Muscle System and Muscle Tissue
Muscles
Muscles represent approximately 50% of body weight.
Study of muscles is called Myology.
Important in physical therapy and intramuscular injection practices.
There are three types of muscle: Skeletal, Cardiac, and Smooth.
Muscles convert energy from ATP to mechanical movement.
Functions of Muscles
Movement:
Move body parts and contents.
Involved in breathing, circulation, digestion, and communication (such as speech and facial expressions).
Stability:
Maintain posture and stabilize joints.
Control of Body Openings and Passages:
Control food intake, waste elimination, and movement of materials.
Control admission of light to the eye.
Heat Production (Thermogenesis):
Skeletal muscles produce 20% to 30% of body heat at rest; up to 85% during exercise.
Glycemic Control:
Skeletal muscles absorb, store, and utilize a significant portion of the body’s glucose, stabilizing blood glucose levels.
Characteristics of Muscle Tissue
Excitability (Responsiveness): Ability to respond to chemical signals, stretch, and electrical changes across the plasma membrane.
Conductivity: Local electrical excitation propagates along the muscle fiber.
Contractility: Muscle fibers shorten when stimulated.
Extensibility: Capacity to stretch between contractions.
Elasticity: Can return to original length after being stretched.
Trivia
Longest Muscle: Examples: Sartorius at ~60 cm (longest).
Strongest Muscle by Weight: Stapedius muscle in the ear.
Types of Muscle Tissue
Smooth Muscle
Lacks striations; appears smooth.
Found in the walls of hollow organs (e.g., blood vessels, visceral organs).
Functions:
Push contents through body cavities via peristalsis.
Regulates pressure and flow of blood and air.
Involuntary control.
Cardiac Muscle
Located in heart walls.
Striated appearance.
Involuntary contraction; can contract without nerve stimulus.
Utilizes intercalated discs featuring gap junctions and desmosomes for synchronized contraction.
Pacemaker cells initiated by the sinoatrial (SA) node; nervous system influences heart rate.
Skeletal Muscle
Muscle fiber (myofiber) defined as skeletal muscle cell.
Attaches to and surrounds the skeleton; appears striated and is primarily voluntary, though can also function involuntarily.
Contributes to overall body mobility; highly adaptable with a rich blood supply.
Human body possesses approximately 600 skeletal muscles.
Skeletal Muscle Connective Tissues
Epimysium: Surrounds the entire muscle.
Perimysium: Bundles muscle fibers into fascicles; houses larger nerves and vessels; contains stretch receptors.
Endomysium: Surrounds each muscle fiber, providing space for innervation and nourishment.
Connective Tissue Structural Relationships
Fascia: Connective tissue bands between muscles/groups that help in compartmentalization.
Functional Examination Factors:
Adipose tissue presence, muscle tone with aging, use/disuse evident through comparisons (e.g., triathletes vs sedentary individuals).
Fascicle Orientation & Muscle Classification
Orientation affects muscle strength and direction of pull.
Fusiform Muscles: Thick in the middle with tapering ends.
Parallel Muscles: Uniform width with parallel fascicles.
Triangular (Convergent) Muscles: Broad origin converging on a narrow insertion.
Types of Pennate Muscles
Pennate Muscles: Feather-shaped; fascicles attach at an angle to a central tendon.
Unipennate: Fascicles attach from one side.
Bipennate: Fascicles attach from both sides.
Multipennate: Several fascicles come to a single tendon.
Circular Muscles (Sphincters): Form rings around openings or passages of the body.
Muscle Attachments
Distinctions between origin and insertion: refers to stationary and moving ends of muscles.
Direct Attachment: Muscle fibers fusing directly to periosteum.
Indirect Attachment: Muscle fibers attach to tendons connecting to the periosteum and bone matrix.
Aponeurosis: Broad flat sheet of tendon anchors muscles.
Retinaculum: Connective band where tendons pass beneath.
Functional Groups of Muscles
Muscles may be categorized as intrinsic (contained within a region) or extrinsic (originate from outside but act within a region).
Muscle Actions: Categorized into four functional groups.
Prime Mover: Primary muscle executing movement; e.g., brachialis for elbow flexion.
Synergist: Assists the prime mover; e.g., biceps brachii with brachialis.
Antagonist: Opposes the prime mover; e.g., triceps brachii for elbow extension.
Fixator: Stabilizes joints; e.g., rhomboid muscles stabilize the scapula during bicep contraction.
