MIDTERM #2

MUSCULAR SYSTEM 

10-1 Identify the common properties of muscle tissues and the primary functions of skeletal muscle.

10-2 Describe the organization of muscle at the tissue level.

10-3 Describe the characteristics of skeletal muscle fibers and identify the components of a sarcomere.

10-4 Identify the components of the neuromuscular junction, and summarize the events involved in the neural control of skeletal muscle contraction and relaxation.

10-5 Describe the mechanism responsible for the different amounts of tension produced in a muscle fiber.

10-6 Compare the different types of skeletal muscle contraction.

10-8 Relate the types of muscle fibers to muscle performance, discuss muscle hypertrophy,atrophy, and aging, and describe how physical conditioning affects muscle tissue

Muscle Tissue 

• Cells are specialized for contraction 

• Skeletal muscles move the noyd by pulling on bones

• Cardiac and smooth muscles control movements inside the body 

Common properties include

Excitability (responsiveness) 

Contractility (ability of cells to shorten) 

Extensibility (stretching) 

Elasticity (recoil)

Functions of Skeletal System 

• producing movement 

• Maintaining posture and body position 

• Support soft tissues 

• Guarding body entrances and exits 

• Maintaining body temperature 

• Storing nutrients 

What do skeletal muscles contain?

• Skeletal muscle tissue (primarily) 

• Connective tissue 

• Blood vessels 

• Nerves 

Skeletal muscles have 3 layers of connective tissue 

Epimysium 

• Layer of collagen fibers that surrounds the muscle 

• Connected to deep fascia 

• Separates muscles from surrounding tissues 

Perimysium 

• Surround muscle fiber bundle (fascicles) 

Contains: 

• Collagen fibers 

• Elastic fibers 

• Blood vessels 

• Nerves  

Endomysium 

• Surrounds individual muscle cells (muscle fibers) 

Contains: 

• capillary networks 

• Myosatellite Cells (stem cells) that repair damage - often simply called satellite cells 

• Nerve fibers 

Collagen fibers of epimysium, permitsm, and endomysium come together at the ends of muscles to form 

• A tendon (bundle) 

• Aponeurosis (sheet) 

• To attach skeletal muscle to bones 

Skeletal Muscles 

Have extensive vascular networks that 

• Delivery oxygen and nutrients 

• Remove metabolic wastes 

Contract only when stimulated by central nervous system 

• Often called voluntary muscles 

• The diagram usually work subconsciously 

Skeletal Muscle Fibers 

• Are enormous compared to other cells 

• Contain hundreds of nuclei (multinucleate) 

• Develop by fusion of embryonic cells (myoblasts) 

• Also known as striated muscles cells due to striations 

Sarcolemma

• Plasma membrane of a muscle fiber

• Surrounds the sarcoplasm (cytoplasm of muscle fiber) 

• A sudden change in membrane potential initiates a contraction 

Transverse Tubules (T Tubules) 

• Tubes that extend from surface of muscle fiber deep into sarcoplasm 

• Transmit action potential from sarcolemma into cell interior and action potentials trigger contraction

Sarcoplasmic 

• A tubular network surrounding each myofibril 

• Similar to smooth endoplasmic reticulum 

• Form chamber (terminal cisternae) that attach to T tubules 

• Two terminal cisternae plus a T tubule forms a TRIAD 

• Specialized for storage and release of calcium ions Ca2+ 

• Ca2+ ions are actively transported from cytosol into terminal cisternae

Myofibrils 

• Lengthwise subdivisions with a muscle fiber 

• Responsible for muscle contraction 

• Made of bundles of protein filaments (myofilaments)

Two types:

Thin Filaments

• Composed primarily of actin 

Thick Filaments 

• Composed primarily of myosin 

Sarcomeres 

• Smallest functional units of a muscle fiber 

• Interactions between filaments produces contraction 

Arrangement of filaments accounts for striated pattern of myofibrils 

• Dark bands (A Bands) 

• Light Bands (I Bands) 

The A Band 

M Line 

• In center of A 

• Proteins stabilize positions of thick filaments 

H Band 

• On either side of M line 

• Has thick filaments but no thin filaments 

Zone of Overlap 

• Dark region 

• Where thick and thin an filaments overlap 

The I band 

• Contains 

Z lines 

•  Bisect I bands 

• Mark boundaries between adjacent sarcomeres 

Titin 

• Elastic protein 

• Extends from tops of thick filaments to the Z line 

• Keep filaments in proper alignment 

Thin Filaments 

• Contain F-actin, nebulin, tropomyosin, troponin proteins 

Filamentous actin (F-actin)

• Twisted strand composed of two rows of globular G-actin molecules 

• Active Sites on G-actin bind to myosin 

Nebulin 

• Holds F-actin strand together 

Tropomyosin 

• Covers active sites on G-actin 

• Prevents actin-myosin interaction 

Troponin 

• A globular protein 

• Binds tropomyosin, G-actin, and Ca2+

Thick Filaments 

• Each contains about 300 myosin molecules 

• Each myosin molecules consists of 

Tail 

• Binds to other myosin molecules 

Head 

• Made of two globular protein subunits 

• Projects toward nearest thin filament 

• Core of titin recoils after stretching 

Excitable Membranes

• Are found in skeletal muscle fibers and neurons 

• Depolarization and rePolarization events produce action potentials (electrical impulses) 

• Skeletal muscle fibers contract due to stimulation by motor neurons 

Neuromuscular Junction (NMJ) 

• Synapse between a neuron and a skeletal muscle fiber 

• Axon terminal of the motor neuron release a neurotransmitter into the synaptic cleft, neurotransmitter is acetylcholine (ACh) 

