function of muscles

Functions of Muscles

Myology: The scientific study of muscles.

Functions of Muscles

Producing Body Movements: Muscles are responsible for all types of movements from gross body motions to fine motor skills.
Stabilizing Body Positions: Muscles stabilize joints and help maintain body postures such as standing and sitting.
Storing and Moving Substances:
- Storage: Circular muscles, known as sphincters, prevent the outflow of contents from hollow organs, such as the stomach or urinary bladder.
- Flow of Substances:
- Cardiac Muscles: Pump blood throughout the body.
- Muscles within Blood Vessels: Regulate blood flow rate.
- GI Tract Muscles: Wavelike contractions help move food, urine, and lymph through their respective systems.
Generate Heat: Known as Thermogenesis, muscular tissue produces heat through contractions; shivering is a critical component used in thermoregulation.

Classification of Muscle Tissue (MT)

Skeletal Muscle

Voluntary Control: Conscious control over contraction.
Appearance: Fibers are striated (thin, striped appearance alternating light/dark).
Nucleus Count: A single fiber can have many nuclei (multinucleated).
Location: Primarily attached to bones.
Speed of Contraction: Very fast contraction due to nerve impulse stimulation.

Cardiac Muscle

Involuntary Control: No conscious control over contraction.
Appearance: Striated fibers that are more branched than skeletal muscle.
Nucleus Count: Usually has one nucleus per cell (possibly two).
Intercalated Disks: Transverse thickenings of the sarcolemma.
Location: Found only in the heart.
Contraction: Rhythmic due to autorhythmicity from specialized nodes in the heart.

Smooth Muscle

Involuntary Control: No conscious control over contraction.
Appearance: No striations (hence called 'smooth').
Nucleus Count: Each cell has one nucleus.
Cell Shape: Cells are not elongated and taper at the ends.
Speed of Contraction: Slow contraction controlled by the autonomic nervous system.

Properties of Muscular Tissue

Electrical Excitability: Ability to respond to stimuli by producing electrical signals called action potentials (impulses).
- Types of Stimuli:
- Autorhythmic Signals: Originating from the muscle tissue itself (e.g. cardiac muscle).
- Chemical Signals: Include hormones, neurotransmitters, or changes in pH.
Contractility: Ability of muscle tissue to shorten and contract forcefully.
Extensibility: Ability to stretch, within limits, without damage (smooth muscle can stretch the most).
Elasticity: Ability to return to its original length and shape after contraction or extension.

Skeletal Muscle Tissue

Overall Structure: Each skeletal muscle is a separate organ consisting of hundreds to thousands of muscle fibers (cells).
Support Needs: Skeletal muscle cannot function without adjacent connective tissue, nerves, and blood vessels.

Connective Tissue Wrappings

Layers of Tissue

Subcutaneous Layer:
- Just beneath the skin, composed of adipose and areolar connective tissue.
- Functions: Connects skin to muscle, stores triglycerides, insulates against heat loss, protects from physical trauma, and serves as a pathway for nerves and blood vessels.
Fascia: Dense sheet of irregular connective tissue surrounding muscles.
- Functions:
- Helps muscles to function cohesively as a unit.
- Allows free movement by providing a slick surface.
- Carries blood vessels, nerves, and lymphatic vessels.
- Fills spaces between muscles and attaches muscle to bone.

Additional Connective Tissue Layers

Epimysium: Outermost layer of dense irregular connective tissue encircling an entire muscle.
Perimysium: Middle layer of dense irregular connective tissue surrounding groups of 10–100 muscle fibers, forming bundles called fascicles.
Endomysium: Reticular fibers that surround each individual muscle fiber within a fascicle.

Muscle Attachment

The connective tissues (epimysium, perimysium, endomysium) extend beyond the muscle to serve as attachment points to other structures, forming tendons and aponeuroses.
- Tendon: Ropelike structures that connect muscle to bone.
- Aponeurosis: Sheet-like structure that connects muscle to another muscle or bone.

Organization of Muscle Tissue

Aponeurosis: A specific type of fascia that links two muscle bellies, exemplified by the epicranial aponeurosis, which connects the occipitalis and frontalis muscles to function as one muscle (the occipitofrontalis).

Microscopic Anatomy

Muscle Fibers

Muscle cells are termed “fibers” due to their elongated and narrowed shape.
Development: Muscle fibers arise from myoblasts, which are fused bundles of mesodermal cells, leading to multinucleated skeletal muscle fibers.
Division Capability: Skeletal muscles do not divide post-development; the number of muscle cells is set at birth.

