Anatomical Positions Textbook Notes PDF

114 4 The Locomotor System (Musculoskeletal System) 쮿 Axes, Planes, and Orientation Axes and Planes of the Body Any number of axes and planes may be drawn through the human body. It is customary, however, to define three main axes running perpendicu- lar to each other in three spatial coordin...

114 4 The Locomotor System (Musculoskeletal System) 쮿 Axes, Planes, and Orientation Axes and Planes of the Body Any number of axes and planes may be drawn through the human body. It is customary, however, to define three main axes running perpendicu- lar to each other in three spatial coordinates (Fig. 4.1): 쮿 A longitudinal axis (vertical axis, cephalocaudal axis) of the body, which in the upright posture runs perpendicular to the base 쮿 A horizontal axis (transverse axis) running from left to right and per- pendicular to the longitudinal axis 쮿 A sagittal axis running from front to back and perpendicular to both the other axes Hence it is possible to define three principal planes: 쮿 A sagittal plane, defined as any plane that is oriented along the sagit- tal axis (the vertical plane that divides the body into two equal halves is called the median plane) 쮿 A transverse plane, defined as any plane running transversely across the body 쮿 A frontal plane (coronal plane) that includes all planes oriented par- allel to the forehead Nomenclature of Positions and Directions The following designations of positions and directions serve to describe accurately the positions of parts of the human body: 쮿 For the trunk Cranial, cephalad, or superior: Toward the head Caudad or inferior: Toward the coccyx (tailbone) Ventral or anterior: Toward the front (abdomen) Dorsal or posterior: Toward the rear (back) Medial: Toward the median plane Lateral: Away from the median plane Internal: Inside the body External: Outside the body Peripheral: Away from the trunk Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  Axes, Planes, and Orientation 115 Fig. 4.1 Major axes and planes of the human body. Median Left anterior view (sagittal) plane Longi- tudinal axis Hori- zontal (transverse) plane Coronal plane Sagittal axis Transverse axis 쮿 For the extremities Proximal: Toward the trunk Distal: Toward the extremities of the limbs Radial Toward the radius (thumb side) Ulnar: Toward the ulna (side of the little finger) Tibial: Toward the shin (side of the big toe) Fibular: Toward the fibula (side of the little toe) Palmar (volar): Toward the palm of the hand Plantar: Toward the sole of the foot Dorsal (of hand and foot): Toward the back of the hand or foot (upper side of foot) Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  116 4 The Locomotor System (Musculoskeletal System) 쮿 General Anatomy of the Locomotor System The skeleton, the supporting framework of the body, is formed by bony and cartilaginous elements, connected by connective tissue structures. Its parts are moved or held in defined positions or postures by the skeletal musculature. The overarching term locomotor system includes the skeleton and the musculature. The passive locomotor system con- sists of the skeleton and its joints (articulations), while the active motor system includes the striated muscles, the tendons, and their auxiliary structures (muscle fasciae, bursae, tendon sheaths, and sesamoid bones). Beside their supporting function, the skeletal elements and their joints serve to provide levers for the muscles during locomotion. The skeletal elements, joints, and skeletal musculature together form the organs of locomotion. In addition, the skeletal elements function to protect other organ systems (bones of the skull, vertebral canal, chest cage). The Bones The bony skeleton consists of bones of various structures and shapes. In the adult human, the skeleton is composed of about 200 individual bones, which are connected by cartilaginous, fibrous, and synovial joints. Each bone, with the exception of the cartilaginous joint surfaces and areas where flat tendons are attached, is enclosed in a connective tissue sheath, the periosteum, like a stocking. The shape of each bone is determined genetically, but its structure depends largely on the type and extent of the mechanical demands placed on it. According to their external shape, bones are divided into long, short, flat, and irregular bones. Examples of long bones (pipe bones) are the bones of the free extremities, with the exception of the wrist and ankle bones. Long bones are composed of a shaft (diaphysis) and an epiphysis at each end. During growth, each diaphysis and the corre- sponding epiphysis are separated by the so-called epiphyseal cartilage (epiphyseal plate) (see Fig. 3.10, p. 85). The short bones include the cube- shaped bones of the wrist and ankle. Among the flat bones are the ribs, the breast bone, the shoulder blade, and the bones of the skull. The irregular bones include the vertebrae and Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  General Anatomy of the Locomotor System 117 the bones at the base of the skull. Some of the bones in the skull (frontal, cribriform