axial skeleton has 80 bones, about 40 percent of the bones in the human body: The skull (8 cranial bones and 14 facial bones). Bones associated with the skull (6 auditory ossicles and the hyoid bone). The vertebral column (24 vertebrae, the sacrum, and the coccyx). The thoracic cage (the sternum and 24 ribs). The axial skeleton provides a framework that supports and protects the brain, the spinal cord, and the thoracic and abdominal organs. It also provides an extensive surface area for the attachment of muscles that (1) adjust the positions of the head, neck, and trunk; (2) perform respiratory movements; and (3) stabilize or position parts of the appendicular skeleton, which supports the limbs. 46 Figure 21: Axial Skeleton 47 1. Cranial and Facial Subdivisions of the Skull 48  Figure 22: Cranial and Facial Subdivisions of the Skull 2. Vertebral Columns The adult vertebral column, or spine, consists of 26 bones: the vertebrae (24), the sacrum (1), and the coccyx (1), or tailbone. The vertebrae provide a column of support, bearing the weight of the head, neck, and trunk and ultimately transferring the weight to the appendicular skeleton of the lower limbs. The vertebrae also protect the spinal cord and help maintain an upright body position, as when we sit or stand. The total length of the vertebral column of an adult averages 71 cm (28 in.). 49  Figure 23: The Vertebral Column 3. Kyphosis, Lordosis, and Scoliosis The vertebral column must move, balance, and support the trunk and head. Conditions or events that damage the bones, muscles, and/or nerves can result in distorted shapes and impaired function. In kyphosis, the normal thoracic curvature becomes exaggerated posteriorly, producing a “round-back” appearance. This condition can be caused by (1) osteoporosis with compression fractures affecting the anterior portions of vertebral bodies, (2) chronic contractions in muscles that insert on the vertebrae, or (3) abnormal vertebral growth. In lordosis, or “swayback,” both the abdomen and buttocks protrude abnormally. The cause is an anterior exaggeration of the lumbar curvature. This may occur during pregnancy or result from abdominal obesity or weakness in the muscles of the abdominal wall. Scoliosis is an abnormal lateral curvature of the spine in one or more of the movable vertebrae. Scoliosis is the most common distortion of the spinal curvature. 50  Figure 24: Vertebral Column Abnormal Conditions 4. Thoracic Cage Figure 25: Thoracic or Rib Cage J. Appendicular Skeleton The appendicular skeleton includes the bones of the limbs and the supporting bone (pectoral and pelvic) girdles that connect them to the trunk). To appreciate the role 51 of the appendicular skeleton in your life, make a mental list of all the things you have done with your arms or legs today. Standing, walking, writing, turning pages, eating, dressing, shaking hands, and texting—the list quickly becomes unwieldy. Your axial skeleton protects and supports internal organs and takes part in vital functions, such as breathing. But your appendicular skeleton lets you manipulate objects and move from place to place. Figure 26: Appendicular Skeleton 1. Clavicle and Scapula The clavicles are S-shaped bones that originate at the superior, lateral border of the manubrium of the sternum, lateral to the jugular notch. From the pyramid-shaped 52 sternal end, each clavicle curves laterally and posteriorly for about half its length. It then forms a smooth posterior curve to articulate with a process of the scapula, the acromion. The flat, acromial end of the clavicle is broader than the sternal end Figure 27: Right Clavicle The anterior surface of the body of each scapula forms a broad triangle. The three sides of the triangle are the superior border; the medial border, or vertebral border; and the lateral border, or axillary border (axilla, armpit). Muscles that position the scapula attach along these edges. The corners of the triangle are called the superior angle, the inferior angle, and the lateral angle. 53  Figure 28: Right Scapula 2. Humerus, Radius and Ulna The arm, or brachium, contains only one bone, the humerus, which extends from the scapula to the elbow. At the proximal end of the humerus, the round head articulates with the scapula. At the proximal end of the humerus, the round head articulates with the scapula. The prominent greater tubercle is a rounded projection on the lateral surface of the epiphysis, near the margin of the humeral head. The greater tubercle establishes the lateral contour of the shoulder. You can verify its position by feeling for a bump located a few centimeters from 54 the tip of the acromion. The lesser tubercle is a smaller projection that lies on the anterior, medial surface of the epiphysis, separated from the greater tubercle by the intertubercular groove, or