Module 4: Muscle Physiology Flashcards
Identify the different levels of organization of skeletal muscle
Muscle
Muscle fibre
Myofibrils
T/F
Skeletal muscle fibres run the entire length of the muscle
True. woohoo
Muscle fibres run surrounded by what?
Connective tissue
T/F
Muscle fibres (muscle cells) have many nuclei and mitochondria
True! They have lots of both
Muscle fibres are divided into contractile elements called what?
Myofibrils
Describe what a myofibril looks like on the side
Have a pattern of light and dark bands creating its striated pattern
Thick filaments are also known as what?
Myosin
Thin filaments are also known as what?
Actin
List the components of myofibrils
A band
I band
H zone
M line
Z line
The A band is also known as what?
Dark band
What is the A band made up of?
Stacked thick and thin filaments
The border of the A band is defined by what?
The length of the thick filament
T/F
The middle of the A band is darker than the rest. Justify your answer
False! It is lighter.
The thin filaments don’t reach this far from the ends. So there is no overlap of thick/thin to make it look dark
The I band is also known as what?
Light band
What is the I band made up of?
The portion of thin filaments that do not extend into the A band
Describe the H zone
The slightly lighter portion of the A band
What is the H zone composed of?
Only the proteins that hold the thick filaments together in a stack. Contains only the heavy chains of myosin
What is myosin composed of?
2 heavy and 2 light chains
Describe the M line
Proteins that hold the thick filaments together in a stack
The M line runs down what portion of a myofibril?
Down the centre of the H zone
Describe the Z line
The vertical line located in the middle of the I band
The distance from one Z line to the next represents what?
One sarcomere
What is a sarcomere?
The functional unit of skeletal muscle
When the muscles are growing, how do they extend in length?
New sarcomeres are added onto the ends
Myosin is what kind of protein?
A motor protein
Describe the structure of a myosin molecule
A dimer with two subunits
Describe the structure of the subunits in a myosin molecule
Looks like a golf club: has a long shaft and globular head
What occurs when myosin dimers come together
The shaft/tail portions of the two dimers wrap around one another and stack with other myosin molvecules
The head of myosin sticks out and contains what two important sites?
An actin binding site and a myosin ATPase site
Thin filaments are made up of what proteins?
Actin, tropomyosin, and troponin
Describe the main structural component of thin filaments
2 actin filaments
Describe the structure of actin filaments
Made up of individual spherical actin molecules that come together to form a double helix structure
Describe the structure and location of tropomyosin
A thin, double helix protein that lies end to end along the actin helix structure
Describe the function of tropomyosin
A regulatory protein that covers the active binding sites, preventing the interaction of actin and myosin
Describe the function of troponin
A regulatory protein complex that binds to tropomyosin, actin, and calcium ions
What makes up troponin?
Made up of 3 polypeptides
What makes up the basis of the sliding filament mechanism?
The cross-bridging between myosin and actin
In simple terms, what occurs during the contraction of a skeletal muscle regarding the myofibrils? What is another word for this type of contraction?
The thin filaments move inwards over the thick filaments
Concentric contraction
What happens to the different parts of the myofibril during concentric contraction?
The A band stays the same width
The I band shortens
The H zone shortens
The sarcomere shortens
What does the power stroke refer to?
The interaction between myosin and actin that leads to a shortening of the sarcomere
When does a power stroke occur?
Occurs when the cross-bridge bends, pulling the thin myofilaments inwards toward the centre of the thick filament
List the 4 steps of the cross-bridge cycle
Binding
Power Stroke
Detachment
Binding
Describe the binding(1) phase of the cross bridge-cycle
Myosin cross-bridge binds to actin molecule
Describe the power stroke phase of the cross bridge-cycle
The myosin head binds, pulling the thin myofilament inwards
Describe the binding(2) phase of the cross-bridge cycle
The cross-bridge binds to more a more distal actin molecule and the cycle repeats
Each myosin molecule is surrounded by how many actin molecules on each end?
6 actin molecules
T/F?
