Physiology of the skeletal muscle

Objectives

• 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

- 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

• 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

Organization of the skeletal muscle

A I I

H

http://www.sport-fitness-advisor.com/images/actin_myosin.jpg

Myofilaments

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

Thick filament

• myosin molecules

– in shape of golf clubs

– long tails bundled together,

– „heads“ sticking out

Neuro-muscular junction

The mechanism of AP transmission from a nerve to a muscle

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

- 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

• 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

Excitation – contraction coupling

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

• 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

Muscle tone

https://figures.boundless-cdn.com/32705/large/uvqvhggorgilmckvon5b.jpe

The force of contraction depends on:

1. Motor unit recruitment

2. Increase in firing frequency

3. Muscle length - tension relationship

4. The graded strength principle

5. Type of muscle fibres

Muscle contraction strength mechanisms

1. Motor unit recruitment

• small motor units

– motor neuron is attached to fewermuscle 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

• 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

http://www.muaythaischolar.com/motor-unit/

• If more motor units are recruited to contract - muscle strength increases

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

2. Increase in firing frequency

• 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

• 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://ffden-

2.phys.uaf.edu/211_fall2004.web.dir/Katherine_

Van_Duine/actin%20and%20myosin.jpg

3. Muscle Length – Length tension relationship

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 cross-

bridges 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 threshold suprathreshold

stimulus stimulus stimulus

no response maximum contraction maximum contraction

The graded strength principle in a muscle

• 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

• 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)

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-Side-

View_545_320x320.jpg

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

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.

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 post-

synaptic muscle fibre

- acetylcholine synthesis slower than required by fast firing rate

• 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)

• 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)

Moss ergograph

serves for

• fixing the forearm and hand

• fixing the cable with load

• recording the contractions

Principle

• the volunteer lifts 2 kg load with m. flexor digitorum superficialis

• signs of fatigue are observed

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

Task: Determination of work and fatigue in a human

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)

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

Work done (J) = load (kg) . gravitational acceleration (m.s-2) . trajectory (m)

trajectory = count of contractions . size of 1 contraction

count of contractions

size of

contractions

beginning fatigue encoura-

ging

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

The smooth muscle

http://faculty.ccri.edu/kamontgomery/muscle%20tissues.jpg

Types of muscles

Skeletal muscle Smooth muscle

Bigger cells, long and thin Smaller cells, spindle shaped

Syncytium (multinuclear cells) Single nucleus

Stiated muscle, sarcomere – basic unit No striations, no sarcomeres

Voluntary control Involuntary control

• Skeletal muscle

• Smooth muscle

• Cardiac muscle

http://faculty.ccri.edu/kamontgomery/muscle%20tissues.jpg

Skeletal muscle and smooth muscle – basic comparison

Types of the smooth muscles

Multi unit smooth muscle

• composed of separate smooth muscle fibers(separated by a glycoprotein/collagen layer)

• only a few fibres innervated by a single nerve ending

• small units of fibers can contract independently of the others

• their control is exerted mainly by nerve signals.

e.g. iris, ciliary muscle, erectores pili

Single unit (unitary) smooth muscle

• mass of hundreds/ thousands of smooth muscle

cells that contract together as a single unit.

• syncytium - cell membranes joined by gap

junctions - ions can flow freely from one cell to

another and cause depolarisation /contraction

• Often controled by non-nervous stimuli (e.g.hormones)

e.g. in the viscera, vessels

Innervation of the smooth muscle

Skeletal Smooth

Motor neurons Autonomic nerves

Synapse - motor end plate Synapse – varicosities

1muscle cell – 1 synapse 1 muscle cell – may have several

synapses

Smooth muscle contraction

Stimuli for a smooth muscle cells:

• Nervous

• Humoral – norepinephrine, epinephrine, acetylcholine, oxytocin, etc.

• Passive stretch

• Local tissue factors – excess of H+, CO2, lactate, deficit of O2

• Spike potentials

• Slow wave potentials - GIT

• Potentials with plateau – ureters,

uterus, some vessels

- Calcium channels play the role

(instead of Na) – slow channels

– therefore slow AP

- Ca ions only from ECT, not form

sarcoplasmic reticulum

Types of potentials in the smooth muscle

Contractile mechanism in the smooth muscle

• contraction – interaction of actin and myosin

filaments (different arrangement of the filaments

than in the skeletal muscle)

• actin filaments attached to the dense bodies (in

the cell membrane, inside the cell)

• the smooth muscle – does not contain troponin

• calmodulin is the regulatory protein - initiates

contraction in a different manner

This activation and contraction occur in the following sequence:

1. The calcium ions bind with calmodulin

2. The calmodulin-calcium combination joins with and activates myosin kinase, a

phosphorylating enzyme

3. Myosin heads become phosphorylated and are capable of interaction with actin

http://www.interactive-biology.com/wp-content/uploads/2012/04/Muscle-cells-

1024x1024.jpg

Skeletal muscle contraction Smooth muscle contraction

by 30% of their lenght by 80% of their lenght

Fast (10 – 300x) Slow (cycling of the cross bridges, long

lasting attachment of A-M, less ATP-ase

activity)

More energy required Less energy required to sustain the

contraction

Lower force of contraction Greater force of contraction

Other differences in the smooth muscle contraction