intertubercular sulcus Figure 29: Right Humerus The radius is the lateral bone of the forearm). The disc shaped radial head, or head of the radius, articulates with the capitulum of the humerus. During flexion, the radial head swings into the radial fossa of the humerus. A narrow neck extends from the radial head to the radial tuberosity. This tuberosity marks the attachment site of the biceps brachii muscle, a large muscle on the anterior surface of the arm. The ulna and radius are parallel bones that support the forearm or antebrachium. In the anatomical position, the ulna lies medial to the radius. The olecranon, the superior end of the ulna, is the point of the elbow. On the anterior surface of the proximal epiphysis, the trochlear notch of the ulna articulates with the trochlea of the humerus at the elbow joint. shows the trochlear notch of the ulna in a lateral view. Figure 30: Right Radius and Ulna 3. Carpal Bones The carpus, or wrist, contains eight carpal bones. These bones form two rows, one with four proximal carpal bones and the other with four distal carpal bones. 55  Each hand has 14 finger bones, or phalanges. The first finger, known as the pollex, or thumb, has two phalanges (proximal and distal). Each of the other four fingers has three phalanges (proximal, middle, and distal). Figure 31: Bones of the Right Wrist and Hand 4. Pelvic Girdle The pelvic girdle consists of the paired hip bones, also called the coxal bones or pelvic bones. Each hip bone forms by the fusion of three bones: an ilium, an ischium, and a pubis. 56  Figure 32: Right Hip Bone. The Right and Left Hip Bones make up the pelvic girdle.and right hip bones make up the pelvic girdle. Male Female 5. Figure Femur and 33: Pelvis Patella difference between Male and Female right hip bones make up the pelvic girdle. The femur is the longest and heaviest bone in the body. It articulates with the hip bone at the hip joint and with the tibia of the leg at the knee joint while 57 patella is a large sesamoid bone that forms within the tendon of the quadriceps femoris, a group of muscles that extend (straighten) the knee. Figure 34: Right Femur right hip bones make up the pelvic girdle. Figure 35: Right Patella right hip bones make up the pelvic girdle. 58 6. Tibia and Fibula The tibia, or shinbone, is the large medial bone of the leg. The medial and lateral condyles of the femur articulate with the medial and lateral tibial condyles at the proximal end of the tibia. The slender fibula parallels the lateral border of the tibia. The head of the fibula articulates with the tibia. Figure 36: Right Tibia and Fibulat hip bones make up the pelvic girdle. 7. Tarsal Bones The ankle, or tarsus, consists of seven tarsal bones. The large talus transmits the weight of the body from the tibia toward the toes. The calcaneus, or heel bone, is the largest of the tarsal bones. When you stand normally, most of your weight is transmitted from the tibia, to the talus, to the calcaneus, and then to the ground. The posterior portion of the calcaneus is a rough, knob-shaped projection. This is the attachment site for the calcaneal tendon (Achilles tendon), which arises at the calf muscles. 59  The metatarsal bones are five long bones that form the distal portion of the foot, or metatarsus. The phalanges, or toe bones, have the same anatomical organization as the fingers. The toes contain 14 phalanges. The hallux, or great toe, has two phalanges (proximal and distal). The other four toes have three phalanges apiece (proximal, middle, and distal). Figure 37: Bones of the Ankle and Foot bones make up the pelvic girdle. Link to Video Recording: Topic Website/s https://www.youtube.com/watch?v=f-FF7Qigd3U https://www.innerbody.com/image/skelfov.html https://www.healthline.com/human-body- Skeletal System maps/skeletal-system#conditions https://www.khanacademy.org/science/high- school-biology/hs-human-body-systems/hs-the- 60  Lesson 4 Joints and Body Movements INTRODUCTION In this chapter we consider the ways bones interact wherever they interconnect. In the last chapter, you learned the individual bones of the skeleton. These bones provide strength, support, and protection for softer tissues of the body. However, your daily life demands more of the skeleton—it must also facilitate and adapt to body movements. Think of your activities in a typical day: You breathe, talk, walk, sit, stand, and change positions countless times. In each case, your skeleton is directly involved. Movements can occur only at joints, or articulations, where two bones meet, because the bones of the skeleton are inflexible. The characteristic structure of a joint determines the type and amount of movement that may take place. Each joint reflects a compromise between the need for strength and the need for mobility. In this chapter we compare the relationships between articular form and function. We consider several examples that range from relatively immobile but very strong joints (the intervertebral joints) to a highly mobile but relatively weak joint (the shoulder). LEARNING OBJECTIVES 1. Contrast the major categories of joints and explain the relationship between structure and function for each category. 