Musculoskeletal System
19.1 Overview of the Human Skeleton
The skeleton provides attachment sites for the muscles.
Muscle contraction facilitates movement of bones.
Enables activities such as walking, playing sports, and holding objects.
The musculoskeletal system encompasses both bones and muscles.
Organization of Tissues in the Skeleton
Bones are classified based on their shape:
Long
Short
Flat
Irregular
Round
Structural organization of bone tissues:
Enclosed by the periosteum, a tough, fibrous connective tissue.
The periosteum is continuous with ligaments and contains blood vessels that supply the bone.
Main structural components of long bones:
Epiphysis: The expanded end of a long bone.
Diaphysis: The shaft between the epiphyses.
Joints between bones are covered by cartilage.
Anatomy of a Long Bone
Primary connective tissues of the skeleton include:
Bone
Cartilage
Dense fibrous connective tissue
Characteristics of bone
Composed of cells separated by a matrix with fibers.
Strength is due to collagen fibers and mineral salts like calcium phosphate.
Bone Structure and Types
Compact Bone:
Highly organized structure comprising osteons (tubular units).
Osteocytes (bone cells) reside in lacunae, arranged in concentric circles around a central canal.
Central canals include blood vessels, lymphatic vessels, and nerves.
Canaliculi form connections between lacunae and the central canal.
Spongy Bone:
Appears unorganized; osteocytes are found in trabeculae (thin plates with unequal spaces).
Plates align along lines of stress, providing strength.
Spaces within spongy bone are filled with red bone marrow.
Red marrow produces blood cells.
Infants have red marrow in all bones; in adults, it is restricted to certain bones.
Anatomy and Structure of Long Bones
The Epiphysis of a long bone predominantly comprises spongy bone.
The Diaphysis contains:
Compact bone as a protective border.
Medullary cavity housing yellow bone marrow, which contains a substantial amount of fat.
Metaphysis:
This small region separates the diaphysis from the epiphysis and contains the epiphyseal plate (growth plate), which consists of cartilage.
Cartilage
Definition: A flexible connective tissue that is not as strong as bone.
Characteristic features:
Gel-like matrix comprised of collagen and elastic fibers.
Cells resided within lacunae arranged irregularly.
Lacks blood vessels.
Types of Cartilage
Hyaline Cartilage:
Firm yet flexible; found at the ends of long bones, nose, ends of ribs, larynx, and trachea.
Fibrocartilage:
Composed of thicker collagen fibers; strong and can withstand pressure and tension.
Commonly found in intervertebral discs and knees.
Elastic Cartilage:
Characterized by extensive elastin fibers, making it highly flexible.
Present in structures such as the ear flaps and epiglottis.
Dense Fibrous Connective Tissue
Composed of densely packed fibroblasts set apart by bundles of collagen fibers.
Key functions include:
Forming the flared sides of the nose.
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Ligaments and Tendons
Ligaments connect bone to bone.
Tendons connect muscle to bone.
Bone Growth and Remodeling
Bones initially appear as hyaline cartilage (with skull bones being the notable exception).
Endochondral ossification:
Replacement of cartilage with bone through a gradual process.
Starts with a hyaline cartilage template; cartilage is substituted by bone over time.
The process involves:
Breakdown of cartilage in the center of a long bone, allowing periosteum to develop.
Stages of Bone Development and Growth
Primary Ossification Center:
Osteoblasts invade and produce spongy bone.
Other osteoblasts lay compact bone beneath the periosteum.
Osteoclasts deconstruct spongy bone to create the medullary cavity.
Secondary Ossification Centers:
Appear at the ends of developing bones where spongy bone forms and remains intact.
The growth plate persists between primary and secondary ossification centers, allowing for continued lengthening until ossification occurs, halting growth.
Remodeling of Bones
Adult bones undergo continuous breakdown and rebuilding.
Osteoclasts break down the bone matrix releasing calcium into the bloodstream.
Osteoblasts absorb calcium and deposit it within the new bone matrix.
Certain osteoblasts become trapped in the matrix, evolving into osteocytes.
Remodeling can alter bone thickness, influenced by hormones and physical activity.
