11 - Types Of Skeletal Muscle

Contractions

Taft College Human Physiology

Types Of Skeletal Muscle Contractions

• Isotonic Contractions: Tension produced and overall shortening of the muscle as a load is moved through the range of motion of the joint .

– Isotonic contractions serve to bring about movement or change in body position. Example = flexion, extension, adduction, abduction, etc.

• Isometric Contractions: Tension but no shortening of the muscleoccurs. Energy is still used!

– Example:contractions that serve to keep the body fixed in position as in 1. maintaining posture, 2. maintaining balance, 3. fixing a proximal joint so a distal joint may move, 4. maintaining muscle tone.

• Most body activities involve both isotonic and isometric contractions.• Twitch: A single isotonic response as a result of a single threshold (liminal)

stimulus. (This is not the type of twitch you feel in your body due to being tired or a chemical imbalance).

• The muscle contracts quickly and then relaxes.• A twitch can be demonstrated with an instrument that produces a

myogram= a tracing of a muscle contraction or activity.

Types Of Skeletal Muscle ContractionsIsotonic Contraction =

Tension + Shortening of muscle

Isometric Contraction =

Tension, no Shortening of muscle

A Myogram of 3 Phases (Periods) of a Muscle Twitch1. Latent Period – the time from stimulation of the muscle until shortening of the muscle

begins.The latent period is a “lag time”.Duration = about 2 ms.During this period of time the following events of muscle contraction are occurring:

a. Depolarizationb. Diffusion of Ca+2 c. Establishment of actin/myosin bonding

2. Contraction Period – Tension and shortening of the muscle occurs. The upward tracing represents this phase. Duration = 10-100 ms.

3. Relaxation Period – Muscle goes back to it resting state.The Ca++ is actively transported back into the SR which results in relaxation. The downward tracing represents this phase. Duration = 10-100 ms.

•Muscles differ in the amount of time it takes to through 3 phases.Ex.eye muscles 12 millisec= Fast Twitch Musclegastrocnemius 36 millisec, soleus 100 millisec= Slow Twitch Muscle

3 Types of Skeletal Muscle Fibers• In humans we see 3 types of skeletal muscle fibers

classified based on :• 1. How fast a muscle will twitch (due to how fast it splits

ATP) and,• 2. Metabolic pathway used to generate ATP (glycolytic

(anaerobic) or oxidative).• We will concentrate on only 2 fiber types a. slow twitch =

slow oxidative fiber and b. fast twitch = fast glycolytic fiber.

Fast & Slow Twitch Muscle FibersFast Fast GlycolyticGlycolytic

Fast TwitchFast TwitchSlow OxidativeSlow Oxidative

Slow TwitchSlow Twitch

Color White = Light Red = Dark

Size Large fibers Small fibersfew mitochondria more mitochondria

Reaction Fast (anaerobic) Slow (aerobic)

Function Speed Endurance

Location Eye muscles & Postural muscles &upper appendages lower appendages

Rate of fatigue Fast SlowExamples: eye muscles 12 millisec gastrocnemius 36 millisec

soleus 100 millisec

Table of Characteristics of Skeletal Muscle Types Slow Twitch Fast Twitch Fast Twitch

Structural Features Slow Oxidative Fast Oxidative Fast GlycolyticDiameter or Fiber Smallest Intermediate LargestColor/Myoglobin Red/large Red-pink/large White/smallMitochondria Many Many FewFunctional Features Slow Oxidative Fast Oxidative Fast GlycolyticATP Production Aerobic (O2) process Aerobic (O2) process Glycolysis (anaerobic) ATP use/Velocity

of Contraction Slow Fast FastResists Fatigue High Intermediate LowGlycogen Stores Low Intermediate HighOrder of recruitment 1st 3rd 2nd

Skeletal Muscle Types

• Slow Oxidative: Maintain posture (anti-gravity muscles), endurance running. Back and neck muscles.

• Fast Oxidative: Walking, sprinting. Leg muscles.

