Lecture 16- How do muscles contract? Flashcards
What does the skeleton and muscles make up?
The musculoskeletal system
What are embryonic muscle cells called?
Myoblasts
What are muscle fibres?
Multinucleate muscle cells
What do muscles consist of?
Muscle fibres bundled together by connective tissue
What does a single muscle fiber consist of?
Many nucleus
Plasma membrane (sarcolemma)
Mitochondria
Myofibrils
Muscle contraction is the result of interactions between which two contractile proteins?
Actin- thin filaments
Myosin- thick filaments
What are myofibrils?
Bundles of actin and myosin filaments
What are myosin filaments held in place by?
Titin
What are the repeating units that make up a myofibril called?
Sacromeres
What does titin connect to?
The full length of the sarcomere, from Z to Z line
Where does resistance to stretch come from in relaxed muscle?
Titin
What is anchored to the Z line, other than titin?
Actin
What is the A band?
The band that contains all of the myosin filaments
What is in the H zone?
Region where the actin and myosin does not overlap (myosin side)
What is in the I zone?
Region of actin where myosin does not overlap
What is the M band?
Contains a protein to help hold the myosin filaments in their regular arrangements
What happens when muscles contract to the sarcomeres?
They shorten
What happens to the H and I zone when muscles contract?
They shorten
What happens to the z lines as a muscle contracts?
Move towards the A band (A band is where they overlap)
What is the name of the theory of muscle contraction called?
Sliding filament theory
What causes filaments to slide?
Actin-myosin interactions
How is muscle contraction initiated?
Action potential from a motor neuron at the neuromuscular junction
What is a motor unit?
All the muscle fibers activated by one motor neuron
How does actin-myosin interactions cause muscles to slide past each other?
Myosin heads bind to specific sites on actin molecules to form cross bridges
Myosin changes conformation, actin filament slides 5-10 nm
What may one muscle have?
Multiple motor units
How is strength of muscle contraction increased?
- Increased rate of firing of motor neuron
- More motor neurons firing
What does a myosin filament consist of?
Myosin molecules which consist of a linear polypeptide chain and a globular head
What does an actin filament consist of?
Actin monomer, tropomyosin, troponin
What does it mean that muscle cells are excitable?
The plasma membrane can conduct action potentials
What molecule is released at the motor neuron to open ion channels in the motor end plate?
Acetylcholine- a neurotransmitter
How do action potentials travel deep within muscle fibers?
T tubules
What are T tubules?
Transverse tubules
Descend into the sarcoplasm (muscle fiber cytoplasm)
What do T tubules run close to?
Sacroplasmic reticulum
What is in the sarcoplasmic reticulum?
A high concentration of Ca2+ at rest
Ca2+ pumps
What happens when an action potential reaches the receptor proteins?
The Ca2+ channels open, calcium ions flow out of sarcoplasmic reticulum, triggers actin-myosin interactions
What is troponin?
A protein with three sub units: one binds actin, one binds myosin, one binds Ca2+
What does tropomyosin do at rest?
Blocks binding sites on actin
What happens to troponin when Ca2+ is released?
They bind, conformation of troponin change
What happens once troponin has bound to Ca2+?
Tropomyosin and troponin bind, tropomyosin twists to expose actins active site
How does contraction of a muscle stop?
Ca2+ pumps remove Ca2+ from the sarcoplasm, contraction stops
How is cardiac muscle similar and different to skeletal muscle?
Also striated
Cells are smaller and only have one nucleus
Cells branch and interdigitate to withstand high pressures
What provides mechanical adhesions between cardiac muscle cells?
Intercalated discs
What initiates coordinated heart contractions?
Pacemaker, conducting cells
What does it mean that the heart is myogenic?
It generates the heartbeat its self
How else is heart rate determined?
The autonomic nervous system
How do cardiac muscles contract?
DHP proteins in T tubules are Ca channels
ryanodine receptors are ion gated calcium channels, sensitive to calcium ions
Action potential causes Ca2+ to flow into sarcoplasm from T tubules, increase opens ca channels in sarcoplasmic reticulum, initiates contraction
TL;DR, how do cardiac muscles contract?
Ca2+- induced Ca2+ release
How are smooth muscle cells arranged?
In sheets, electrical contact is via gap junctions- action potential in one cell spreads to others in sheet
What happens when smooth muscles are stretched?
Cells depolarise and fire action potentials to start contraction
How does smooth muscle contract?
Ca2+ influx to sarcopasm stimulated by hormones, stretching or action potentials
Ca2+ binds to calmodulin- activates myosin kinase which phosphorylates myosin heads- bind and release actin
Explain how the hypothesis that stretch and parasympathetic stimulation induce contraction in gut smooth muscle can be tested experimentally.
Incubate a strip of smooth muscle in a saline bath
Measure action potentials and force of contraction.
Explain the method of this experiment.
The muscle is anchored to a device which applies a stretch force
An electrode detects membrane and action potentials which is recorded
In experiment 2, acetylcholine or norepinephrine is dripped on
What are the results of this experiment?
Stretching depolarises the membrane which causes action potentials which activates contractile membrane
Neurotransmitters alter membrane resting potential to alter rate at which action potentials are fired
What is the minimum unit of skeletal contraction?
A twitch
What factors determine the force generated by an entire muscle?
- Number of activated motor units
- Frequency of firing of motor units
