0429 Excitation-contraction coupling in muscle

Question 1

Briefly describe the synaptic transimission from axon hillock of a motor neuron to the nerve terminal (use ACh as a NT)

Answer 1

Motor neuron fires, causing influx of Ca2+ via depolarisation at the nerve terminal. This signals vesicle fusion with the pre-synaptic membrane, releasing ACh in vesicles into synaptic cleft. ACh binds to nicotinic receptors on the post-synaptic membrane, opening Na+/K+ channels and causing EPSP. ACh is degraded in the synaptic cleft by AChE and choline is transported back into terminal

Question 2

What is excitation-contraction coupling

Answer 2

It is the excitation of a muscle membrane (due to NMJ synaptic transmission) that leads contraction of muscles

Question 3

What determines the innervation ratio of a motor neuron in muscle

Answer 3

The function of the neuron. E.g. a fine movement generally involves 3 fibres/neuron. A strong, course movemnt canuse 100 to 1000 fibres/neuron

Question 4

What is the role of the Sarcolemma, T tubule and Sarcoplasmic reticulum in excitation-contraction coupling

Answer 4

Sarcolemma = plasma membrane of the cell down with AP propagates. The T tubules allow AP propagation to distribute throughout the myofibrils. SR contains the Ca2+ ions that a released into the sarcoplasm and initiate the cross bridge cycling

Question 5

What is the triad? What receptors are found on it and what are their roles

Answer 5

A triad = terminal cisternae + T tubule + terminal cisternae (part of SR). Dihyropyridine channels (DHP) are found on the T tubules. They are coupled to Ryanodine receptors on the SR. Activation of DHP (due to AP) stimulates ryanodine receptrs which release Ca2+ stored in SR into the sarcoplasm to initiate cross bridge cycle

Question 6

What are the main players in relaxation of the muscle (i.e. removal of intracellular Ca2+

Answer 6

SERCA (sarco/endoplasmic reticulum calcium ATPase). Calsequrestrin and sarcalumenin

Question 7

Describe the structure of skeletal muscle (fasicle is made of …., made of…..) down to the thick and thin filaments

Answer 7

Fasicle is made of muscle bundles. Muscle bundle is made of muscle fibres. Muscle fibres are myofibrils. Myofibrils are made of thick and thin filaments

Question 8

Describe the Z lines, I,A and H bands

Answer 8

Z line demarcate one sarcomere (distance between 2 Z lines). I band contains actin (thin) filaments and no myosin. A band marks the length of myosin filaments. H band contains myosin (thick) filaments, no actin.

Question 9

During contraction which bands shorten and which bands remain the same length

Answer 9

The A band (length of thick, myosin filaments) stays the same. The H and I bands (myosin and actin only sections) shorten.

Question 10

Describe the structure of the thin, actin filaments (include actin, troponin, tropomyosin) and their component functions

Answer 10

Actin filaments back the backbone of the filament. Troponin/troponin complex (T,I,C) controls the position of tropomyosin. Tropomyosin wraps around the actin and blocks the myosin binding sites

Question 11

Where do calcium ions from the sarcoplasm bind to on the thin filaments

Answer 11

The troponin complex (specifically troponin C)

Question 12

Describe the structure of the thick filaments (tail and cross bridge region) and their component functions

Answer 12

Myosin consists of a tail region (structure of thick filament) and the cross-bridge region. CB region contains the neck and globular heads which binds to actin and makes a cross bridge

Question 13

Briefly describe the cross bridge cycle (after calcium has bound to troponin, exposing the myosin binding site on actin)

Answer 13

Resting myosin heads contain ATP. Exposure of binding site means myosin and actin join to form a cross bridge. Once bound, myosin heads release phosphate (ATP to ADP) and pull the actin filament towards centre of sarcomere. Myosin release ADP (remains attached). Finally ATP binds to myosin which releases it from actin. Repeat cycle

Question 14

What would happen if there is no ATP available in muscle

Answer 14

Then the actin and myosin filaments would be stuck in attachment state (contraction but no relaxation of muscle). An example of this is rigor mortis

Question 15

What are the differences between fast and slow twitch skeletal muscles in terms of contraction (think cross bridges and Ca2+ elimination)

Answer 15

Fast twitch (type 2) muscle fibres have faster cross bride cycles and SERCA 1. Slow tiwtch (type 1) fibres have slower cross bridge cycles and SERCA 2

Question 16

List some differences between cardiac muscle and skeletal muscle in terms of contraction and contraction components

Answer 16

Cardiomyosites have intercalated discs (electrical and mechanical syncytium). The SR is less developed, the sinoatrial and atrioventricular nodes regulate contraction pace and frequency, and there is a higher [mitochondria]. Role of Ca2+ (calcium release in Sr, removal, force of contraction)

Question 17

Skeletal/cardiac muscle contraction is troponin regulated by binding interaction. What is smooth muscle regulated by

Answer 17

Smooth muscle is myosin regulated by phosphorylation

Question 18

What are the 2 types of smooth muscle characterised by contraction pattern?

Answer 18

Phasic smooth muscle (contracts rhythmically/intermittently) and tonic smooth muscle (contracts continuously)

Question 19

What are some types of junctions that coordinate contraction in smooth muscle

Answer 19

Gap junctions, adherens junctions, Tight junctions

Question 20

What is the name given to Smooth muscle cells with and without electrical coupling (i.e. gap junctions between them)?

Answer 20

Without gap junctions = multiunit smooth muscle. With gap junctions = single-unit smooth muscle