Skeletal Muscle

SKELETAL MUSCLE Dr. Hoe See Ziau Department of Physiology Faculty of Medicine University of Malaya

Muscle Tissues Muscles in human body     

Specialised excitable tissues ~ 50 % body weight Ability to contract Contractions provide movements Do work  Move body or limbs  Push, pull or hold an external load or object  Mix or move food through the gastrointestinal track  Pump blood out of the heart to the blood vessels  Contract uterus for birth of foetus  Micturition and defaecation

Types of Muscle Three types of muscle: 1. Skeletal muscle 2. Cardiac muscle 3. Smooth muscle

Types of Muscle

Skeletal Muscle

Cardiac Muscle

Smooth Muscle

 striated

 striated

 non-striated

 voluntary

 involuntary

 involuntary

Basic Characteristics of Muscle Tissues  Excitability  Response to stimuli

 Conductivity  Able to conduct action potential

 Contractibility  Able to shorten in length

 Extensibility  Stretches when pulled

 Elasticity  tends to return to original shape & length after contraction

or extension

Skeletal Muscle  Attached to bones & moves skeleton

 Makes up 40% of BW in men and

32% of BW in women  Main functions of skeletal muscle:  Initiate movements  Perform work  Maintain posture  Stabilise joints  Generate heat

Level of Organisation in Skeletal Muscle Skeletal Muscle (organ)

Fascicle

(bundle of muscle fibres)

Muscle Fibre (cell)

Myofibril Sarcomere Filaments

(Thin – actin) (Thick - myosin)

(fascicle)

Membranes of Skeletal Muscle    

Muscle surrounded by epimysium Bundles of fibres (fascicles) surrounded by perimysium Muscle fibre surrounded by endomysium These connective tissues extend beyond the ends of muscle to form tendons that attach muscle to bones

Skeletal Muscle Fibre  Large, elongated, shape like

cylinder  10 – 100 µm in diameter, up

to 750,000 µm (0.75 m) in length (extend entire length of muscle)  Multinucleated with

abundant of mitochondria  Sarcolemma (cell membrane)  Sarcoplasm (muscle cell

cytoplasm)  Sarcoplasmic reticulum

(modified ER)

 Transverse tubules (T-tubules)

– internal conduction system  Myofibrils for contraction  Sarcomeres – regular

arrangement of thin (actin) & thick (myosin) filaments  Actin filaments interdigitate

with myosin filaments  Appears striated under

microscope

Structure of a Skeletal Muscle Fibre

Electron Micrograph of Skeletal Muscle

Sarcomere • The functional unit of skeletal muscle • Multi-protein complexes composed different filament systems: 

Thin filament system



Thick filament system

Sarcomere sarcomere

Sarcomere

Sarcomere

Sarcomere

Sarcomere 

A band (dark band) 



I band (light band) 



The lighter area in the centre of A band where the thin filaments do not overlap with thick filaments

M line 



Consists of the array of thin filaments, and is the region where they do not overlap the thick filaments

H zone 



consists of a stacked set of thick filaments

Consists of supporting proteins that hold the thick filaments together vertically within each stack

Z line 

Consists of supporting proteins that hold the thin filaments together vertically within each stack



Area between two Z lines is called a sarcomere

Thin Filament 





Actin 

Spherical in shape, with a special binding site for attachment with myosin cross bridge



Joined into two strands and twisted together to form the backbone of a thin filament

Tropomyosin 

Threadlike proteins that lie end-to-end alongside the groove of the actin spiral



Covers active sites of actin

Troponin complex 

binds to actin & holds tropomyosin in place

Thin Filament

Thin Filament Troponin Complex

  

TnT – binds to tropomyosin TnC – binds to Ca2+ TnI – binds to actin

Thick Filament  Each thick filament is composed

of several hundred myosin molecules packed together  A single myosin protein looks like

2 golf clubs with shafts twisted about one another  Myosin molecules have

elongated tails & globular heads  Heads form cross-bridges

between thick and thin filaments during contraction

Thick Filament Cross Bridges

 Each cross bridge has two

important sites:  An actin-binding site  A myosin ATPase site

Organisation of Actin and Myosin  Thin filaments are arranged

hexagonally around thick filaments Cross bridges

 Each thin filament is surrounded

by 3 thick filaments  Cross bridges project from each

thick filament in all 6 directions toward the surrounding thin filaments

Contraction of Muscle Fibres  Done by sliding actin filaments

Contraction of Muscle Fibres

Contraction of Muscle Fibres Sliding Filament Theory  Contraction occurs by actin filaments sliding into

myosin filaments  Actin filaments move, myosin filaments remain

stationary  Sarcomeres shortened  Cause whole muscle to contract

Contraction of Muscle Fibres Role of Calcium  Ca2+ released from

sarcoplasmic reticulum  Ca2+ binds to troponin C

 Troponin turns, moves tropomyosin & exposes actin active site

Contraction of Muscle Fibres Role of Calcium  Myosin head binds to actin

active site, form cross-bridge, move & produces powerful strokes  Actin slides in – muscle fibre contracts

