Lecture 5: Skeletal Muscles Flashcards
Summary if contractile structures in skeletal muscle
Slide 16
Skeletal Muscle- fascicles - muscle fibres- myofibrils- sarcomeres
Thick filaments. Thin filaments. Titin.
Myosin. Actin, troponin, tropomyosin
Describe the structure if a myofibril
Draw a myofibril
Slide10
There are multiple sarcomeres lined up along a myofibril
Sarcomere= organisational/contractile unit of a myofibril
There are 2 important lines in a sarcomere:
Z lines: the ends of a sarcomere, zig zag appearance, anchor thin (actin) filaments
M lines: middle of sarcomere, anchor thick (myosin) filaments
Z lines anchor actin
M lines anchor myosin
Sarcomere a have three main components, what are they?
Thick filaments
Thin filaments (Both involved in muscle conaction)
Titin (involved in muscle stability and relaxation)
Thin filaments have 2 main components what are they? Describe them all.
- Contractile protein (actin)
- Regulatory proteins (tropomyosin, troponin)
Actin:
- have a myosin binding site
- monomers combine to form strands
- 2 strands form a double helix in the thin filament
Troponin:
-Ca2+ binding protein
-has 3 subunits
-troponin T binds tropomyosin
-troponin I binds actin
-troponin C binds Ca2+
Troponin and tropomyosin start and stop (regulate) contraction
Thick filament structure
Basic components is myosin
- myosin molecule has a head with an actin binding site and an ATPase site.
- 2 myosin molecules join tail-to-tail, then form filaments with other myosin molecules with heads
- middle of thick filament has no head called ‘bare zone’
Titin structure and function
- huge elastic molecule
- single Titin molecule connects M line to neighbouring Z line
Functions:
- Stabilises position of contractile filaments
- elastic recoil- helps muscles return t resting length after contraction
Titin is the largest known protein
What are the major steps in muscle function
Excitation: triggering of a muscle action potential –>
Excitation-contraction coupling: initiation of contraction by the muscle action potential –>
Contraction (crossbridge cycle): movement and/or force generated by muscle fibres –>
Relaxation: termination of movement and/or force generation by muscle fibres
Describe the major steps in muscle function in detail
- Excitation:
-skeletal muscle require synaptic activation from a somatic NMJ to contract (neurogenic) slide 19
-single muscle fibre innervated by single motor neuron
-single motor neuron innervates many muscle fibres
Motor unit= A motor neuron and all the muscle fibres it innervates - Excitation-contraction coupling
-events between firing of muscle AP and start of contraction
-AP arrives in T tubules
-Change shape of dihydropyridine receptors (voltage sensors)
-DHP receptors open ryanodine receptors which are Ca2+ channels
-Ca2+ diffuses down its concentration gradient from the SR to the cytoplasm
-initial release of Ca2+ from SR triggers opening if additional Ca2+ channels in SR (calcium-induced calcium release)
Focus on regulation of contraction by troponin and tropomyosin
Relaxation:
-tropomyosin covers myosin binding sites on actin
-no actin/myosin thus cross ridges can form
Contraction:
-Ca2+ released from SR bond to troponin
-troponin moves tropomyosin away from myosin binding sites on actin
-allows actin/myosin interaction (crossbridges to form)
Summary is on slide 24 - Contraction:
- thin filaments slide past thick filaments
- sarcomeres shorten
- know the diff between isometric and isotonic contractions slide 27
- understand the crossbridge cycle slide 28 - Relaxation- how to make it stop
-Crossbridge cycling persists as long as cytoplasmic Ca2+ is high
Cytoplasmic Ca2+ drops–> Ca2+ dissociates from troponin –> tropomyosin covers myosin binding sites on actin
-cytoplasmic calcium concentration depends on balance between Ca2+ release from and Ca2+ uptake to the SR
Slide 31 remember it!
What are the factors affecting force production?
Muscle twitch: bare minimum–> mechanical sponge of muscle cell, motor unit, or whole muscle to a single action potential. Slide 34
1. Frequency of stimulation
-muscle fibres can fire many APs during single twitch
-repeated APs release Ca2+ faster than it can be taken up into the SR
-higher frequency AP firing–> more frequent Ca2+ release from the SR –> high level if cytoplasmic Ca2+–> more crossbridges cycling–> greater force
2. Fibre diameter:
-force generation capacity not related to number of sarcomeres in series (longer muscles aren’t stronger)
-force-generating capacity depends in number of sarcomeres in parallel (fibre diameter)
Ie bulkier muscle are stronger
3. Resting muscle length
-force o muscle contraction depends on length of muscle before contraction starts (how stretched it is)
-stret hung changes overlap between actin/myosin filaments
How is a voluntary muscle contraction carried out. In detail
- Neuron in motor cortex trigger action potentials in upper motor neurons
- These action potential spread down the white matter of spinal cord along axon of the upper motor neurons and trigger transmitter release from nerve terminals.
- This transmitter triggers an AP in a lower motor neuron
- The axon of lower motor neuron leaves spinal cord and forms a synapse with a skeletal muscle fibre (neuromuscular junction)
- When AP reaches nerve terminal, triggers release of neurotransmitter acetylcholine (Ach)
- Ach binds to nicotonic cholinergic receptors on the skeletal muscle membrane, producing an end plate potential.
- The EPP triggers action potential firing in the muscle cell, which in turn releases Ca2+ ions from the sarcoplasmic reticulum of muscle fibre and initiates contraction.
