11. 12.Skeletal muscle, contraction, electromechanical coupling , muscle fatigue Flashcards
Basic characteristics of skeletal muscles
Attached at skeletal bones
striated
Innervated by somatic nervous system
Contraction under voluntary control
What are functions of skeletal muscle
Reflex and voluntary movements
Respiration-contraction of diaphragm and external intercostal
Regulate body temperature (thermogenesis)- heat generated during contraction - shivering when body temp low
Controls entrance (mouth) and exit (anus) orifices- voluntary control of food and fluids and rid urine/faeces
Help movemnt of venous blood to heart
Support and protect visceral organs
They are made of muscle fibers. what are these called
myofibers (each fiber is a single cell)
- Elliptical, multinucleated and nuclei just below sarcolemma
*The cytoplasmic surface of the cell membrane is
covered by a protein called dystrophin, which
provides mechanical stabilization during cell
contractions.
Composition of a myofiber
Individual fibres surrounded by connective tissue ENDOMYSIUM
Muscle fibers grouped together in fascicles surrounded by PERIMYSIUM
Fascicles bundle together to form whole muscle surrounded by epimysium
What do myofibers (myocytes) contain in their cytoplasm that helps with contractility
myofibrils
1 micrometer thick
divided into thick and thin myofilaments
What are myofibrils
What is their main structural and functional unit
Cell organelles responsible for muscle contraction
Sarcomere
What are the components of myofibers
cell memebrane= sarcolemma (surrounded by ECM forms basal lamina)
Cytoplasm= sarcoplasm ( has glyccogen granules, myoglobin, sarco retic and mysofibrils)
ER= Sarcoplasmic reticulum (storage for Ca2+
T tubules ( sarcolemmal invaginations conducts excitation from sarcolemma to myofibrils)
What causes the striated appearance
repeating dark and light bands in myofibrils
What are the thick and thin filaments of sarcomeres
thick= myosin
thin= actin
myosin composition
6 polypeptide chains
2 heavy 4 light
heavy chain = long tail and globular head. the tails form double helix
What are properties of myosin heads
- Bind to active center of actin
- Binding site for ATP
- Binding site for light chains
Actin is a globular protein froming double helix
What blocks its active center
Tropomyosin
Bound to tropomyosin is troponin (3 subunits I,C,T)
I= prevents removal of tropomysoin when there no signal
C= receives signal from calcium ions
T= bind the other sununits to topomyosin
*Troponin moves tropomyosin when there is a impulse
As well as contractile proteins what other proteins are there
Cytoskeletal proteins
Corrects arrangement of thick and thin filaments
Titin and nebulin are longitudinally arranged cytoskeletal protein
What are the 6 physiological properties of skeletal muscle
- excitability
- Conductivity
- Contractility
- elongation
- Elasticity
- Generation of heat
How are skeletal muscles excited
nerve impulses reach them along motor neurones of somatic system
The axons reach the muscle by what site
Neuromuscular juntion
Sarcomere bands
Z lines= ends of the sarcomere
DARK: a-actinin homodimer
I band= either side of z line
LIGHT: thin actin filaments
A band=located in middle of sarcomere
large diameter so restricts light
DARK: Thick myosin
M line= where the myosin filamets join tail
to tail
H zone= where thick and thin filaments
dont overlap (A and I band)
LIGHT
What bit of the sarcomere contracts
Only I band shortens
What does it mean the myo-neuronal synapse is 1:1?
Each impulse in the presynaptic cell triggers an action potential in the postsynaptic structure without the need for temporal or spatial summation, as in most CNS cells.
