4. Muscle contraction Flashcards
What are the three types of muscle?
Skeletal
Smooth
Cardiac
What is meant by the term contraction?
The ability of a muscle to actively shorten and develop tension
What are the structures that make up the muscle?
Muscle Fasciculus Fibre/cell Myofibril Sarcomere Filaments
Describe the actin filaments in the relaxed state?
The ends of actin filaments from Z discs barely overlap
Describe the actin filaments in the contracted state?
Actin filaments pulled inward, overlap to maximum extent
What are the features of the actin filaments?
Double stranded F-actin molecules comprising of polymerised G-actin molecules
2 strands of tropomyosin molecules
Troponin complex on each tropomyosin for contraction (binds to Ca2+)
How many myosin and actin filaments on each myofibril?
1500 myosin
3000 actin
Describe the first three steps of the sliding-filament mechanism
- Low Ca2+ conditions, tropomyosin blocks access to the myosin binding site of the actin
- In high Ca2+ conditions, Ca2+ binds to troponin
- The positions of troponin and topomyosin are altered on actin + myosin can now access its binding site on actin
complete the full theory
Describe what happens when the sarcomere length decreases
Unstretched muscle
Describe what happens when the sarcomere is at optimal resting length
Muscle can develop maximal tension
Describe what happens to increased sarcomere length?
Actin is pulled to end of myosin with no overlap. No tension
Describe excitation-contraction coupling in skeletal muscles
Action potential travels along a motor nerve to the neuromuscular junction
Nerve secretes small amount of acetylcholine
ACh opens channels on muscle membrane
Na+ flows into the cell – initiates an action potential
AP travels along muscle fibre membrane
Describe the T-tubule endoplasmic reticulum system?
Muscle membrane depolarized, AP flows rapidly through centre of fibre via the T-tubules. Sarcoplasmic Reticulum releases Ca2+
Rise in intracellular Ca2+ enables Ca2+-binding to troponin, tropomyosin moves and cross-bridge cycling occurs
Ca2+ actively pumped back into SR store. Relaxation
What causes the release of Calcium ions?
Action potential from Sarcoplasmic reticulum
What causes re-uptake of calcium ions?
Calcium pump
What is the force of muscle contraction regulated by?
Increasing the number of motor units contracting simultaneously
Increasing the stimulation frequency
What is meant by tetany?
A continuous contraction occurring at high stimulation frequency
Describe concentric contraction
Isotonic- constant tension- change in length
Concentric - Muscle shortens
Describe eccentric contraction
Isotonic and eccentric (muscle lengthens)
Describe isometric contraction
Isometric- constant length, produces tension
Muscle length stays constant
What are the two classifications of skeletal muscle fibres?
Slow twitch type I and fast twitch type II
Give an example of slow twitch type I
Postural muscles of the back
What are the features of the slow twitch type I?
Slow, sustained contractions Oxidative metabolism Many mitochondria Red-myoglobin Resist fatigue
What are the two fast twitch, type II?
Type IIA and Type IIB
Give an example of Type IIA
Soleus in calf
Give an example of type IIB
Extraocular muscle
Describe the features of Type IIA fast twitch
Fast contractions Oxidative metabolism Many mitochondria Red-pink myoglobin Intermediate fatigue resistance
Describe type IIB fast twitch features
Rapid contractions Anaerobic glycolysis Few mitochondria White - little myoglobin High glycogen content
Where is smooth muscle found?
Found in the gastrointestinal tract, uninary tract and walls of blood vessels
What are the features of smooth muscle?
Single nuclei
No striations
Describe the structure of smooth muscle
Actin filaments radiating from dense bodies
What innervates smooth muscle?
Automatic nerve fibres
Describe smooth muscle excitation
Typical smooth muscle action potential elicited by an external stimulus
Describe smooth muscle spontaneous activity
Repetitive spike potentials, occur spontaneously in the smooth muscle of the wall
Describe the smooth muscle excitation- contraction coupling
Has spontaneous electrical and mechanical activity
Activity can be phasic or tonic
Activity is modified by circulating hormones and neurotransmitters
Contraction velocity is less than skeletal muscle
Smooth muscle can maintain prolonged contractions at less energy cost of the other muscle types
What are the features of skeletal muscles?
Striations
Troponin
Cross-bridge cycling
Neurogenic stimulation only excites in skeletal muscle
E-C coupling- depolarisation Ca2+ release from SR
What are features cardiac muscle?
Striations troponin Cross-bridge cycling Spontaneous activity Neurogenic stimulation- modultaes cardiac muscle contraction E-C coupling- depolarisation Ca2+
What are the features of smooth muscle?
Calmodulin
Spontaneous activity
Neurogenic stimulation- modulates smooth muscle contraction
E-C coupling - smooth muscle -depolarisation -Ca2+ - influx and CICR second messenger system