 Cross-bridge action continues while Ca2+ is present  When action potential stops, Ca2+ is pumped back to SR  Tropomyosin covers back actin’s active site  Relaxation occurs

Contraction of Muscle Fibres Role of Calcium

Contraction of Muscle Fibres Role of ATP

 ATP split by myosin ATPase ; ADP and Pi

remain attached to myosin; energy is stored within the cross bridge  Mg2+ must be attached to ATP before ATPase 2

can split the ATP  Ca2+ released on excitation, removes

1

3

inhibitory influence from actin → energised myosin cross bridge bind with actin  Cross bridge bends and causes power stroke  ADP and Pi are released after power stroke is

completed 4

 ATPase site is free for attachment of another

ATP  Attachment of new ATP permits detachment

of cross bridge

Contraction of Muscle Fibres

Contraction of Muscle Fibres

 All the cross bridges’ power strokes are directed

toward the centre of the sarcomere  All 6 of the surrounding thin filaments on each end

of the sarcomere are pulled inward simultaneously

Contraction of Muscle Fibres Rigor Mortis

 “Stiffness of death” – a generalised

locking in place of skeletal muscle that begins 3 to 4 hours after death  Following death, [Ca2+]i begins to rise  This Ca2+ moves the regulatory proteins

aside, permitting actin bind with the myosin cross bridges, which were already charged with ATP before death  No fresh ATP available after death, actin

and myosin remain bound in rigor complex  Resulting in stiffness condition of dead

muscles

Electrical Properties of Muscle Fibres  When an adequate stimulus is given

→ action potential  Maximum potential: +30mV  Depolarisation is due to influx of Na+

 Time taken: 1 – 2 msec  Absolute refractory period & relative

refractory period present  Action potential results in muscle

contraction

Membrane potential (mV)

 Resting membrane potential: -90mV

-90

Action Potential and Muscle Twitch

Membrane potential (mV)

Tension

 Latent period  The delay between stimulation and the onset of contraction (a few msec)  Contraction time  The time from the onset of contraction until peak tension is developed (average ~ 50 msec)  Relaxation time  The time from peak tension until relaxation (~ 50 msec or more)  A single contraction/relaxation -90

cycle is called a muscle twitch

Excitation-Contraction Coupling  Refers to the series of events linking muscle

excitation (electrical events) to muscle contraction (mechanical events)  Electrical events – presence of action potential

 Mechanical events – cross-bridge activity  Electrical events come first before mechanical

events  Ca2+ is the link between excitation and contraction

Excitation-Contraction Coupling Transverse Tubules (T tubules)  The surface membrane at

each junction of A band and I band dips into muscle fiber to form a T tubule  Action potential on the

surface membrane spreads down into the T tubule  The presence of local action

potential in T tubule induces permeability changes in the sarcoplasmic reticulum

Excitation-Contraction Coupling Sarcoplasmic Reticulum (SR)  Modified endoplasmic

reticulum  Consists of a fine network of

interconnected compartments surrounding each myofibril  Separate segments of SR are

wrapped around each A band and each I band  The ends of each segment

expand to form lateral sacs, which store Ca2+

Excitation-Contraction Coupling Release of Ca2+ from SR  When action potential is

propagated down the T tubules, local depolarisation activates the voltage-gated dihydropyridine receptors in T tubule  These activated receptors in turn

trigger the opening of Ca2+release channels (alias ryanodine receptors) in adjacent lateral sacs of SR  Ca2+ is released into the

surrounding sarcoplasm

Relaxation of Muscle Fibres  When ACh is removed from the neuromuscular junction, the muscle

fibre action potential ceases  No longer a local potential in T tubules to trigger Ca2+ release  Released Ca2+ is pumped back into the lateral sacs by Ca2+-ATPase

pump  Removal of sarcoplasmic Ca2+ allows the troponin-tropomyosin

complex to slip back into its blocking position  Actin and myosin are no longer able to bind at the cross bridges  Thin filaments are able to return passively to their resting position

