Skeletal Muscle Flashcards
What do the the muscles do?
- they allow movement = walking
- protect the viscera = body wall
- heat production = 37 degrees Celsius
- posture, fascial expression
What are the characteristics of muscle?
- excitable = respond to stimuli and produce ATP
- contractile = can shorten and thicken
- extensible = they stretch when pulled
- elastic = can go back to original after contraction and extension
In a neuromuscular junction, how many neurone(s) innervates each muscle fibre(cell)? Explain.
- one
- the axon of a motor neurone branches out to innervate several muscle fibres
How many muscle fibres can a singular motor neurone innervate?
- about 150 muscle fibres within the same whole muscle
What is a motor unit?
- a single neurone + all the muscle fibres it innervates
Explain the structure of a motor unit.
- presynaptic cell (neurone) that releases nt = ACh in vesicles
- postsynaptic cell (muscle) membrane (sarcolemma) = specialized with ACh
- the two membranes are separated by a synaptic cleft
Explain the steps of impulse transmission at a neuromuscular junction.
1) AP reaches the axon terminal and the synaptic end bulb
2) the voltage Ca2+ gates open and Ca2+ diffuse in
3) Rising Ca2+ causes exocytosis of nt AcH
4) AcH binds to AcH receptors on the motor end plate of the muscle cell
5) chemical gates Na+ open and Na+ diffuse in causing EPP (End Plate Potential = a depolarizing GP)
6) EPP causes Na+ voltage gates open on adjacent sarcolemma –> more Na+ diffuse in causing an AP
7) AP travels long the sarcolemma
In a neuromuscular junction, will an AP occur always? why
- yes because it’s stimulus is always a critical stimulus due to the large concentration of AChr eleased accompanied by the abundant amount of ACh receptors found on the motor end plate.
So how can the skeletal muscle be inhibited?
- by inhibiting the motor neurone
In a resting muscle, what is the state of the filaments in the muscle fibre?
- tropomyosin covers the myosin binding site on the actin
- this activates the myosin head
In a myosin head activation, what happens.
- this is still in the resting phase of the muscle,
- ATP is found on the myosin head which later breaks down to form ADP and release energy.
- the energy, ADP and Pi however remains in the myosin head.
Generally, what happens when the AP or impulse reaches the muscle fibre?
1) Excitation of muscle fibre (electrical event)
2) Excitation-contraction coupling (electrical to mechanical event)
3) Contraction or the Sliding Filament Mechanism (mechanical event)
In details, explain the excitation of the muscle fibre.
- the sarcolemma gets depolarized ( EPP -> AP)
- AP travels down to the T-tubules to deep within the fibre
In details, explain what happens in the Excitation-Contraction coupling
- the AP reaches the T-tubules causing the release of Ca2+ via mechanical gates found on the terminal cisternae of the sarcoplasmic reticulum
- the Ca2+ binds to the troponin
- the troponin-tropomyosin complex moves and exposes the myosin binding site on the actin
In details what happens in the Sliding filament mechanism?
- the activated myosin head attaches to the myosin binding site on the actin, as a result a cross bridge formation occurs (myosin head becomes the bridge between the myosin and the actin)
- the energy stored in the activated myosin head is released when the myosin head pivots (called power stroke), this causes the ADP and Pi to be released as well. As a result, the actin moves over the myosin towards the center of the sacromere (M-Line)
- ATP attaches to the myosin head causing it to unpivot and detach from the actin (called recovery stroke)
- the myosin head gets reactivated again and the cycle continues as long as there is high concentration of Ca2+ in the cytosol.
To shorten the sarcomere, the cycle has to happen _____ times.
many
What is the power stroke?
when the activated myosin head releases it’s energy, ADP + Pi by pivoting
What is a recovery stroke?
when ATP attaches with the myosin head to detach and unpivot it from the actin
Describe the state of the sarcomere during the Sliding Filament Mechanism
- in a sarcomere:
- the H zone and the I band shorten
- A band remains the same length
- the thick (myosin) and the thin (actin) myofilaments remain the same length
When the myofibril shortens during the sliding filament mechanism, what happens to the muscle?
- it shortens as well
Explain the steps to muscle fibre relaxation.
1) ACh is broken down by the AChE and produces acetic acid (used in Krebs cycle as Acetyl CoA) and choline which is recycle
2) The sarcoplasmic reticulum takes up Ca2+ via Ca2+ ATPase
3) the tropomyosin covers the myosin binding site on the actin
4) ATP binds to the myosin head
What is the role of ATP in skeletal muscles?
1) fuels up Na+/K+ ATPase
2) pumps Ca2+ into to the sarcoplasmic reticulum via Ca2+ ATPase
3) deactivates cross bridge
4) Activates myosin head
What happens with improper canning? And what causes this?
- botulism
- caused by clostridium botulinum which is very potent
How can botulism affect skeletal muscle?
- prevents exocytosis of ACh which causes flaccid paralysis
what is the medical use of botulism?
treat uncontrolled blinking eyes and crossed eyes
What is the cosmetic use of botulism?
- botox (wrinkles and sweating)
What is muscle tension?
