PT15 Midterms

length-tension relationship

tension generated by a muscle, & the force of contraction, depends on how stretched or contracted the muscle was at the start

muscle tension

force to make skeletal muscle do work

sarcomeres at <60% or >175% of optimal length

develop no tension at all in response to a stimulus

overly contracted

thick filaments would be close to the Z discs & butt against them; contraction would be very brief & weak

overly stretched

less overlap; myosin heads can't grip onto actin filaments and results in weak contractions

optimum resting length

produces greatest force when muscle contracts

100% optimal length

medial edges of the actin filaments are almost at the most medial of the myosin heads

reasons why a muscle is never overly stretched or contracted

- attachments of muscles to bones & limitations on bone movement restrict muscle contraction
- CNS continually monitor & adjusts the length of resting muscle to maintain a state of partial contraction
- presence of accessory proteins as well as connective tissues opposing extreme stretching

skeletal muscle tone

constant, partially, slightly contracted state of all muscles at rest, which does not produce active movements

functions of skeletal muscle tone

- keeps the muscles firm, healthy, & ready to respond to a stimulus
- stabilize joints & maintains posture

spinal reflexes account for muscle tone by:

- activating one motor unit & then another in a systematic manner
- responding to activation of stretch receptors in muscles & tendons

hypotonia

- absence of low level contractions
- damage in CNS or loss of innervations
- appear flaccid and display functional impairments

hypertonia

- excessive muscle tone; overreactive or excessive reflexor response (hyperreflexia)
- damage in CNS @ upper portion
- muscle rigidity & spasticity

Muscle twitch

response of a muscle to a single, brief threshold stimulus

elicits muscle twitch

electrical excitation of nerve to muscle / short electrical stimulus through the muscle itself

Threshold

minimum voltage necessary to generate an AP in a muscle fiber

myogram

a chart of the timing and strength of a muscle's contraction

three phases of a muscle twitch

- latent period
- period of contraction
- period of relaxation

latent period

- delay of about the first 2 milliseconds between the onset of stimulus & the onset of twitch
- phase where AP is already being propagated in the sarcolemma
- calcium ions released from SR

internal tension

force generated during latent period and no shortening of the muscle occurs (FLAT line on myogram)

period of contraction

- cross bridges actively form, muscle tension rises & muscle shortens
- calcium binds with TMC & pulls it away from actin's active binding site
- short-lived bc SR quickly reabsorbs calcium before muscle develops maximal force

external tension

develops in the muscle as the elastic components become taut & move a resisting object or load

period of relaxation

Ca2+ is quickly reabsorbed into the SR, before the muscle develops maximal force & muscle tension goes to zero

asymmetry of myogram

- explains muscle contracts more quickly than it relaxes (Ca2+ reabsorption is faster than release)

causes of difference between muscle twitch response

type of muscle fiber; metabolic properties of myofibrils and enzyme variations

contraction

activation of myosin's cross bridges; series of action potentials

"all-or-none" law of muscle physiology in skeletal muscles

applies to the muscle fiber but not the whole muscle

single twitch

not strong enough to do any useful work

increasing more the stimulus intensity

produces twitches no stronger than those at threshold

Summation

- adding together of individual twitch contractions to increase the intensity of overall muscle contraction
- each new stimulus arrives before the previous twitch is over, & each twitch "rides piggyback / hitches onto the previous one & generates higher tension

graded muscle contraction

uses varying amounts of force

Frequency Summation of action potential

by changing (increasing) the frequency of muscle stimulation but can lead to tetanization

tetanization

- characteristic of skeletal muscle
- muscle will be unable to relax before new stimulus is introduced
- muscle contracts continuously w/o relax

Multiple Fiber Summation

by changing (increasing) the number of muscle fibers/motor units being stimulated at one time

Recruitment or Multiple Motor Unit (MMU) Summation / Size Principle

- recruitment starts from smallest muscle fiber to bigger size as the intensity of stimulus increases
- contributes to contractile force & produces smooth, continuous muscle contractions by rapidly stimulating specific number of muscle cells

reasons why twitches vary in strength (at a constant voltage)