ACh binds to and opens a nicotinic receptor on the muscle fiber 

• Na+ enters cells and dePolarizes motor and end plate 

• An action potential is generated 

Excitation-Contraction Coupling 

• Action potential travels down T Tubules to triads 

• Ca 2+ is released from terminal cisternae of sarcoplasmic reticulum 

• Ca2+ binds to troponin and changes its shape 

• Troponin-tropomyosin complex changes position, exposes active sites on thin filaments, contraction cycle is initiated 

Contraction Cycle (details in the image below) 

1. Contraction cycle begins

2.  Active-site exposure 

3. Cross-bridge formation (myosin binds to actin) 

4. Myosin head pivoting (power stroke) 

5. cross -bridge detachment 

6. Myosin reactivation

Generation of Muscle Tension 

• When muscle cells contract, they produce tension (pull) 

• To produce movement, tension must overcome the load (resistance) 

• The entire muscle shortens at the same rate 

• Because all sarcomere contract together 

• Speed of shortening dpEns on cycling rate (number of power strokes per second) 

Duration of contraction depends on 

• Duration of neural stimulus 

• presences of free calcium ions in cytosol 

• Availability of ATP 

• As Ca2+ is pumped back into SR and Ca2+ concentration cytosol falls 

• 1. Ca2+ detaches from troponin 

• Troponin returns to original position 

• Active sites are received by tropomyosin and the contraction ends  

Rigor Mortis 

• Fixed muscular contraction after death

• Results when ATP runs out and ion pumps cease to function 

• Calcium ions build in cytosol 

Tense Production 

**# of sarcomeres that contract in a muscle fiber is “all-or-none”

• So a muscle fiber is either producing tension, or relaxed 

What does the amount of tension produced depend on? 

• Number of power strokes performed 

• Fibers resting length at time of stimulation 

• Frequency of stimulation

Length-Tension Relationship

• Tension produced by a muscle fiber relates to the length of the sarcomeres 

What does the amount of tension produced depend on? 

• Number of power strokes performed by cross-bridges 

• Amount of overlap between thick and thin filaments

Maximum tension is produced when the maximum number of cross-bridges are formed, occurs when zone of overlap is large 

Treppe 

• stair-step increase increase in tension 

• Caused by repeated stimulation immediately after relaxation phase 

• Stimulus frequency of less than 50/second 

• Produces a series of contractions with increasing tension 

• Typically seen in cardiac muscle and not skeletal muscles 

Wave Summation 

• Increasing tension due to summation of twitches 

• Caused by repeated stimulation before the end of relaxation phase 

• Stimulus frequency of greater than 50/second

Tetanus is maxim tension 

Incomplete Tetanus 

• Muscles produce near-maximum tension 

• Caused by rapid cycles of contraction and relaxation 

Complete tetanus 

• Higher stimulation frequency eliminates relaxton phase 

• Muscle in in continuous contraction 

• All potential cross-bridges form 

Muscle Contractions 

Tension production by skeletal muscles 

• Depends on the number of stimulated muscle fibers 

Motor unit is a motor neuron and all of the muscle fibers it controls 

• May contain a few muscle fibers or thousands 

• All fibers in a motor unit contract at the same time 

Fasciculation 

• Involuntary “muscle twitch” 

• Unlike a true twitch, involves more than one muscle fiber 

Recruitment 

• Increase in the number of active motor units 

• Produces smooth, steady, increase in tension 

• Maximum tension is achieved when all motor units rewatch complete tetanus - can be sustained for a very short time 

Sustained contractions 

• Produce less than maxim tension 

Muscle Tone 

• The normal tension and firmness of a muscle at rest 

• Without causing movement, motor units activities 

• Stabilize positions of bones and joints 

• Maintain balance and posture

• Elevated muscle tone increases resting energy consumption 

Types of muscle contraction 

• Contractions are classified based on their pattern of tension production

• Isotonic or isometric 

Isotonic Contractions 

 Skeletal muscle changes length 

• Resulting in motion 

Isotonic Concentric Contraction 

• Muscle tension > load (resistance) 

• Muscle shortens 

Isotonic Eccentric Contraction 

• Muscle tension < load 

• Muscle elongates 

Isometric Contractions 

• Skeletal muscle develops tension that never exceed the load 

• Muscle does not change length 

Load and Speed contractions are inversely related 

• The heavier the load, the longer it takes for movement to begin 

• Tension must exceed the load before shortening can occur 

Muscle Relaxation and return to resting length 

Elastic Forces 

• Tendons recoil after contraction 

• Helps retain muscle fibers to resting length 

Opposing Muscle Contractions 

• Return a muscle to resting length quickly 

Gravity 

• Assist opposing muscles

Muscle Performance 

Force 

• Maximum amount of tension produced 

Endurance 

• Amount of time an activity can be sustained 

Force and endurance depend on 

• Types of muscle fibers 

• Physical conditioning 

Three types of skeletal muscle fibers 

Fast Fibers 

• Majority of skeletal muscle fibers 

• Contracts very quickly 

• Large diameter 

• Large glycogen reserves 

• Few mitochondria 

• Produce strong contractions, but fatigue quickly 

Slow Fibers 

• Slow to contract and slow to fatigue 

• Small diameter 

•  Numerous mitochondria 

• High oxygen supply from extensive capillary network 

• Contain myoglobin (red pigment that binds oxygen)

Intermediate Fibers 

• Mid–sized 

• Little myoglobin 

• Slower to fatigue than fast fibers

White Muscles 

• Mostly FAST fibers 

• Pale (ex:chicken breast) 

Red Muscles 

• Mostly SLOW 

• Dark (ex: chicken legs) 

Most Human Muscles 

• Contain a mixture of fiber types and are pink 

Muscle Hypertrophy 

Muscle growth from heavy training that causes and increase in:

• Diameter of muscle fibers 

• Number of myofibrils 

• Number of mitochondria 

• Glycogen reserves 

Muscle Atrophy 

• Reduction of muscle size, tone, and power due to lack of activity 

Changes in muscle tissue as we age 

• Skeletal muscle fibers become smaller in diameter and less elastic 

• Fibrosis - increase in fibrous connective tissue 

• Tolerance for exercise decreases and the ability to recover from muscular injuries decreases 