2. Describe the basic structure of a synovial joint and describe common synovial joint accessory structures and their functions. 3. Describe how the anatomical and functional properties of synovial joints permit movements of the skeleton. Describe the joints between the vertebrae of the vertebral column. 4. Describe the structure and function of the shoulder joint and the elbow joint. 5. Describe the structure and function of the hip joint and the knee joint. 6. Describe the effects of aging on joints, and discuss the most common age-related clinical problems for joints; and 7. Explain the functional relationships between the skeletal system and other body systems. A. Joints We use two classification methods to categorize joints. The first is the one we will use in this chapter. It is a functional scheme because it is based on the amount of movement possible, a property known as the range of motion (ROM). Each functional 64 group is further subdivided primarily on the basis of the anatomical structure of the joint a. An immovable joint is a synarthrosis. A synarthrosis can be fibrous or cartilaginous, depending on the nature of the connection. Over time, the two bones may fuse. b. A slightly movable joint is an amphiarthrosis. An amphiarthrosis is either fibrous or cartilaginous, depending on the nature of the connection between the opposing bones. c. A freely movable joint is a diarthrosis, or synovial joint. Diarthroses are subdivided according to the movement permitted. The second classification scheme relies solely on the anatomy of the joint, without regard to the degree of movement permitted. Using this framework, we classify joints as fibrous, cartilaginous, bony, or synovial. Bony joints form when fibrous or cartilaginous joints ossify. The ossification may be normal or abnormal, and may occur at various times in life. The two classification schemes are loosely correlated. We see many anatomical patterns among immovable or slightly movable joints, but there is only one type of freely movable joint—synovial joints. All synovial joints are diarthroses. We will use the functional classification rather than the anatomical one because our primary interest is how joints work. 65  Figure 38: Functional and Structural Classification of Jointsones make up the pelvic girdle. 1. Synovial Joints Synovial joints are freely movable and classified as diarthroses. A two-layered joint capsule, also called an articular capsule, surrounds the synovial joint. Under normal conditions, the bony surfaces at a synovial joint cannot contact one another, because special articular cartilage covers the articulating surfaces. Figure 39: Structure of a synovial jointones make up the pelvic girdle. 66 2. Synovial Fluid, Ligaments, Tendons and Bursae Synovial fluid is a clear, viscous solution with the consistency of egg yolk or heavy molasses. Synovial fluid resembles interstitial fluid, but contains proteoglycans with a high concentration of hyaluronan (hyaluronic acid) secreted by fibroblasts of the synovial membrane. The capsule that surrounds the entire joint is continuous with the periostea of the articulating bones. Accessory ligaments support, strengthen, and reinforce synovial joints. Capsular ligaments, or intrinsic ligaments, are localized thickenings of the joint capsule. Extrinsic ligaments are separate from the joint capsule. These ligaments may be located either inside or outside the joint capsule, and are called intracapsular or extracapsular ligaments, respectively. Tendons are not part of the joint itself, but tendons passing across or around a joint may limit the joint’s range of motion and provide mechanical support for it. For example, tendons associated with the muscles of the arm help brace the shoulder joint. Bursae are small, thin, fluid filled pockets in connective tissue. They contain synovial fluid and are lined by a synovial membrane. Bursae may be connected to the joint cavity or separate from it. They form where a tendon or ligament rubs against other tissues. 3. Types of Movements at Synovial Joints Gliding Movement In gliding, two opposing surfaces slide past one another. Gliding occurs between the surfaces of articulating carpal bones, between tarsal bones, and between the clavicles and the sternum. The movement can occur in almost any direction, but the amount of movement is slight, and rotation is generally prevented by the capsule and associated ligaments. 