Healing Fractured Bone
Reactive Phase: An inflammatory response occurs immediately after injury.
Reparative Phase: Involves the formation of a bone callus, a temporary connective tissue bridge that eventually gets replaced.
Remodeling Phase: Osteoclasts and osteoblasts gradually replace temporary bone with compact bone.
Functions of the Skeleton
Support body structures, especially pelvis and leg bones.
Protect soft body parts (e.g., skull protects the brain; rib cage guards the heart and lungs).
Produce blood cells.
Store minerals and fat.
Facilitate muscle movement.
Classification of the Bones
Total of approximately 206 bones categorized into two divisions:
Axial Skeleton: Comprises the midline bones of the body.
Appendicular Skeleton: Encompasses the bones of limbs and their girdles.
Classification by shape includes:
Long Bones: Longer than they are wide (e.g., limbs, phalanges).
Short Bones: Cube-shaped bones (e.g., carpals, tarsals).
Flat Bones: Plate-like structures (e.g., skull, scapula).
Irregular Bones: Bones with irregular shapes (e.g., vertebrae, facial bones).
Round Bones: Spherical in shape (e.g., patella).
The Axial Skeleton
Comprises the skull, hyoid bone, vertebral column, rib cage, and ossicles.
The Skull:
Includes the cranium (braincase) and facial bones.
The cranium protects the brain and consists of eight tightly fitted bones, not completely ossified in infants.
Fontanels: Gaps in an infant's skull that close by 24 months through intramembranous ossification.
Sinuses: Air cavities lined with mucous membranes to reduce skull weight and provide resonance to the voice.
The Cranium
Bones named according to their corresponding brain lobes:
Frontal Bone: Forehead region.
Parietal Bones: Top and sides of the braincase.
Temporal Bones: Situated below parietals, containing external auditory canal and styloid process.
Occipital Bone: Base of the skull; contains the foramen magnum, allowing the spinal cord’s passage.
Sphenoid Bone: Forms the cranial floor, contributing to the orbits.
Ethmoid Bone: Forms part of the orbits and nasal septum.
Facial Bones
Mandible: The lower jaw; the only movable skull bone.
Maxillae: Upper jaw and anterior hard palate.
Palatine Bones: Posterior segment of hard palate; form floor of the nose.
Zygomatic Bones: Cheekbones.
Nasal Bones: Bridge of the nose.
Lacrimal Bones: Contain nasolacrimal canals.
The Hyoid Bone
The hyoid is unique in being the only bone that does not articulate with another bone.
It attaches to the larynx through membranes and to temporal bones via muscles and ligaments.
Serves to anchor the tongue and attach swallowing-associated muscles.
The Vertebral Column
Comprises thirty-three vertebrae and possesses four normal curvatures:
Abnormal curvatures include:
Scoliosis: Abnormal lateral curvature.
Kyphosis: Hunchback appearance.
Lordosis: Swayback posture.
The spinal cord traverses through the vertebral canal; spinal nerves exit via intervertebral foramina.
Spinous and transverse processes serve as muscle attachment sites.
Types of Vertebrae
Seven Cervical Vertebrae: Form the neck region (first two specialized as Atlas and Axis).
Atlas: Facilitates the "yes" motion.
Axis: Facilitates the "no" motion.
Twelve Thoracic Vertebrae: Feature long, thin spinous processes; articular facets for rib attachments.
Five Lumbar Vertebrae: Characterized by large bodies and thick processes.
Five Sacral Vertebrae: Fused to form the sacrum.
Coccyx: Comprised of four fused vertebrae, also referred to as the tailbone.
Vertebral Counting in Other Species
Cats and Dogs:
C7, T13, L7, S3, Cd20-ish.
Horses:
C7, T18, L6, S5, Cd20-ish.
Cows:
C7, T13, L6, S5, Cd20-ish.
Birds (e.g., Chickens):
C14, T7, LS14, Cd6; variations exists among different species.
Intervertebral Discs
Located between vertebrae, acting as padding to absorb shock and prevent grinding.
Composed of fibrocartilage.
