7. Muscle Flashcards
Describe skeletal muscle
Attached to bone
Produces body movement
Found in antagonistic muscle pairs: flexors and extensors
What are the 2 types of muscle contraction and how do they differ?
Isotonic: tension stays the same and length changes
Isometric: tension changes and length remains the same
What are the 2 subtypes of isotonic contraction?
Concentric: shortening
Eccentric: lengthening
Skeletal muscle consists of
Bundle of myofibres
Describe myofibres
Large and Cylindrical
Multinucleate
Packed with myofibrils
Appearance of myofibrils
striated due to light and dark bands
What is a sarcomere?
Functional unit of muscle
Lies between 2 Z-lines
Describe the process of excitation-contraction coupling of skeletal muscle
An AP propagates along sarcolemma and T tubules
Reaches the Dihydropyridine receptors
Depolarisation causes a conformational change in the DHPRs
Change transmitted to Ryanodine Receptors on sarcoplasmic reticulum
RyR opens: causing Ca2+ release from intracellular stores
Causes depolarisation due to increase in intracellular Ca2+
T-Tubules
Membrane invaginations that contact the extracellular fluid
Found in myofibres
Sarcoplasmic reticulum (SR)
extensive network of Ca2+ stores surrounding each myofibril
What are the different components of a sarcomere?
Z-line Actin filaments Myosin Titin Nebulin Tropomyosin CapZ and Tropomodulin
What is a Z line?
Defines lateral boundaries of sarcomere
What is actin?
Polymeric thin filament composed of 2 twisted alpha-helices
Displays polarity
What is myosin?
Thick filaments
‘motor proteins’
Contain numerous ‘globular heads’ that interact with actin
What is titan?
Very large ‘spring-like’ filaments
Anchor myosin to the Z-line
What is nebula?
Large filaments associated with actin
What is tropomyosin?
Elongated protein bound to actin
What are CapZ and Tropomodulin?
Associated with +ve and –ve ends of actin, respectively
Describe the sliding filament theory
Calcium binds to Troponin, causing tropomyosin chain to move
Movement exposes myosin binding site on surface of actin chain
‘Charged’ myosin heads bind to exposed binding sites
Binding and discharge of ADP caused myosin head to pivot (Power stroke), pulling actin filament towards centre of sarcomere
ATP binds to myosin head, releasing it from the chain
ATP hydrolysis provides energy to ‘recharge’ the myosin head
What is the relationship between muscle tension and load in isotonic and isometric contraction?
Isotonic: Tension > Force exerted by load
Isometric: Tension = Force exerted by load
What does the tension-load relationship in isotonic contraction cause?
Muscle to contract Fibres shorten Energy expenditure (ATP): ‘recharging’ of myosin heads
What does the tension-load relationship in isometric contraction cause?
Muscle DOES NOT contract: myosin heads reattach to the same point on actin chain Energy expenditure (ATP): ‘recharging’ of myosin heads
What are cardiomyocytes?
Cells in heart that contract causing movement of blood
What are the pacemaker cells in the heart?
Sinoatrial node
Atrioventricular node
Describe sinoatrial nodal cells
Small, ‘empty’, spindle shaped cells
spontaneously active
Describe atrioventricular node
spindle-shaped network of cells located at base of right atrium
Conducting fibres in the heart
Bundle of His
Purkinje fibres
What is the bundle of his?
fast conducting cells adjoining the AV node and Purkinje fibres
What are purkinje fibres?
large cells that rapidly conduct electrical impulses
Describe the appearance of Cardiomyocytes
striated muscle cells that are distinct but are linked by intercalated discs
What are intercalated discs?
Specialised regions connecting individual cardiomyocytes
Contain numerous gap junctions: allow action potentials to spread rapidly from cell to cell.
What are sarcomeres (in the heart)?
Contractile units of cardiomyocytes
Mechanism of contraction is same as skeletal muscle
Describe the process of excitation-contraction coupling of cardiac muscle.
Action Potential propagates down T-tubules and leads to the opening of VGCCs: leads to influx of Ca2+
Ca2+ then binds to the RyR making them open leading to Ca2+ release from the SR and a further increase in intracellular Ca2+ (CICR)
Ca2+ initiates contraction by binding to troponin
Ca2+ causes further depolarisation
What are the 2 types of receptors involved in excitation-contraction coupling of skeletal muscle?
Dihydropyridine receptor
Ryanodine receptor
Which receptors are involved in excitation-contraction coupling of cardiac muscle?
Voltage Gated Calcium Channels
Ryanodine Receptors
Describe smooth muscle
present within walls of hollow organs e.g. Blood vessels
Do not contain regular actin/myosin arrangement
Acts slowly
No control over smooth muscle
Describe the process of excitation-contraction coupling of smooth muscle
Action potential reaches VGCC and makes it open leading to Ca2+ influx
Ca2+ binds to Calmodulin forming a Ca2+-CaM complex
This complex activates myosin light chain kinase (MLCK)
MLCK phosphorylates myosin light chains (MLC20) that form cross bridges with actin filaments
Leads to smooth muscle contraction