During any given time, all cross-bridges actively pulling actin
False.
Some are just holding the actin in position while others prepare for the next power stroke
What is excitation-contracting coupling?
The process of converting an electrical signal into an actual contraction
What membrane structures do skeletal muscle haves that help transmit electrical signal to the muscle fibres?
The sarcoplasmic reticulum (SR)
T-tubules
The SR is called what in non-muscle cells?
The endoplasmic reticulum (ER)
T/F
The sarcoplasmic reticulum is a membranous structure
True
T/F
The sarcoplasmic reticulum runs perpendicular to the fibres?
False
It runs parallel to the muscle fibres
What is stored in the sarcoplasmic reticulum?
Calcium ions
What is the full name for T-tubules?
transverse tubules
What are T-tubules?
Invaginations of the plasma membrane
At the junction of the A and I bands, what do T-tubules do?
They dip into the fibre and run perpendicular to the fibres
What is the relationship between T-tubules and the SR in terms of depolarization
- When the plasma membrane depolarizes, this wave of depolarization goes deeper into the cells by spreading down the T-tubules
- Because they are in close proximity, this electrical impulse is also transmitted from the T-tubule to the SR
What is on the surface of T-tubules. What kind of receptors are they? What do they do?
Dihydropyridine receptors
Voltage-gated
Sense the wave of depolarizatin as it makes its way down to the T-tubules
What is the difference between dihydropyridine receptors and ryanodine receptors?
dihydropyridine receptors are on the T tubules
Ryanodine receptors are on the sarcoplasmic reticulum
Describe what happens after the wave of excitation enters the T-tubules?
The dihydropyridine receptors sense the wave of excitation and influence the ryanodine receptors on the SR to undergo a conformational change
What kind of receptor is the ryanodine receptor?
A calcium channel. When it’s activated, they open to allow calcium ions to enter the cytoplasm
What is the importance of calcium release?
It is the primary trigger to allow skeletal muscles to contract
In a relaxed muscle, why can contraction not take place?
Tropomyosin and troponin are positioned in a way to prevent corss-bridge formation by blocking the myosin binding site on the actin molecules
What does calcium do when a muscle becomes excited?
It binds to troponin and causes a conformational change that results in the tropomyosin moving out of the way to expose the myosin binding sites on the actin molecules
What is the cause of muscle relaxation?
Decreased nerve activity at the neuromuscular junction
What happens to ATP when it binds to the ATPase site on the myosin head?
It splits into ADP and inorganic phosphate (Pi)
What occurs when the Pi is removed from the ATP? What is the importance of this?
Stored energy is released and transferred to the myosin cross-bridge
The cross-bridge is now “cocked” and is ready to “fire” once triggered
What action in cross-bridge cycling “pulls the trigger”? allowing for a power stroke to occur?
The presence of calcium ions that move the troponin-tropomyosin complex exposing the actin molecules so the cross-bridge can bind to the actin
T/F
Even if there is an absence of calcium ions, if the cross-bridge is “cocked” there will always be a power stroke
False!
The power stroke cannot occur if the cross bridge cannot reach the binding site for actin, which occurs when calcium ions cause this binding site to be uncovered
What causes the cross-bridge to detach and return to its “uncocked” shape?
The binding of a new ATP molecule
How does rigor mortis occur?
The calcium concentration increases in cells and because the cross-bridges were already “cocked”, the muscles will start contracting until they run out of ATP, and then they will remain contracted
List the stages in a skeletal muscle potential (3)
Latent Period
Contraction Time
Relaxation Time
Describe the latent period in a skeletal muscle contraction
The delay before the contraction starts
When cross-bridging cycling is beginning
Describe the contraction time in a skeletal muscle contraction
The phase of time when the actin filaments slide along the myosin filaments creating greater tension
Describe peak tension
The greatest tension that can be reached between the sliding of actin and myosin filaments while still creating force against an outside load
Describe the relaxation time in a skeletal muscle contraction
The time it takes for until all calcium ions have been removed since peak tension
Muscle twitch occurs at what stage of the skeletal muscle contraction
At the end of contraction time, during peak tension
What is the basic unit of a muscle contraction?
