Lecture 8: mechanical properties of skeletal muscle Flashcards
what are the** functions** of skeletal muscle?
- to move the body by creating motion
- to** provide strength** by generating force
- to protect joints by absorbing shock
what are the specific conective tissue functions within the muscle?
- to provide gross structure to the muscle
- to generate** passive tension against strength**
- to transmit force to the bone & across the joint
Describe the properties of a skeletal muscle fiber?
- their** extensibility** - their ability to be stretched / increase in length
* elasticity- their ability to return to their original length after a stretch
* contractibility - their ability to develop tension
* irritability -their ability to respond to a stimulus eg action potential or mechaical force
what is the fascia?
- the band of sheet of fibrous connective tissue (normally collagen)
- they surround muscles
what is a muscle twitch?
the response of a muscle to a single stimulus
what is the twitch tension?
the** force developed** as a result of a muscle twitch
What are the** mechanics** of a single muscle fiber contraction?
- single muscle fiber contraction refers to the ‘turning on’ of the cross - bridge cycle
- the thin (actin) and thick (myosin) filaments do not shorten, but they increase their overlap
After the activation of a muscle fibre, what are the 3 types of muscle contractions that can occur?
- isometric contraction (iso=same, metric=distance) - the muscle generates tension but does not change length EG holding a weight
-
isotonic contraction/concentric - the muscle generates tension (force) and it shortens which moves a load - eg bicep curl
3.** lengthening contraction/eccentric** - the muscle is trying to shorten by generating tension but in fact it is lengthening due to the overpowering external force being applied to the muscle EG lifting weight thats too heavy
what does the movement around a joint require?
it requires** 2 antagonistic groups** of the muscles - 1 flexes the limb at the joint, the other extends the limb
Describe how a muscle shortens/relaxes
- when a muscle fibre is activated, the process of a contraction begins with an AP passing across its length
- initial phase of acceleration during which the tension in the muscle increases until it equals that of the load to which it is attached
- the fibre the begins to shorten at a constant rate until it decelerates
- finally the muscle relaxes and the tension declines to zero
What are the 3 types of skeletal fibers?
- Type I - slow oxidative twitch fibers (need oxygen)
- Type IIa - fast oxidative twitch fibers (intermediate) (fast but still use o2)
- Type IIb - fast glycolytic twitch fibers - anaerobic
What are the 2 characteristics that determine the type of skeletal muscle fiber?
- differences in maximal shortening velocites - due to different myosin enzymes with high or low ATPase activites
- **diameter **- a larger diameter produces greater tension
What is a motor unit?
- a single neuron and all the muscle fibers it innervates
Can motor units contain a mix of all 3 types of fibres?
- No
- but most muscles do contain the 3 types, so we have many motor units
Describe type I/ red muscle fibers
think the chicken leg - red
- red muscles contain predominately red muscle cells
- red muscle fibers/cells contain large amounts of mitochondria and myoglobin (protein containing oxygen)
- red muscle cells contain a myosin enzyme with low ATPase activity - hence a slow contraction
- examples - posture muscles -a sustained production of force is nessessary
Describe **type II fibers/white muscle cells **
Note diff between type IIa and type IIb
- white muscles contain predominately white muscle cells
- they are** thicker** cells and contain less myoglobin
* ATPase activity is high and hence contraction is fast - type IIa (red) contain **many mitochondria **& and are available for both sustained contraction & short lasting contractions
- type IIb (white) contain a few mitochondria and they rely on anaerobic glycolysis to generate ATP needed for contraction
Which muscle fibre type is most prone to fatigue?
type IIb
What is recruitment?
- increasing the number of active motor units in a muscle
- slow oxidative (type I) fibers are recruited first, then fast oxidative, then fast glycolytic
what is the size principle in relation to recruitment?
- the motor units are recruited according to size - ie type I first, then type IIa and then type IIb
what effect does increasing motor unit recruitment have on the muscle?
it increases the velocity at which a muscle will move a given load
what is the rate principle?
- increasing the frequency of action potentials in a muscle fiber
- this will increase the mechanical response ( tension and shortening ) up to the level of maximal tetanic shortening
what **2 mechanisms **control muscle contraction?
- recruitment
- rate principle
what is a tetanus muscle contraction ?
a sustained muscle contraction - action potentials are being emmitted from a nerve at a **high rate **
what is the difference between a fused and unfused tetanus contraction?
- unfused tetanus occurs when muscle fibers** do not completely relax before the next stimulus** because they are being stimulated at a fast rate
- fused tetanus - the muscle fibers have reached maximum tension
what is muscle fatigue?
the reduction of
* muscle force production capability
* contraction velocity
and prolonged relaxation of motor units between recruitment
what does muscle fatigue depend on?
- the muscle itself
- exercise duration
- fibre type composition
*
what are the causes of muscle fatigue?
- inorganic phosphate which increases during fatigue due to breakdown of creatine phosphate
- reduction in the rate of intracellular Ca2+ release and uptake by sarcoplasmic reticulum
- inadequate oxygen availibility - NOT A LACK OF ATP
what does long duration, low intensity exercise do to a muscle fiber?
increases the muscle fiber’s capacity for ATP production by increasing the no of mitochondria, blood vessels in the muscle which increases endurance
what does short duration and high intensity exercise (eg weight lifting) do to a muscle fiber?
- increases fiber diameter as a result of increased synthesis of actin and myosin, resulting in increased strength
Describe the length - tension curve
- optimal length - actin and myosin are at ideal positions for max cross bridge binding and therefore max tension
- increase or decrease in stretch from optimal length results in a deceased tension developed
