10 - Skeletal Muscle Physiology Flashcards
What are the three types of muscles? Which type is voluntary?
Muscle comprises largest group of tissues in body
– Cardiac muscle – found only in the heart
– Smooth muscle – appears throughout the body systems as components of hollow organs and tubes
– Skeletal muscle – makes up muscular system
What are skeletal muscles made of and how are they organized?
Made of groups of muscle fibres bundled together and attached to bones.
Connective tissue covering muscle divides muscle internally into bundles.
(Connective tissue extends beyond ends of muscle to form tendons - tendons attach muscle to bone)
What is a single skeletal muscle cell known as
A muscle fibre
– Multinucleated
– Large, elongated, and cylindrically shaped
– Fibres usually extend entire length of muscle
Actin (thin) and Myosin (thick) are interleaved and this shape allows for shortening
Describe the relaxed and excited muscle states?
How actin, troponin & myosin explain how the muscle shortens
- Ca2+ is released and binds with troponin that exposes the cross-bridge binding site
- Actin binds to Myosin cross-bridge and triggers power stroke that pulls thin filament inward during contraction
Describe what regions change in muscle excitement.
Sacromere, H-zone and I band shorten
A band is same width
Why is Calcium Concentration important?
The presence of Ca2+ in the myofibrils is crucial to forming cross-bridges between actin and myosin and generating muscle contractions.
What mechanism brings about muscle contraction?
Sliding filament mechanism
- Increase in Ca2+ starts filament sliding
- Decrease in Ca2+ turns off sliding process
- Thin filaments on each side of sarcomere slide inward over stationary thick filaments during contraction (Sarcomere shortens)
What is a motor unit? What is the point of motor unit recruitment?
Motor unit: One motor neuron and the muscle fibres it innervates
of muscle fibres varies among different motor units
- muscles for precise, delicate movements contain fewer fibres per motor unit
- muscles for powerful, coarsely controlled movement have a greater # of fibres per motor unit
Asynchronous recruitment of motor units helps delay or prevent fatigue.
What is fatigue? How does asynchronous recruitment of motor units work?
Fatigue: inability to maintain muscle tension at a given level during sustained contraction
Some muscle motor units rest while others are in use and they switch up
When is it impossible to to use motor unit recruitment?
- During maximal muscle contraction, all muscle fibres must participate, to motor units cannot be alternated
- Only possible during submaximal contractions
What are twitch muscle contractions?
Twitch
- Brief, weak contraction
- Produced from single action potential
- Too short and too weak to be useful
- Normally does not take place in body
What is twitch summation?
Happens when muscle fibre is re-stimulated before it has relaxed, so second twitch is added to first.
Results from sustained elevation of calcium in the intracellular environment.
What is tetanus?
It is a maximal sustained contraction
– Occurs if muscle fiber is stimulated so rapidly that it does not have a chance to relax between stimuli.
– Contraction is usually three to four times stronger than a single twitch.
– 2 types: unfused and fused.
What is unfused tetanus?
Type of tetanus where the stimulation rate of the muscle fibre is not at a maximum value and the fibre relaxes slightly between the stimuli
What is fused tetanus?
Type of tetanus where the stimulation rate is so fast that the muscle fiber does not have time to relax between stimulations.
Maximum tension in the muscle fiber is achieved with no period of relaxation.
The maximum number of cross-bridge binding sites remain uncovered so that cross-bride cycling and tension develop
What does muscle tension depend on? Where is it produced?
of fibres contracting
Tension developed by each contracting fibre
Muscle tension is produced in the sarcomeres
How is muscle attached between two bones?
Muscle is typically attached to at least two different bones across a joint
- Origin: End of muscle attached to more stationary part of skeleton.
- Insertion: End of muscle attached to skeletal part that moves
Describe the Length Tension Relationship
A: Maximal Tetanic Contraction
- Achieved when a muscle fiber is at its optimal length (lo) before contraction
- There is optimal overlap of thick-filament cross bridges and thin-filament cross bridge binding sites
B and C
- The % maximal tetanic tension that can be achieved decreases when the muscle fibre is longer than Io before contraction
- When longer, fewer thin-filament binding sites are accessible for binding with thick filament cross bridges, because the thin filaments are pulled out from between the thick filaments
D:
- The % maximal tetanic tension that can be achieved decreases when the muscle fibre is shorter than lo before contraction
- When the fibre is shorter, fewer thin-filament binding sites are exposed to thick filament cross bridges because the thin filaments overlap
Limits
- The resting muscle length is at lo
- Skeletal attachments impose restrictions
- Muscles cannot vary beyond 30% of their lo in either direction
- At the outer limits of this range, muscles still can achieve about 50% of their maximal tetanic contraction
What are the ‘series-elastic elements’?