plate, upper jaw) contain air-filled cavities. Sesamoid bones are bones embedded in tendons (e. g., the kneecap). Finally, certain extra bones, occurring especially in the hand and foot, are called accessory bones. Their presence in a radiographic image can lead to diagnostic er- rors (as displaced fragment due to a fracture). The Joints Joints are connections between cartilaginous and/or bony parts of the skeleton. They enable movement between the individual segments of the trunk and the extremities, and transmit force. They are divided, ac- cording to the type of connection, into immovable and movable. Immovable Joints (Synarthroses) So-called immovable joints, or synarthroses, are those joints in which parts of the skeleton are separated by a different tissue such as cartilage or connective tissue. According to the intervening tissue these are divided into (Fig. 4.2): 쮿 Syndesmoses (fibrous joints) 쮿 Synchondroses (cartilaginous joints) 쮿 Synostoses (bony joints), which are not true joints but bony fusions 쮿 Syndesmoses. In syndesmoses, two bones are connected by connec- tive tissue (Fig. 4.2). Examples are the interosseous membrane between the ulna and radius of the lower arm; the membranous fontanelles of the newborn skull, and the sutures between the bones of the skull. The connective tissue anchoring of the roots of teeth in the upper and lower jaw, known as a peg and socket joint (gomphosis), is also a syndesmo- sis. 쮿 Synchondroses. The connecting tissue in synchondroses is cartilage (Fig. 4.2). Examples are the fibrocartilaginous intervertebral disks be- tween vertebrae or the symphysis pubis at the junction of the two pubic bones. The connection of the bony diaphysis of a juvenile long Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  118 4 The Locomotor System (Musculoskeletal System) Fig. 4.2 Simplified repre- Bone sentation of the different immovable joints (synar- Syndesmosis throses) (fibrous joint) Connective tissue Synchondrosis (cartilaginous joint) Cartilage Synostosis (bony joint) Bone bone to the epiphysis by its cartilaginous epiphyseal plate is also a syn- chondrosis. 쮿 Synostoses. In a synostosis individual bones are fused secondarily by bone tissue (Fig. 4.2). A typical example is the sacrum, which originally consists of five separate vertebrae that fuse to each other when growth is complete. Another example is the hip bone, which, until growth is completed, consists of three separate bones: the pubis, the ilium, and the ischium. Movable Joints (Synovial Joints, Diarthroses) In synovial joints the bones are separated by a joint space (Fig. 4.3). They are also distinguished by hyaline cartilage covering the joint surfaces and by a joint capsule that encloses a joint cavity. Some joints feature interar- ticular disks (menisci), articular lips, or intra-articular ligaments. For in- stance, the menisci of the knee are made of fibrocartilage, are semilunar in shape, and incompletely partition the knee joint. Disks are also a fea- ture of the mandibular joint and the sternoclavicular joint among others. The function of disks in joints is to increase the contact between two op- posing surfaces. Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  General Anatomy of the Locomotor System 119 Cancellous bone Joint space Hyaline cartilage of joint Bone marrow cavity Compact bone Fibrous joint capsule Tendon sheath Joint cavity Inner joint membrane Long flexor tendon (synovial membrane) Fig. 4.3 Structure of a movable joint as exemplified in the metatarsophalangeal joint of the big toe 쮿 Joint Cartilage. The smooth surface of joint cartilage consists mostly of hyaline cartilage (Fig. 4.3), the mechanical and “shock absorbing” prop- erties of which are essentially due to its extracellular matrix. Important constituents of extracellular matrix are collagen fibers, macromolecules (protein saccharides), and water. The thickness of joint cartilage varies considerably. It averages 2−3 mm, but in some places (joint surface of the patella) joint cartilage can reach 8 mm. Since this cartilage does not con- tain blood vessels, it must receive nutrients by diffusion from the syn- ovial fluid. Optimal nutrition requires regular movement (loading and unloading) of the cartilage, so that the synovia is pressed into the car- tilage. Lack of movement and unphysiologically high tensions lead to degenerative changes (osteoarthritis) in joint cartilage, especially in older Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  120 4 The Locomotor System (Musculoskeletal System) people. Because there is no perichondrium, the regenerative power of joint cartilage is insignificant (see p. 79). 