Structure of Muscle Fiber

Sarcolemma: Plasma membrane of a muscle fiber.
Transverse Tubules (T tubules): Inward extensions of the sarcolemma filled with interstitial fluid that facilitate action potential travel through the muscle fiber.
Sarcoplasm: Cytoplasm of muscle cells containing glycogen (for ATP production) and myoglobin (oxygen-storing reddish pigment).
Mitochondria: Arranged in rows throughout muscle fibers, close to contractile fibers.
Myofibrils: Contractile organelles running the length of the fiber.
Sarcoplasmic Reticulum: Modified smooth endoplasmic reticulum surrounding myofibrils, storing calcium.
Terminal Cisterns: Enlarged ends of the sarcoplasmic reticulum linked to T tubules, releasing calcium ions crucial for contraction.

Structure of Myofibrils

Myofilaments

Myofilaments: Proteins within myofibrils responsible for contraction; they are not as long as myofibrils but are arranged in units called sarcomeres.
Thin Filaments: Mainly composed of actin, along with proteins tropomyosin and troponin.
Thick Filaments: Primarily made of myosin, functioning as a motor protein across all muscle types.

Details of Filament Structure

Thin Filament: Composed of Actin, Troponin, Tropomyosin, Z disc structure for binding. The myosin binding site on actin is initially covered by tropomyosin.
Thick Filament: Composed mainly of myosin.
Sarcomere: Functional contractile unit stretching from Z disc to Z disc, includes the following zones:
- A Band: Dark band with the total length of myosin.
- H Zone: Lighter area of the A band indicating distance between thin filaments.
- M Line: Center of the H zone.
- I Band: Light band indicating space between myosin containing thin filaments.
- Z Line: Midline of the I band.

Proteins within Myofibrils

Contractile Proteins

Contractile Proteins: Generate force; includes myosin (thick filament) and actin (thin filament).

Regulatory Proteins

Regulatory Proteins: Control the contraction process.
- Tropomyosin: Attached to actin, covers myosin-binding sites.
- Troponin: Holds tropomyosin in place; in the presence of Ca²⁺, it changes shape, moving tropomyosin and exposing binding sites.

Structural Proteins

Structural Proteins: Stabilize and maintain proper alignment of contractile proteins.
- Titin: Connects Z disc to M line; can stretch up to four times its resting length.
- Myomesin: Forms the M line.
- Nebulin: Wraps around thin filaments, anchoring them to the Z disc.
- Dystrophin: Links thin filaments to the sarcolemma, reinforcing it and helping transmit tension to tendons.

Muscle Contraction

Muscle Contraction: Shortening of sarcomeres, causing overall muscle shortening.
Sliding Filament Mechanism: Myosin heads attach to actin, pulling thin filaments toward the M line while they slide past thick filaments, without either filament actually shortening.

Contraction Cycle Steps

ATP Hydrolysis: ATP binds to myosin, followed by hydrolysis which releases energy.
Attachment: Myosin forms cross-bridges with actin.
Power Stroke: Release of ADP generates force, pulling cross-bridges toward the center of the sarcomere.
Detachment: Myosin heads remain attached until another ATP binds, allowing detachment from actin.

Repetition: The contraction cycle continues as long as ATP is available and Ca²⁺ levels remain high, facilitating the exposure of actin binding sites through troponin.

Rigor Mortis

Rigor Mortis: Post-mortem condition where muscles become rigid. Occurs as calcium leaks from the sarcoplasmic reticulum, allowing myosin to bind to actin without ATP for detachment, resulting in stiff muscles.

Neuromuscular Junction

Skeletal Muscle Control: Stimulated by nerve impulses.
Somatic Motor Neuron: Carries impulses from brain or spinal cord to muscle.
Neuromuscular Junction (NMJ):
- Components: Includes the synaptic cleft (gap between nerve and muscle), axon terminal (contains synaptic end bulbs with neurotransmitters), motor end plate (region of sarcolemma with ACh receptors).
Neurotransmitter: Acetylcholine (ACh) is released upon nerve impulse arrival, binding to receptors on the motor end plate, increasing sodium permeability and generating an action potential.

Response of Muscle

Action Potential Generation: Once ACh binds, sodium floods into the fiber, generating an action potential that travels along the sarcolemma and into T tubules, triggering the sarcoplasmic reticulum to release Ca²⁺ for muscle contraction.
ACh Decomposition: Enzyme acetylcholinesterase decomposes ACh to terminate the muscle action potential and prevent sustained contraction.