- the force exerted by a muscle or a muscle fibre
What determines muscle tension?
number of cross bridges formed
Muscle tension in a muscle fibre is affected by:
1) Frequency of Stimulation
2) Fatigue
3) Fibre length
4) Size of fibre
What are some factors that affect the frequency of the stimulus?
a) Single Stimulus
b) 2nd stimulus arrives before complete relaxation of 1st stimulus
c) Rapid sequence of stimuli
d) High frequency of stimuli
In a single stimulus, what happens in muscle tension
- single stimulus = causes twitch (not normally occurring)
- for 1 stimulus = 1 AP fires up which lasts 1-2 msec
- latent period comes which lasts for about 2msec = excitation-contraction coupling
- contraction period which lasts about 10-100 msec = muscle tension increases due to cross bridges attachment & sliding filaments
- relaxation = decreases muscle tension
After the single stimulus, what comes next?
second stimulus appears before the first stimulus completely relaxes
- the first stimulus has not relaxed completely because uptake of Ca2+ in SR has not been finished
- the second stimulus increases availability of Ca2+ which causes anther contraction
- this is called wave summation
Is there refractory period in contractions?
-no
How does rapid sequence of stimuli affect muscle tension?
- it increases muscle tension because it increases availability of Ca2+ which causes wave summation
What is the partial relaxation that occurs between contractions and causes quivering?
incomplete tetanus
what causes high frequency of stimuli?
- all troponin have been saturated with Ca2+ and fibre warmth (from ATP synthesis) = works faster
what is a complete tetanus?
no relaxation in between muscle contractions
How does fibre length affect muscle tension?
- resting fibre length is optimal because max number of crossbridges upon stimulation will maximize tension
- tension decreases with shorter or longer fibre
= short fibre = the thin filaments overlaps disrupting the the crossbridge attachment (min length = 70% of optimal)
= long fibre = not all myosin head attaches with the binding sites on the actin (max length = 130% of optimal0
How does size of fibre affects muscle tension?
- the more myofibrils = the thicker the fibre is
- the thicker the fibre = the greater the tension is
How can muscle fibre size be increased?
- exercising and testosterone
How does fatigue affect muscle tension?
- muscles cannot contract because theres no ATP and low Ca2+ which causes 0 tension.
What kind of fibre is present in muscle that contracts an relaxes rapidly?
- white fibre with little myoglobin
What kind of fibre is present in muscle that contracts slowly and relaxes slowly? give an example
- red fibre with more myoglobin
- postural muscle
In a whole muscle, what affects muscle tension?
1) number of fibres contracting
2) number of fibres/motor unit
3) muscle size
4) fatigue
How does the number of fibres contracting affect muscle tension?
- more number of fibres contracting the more fibres/motor unit = increased muscle tension
How does number of fibres/motor unit affect muscle tension?
- increases muscle tension
which motor unit will have greater tension, 1 neurone innervated to 10fibres or 1 neurone innervated to 1000 fibres?
1 neurone innervated to 1000 fibres
how does muscle size affect muscle tension
- the bigger the muscle, the more fibres contrasting = greater muscle tension
what causes muscle tone?
when low level of tension in a few fibres become a separate group of motor units are stimulated alternately overtime
what does muscle tone do?
give firmness to muscle
What are two types of muscle contraction?
isotonic & isometric
What is an isotonic muscle contraction? give an example
- uses ATP
- muscle changes in length
- tension exceeds the resistance of the load lifted
- flexion of elbow = tension > weight of forearm
What is an isometric muscle contraction?
- uses ATP
- the tension is lower than the whats required to move load
- tension increases because of cross bridge formation but no shortening (no changes muscle length)
when lifting a book what muscle contraction is taking place?
- isotonic
When holding a book, what muscle contraction is working?
isometric
Where does the muscle get its energy during resting conditions
- fatty acids used to make ATP (aerobic)
- use storage of glycogen and creatine phosphate
Where does the muscle get its energy during short term exercise. eg. 1 minute or less like sprinting.
- uses creatine phosphate to produce ATP (lasts about 15 secs)
- uses glycogen –> glucose –> pyruvic acid –> via anaerobic pathway –> lactic acid (lasts about 30 sec)
- mostly anaerobic reaction is used
Where does muscle get its energy during long term exercise (1 min to hours)?
- ATP - from aerobic pathway
- glucose in liver
- fatty acids - used more as exercise continues
- O2: from blood hemoglobin and muscle myoglobin
- but sometimes they get energy from anaerobic pathways n
What are the kinds of muscle fatigue?
Physiological Fatigue & Psychological Fatigue
What is physiological fatigue and what does it do?
- the muscle’s inability to maintain tension - not widely understood
- decreases ATP therefore crossbridges can’t be released.
What causes physiological fatigue?
1) depletion of sources of energy such as glycogen
2) the build of end products
- H+ from lactic acid - muscle contractions compress blood vessels which decreases O2 = therefore remains anaerobic for periods even in long term exercise
- Pi from ATP binds to Ca2+ = less binds with the troponin and this slows down the release of phosphate from myosin and slows crossbridges release
3) Failure of APs
- increase in K+ in small T-tubules during rapid stimuli disrupts the MP, which stops the release of Ca2+ from the SR
- long term: neurone runs out of ACh
What is psychological fatigue?
the failure of CNS to send commands to muscles probably due to lactic acid
what does EPOC stand for and what is it?
- Excess Post-exercise oxygen consumption
= recovery from O2 consumption (deep rapid breathing)
What is O2 used for?
- replenish storage of glycogen, creatine phosphate, O2 in hemoglobin and myoglobin
- used convert lactic acid to pyruvic acid in Krebs cycle, or lactic acid to glucose in liver
increase of body temperature from exercise equals _________
increase in O2 demand
