Structure and function of muscles Flashcards
What are the 3 types of muscles?
Skeletal muscles
- locomotion & posture
- voluntary control
- multi-nucleated
- striated (striped appearance)
Smooth muscles
- lines blood vessels & organs, & controls diameter of blood vessels
- involuntary controlled
- bi-nucleated
- non striated
Cardiac muscles :
- heart contraction
- involuntary control
- uni-nucleated
- striated
What is the function of muscles?
- Movement
- providing support & protection of organs
- Maintain posture
General structure of muscles? What is sarcolemma?
Muscles contain several fascicles
- muscles are covered in thin layer of epimysium
fascicles contain several muscle fibres
- fascicles are bound by perimysium
Muscle fibres contains several myofibrils
- Muscle fibres are bound by endomysium
- contain several thick & thin filaments
Describe the structural components of myofibrils
Contain thin & thick filaments which go across fibre & cause contraction
Thin filament= actin
Thick filament= myosin
Have lots of nuclei & mitochondria for ATP
Sarcolemma- membrane of muscle fibres- has many receptors & channels on its surface
T-tubules- tubes that run around outside of myofibrils- made of sarcolemma
- Along its length, the tubule associated w/ 2 terminal cisternae- specialised regions of sarcoplasmic reticulum
- T tubule + 2 terminal cisternae = triad junction
Sarcoplasmic reticulum- surrounds t- tubules- contains Ca ions
Sarcomere:
- a repeating unit on a myofibril
- it is the distance of 1 z line to another on a myofibril
- z line goes across fibre & thin filaments are inserted here
- M-line= Contains thick filaments
- A-band= where thick and thin filaments overlap
- I-band= Only contains thin filaments
- H zone- thick
Sarcoplasm- cytoplasm of muscle fibre- contains ions; potassium, magnesium & phosphate, enzymes & mitochondria.
NOTE: view diagram for structure of sarcomere on notes- very important!
https://www.notion.so/PBL-week-11-92f7a1365c35422b8d2810696c535658
Describe the structure of myosin.
Contains:
- myosin head for actin binding
- hinge for head movement
- 2 heavy intertwined chains
- 2 alkali light chains to stabilise myosin head
- 2 regulatory light chains for ATPase activity
Describe the structure of actin.
have actin binding site- myosin head binds to actin
also contains tropomyosin- blocks myosin receptors
tropomyosin have protein complex of troponin, C, I & T
- troponin= controls tropomyosin position
- C= binding of Ca2+ to high affinity sites causes conformation change in troponin complex
- I = moves away from active filament
- T= pushes tropomyosin away from myosin binding site on actin
What is a motor unit? What is a neuromuscular junction?
Neuromuscular junction- point where neurone & muscle connect
Motor unit- Group of muscle fibres innervated by branches of a single motor neuron
How does a nerve impulse lead to muscle contraction? i.e. excitation-contraction coupling
Muscles only contract when an A.P reaches the axon terminal (end) of a motor neuron
- Axon terminal of neurone communicates w/ muscle fibres- neuromuscular junction
Before contraction
1. Action potential causes acetylcholine release
- Acetylcholine released from axon terminal binds to receptors on sarcolemma
- Receptors open, Na ions come inside & open voltage gated L-type Ca²⁺ channels on t tubules - causes action potential across sarcolemma which travels down t tubule
- Ca released from sarcoplasmic reticulum into sarcoplasm of myofibril in response to changes in voltage
- Ca binds to troponin- cross-bridges form between actin & myosin- activates contraction
After contraction:
6. Acetylcholinesterase removes acetylcholine from synaptic cleft
- Ca is transported back into sarcoplasmic reticulum
- Tropomyosin binds active sites on actin causing cross-bridge to detach
Explain the process of contraction i.e. cross-bridge cycling
- When relaxed, myosin heads are not attached to actin
- During contraction, myosin heads become attached to actin filaments
- ATP is hydrolysed by ATPase on myosin head which provides energy for myosin head to tilt & drag actin fibre w/ it- called power stroke.
- This leaves ADP at binding site & extra phosphate is released.
- The filaments slide past each other from this- sliding filament theory - There is a greater overlap of thin filaments & thick filaments
- This brings the Z-lines closer together = shortening = contraction
NOTE: look at diagram on notes! v important!
How does Ca²⁺ initiate cross-bridge cycling?
- Ca²⁺ binds to Troponin C causing conformational change in troponin complex (which includes Troponin C, I, T & Tropomyosin).
- Troponin I moves away from actin filament.
- Troponin T pushes tropomyosin away from myosin binding site on actin.
- Frees up binding site, allowing Myosin head to bind to actin.
NOTE: https://www.notion.so/PBL-week-11-92f7a1365c35422b8d2810696c535658
How does termination of a contraction occur?
Ca²⁺ must be removed from the sarcoplasm for contraction to cease and relaxation to occur.
3 methods:
1. SERCA-type Ca pump. - involves active transport of Ca across the sarcoplasmic reticulum. Uses Ca-ATPase. MAJOR
- Na/Ca exchanger - leads to facilitated diffusion across the sarcolemma membrane & into extracellular space i.e. completely leaves cell. MINOR
- Ca-ATPase active transport - this moves Ca²⁺ across sarcolemma membrane & into extracellular space. MINOR
Process:
1. Na-Ca exchanger and calcium pump in membrane force out calcium from cell
2. Calcium pump hides the calcium in sarcoplasmic reticulum
3. Calcium is bound to sarcoplasmic reticulum by calreticulin and calsequestrin
What are two types of contractions?
Isometric- when muscle contracts but no shortening of limb (maintaining posture)
Isotonic- muscle shortens during contraction
What are the different types of muscle fibres and their properties
- Red fibres - slow twitch - use oxygen & don’t fatigue easily.
- high mitochondria
- low glycogen - White fibres - fast twitch - use glycogen & fatigue easily.
- Less mitochondria
- high glycogen - Intermediate fibres - intermediate fast twitch has a mixture of properties from red & white muscle.
