Download Muscle physiology

Physiology of the skeletal muscle
Contents
• Mechanical properties of the skeletal muscle
• Physiological properties of the skeletal muscle
• Organization of the skeletal muscle
• Mechanism of muscle contraction and relaxation
• Tetanus
• The all or nothing law in skeletal muscle
• Types of muscle fibres
• Mechanisms of skeletal muscle strength
Practical tasks
Determination of work and fatigue in human
Determination of skeletal muscle strength in a human
 Katarína Babinská, MD, PhD. 2016
Physiological properties of skeletal muscle tissue
1. Excitability (irritability)
-
refers to the ability of a muscle to respond to stimulation
-
in the human body the muscle activity is regulated by the nervous
system and in some muscle types by the endocrine system
-
(in experiment the electric current is a suitable stimulation)
2. Contractility
-
refers to the capacity of muscle to contract (shorten)
-
contraction = response of a muscle to stimulation
Skeletal muscle mechanical characteristics
1. Strength (firmness)
•
can be expressed as a maximum weight that can be kept by contracted
muscle (or group of muscles) in balance against gravity
2. Elasticity (Extensibility)
- ability to return to the resting length after contraction
- if load is put on the muscle gradually, it becomes progressively longer
- the increase of length per each kilo added gradually decreases,
(the muscle resists the stretch at first slowly, then more rapidly)
- when taking the weight off the muscle becomes shorter, however it
reaches its resting length just after some time
0
kg
kg + kg +
kg +
kg +
kg
removing
the load
Organization of the skeletal muscle
- muscle
- fascicles
- fibres (cells)
- myofibrils
- myofilaments
(actin, myosin)
• myofibrils – a sequence of sarcomeres
• a sarcomere – a basic longitudinal
contractile unit of the striated muscle
• demarcated by two successive Z lines
H
I
A
I
• main components (myofilaments)
– thin filaments (actin)
– thick filaments (myosin)
(fixed by titin to the Z lines)
• cross striae formed by:
– I band - actin
– A band
• actin + myosin overlapped
• H band – myosin only
http://www.sport-fitness-advisor.com/images/actin_myosin.jpg
Myofilaments
Thick filament
• Myosin molecules
– in shape of golf clubs
– long tails bundled together,
– „heads“ sticking out
Thin filament
• actin globules arranged into fibres - helix of 2 filaments
• tropomyosin spreads along actin and covers the binding sites for myosin
• troponin (C, I, T) complex is present on each tropomyosin dimer
Neuro-muscular junction
The mechanism of AP transmission from a nerve to a muscle
- motor end-plate - the point of junction of a motor nerve fibre and a muscle fibre
- modified area of the muscle fibre membrane at which a synapse occurs
1. nerve impulse reaches the end of a motor neuron
2. Ca2+ volt. gated canals in the axon terminal open, Ca2+ moves inside the terminal
3. this triggers release of acetylcholine into the synaptic cleft
4. acetylcholine binds to
receptors on the motor endplate
5. this causes opening of Na+
channels in motor endplate
6. motor endplate
potential is generated
7. action potential is generated
that travels along the
sarcolemma
Excitation – contraction coupling
- the AP travels along the membrane of the
muscle cell (sarcolemma)
- through T tubules (invaginations of the
sarcolemma) pass to sacs of sarcoplasmic
(endoplasmic) reticulum = store of Ca2+
- Ca++ is released from sarcoplasmic reticulum
into the sarcoplasm
- Ca++ binds to troponin molecules
- tropomyosin fibres shift and
expose the actin´s active sites
http://t1.gstatic.com/images?q=tbn:ANd9GcS2B26VzQ_y1GpW03ZDKL-1LRwnKF9qKSS9MMe3J0RJULQ8xeOG
http://www.blobs.org/science/cells/sr.gif
Actin – myosin interaction
https://classconnection.s3.amazonaws.com/216/flashcards/1
042216/jpg/power_stroke1327356421108.jpg
The mechanism of muscle relaxation
- after the impulse is over, the
sarcoplasmic reticulum begins
actively pumping Ca++ into sacs
- as Ca++ is released from troponin,
tropomyosin returns to its resting
position blocking actin´s active
sites
- myosin cross bridges are
prevented
- the contraction can no longer
sustain
- the muscle returns to its resting
length
http://www.blobs.org/science/cells/sr.gif
Types of muscle contractions
Isotonic contractions - generate force by
changing the length of the muscle
• a concentric contraction causes muscles to
shorten, thereby generating force
• eccentric contractions cause muscles to elongate
in response to a greater opposing force
Isometric contractions generate force without
changing the length of the muscle
Auxotonic contraction
• combination of isotonic and isometric contraction
• this type occurs mostly in real life
Muscle tone
•
•
•
•
a continual partial contraction of the muscle
involuntary activation of a small number of motor units causes
small contractions that give firmness to the muscle
important for maintaining posture
higher when awake
https://figures.boundless-cdn.com/32705/large/uvqvhggorgilmckvon5b.jpe
Muscle contraction strength mechanisms
The force of contraction depends on:
1. Motor Unit Recruitment
2. Increase in firing frequency
3. Muscle Length – Length tension relationship
4. The graded strength principle
5. Type of muscle fibres
1. Motor unit recruitment