• Changes can begin as early as 25 years olds 

• Strength can be reduced by 30-50% by 80 years old - 3-5% per decade after age 3o 

• Fast-twitch fibers decrease in number more rapidly than slow-twitch fibers  - contributes to loss of strength and speed

• Surface area of neuromuscular junction decreases - results in fewer action potentials produced in muscle fibers

• Number Of motor units also decreases - remaining motor units take up extra fibers (may result in less precision) 

• Decrease in density of capillaries in skeletal muscle - reduced blood flow to muscles and longer recovery period following exercise 

• Resistance Training can slow the process 

Muscle Fatigue 

• When  muscles can no longer perform at a required level they are fatigued 

Correlated with 

• depletion of metabolic reserves 

• Image to sarcolemma and sarcoplasmic reticulum 

• Decline in pH which affects calcium ion binding and alters enzyme activities 

• Weariness due to low blood pH and pain 

Physical Conditioning improves power and endurance

Anaerobic Endurance (ex: 50-meter dash, weightlifting)

• Uses fast fibers and stimulates hypertrophy 

• Improve dab frequency, brief intensive workouts 

Aerobic Endurance (prolonged activities) 

• Supported by mitochondria 

• Does not stimulate muscle hypertrophy  

• Training involves sustained, low level of activity 

Effects of Training 

Improvements in aerobic endurance result form things like: 

• Alterations in the characteristics of muscle fibers 

• Improvements in cardiovascular performance 

Spinal Reflexes

Stretch Reflex (like patellar reflex)

• Regulates skeletal muscle length throughout the body 

• Very rapid (large myelinated fibers) 

Steps in a stretch reflex 

1. Stimulus = muscle stretching 

2. Distortion of receptor sends action potential through sensory neuron 

3. Sensory neuron synapses with motor neurons in spinal cord 

4. Motor neurons send signals to motor units; trigger reflexive contraction of stretched muscle 

Muscle Spindles are the receptors in the stretch reflexes 

• Made of bundles of small, specialized intrafusal muscle fibers 

• Innervated by sensory and motor neurons - which are called gamma motor neurons; their axons are called gamma efferents 

• Muscle spindle is surrounded by extrafusal muscle fibers - maintain muscle tone and contract muscle 

• Dendrites of sensory neurons wind around central region of intrafusal fibers 

• Sensory neuron axon enters CNS in posterior root of spinal cord 

• Sensory neurons synapse in spinal cord dirt club with motor neurons 

• Gamma efferents complete reflex arc by synapsing back at the intrafusal fibers 

• Muscle contracts back to resting length

TENDON REFLEX 

• Prevents skeletal muscles from:

• Developing too much tension

• Tearing or breaking tendons

• Sensory receptors are Golgi tendon organs

• Stimulated when collagen fibers are overstretched

• Stimulate inhibitory interneurons in spinal cord to relax the muscle to prevent tendon damage

• More muscle tension leads to more muscle inhibition

Reflex arcs

Ipsilateral reflex arcs

• Occur on same side of body as stimulus

•  Stretch, tendon, and withdrawal reflexes

• Crossed extensor reflexes involve contralateral reflex arcs

• Occur on side opposite stimulus

Postural reflexes

• Include both stretch reflexes and tendon reflexes

• Maintain normal upright posture

• Often involve multiple muscle groups (e.g., back and abdominal muscles

• Maintain firm muscle tone

• Extremely sensitive receptors allow constant fine adjustments to be made as needed

Lecture Specific Notes 

Skeletal Muscles 

• Multinucleated

• Striated 

Cardiac Muscle 

• Found in heart tissue 

• Branched and striated 

• Intercalated discs 

Functions of Muscles 

• Produce skeletal movement 

• maintain posture and body position 

• Support soft tissue 

• Guard exits and entrances 

• Maintain body temperature 

• Store nutrient reserves 

Properties of Skeletal Muscles***

• Contractility: ability to shorten with force 

• Excitability: capacity to respond to a stimulus 

• Extensibility: ability to be stretched beyond normal resting length 

• Elascity: ability to return to normal resting length after being stretched 

Epimysium (organ level) 

Perimysium (tissue level) 

Endomysium (cellular level)

Sarcolemma 

Transverse Tubules 

Sarcoplasmic Reticulum 

• Calcium 

Sarcomere 

• Thick and thin filaments produce a banded, or striated appearance 

Thin Myofilaments 

• actin: contains active site; creates cross bridge with myosin 

• troponin : binds to Ca2+ released from sarcoplasmic reticulum

• Tropomyosin: blocks the active site until moved when troponin bind Ca2+

Myosin- two main parts 

• Head - acts as an ATPases - breaks down ATP an forms cross-bridges with actin 

• Tails - bind together forming thick filament 

• Hinge region is found between the head and tail 

Titin - stabilizes sarcomere and recoils after stretching 

• 300 myosin molecules per thick filament 

Sliding Filament Theory 

• During muscle contraction, actin and myosin filaments slide over each other 

• Cross bridges create tension (a pulling force) - muscles are like ropes, only pull no pushing 

Neuromuscular Junction (NMJ) 

• Synapse between motor neuron and skeletal muscle 

• ACH is the neurotransmitter - nicotinic receptors are chemically gated Na+ channels 

• depolarize the muscle leading to contraction 

Excitation-Contraction Coupling 

• Synaptic cleft is a narrow space that separates the axon terminal of the neuron from the opposing motor end plate  

Contraction Cycle 

1. Contraction cycle begins when Ca2+ arrives at the zone of overlap 

2. Active site exposure occurs when Ca2+ binds to troponin and tropomyosin is moved, allowing interaction with energized myosin head 

3. Cross-bridge formation occurs between the active site on actin and the energised myosin head 

4. The power stroke uses the stored energy in the myosin head to pivot the hinge region moving the head towards the M line - ADP and phosphate are released

