Introduction to Skeletal Muscle Anatomy

Introduction to Skeletal Muscle

• Skeletal muscles move the body by pulling on bones.
• They maintain posture by continuous contraction.
• They stabilize joints by controlling movement.
• They generate heat during contraction, essential for maintaining body temperature.

Characteristics of Skeletal Muscle Tissue

• Contractility: Muscle cells shorten and generate force.
• Excitability: Muscle cells respond to nerve signals, producing electrical impulses leading to contraction.
• Extensibility: Muscle tissue can be stretched by opposing muscle contraction.
• Elasticity: Muscle tissue recoils to its resting length after stretching.

Gross Anatomy of Skeletal Muscle

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Perimysium: Connective tissue surrounding fascicles, bundles of muscle fibers.

• Endomysium: Fine connective tissue sheath wrapping individual muscle fibers within a fascicle.

• Tendon: Dense, regular connective tissue attaching muscle to bone. Composed of tightly packed collagen fibers.

• Aponeurosis: Flat, sheet-like structure similar to tendon, providing broad muscle attachment to bones or fascia.

• Blood vessels: Supply muscle with oxygen and nutrients, remove waste.

• Nerves: Stimulate muscle contraction.

Microscopic Anatomy of Skeletal Muscle

• Sarcolemma: The plasma membrane of a skeletal muscle fiber.

• Sarcoplasmic Reticulum: Interconnected tubules surrounding each myofibril, storing and releasing calcium ions for contraction.

• T-tubules: Invaginations of the sarcolemma, extending into the muscle fiber, transmitting nerve impulses deep within the muscle.

• Thick Filaments: Composed of myosin molecules, with ATPase enzymes for energy release during contraction. Sporting myosin heads that bind to actin.

• Thin Filaments: Contain actin molecules and other proteins.

• Myofibrils: Organelles within muscle fibers containing myofilaments.

• Sarcomere: Segment of a myofibril, the basic functional unit of contraction.

• Z-disc: Boundary marking the ends of a sarcomere, where thin filaments attach.

• A-band: Region of thick filaments, along with the overlapping thin filaments. This dark band.

• H-zone: Center of the A-band, where only thick filaments exist, appearing lighter.

• M-line: Center of the H-zone, with tiny rods holding thick filaments together.

• I-band: Region of thin filaments only, extending from one A-band to the next, giving a lighter appearance.

Skeletal Muscle Fiber Types

• Muscle Fiber Types: Classified based on how fast (speed of contraction) and how long (duration of contraction) they contract:
  • Slow Oxidative (SO) Fibers: Slow contraction, resistant to fatigue, high myoglobin content, generate ATP aerobically.
  • Fast Oxidative-Glycolytic (FOG) Fibers: Fast, intermediate resistance to fatigue, high myoglobin and glycogen content, generate ATP aerobically and anaerobically.
  • Fast Glycolytic (FG) Fibers: Fast contraction, quickly fatigue, low myoglobin content, generate ATP anaerobically.

Criteria for Classification of Muscles - Fascicle Arrangement

• Circular: Fascicles arranged in concentric rings around external openings, like sphincters. (e.g., Orbicularis oris, Orbicularis oculi)
• Convergent: Fascicles converge towards a single insertion tendon, triangular or fan shaped. (e.g., Pectoralis major)
• Parallel: Fascicles run parallel to the long axis of the muscle, strap-like. (e.g., Sartorius).
• Fusiform: A modification of parallel, spindle-shaped with an expanded belly. (e.g., Biceps brachii)
• Pennate: Fascicles attach obliquely to a central tendon, feather-like.
  • Unipennate: Fascicles insert into one side of the tendon. (e.g., Extensor digitorum longus)
  • Bipennate: Fascicles insert into opposite sides of the tendon. (e.g., Rectus femoris)
  • Multipennate: Fascicles insert into the tendon from various sides. (e.g., Deltoid)

Classification of Muscles

• Location: Named after the bone or body region they are associated with.
• Shape: Named based on their shape. (e.g., Deltoid - triangle)
• Size:
  • Maximus: Largest
  • Minimus: Smallest
  • Longus: Long
• Direction of Fibers:
  • Rectus: Fibers run parallel to a midline or bone (straight).
  • Transversus: Fibers run perpendicular to the midline.
  • Oblique: Fibers run at angles to an imaginary defined axis.
• Number of Origins:
  • Biceps: Two origins.
  • Triceps: Three origins.
• Location of Attachments: Named for their origin or insertion. (e.g., Temporalis)
• Action: Named for their action, such as flexor, extensor, adductor.

Skeletal, Cardiac, and Smooth Muscle Tissue

• Similarities:
  • All muscle cells are elongated fibers.
  • Contractions are dependent on actin and myosin myofilaments.
  • Similar terminology.
• Differences:
  • Skeletal Muscle: Striated, voluntary, attaches to bone.
  • Cardiac Muscle: Striated, involuntary, found in the heart.
  • Smooth Muscle: Non-striated, involuntary, found in the walls of internal organs and blood vessels.