쮿 Joint Capsule and Synovial Fluid. The joint capsule (Fig. 4.3) is a con- tinuation of the periosteum. It is made up of an outer dense white fibrous layer (fibrous membrane) and an inner loosely structured mem- brane rich in vessels and nerves (synovial membrane), which may con- tain a varying amount of fat. The outer fibrous membrane is often rein- forced with ligaments, which may reinforce the capsule, guide move- ment, or prevent hyperextension of a joint. When a joint is immobilized over a prolonged period of time, the connective tissue fibers shorten, the joint capsule shrinks, and the mobility of the joint can be severely com- promised (joint contracture). From the inner synovial membrane folds and protrusions project into the joint. This membrane abuts on the joint cavity with specialized connective tissue cells that are responsible for the secretion (production) and reabsorption (resorption) of the synovial fluid. The glairy, thick (viscous) synovial fluid not only nourishes the joint cartilage, but serves as a lubricant to reduce friction between the joint surfaces. Slightly Movable Joints (Amphiarthroses) Some joints are severely limited in their mobility by the shape of their facets and strong ligaments. Such joints include the tibiofibular joint and the sacroiliac joint between the sacrum and the ilium. Types of Joint Joints may be classified from different points of view, for example, by the number of axes of mobility, of degrees of freedom, or of components of the joint. The following is a classification by shape and configuration of the joint surfaces (Fig. 4.4): 쮿 Ball-and-socket joints 쮿 Condyloid joints 쮿 Hinge joints 쮿 Pivot joints 쮿 Saddle joints 쮿 Plane joints Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  General Anatomy of the Locomotor System 121 Ball-and-socket joint Condyloid joint Hinge joint Pivot joint Saddle joint Plane joint Fig. 4.4 Types of joint. The arrows show the direction in which the skeletal parts can move around each axis. 쮿 Ball-and-Socket Joints (Spheroid Joints) consist of a ball-shaped head and a correspondingly concave socket. They have three main axes per- pendicular to each other and allow six main movements. Typical ball- and-socket joints are the hip and shoulder joints. Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  122 4 The Locomotor System (Musculoskeletal System) 쮿 Condyloid Joints (Condylar Joints) have an elliptical head fitted into a convex and a concave socket. They have two main axes perpendic- ular to each other, and they allow four main movements. Examples are the joint between the bones of the forearm and the wrist (pro- ximal wrist joint) and the joint between the atlas and the occipital condyles. 쮿 Hinge Joints (Ginglymus Joints) and Pivot Joints (Trochoid Joints) are also known as trochlear joints. In hinge joints, a cylindrical bone end is applied to a gutterlike depression in a hollow skeletal cyl- inder. Because of this shape, hinge joints have only one axis of movement and two main movements (elbow joint). In pivot joints, a cylindrical part of the skeleton is fitted into a corresponding hollow cylinder and a ring-shaped ligament. A typical example is the su- perior radioulnar joint and its annular ligament. Such a joint allows rotation around one axis and two main movements. 쮿 Saddle Joints consist of two concave curved surfaces with two main axes of movement that are perpendicular to each other and allow four main movements. An example is the joint in the wrist be- tween the first metacarpal bone and the trapezium. 쮿 Plane Joints (Gliding Joints) allow gliding movements of plane joint surfaces, as in the small joints of the vertebrae. Joint Mechanics The direction of movement in a joint is determined not only by the shape of the joint surfaces but also by the arrangement of the muscles and ligaments. Human joints cohere by force: their integrity is ensured by muscular forces, which also determine the direction and type of their movement. The shape of the joint, the muscles, ligaments, and soft tissues limit the extent of movement. Hence, limitations may be divided into bony, muscular, ligamentous, and soft tissue types. Joints move around movement axes: the direction of movement is determined by the relationship of the muscles to the axes. The body can be considered to have three main axes, running perpendicular to each other (p. 115). In addition there are axes relating specifically to the Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  General Anatomy of the Locomotor System 123 movement of each joint, named according to its movement, e. g., the pronation/supination axis of the proximal and distal radioulnar joints around which the hand may be rotated inward and outward (pronation and supination). Two opposite movements can occur around each axis. Examples are: 쮿 Bending—extending (flexion—extension), e. g., the elbow joint 쮿 Pushing out—pulling in (abduction—adduction), e. g., the hip joint 쮿 Rolling inward—rolling outward (inner rotation—outer rotation), e. g., the shoulder joint 쮿 Forward motion—backward motion (flexion—extension), e. g., hip joint 쮿 Opposition—reposition, e. g., the saddle joint of the thumb The effect of a muscle on a joint depends on the lever arm, that is, the vertical distance of its insertion to the axis of its joint (force arm). Force and load are in equilibrium when force × force arm = load × load arm. The product of force with force arm and of load with load arm is called the torque (Fig. 4.5). Function and Structural Principles of Skeletal