Skeletal muscle physiology Flashcards
What are the 5 main functions of skeletal muscle?
- Maintenance of posture
- Purposeful movement in relation to external environment
- Respiratory movement
- Heat production
- Contribution to whole body metabolism
What are the 3 types of muscle?
Cardiac
Smooth
Skeletal
Which types of muscle are striated?
Skeletal
Cardiac
Which types of muscle are involuntary?
Smooth
Cardiac
Which type of muscle is voluntary?
Skeletal
What causes striation of muscle?
Alternation of dark bands (Myosin filaments) and light bands (Actin filaments)
Where does skeletal muscle receive its calcium for excitation contraction coupling?
Entirely from the sarcoplasmic reticulum
Where does cardiac muscle receive its calcium for excitation contraction coupling?
ECF and sarcoplasmic reticulum (Calcium induced calcium release)
What is meant by a motor unit?
This is a single alpha motor neurone and all the skeletal muscles fibres that it innervates
How many muscle fibres will be contained in a motor unit with fine movement requirements?
Only around 10
How many muscle fibres will be contained in a motor unit with power movement requirements?
100s to 1000s
What are some examples of muscles with fine motor requirements?
External eye muscles
Muscles of facial expression
Intrinsic hand muscles
What is meant by a muscle fibre?
One single muscle cell
How is a muscle fibre organised?
It is a bundle of myofibrils surrounded by a sarcoplasm, containing multiple nuclei and many mitochondria, all surrounded by a sarcolemma
What is the smallest contractile unit of the myofibril?
Sarcomere
What are the 5 zones of the sarcomere?
A-band
H-zone
M-line
I-band
Z-line
What is the A band?
This is the overlap between thick and thin filaments
What is the H zone?
This is the section containing only myosin thick filaments
What is the M line?
This is the line that passes vertically down the H zone at the centre of the thick filaments
What is the I band?
This is the section containing only actin thin filaments
What is the Z line?
This is the vertical line connecting the actin filaments
What is the sliding filament theory?
The theory that muscle tension is produced by the sliding of actin filaments over the myosin filaments
Describe the contraction stages of the sliding filament theory
Ca2+ binds to troponin, on tropomyosin fibres (Wrapped around the actin filaments), causing a conformational change allowing the uncovering of myosin binding sites on the actin filament
Ca2+ also causes excitation allowing the binding of myosin cross-bridges to myosin binding sites on actin
The actinomyosin cross-bridge then bends, via breakdown of ATP, allowing for a “power stroke” causing the release of ADP and Pi, as well as energy
Describe the relaxation stage of the sliding filament theory
ATP then binds to the cross-bridge, allowing for detachment of the cross-bridge from actin
The breakdown of this ATP into ADP and Pi then allows for resetting where the cross-bridge returns to its normal place
Ca2+ is actively taken up by the sarcoplasmic reticulum when there is no longer an action potential, meaning that tropomyosin can return to its normal place, preventing myosin binding to actin filaments
What causes rigour mortis in death?
Rigor mortis is caused by a lack of ATP due to cessation of metabolism, meaning that myosin cross-bridges cannot detach from actin, and Ca2+ cannot be pumped back into the sarcoplasmic reticulum, so muscle remain contracted
What is meant by excitation contraction coupling?
This is the process whereby the surface action potential results in activation of the contractile structures of the muscle fibre
Describe the process of excitation contraction coupling in skeletal muscle fibres
Ca2+ is released from the lateral sacs of the sarcoplasmic reticulum when the surface action potential spreads down the transverse T-tubules
What are T-tubules?
These are extensions of the surface membrane that dip into the muscle fibre
What are the 2 primary factors that affect skeletal muscle tension gradation?
Number of muscle fibres contracting
Tension developed by each contracting muscle type
What is meant by asynchronous motor unit recruitment?
This means that while some motor units are contracting, others aren’t, resulting in a sub-maximal contraction, protecting against muscle fatigue
What factors affect the tension developed by each contracting muscle fibre?
Frequency of contractions
Summation of contractions
Length of muscle fibre
Thickness of muscle fibre
What is meant by twitch summation?
In skeletal muscle, the duration of action potential is much shorter than the duration of the resulting twitch
It is therefore possible to summate twitches to bring about stronger contraction through repetitive fast stimulation of skeletal muscle
This is because the latent period of the action potential will have passed before the twitch ends
What will occur if a muscle fibre is re-stimulated after it has completely relaxed?
The second resulting twitch will be of the same magnitude as the first, as there is no summation of contractions
What will occur if a muscle fibre is re-stimulated before it has completely relaxed?