- Non contractile components found in all muscle tissues, in tendons and connective tissue (elastic cytoskeletal proteins)
- The tension is transmitted to the bone by the stretching and tightening of the muscle’s elastic connective tissue and tendon as a result of sarcomere shortening brought about by cross- bridge cycling
- All of these elastic components behave as if they were connected in series to the contractile elements of the muscle
What are the types of contraction?
1. Isotonic: Muscle tension remains constant as muscle changes length
Two types
- Concentric contractions: Muscle shortens
- Eccentric contractions: Muscle lengthens
2. Isometric: Muscle is prevented from shortening
- Tension develops at constant muscle length
What are Isotonic Contractions
- Muscle tension is constant throughout the range of motion
- Muscle length changes through the range of motion
- This is a measure of dynamic strength
- Creates force and moves a load
What are Concentric vs Eccentric Contractions?
- Types of isotonic contractions: Dynamic contractions
- Concentric: Produces tension during a shortening motion
- The actin filaments are pulled together by the myosin filaments which move the Z lines closer together
- The sarcomere shortens and this results in a shortening of the entire muscle
- Tension within the muscle is proportional to the externally applied load
- eg: elbow moving toward increasing flexion portion of the biceps curl with free weights
- Eccentric: tension produced while muscle is lengthened
- associated with delayed muscle soreness (ie stretching) but also injury
- Sarcomere lengthened
What are isometric contractions?
- A static contraction (static strength)
- tension or force is created to overcome a load, but no movement of the load occurs
- Muscle tension is generated but the length of the muscle remains unchanged (sarcomeres shorten, but elastic elements stretch)
- ex: pushing against an immovable object
What provides energy for muscle contractions?
- Energy source: ATP
- For contractile activity to continue, ATP must be constantly supplied.
- Skeletal muscles consume ATP rapidly
- Transfer of high-energy phosphate from creatine phosphate to ADP
- Oxidative phosphorylation
- Glycolysis
What is Oxidative Phosphorylation?
Aerobic process to provide ATP energy for low energy demands (ex: walking to jogging)
Needs to have adequate O2 and nutrients
What is glycolysis?
When theres not enough O2 or if oxidative phosphorylation cannot keep pace, the muscle fibers rely increasingly on glycolysis to generate ATP (anaerobic or high intensity exercise)
During glycolysis two options for synthesis of energy:
- Can yield ATP anaerobically
- Glucose is broken down to pyruvate. Pyruvate produce ATP via oxidative phosphorylation if sufficient O2 is available.
Although glycolysis extracts fewer ATP molecules from each nutrient molecule processed, it can proceed much more rapidly
It outproduces oxidative phosphorylation over a given period of time if enough glucose is present
What are 2 consequences of producing energy via glycolysis?
- Large amounts of nutrient fuel must be processed as it is less efficient than oxidation phorphylation (yields only 2 ATP vs. 36 molecules)
- End product of anaerobic glycolysis, pyruvic acid, if not processed by oxidative phosphorylation, is converted to lactic acid
Why is lactic acid bad?
- Associated with muscle soreness
- Occurs when intense exercise is actually taking place
- Enters the circulatory system and can produce a metabolic acidosis
- High intensity anaerobic exercise can be sustained for only a short duration, in contrast to the body’s prolonged ability to sustain aerobic endurance-type activities
Why is Muscle Fatigue considered a defense mechanism?
Happens when muscle can no longer respond to stimulation with same degree of contractile activity
What are the major types of Muscle Fibres?
On basis of contractile and metabolic function: Muscle fibres within a single motor unit are same Muscle fibres from different motor units are different
Classifications:
1. Contractile (twitch) properties: fast-twitch, slow-twitch
- Metabolic properties: Slow-oxidative (SO), Fast-oxidative glycolytic (FOG), Fast-glycolytic (FG)
What are the 3 types of skeletal muscle fibres?
- Slow-twitch oxidative red muscle (Fatigue resistant)
- Fast-twitch oxidative glycolytic red muscle (Fatigue resistant)
- Fast-twitch glycolytic white muscle (Fatigues easily)
How can Muscle Fibres Adapt to different types of exercise?
- They can adapt by changing their ATP synthesizing capacity and changing their diameter.
- Depending on the pattern of neural activity, long-term adaptive changes occur in the muscle fibres, enabling them to respond most efficiently to the types of demands placed on the muscle.
- Skeletal muscle has a high degree of plasticity
