Muscles

Muscles

1. Skeletal Muscles

   • Muscle Contractions
     • Process of Muscular Contraction
       • 1. At the nerve-muscle junction, the neurotransmitter acetylcholine (ACh) is released by the nerve that bind to receptors on sacrolemma
         • 2. This binding changes the voltage difference across the sacrolemma, producing a rapid voltage pulse (an action potential) that spreads across the whole muscle fibre and along the T tubules
           • 3. The action potential is passed into the fibre, causing calcium ions to be released by the sacroplasmic reticulum into the sacroplasm
             • 4. Calcium Ions bind to the protein troponin, exposing active sites on actin myofilaments inside the myofibrils. Cross bridges form when myosin heads bind to active sites on actin
               • 5. Muscle Contraction begins as repeated cycles of cross-bridge binding, pivoting, and detachment occur, powered by splitting of Adenosine Tri-Phosphate (ATP), to produce filament sliding
     • Muscle Relaxation process
       • 1. Acetylecholine is broken down by the enzyme acetylcholinesterase (AChE), ending action potential generation
         • 2. The sacroplasmic reticulum absorbs calcium, decreasing the concentration of calcium ions in the sacroplasm
           • 3. Active sites on actin are covered
             • 4. Filament sliding cannot occur and muscle contraction ends
               • 5. Muscle relaxation occurs and the muscle passively returns to its resting length
     • Types of Skeletal Muscle Contractions
       • Isokinetic
         • Change in muscle length occurs
           • Concentric
             • Shortening - decreasing distance between joints
           • Eccentric
             • Lengthening - increasing distance between joints
       • Isometric
         • No change in muscle length
     • Mechanisms of Contraction
       • During Contraction the I ban (Light - Actin (Thin)) shortens, but the A band (Dark - Myosin (Thick)) stays the same
       • Sliding Filament Model
         • Myosin filaments stay still while actin filaments move into the A band
         • Z lines move closer together
       • Cross-bridge cycle
         • Filament sliding occurs by cyclical attachment and detachment of myosin to actin such that myosin heads pull the actin filaments towards cetnre of A band, detach from actin, and creates a power stroke to bind to next actin molecule
   • Structure of skeletal Muscles (Superficial to deep)
     • Skeletal muscle
       • Surrounded in epimysium
       • Contains Muscle Fascicles
       • Muscle Fascicle
         • Contains Muscle Fibres
         • Surrounded by Perimysium
         • Muscle Fibres
           • Contains Myofibrils
           • Surrounded by Endomysium
           • Myofibrils
             • Contains Sacromeres (Z line to Z line)
             • Sacromere
               • M line - A protein ring within H band that supports myosin
               • Z line - Line joining adjacent sacromeres
               • A band - Myosin (Thick Filaments)
               • H band - length of myosin without actin overlaps
               • I band - Actin (Thin Filaments)
             • Surrounded by Sacroplasmic Reticulum
   • Types of Skeletal Muscles
     • Parallel
       • Fusiiform
         • e.g. Bicep Brachii
       • Radiate (Triangular)
         • e.g. Trapezius
       • Strap-like
         • e.g. Sartorious
       • Flat (Quadrilateral)
         • e.g. Pronator Quadratus
       • Muscles with Parallel form are better adapted to produce LARGE RANGE OF MOVEMENT AT JOINTS
       • Muscle fibres run from one attachment of muscle to the skeleton to the other attachment
     • Convergent
       • e.g. Pectoralis Major
     • Pennate
       • Uni-pennate
         • Extensor Digitorum
       • Bi-pennate
         • Rectus Femoris
       • Multi-pennate
         • Deltoid
       • Muscle fibres run oblique to line of pull of muscle
       • Pennate form muscles are better adapted to produce forceful (powerful) contractions
     • Circular
       • e.g. Orbicularis oris
       • Around entrances and/or exits
   • Types of Skeletal Muscle Attachments
     • Tendinous
       • Cord of fibrous tissue attaching muscle fibres to bone
     • Raphe
       • Muscle to Muscle
     • Aponeurosis
       • Sheet of fibrous tissue attaching muscle fibres to bone
     • Epimysium
       • Muscle to Bone
   • Types of Skeletal Muscle Fibres
     • Slow
       • Small cross-sectional diameter
       • Low Muscular Tension
       • Slow Contraction Speed
       • Highly Faitgue Resistant
       • Red (Oxidated)
       • High Myoglobin Content
       • Dense Capillary Supply
       • Many Mitochondria
       • Low Glycolytic enzyme concentration in sacroplasm
       • Fuel: Lipids, Carbohydrates & Amino Acids
     • Intermediate
       • Intermediate Cross-sectional Diameter
       • Intermediate Muscular Tension
       • Fast Contraction Speed
       • Intermediate Fatigue resistance
       • Pink
       • Low Myoglobin Content
       • Intermediate Capillary supply
       • Intermediate Mitochondria
       • Low Glycolytic enzyme concentration in sacroplasm
       • Fuel: Primarily Carbohydrates
     • Fast
       • Large Cross-sectional Diameter
       • High Muscular Tension
       • Fast Contraction Speed
       • White (unoxidised)
       • Low Fatigue Resistance
       • Low Myoglobin Content
       • Scarce Capillary Supply
       • Few Mitochondria
       • High Glycolytic enzyme concentraction in sacroplasm
       • Fuel: Carbohydrates
   • Myotendinous Junction (MTJ)
     • Force generated by muscle fibres is transmitted through the MTJ
     • Characteristics
       • Potential site of weakness (injury prone)
       • Muscle cell membrane (sacrolemma) folded into finger-like projections to increase inter-surface area for force transmission
       • Perimysium of the muscle is continuous with collagen fibres of the tendon
       • The MTJ receives blood supply from the muscle (hypovascular)
       • Receives sensory informatino on muscle length (Spindles), tension (golgi organs) and pain
   • Muscle Functions
     • Action = Movement(s) produced when a muscle contracts concentrically in isolation
     • Agonist (mover)
       • Muscle producing desired movement contracting either concentrically (action) or eccentrically
     • Antahonist
       • Muscle which must relax to allow desired movement to occur
     • Stabiliser
       • Muscle that supports a body part so that another muscle will have a firm base from which to act
     • Synergist
       • Muscle that eliminates an undesired movement that would otherwise be produced by movers
   • Muscle Recruitment Patterns
     • Effects of Load
       • Increasing load resutls in an increase of muscle fibre activation
     • Effects of Speed
       • Increasing speed causes instability and requires the recruitment of shunt muscles and some co-contraction
     • Effects of Joint position
       • Changing joint position changes gravity effects and therefore changes the muscles used in actions
     • Laws and Principles
       • Law of minimal muscle
         • Muscles are recruited in such a way that synergistic activity is minimised
           • E.g. The degree of elbow flexion and/or forearm pronation associated with the activity
       • Factors determining main agonist
         • Joint Position (large movement limb & muscle-length relationship)
         • Minimising synergist activity
         • Large Cross-sectional Areas (CSAs)
         • Uni/Bi/Poly -articular - based on what movement desired (amount of articular bases or surfaces required for desired movement)

2. Cardiac Muscles

3. Smooth Muscles