Risks include rupture and herniation, which can exert pressure on spinal cord.
The Rib Cage
Also known as the thoracic cage; a component of the axial skeleton.
Composed of thoracic vertebrae, ribs, associated cartilages, and sternum.
Protective and flexible in functioning.
Safeguards the heart and lungs while allowing expansion during breathing.
The Ribs
Twelve Pairs of Ribs: Flattened bones that curve outward, then forward and downward.
“True Ribs”: Seven pairs connect directly with the sternum via costal cartilage.
“False Ribs”: Three pairs that joint to sternum through shared cartilage.
“Floating Ribs”: Last two pairs, not connected to the sternum.
General pattern for mammals dictates that the number of ribs generally matches that of thoracic vertebrae.
The Sternum
Also called the breastbone, in conjunction with the ribs, it safeguards the heart and lungs.
Formed from three bones fused during fetal development.
Manubrium: Articulates with clavicle and first rib pair.
Body: Junction point with the manubrium, serves as a landmark for rib counting.
Xiphoid Process: Provides attachment for the diaphragm.
The Appendicular Skeleton
Comprises the bones within the pectoral and pelvic girdles along with associated limbs.
Pectoral girdles and upper limbs are built for flexibility.
Pelvic girdle and lower limbs are structured for strength.
The Pectoral Girdles and Upper Limbs
Pectoral Girdle (shoulder girdle)
Clavicle: Spans across the top of the thorax, connecting with the sternum and acromion process of scapula.
Mammals may have a rudimentary, free-floating, or absent clavicle.
Species with adapted forelimbs exhibit variations in clavicle development, like the fused clavicle (wishbone) seen in birds which aids flight.
Scapula
Glenoid Cavity: Articulates with the humerus.
Rotator Cuff: Formed by encircling tendons around the shoulder joint.
Coracoid Process: Provides an attachment point for arm and chest muscles.
Upper Limb Structure
Humerus: The bone of the upper arm.
Radius and Ulna: Forearm bones.
Hand Composition:
Carpal Bones: Eight wrist bones.
Metacarpals: Five palm bones.
Phalanges: Bone structures in the fingers (digits).
The Pelvic Girdle and Lower Limbs
Pelvic Girdle
Composed of two coxal bones (hip bones), each formed from the fusion of three bones:
Ilium: The most significant among the three.
Ischium: Contains the ischial spine on its posterior side.
Pubis: Joins with the opposite pubis at the pubic symphysis.
The pelvis includes the pelvic girdle, sacrum, and coccyx, serving critical roles in protecting internal organs, supporting body weight, and functioning as attachment points for lower limbs.
Lower Limb Structure
Femur: The largest bone, whose head articulates with the acetabulum (the hip socket where the coxal bones converge).
Tibia: The primary weight-bearing bone of the lower leg.
Patella: Known as the kneecap.
Fibula: Slender bone along the lateral side of the tibia.
Tarsals: The ankle bones including talus and calcaneus (heel bone), which support body weight.
Metatarsals: Form the instep of the foot.
Phalanges: Compose the bones of the toes (digits).
Joints
Joints connect bones and are categorized as follows:
Fibrous Joints: Immovable, such as sutures between cranial bones.
Cartilaginous Joints: Slightly movable, connected by cartilage (e.g. ribs/sternum and intervertebral discs).
Synovial Joints: Freely movable and characterized by synovial fluid production.
Synovial Joints
Features include:
Ends of bones capped by articular cartilage.
Bones separated by a cavity.
Ligaments stabilize the joint and form a joint capsule.
Synovial membrane lines the capsule, producing lubricating synovial fluid.
Major Synovial Joints
Examples include the shoulder, elbow, hip, and knee; specific characteristics of the knee include:
Presence of menisci made of fibrocartilage, offering added stability and functioning as shock absorbers.
13 fluid-filled sacs (bursae) reduce friction between tendons and ligaments, which can lead to bursitis if inflamed.
Types of Synovial Joints
Hinge Joint: Permits movement in one direction only (e.g., knee).
Pivot Joint: Allows rotational movement (e.g., between radius and ulna).