The second twitch is added onto the first twitch, resulting in summation, and thus a greater level of contractility
What will occur is the muscle fibre is stimulated so rapidly that it does not have an opportunity to relax between stimuli?
A maximal sustained contraction known as tetanus will occur
What happens after a prolonged period of tetanus?
Tetanus cannot be sustained for long periods of time and fatigue will begin, unless the stimulus is stopped, meaning that contractility will decrease
Why can cardiac muscle not be tetanised?
Cardiac membrane potential has a much longer refractory period
What is meant by optimal length for maximal tetanic contraction?
This is the optimal length of a muscle so that there is an adequate overlap between myosin and actin, thus that maximal tetanic contraction can occur
Why does percentage of maximal tetanic contraction decrease if the muscle fibre is too long?
Fewer thin-filament binding sites are accessible for binding with thick filament cross bridges
Why does percentage of maximal tetanic contraction decrease if the muscle fibre is too short?
Fewer thin-filament binding sites are exposed to thick filaments because the thin filaments overlap with each other
What are the 2 components of a skeletal muscle?
Contractile component
Series-elastic component
What makes up the contractile component of the skeletal muscle?
The cross-bridge cycling with sarcomeres
What makes up the series-elastic component of a skeletal muscle?
Transmission of tension to bone via the stretching and tightening of muscle connective tissue and tendons
What are the 2 main types of skeletal muscle contraction?
Isotonic contraction
Isometric contraction
What are the uses of isotonic muscle contractions?
Body movements and movement of objects
What are the uses of isometric muscle contractions?
Supporting objects in fixed positions
Maintaining body posture
What is meant by isotonic contraction?
Muscle tension remains constant as muscle length changed
What is meant by isometric contraction?
Muscle tension develops at constant muscle length
What are the 3 main differences between types of skeletal muscle fibre?
Enzymatic pathways for ATP synthesis
Resistance to fatigue
Activity of myosin ATPase
What effect does myosin ATPase have on muscle contraction?
It determines the speed at which energy is made available for cross-bridging
What is the usual number of types of muscle fibres contained in 1 motor unit?
1
What are the 3 types of skeletal muscle fibre?
Slow oxidative (Type I)
Fast oxidative (Type IIa)
Fast glycolytic (Type IIx/IIb)
What are the 3 main metabolic pathways for ATP production
- Transfer of high energy phosphate from creatinine phosphate to ADP to form ATP
- Oxidative phosphorylation (aerobic)
- Glycolysis - Pyruvic acid to lactic acid (Anaerobic)
What are the characteristics and uses of slow oxidative skeletal muscle fibres?
Slow contraction speed
High resistance to fatigue
High capacity for oxidative phosphorylation
Many mitochondria
Low myosin ATPase
Used in prolonged, low work, aerobic activity (e.g. maintenance of posture, walking)
What are the characteristics and uses of fast oxidative skeletal muscle fibres?
Fast speed of contraction
Intermediate resistance to fatigue
Intermediate levels of enzymes fro anaerobic glycolysis
High level of ATPase activity
High oxidative phosphorylation capacity
Used for both aerobic and anaerobic activity and are useful in prolonged relatively moderate work (e.g. jogging)
What are the characteristics and uses of fast glycolytic skeletal muscle fibres?
Fast speed of contraction
Low resistance to fatigue (Lactic acid)
low oxidative phosphorylation capacity
High levels of anaerobic enzymes
Used in anaerobic activity such as short term, high intensity exercise (e.g. jumping)
What is a reflex?
A stereotyped response to a specific stimulus
What is the purpose of a stretch reflex?
This serves as a negative feedback that resists passive change in muscle length to maintain optimal resting muscle length
Describe the pathway of the patellar tendon reflex?
When a hammer hits the patellar tendon, the sensory receptor is the muscle spindle itself and is activated by the muscle stretch
This causes the firing of the afferent neurones which synapse in the spinal cord with the alpha motor neurones that innervate the stretched muscle
Activation of the reflex results in contraction of the stretched muscle, in this case, the quadriceps femurs
What are muscle spindles?
Muscle spindles are a collection of specialised muscle fibres that act as the sensory receptors for stretch reflex
Where are muscle spindles found?
These are found within the belly of the muscles and run parallel to ordinary muscle fibres
How are muscle spindles specialised?
They have sensory nerve endings known as annulospiral fibres which increase firing as the muscle is stretched
What is the motor supply to the muscle spindles called?
Gamma motor neurones
How do gamma motor neurones affect muscle spindles?
They just the level of tension in the muscle spindles to maintain their sensitivity to stretch (Stop them going loose)
