Excitation - Contraction Coupling

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  • Created by: LBCW0502
  • Created on: 19-01-19 12:29
Outline the pathway of the electrical signal to the muscle for contraction
Signal from brain – spinal cord – motor nerves – response in muscle (contraction)
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Describe features of innervation of skeletal muscle
By motorneurons of the somatic NS. APs set up in axon hillock of a motor neuron are propagated along the axon to its terminals. These terminals make synaptic contacts with muscle fibres at NMJ so each nerve AP can set up an AP in the muscle fibre
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What is excitation-contraction coupling?
Process by which muscle AP causes rise of intracellular Ca 2+ and actin-myosin interaction (new signal generated in NMJ). Electrical signal causes mechanical contraction
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What is involved in EC coupling?
Myofibrils (highly organised bundles of thick/thin filaments which generate contraction). Calcium ions (needed to allow the interaction between contractile filaments, muscle contraction is calcium dependent).
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What are the membrane systems?
Excitable fibre membrane to propagate APs along fibre (surrounded myofibrils). Transverse tubular system (propagates APs deep into muscle fibre) invaginations in the muscle fibres. SR (stores Ca 2+ and releases it on demand)
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What are triads?
Composed of a segment of transverse (T) tubule sandwiched between 2 sacs of SR. Site of physical and functional contact between T-tubule and SR membranes via feet/pillars
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Outline the process of E-C coupling (1)
AP runs along sarcolemma from NMJ towards ends of muscle fibre. AP dives into centre of fibre along T-tubules. AP reaches triads, where voltage sensors in T-tubules interact with Ca 2+ release channels of SR
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Outline the process of E-C coupling (2)
Ca 2+ release channels in SR membrane open. Ca floods out of SR down its conc gradient so sarcoplasmic Ca rises. Ca ions bind to thin filaments, allowing cross-bridging cycling to start. Relaxation when Ca pumped back into SR
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Describe the events which take place in the triad
DHP receptor changes shape as AP travels along T-tubule membrane. As DHP and RyR molecules are in physical contact, this causes a change in the RyR which allows Ca to exit through the open channel made by the RyR molecules
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What are the three components of the troponin complex?
Tn C (binds to Ca), Tn T (attaches to tropomyosin), Tn I (holds troponin to tropomyosin)
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Describe the steric blocking mechanism (1)
Enables Ca to operate the on-off switch at molecular level. Depends on presence of regulatory proteins on thin/actin filament. Ca 2+ binds to troponin which changes shape.
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Describe the steric blocking mechanism (2)
This moves tropomyosin out of the way of myosin heads which can then bind to actin molecules and pull. Flexible neck stretched has myosin head changes shape and rotates (pulls actin)
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Describe features of pharmacology and pathology of EC coupling (1)
Malignant hyperthermia/hyperpyrexia (MH) - genetic cause (defective RyR gene, or defect in DHPR gene), normally people affected have no idea that anything is wrong
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Describe features of pharmacology and pathology of EC coupling (2)
But defects result in excessive Ca release when patient is exposed to some gaseous GAs such as halothane. Results in generalised muscle contractions, resulting in large rise in body temperature, massive release of lactate (acidosis)
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Describe features of pharmacology and pathology of EC coupling (3)
Release of K from muscle, raising blood [K+]. Problems develop very rapidly, but prompt treatment with dantrolene (blocks Ca release) rapidly stops contractions before too much damage is done. Alternative GAs needed if these patients require surgery
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What is a twitch? (1)
Brief mechanical response (contraction usually recorded under isometric conditions) that occurs in response to a single AP. Always much longer in skeletal muscle than the AP. Has a speed/duration depending on the type of muscle fibre
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Why do fast fibres shorten faster?
Because their myosin has a faster ATPase and so does more X-bridge cycles per second
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Describe the individual differences in muscle fibre type composition for a higher jumper and a marathon runner
More faster fibres than slow fibres for high jumper. More slow fibres than fast fibres for marathon runner
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Why is the twitch much slow than the AP which triggered it?
Time needed for SR Ca channels to open, Ca to diffuse out of SR, Ca to bind to TN and shift TM, X-bridges to start interacting with thin/actin filaments, tightening up of slack elastic structures in the muscle before force can be recorded
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What is a tetanus? (1)
Sustained mechanical response to a series of APs. Summation (what happens to twitches when they occur in such quick succession that they overlap. Tetanus is the ultimate form of summation
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What is a tetanus? (2)
(series of mechanical responses occurring in such quick succession that their overlap gives a prolonged smooth contraction). Force produced in a tetanus is 3-5x that of a twitch
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What is a fused tetanus?
High frequency stimulation resulting in complete fusion of the contractile responses
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Describe features of motor units (1)
Every muscle is innervated by many motor neurons. Each muscle fibre is innervated by one motor neuron. Each motor neuron innervates a different set of muscle fibres. A motor neuron and its muscle fibres (motor unit)
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Describe features of motor units (2)
Normal muscles have a mix of motor units of different properties. All fibres in the unit will contract together (functional unit), even though they are scattered across the muscle
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Describe features of motor units (3)
Never get motor unit with twitch or tetanus (either one or the other). Speed of contraction (fast twitch fibre – larger diameter but same force as slow twitch fibre)
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What is the force output of a motor unit determined by?
Number of muscle fibres it contains (varies in different muscles) and the diameter of its muscle fibres (force is proportional to fibre XSA). Other properties (speed, resistance to fatigue) depend on type of muscle fibres it contains
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How can you regulate muscle force (force proportional to area of active muscle fibres)? (1)
Short term (moment to moment) regulation is by: motor unit recruitment (CNS determines how many of units are activated at any one moment - more units active/bigger force). Rate recruitment (CNS determines frequency of stimulation)
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How can you regulate muscle force (force proportional to area of active muscle fibres)? (2)
(determine degree of summation). Tetanic contractions produce more force than twitches (force can also be increased by muscle hypertrophy but is a long term effect - takes weeks/months)
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Card 2

Front

Describe features of innervation of skeletal muscle

Back

By motorneurons of the somatic NS. APs set up in axon hillock of a motor neuron are propagated along the axon to its terminals. These terminals make synaptic contacts with muscle fibres at NMJ so each nerve AP can set up an AP in the muscle fibre

Card 3

Front

What is excitation-contraction coupling?

Back

Preview of the front of card 3

Card 4

Front

What is involved in EC coupling?

Back

Preview of the front of card 4

Card 5

Front

What are the membrane systems?

Back

Preview of the front of card 5
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