Ball-and-Socket Joint: Permits movement in all planes (e.g., hip joint).
19.3 Human Skeletal Muscle
Three types of muscle tissue:
Smooth Muscle
Cardiac Muscle
Skeletal Muscle: Dominates muscle tissue composition, characterized as voluntary due to its contractions being consciously stimulated and controlled by the nervous system.
Skeletal Muscles Work in Pairs
Skeletal muscles are covered by connective tissue layers termed fascia.
Fascia extends beyond the muscle to form a tendon, connecting muscle to bone.
Upon muscle contraction, one bone remains stationary (origin), and the counterpart moves (insertion).
Antagonistic Muscles
Antagonistic muscle pairs function in opposite movement directions:
Flexion: Brings bones closer at a joint.
Extension: Moves bones further apart at a joint.
Abduction: Moves limbs away from the midline of the body.
Adduction: Brings limbs towards the midline.
Rotation: Moves parts around an axis (twist).
Naming of Skeletal Muscles
Characteristic bases for naming skeletal muscles include:
Size: Words like maximus indicate "largest"; for example, the gluteus maximus is the largest muscle in the buttocks.
Shape: Shapes reference (e.g., trapezius for trapezoid shape; deltoid is triangle-shaped).
Location: Includes names like frontalis (overlying frontal bone).
Direction of Muscle Fibers: Includes terms like rectus (straight) as in rectus abdominis.
Number of Attachments: Bi- in biceps brachii indicates two attachments.
Action: For instance, extensor digitorum indicates extension of fingers.
Major Skeletal Muscles
Approximately 650 skeletal muscles exist in the human body.
Head Muscles: Involved in facial expression and chewing.
Neck Muscles: Facilitate head movement.
Trunk and Upper Limb Muscles: Responsible for arm, forearm, and finger movement.
Buttocks and Lower Limb Muscles: Manage movement in thigh, leg, and toes.
Major Human Muscles: Anterior and Posterior Views
Includes tables and figures illustrating the major muscle groups and their respective actions for both anterior and posterior views.
Mechanism of Muscle Fiber Contraction
Skeletal muscle tissues exhibit a striated appearance due to alternating light and dark bands reflecting the arrangement of myofilaments.
Structure of a Muscle Fiber
Muscle Fiber:
Contains typical cellular components assigned special terms:
Sarcolemma: The muscle cell's plasma membrane.
Sarcoplasmic Reticulum: Functions as the muscle fiber's endoplasmic reticulum.
Sarcoplasm: The muscle fiber's cytoplasm.
Notable structures include T (Transverse) tubules that extend into the cell and contact sarcoplasmic reticulum portions.
Sarcoplasmic Reticulum:
Expanded sections store calcium, essential for muscle contraction, enclosing multiple myofibrils.
Mitochondria are situated in the sarcoplasm between myofibrils, alongside glycogen (energy source) and myoglobin (oxygen-binding pigment).
Components of a Muscle Fiber
Table 19.3 Components of a Muscle Fiber:
Interaction of structures like sarcolemma, sarcoplasm, glycogen, myoglobin, T tubule, sarcoplasmic reticulum, myofibril, and myofilament is necessary for muscle contraction and function.
Myofibrils and Sarcomeres
Myofibrils: Cylindrical structures that extend the length of muscle fibers; their striated appearance is due to the arrangement of myofilaments.
The Sarcomere units stretch from Z-line to Z-line and house two types of protein myofilaments:
Thin Filaments: Comprised of actin.
Thick Filaments: Composed of myosin.
Sarcomere Structure
I Band: Light band containing only actin filaments, attached to Z line.
A Band: Band that features overlapping actin and myosin filaments.
H Band: Contains only myosin filaments.
Myofilaments
Thick Filaments: Comprised of numerous myosin molecules.
Each molecule has a club-like shape with heads on both ends, except at the center of the sarcomere.
Thin Filaments: Composed mainly of two intertwined strands of actin, with tropomyosin and troponin as associated proteins.
Sliding Filament Model of Contraction
Muscle stimulation triggers impulses traveling down T tubules, releasing calcium from the sarcoplasmic reticulum.