5. Cross-bridges are detached when the ATP molecule binds to the myosin head, exposing the active site on actin for another cross-bridge 

6. Myosin reactivation occurs when the ATP is broken down into ADP and phosphate - the energy is used to re-cock the myosin head
   
   

3 things that will start muscle contraction 

• ATP present 

• Calcium release

• Action potential 

Contraction cycle continues if:

• Neural stimulus continues 

• Free calcium ions in cytosol 

• ATP is available 

The contraction will continue if there is:

• Stimulus at the NMJ 

• Ca2+ is available at the zone of overlap 

• ATP is available 

Muscle Relaxation 

Return to resting conditions after excitation-contraction coupling ends 

• Acetylcholinesterase (AChE) break down the ACh in NMJ 

• Depolarization of the muscle ends 

Ca2+ is removed from the zone of overlap 

• Active transport of Ca2+ in sarcoplasmic reticulum (major) 

• Active transport of Ca2+ across sarcolemma into the extracellular fluid (minor)

Rigor Mortis 

• After death, circulation stops and the muscle is deprived of nutrients and oxygen 

• After a few hours, muscle fibers run out of ATP 

• Muscles can't pump Ca2+ out of the cytosol and Ca2+ levels rise 

• Without ATP, cross bridges cannot be released and the muscles become “lock in the contracted position” 

• Begins 2-7 hours after death and can last 1-6 days depending on conditions 

Tension Production 

• Tension =  force 

• Tension is determined by number of pivoting cross-bridges 

Tension produced is dependent on

• Number of power strokes performed 

• Resting muscle fiber length 

• Frequency of stimulation 

Frequency of Stimulation 

• A muscle twitch is produced when a single action potential initiates contraction - twitch lasts 7-1000 msec

• Sustained contractions require repeated stimulation 

• Myogram is a graph of the tension produced by a single twitch

Lecture Specific Notes February 3rd 

Muscle Contraction 

• Most amount of cross bridges are at the resting length and have maxim force 

• Sarcomere are pulled further apart 

• Less cross bridges because less overlap leads to less 

• A muscle twitch is produced when a single action potential initiates contractions 

• A twitch lasts 7-100 msec

• Sustained contractions require repeated stimulation 

• a myogram is a graph of the tension produced by a single twitch 

• Y axis = tension developed 

• X axis is time 

Frequency of Stimulation

1. Treppe is an increase in peak tension with each successive stimulus delivered shortly after the completion of the relaxation phase of preceding twitch, the fibers maximum potential tension is not reached until tetanus 

2. Wave Summation Frequency of stimulation - not enough time for the next stimulus to arrive 

3. Incomplete tetanus - has wiggly line, coming in rapid succession, next stimulus comes to contract more, muscle is firing at high tension, lots of force, but brief dips, some relaxation 

4. Complete Tetanus - maximum force that a muscle can produce, reaches a plateau, 

• Are all individual muscle fibers, needs to link multiple twitchs to get a muscle contraction 

Muscle Contractions 

Whole muscle is determined by: 

• Tension produced by individual muscle fibers 

• Number of muscle fibers 

Sum of tension generated by each muscle fiber is the total tension of tension of the muscle 

Motor Units 

Motor Unit = motor neuron and all the muscle fibers that it innervates 

Motor units vary in size from - 4-6 fibers per motor neuron in eye muscles, 1000-2000 fibers per motor neuron in the quadricep muscles 

• Recruitment is used to increase the number of active motor units 

• Asynchronous motor unit summation is used to rest in a rotation 

Muscle Tone 

• Muscle tone = resting tension 

• Without causes movement tone: - stabilizes bones and joints, maintain balance and posture 

• Greater muscle tone = higher metabolism 

Type of Muscle Contraction 

• Classified based on the muscles pattern of tension production 

• Isotonic = equal tension; muscle shortens or lengthens

• Isometric = equal length, muscle length does not change 

Concentric Contraction 

• Muscle tension exceeds load and muscle shortens 

Eccentric Contraction 

• Muscle tension is less than the load and muscle lengthens 

Isometric Contractions 

• Tension never exceeds the load 

• Muscle doesn't change length 

Muscle Performance 

• Skeletal muscle has a huge capacity to adapt to stresses placed upon it 

Force: maximum amount of tension produced 

Endurance: amount of time an activity can be sustained 

Both force and endurance are impacted by:

• Physical conditioning 

• Types of muscle fibers

 3 types of muscle fibers exist in humans and are classified in several ways:

• Type of myosin (Type 1 or Type 2)

• Speed of contraction (fast, slow, intermediate)

• Oxidative capacity and a mix of other physical characteristics 

• Type 2 are fast and use ATP, and generate more force 

Slow Twitch Fibers 

• AKA: Type 1, slow oxidative (SO), slow-twitch oxidative, red fibers 

• Coloring is based on blood vessels 

Physical Characteristics 

• Small diameter 

• Small glycogen reserves/ample lipid supply 

• Many mitochondria 

• Rich capillary supply 

• Abundant myoglobin

Fast Twitch Fibers 

• AKA: Type llb(x), fast fatigable, fast glycolytic (FG), white fibers 

• Use glycolysis and use ATP 

• White because they have less blood supply 

Physical characteristics 

• Large diameter 

• Many myofibrils 

• Large glycogen reserves 

• Few mitochondria 

• Low capillary supply 

• Able to reach peak tension very quickly and produce strong contractions 

Intermediate Fibers 

• AKA: Type lla, fast oxidative glycolytic (FOG), fast resistant 

Physical Characteristics: 

• Intermediate diameter 

• Intermediate mitochondria supply 

• Intermediate capillary supply 

• Fast contraction speed, strong contractions 

Fiber Type Distribution 

Some animals have segregated muscles: 

