WEEK 9 - MUSCLE TISSUE Flashcards
MUSCLE TISSUE
muscle tissue
- primary function of muscle - generate force or movement
- three types of muscle :
- skeletal muscle - striated, voluntary movement for locomotion, controls breathing cycle via diaphragm
- cardiac muscle - striated, involuntary, specific to the heart
- smooth muscle - unstriated, involuntary, mechanical control of organs
e.g. digestive and urinary tracts, blood vessels and airway passages of the lungs
terminology for muscle tissues
- Muscle cells – fibres
- Cytoplasm – sarcoplasm
- Cell membrane - sarcolemma
skeletal muscle fibres
- striated, fast contraction, voluntary control, many nuclei/cells, contraction controled through nerves and its function is body movement
- Giant cells/fibres –up to 30cm long & 100μm diameter
- Comprised of multiple myofibrils
- Elongated cylindrical contractile threads
- Gives a striated appearance, light (I) & dark (A) bands
- Multi-nucleated
- Nuclei – flattened, located on fibre periphery, just inside the sarcolemma
skeletal muscle organisation
- Muscle → muscle fibre bundle → muscle fibre → myofibrils → myofilaments
- Myofilaments
- Thick –primarily myosin
- Thin –primarily actin
cardiac muscle
- striated, intermediate contractions, involuntary control, single nuclei/cells, spontaneous contractions (modulated by nerves) and functions cardiac contractions
- Cardiac fibres
- Cylindrical cardiac myocytes with single central nucleus
- Long chain of myocytes, branch and intertwine to connect with other cardiac myocytes
- linked by cell junctions –intercalated discs
- Gap junctions -link cells electronically; action potentials
- Desmosomes –link cells mechanically
- Contraction result of electrical stimulation from pacemaker (also known as sinoatrial node)
- Striated –contains thick and thin filament
sarcomere
- Sarcomere –repeating unit between z disks
- Give skeletal & cardiac muscle striped/striated appearance
- Myofibril –sarcomeres stacked end to end
- Relaxed –little over lapping of actin and myosin filaments –large H band
- Contracted –vast overlapping –small H band
- z disks both contained in cardiac and skeletal
- H bands are the area (distance) between actin filaments, the width of an H band will vary depending on if the muscle is undergoing contraction or relaxation.
Ca2+ - key to unlocking muscle contraction
- Calcium (Ca2+) is stored in sarcoplasmic reticulum (SR)
skeletal muscle :
* axon terminal creates a junction with the skeletal muscle which will have lots of receptors
* neurone release neurotransmitter acetylcholine (ACH) which travels down the terminal
* ACH binds to nicotinic receptors on muscle that is coupled to an ion channel that when ACH binds with the receptor the channel will open
* influx of sodium and calcium ions
* influx of sodium causes an increase in membrane potential which will go along the sarcolemma until it reaches the T tubule
* spread of increase in membrane potential causes the activation of voltage-gated calcium channel on the sarcoplasmic reticulum to open and release calcium ions (calcium-calcium induced release)
* calcium ions get released into the sarcoplasm and binds to troponin which will cause tropomyosin to undergo a conformation change (shape change)
*at rest tropomyosin covers the myosin and actin binding points but when calcium binds, tropomyosin moves out the way
* actin and myosin can interact with each other allowing contractions to take place
* for relaxation to occur calcium pumps pump calcium ions back into the sarcoplasmic reticulum
Ca2+ - key to unlocking muscle contraction pt2
Cardiac myocytes–gap junctions :
* pacemaker is going to send an electrical chemical signal which will pass along the sarcolemma and pass through gap junctions from one myocyte to the next otherwise the electrochemical signal will spread along the sarcomere
* spread of depolarisation going around the outside which will lead to the activation of any voltage gated calcium channels
* depolarisation will also spread down the tubule (similar to skeletal muscle) and will cause the activation of its calcium gated channels
* the calcium that comes into the site from the extracellular fluid will act in the same way as skeletal muscle and will bind to the calcium channels and activate the calcium-induced calcium released
* sarcoplasm releases flood of calcium ions which will bind to troponin (same as skeletal)
* to go back into relaxation, activation of pumps pump against conc gradient through the assistance of ATP and pump calcium ions back out into the sarcolemma and some into the extracellular fluid
muscle contraction - skeletal and cardiac
at rest :
* tropomyosin is lying over region where the myosin would attach itself to action so they can’t react
at contraction :
* as calcium conc starts to increase it binds to the troponin creating a calcium troponin complex causing tropomyosin to shift (conformational change)
* myosin can bind to actin causing the actin filament to move and contractions to take place
Cross-bridge cycle in skeletal & cardiac muscle
- relaxed state, ATP binds to myosin head, causing the dissociation of the actin-myosin complex
- ATP is hydrolysed into ADP + Pi, causing myosin head to return to their resting conformation state
- a cross-bridge forms and the myosin head binds to a new position on actin further along
- a Pi (phosphate) is released, enable a strong cross-bridge to occur
- a conformational change in the myosin head causes the power stroke. the actin and myosin filaments slide past each other, narrowing the H band
- ADP is released and ATP can bind to the myosin again
- cycle will continue as long as there is enough ATP and that calcium levels are high enough so that tropomyosin is moved out the way
smooth muscle
- not striated, slow contraction speed, involuntary control, single nuclei/cell, contraction through nerves/hormones/stretch and its function is visceral and circulatory
- Fibres are spindle shaped
- Large central nucleus
- Fibres are not striated
- Shorter fibres than skeletal fibres
- Cells occur individual fibres or in groups as sheet or bands
- Gap junctions – spread electrical excitation
Actin & myosin in smooth muscle
- Filaments irregularly arranged
- Actin attached to dense bodies in sarcoplasm & sarcolemma
- Myosin filaments between actin filaments
- Whole cell contracts
- due to anchored actin
Smooth muscle contraction
- intermediate filaments and protein dense bodies form a cytoskeleton. Actin attaches to the dense bodies. Each myosin molecule is surrounded by actin filaments
- smooth muscle myosin has hinged heads all along its length and the myosin filaments in smooth muscle are longer than that in skeletal muscle. This is unique to smooth muscle as it enables it to stretch while still maintaining enough overlap to create optimum tension
- important for organs such as bladder
- calcium enables the contraction by binding to calmodulin and then calmodulin will activate myosin light chain kinase (MLK) which will phosphorylate myosin and that will increase the ATPase activity
During depolarisation
Na+ enters the cell
Depolarisation is when the membrance potential becomes more positive. Remember sodium starts/stimulates an action potential. Na+ is a positive ion so to make the membrane potential more positive it needs to enter the cell by passing down it’s concentration gradient.
Na+ channels close and K+ channels open in
Repolarisation
When K+ channels open, K+ will leave the cell as it passes down it’s concentration gradient. As K+ is a positive ion the membrane potential will fall/plummet. Repolarisation is returning to the resting potential!
The sarcolemma of a muscle fibre is
cell membrane
Actin, myosin, ATP and Ca2+ are essential for
muscle contraction
What relays electrical stimulation
Gap junctions
Put the following in the correct order for the series of events in an action potential
- Initial local stimulus
- Threshold potential reached
- Depolarisation
- Repolarisation
- hyperpolarisation
- return to resting potential
