Musculoskeletal System: Structure and Function

Introduction

• Musculoskeletal system provides body with form, stability, and movement
• Two main components comprise the musculoskeletal system: the skeletal and muscular systems
• Bones and joints structurally support and protect
• Skeletal muscles are responsible for voluntary movements
• Tendons, ligaments, cartilage, and other connective tissues support and connect the skeletal and muscular systems

Learning Objectives

• Define musculoskeletal system and its primary functions
• Identify the major components of the musculoskeletal system
• Describe the structure and function of bones, muscles, joints, and connective tissues
• Understand naming of structures in the musculoskeletal system
• Understand interplay between bones and muscles in movement
• Identify the muscles of the upper and lower limbs
• Understand development of the limbs

Bones

• Skeletal system consists of approximately 206 bones
• Axial skeleton includes bones of the skull, ossicles of the middle ear, hyoid bone, vertebral column, and thoracic cage
• Appendicular skeleton includes bones of the upper and lower limbs
• Cancellous bone is denser, smooth, and white on the outer layer
• Spongy bone is porous, highly vascularized inner layer

Bone Types

• Long bones include the femur and humerus which are involved in movement
• Short bones include carpals and tarsals; provide stability and support
• Flat bones include the skull and ribs; protect vital organs and provide muscle attachment points
• Irregular bones include vertebrae that have specialized functions
• Sesamoid bones include the patella which reduces friction and modifies pressure

Joints

• Classified based on movement capability Fibrous Joints (Immovable)
• Examples of fibrous joints include sutures of the skull
• Cartilaginous Joints are Partially Movable
• An example of Cartilaginous Joints include intervertebral discs.
• Synovial Joints are Freely Movable
• Examples of Synovial Joints include the knee and shoulder
• Types of Synovial Joints
• Hinge joints allow movement in one direction, example elbow and knee
• Ball and socket joints facilitate multidirectional movement, example shoulder and hip
• Pivot joints allow rotational movement, example atlanto-axial joint
• Gliding/plane joints permit sliding movement, example intercarpal joint
• Saddle joints allow greater movement than hinge joints, example thumb joint
• Condyloid joints include the metacarpophalangeal joint

Connective Tissues

• Tendons attach muscles to bones; for example, the Achilles tendon
• Ligaments connect bones to other bones at joints; for example, the ACL in the knee
• Cartilage provides cushioning at joints; for example, the meniscus in the knee

Muscles

• Responsible for movement, posture, and heat production
• Three types: skeletal, smooth, and cardiac
• Skeletal Muscle is voluntary and attached to bones via tendons, enabling movement
• Smooth Muscle is involuntary and found in internal organs
• Cardiac Muscle is specialized and found in the heart

Smooth Muscles

• Found in walls of hollow organs such as intestines, uterus, stomach, blood vessels, and urinary and repro systems
• Smooth muscles control involuntary body functions
• They surround arteries to adjust blood flow
• Found in intestines where they contract to move food and feces
• Visceral nervous system controls it, influenced by neurotransmitters such as norepinephrine and acetylcholine
• Controlled by hormones like estrogen and oxytocin and tissue hormones like prostaglandins and histamine

Cardiac Muscle

• Cardiac muscle forms the heart, not part of musculoskeletal system
• Similar to skeletal muscle, has a regular pattern of fibers, appears striped under microscope
• Contracts and relaxes rhythmically without conscious awareness
• Muscle fibers joined by intercalated discs, have gap junctions

Skeletal Muscles

• Voluntary in action, responsible for posture and movement
• Attached to bones, arranged in opposing groups around joints
• Grouped as muscles of the head and neck, trunk, upper limbs, and lower limbs

Skeletal Muscle Structure

• These tissues contain muscle fibers, blood vessels, nerve fibers, and connective tissue
• Ensheathed by the epimysium of dense, irregular connective tissue that wraps muscle fibers that allow muscle to contract powerfully while maintaining structural integrity
• Bound together into muscle fascicles by the perimysium
• Fascicles contain 10-100+ muscle fibers.
• Enclosed in each fascicle is fibers within a thin connective tissue layer of collagen and reticular fibers, known as the endomysium