• White muscles are mostly flat fibers 

• Red muscles are mostly slow fibers 

Human Muscles have a mix of all 3 fibers 

• Are pink in colour as a result 

Size Principle 

• Describes recruitment patterns during muscle contraction 

• Small motor units have lowest threshold and are recruited first 

• Are force requirements increase, larger motor requirements are recruited 

• Smaller motor units are associated with slow twitch (l) fibers, large motor units are associated with fast twitch 

Muscle Tissues 

• Primary tissue 

3 types 

Skeletal 

• Striated and multinucleated 

• Moves and stabilizes position of skeleton 

• Guards entrances and exits to digestive respiratory and urinary tracts 

Cardiac

• Striated and single nucleus 

• Found in heart 

• Circulates blood, maintains blood pressure 

Smooth 

• Short and nonstriated 

• Found in blood vessels and digestive, respiratory and urinary tract 

Functions of Muscles 

• Produce skeletal movement 

• Maintain posture and body position 

• Support soft tissue 

• Guard entrances and exits 

• Maintain body temperature 

• Store nutrient reserves 

Properties of Muscles 

• Excitability: responsiveness or a capacity to respond to a stimulus 

• Contractility: ability of cells to shorten with force 

• Extensibility: ability to be stretched beyond normal resting length (stretching) 

• Elasticity: ability to return to normal resting length after being stretched  (recoil) 

Muscle Tissue 

• Cells are specialised for contraction 

• Skeletal muscles move the body by pulling on bones 

• cardiac and smooth muscles control movement inside the body 

Organization of Skeletal Muscle 

• Epimysium (organ level)

• Perimysium (tissue level) 

• Endomysium (cellular level)

Skeletal Muscle Fibers 

Key terminology: 

• Sarcolemma 

• Transverse tubules 

• Sarcoplasmic reticulum 

Skeletal muscle fibers (cells) are packed with bundles of protein called myofibrils 

Myofibrils are made of two types of myofilaments 

• Thin filaments = ACTIN and regulatory proteins 

• Thick filaments = contain MYOSIN 

Myofibrils actively shorten = contraction \

Sarcoplasmic Reticulum 

• Tubular network that surrounds myofibril and sequesters CALCIUM (Ca2+)

Sarcomere 

• Thick and thin filaments produce a banded, or striated appearance 

Thin Myofilaments 

• Actin: contains the active site, creates cross-bridges with myosin 

• Troponin: Binds Ca2+ released from sarcoplasmic reticulum 

• Tropomyosin: locks the active site until moved when troponin binds Ca2+

Thick Myofilaments 

Key Components:

Myosin: two main parts 

• Heads: acts as ATPases and form cross-bridges with actin 

• Tails: bind together forming thick filament - the hinge region is found between the head and tail 

• Titin: stabilizes sarcomere and recoils after stretching 

• 300 myosin molecules per thick filament 

Sliding Filament Theory 

• During muscle contraction, actin and myosin filaments slide over each other 

• Cross-bridges creates tension (pulling force) 

**muscles are like ropes, they can only pull, never push

Neuromuscular Junction (NMJ) 

• Synapse between motor neuron and skeletal muscle 

• Ach is the neurotransmitter 

• Nicotinic receptors are chemically gated Na+ channels 

• Depolarize the muscle leading to contraction 

Contraction Cycle 

1. Contraction cycle begins when Ca2+ arrives at the zone of overlap 

2. Active site exposure occurs when Ca2+ binds to troponin and tropomyosin is moved, allowing interaction with energized myosin head 

3. Cross-bridge formation occurs between the active site on actin and the energized myosin head 

4. Power stroke uses the stored energy in the myosin head to pivot the hunger region moving the head towards the M line, ADP and phosphate (P) are released 

5. Cross-bridges are detached when an ATP molecule binds to myosin head, exposing the active site on actin for another cross-bridge 

6. Myosin reactivation occurs when the ATP is broken down into ADP and phosphate, the energy released is used to re-cock the myosin head 

Contraction Cycle continues if: 

• Neural stimulus continues 

• Free calcium ions in cytosol 

• ATP is available 

• Stimulation at the neuromuscular junction 

• Ca2+ is available at the zone of overlap 

• ATP is available 

Muscle Relaxation 

Return to resting conditions after relaxation-contraction ends 

• Acetylcholine (AChE) breaks down ACh in neuromuscular junction 

• Depolarization of the muscle ends 

Ca2+ is removed at the zone of overlap 

• Active transport of Ca2+ in sarcoplasmic reticulum (major) 

• Active transport of Ca2+ across sarcolemma into ECF (minor)

Rigor Mortis 

• After death, circulation stops and the muscle is deprived of nutrients and oxygen 

• After a few hours, muscle fibers run out of ATP 

• Muscles can’t pump Ca2+ out of the cytosol and Ca2+ levels rise 

• Without ATP, cross bridges cannot be released and the muscles become “locked in the contracted position” 

• Begins 2-7 hours after death and can last 1-6 days depending on conditions 

Tension Production 

• Tension = force 

• Tension is determined by number of pivoting cross-BRIDGES 

Tension produced is dependent on:

• Number Of power strokes performed 

• Resting muscle fiber length 

• Frequency of stimulation

Frequency of Stimulation 

• A muscle twitch is produced when a single action potential initiates contraction - twitch lasts 7-100sec

• Sustained contraction require repeated stimulation 

• A myogram is a graph of tension produced by a single twitch 

Muscle Contractions 

Whole muscle tension is determined by: 

• Tension produced by individual muscle fibers 

• Number Of muscle fibers stimulated 

Sum of the tension generated by each muscle fiber is the total tension of the muscle 

Motor Units 

• Motor unit = motor neuron and all the muscle fibers that it innervate s

Motor units vary in size from: 

• 4- fibers per motor neuron in eye muscles

• 1000-2000 fibers per motor neuron in the quadriceps muscles 

Recruitment is used to increased the number of active motor units 

Asynchronous motor unit summation is used to rest in a “rotation”