Excitation-Contraction Coupling

Nerve impulse reaches axon terminal of motor neuron, releasing ACh.
ACh binds to receptors, triggering a muscle action potential (AP).
AP travels along T tubules opening Ca²⁺ release channels in the sarcoplasmic reticulum.
Ca²⁺ binds to troponin, exposing myosin binding sites on actin, leading to contraction.
Muscle Relaxation: Ca²⁺ is actively transported back into the sarcoplasmic reticulum, blocking myosin binding sites once more.

Muscle Metabolism

ATP Requirements: Substantial ATP is necessary for muscle contraction, Ca²⁺ pumping, and other metabolic activities during contraction.

Sources of ATP

Creatine Phosphate: High-energy phosphate compound synthesized when ATP is abundant; utilized instantly for ATP formation.
- Provides energy for about 15 seconds of contraction.
Anaerobic Respiration: Produces ATP without oxygen, converting glucose to pyruvic acid (glycolysis) and subsequently to lactic acid in low oxygen environments. Supplies energy for 30-40 seconds of intense activity.
Aerobic Respiration: Occurs with oxygen, converting pyruvic acid into ATP, carbon dioxide, and water, providing energy for prolonged activities.

Muscle Fatigue

Muscle Fatigue: The inability to maintain force of contraction; caused by:
- Lowered Ca²⁺ levels in the sarcoplasm.
- Insufficient oxygen supply.
- Depletion of glycogen.
- Buildup of lactic acid and ADP.

Oxygen Debt

Oxygen Debt: Increased breathing and blood flow needed to restore metabolic conditions post-exercise. Requirements of repayment include:
- Converting lactic acid back to glycogen.
- Resynthesizing creatine phosphate.
- Replacing oxygen bound to myoglobin.

Muscle Tension

Motor Unit: A motor neuron and all skeletal muscle fibers it controls.
Contractions are all-or-none for individual fibers, while entire muscles can exhibit varied tension through motor unit recruitment and frequency modulation.

Types of Muscle Tension

Twitch Contraction: Brief contraction of all muscle fibers in a motor unit due to a single action potential.
- Latent Period: Time between action potential onset and muscle contraction.
- Contraction Period: Ca²⁺ binds and cross-bridges form leading to contraction.
- Relaxation Period: Ca²⁺ detaches as it is pumped back into the sarcoplasmic reticulum.
- Refractory Period: Phase post-stimulus where muscle cannot respond.
Wave Summation: A second stimulus can result in a stronger contraction if applied after the refractory period.
Tetanus Types:
- Unfused Tetanus: Partial relaxation occurs between stimuli.
- Fused Tetanus: No relaxation occurs, leading to a sustained, stronger contraction.

Motor Unit Recruitment

Motor Unit Recruitment: Gradual activation of additional motor units to increase muscle tension; starts with the weakest units and progresses to stronger units as needed.
Muscle Tone: Continuous involuntary contraction of small groups of motor units, providing stability.
Flaccid: Condition when muscle tone is lost, indicating potential nerve damage.

Categories of Contraction

Isotonic Contractions: Muscle tension remains constant while changing length.
- Concentric: Shortening of muscle (lifting).
- Eccentric: Lengthening of muscle (lowering).
Isometric Contraction: Muscle tension does not cause a change in length (holding an object steady).

Muscle Use

Muscular Atrophy: Wasting away due to inactivity, leading to loss of myofibrils.
- Disuse Atrophy: Muscle not used due to immobilization but retains nerve supply.
- Denervation Atrophy: Loss of nerve supply replaced by fibrous tissue.
Muscular Hypertrophy: Increase in muscle fiber diameter due to stress, leading to stronger contractions.

Types of Muscle Fibers

Slow Oxidative (SO) Fibers: Smallest diameter, least powerful, rich in myoglobin, resist fatigue, suited for endurance activities.
Fast Glycolytic (FG) Fibers: Larger fibers with rapid, powerful contractions, fatigue quickly; predominantly use glycolysis for energy.
Fast Oxidative-Glycolytic (FOG) Fibers: Intermediate fibers, fatigue-resistant, utilize both cellular respiration and glycolysis for ATP production.

Variations in Fiber Types: The proportion of these fibers among individuals affects athletic performance and susceptibility to fatigue.