A muscle twitch
A muscle can develop tension from which two ways?
Motor Unit recruitment
Frequency of Stimulation
T/F
The greater number of muscle fibres recruited to contract results in greater muscle tension
True
Describe motor unit recruitment
The motor neuron in a muscle branches out and innervates multiple muscle fibres, resulting in a motor unit. Multiple motor units need to be activated in order for a strong contraction to occur
How do muscles manage to maintain a sustained contraction without fatigue
The body selectively rotates the activation of motor units so some can rest while others take over
T/F
Membrane potential takes a while to recover in skeletal muscles
False
It recovers quickly
Describe the concept of frequency of stimulation
Multiple action potentials increase contractile ability
Describe what happens to the magnitude of a second muscle twitch if a muscle fibre is re-stimulated after it has completely relaxed
The second twitch is the same magnitude of the first twitch
Describe what happens to the magnitude of a second muscle twitch if a muscle fibre is re-stimulated before it has completely relaxed. What is this called?
The second twitch magnitude is added onto the fist twitch
Called twitch summation
What is the length-tension relationship?
The amount of tension that can be generated at tetanus is dependent upon the length of the muscle at onset of contraction.
Describe the length-tension relationship at less than optimal length?
As the fibres shorten with contraction, the thin filament overlap with the part of the filament without cross-bridges, leading to an decrease in contraction efficiency and tension
Describe the length-tension relationship at optimal length
The maximal number of cross-bridge binding sites are available to the cross-bridges for binding. Allows for maximal contraction
Describe the length-tension relationship at greater than optimal length
Occur during passive stretch. The distance between the z-lines increase and the amount of overlap of the thick and thin filaments decreases.
Less overlap results in less cross-bridges available and less tension can be developed
T/F
For a muscle to shorten during contraction, it must exceed the forces that oppose the movement of the bone
True
Give an example of muscle tension with a load
Biceps must exceed the amount of force that is pulling the bone and triceps down in order to contract and pull it up
What is another name for muscle soreness? What is it caused from?
myalgia
caused due to overexertion and improper rest
Describe common athletic injury “contusion”:
Occurs when a muscle is subject to sudden, heavy extrinsic compressive force
Describe common athletic injury “strain”:
Occurs when the muscle fibres are exposed to an excessive force caused by intrinsic tension
Describe common athletic injury “laceration”:
Occurs from a deep cut or tear of muscle
At the level of the motor unit, what are the two types of muscle contraction?
Isotonic contraction and isometric contraction
Describe isotonic contraction
The muscle fibre tension remains constant as it changes length
Describe isometric contraction. What is another name for this?
The muscle fibre tension increases as it remains at the same length. AKA static muscle contraction
At the level of the whole muscle, what are the two types of muscle contractions?
concentric or eccentric
Describe concentric contractions
Produce tension while the muscle shortens
Give an example of concentric contraction
The lifting of an object by the biceps muscle
Describe eccentric contractions
Produce tension while the muscle lengthens
Give an example of eccentric contractions
The controlled lowering of an object using the biceps muscle
Describe what happens to the components of the myofibrils in concentric muscle contraction (sarcomere, A-band, I-band, H-zone, muscle length)
Sarcomere: shortens
A-band: no change
I-band: shortens
H-zone: shortens
Muscle length: shortens
Describe what happens to the components of the myofibrils in eccentric muscle contraction (sarcomere, A-band, I-band, H-zone, muscle length)
Sarcomere: lengthens
A-band: no change
I-band: lengthens
H-zone: lengthens
Muscle length: lengthens
Describe what happens to the components of the myofibrils in static/isometric muscle contraction (sarcomere, A-band, I-band, H-zone, muscle length)
Sarcomere: shortens
A-band: no change
I-band: shortens
H-zone: shortens
Muscle length: no change
Describe fatigue
When contractile activity cannot be maintained and tension of a muscle declines
What are the two types of muscle fatigue?