Muscle Tone 

Muscle tone = resting tension 

Without causing movement muscle tone: 

• Stabilizes bones and joints 

• Maintain balance and posture 

Greater muscle tone = higher metabolism 

Types of Muscle Contraction 

• Classified based on the muscles patter of tension production

• Isotonic = a equal tension; the muscle changes length 

• Isometric - equal length; the muscle does not change length

Isotonic Contractions 

Muscle length changes 

Two types: 

Concentric contraction 

• Muscle tension exceeds load and muscle shortens 

Eccentric Contraction

• Muscle tension is less than the load and muscle lengthens 

Isometric Contraction 

• Tension never exceeds the load 

• Muscles does not change length 

Muscle Performance 

• Skeletal muscle has a huge capacity to adapt to the stressor upon it 

• Force : maximum amount of tension produced 

• Endurance: amount of time an activity can be sustained

Both force and endurance are impacted by: 

• Physical conditioning 

• Types of muscle fibers 

3 types of muscle fibers exist in humans and are classified in several ways 

• Type of myosin (Type I or Type II) 

• Speed of contraction (Fast, Slow, Intermediate) 

• Oxidative capacity and a mix of other physical characteristic 

Slow Twitch Fibers 

AKA: Type 1, slow oxidative (SO), slow twitch oxidative, red fibers 

Physical Characteristics: 

• Small diameter 

• Small glycogen reserves/ample lipid supply 

• Many mitochondria 

• Rich capillary supply 

• Abundant myoglobin 

Half the diameter of fast-twitch fibers and 3x times slow to reach peak tension

Fast Twitch Fibers 

AKA: Type llb (x), fast fatigable, fast glycolytic (FG), white fibers 

Physical Characteristics 

• Large diameter

• Many myofibrils 

• Large glycogen reserves

• Few mitochondria 

• Low capillary supply 

Able to reach peak tension very quickly and produce strong contractions 

Intermediate Fibers 

AKA: Type lla, fast oxidative glycolytic (FOG), fast resistant 

Physical Characteristics closely resembles fast twitch in appearance 

• Intermediate diameter 

• Intermediate mitochondria supply 

• Intermediate capillary supply

Fast contraction speed, strong contractions 

Fiber Type Distribution 

Some animals have segregated muscles 

• White muscles are mostly fast fibers 

• Red muscles are slow fibers

Humans muscles have a mix of all 3 muscle fibers 

• Pink color is a result of it 

Size Principle 

• Describes recruitment patterns during muscle contraction

• Small motor units have lowest threshold and are recruited first 

• As force requirement increase, larger, motor units are recruited 

• Small motor units are associated with slow-twitch (l) fibers, large motor units are associated with fast-twitch

Effects of Exercise 

It’s not easy to change one fiber type to a different fiber type 

• Type l remain Type l 

• Type llb (x) can become Type lla 

• Training can increase size and capacity of both types (l & ll)

Effects of Exercise 

• High intensity training increases muscular strength and mass in Type ii fibers 

• Endurance training enlarges Type l fibers

•  Endurance training can also convert some Type llb (x) fibers to Type lla fibers

Hypertrophy: increase in muscle size 

• Due to increase in number of myofibrils 

• Also results in an increase in blood supply and mitochondria

Atrophy: decrease in muscle size 

Hyperplasia: increase in number of skeletal muscles 

*doesn’t happen in humans 

Strengthening Exercises 

• Strength increases from hypertrophy are not only achieved by increases in muscle size 

• Increases in muscle strength is also archives by change sin the nervous system 

• Nervous system in a trained person can recruit a large number of motor units than an untrained person 

• Neural factors account for rapid and significant strength increases early in training 

• Often occurs without an increase in muscle size and cross-sectional area 

Neural Factors in Strength Gains 

• Greater efficiency in neural recruitment patterns 

• Increased central nervous system activation 

• Increased motor unit synchronization 

• Lower neural inhibitory reflexes 

Endurance Exercises 

Gained by things like: 

• Improved metabolism 

• Increased mitochondrial content 

• Increased number of capillaries 

• More efficient respiration 

• Greater cardiac output 

Aging and Muscles 

• Changes can begin as early as 25 years old 

• Strength can be reduced 30-50% by 80 years old which is 3-5% per decade after 30 

Why does Aging occur?

• Fast-twitch fibers decrease in number more rapidly than slow twitch fibers, contributes to the loss of strength and speed 

• Surface area of neuromuscular junction decreases, which results in fewer action potentials produced in muscle fibers 

• Number of motor units also decreases, remaining motor units take up extra fibers (can result in less precision)

• Decrease in density of capillaries in skeletal muscle - reduced blood flow to muscles and longer recovery period following exercise 

BUT 

• These changes can be slowed by remaining active 

Fatigue: when a muscle can no longer perform at the required level 

Many complex factors involved including 

• ATP availability 

• Declining pH (increased acidity) 

• Damage to membranes 

Endurance and resistance training can prolong performance and delay fatigue 

Spinal Reflexes 

• Reflexes are “automatic” responses to stimuli 

• Two types of receptors in muscle play an important role 

• Muscle spindles are involved in stretch reflex 

• Golgi tendon organs are involved in the tendon reflex 

Muscle Spindles 

• Monitors muscle & speed of contraction 

Includes:

• Intrafusal fibers 

• Gamma motor neuron (gamma efferents)

• Afferent sensory nerves

Metabolism 

Metabolism, Nutrition & Energetics 

Energetics: the study of the flow of energy and its changes from one form to another 

• We get our energy from the nutrients (fats, proteins, carbohydrates we eat) 

Energy to Power Contractions 

• At rest, skeletal muscles produce more ATP than they need 

• ATP transfers energy to creatine - becomes creatine phosphate 

Recovery from Exercise 

• Following a bout of exercise several byproducts and processes need to be dealt with 