Naming of Naming of Skeletal Muscles

• By direction of fibers, example rectus (straight) or oblique (at a slant) to an imaginary line
• By size, example maximus (largest), minimus (smallest), and longus (long)
• By location, example frontalis (frontal) and temporalis (temporal)
• By number of origins, example biceps (two), triceps (three), or quadriceps (four) heads
• By origin and insertion, example sternocleidomastoid connects the sternum, clavicle, and mastoid process
• By shape, some have shapes that help in their identification

Skeletal Muscle Interactions

• Muscles arranged so one muscle can reverse the action of another, allowing varied movements around muscles
• Prime movers have major responsibility for causing particular movement
• Example triceps causes elbow extension
• Antagonists oppose or reverse a movement
• When a prime mover is active, antagonist is stretched and relaxed
• Example hamstrings is related to relation of the quadriceps femoris
• Synergists help prime movers by producing same movement, reduce undesirable movements
• Example anconeus for elbow extension
• Fixators are specialized synergists
• They hold a bone still or stabilize origin of prime mover so all tension can move insertion bone

Skeletal Muscle Abnormalities

• Sarcopenia involves the gradual loss of skeletal muscle mass
• Loss can be related to aging or other disease conditions
• Muscular dystrophy is a group of diseases causing progressive degeneration of skeletal muscle fibers
• Muscular dystrophy is inherited from an abnormal gene
• Myasthenia gravis (MG) is an autoimmune disease that impedes communication between muscles and nerves and leads to severe muscle weakness and fatigue.

Limb development

• Upper and lower limbs begin development in the 4th week of gestation
• Upper limb development begins first, followed by lower limb development about 2-3 days later
• Stages include initiation, specification of limb pattern, tissue formation/limb morphogenesis, and growth
• The limb will form buds in limb bud initiation
• Specification of limb pattern and, growth of the 3 axis
• In tissue formation and limb elements form and limb morphogenesis
• Growth occurs into early adulthood

Limb Bud Formation

• Limb buds first appear in the 4th week as small elevations on the ventrolateral body wall.
• Initially, limb bud comprised of homogeneous mesenchymal cells and a thin ectodermal covering.
• Distal tip of the ectoderm condenses into the Apical Ectodermal Ridge (AER)
• The AER marks the doral-ventral boundary of the limb
• Three steps involved in AER formation: induction of precursors, migration of precursors, and compaction of ridge.

Mesenchyme

• Mesenchyme directly below ridge is called the PROGRESS ZONE (PZ)
• PZ is the site of cell division/elongation of the limb
• Progress zone (PZ) is characterized as unspecified, actively dividing mesenchymal cells
• The AER has an inductive influence on the PZ and is essential for the elongation process of the limbs
• The limb buds elongate by proliferation of the mesenchyme, which condenses to form cartilage/bone and connective tissue of the dermis.

Digit Formation

• Early development of hands and feet appear like paddles
• Digits (fingers and toes) are initially "webbed"
• Cells in the webbing die via programmed cell death to form separate digits - interdigital apoptosis

Limb Rotation

• By end of the 6th week, limbs extend and lie in the coronal plane.
• Then, undergo rotations to reach final positions.
• The upper limb rotates dorsally and laterally
• The thumb moves laterally
• Flexors are anterior and extensors are posterior
• Elbow flexors now are located anteriorly and the elbow flexes to the anterior.
• The lower limbs rotate ventrally and medially
• The big toe moves medially
• Extensors have moved anteriorly and flexors are posterior
• Knee flexors located posteriorly, so the knee flexes to the posterior

Summary

• The musculoskeletal system consists of bones and muscles
• It is critical for giving stability and protection to human bodies
• Muscle actions give rise to movements in various directions
• Bones and muscles can be of different shapes and sizes