10. Skeletal Muscle Physiology

Question 1

Muscle Physiology basics • Three types of muscle

Answer 1

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

Question 2

Skeletal Muscle consists of…

Answer 2

…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

Question 3

Structure of Skeletal Muscle

Answer 3

A single skeletal muscle cell is known as a muscle fibre

– Multinucleated
– Large, elongated, and cylindrically shaped
– Fibres usually extend entire length of muscle

Question 4

Answer 4

Diagram for the structure of a muscle fibre

myofibrils are key

• interweaved muscle arrangements

Question 5

Answer 5

Actin (thin) and Myosin (thick) are interleaved and this shape allows for shortening

Question 6

Answer 6

How actin, troponin & myosin explain how the muscle shortens

• A and M in parallel and excitation binding sites are released and two can bind together and M pulls on A and whole arrangement gets shorter

Question 7

Answer 7

Sarcomere and band zones

Sacromere shortens and H zone shortens and same with I band slightly

Question 8

Control of Calcium Concentration

Answer 8

The presence of Ca2+ in the myofibrils is crucial to forming cross-bridges between actin and myosin and generating muscle contractions.

Question 9

Answer 9

Muscle action potentials and shortening of muscles

Question 10

Contraction Mechanism

Answer 10

Cross-bridge interaction between actin and myosin brings about muscle contraction by means of the 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
• As thin filaments slide inward, they pull Z lines closer together
• Sarcomere shortens
• All sarcomeres in a muscle fiber shorten simultaneously

Question 11

Motor Unit Recruitment

Answer 11

All fibres connected to an axon receive action potentials at the same time

of muscle fibres varies among different motor units.

of muscle fibres per motor unit and # of motor units per muscle vary widely.

– Muscles that produce precise, delicate movements contain fewer fibres per motor unit

– Muscles performing powerful, coarsely controlled movement have larger number of fibres per motor unit.

Asynchronous recruitment of motor units helps delay or prevent fatigue.

Question 12

Motor unit

Answer 12

One motor neuron and the muscle fibres it innervates.

Question 13

Fatigue

Answer 13

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

Question 14

Asynchronous recruitment of motor units

Answer 14

The body alternates motor unit activity, like shifts in a factory, to give motor units that have been active an opportunity to rest while others take over.

Changing of the shifts is carefully coordinated, so the sustained contraction is smooth rather than jerky.

Question 15

Muscle Fatigue

Answer 15

Asynchronous motor unit recruitment is possible only for submaximal contractions, during which only some of the motor units must maintain the desired level of tension.

During maximal contractions, when all the muscle fibres must participate, it is impossible to alternate motor unit activity to prevent fatigue.

This is one reason why you cannot support a heavy object as long as a light one.

Question 16

Muscle Contractions

Answer 16

• Contractions of whole muscle can be of varying strength
• Twitch
  
  • Brief, weak contraction
  • Produced from single action potential
  • Too short and too weak to be useful
  • Normally does not take place in body

Question 17

Answer 17

Blue - mechanical

Red - action potential (electrical)

Depends on the spacing between of action potentials causing 3 different scenarios

Question 18

Twitch Summation

Answer 18

Results from sustained elevation of calcium in the intracellular environment.

Question 19

Tetanus

Answer 19

– 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.

Question 20

Unfused tetanus

Answer 20

the stimulation rate of the muscle fibre is not at a maximum value and the fibre relaxes slightly between the stimuli

Question 21

Fused Tetanus

Answer 21

When 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

Question 22

Answer 22

Whole Muscle Tension

Question 23

Muscle Tension

Answer 23

Tension is produced internally within sarcomeres.

Tension must be transmitted to bone by means of connective tissue and tendons before bone can be moved.

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

Question 24

Length Tension Relationship

Answer 24

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

Question 25

Muscle Tension

Answer 25

• 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
• Tendons (and connective tissue) have a certain degree of passive elasticity
• These non-contractile tissues are called the series-elastic elements

Question 26

Series Elastic Elements

Answer 26

Elastic fibers are found in:

• All muscle tissues
• Tendons
• Connective tissue between muscle fibers
  
  • Elastic cytoskeletal proteins between myofibrils and in the sarcomeres

All of these elastic components behave as if they were connected in series to the contractile elements of the muscle

They are called ‘series elastic elements of the muscle’

Question 27

Answer 27

• Bones function as levers
• Joints function as fulcrums
• Skeletal muscles provide force to move bones