 Relaxation occurs

Excitation-Contraction Coupling

Excitation-Contraction Coupling and Relaxation Summary of Events 1. Ach released from the terminal of a motor neuron initiates an action potential in the muscle fibres

6. Actin slides in, muscle fibre contracts resulting in contraction of whole muscle

2. Muscle action potential travels down T tubule

7. ADP and Pi are released after the power stroke is complete

3. Causes SR to release Ca2+ into sarcoplasm

8. New ATP binds to myosin head; detachment of the cross bridge

4. Ca2+ binds to troponin, exposing actin’s cross-bridge binding sites

9. Cross-bridge action continues while Ca2+ is present

5. Myosin head binds to active site, form cross-bride, moves and produces power stroke

10. When action potential stops, Ca2+ pumped back to SR

11. Tropomyosin covers back active sites 12. Relaxation occurs

Contraction of Whole Muscles  Even when muscle are at rest, certain amount of

tautness usually remain → muscle tone  Results from a low rate of nerve impulses coming

from the spinal cord  To maintain a normal posture

Contraction of Whole Muscles  Whole muscles are groups of muscle fibres bundled

together  Muscle fibres in each muscle can function

cooperatively to produce contractions of variable grades of strength  When the whole muscles contract, tension is created  Gradation of whole muscle tension depends on  The number of muscle fibres contracting within a muscle  The tension developed by each contracting fibre

Motor Unit  Each whole muscle is innervated by a

number of different motor neurons  One motor neuron innervates a number

of muscle fibers  Each muscle fiber is supplied by only

one motor neuron  A motor neuron plus all the muscle

fibres it innervates is called a motor unit  When a motor neuron is activated, all

the muscle fibres in that motor unit are stimulated to contract simultaneously  Each muscle consists of a number of

intermingled motor units

Motor Unit  Precise control of movement determined by

number and size of motor units  The number of muscle fibres innervated by one motor neuron – innervation ratio  The bigger the ratio of nerve to muscle fibres, the

coarser the movement will be  Examples:  1:4 – fine movements (external eye muscle)

 1:200-300 – Coarse movements (back muscle)  1:150 – on average

Motor Unit

Motor Unit Recruitment  For a weak contraction of the whole muscle,

only a few of its motor units are activated  For stronger and stronger contractions, more and more motor units are stimulated to contract → motor unit recruitment

Motor Unit Recruitment  At minimum stimulus strength  only motor units with low threshold will contract

 At maximum stimulus strength  all motor units contract

 ↑ strength of stimulus  ↑ recruitment of motor units  ↑ contraction

Mechanical Properties of Skeletal Muscle Muscle Twitch  A twitch is a single contraction/ relaxation cycle

Mechanical Properties of Skeletal Muscle Muscle Twitch  If the muscle has completely relax before the next stimulus

takes place  A second twitch of the same magnitude as the first occurs

Maximum tension (in treppe)

• When a muscle begins to contract, its initial strength of contraction may be as little as ½ of its strength 10 to 50 muscle twitches later • The strength of contraction increase to plateau

(Treppe)

Mechanical Properties of Skeletal Muscle Summation and Tetanus

 Summation  If the muscle is restimulated before it has completely relaxed, the

2nd twitch is added on to the 1st twitch, resulting in summation

 Tetanus  When the muscle is stimulated so rapidly that it does not have an opportunity to relax between stimuli, a maximal sustained contraction occurs → tetanus

Mechanical Properties of Skeletal Muscle Summation and Tetanus  When muscle is stimulated, Ca2+ is released from SR → cross-

bridges → contraction  When stimulation ceases, Ca2+ is pumped back into SR  If the 2nd stimulation occur far enough apart in time for all the

released Ca2+ from the 1st contractile response to be pumped back into SR → an identical twitch response occurs  With rapid stimulation, there is not enough time between

successive stimulations to remove all the Ca2+ from the sarcoplasm → Ca2+ levels in the sarcoplasm increase → more active crossbridges → a stronger contraction → summation occurs

Mechanical Properties of Skeletal Muscle  Response of muscle to repeated stimulation  As strength of stimulation increases gradually, more

& more motor unit will be activated → recruitment  As frequency of stimulation increases gradually,

contraction will increase more & more, then become sustained → summation & tetanus

Mechanical Properties of Skeletal Muscle Muscle Fatigue

 When the stimulation is given repeatedly at a fast rate  Contraction becomes weaker & weaker gradually