- dependent on how stretch muscle was before stimulation (length-tension relationship)
- become weaker as muscle fatigues
- vary with temperature of muscles (heat -> myosin heads work more quickly)
- varies with muscles state of hydration (affects spacing between myofilaments)
- varies with stimulus frequency (arriving closer together produce stronger twitches)

relationship between stimulus intensity (voltage) & muscle tension

the higher the stimulus intensity or the voltage, more nerve fibers are stimulated, & hence, more motor units are stimulated to contract

threshold stimulus

stimulus strength at which the first observable muscle contraction occurs

maximal stimulus

strongest stimulus that produces increased contractile force where all muscle's motor unit are recruited

mechanism of size principle of recruitment

- weak signal from CNS stimulates smaller motor units; sufficient for delicate tasks & refined movements
- as signal strength increased, larger motor units are excited

importance of size principle of recruitment

- allows gradation of muscle force
- provides smooth contraction even at low frequencies of nerve signals

high-frequency stimulation

produce stronger muscle twitches

wave summation

results from one wave of contraction added to another

temporal summation

greater frequency of stimulation leads to stronger muscle contraction

incomplete or unfused tetanus

- muscle's response at a stimulus frequency within normal physiological range
- state of sustained fluttering contraction; only partial relaxation between stimuli

complete or fused tetanus

- muscle's response at an unnaturally high stimulus frequency
- smooth continuous, sustained contraction without any evidence of relaxation, as a result of a very rapid rate of stimulation given quickly at higher frequencies; no relaxation at all

difference of tetanus vs summation

tetanus is muscle response, while summation refers to the stimulus

spinal cord inhibition of complete tetanus

inhibitory mechanism in the SC that prevents motor neurons to fire impulses at a faster rate, protecting the muscles by preventing complete tetanus

tetanus/lockjaw

- differ from tetanic contractions
- a bacterial toxin made by a Clostridium bacterium
- causes muscles to go into uncontrollable spasms and may cause respiratory arrest

isotonic contractions

- same tone
- muscle changes in length
- tension remains constant

concentric contractions

muscle shortens & does work; muscle shortens as it maintains tension

eccentric contractions

muscle contracts as it lengthens; muscle lengthens as it maintains tension

isometric contractions

- same measurement or length
- produced wherein the muscle contracts without a change in length
- occurs if load > tension
- NOT a prelude to movement

physiological classification of skeletal muscle fibers

slow-twitch & fast-twitch

functional types of muscle fibers based on speed of contraction

- determined by speed of ATP hydrolysis
- slow & fast muscle fibers

functional types of muscle fibers based on major ATP-forming pathways

oxidative & glycolytic fibers

oxidative fibers

aerobic pathways

glycolytic fibers

anaerobic pathways

three categories of muscle fibers

- slow oxidative
- fast oxidative
- fast glycolytic

metabolic characteristics of slow oxidative fibers

- contract slowly
- have slow acting myosin ATPases
- fatigue-resistant
- well-adapted for endurance

structural characteristics of slow oxidative fibers

- small, thin muscle fibers innervated by smaller nerve fibers
- more extensive blood vessel system & capillaries
- inc # of mitochondria
- large amounts of myoglobin

myoglobin

- iron-containing protein in muscle cells that controls oxygen uptake from red blood cells
- gives slow muscles bright reddish appearance

characteristics of fast oxidative fibers

- contract quickly
- have fast myosin ATPases
- moderately resistant to fatigue
- rare in humans except endurance-trained athletes

metabolic characteristics of fast glycolytic fibers

- contract quickly
- have fast myosin ATPases
- easily fatigued

structural characteristics of fast glycolytic fibers

- extensive SR of rapid release & reabsorption of Ca2+
- large amounts of glycolytic enzymes for rapid release of energy by the glycolytic process
- high conc of glycogen & CP
- less extensive blood supply
- fewer mitochondria & myoglobin

affects force of muscle contraction

- # of muscle fibers contracting
- relative size of muscle
- series-elastic elements
- degree of muscle stretch