67 Figure 40: Classification of Synovial Jointsup the pelvic girdle. Angular Movement Examples of angular movement include flexion, extension, abduction, adduction, and circumduction. Descriptions of these movements are based on reference to an individual in the anatomical position. Flexion and Extension. Flexion is movement in the anterior–posterior plane that decreases the angle between articulating bones. Extension occurs in the same plane, but it increases the angle between articulating bones Abduction and Adduction. Abduction is movement away from the longitudinal axis of the body in the frontal plane. For example, swinging 68  the upper limb to the side is abduction of the limb. Moving it back to the anatomical position is adduction. Adduction of the wrist moves the heel of the hand and fingers toward the body, whereas abduction moves them farther away. Circumduction. Recall the special type of angular movement, circumduction, from our model. Moving your arm in a loop is circumduction, as when you draw a large circle on a whiteboard. Your hand moves in a circle, but your arm does not rotate. Figure 41: Angular Movements make up the pelvic girdle. 69 Rotational Movement Rotation of the head may involve left rotation or right rotation. Limb rotation by reference to the longitudinal axis of the trunk. During medial rotation, also known as internal rotation or inward rotation, the anterior surface of a limb turns toward the long axis of the trunk. The reverse movement is called lateral rotation, external rotation, or outward rotation. The proximal joint between the radius and the ulna permits rotation of the radial head. As the shaft of the radius rotates, the distal epiphysis of the radius rolls across the anterior surface of the ulna. This movement, called pronation, turns the wrist and hand from palm facing front to palm facing back. The opposing movement, in which the palm is turned anteriorly, is supination. The forearm is supinated in the anatomical position. Figure 42: Rotational Movements make up the pelvic girdle. Special Movements Inversion is a twisting movement of the foot that turns the sole inward, elevating the medial edge of the sole. The opposite movement is called eversion. Dorsiflexion is flexion at the ankle joint and elevation of the sole, as when you dig in your heel. Plantar flexion, the opposite movement, extends the ankle joint and elevates the heel, as when you stand on tiptoe. However, 70  it is also acceptable (and simpler) to use “flexion and extension at the ankle,” rather than “dorsiflexion and plantar flexion.” Opposition is movement of the thumb toward the surface of the palm or the pads of other fingers. Opposition enables you to grasp and hold objects between your thumb and palm. Reposition is the movement that returns the thumb and fingers from opposition. Protraction is moving a body part anteriorly in the horizontal plane. Retraction is the reverse movement. You protract your jaw when you put your chin forward, and you retract your jaw when you return it to its normal position. Elevation and depression take place when a structure moves in a superior or an inferior direction, respectively. You depress your mandible when you open your mouth, and you elevate your mandible as you close your mouth. Another familiar elevation takes place when you shrug your shoulders. Lateral flexion occurs when your vertebral column bends to the side. This movement is most pronounced in the cervical and thoracic regions. Figure 43: Special Movements make up the pelvic girdle. 71 B. Shoulder and Elbow 1. The Shoulder Joint The shoulder joint, or glenohumeral joint, permits the greatest range of motion of any joint. It is also the most frequently dislocated joint, demonstrating the principle that stability must be sacrificed to obtain mobility. The shoulder is a ball-and socket diarthrosis formed by the articulation of the head of Figure 44: Shoulder Jointmake up the pelvic girdle. the humerus with the glenoid cavity of the scapula 2. The Elbow Joint The elbow joint is a complex hinge joint that involves the humerus, radius, and ulna. The largest and strongest articulation at the elbow is the humeroulnar joint, where the trochlea of the humerus articulates with the trochlear notch of the ulna. This joint works like a door hinge, with physical limitations imposed on the range of motion. The elbow joint is extremely stable because (1) the bony surfaces of the humerus and ulna interlock, (2) a single, thick articular capsule surrounds both the humeroulnar and proximal radioulnar joints, and (3) strong ligaments reinforce the articular capsule. Figure 45: Elbow Jointmake up the pelvic girdle. 