 Contraction becomes more irregular  Until no contraction occur  Fatigue occurs

Mechanical Properties of Skeletal Muscle Fatigue Type  Muscle fatigue  Occurs when an exercising muscle can no longer respond to stimulation with the

same degree of contractile activity  Causes: 

Accumulation of lactic acid



Depletion of energy stores



supply of O2 and nutrients

 Central fatigue  Occurs when the CNS no longer adequately activates the motor neurons

supplying the working muscles  Neuromuscular fatigue  Depletion of acetylcholine

Mechanical Properties of Skeletal Muscle Length-tension relationship  At each determined muscle length:  Without stimulation → passive tension  With stimulation → total tension  Active tension = total tension – passive tension

Mechanical Properties of Skeletal Muscle Length-tension relationship  In general, tetanic tension develop muscle length (within limit)

initial

 For every muscle → an optimal length at which maximal force can be developed

 In the body, relaxed length of muscle are also the optimal length  Capable of obtaining maximal tetanic

contractions & maximal force

Mechanical Properties of Skeletal Muscle Length-tension relationship

Mechanical Properties of Skeletal Muscle Types of Contraction 1.

Isotonic contraction Tension developed – constant  Muscle length – changes  For 

 

2.

Body movement Moving an external load or object

Isometric contraction Muscle length – constant  Tension developed – changes  For 



Holding a load or object

Mechanical Properties of Skeletal Muscle Isotonic Contraction

 The muscle length changes to move a load

Mechanical Properties of Skeletal Muscle Isometric Contraction

 Tension in the muscle increases but the muscle fibres neither shortened or lengthened

Mechanical Properties of Skeletal Muscle

Isotonic

Isometric

Skeletal Muscle Metabolism  Contraction-relaxation process requires ATP in

three different steps: 1. Splitting of ATP by myosin ATPase provides energy for the

power stroke of the cross bridge 2. Binding of fresh ATP molecule to myosin permits

detachment of the bridge from actin filament at the end of power stroke 3. The active transport of Ca2+ back into the SR during

relaxation

 ATP must constantly be supplied for contractile activity to continue

Skeletal Muscle Metabolism  Additional ATP is supplied by three pathways: 1. Creatine phosphate Creatine phosphate + ADP 

Creatine + ATP

First source for supplying additional ATP

2. Oxidative phosphorylation 

Takes place within the muscle mitochondria if sufficient O2 is present



Fueled by glucose and fatty acids



Relatively slow because involves many steps

3. Glycolysis 

Synthesis ATP in the absence of O2



Uses large amounts of stored glycogen and produces lactic acid in the process

Skeletal Muscle Metabolism

Types of Skeletal Muscle Fibres  Three types of muscle fibres are classified by:  The pathways they used for ATP synthesis  

Oxidative Glycolytic

 The Speed of their contraction  

Fast Slow

1. Slow-oxidative (type I) fibres 2. Fast-oxidative (type IIa) fibres 3. Fast-glycolytic (type IIb) fibres

Types of Skeletal Muscle Fibres Fast vs slow fibres  Fast fibres have higher myosin ATPase (ATP-splitting)

activity  ATP is split More rapidly  The rate at which energy is made available for cross-bridge cycling

is faster  Results in a fast twitch

 Fast fibres are activated by large-diameter motor neurons  Slow fibres are activated by small-diameter motor neurons

Types of Skeletal Muscle Fibres Oxidative vs glycolytic fibres  Oxidative fibres have a greater capacity to form ATP  More ATP is yielded from each nutrient molecule processed → does not readily deplete energy stores  Does not result in lactic acid accumulation  More resistant to fatigue • Oxidative fibres have a high myoglobin content →

red fibres

Types of Skeletal Muscle Fibres  Most skeletal muscles contain a

mixture of all three fibre types  A single motor unit always

contains one type or the other  The percentage of each type

determined by the type of activity for which the muscle is specialised

Muscle Hypertrophy  Enlargement (increase in diameter) of muscle

 Total mass of muscle increases  Results from increased synthesis of actin and myosin

filaments in each muscle fibre  Occurs when the muscle undergoes regular bouts of

anaerobic, short-duration, high-intensity resistance training

Muscle Atrophy  Muscle becomes smaller and weaker

 Total mass of muscle decreases  Results from decrease of actin and myosin content  Disuse atrophy

 Occurs when a muscle is not used for a long

period of time even though the nerve supply is intact  Denervation atrophy

 Occurs after the nerve supply to a muscle is lost