Functions of Skeletal Muscles

• Skeletal muscles move the body by moving bones
• They aid in maintaining posture and stabilization of joints
• Muscle contractions also generate heat, contributing to maintaining normal body temperature

Characteristics of Skeletal Muscle Tissue

• Contractility allows muscles to contract forcefully, shortening the cells and generating pulling force
• Excitability means muscle cells can be stimulated by nerve signals or other factors, triggering electrical impulses that lead to contraction
• Extensibility enables muscle tissue to stretch when opposing muscles contract
• Elasticity allows stretched muscle tissue to passively recoil to its resting length after being stretched

Gross Anatomy of Skeletal Muscle

• Epimysium, a dense irregular connective tissue layer, surrounds the entire skeletal muscle

• Perimysium is a fibrous connective tissue layer surrounding fascicles, groups of muscle fibers within a muscle

• Endomysium is a fine reticular fiber sheath that wraps around each individual muscle fiber within a fascicle

• Tendon is a dense regular connective tissue, resembling a rope, that transfers muscle force to bones

• Aponeurosis is a flat, sheet-like structure, similar to a tendon, attaching muscles broadly to bones or fascia

• Blood vessels and nerves supply muscles with oxygen, nutrients, and nerve impulses necessary for contraction

Microscopic Anatomy of Skeletal Muscle

• Sarcolemma is the specialized plasma membrane of a skeletal muscle cell

• Sarcoplasmic reticulum is a network of interconnected tubules surrounding myofibrils, storing calcium ions needed for muscle contraction

• T-tubules are extensions of the sarcolemma that penetrate the cell's interior, conducting nerve generated impulses

• Thick filaments are composed of bundled myosin molecules, containing ATPase enzymes that split ATP for muscle contraction

• Thin filaments are made of actin molecules and other proteins

• Myofibrils are organelles within muscle cells that contain myofilaments

• Sarcomeres are segments of myofibrils, representing the smallest contractile unit of a muscle

• Z discs mark the boundaries of each sarcomere, with thin filaments attached to them

• A band is the central region of the sarcomere, encompassing the full length of thick filaments and the inner ends of thin filaments

• H zone is the center of the A band, containing only thick filaments

• M line lies within the H zone, where tiny rods hold thick filaments together

• I band is the region at the two ends of the A band, where only thin filaments are present

Skeletal Muscle Fiber Types

• Skeletal muscle fibers are classified based on speed of contraction, metabolic pathways used to generate ATP, and resistance to fatigue

Criteria for Classification of Muscle – Fascicle Arrangement

• Circular muscle fascicles are arranged in concentric rings, typically found around body openings and called sphincters - Ex: orbicularis oris and orbicularis oculi
• Convergent muscle fascicles converge towards a single insertion tendon, resulting in a triangular or fan-shaped muscle - Ex: pectoralis major
• Parallel muscle fascicles run parallel to the muscle's long axis, producing a strap-like muscle - Ex: sartorius
• Fusiform muscles are spindle-shaped with an expanded belly, resembling a modification of parallel arrangement - Ex: biceps brachii
• Pennate muscles have "feather-like" arrangement with short fascicles attaching obliquely to a central tendon. They can be:
  • Unipennate: fascicles insert into one side of the tendon -Ex: extensor digitorum longus
  • Bipennate: fascicles insert into opposite sides of the tendon - Ex: rectus femoris
  • Multipennate: fascicles insert into the tendon from various sides - Ex: deltoid

Classification of Muscles

• Muscle names can be based on their:
  • Location: referencing the associated bone or body region
  • Shape: e.g., deltoid (triangle)
  • Size: maximus (largest), minimus (smallest), longus (long)
  • Direction of fibers: rectus (parallel), transversus (perpendicular), oblique (angled)
  • Number of origins: biceps (two origins), triceps (three origins)
  • Location of attachments: named based on origin or insertion - Ex: temporalis
  • Action: flexor, extensor, adductor, etc., reflecting the muscle's action

Skeletal, Cardiac and Smooth Muscle Tissue

• Similarities:*

• Skeletal and smooth muscle cells are elongated, referred to as muscle fibers

• Contraction in all muscle types depends on actin and myosin myofilaments

• Similar terms are used across muscle types, with sarcolemma referring to muscle plasma membrane and sarcoplasm to muscle cell cytoplasm

• Prefixes "myo," "mys," and "sarco" are related to muscles

• Differences:*

• Skeletal Muscle:

  • Voluntary control
  • Striated appearance due to organized arrangement of myofilaments
  • Found attached to bones

• Cardiac Muscle:

  • Involuntary control
  • Striated appearance
  • Found only in the heart, forming the walls of the heart chambers

• Smooth Muscle:

  • Involuntary control
  • Non-striated appearance
  • Found in walls of hollow organs like the stomach, bladder, and blood vessels

Introduction to Skeletal Muscle

• Skeletal muscles are attached to the skeleton and move the body by moving bones.
• Skeletal muscles contract continuously to maintain posture, enabling the body to remain standing or seated.
• Skeletal muscles stabilize joints.
• Muscle contractions produce heat that helps maintain normal body temperature.