Muscle A skeletal muscle is divided into a fleshy, variously shaped muscle belly and the usually markedly thinner tendons. The latter are attached to structures in the skeleton or connective tissue of the locomotor system (fasciae, interosseous membrane) and transmit the muscle pull directly or indirectly to parts of the skeleton. In the extremities, the attachment nearest the trunk (proximal) is usually considered the origin and that farthest from the trunk (distal) the insertion of the muscle. In the trunk, the origin of a muscle is always cephalad. The origin and insertion of a muscle are designated arbitrarily and should not be confused with the fixed and mobile points, the latter being where the muscle is attached to the part of the skeleton that is moved, the former to that which is fixed. Although the fixed point and the origin coincide in most movements of the extremities, this is not necessarily the case, since the fixed point could be interchanged with the mobile point in the course of a move- ment. Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  124 4 The Locomotor System (Musculoskeletal System) Head of the humerus Glenoid cavity (joint cavity of the shoulder) M. biceps brachii (two- headed muscle of the upper arm) M. triceps brachii (three-headed muscle of the Radial tuberosity upper arm) (insertion of the biceps into the radius) Arm bone Radius (humerus) Insertion of the triceps m. at the elbow end of the ulna (olecranon) Ulna Force Load Force arm Load arm Fulcrum Fig. 4.5 Effect of the flexors and extensors of the upper arm on forearm movement. Flexor at the elbow joint: m. biceps brachii (two-headed muscle of the upper arm). Exten- sor at the elbow joint: m. triceps brachii (three-headed muscle of the upper arm). The me- chanics of muscle power are shown: load × load lever = force × force lever (the product of force × force lever and load × load lever constitute the current torque). Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  General Anatomy of the Locomotor System 125 At the origin of a muscle there is often a head (caput) that transitions into a belly (venter). A muscle with several origins may be two-, three-, or four-headed, all joining to form a single belly and ending in a single ten- don. A muscle with a single head but one intersecting tendon is called a digastric muscle. A muscle may have several such intervening tendons (Fig. 4.6). Muscles that extend over two or more joints are called diarthric or polyarthric (multiarticular), respectively. Muscles that work together in one movement are synergists, while those with opposing actions are antagonists. Muscles are also classified according to the way the muscle fibers are inserted into the tendons (e. g., pennate muscles from penna = feather) (Fig. 4.6). A muscle with parallel fibers can achieve considerable height of lift with relatively little force, but because of the small total cross-section of its muscle fibers (physiological cross-section), their lifting strength is rather small. The fibers of a unipennate muscle are inserted on only one side of the tendon of origin and insertion. This results in a large physio- logical cross-section and considerable muscular strength. Because the fibers of such a muscle are short, the height of lift is small. In a bipennate muscle the muscle fibers originate from a bifurcated tendon and run alongside both sides of the tendon of insertion. The physiological cross- section and so its ability to develop power is here even greater than in a unipennate muscle. The fine structure of a muscle is determined not only by its striated muscle fibers but also by its connective tissue structures, which form the boundaries between the individual components of each skeletal muscle and are the conduit for the vessels and nerves supplying the muscle fibers (see Fig. 3.12a−e). Loose alveolar tissue in the form of endomysium forms a sheath around the individual fibers, which in their turn are grouped together by denser white connective tissue (perimysium). Several primary bundles are pulled together by another strong connec- tive tissue sheath, the epimysium, into the secondary bundles (fleshy fibers) that are visible to the naked eye. A stout connective tissue sheath, the muscle fascia, envelops the whole muscle. Loose areolar tissue (epimysium) separates the muscle from the fascia. Several individual muscles can be enclosed by a common fascia. Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.  126 4 The Locomotor System (Musculoskeletal System) Unipennate muscle (e.g., m. semi- Muscle with parallel membranosus) fibers (e.g., m. palmaris longus) Bipennate muscle (e.g., m. tibialis anterior) Insertion ends of tendons Tendinous Tendon of origin intersection Aponeurosis (tendon sheet) Muscle bellies Insertion end of tendon Muscle with several bellies Two-headed (e.g., m. rectus muscle (e.g., m. Flat muscle abdominis) biceps brachii) (e.g., m. trapezius) Fig. 4.6 Various types of muscle. (After Platzer) Faller, The Human Body © 2004 Thieme All rights reserved. Usage subject to terms and conditions of license.