72 3. Hip and Knee Joint The hip joint is a sturdy ball-and-socket diarthrosis that permits flexion, extension, adduction, abduction, circumduction, and rotation. Figure 46: Right Hip Jointmake up the pelvic girdle. The knee joint functions as a hinge, but the joint is far more complex than the elbow or even the ankle. The rounded condyles of the femur roll across the superior surface of the tibia, so the points of contact are constantly changing. The joint permits flexion, extension, and very limited rotation. 73  Figure 47: Right Knee Jointmake up the pelvic girdle. 4. Joints of the Appendicular Skeleton Figure 48: Joints of the Appendicular Skeletonmake up the pelvic girdle. 74  Lesson 5 Muscular System INTRODUCTION In this chapter we describe the gross anatomy of the muscular system and consider functional relationships between muscles and bones of the body. Most skeletal muscle fibers contract at similar rates and shorten to the same degree, but variations in microscopic and macroscopic organization can dramatically affect the power, range, and speed of movement produced when muscles contract. In addition, this chapter will also discuss muscle tissue, one of the four primary tissue types, with particular attention to skeletal muscle tissue. We examine the histological and physiological characteristics of skeletal muscle cells and relate those features to the functions of the entire tissue. We also give an overview of the differences among skeletal, cardiac, and smooth muscle tissues. LEARNING OBJECTIVES 1. Specify the functions of skeletal muscle tissue. 2. Describe the organization of muscle at the tissue level. 3. Identify the structural and functional differences between skeletal muscle fibers and cardiac muscle cells. 4. Identify the structural and functional differences between skeletal muscle fibers and smooth muscle cells and discuss the roles of smooth muscle tissue in systems throughout the body. 5. Describe the classes of levers and explain how they make muscles more efficient. Predict the actions of a muscle based on its origin and insertion and explain how muscles interact to produce or oppose movements. 6. Explain how the name of a muscle can helps identify its location, appearance, or function. 7. Identify the principal axial muscles of the body, plus their origins, insertions, actions, and innervation. 78  8. Identify the principal appendicular muscles of the body, plus their origins, insertions, actions, and innervation, and compare the major functional differences between the upper and lower limbs. 9. Identify age-related changes of the muscular system; and 10. Explain the functional relationship between the muscular system and other body systems and explain the role of exercise in producing various responses in other body systems. A. Primary Functions The muscular system performs six critical functions for the human body. It produces skeletal movement, helps maintain posture and body position, supports soft tissues, guards body entrances and exits, helps maintain body temperature, and stores nutrients. B. Skeletal Muscles Muscle tissue consists chiefly of muscle cells that are highly specialized for contraction. Our bodies contain three types of muscle tissue: (1) skeletal muscle, (2) cardiac muscle, and (3) smooth muscle. Skeletal muscles are organs composed mainly of skeletal muscle tissue, but they also contain connective tissues, nerves, and blood vessels. Each cell in skeletal muscle tissue is a single muscle fiber. Skeletal muscles attach directly or indirectly to bones. Figure 49: Organization of Skeletal Musclesmake up the pelvic girdle. 79  1. Layers of Muscle Connective Tissues Each muscle has three layers of connective tissue: (1) an epimysium, (2) a perimysium, and (3) an endomysium. The epimysium is a dense layer of collagen fibers that surrounds the entire muscle. It separates the muscle from nearby tissues and organs. The epimysium is connected to the deep fascia, a dense connective tissue layer. The perimysium divides the skeletal muscle into a series of compartments. Each compartment contains a bundle of muscle fibers called a fascicle. In addition to collagen and elastic fibers, the perimysium contains blood vessels and nerves that supply the muscle fibers within the fascicles. Each fascicle receives branches of these blood vessels and nerves. Within a fascicle, the delicate connective tissue of the endomysium surrounds the individual skeletal muscle cells, called muscle fibers, and loosely interconnects adjacent muscle fibers. This flexible, elastic connective tissue layer contains (1) capillary networks that supply blood to the muscle fibers; (2) myosatellite cells, stem cells that help repair damaged muscle tissue; and (3) nerve fibers that control the muscle. All these structures are in direct contact with the individual muscle fibers. C. The nervous system communicates with skeletal muscles at the neuromuscular junction Skeletal muscle fibers begin contraction with the release of their internal stores of calcium ions. That release is under the control of the nervous system. Communication between a neuron and another cell occurs at a synapse. When the other cell is a skeletal muscle fiber, the synapse is known as a neuromuscular junction (NMJ), or myoneural junction. The NMJ is made up of an axon terminal (synaptic terminal) of a neuron, a specialized region of the sarcolemma called the motor end plate, and, in between, a narrow space called the synaptic cleft. Motor neurons of the central nervous system (brain and spinal cord) carry instructions in the form of action potentials to skeletal muscle fibers. 