Characteristics of Skeletal Muscle Tissue

• Muscle cells shorten and generate a strong pulling force as they contract.
• Nerve signals or other factors excite muscle cells, causing electrical impulses to travel along the cells’ plasma membrane.
• Muscle tissue can be stretched by the contraction of an opposing muscle.
• After being stretched, muscle tissue can recoil passively and resume its resting length.

Anatomy of Skeletal Muscle

Gross Anatomy of Skeletal Muscle

• Epimysium is an overcoat of dense, irregular connective tissue surrounding the whole skeletal muscle.
• Perimysium is a layer of fibrous connective tissue surrounding each fascicle, a bundle of muscle fibers within a skeletal muscle.
• Endomysium is a fine sheath of connective tissue consisting mostly of reticular fibers surrounding each muscle fiber within a fascicle.
• A tendon is a dense, regular connective tissue structure that attaches muscle to bone, composed of parallel collagen fibers.
• An aponeurosis is a flat, sheet-like structure composed of dense fibrous connective tissue, acting as a broad attachment point for muscles to bones or fascia.
• Blood vessels and nerves serve muscle by supplying oxygen, nutrients, and enabling contraction through nerve impulses.

Microscopic Anatomy of Skeletal Muscle

• Sarcolemma is the specialized plasma membrane of a skeletal muscle cell.
• Sarcoplasmic reticulum is an interconnected network of tubules that surround each myofibril.
• T-tubules are continuous with the sarcolemma, penetrating the cell's interior at each A-band and I-band junction.
• Thin filaments contain actin molecules, while thick filaments contain bundled myosin molecules.
• Myosin heads are knobs on thick filaments that split ATP to release energy for muscle contraction.
• Myofibrils are organelles of the muscle cell that contain myofilaments.
• A sarcomere is a segment of a myofibril, the smallest contractile unit of the muscle.
• Z discs are the boundaries at the ends of each sarcomere.
• Thin (actin) filaments attach to each Z disc extending toward the center of the sarcomere.
• The central region of the sarcomere is called the A band, containing the entire length of thick filaments and the inner ends of thin filaments.
• The center of the A band, where no thin filaments reach, is called the H zone.
• The M line is the center of the H zone where tiny rods hold the thick filaments together perpendicular to the filaments.
• The I band is the region at the two ends of the A band, where there are no thick filaments.

Skeletal Muscle Fiber Types

Criteria for Classification of Muscle Fiber Types:

• Thin and Thick Filaments: Muscle fibers contain either thin (actin) or thick (myosin) filaments.
• I and A Bands: Light (I band) and dark (A band) regions appear on muscle fibers, reflecting the arrangement of thin and thick filaments.
• Sarcomere: The area between two consecutive light bands is called a sarcomere, the smallest contractile unit of the muscle.

Criteria for Classification of Muscle – Fascicle Arrangement

• Circular: Fascicles are arranged in concentric rings, typically surrounding external body openings, like the orbicularis oris and orbicularis oculi muscles.
• Convergent: Fascicles converge towards a single insertion tendon, usually triangular or fan-shaped, like the pectoralis major muscle.
• Parallel: Fascicles run parallel to the long axis of the muscle, resulting in a strap-like structure, like the sartorius muscle.
• Fusiform: Modified parallel arrangement with a spindle-shaped muscle with an expanded belly, like the biceps brachii muscle.
• Pennate: Fascicles attach obliquely to a central tendon, resembling feathers.
  • Unipennate: Fascicles insert into only one side of the tendon, like the extensor digitorum longus muscle.
  • Bipennate: Fascicles insert into opposite sides of the tendon, like the rectus femoris muscle.
  • Multipennate: Fascicles insert into tendon from several different sides, like the deltoid muscle.

Classification of Muscles:

• Location: Refers to the bone or body region associated with the muscle.
• Shape: Describes the muscle's shape like "deltoid" for a triangle.
• Size: Terms like "maximus" for largest, "minimus" for smallest, and "longus" for long describe muscle size.
• Direction of Fibers:
  • "Rectus" indicates fibers running parallel to an imaginary line.
  • "Transversus" means fibers run perpendicular to the midline.
  • "Oblique" indicates fibers running at angles to an imaginary axis.
• Number of Origins: "Biceps" for two origins, "triceps" for three origins.
• Location of Attachments: Named according to origin or insertion, like "Temporalis".
• Action: Describes the muscle's action, such as "flexor" or "extensor".

Skeletal, Cardiac and Smooth Muscle Tissue:

• Similarities:
  • Skeletal and smooth muscle cells are elongated and called muscle fibers.
  • Muscle contraction depends on actin and myosin myofilaments.
  • Similar terminology like sarcolemma (muscle plasma membrane) and sarcoplasm (cytoplasm of a muscle cell).