Central fatigue
Muscle Fatigue
How does central fatigue occur?
CNS decreases its activation of motor neurons
What characterizes central fatigue?
A slowing down and cessation of activity even though the muscle fibres themselves are not fatigued
Why does central fatigue occur?
Can be caused when a person feels bored with the activity, are tired, or lack motivation
Describe muscle fatigue
Mechanism used to protect muscle cells
What characterizes muscle fatigue
reduction in contractile activity before ATP supplies run out, otherwise rigor mortis would occur
What are the 4 causes of muscle fatigue?
Local accumulation of ADP and Pi from ATP hydrolysis
Accumulation of lactic acid
Accumulation of extracellular K+
Depletion of glycogen
Describe how local accumulation of ADP and Pi from ATP hydrolysis causes muscle fatigue
When ATP metabolic contractions get too high they interfere with cross-bridge cycling
Describe how accumulation of lactic acid causes muscle fatigue
Inhibits the enzyme of glycolysis, reducing ATP production
Interferes with excitation-contraction coupling
Describe how accumulation of extracellular K+ causes muscle fatigue
Without ATP, the Na-K pump cannot function to restore Na and K ion gradients. Accumulation of extracellular K+ causes membrane depolarization and makes muscle fibres less excitable
Describe how the depletion of glycogen causes muscle fatigue
Depletion of glycogen stores can occur during extreme exercise
What are the two types of muscle fibres?
Slow twitch (type I) muscle fibres
Fast Twitch (type II) muscle fibres
Describe the speed of contraction in slow twitch muscle fibres
Both contract and relax at slower rates than type II
Describe the innervation of slow twitch muscle fibres
Innervated by a2 motor neurons, which have lover activation threshold and slower conduction speeds
Describe the metabolic properties of slow twitch muscle fibres
Produce their ATP by aerobic processes
Describe the speed of contraction of fast twitch muscle fibres
Contract and relax at faster rates than type I
Describe the innervation of fast twitch muscle fibres
Innervated by a1 motor neurons, which have a higher activation threshold and faster conduction speeds
What are the two types of fast twitch fibres? What do they differ in?
- Fast oxidative glycolytic (FOG)
- Fast glycolytic (FG) fibres
- Differ in their metabolic activity. FOG fibres produce ATP by both aerobic and anaerobic respiration. FG fibres produce ATP by anaerobic processes
The colour of muscle fibres is determined by what?
How they produce their energy and if they have/don’t have myoglobin
Describe red fibres. What muscle fibres land under this category?
Contain many mitochondria and are highly vascularized
Contain myoglobin to help support the high use of oxygen
Slow oxidative and fast oxidative glycolytic muscle fibres
describe white fibres. What muscle fibres land under this category?
Rely mainly on anaerobic metabolism. Have few mitochondria and no myoglobin.
Fast glycolytic fibres
What is proprioception?
Your awareness of your body in the environment
What 2 muscle receptors allow for proprioception?
Muscle Spindles
Golgi Tendon Organs
Describe the function and location of muscle spindles
Monitor changes in muscle length and play a role in stretch reflexes
Located throughout the muscles within CT
What is another name for muscle spindles?
Intrafusal fibres
What is another name for extrafusal fibres?
Regular muscle fibres
T/F
The ends of intrafusal fibres are contractile and the middle region is not
True
Muscle spindles are innervated by what kind of motor neuron?
Gamma motor neurons
Describe the function and location of Golgi Tendon organs
Purpose is to respond to changes in muscle tension
Found at the junction of tendons and muscle fibres
Respond to both stretch and contraction of the muscle
The middle region of a muscle spindle contains sensory afferent fibres that are activated by what movement? What information does it relay to the CNS?
Stretch
Muscle length and rate of stretch
What kind of movement do the golgi tendon organs detect?
tension within in the muscles
How do the golgi tendon organs detect tension within the muscles?