80 Figure 50: Steps Involved in Skeletal Muscle Contraction and Relaxation. 81 D. Muscle Contractions 1. Isotonic and Isometric Contractions We can classify muscle contractions as isotonic or isometric on the basis of their pattern of tension production. Isotonic Contractions. In an isotonic contraction, tension increases and the skeletal muscle’s length changes. Lifting an object off a desk, walking, and running involve isotonic contractions. There are two types of isotonic contractions: concentric and eccentric. In a concentric contraction, the muscle tension exceeds the load and the muscle shortens. In an eccentric contraction, the peak tension developed is less than the load, and the muscle elongates due to the contraction of another muscle or the pull of gravity Isometric Contractions. In an isometric contraction (metric, measure), the muscle does not change length, and the tension produced never exceeds the load. Examples of isometric contractions include carrying a bag of groceries and holding our heads up. Many of the reflexive muscle contractions that keep your body upright when you stand or sit involve isometric contractions of muscles that oppose the force of gravity. 2. Adenosine Triphosphate (ATP) provides energy for muscle contraction Adenosine triphosphate, also known as ATP, is a molecule that carries energy within cells. It is the main energy currency of the cell, and it is an end product of the processes of photophosphorylation (adding a phosphate group to a molecule using energy from light), cellular respiration, and fermentation. A single muscle fiber may contain 15 billion thick filaments. When that muscle fiber is actively contracting, each thick filament breaks down around 2500 ATP molecules per second. Even a small skeletal muscle contains thousands of muscle fibers, so the ATP demands of a contracting skeletal muscle are enormous. In practical terms, the demand for ATP in a contracting muscle fiber is so high that it would be impossible to have all the necessary energy available as ATP before the contraction begins. Instead, a resting muscle fiber contains only enough ATP and other high-energy compounds to sustain a contraction until additional ATP can be generated. Throughout the rest of the contraction, the muscle fiber will generate ATP at roughly the same rate as it is used. 82 E. Classification of Skeletal Muscles 1. Parallel muscle – the fascicles are parallel to the long axis of the muscle. Most of the skeletal muscles in the body are parallel muscles. 2. Convergent muscle – muscle fascicles extending over a broad area come together, or converge, on a common attachment site. 3. Pennate muscle – the fascicles form a common angle with the tendon. Because the muscle fibers pull at an angle, contracting pennate muscles do not move their tendons as far as parallel muscles do. 4. Circular muscle, or sphincter – the fascicles are concentrically arranged around an opening. When the muscle contracts, the diameter of the opening becomes smaller. Figure 51: Muscle Type Based on Pattern of Fascicle Organization F. Classes of Levers Skeletal muscles do not work in isolation. For muscles attached to the skeleton, the nature and site of the connection determine the force, speed, and range of the movement. Attaching the muscle to a lever can modify the force, speed, or direction of movement produced by muscle contraction. A lever is a rigid structure—such as a board, a crowbar, or a bone—that moves on a fixed point called the fulcrum. A lever moves when pressure called an applied force 83 is sufficient to overcome any load that would otherwise oppose or prevent such movement. In the body, each bone is a lever and each joint is a fulcrum. Muscles provide the applied force. The load can vary from the weight of an object held in the hand to the weight of a limb or the weight of the entire body, depending on the situation. The important thing about levers is that they can change (1) the direction of an applied force, (2) the distance and speed of movement produced by an applied force, and (3) the effective strength of an applied force. Figure 52: he Three Classes of Levers There are three classes of levers. We find examples of each in the human body. A pry bar or crowbar is an example of a first-class lever. In such a lever, the fulcrum (F) lies between the applied force (AF) and the load (L). The body has few first-class levers. In a second-class lever, the load lies between the applied force and the fulcrum. A familiar example is a loaded wheelbarrow. The weight is the load, and the upward lift on the handle is the applied force. 