Calcium’s role activates muscle fiber contraction, facilitating the sliding of actin past myosin filaments, leading to sarcomere shortening.
I Band shortens, H Band may diminish altogether; the sarcomere itself contracts as filaments come near.
Myosin filaments perform work by breaking down ATP and forming cross-bridges.
Skeletal Muscle Contraction Mechanisms
Neuromuscular Junction: Site where a motor neuron synapses with a muscle fiber. Axons contain synaptic vesicles filled with acetylcholine (ACh).
Upon a stimulus reaching the axon terminal, ACh releases into the synaptic cleft, diffuses across, and binds to receptors on the sarcolemma.
Generates impulses that excite T tubules and prompt calcium release from the sarcoplasmic reticulum, initiating contraction processes.
Molecular Mechanism of Contraction
Tropomyosin winds around actin filaments, blocking myosin-binding sites.
When calcium binds to troponin, tropomyosin shifts to expose these binding sites.
Myosin heads attach to these sites leading to:
ADP and inorganic phosphate release as a power stroke pulls actin fibers toward the center of the sarcomere.
New ATP binding detaches myosin heads and permits the cycle to begin anew.
Role of Calcium and ATP in Muscle Contraction
Muscle contraction relies heavily on calcium and ATP.
Energy for Muscle Contraction
ATP production occurs via several pathways:
Creatine Phosphate Breakdown: Quick source of ATP for intense activity, lasting approximately 8 seconds.
Cellular Respiration (Aerobic): Utilizes glucose and fatty acids in the presence of oxygen, providing sustained energy.
Fermentation (Anaerobic): Provides ATP when oxygen delivery fails, generating lactic acid and contributing to fatigue if prolonged.
Oxygen Debt
Oxygen debt occurs post-anaerobic activity where creatine phosphate or fermentation fulfill energy demands.
Repaying this debt entails:
Replenishing creatine phosphate reserves.
Eliminating lactate either through mitochondrial processing or conversion back to glycogen by the liver.
Whole Muscle Contraction: In the Laboratory
Isolated muscle fibers exhibit all-or-none law responses to stimuli, completely contracting.
Whole muscle contractions are recorded on a myogram, detailing varied contraction levels influenced by stimulation intensity.
Muscle Twitch Stages:
Latent Period: Time between stimulus and contraction onset.
Contraction Period: Active shortening of muscles.
Relaxation Period: Muscle returns to resting length.
Whole Muscle Adjustments
Whole muscle contractions can vary in strength based on stimulation; stronger stimuli activate more fibers.
The sustained contraction (tetanus) results from summation, where twitches fuse into a continuous contraction until fatigue arises.
Physiology of Skeletal Muscle Contraction
The myogram reflects stages of simple muscle twitch and demonstrates summation and tetanus phenomena when stimulated at higher intensities.
Athletics and Muscle Contraction
Disuse leads to muscle atrophy; muscle gains from forceful activity appear through hypertrophy, which involves a minimum of 75% maximum tension during contractions.
Types of Muscle Fibers
Slow-Twitch Fibers:
Aerobic, enduring, more mitochondria, colored dark by myoglobin.
Exhibit a steadier contractile force; suit activities like long-distance running.
Fast-Twitch Fibers:
Anaerobic, less durable, lighter due to fewer mitochondria.
Generate maximum strength in short bursts; suited for spikes in activity like sprinting or weightlifting.
Disorders of the Musculoskeletal System
Disorders of the Skeleton and Joints
Fractured Bones: Classification into complete vs. incomplete, simple vs. compound, impacted vs. spiral, and stress fractures.
Osteoporosis: Characterized by reduced bone mass, increased fracture risk, typically affecting women.
Arthritis: Includes osteoarthritis (degenerative) and rheumatoid arthritis (autoimmune).
Disorders of Muscles
Muscular Dystrophy (MD): A group of genetic disorders affecting muscles, most notably Duchenne MD, linked to dystrophin gene mutations.
Hyperkalemic Periodic Paralysis (HYPP): Genetic disorder in which abnormal sodium channels affect muscle contractions, notably in horses descended from the stallion "Impressive."