Mechanism of neuromuscular junction
- AP travels down motor neuron and causes depolarization
- Depolarization causes Ca2+ channels to open and calcium to enter presynaptic terminal
- Ca2+ and snare complex Ach is extruded into synaptic cleft
- Ach binds to receptors on post synaptic membrane ( mototr end plate)
- This causes Na+/ K+ channels to open = depolarization= EPSP=AP in adjacent muscle tissue
Excitation/contraction coupling
AP in mucle cell travels along sarcolemma until it reaches T TUBULES
DIHYDROPYRIDINE RECEPTORS are located in t tubules
AP causes conformational change to DHP
Causing Ca2+ channels to open ( RYANODINE RECEPTORS) or sarcoplasmic reticulum
Calcium ions go into bind to troponinc to move tropomyosin and expose actin myosin head
What is the result of the coupling mechanism
Ca2+ leave SR and bind to tropinin C
Causes removal of tropomyosin from active sites of actin
So which is the link between excitation and contraction
- Release of Ca2+ from SR
- Binding of actin to myosin =contraction
Sliding filament theory of Huxley and Niedergerke
(baso how is the contraction executed)
Tropomysosin is removed from the active centers
ATP molecule binds to myosin head then is degraded to ADP and P by myosin ATPASE
Possible binding of myosin to actin = CROSS BRIDGES
These cross bridges increase 200X the
ATPase activity of myosin head.
When ADP and P detatch from myosin head, it tilts 45’.
This tilt causes the bound actin to slide towards centre of sarcomere
How is the acto myosin head disassembled
When new ATP molecule binds to myosin head
This is then degraded to ADP and P and process starts again
Depending whether tension or length changes, what are the types of contractions
ISOMETRIC CONTRACTION=No movement Only tension changes at steady length, muscle doesnt chnage length but tension increases
ISOTONIC CONTRACTION= length changes under steady tension.
What is is Lymn Taylor cycle
It represents the sequence of
biochemical reactions that convert
the energy released by ATP into
motion.
Therefore, what are the 2 prerequisites for binding of actin to myosin
- Ca2= needed to displace tropomyosin from actin active centre
- Breaking down ATP on myosin head to ADP and P. Then release of this causes head to tilt and pull actin filaments for contraction
What are the 2 types of isotonic contraction
concentric= muscle shortens
Excentric= muscle lengthens
What happens during concentric contraction
Lifts a load of a weight lesser than the max tension of the muscle
Muscle length shortens
Like doing bicep curl
What happens during eccentric contraction
Lifts a load that exceeds 1.6-1.8 times the max tension of the muscle
Length of muscle increases even though active tension greater
E.g Descending down the stairs or lowering a load in hand
Max muscle active tension is greater during eccentric contraction eg can lower heavier load than lift it
Therefore can cause muscle damage like muscle fever
What damage to muscles can eccentric contraction
Delayed onset muscle soreness ( muscle fever)
Bc you can lower a heavier load with your hand than you can lift
What happens during isometric contraction
muscles increases tension muscle length does not
These static contractions are performed by muscles that maintain upright position of body or stabilize joint position
How do both these contractions occur during lifting of a load
When a load is to be lifted, the muscle first contracts isometrically until it develops tension EQUAL to the load
Then it lifts the load by contracting isotonicaly ( muscle shortens and lengthens)
Relationship btw speed and concentric contraction
As the lod increases, speed of contraction decreases
when the load becomes
equal to the maximum isometric tension
(maximum force), the speed becomes zero.
eg holding a heavy dumbell but cant flex it up)
How is active tension generated
when actin filaments bind to mysoin filaments ]
It is greatest when the isometric
contraction occurs at the optimal length
of the muscle, which corresponds to the
length at rest.
How is passive tension generated
Generated by structural proteins called Titin, elastic and collagen fibers
It is generated by stretch
baso u reach optimal length of muscle and u try to stretch it further
When is the highest active tension generated
at sarcomere lenth 2-2.2 micrometres
This is where max overlap of thin and thick filaments
2.2 micrometres is length of sarcomere at rest
Depending on frequency of impulses to muscle, what type of contractions can we get
Muscle twitch= single isotonic contration
Impulse is longer than contration and relaxation duration. Each impulse finds the muscle in relaxed state and makes new contraction
Tetanus= impulse excites muscle before it completely relaxes from previous contraction so increases amplitude tetanic contraction
Difference btw incomplete and complete tetanus
Incomplete= Impulse finds muscle in relaxation phase
serrated graph
Complete= when impulse finds muscle in contracted state
smooth graph
Phases of single isotonic contraction
Latency period - this is the time from
the beginning of the stimulus to the
beginning of contraction.
b. Contraction phase - the ascending part
of the curve; and
c. Relaxation phase - the descending part
of the curve.