84  In a third-class lever, such as a catapult, the applied force is between the load and the fulcrum. Third-class levers are the most common levers in the body. The effect is the reverse of that for a second-class lever: Speed and distance traveled are increased at the expense of effective force. G. Origin, Insertion and Actions of Muscles The place where the fixed end attaches to a bone, cartilage, or connective tissue is called the origin of the muscle. The site where the movable end attaches to another structure is called the insertion of the muscle. The origin is typically proximal to the insertion. When a muscle contracts, it produces a specific action, or movement. We describe muscles as follows, based on their functions: An agonist, or prime mover, is a muscle whose contraction is mostly responsible for producing a particular movement. The biceps brachii muscle is an agonist that produces flexion at the elbow. An antagonist is a muscle whose action opposes that of a particular agonist. The triceps brachii muscle is an agonist that extends the elbow. For this reason, it is an antagonist of the biceps brachii muscle. Likewise, the biceps brachii is an antagonist of the triceps brachii. Agonists and antagonists are functional opposites. If one produces flexion, the other produces extension. When an agonist contracts to produce a particular movement, the corresponding antagonist is stretched, but it usually does not relax completely. Instead, it contracts eccentrically, with just enough tension to control the speed of the movement and ensure its smoothness. When a synergist contracts, it helps a larger agonist work efficiently. Synergists may provide additional pull near the insertion or may stabilize the point of origin. A fixator is a synergist that assists an agonist by preventing movement at another joint, thereby stabilizing the origin of the agonist. H. How Muscles Are Named The name of a muscle may include descriptive information about its location in the body, origin and insertion, fascicle organization, position, structural characteristics, and action. 1. Location in the Body Regional terms are most common as modifiers that help identify individual muscles. In a few cases, a muscle is such a prominent feature of a body region 85  that a name referring to the region alone will identify it. Examples include the temporalis muscle of the head and the brachialis muscle of the arm. 2. Origin and Insertion Many muscle names include terms for body places that tell you the specific origin and insertion of each muscle. In such cases, the first part of the name indicates the origin, the second part the insertion. The genioglossus muscle, for example, originates at the chin (geneion) and inserts in the tongue (glossus). 3. Fascicle Organization A muscle name may refer to the orientation of the muscle fascicles within a particular skeletal muscle. Rectus means “straight,” and rectus muscles are parallel muscles whose fibers run along the long axis of the body. For example, the rectus abdominis muscle is located on the abdomen, and the rectus femoris muscle on the thigh. 4. Position Muscles visible at the body surface are often called externus or superficialis. Deeper muscles are termed internus or profundus. Superficial muscles that position or stabilize an organ are called extrinsic. Muscles located entirely within an organ are intrinsic. 5. Structural Characteristics Some muscles are named after distinctive structural features. The biceps brachii muscle, for example, is named after its origin. It has two tendons of origin. Similarly, the triceps brachii muscle has three, and the quadriceps group has four. Shape is sometimes an important clue to the name of a muscle. For example, the trapezius, deltoid, rhomboid, and orbicularis) muscles look like a trapezoid, a triangle, a rhomboid, and a circle, respectively. 6. Action Many muscles are named flexor, extensor, pronator, abductor, and so on. These are such common actions that the names almost always include other clues as to the appearance or location of the muscle. For example, the extensor carpi radialis longus muscle is a long muscle along the radial (lateral) border of the forearm. When it contracts, its primary function is extension at the carpus (wrist). 