Muscle Movements
Joint movements include:
Extension: Increasing the angle between body parts.
Flexion: Decreasing the angle between body parts.
Supination: Turning upwards; palms facing up.
Pronation: Turning downwards; palms facing down.
Plantar Flexion: Foot pointing downwards.
Dorsiflexion: Foot lifting upwards.
Abduction: Movement away from the midline.
Adduction: Movement towards the midline.
Medial Rotation: Turning towards the midline.
Lateral Rotation: Turning away from the midline.
Innervation and Blood Supply
Muscle contraction relies on nerve stimulation.
Diagnosis of muscle function considers nerve integrity, including spinal cord and brainstem injuries.
Cranial Nerves: Connect from the base of the brain through foramina to innervate head and neck musculature (CN 1 - CN 12).
Spinal Nerves: Extend from the spinal cord through intervertebral foramina to innervate muscles below the neck.
Capillaries: Extensive branching through the endomysium allows delivery of oxygen and nutrients to muscle fibers.
Blood Supply and Cardiac Output: Increases from 25% (1.24 L/min) at rest to 75% (11.6 L/min) during vigorous exercise.
Muscle Fiber Structure
Composed of large cells containing multiple nuclei (30 to 80 per mm), aiding in fiber repair.
Contains calcium-regulated molecular motors and storage components like glycogen and glycosomes.
Myoglobin: Red pigment structurally similar to a hemoglobin subunit with one heme.
Specialized Structures
Sarcoplasm: The cytoplasm of muscle fibers containing organelles such as myofibrils.
Transverse (T) Tubules: Extensions of the sarcolemma, facilitating the transmission of electrical signals into muscle fibers.
Sarcoplasmic Reticulum: Smooth endoplasmic reticulum network serving as a calcium reservoir.
Myofibrils and Myofilaments
Myofibrils: Long protein cords occupying most of the muscle fiber, composed of repeating units called sarcomeres.
Myofilaments: Comprise thick (myosin) and thin (actin) filaments essential for muscle contraction.
Thick Filaments: Composed of myosin, with 300 peptides per filament.
Thin Filaments: Made of actin, with tropomyosin blocking binding sites and troponin as a calcium-binding protein.
Contraction Mechanics
During contraction, actin filaments slide past myosin filaments, shortening the sarcomere—concepts central to the cross-bridge cycle involving attachment, pivoting, detachment, and reattachment of myosin heads to actin filaments, generating muscle force.
Muscles, Properties, and Mechanisms
Sarcomere and Cross Bridge Cycle
Sarcomeres are the basic units of muscle fibers.
The cycle of cross-bridge formation and detachment is crucial for muscle contraction.
Neuronal Stimulation
Voluntary Action: Muscles are controlled by neuronal stimulation involving decision-making processes.
Motor Unit: Defined as one nerve fiber and all its innervated muscle fibers.
Typically contains approximately 200 muscle fibers on average.
Muscle fibers are dispersed throughout the muscle, leading to weak contractions over a wide area.
Effective muscle contraction requires multiple motor units.
Small Motor Units: Provide fine control; can have as few as 3-5 muscle fibers.
Large Motor Units: Offer more strength than control, involving up to 1000 muscle fibers.
Excitable Cells: Muscle fibers function as excitable cells that respond to stimuli and change membrane potential (charge).
Develop action potentials when a threshold is reached.
Phases of Contraction and Relaxation
The process of muscle contraction and relaxation consists of four major phases:
Excitation: Action potentials generated in the motor nerve fiber cause action potentials in the muscle fiber.
Excitation–Contraction Coupling: Links the action potentials on the sarcolemma to the activation of myofilaments, setting the stage for contraction.
Contraction: The muscle fiber generates tension and may shorten as a result of the contraction process.
Relaxation: As stimulation ends, the muscle fiber relaxes, returning to its resting length.
Excitation Process
At the neuromuscular junction (NMJ):
Nerve signals arrive at the axon terminal, triggering the release of the neurotransmitter Acetylcholine (ACh).
ACh diffuses across the synaptic cleft and binds to receptors in the NMJ of the sarcolemma.
This process opens ligand-gated Na+ channels, resulting in an influx of Na+ and local depolarization of the membrane (becoming more positive).
Acetylcholinesterase is responsible for removing ACh from the synaptic cleft after its action.
Action Potential Generation
The binding of ACh results in the opening of nearby voltage-gated Na+ and K+ channels, generating an action potential (AP) that spreads away from the junction and is carried inward by T-tubules.