• in the human body skeletal muscles are stimulated by signals (action potentials)
transmitted via motor neurons to muscles
• axon of a single motor neuron branches and is attached to more muscle fibres
motor unit = one motor neuron + muscle fibres to which it is attached
• when a motor unit is activated, all of its fibres contract
• small motor units
– motor neuron is attached to fewer
muscle fibres (eye, face, fingers)
– allow for fine and precise movements
– they produce little force
• large motor units
–
–
–
–
involve – several hundreds of muscles
e.g. in postural muscles
produce large force
allow for less precise movements
http://www.muaythaischolar.com/motor-unit/
•
If more motor units are recruited to contract - muscle strength increases
2. Increase in firing frequency
The twitch contraction
• is a quick jerk (contraction) of the muscle fibre that occurs
after stimulation (e.g. electric stimulation)
• can be recorded by myograph (see picture)
• the curve of a twitch contraction includes 3 phases:
– latent period – time between stimulation and beginning
of contraction
– contraction phase
– relaxation phase
http://t2.gstatic.com/images?q=tbn:ANd9GcQ0QYA15DFQeIetqELpVYgRUnlzt_iXRYtCfXUxrcI_41B3rUVA
• if a series of stimuli come in longer intervals, the muscle has enough
time to relax completely before next contraction
• a series of individual twitch contractions can be observed
• if stimulation is fast, and the
next stimulus arrives before the
relaxation phase has ended
– summation of twitches occurs
– muscle is in tetanus
Tetanus
- sustained contraction of a skeletal muscle, result of stimulation with high frequency
of stimuli
Incomplete tetanus
• the next stimulus arrives before the relaxation phase has ended
• muscle gets only partially relaxed
• summation of twitches occurs and the force of contraction increases
Complete tetanus
• the next stimulus comes at the peak of the previous contraction
• the muscle is instantly contracted – strength of contraction increases even more
Incomplete tetanus Complete tetanus
Increase in firing frequency – increases the strength of contraction
• single stimulus → release of Ca++ from sarcoplasmic reticulum → twitch
• twitch is terminated by reuptake of Ca++ into sarcoplasmic reticulum
• repeated stimulation in high frequency
– insufficient time to reaccumulate Ca++ into sarcoplasmic reticulum
– remains in sarcoplasm – sustained contraction = tetanus
• incomplete – next stimulus occurs in relaxation period of a twitch
• complete – next stimulus comes on the top of the twitch
Incomplete tetanus Complete tetanus
3. Muscle Length – Length tension relationship
•
the strength or maximum force produced by a muscle depends on the number
of cross bridges per unit area
•
to increase the maximum force, increase the number of cross bridges
•
the number of cross bridges depends on the starting position of actin and
myosin
http://ffden2.phys.uaf.edu/211_fall2004.web.dir/Katherine_
Van_Duine/actin%20and%20myosin.jpg
Optimum muscle lenght – greatest force
- when the muscle is at an optimal length = indicated by the greatest possible
overlap of thick and thin filaments, maximal strength is produced.
-CNS maintains optimum length producing adequate muscle tone
Overly contracted
- thick filaments too close
to Z discs – cannot slide
more
Too stretched
-little overlap of thin and
thick filaments does not
allow for very many crossbridges to form
4. The graded strength principle
The „all or nothing“ law in skeletal muscle
• individual muscle fibres operate according the all or nothing law:
– insufficient stimulation (subthreshold stimulus) causes no
contraction (no response)
– sufficient stimulation (threshold or suprathreshold stimulus)
causes maximum contraction
subtheshold
stimulus
no response
threshold
stimulus
maximum contraction
suprathreshold
stimulus
maximum contraction
The graded strength principle in a muscle
•
•
skeletal muscle = a bundle of muscle fibres
muscle as a bundle of fibres operates according to graded strength principle
(fibres respond to the stimulus gradually depending on their sensitivity)
• subthreshold stimulus – no response
• threshold stimulus – first response
• then
– the stronger the stimulus, the stronger
response (still more muscle fibres are
recruited and respond)
• maximal stimulus – all the muscle fibres
respond
• further increase of intensity of the
stimulus (supramaximal stimulus) does
not increase the response
5. Muscle fibre type
Fast twith fibres
(type II, white)
Slow twitch fibres
(type I)
Contraction velocity
High
Low
Capillarization
Low
high
Myoglobin content
Low
High
Mitochondrial content
Low
High
Aerobic energy production
Low
High
Anaerobic energy production
High
Low
Glycogen stores
High
Low
Fatigue
Fast
Slow
Generation of speed and power
High
Low
Suited for
Explosive sports
Endurance sports
- Muscle fibre type is determined genetically + by training
1. type I muscle fibers (slow-twitch fibers, red) – typically smaller motor units
2. type II fibers (fast-twitch fibers, white) - typically larger than motor units
containing type I fibers
-
-
i.e. when a single type II motor unit is stimulated, more muscle fibers
contract
since more fibers are stimulated to contract in type II motor units, more force
is produced by type II fibers.