86  Figure 53: Muscle Terminology I. Axial and Appendicular Muscles The separation of the skeletal system into axial and appendicular divisions serves as a useful guideline for subdividing the muscular system as well: The axial muscles arise on the axial skeleton. This category includes approximately 60 percent of the skeletal muscles in the body. They position the head and spinal column and also move the rib cage, assisting in the movements that make breathing possible. They do not play a role in movement or support of either the pectoral or pelvic girdle or the limbs. The appendicular muscles stabilize or move components of the appendicular skeleton. These muscles include the remaining 40 percent of all skeletal muscles. 87 Figure 54: Anterior View of Major Skeletal Muscles 88 Figure 55: Posterior View of Major Skeletal Muscles 89 1. Axial muscles are muscles of the head and neck, vertebral column, trunk, and pelvic floor a. Muscle of Facial Expression Figure 56: Muscles of Facial Expression 90  b. Muscles of the Vertebral Column Figure 57: Muscles of the Vertebral Column c. Muscles of the Extrinsic Eyes, Mastication, Tongue, Pharynx, Anterior Neck, Diaphragm and Pelvic Floor (See media support at the end of this chapter) 91  2. Appendicular Muscles are muscles of the shoulders, upper limbs, pelvis, and lower limbs a. Appendicular Muscles of the Trunk Figure 58: Anterior Overview of the Appendicular Muscles of the Trunk 92 Figure 59: Posterior Overview of the Appendicular Muscles of the Trunk b. Muscles that move the arm The muscles that move the arm are easiest to remember when they are grouped by their actions at the shoulder joint. The deltoid muscle is the major abductor, but the supraspinatus muscle assists at the start of this movement. The subscapularis and teres major muscles produce medial rotation at the shoulder, whereas the infraspinatus and the teres minor muscles produce lateral rotation. All these muscles originate on the scapula. 93 94 Figure 60: Muscles that Move the Arm c. Muscles that move the hand and fingers 95 Figure 61: Muscles that Move Hands and Fingers d. Muscles that move the leg 96 Figure 62: Muscles that Move the Leg e. Muscles that position pectoral girdle (See media support at the end of this chapter) f. Muscles that move the thigh, foot and toes (See media support at the end of this chapter) J. Effects of Aging on the Muscular System 1. Skeletal Muscle Fibers Become Smaller in Diameter. 2. Skeletal Muscles Become Less Elastic. Aging skeletal muscles develop increasing amounts of fibrous connective tissue, a process called fibrosis. Fibrosis makes the muscle less flexible, and the collagen fibers can restrict movement and circulation. 3. Tolerance for Exercise Decreases. A lower tolerance for exercise comes in part from tiring quickly and in part from reduced Thermoregulation, Individuals over age 65 cannot eliminate the heat their muscles generate during contraction as effectively as younger people can. For this reason, they are subject to overheating. 4. The Ability to Recover from Muscular Injuries Decreases. The number of satellite cells steadily decreases with age, and the amount of fibrous tissue increases. As 97  a result, when an injury occurs, repair capabilities are limited. Scar tissue formation is the usual result. Regular exercise helps control body weight, strengthens bones, and generally improves the quality of life at all ages. Extremely demanding exercise is not as important as regular exercise. In fact, extreme exercise in the elderly can damage tendons, bones, and joints. K. Exercises Produces Response in Multiple Body Systems To operate at maximum efficiency, the muscular system must be supported by many other systems. The changes that take place during exercise provide a good example of such interaction. As noted earlier, active muscles consume oxygen and generate carbon dioxide and heat. The immediate effects of exercise on various body systems include the following: Cardiovascular System: Blood vessels in active muscles and the skin dilate, and heart rate increases. These adjustments speed up oxygen and nutrient delivery to and carbon dioxide removal from the muscle. They also bring heat to the skin for radiation into the environment. Respiratory System: Respiratory rate and depth of respiration increase. Air moves into and out of the lungs more quickly, keeping pace with the increased rate of blood flow through the lungs. Integumentary System: Blood vessels dilate and sweat gland secretion increases. This combination increases evaporation at the skin surface and removes the excess heat generated by muscular activity. Nervous and Endocrine Systems: The responses of other systems are directed and coordinated through neural and endocrine (hormonal) adjustments in heart rate, respiratory rate, sweat gland activity, and mobilization of stored nutrient reserves. Even when the body is at rest, the muscular system has extensive interactions with other systems. 98