• Lactate is removed and recycled (CORI CYCLE)

• Excess Postexercise Oxygen Consumption (EPOC)

• Heat production 

• Hormones

Metabolism and Energetics 

Catabolism: breakdown of organic molecules 

• Releases energy that is used to synthesis ATP 

• Proceeds in a series of steps 

• Most ATP is formed in mitochondria 

Anabolism: synthesis of new organic molecules 

• Carry out structural maintenance & repairs 

• Support growth 

• Store nutrients 

Energy is harvested from nutrients by breaking chemical bonds 

Two critical chemical reactions are:

• Oxidation: the loss of a hydrogen atom or electrons 

• Reduction: gaining a hydrogen atom or electrons 

LEO says GER 

• Lose Electrons Oxidation, Gain Electrons Reduction 

Two critical coenzymes acts as intermediates in the flow of energy in the cell 

They accept electrons from one molecule and transfer them to another 

• Called redox reactions 

NAD (Nicotinamide Adenine Dinucleotide) 

• Oxidized form is positive NAD+ 

• Accepts 2 hydrogen atoms - gains 2 electrons, ejects a proton 

• Reduced form is NADH 

FAD (Flavin Adenine Dinucleotide) 

• Accepts 2 hydrogen atoms - gains2 electrons 

• Reduced form is FADH2 

Carbohydrate Metabolism 

• Macromolecule with Carbon, Hydrogen and Oxygen in the ratio 1:2:1 

• glucose = C6H12O6

• Sugars and starches come from plants 

• Glycogen is the storage form in animals 

• Cellular respiration: glucose, oxygen, carbon dioxide, water 

• Glucose is used to generate new ATP 

A multistep process involving 

• Glycolysis 

• Citric acid cycle 

• Electron transport chain 

Catabolism of 1 glucose molecule = 32 ATP

Glycolysis 

• Glucose (6 Carbon) - 2 x pyruvate (3 carbon) 

• Occurs in a series of 7 metabolic steps in the cytosol 

Requires: 

• Glucose, enzymes, ATP & ADP, inorganic phosphate, (Pi) NAD 

• Anaerobic = does not require oxygen (O2) 

Glycolysis 

Phase 1 - Energy Investment 

Steps 1 & 2 - Phosphorylation 

• Costs the cell 2 ATP molecules 

Step 3 - “split” 

• Creates two 3-carbon fragments

Phase 2 - Energy Harvesting 

Step 4 - Reduction of NAD+ 

• Occurs for both 3 carbon fragments 

• 2 NAD+ - 2 NADH 

Step 5 - Formation of 2 ATP molecules 

Step 6 - Formation of 2H2O molecules 

Step 7 - Formation of 2 ATP molecules 

• End point of glycolysis is reached, and 2 molecules of pyruvate are formed 

Glycolysis is an anaerobic process 

• Doesn’t require O2 

Occurs in the cytoplasm of the cell 

Results in: 

• Net Gain = +2 ATP molecule s

• 2 NADH 

• 2  H2) and 

• 2 NADH 

Pyruvate 

• Still contains lots of energy in its bonds 

• To capture the energy…we need aerobic metabolism - oxygen must be available 

• If oxygen is available, we now move into the mitochondria for more metabolism 

• Occurs in mitochondria 

• Results in a molecule of 1 Acetyl CoA, 1 CO2 and 1 NADH 

Citric Acid Cycle 

• AKA: Tricarboxylic Acid Cycle - TCA cycle or Krebs cycle 

• Goal is to remove hydrogen atoms from molecules and transfer them to coenzymes (NAD & FAD) 

• 8 steps that start and end at the same place 

Citric Acid Cycle 

• Acetyl CoA (2 Carbon) combines with oxaloacetate (4 carbon) to form citrate (6 carbon) 

Electron Transport Chain 

• Series of redox reactions in the inner mitochondrial membrane 

• Produces more than 90% of the body’s ATP via - oxidative phosphorylation 

• Coenzymes NADH and FADH2 donate electrons to respiratory complexes 

• Electrons are eventually passed to O2 forming H2O

Electron Transport Chain 

• Respiratory complexes use electron energy to pump H+ into the intermembrane space - creating a concentration gradient 

• H+ re-enters the matrix through the ATP synthase complex 

• Chemiosmosis - generation of ATP using kinetic energy of passing hydrogen ions 

• Each NADH donates to respiratory complex I - pumps 6 H+ - generates 2.5 ATP 

• Each FADH2 donates to respiratory complex II - pumps 4 H+ - generates 1.5 ATP 

• ATP generation is limited by the availability of either oxygen or electrons (NADH, FADH2) 

• No oxygen - no ATP production in the mitochondrial - pyruvate cannot be used - converted into lactate (more water) 

**LYASE = BREAKDOWN 

Lipid Metabolism 

• 95% of lipid in our diet are triglycerides 

Triglycerides: 

• 3 fatty acids (saturated or unsaturated fats) 

• Glycerol backbone 

Beta Oxidation 

• The part of the lipid molecule that is used for energy production is the free fatty acid 

• Free fatty acid (FFA) are metabolized by a process called beta oxidation  - produces Acetyl CoA in the mitochondria 

Most Acetyl CoA enters the Citric Acid Cycle 

• Each one generates 2 CO2, 3 NADH, 1 FADH2 and 1 GTP - ATP 

Lipids are responsible for 99% of the body’s energy storage 

During rest, nearly 60% of the energy supply is provided by the metabolism of fats 

Lipid Metabolism

Most Acetyl CoA enters the Citric Acid Cycle 

• Each one generates 2 CO2, 3 NADH, 1 FADH and 1 GTP - ATP 

• Lipids are responsible for 99% of the body’s energy storage 

• During rest, nearly 60% of the energy supply is provided by the metabolism of fats 

