Excitatory Tissue- Muscle Flashcards
Structure of Skeletal Myocytes
Multiple peripheral nuclei with striations formed from the myosin and actin filaments.
Long cylindrical shape- 35cm long and 0.1mm wide.
Sarcoplasmic reticulum present and main source of Ca2+ for contractions. Releases Ca2+ by voltage-sensitive protein.
T-tubule present at interface between A and I bands. Invaginations of the sarcolemma used to conduct action potential into the myocyte.
Thick filament structure
Two intertwined myosin filaments. Tails intertwine to form a helix while heads act as an ATPase and is tilted back towards the M-line.
Thin filament structure
Made of many globular actins joined in a chain. Two of these chains form a helix.. Actin proteins have binding sites for myosin heads which are covered by tropomyosin. Tropomyosin held in place by troponin, which undergoes conformational change when Ca2+ binds.
Sliding Filament Model
1) ATP binds to myosin head which is attached to actin. Causes detachment of myosin.
2) ATP is hydrolysed by the ATPase and bends backwards into the energised position.
3) Myosin head binds to actin while still bound to ADP and Pi.
4) ADP and Pi detach and causes myosin head to rotate towards the centre of the sarcomere, pulling actin along.
What is a Motor Unit
A motor neuron and all the muscle fibres it innervates.
Process of Excitation Contraction Coupling.
Presynaptic neuron releases Ach. Ligand gated Na+ channels open at the motor end-plate and ALWAYS results in an action potential.
Depolarisation travels around the myocyte and into t-tubules which stimulates Ca2+ voltage gated channels to open on the sarcoplasmic reticulum. Entry of Ca2+ stimulates contraction by binding to troponin.
Mechanism of Relaxation-Skeletal
Ca2+ is actively transported by CaATPase into the SR. Reduces troponin binding and available binding sites.
Total Tension
Total tension is the sum of active tension from contraction and passive tension from the energy stored in the connective tissue around muscle.
Effect on Total Tension by Muscle Length
< Optimum: Minimal active and passive tension. Can be easily stretched. Low total tension.
Optimum: Maximum active tension but low passive tension. High total tension-resists stretching.
> Optimum: Reduced active tension but higher passive tension. Total tension actually lower so stretching not as resisted.
» Optimum: Near zero active tension but very high passive tension. High total tension and strong resistance against stretching.
Isotonic and Isometric Contractions
Isotonic: Muscle tension remains constant but length changes. Used when movement is required. Occurs when the force exerted exceeds inertia of object.
Isometric: Muscle tension increases but length remains constant. Used to maintain posture and when force exerted does not exceed inertia.
Sources of ATP in muscles
Creatine phosphate. Stores phosphates and phosphorylates ATP to provide a short term energy supply (15 seconds).
Aerobic Respiration: Slow but effective. Produces 16x more than anaerobic respiration. Used when a slow steady supply of energy is needed such as for maintaining posture.
Anaerobic respiration: Fast inefficient source of ATP. Used when a large quantity of ATP is required suddenly. Used for vigorous exercise and lasts up to 120 seconds.
Types of Muscle Fibres
Slow Oxidative Type I: Smaller diameter to improve SA:V ratio to improve rate of diffusion of oxygen into the cell. Mainly forms ATP aerobically. Ca2+ channels on SR and myosin less efficient.
Fast Glycolytic type II: Poor blood supply and does not require efficient oxygen supply so has large diameter. SR Ca2+ channels and myosin heads both very efficient so cross bridges are quickly formed.
Summation of Contractions
A single stimulus/AP will cause one contraction which lasts 50x longer.
Low frequency stimuli will cause unfused tetanus, where the next contraction occurs before the muscle is fully relaxed, causing gradually increasing tension.
High frequency of stimuli will caused fused tetanus, where there is sustained and strong contractions- stronger than unfused tetanus as more contractions are summed.
Recruitment of Muscle Fibres
Type I (small) fibres are first recruited as they are fatigue resistant but generate the least amount of tension. Type II fibres (medium/large) are only recruited if the small fibres are insufficient. Only active for short periods to provide sudden high output of power. Increasing stimulus intensity will result in increasing recruitment and contraction as more neurons are activated.
Cardiac Muscle Structure
Branched and striated with 1-3 central nuclei. 1 t-tubule at z-disc. Many mitochondria and myoglobin. Joined by desmosomes and gap junctions. Electrically coupled.