Determination of skeletal muscle strength in a human
Muscle strength
• can be expressed as a maximum weight that can be kept by
contracted muscle (or group of muscles) in balance against gravity
• can be measured by hand dynamometer
• average (normal) value of the dominant hand
– 50 – 55 kg in an adult male
– 31 – 36 kg in an adult female
– value of the non-dominant hand – approx. 10 % less
http://www.getprice.com.au/images/uploadimg/2434/
Hydraulic-Hand-Dynamometer-Left-SideView_545_320x320.jpg
Procedure
1. Rotate the peak-hold needle counter to 0
2. Let the right upper extremity with dynamometer hang freely along the body (in
the standing position).
3. Compress the dynamometer by the right hand maximally
4. Record the value in kg (the peak-hold needle records the max force)
5. Repeat the measurement 3 times and calculate average value.
6. Repeat for the left hand.
Report
A. Which of your hands is dominant?
B. Write down the measured and average values:
Right hand
Left hand
measurement-1:
measurement-1:
measurement-2:
measurement-2:
measurement-3:
measurement-3:
average value:
average value:
C. Compare your average value for dominant hand with normal values
Muscle fatigue
- the transient/reversible decrease in performance capacity of muscles induced
by eercise
- evidenced by a failure to maintain or develop a certain expected force or power, or
to sustain the task
- depends on




the intensity/duration of the performance
aerobic/anaerobic metabolism
types of muscle fibres
personal fitness
- experienced mainly in sustained and/or close to maximum activities
Sites, causes and mechanisms of fatigue:
 Neuromuscular depression (fatigue of synapses)
- synapse – most prone to fatigue
- every successive stimulation of a motor nerve causes weaker response in the postsynaptic muscle fibre
- acetylcholine synthesis slower than required by fast firing rate
 Cellular fatigue
- accumulation of extracellular K+ - due to repeated action potentials and the Na+K+ pump can not rapidly transport K+ back to the muscle - failure to reestablish the
resting membrane potential on a synapse
- rise in lactic acid concentration = lowering of pH – inhibits the cross-bridge
formation
- depletion of glycogen/glucose
- decrease in availability of Ca2+ ions - results in decreased Ca2+ release from
sarcoplasmic reticulum
- excessive accumulation of inorganic phosphate (ATP breakdown in cross-bridge
formation) in cytoplasm
- glycolytic fibres more prone to fatigue (slower Ca uptake)
 Central fatigue
- subjective feeling of tiredness and a desire to stop the activity
- lack of motivation due to failure of cerebral cortex to send excitatory signals to
the motor neurons
- low pH seems to play role (in close to maximum physical activities)
Task: Determination of work and fatigue in a human
Principle
• the volunteer lifts 2 kg load with m. flexor digitorum superficialis
• signs of fatigue are observed
Moss ergograph
serves for
• fixing the forearm and hand
• fixing the cable with load
• recording the contractions
Procedure
• the forearm of examinee is fixed in the ergograph holder
• the examinee holds the handle with his hand
• a leather ring is put on the second finger
• the examinee lifts a 2 kg load in pace given by a metronome
• the series of contractions are registered and evaluated
1.
ask the subject for any feelings of fatigue (pain, weakness of the finger…)
2.
observe signs of fatigue on the record – the curve is flattened, the
contractions are irregular (the examinee continues to lift the load for
another 30-60 s)
3.
start to encourage the volunteer – observe the effect of motivation on the
performance
4.
calculate the work done per unit of time
- at the beginning of performance
- the period when sifns of of fatigue are visible
- in the period when the subject is encouraged
- unit of time = a segment, e.g. 15 cm
- select 3 segments from the beginning of the performance and the end - when
signs of fatigue are seen)
beginning
size of
contractions
count of contractions
fatigue
encouraging
frequency
13
11
9
size
0,02
0,015
0,02
trajectory
0,26
0,165
0,18
gravitational
acceleration
10
10
10
load
2
2
2
work done
5,2 J
3,3 J
3,6 J
Work done (J) = load (kg) . gravitational acceleration (m.s-2) . trajectory (m)
trajectory = count of contractions . size of 1 contraction
Positive dynamic work
- work done during muscle contraction
- e.g. load ligting
Negative dynamic work
-prohibits falling down (e.g. load releasing, going downstairs)
–this is not taken into account in this task