Excitation–Contraction Coupling Details
The activation of the T-tubule membrane by AP triggers the release of Ca2+ from the sarcoplasmic reticulum.
Ca2+ binds to troponin, causing it to twist tropomyosin and uncover myosin binding sites on actin filaments.
Process Flow:
T-tubules propagate action potentials leading to continuous Ca2+ release from terminal cisternae of the sarcoplasmic reticulum, exposing actin binding sites and facilitating cross-bridge formation in muscle contraction.
Contraction and the Cross Bridge Cycle
ATP Binding: Myosin heads have ATP bound when the muscle is in a relaxed state.
When ATP is hydrolyzed, it breaks down into ADP and inorganic phosphate (P), activating the myosin head conformation.
Cross Bridge Formation: Myosin heads bind to active sites on actin, and during the power stroke, ADP + P are released causing the actin to slide past the myosin.
Muscle Relaxation
The process of muscle relaxation occurs in several steps:
Nervous stimulation ceases, leading to a cessation of ACh release.
Acetylcholinesterase (AChE) removes ACh from the synaptic cleft.
Ca2+ ions are transported back into the sarcoplasmic reticulum.
Ca2+ unbinds from troponin, causing tropomyosin to twist back and block the myosin binding sites.
The muscle fiber relaxes, returning to its resting length.
Skeletal Muscle Properties
Measuring Muscle Tension
A myogram measures muscle twitch, which is the contraction-relaxation cycle resulting from electrical stimulation.
Phases of a Muscle Twitch:
Latent Phase: Delay that occurs during excitation-contraction coupling and tension formation.
Contraction Phase: The muscle generates external tension.
Relaxation Phase: Calcium is reabsorbed, leading to a decline in tension.
Twitch vs. Graded Contraction:
A twitch refers to a single activation, while graded contraction represents a spectrum of muscle activity.
Graded Contraction
Achieved by increasing stimulus frequency leading to temporal summation, resulting in sustained levels of Ca2+.
Increased Stimulus Strength:
Recruitment: More motor units are activated.
Size Principle: Increasing voltage excites more nerve fibers, recruiting both small and large motor units synchronously.
Factors Affecting Muscle Contraction Strength
Force of Contraction Factors:
Number of cross bridges formed.
Frequency of stimulation.
Number of fibers recruited.
Size of fibers and their cross-sectional area (larger fibers deliver more power).
Degree of muscle stretch affects the degree of overlap in sarcomeres.
Types of Contractions
Isotonic Contractions: Muscles shorten or lengthen while maintaining force.
Concentric: Muscle shortens (e.g. lifting weights).
Eccentric: Muscle lengthens (e.g. lowering weights).
Isometric Contractions: Muscles develop tension without a change in length.
Tension equals resistance.
Summary of Contraction Types
Represents the interplay of active cross-bridge cycling versus relaxation in differing muscle movements (eccentric, concentric, isometric).
ATP Production for Muscle Contraction
Energy Sources:
Very little ATP is stored in muscle cells, requiring continuous production.
Three Pathways of ATP Production:
Phosphorylation of ADP: Catalyzed by creatine phosphate (CP) which donates a phosphate group to ADP, regenerating ATP.
Immediate source for ~6 seconds of high-intensity activity.
Anaerobic Pathway: Involves the catabolism of glucose through glycolysis, leading to the generation of 2 ATP and pyruvic acid. When oxygen delivery is impaired, pyruvic acid converts to lactic acid.
Aerobic Pathway: Provides 95% of ATP through oxidation in the presence of oxygen, producing CO2 and H2O as byproducts with a yield of 32 ATP per glucose molecule.
Factors Affecting Performance
EPOC (Excess Post-exercise Oxygen Consumption): Represents the elevated oxygen uptake following intense exercise for the recovery of muscle cells.
Muscle Fiber Types:
Slow twitch (Type I): High endurance, resist fatigue, rich in capillaries.
Fast twitch oxidative (Type IIa): Moderate sprinting, thicker fibers, red color.
Fast twitch glycolytic (Type IIx): Depend on glycogen, powerful bursts of activity, fewer capillaries and lighter color.
Effects of Exercise on Fiber Types
Regular resistance training converts fast oxidative fibers to a fast glycolytic phenotype.
Regular endurance training promotes changes from fast glycolytic fibers to fast oxidative fibers.