• Fast Glycolytic: Rapid, intense movements of short duration. Ball throwing, weight lifting. Arm muscles.

Implications for Sports

• Number of red (slow twitch) vs. white (fast twitch) muscle fibers is hereditary

– Conditioning and training can do nothing to change the ratio (New research may show otherwise)

– E.g. Alberto Salizaar – New York marathon winner had 92% red vs. 8% white. Other runners tend to have fewer red and more white in comparison.

• Sprinters contain about 60% fast oxidative.• Weight lifters have about equal amounts fast glycolytic and slow

oxidative• Determination of muscle fiber ratios can be done by muscle

biopsy.

Other animals use of muscle cell types

• Concentrations of fast and slow twitch muscles can be observed in other animals as well as humans.

• The light and dark meat of a chicken has to do with concentrations of different types of muscle fibers. – A chicken uses it’s breast muscle (white meat) for

short flight if at all = fast twitch. The legs (dark meat) of a chicken serve for endurance = slow twitch.

– What type of meat would you expect to find in the breast of a migratory duck? Dark meat = slow twitch for endurance.

Physiology Assignment #4

• Name__________________________• Due Date ___________• Internet assignment (Attach article)• What are the implications of fast/slow

twitch muscle in sports (any sport you wish).

• Is it possible to change the percentage of fast twitch to slow twitch muscles you have or is it genetically determined?

• Testosterone (found in men) and human growth hormone influence muscle growth.

• Anabolic steroid drugs are testosterone-like and are abused by athletes to increase muscle size & strength and endurance.

– Moderate doses – no better than optimal training.– Large doses…..damage to body.

Anabolic Steroid Abuse

• Problems:– liver cancer, kidney cancer, heart disease,

aggressive behavior, mood swings.

– Females: sterility, facial hair, deep voice, atrophy of breasts and uterus, menstrual irregularities.

– Males: testes atrophy, less sperm production, baldness.

Anabolic Steroid Abuse

Types of Skeletal Muscle (cont.)• Tetanus – sustained contraction of a muscle due to increased frequency of stimulation.

Result of a summation of twitches (a normal muscle contraction)– When the frequency of the stimulation is such that there is no hint of reduced tension or

force between stimuli, it is called complete tetany or fused tetany. 80-100 stim/sec– When the frequency of stimulation is reduced slightly, you can see partial muscle

relaxation occurring between contractions, this is called incomplete tetany or unfusedtetany. 20-30 stim/sec

• Incomplete tetany can result in trembling (shaky) movements of the limbs observed in some individuals.

– Normal muscle contractions with smooth movement are a result of complete tetaniccontractions.

Twitches

• Treppe – increased strength of contraction as muscle “warms up” due to identical stimuli too far apart for wave summation to occur.

• It is also known as the ‘staircase effect’, as the muscle steps up its strength with each contraction.

Types of Skeletal Muscle (cont.)

Stimulus constantTens

ion

Incr

easi

ng

Trep

pe How does this myogram differ from incomplete tetanus?

How do we know this is not wave summation?

Hint

• Treppe can be explained as follows:

• A progressive buildup of Ca++ in the sarcoplasm probably accumulates because the stimuli release Ca++ faster than the Ca++ pump can move them back in to the SR.– The troponin becomes saturated for maximum binding to myosin

heads. – Eventually the inflow and outflow of calcium ions equalize and the

strength of contraction level off

• In your warming muscles, the sarcoplasm becomes less viscous. With more heat and the internal resistance of the muscle lessened it allows more energy to be directed to muscle shortening and less to overcome resistance.– With heat from ATP splitting and other chemical reactions, the enzyme

systems become more efficient.– This is the basis for the warm-up period for athletes.

Types of Skeletal Muscle (cont.)

Muscle Tone• Tone is a sustained partial state of contraction in

the muscle.

• Tone is maintained in the body without fatigue by the alternation of different motor units. It serves to keep the body in a state of readiness for activity at all times.

• It is believed that the contractions are some type of involuntary spinal reflexes responding to activation of stretch receptors in muscles and tendons.