ATP Tally for 1 - 18 Carbon Saturated FFA 

Beta Oxidation 

• Uses 2 ATP 

• 8 NADH = 20 ATP 

• 8 FADH2 =12 ATP 

9 Acetyl CoA 

• 27 NADH = 67.5 ATP

• 9 FADH2 = 13.5

• 9 GTP = 9 ATP 

Protein Metabolism

• Chains of amino acids (nitrogen containing molecules) 

20 different amino acids 

• Essential amino acids (9) - must be consumed in our diets 

Non-essential amino acids - our body can synthesize as needed 

Used to synthesise 100,000 -140,000 different proteins 

Only some amino acids can also be used as an energy source- based on their structure 

Amino acids enter the processes at different points - ATP tallies vary as a result 

The liver plays a critical role when lipid or glucose reserves are low 

• Removes the nitrogen components 

• Break down proteins for energy metabolism 

Protein catabolism is impractical because: 

• Proteins are harder to breakdown (vs glucose or fats) 

• Nitrogen by-product (ammonia) is toxic to cells 

• More important for structural and functional properties 

Cellular Respiration 

• is both anaerobic and aerobic - not either /or both systems work concurrently 

• When we refer to exercise, anaerobic and aerobic refer to which energy system predominates in providing the most ATP - all systems are active all the time 

ATP Demand and Exercise 

• When you exercise, ATP demand is instantaneous and myosin heads are the largest users 

• Oxygen delivery for aerobic metabolism is delayed by 3-4 minutes 

The Energy Continuum 

• ATP-PC system predominates in activity lasting 5-15 seconds - creatine kinase is activated by ADP 

• Anaerobic metabolism (ATP-PC and glycolysis) predominates in supplying energy for exercise lasting less than 2 minutes 

• ADP and P activate phosphofructokinase (PFK) - rate limiting enzyme for glycolysis 

• Aerobic pathways dominate ATP production 5 minutes into exercise 

• The longer the exercise the more important it becomes 

Lactate 

• Lactate is produced in muscle cells 

• In absence of oxygen, NADH transfers it’s hydrogen to pyruvate lactate 

Lactate and Exercise 

• At higher intensities, glycolysis accelerates, and oxygen availability can be insufficient for aerobic metabolism 

• Lactate is transported out of the muscle and can accumulate in the blood - the lactate threshold is seen during incremental exercise 

• The point during incremental exercise when blood lactate begins to accumulate 

Why is Lactate a Problem?

• Lactates not the problem, because it's a great energy source 

Lactic acid = Lactate + H+ 

• Most is in the form of lactate 

• Causes a rise in H+ 

H+ is the issue,acidification of muscle cells and the blood 

• Causes pain during exercise 

• Contributes to fatigue and decreased performance 

Lactate Removal 

• Lactate is removed from the bloodstream relatively quickly following exercise 

• Generally half of the total lactate is removed in about 15-25 minutes 

• Near-resting levels can be achieved in 30-60 minutes

• 70% of lactate is oxidized for energy by the heart and skeletal muscles 

• Lactate removal occurs more quickly with light exercises during recovery 

• 20% is converted to glucose in the liver via gluconeogenesis 

• Can be released into the blood for use (Cori Cycle) 

• 10% is used to resynthesize glycogen stores 

Measuring Anaerobic Power 

Wingate Anaerobic Power Test 

• Specialized Monark Cycle Ergometer 

• All out sprint for 30 seconds 

• Resistance based on body weight (7.5%) 

• Pedal revolutions are measured 

3 Key Variables Calculated 

Peak Power (peak for 5 seconds) 

Mean Power (average for 30 seconds) 

Fatigue Index (best 5 seconds/worst 5 seconds) 

Training the Anaerobic System (sprint/power training) 

• Increases resting levels of ATP, CP, creatine and glycogen 

• Increases strength 

• Increases numbers of enzymes that control glycolysis 

• Increases capacity to generate high levels of lactate 

• All fibers engaged; Type ll(x) fibres are recruited early 

Measuring Aerobic Capacity 

Oxygen Consumption (VO2) 

• The volume of oxygen consumed per minute 

• Absolute = L/min 

• Relative = mL/kg/min 

• Having a high VO2 max reflects improved health and performance 

Measuring Aerobic Capacity 

Metabolic Equivalent (MET)

• 1 MET is your oxygen consumption at complete rest 

• 3-6 METs is moderate intensity physical activity 

• More than 6 METs is is vigorous intensity physical activity 

Training the Aerobic System (Endurance Training)

• Larger more numerous mitochondria in muscle 

• Dense capillary networks 

• Enhanced breakdown of fat during submaximal exercise - spares muscle glycogen 

• Enhanced ability to breakdown CHO during maximal exercise 

• Engages Type l and Type iia fibres engaged 

• Delays onset of blood lactate during exercise of progressively increasing intensity 

• Body composition changes 

• Performance change 

• Psychological benefits 

Midterm 

30 mc 

10 fill in the blanks 

20 short answer 

Lecture Notes 

Protein Metabolism

• Only some amino acids can be used as energy NOT all 20 

• Different amino acids enter metabolic processes at different stages/places meaning we get different amounts of ATP from them 

• Proteins are harder to breakdown 

• Either we use them or convert them to other things that are useful 

Cellular Respiration 

• Both aerobic and anaerobic 

• Don't think of the 2 systems separately - both occur all the time, just depends which one is using more ATP 

• Myosin head is largest used of ATP - look at 6 steps of contraction cycle 

• Glycolysis churns out energy in 5-10 seconds and is the predominant energy provider for 2 minutes 

• If PFK (phosphofructokinase) is activated then the rest of the process works easily 

• ADP being generated is the stimulus is the activates PFK

Lactate and Exercise 

• Hydrogen ions prevent and block cross bridges 

• Speed is measure of intensity 

• Lactate can be converted back into pyruvate by working muscles and that pyruvyate can go back into the citrc acid cycle and allows us to manage byproducts and provides energy 

• Heart uses lactate