36 Questions
What is the role of calcium in the excitation-contraction coupling of cardiac muscle?
Binds to myofibrils during contraction
How does the length of the contractile cell action potential in cardiac muscle prevent tetanus?
By having a prolonged refractory period
How does the resting sarcomere length in cardiac muscle compare to optimal length?
It is less than optimal length
What differentiates cardiac muscle from skeletal muscle regarding contraction?
It has higher passive tension due to blood filling
Which statement best describes the excitation-contraction coupling of skeletal muscle?
Dependent on calcium binding to myofibrils
What effect does the blood filling in the ventricles have on cardiac muscle?
Increases passive tension due to stretching
What type of muscle fiber has physiological and histological features intermediate between the other two types?
Fast oxidative-glycolytic (Type II a, FOG) fibers
Which type of cells in the heart are responsible for initiating action potentials?
Pacemaker cells
What is the function of intercalated disks in cardiac muscle cells?
Facilitate electrical coupling between cardiac myocytes
What is the primary role of pacemaker cells in the heart's conduction system?
Generating and conducting electrical impulses
Which component is significantly less developed in cardiac muscle cells compared to skeletal muscle fibers?
Endoplasmic reticulum
What percentage of myocardial cells are made up of pacemaker cells in the heart?
Around 1-5%
What is the correct sequence of events at the neuromuscular junction for skeletal muscle contraction?
Excitation, coupling, contraction, relaxation
What is the result of calcium binding to the myofibrils during skeletal muscle contraction?
Powerstroke
What happens when calcium is released from the myofibrils during skeletal muscle relaxation?
Calcium returns to the sarcoplasmic reticulum
What occurs when action potentials are propagated down the somatic motor neuron at the neuromuscular junction?
Depolarization of presynaptic terminal
What role do nicotinic receptors play at the motor-end plate during skeletal muscle contraction?
Binding acetylcholine
What is the effect of the end plate potential spreading across the muscle fiber during skeletal muscle contraction?
Depolarization and firing of an action potential
What happens at the end of the power stroke in muscle contraction?
Myosin releases ADP
In skeletal muscle, what induces a conformational change in the thin filament allowing myosin heads to cross-bridge with actin?
Calcium binding to troponin-tropomyosin complex
Which statement best describes an isotonic contraction?
Muscle tension stays constant and the muscle fiber shortens
What is the role of afterload in the force-velocity relationship in muscle contraction?
Afterload decreases velocity of shortening
What happens during temporal summation in muscle contraction?
Second contraction starts before first relaxes
In skeletal muscle, what is the primary function of spatial summation?
Enhance force through multiple motor unit recruitment
At what stage does ATP binding destabilize the myosin-actin interaction in muscle contraction?
"Power stroke"
What defines the length-tension relationship in muscle mechanics?
"Overlap between actin and myosin filaments"
Which type of muscle tissue makes up approximately 30-45% of the total body weight?
Skeletal muscle
What is the function of the perimysium in skeletal muscle organization?
Enclose the muscle fibers known as fascicles
Which protein wraps around actin filaments and covers the myosin-binding sites in sarcomeres?
Tropomyosin
What is the function of troponin-I (Tn-I) in sarcomeres during muscle contraction?
Inhibit myosin binding site on actin
Where are triads located in skeletal and cardiac muscle cells?
Terminal cisternae and T-tubules
In skeletal muscle, what is the primary function of nebulin and titin?
Support actin strands
Which region of the sarcomere contains only thick (myosin) filaments?
H-zone
What is the primary function of the endomysium in skeletal muscle organization?
Enclose the skeletal muscle cells or myofibrils
Which organelle serves as a major store for calcium in muscle cells?
Sarcoplasmic reticulum (SR)
What is the primary function of transverse tubules (T-tubules) in muscle cells?
Conduct action potentials rapidly throughout the muscle
Study Notes
Skeletal Muscle Organization
- Skeletal muscles make up 30-45% of total body weight
- Under voluntary control and mostly attached to bones
- Functions: produce skeletal movement, maintain posture and body position, protect internal organs, and generate heat
- Three connective tissue layers: epimysium, perimysium, and endomysium
- Each skeletal muscle fiber has a sarcolemma (cell membrane), transverse tubules (T-tubules), and a sarcoplasmic reticulum (SR)
Sarcomere Structure
- Sarcomeres are the contractile units of skeletal and cardiac muscle
- A-band contains thick (myosin) and thin filaments (actin)
- I-band contains only thin filaments
- Z-line anchors thin filaments to the sarcolemma
- H-zone contains only thick filaments
- Sarcomeres shorten during muscle contraction
Actin Filaments
- Actin chains are intertwined with troponin and tropomyosin proteins
- Actin has a myosin-binding site
- Troponin and tropomyosin regulate muscle contraction
Myosin Filaments
- Myosin molecules form a protein chain with a head and a tail
- Myosin head has a heavy chain (HC) and a light chain (LC)
- Heavy chain contains an ATP binding site and a binding site for actin
- Elastic hinge region allows the head to swivel and move (power stroke)
Excitation-Contraction Coupling
- Excitation-contraction coupling is the process in which an action potential causes calcium concentration to increase in the cytosol, leading to contraction of the muscle
- Steps: excitation, coupling, contraction, and relaxation
- Calcium binds to troponin and tropomyosin, allowing myosin heads to cross-bridge with actin
Neuromuscular Junction
- The neuromuscular junction is the synapse between a motor neuron and a muscle fiber
- Sequence of events: action potential propagation, calcium influx, ACh release, binding to nicotinic receptors, and muscle contraction
Muscle Contraction and Relaxation
- Muscle twitch: one cycle of excitation-contraction coupling
- Types of contraction: isometric (no shortening) and isotonic (shortening with constant tension)
- Types of isotonic contractions: concentric (shortening) and eccentric (lengthening)
- Length-tension relationship: the relationship between fiber length and force produced
- Force-velocity relationship: the velocity of shortening as a product of changes in afterload
Cardiac Muscle
- The heart is a hollow muscular pump that pumps blood throughout the vasculature
- Ventricular muscle cells: striated muscle cells with two nuclei, rich in mitochondria, and less developed SR
- Cells are connected end-to-end by gap junctions, forming a functional syncytium
- Pacemaker cells initiate action potentials necessary for cardiac muscle contraction
Excitation-Contraction Coupling in Cardiac Muscle
- Similar to skeletal muscle, but with differences in SR and calcium release
- No tetanus in cardiac muscle due to the length of the contractile cell action potential
- Graded contractions possible due to the heart's ability to increase contractile force under changing conditions
Test your knowledge on the muscle contraction process. From myosin releasing ADP to ATP binding destabilizing the myosin-actin interaction, each step is crucial for muscle fiber shortening.
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