Question 28

Types Of Contraction

Answer 28

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

Question 29

Isotonic Contractions

Answer 29

• 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

Question 30

Concentric Contractions

Answer 30

• A dynamic contraction
• 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

Question 31

Eccentric Contractions

Answer 31

• A dynamic contraction
• Tension as muscle is getting longer
• The actin filaments are pulled apart, moving the Z lines farther from the centre, lengthening the sarcomere
• The length of the whole muscle increases
• Have the greatest potential for high muscle tension and muscle injury
• Associated with delayed muscle soreness
• eg: The elbow moving toward increasing extension of the biceps curl with free weights

Question 32

Isometric Contractions

Answer 32

• 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
• eg: Pushing against an immovable object (such as a wall) or pick up weights and old them stationary in front of you

Question 33

Force is created despite the fact that the length of the muscle does not change

Answer 33

Question 34

Energy for muscle contractions

Answer 34

• ATP is the only energy source that can be directly used for these activities.
• For contractile activity to continue, ATP must be constantly supplied.
• The rate at which ATP is consumed by skeletal muscle is very rapid.

Question 35

Energy for Muscle Contractions (Cnt’d)

Energy sources:

Answer 35

1. Transfer of high-energy phosphate from creatine phosphate to ADP
   
   1. First energy storehouse tapped at onset of contractile activity
2. Oxidative phosphorylation (citric acid cycle and electron transport system
   
   1. Takes place within muscle mitochondria if sufficient O2 is present
3. Glycolysis
   
   1. Supports anaerobic or high-intensity exercise

Question 36

Oxidative Phosphorylation

Answer 36

During light exercise (walking) to moderate exercise (jogging or swimming), muscle cells can form enough ATP through oxidative phosphorylation to keep pace with the modest energy demands of the contracting muscle for prolonged periods of time

To sustain ongoing oxidative phosphorylation, the exercising muscles depend on delivery of adequate O2 and nutrients by the blood to maintain their activity

• This activity is aerobic or endurance type exercise

Question 37

Glycolysis

Answer 37

Glycolysis

When O2 delivery or oxidative phosphorylation cannot keep pace with the demand for ATP formation as the intensity of exercise increases the muscle fibers rely increasingly on glycolysis to generate ATP

During glycolysis 2 options for synthesis of energy:

1. Can yield ATP anaerobically if insufficient O2 is present
2. 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

• anaerobic/ high intensity exercise

Question 38

Lactic acid

Answer 38

when producing energy via glycolysis

1. large amounts of nutrient fuel must be processed as it is less efficient than oxidation phorphylation (yields only 2 ATP vs. 36 molecules)
2. endproduct of anaerobic glycolysis, pyruvic acid, if not processed by oxidative phosphorylation, is converted to lactic acid

Question 39

Lactic acid process

Answer 39

• Accumulation of lactic acid has been implicated in muscle soreness
• Occurs during the time that 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
• Delayed onset pain and stiffness the day after unaccustomed muscular exertion is probably due to reversible structural damage

Question 40

Muscle Fatigue

Answer 40

Occurs when exercising muscle can no longer respond to stimulation with same degree of contractile activity.

Defense mechanism that protects muscle from reaching point at which it can no longer produce ATP.

Question 41

Major Types Of Muscle Fibers

Answer 41

The muscle fibres within a single motor unit are homogeneous in nature when they are compared on the basis of contractile and metabolic function.

However, when muscle fibres from different motor units are compared, there are differences in contractile and metabolic function.

1. Contractile (twitch) properties - fast-twitch, slow-twitch
2. Metabolic properties - Slow-oxidative (SO), Fast-oxidative glycolytic (FOG), Fast-glycolytic FG)

Question 42

3 types of skeletal muscle fibres

Answer 42

• Slow-twitch oxidative red muscle (Fatigue resistant)
• Fast-twitch oxidative glycolytic red muscle (Fatigue resistant)
• Fast-twitch glycolytic white muscle (Fatigues easily)

Question 43

Answer 43

Summary of structure and functional with 2 skeletal muscle fibres

Question 44

Muscle Fiber Adaptation

Answer 44

• Different types of exercise produce different patterns of neuronal discharge to the muscle involved.
• 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.
• Therefore skeletal muscle has a high degree of plasticity.
• They can adapt by changing their ATP synthesising capacity and changing their diameter.