Why can muscle contractions be summated
Because the duration of a single muscle
contraction (50 - 100 ms) significantly exceeds the
duration of a single AP generated in the
sarcolemma (5 ms).
So muscle cells can react to new APs
How we define the motor unit of а muscle?
. muscle fibes innervated by 1 motor neurone
(The finer the movements of a muscle,
like fingers , the smaller the number of muscle cells in the 1 motor unit.)
(large motor units made of thousands of muscle fibres and common in muscles of back)
Are all motor units used during contraction?
. Active units at 40% at any given time
.When stronger contraction is
needed, % of active motor units increases.
What are the types of muscle fibres
slow twitch oxidative (type 1)
fast twitch oxidative glycolytic (type 2 A)
fast twitch oxidative glycolytic (type 2 B)
shortening velocity of mucle fibres is determined by atp breakdown. what it like for each fibre?
slow twitch - breakdown small and slow. takes 3000ms
fast twitch - breakdown high so contraction relaxtion cycle only takes 30ms
Properties of slow twitch fibres
. Performs contractions with little maximal force for long time without fatigue
. energy supplied by aerobic pathway.
. lots of mitochondria and myoglobin and well supplied
. red colour
. fibres small diameter for quick gas exchange
** (long back muscles for posture)
Properties of fast twitch A
. rapid contractions, moderate force
. well vascularized and rich myoglobin
. red colour
. resistant to fatigue (has good oxidative pathwyas for ATP resynthesis
Properties of fast twitch B
. rapid contraction BUT not for long time
. contractions of high maximal force
. ATP from anaerobic glycolysis of glycogen
. little myoglobin and mito
. white colour
.fibres large diameter
** ocular muscles
Which muscle fiber is rare in humans
fast twitch 2a= contract faster, lots of glycogen
Anaerobic pathway and aerobic pathway bc pf MITOCHONDRIA
Fast twitch 2b doesnt hv mitochondria (also called white fibers)
What represents muscle fatigue
REVERSIBLE decrease of muscle performance due to prolonged work
- Increasing the refractory period
- diminishing the force of contraction
- slowing down the velocity of contraction and relaxation
What are the causes of fatigue in fast muscle fibers
Occurs due to the depletion of glycogen stores and accumulation of lactic acid-ACIDOSIS
Inorganic phosphates released during the breakdown of creatine-phosphate
also may lead to muscle fatigue.
Why does acidosis and ingorganic phosphtaes lead to muscle fatigue
Acidosis inhibits glycolysis (main source of energy for contraction)
BOTH also reduce the release of Ca2+ from the sarcoplasmic reticulum and the sensitivity of the contractile system to it.
What causes muscle fatigue in slow fibers
Resistant to muscle fatigue
Mainly occurs if O2 diminshes
What does central fatigue mean
Fatigue can occur throughout the
body as a result of changes of the
pattern of muscle stimulation by CNS
which decreases the neural drive to
the muscles, and leads to reduced
muscle activation.
What does electromyography register
EMG registers electrical potentials generated in the muscle upon its
excitation, not its mechanical response!
What are the types of EMG electrodes used in medicine
needle-like and surface electrodes.
What is the difference between
the electrical potentials detected
by needle-like or surface
electrodes?
Needle-like electrodes can register electrical potentials generated by
one or several motor units.
While through surface electrodes, total potential from multiple motor
units is registered.
What are the adv and disadv of needle electrodes
Needle electrodes can register the electrical potentials generated in one or
several motor units and are therefore preferred in neurological practice.
However, the method is invasive.
What are the adv and disadv of surface electrodes
Surface electrodes register the total electrical potential generated by
multiple motor units.
The overall muscle response to stimulation and the inclusion of new motor
units can be examined.
However, no detailed information is available on the individual motor units.
Clinical application of EMG
- Diagnose muscular and neuromuscular diseases:
* muscular dystophy
* myasthenia gravis (affect neuromuscular junc)
* Carpal tunnel syndrome
* Poliomyelitis (motor neurones) - To guide the needle when injecting botulinum toxin or phenol into the muscles.
- prosthetic implants with myoelectric control
- monitor neuromuscular function during general anesthesia when using
neuromuscular blocking drugs.
(avoid crurare effect)