The contraction of extrafusal fibres pulls on the golgi tendons and the stretch activates the afferent fibres intertwined within the tendons. The stronger the pull of the tendon, the higher the rate of firing of the golgi tendon afferents
Control of motor movement in the afferent neurons occur where?
At the level of the spinal cord
The cortico-spinal motor system controls what?
The fine voluntary movements of the body parts (hands and fingers)
The multi-neuronal motor system is located where and influenced by what?
Located in the brain stem
Influenced by the motor regions of the cortex, cerebellum and basal nuclei
What is the function of the multi-neuronal motor system?
Regulates overall body posture and involuntary movements of the larger muscle groups
Where are smooth muscle cells located
In the walls of hollow organs and tubes
Describe the structure of smooth muscle cells
Lack sarcomeres, have 3 types of filaments
How do the thick myosin filaments in smooth muscle differ from those found in skeletal muscle?
They are longer than those in skeletal muscle
How do the thin actin filaments in smooth muscle differ from those found in skeletal muscle
They contain tropomyosin but not troponin
T/F
Intermediate filaments in smooth muscle cells aid in contraction and the cytoskeletal framework
False
Does not directly support contraction
What is the smooth-muscle equivalent to Z-lines?
Dense bodies positioned throughout the cell as well as on the internal surface of the plasma membrane that serve as anchor points for the intermediate and contractile filaments
Contrast the orientation of thick and thin filaments in skeletal vs smooth muscle cells
In skeletal they are oriented along the length of the cell
In smooth, they are oriented at angles, forming a diamond-like pattern
Does the myosin light chain plays a larger role in skeletal muscle or smooth muscle? Why?
Smooth muscle bc it doesn’t have troponin to aid in its cross-bridge formation for contraction
During excitation of the myosin cross-bridge activation, calcium ions enter the smooth muscle cell and bind to what?
Calmodulin
In myosin cross-bridge activation, what activates the myosin light chain kinase?
The calcium2+ calmodulin complex
The myosin light chain kinase does what to the myosin light chain, allowing the myosin cross-bridge to bind to actin
Phosphorylates
T/F
Smooth muscle cells have T-tubules and plenty sarcoplasmic reticuli
False
They don’t
How does calcium enter the smooth muscle cells?
From the extracellular fluid
From the SR
Describe single unit smooth muscle
The muscle fibres are all electrically connected and become excited and contract as a single unit
Where is single unit smooth muscle found?
In the digestive system, reproductive system, urinary tract, small blood vessels
describe multi unit smooth msucle
Distinct groups of smooth muscle cells that are innervated by nerves to contract
Describe neurogenic stimulation
Where muscles are innervated by nerves of the autonomic nervous system - not motor neurons
Where is multi unit smooth muscle found?
In the walls of large blood vessels, small airways to the lungs, in the eye, and at the base of hair follicles of the skin
Describe myogenic stimulation
Cells are self-excitable and do not require nerve stimulation
T/F
Neurogenic stimulation occurs in single unit smooth muscle cells
False! It occurs in multi-unit smooth muscle
Myogenic stimulation occurs in single unit smooth muscle cells
What are the two types of spontaneous depolarization in single unit smooth muscle?
Pacemaker potentials
Slow-wave potentials
Describe what occurs in pacemaker potentials
The membrane gradually depolarizes until it reaches threshold, then fires an action potential
Describe what causes slow-wave potentials
The active transport of calcium ions across the membrane
T/F
Smooth muscle is only innervated by the parasympathetic nervous system
False
It’s innervated by both!
How does the ANS affect smooth muscle cells?
It affects its rate and strength of contraction
T/F
The action potential sin cardiac muscles last the longest than any other muscle type
True
List some of the similarities between skeletal and cardiac muscle
- Both striated with thick and thin filaments organized into sarcomeres
- Contains troponin and tropomyosin
- Contains T-tubules and well defined SR
- Contain lots of mitochondria
List some of the similarities between smooth and cardiac muscle
- Calcium ions come from both the ECF and SR
- Cells are interconnected by gap junctions
- Innervated by the ANS to modify rate and strength of contractions