Muscle Tone• Hypotonia – Refers to decreased or lost muscle tone,

resulting in flaccid (flattened) shape instead of round.

• Atrophy – Wasting of muscle tissue where muscle fibers decrease in size as myofibrils are lost.

• Hypertrophy – Opposite of atrophy. Refers to an increase in diameter of muscle fibers where myofibrils, mitochondria, and SR are increased. No increase in # of cells. Capillaries servicing muscle fibers are increased too.– Due to forceful, repetitive strength training, which results in

increased capacity for forceful contractions.

Muscle ToneMuscle tone can be lost quickly:

• If muscle usage is prevented by a cast (disuse atrophy), or by a severing of the nerves (denervation atrophy), the muscle fibers begin to atrophy in just a few days.

• Prolonged inactivity can lead to degeneration of the muscle fibers and potential replacement by scar tissue (C.T., including fat), which cannot be reversed when complete.

• Direct stimulation of the inactive muscle using a muscle stimulator may prevent atrophy until the muscle is removed from the cast or the severed nerve fibers can remake connections.

• The important thing to realize is that muscle health is maintained in part by utilization - “use it, or lose it!”

Smooth Muscle• Smooth muscle is also called involuntary muscle (under ANS control)

and nonstriated muscle (lacks organized sarcomeres). – Actin and myosin myofilaments are present but are not regularly

arranged leading to the absence of light and dark bands that cause the striations in skeletal muscle tissue.

Characteristics of smooth muscle in comparison to skeletal muscleSmooth muscle has:1. 7 times less actin & myosin in smooth muscle than skeletal muscle.2. Lower levels of ATP & creatinine phosphate3. Fewer number of mitochondria with slow contractions. 4. Smooth muscle cells lack T-tubules. Slower onset of contraction. 5. Have poorly developed sarcoplasmic reticulum. Takes Ca++ longer to

diffuse. Also delays in transport of Ca ++ for longer contraction.6. Contraction stimulated by a. neurotransmitters (ACh, NE by ANS),

b. hormones c. local chemical changes (pH, O2, CO2), d. stretching.

So smooth muscle is designed for slow reacting, but prolonged contractions.

Abnormal Contractions Of Muscle Tissue• Spasm – A sudden involuntary muscle twitch

(contraction of short duration), usually due to a chemical imbalance.

• Cramp – A sustained, painful, spasmodic (tetanic) contraction of a muscle. – Can last minutes to hours. – Severe cramps usually occur when the muscle is

shortened (when there is little pull on the tendons). – Usually occurs at night or after exercise. – It is not known what actually happens at the level of a

sarcomere during a cramp. However, the pull on the tendons of muscles are constantly monitored by sense organs called golgi tendon organs

Abnormal Contractions Of Muscle Tissue• Golgi tendon organs act to inhibit or “apply the brakes” to

muscular contraction to prevent the development of too great of tensile force that could result in injury to the muscle or tendon. – Learning how to disinhibit (keep the golgi tendon organs

from working) may be an important part of strength training.

– Maximal vigorous contractions in a shortened position seems to increase the probability of cramping as you have maximal innervation with minimal inhibition.

• How can you relieve cramps in light of this information? – Simply forcing the muscle into its longest position

(stretching) will create tension on the golgi tendon organ. • The inhibition caused by the golgi tendon organ will then stop the

cramp.

Abnormal Contractions Of Muscle Tissue

• Convulsions – Violent, involuntary contractions of whole groups of muscles. – Convulsions occur when motor neurons are

stimulated by factors such as fever, poisons, hysteria, and changes in body chemistry due to drug withdrawals.

– The stimulated neurons send seemingly senseless impulses to the muscle fibers.

– This is a nervous disorder not a muscular disorder.

Abnormal Contractions Of Muscle Tissue

• Fibrillation – Uncoordinated contraction of individual muscle fibers so that the muscle fails to contract smoothly. – Cardiac muscle is most prone to this type of

activity and is recorded by electromyography.