Number of muscle fibers contracting

the more motor fibers in a muscle, the stronger the force of contraction

Relative size of the muscle

the bulkier the muscle, the greater tension, greater its strength

Frequency of stimulation

more frequent stimulus, wave summation & tetanus, greater is the tension, stronger is the force of muscle contraction

Degree of muscle stretch

muscles contract strongest when muscle fibers are 80-120% of their normal resting length

Series-elastic elements

non-contractile structures (connective tissue coverings & tendons) of muscles with ability to stretch & recoil

forces brought about by internal tension

generated by contractile elements (myofibrils) stretches the series-elastic elements

forces brought about by external tension

force is transferred to the load which causes recoil & return of muscle to normal resting length

factors that influence velocity & duration of contraction

- type of muscle fiber
- load
- recruitment

fast glycolytic muscle fiber increases

contractile velocity

slow oxidative muscle fiber increases

contractile duration

effect of load on velocity & duration of contraction

- Smaller load, increase contractile velocity & increase contractile duration
- The greater the load, the longer the latent period, the slower the contraction, the
shorter the duration of contraction

effect of recruitment on velocity & duration of contraction

the more motor units that are contracting, the more prolonged the contraction
can be

factors affecting muscle strength

- muscle size
- fascicle arrangement
- size of active motor units
- MMU summation
- temporal summation
- length-tension relationship
- fatigue

fascicle arrangement strength hierarchy

pennate (rectus femoris) > parallel (sartorius) > circular (orbicularis oculi)

treppe or staircase effect

A state wherein the strength of muscle contraction reaches a plateau when a muscle begins to contract after a long period of rest, its initial strength of contraction may be ½ its strength after 10-50 muscle twitches

cause of treppe effect

increasing availability of Ca2+ in the sarcoplasm from SR with successive muscle AP & failure of the SR to recapture the Ca2+ immediately

resistance exercise

contraction of muscles against a load that resists movement

endurance exercise

aerobic or endurance exercise results in changes in skeletal muscles

adaptations to exercise

- Muscles used actively or strenuously increase in size or strength, become more efficient & fatigue resistant
- Inactive muscles lead to muscle weakness & wasting
- No new cells formed when muscle grow but there is an inc/dec of muscle proteins

adaptations to endurance exercises

improves fatigue-resistance of the muscles by enhancing the delivery & use of O2

physiological effects of endurance exercise

- increased blood supply from increase in # of capillaries around muscle fibers
- increased mitochondria, glycogen, myoglobin synthesis, RBC count, & O2 transport capacity of blood

benefits of endurance exercise

helps reach a steady rate of ATP production & improves the efficiency of aerobic respiration

cardiovascular changes from endurance exercises

hypertrophy of heart muscles, increases stroke volume

circulatory changes from endurance exercises

release of fatty deposits from blood vessel walls

respiratory system changes from endurance exercises

more efficient gas exchange in the lungs & improve delivery of oxygen (and nutrients) to all body tissues

muscle growth

is from cellular hypertrophy or cellular enlargement (not cellular division)

adaptations to resistance exercise

stimulates muscle growth

physiological effects of resistance exercise

- more mitochondria & glycogen
- increase in muscle bulk - increase connective tissues between cells

cause of inc of muscle bulk in resistance exercise

- Muscle fibers synthesize more myofilaments & the myofibrils grow thicker
- Myofibrils split longitudinally when they reach a certain size; well-conditioned muscle
has more myofibrils vs. poorly conditioned muscles

cross-training

required for optimal performance & musculoskeletal health; incorporates both aerobic & anaerobic exercise

deconditioned muscles

muscles are not kept sufficiently active, they become weaker & more easily fatigued

anabolic steroids

testosterone; increases muscle mass & power output

erythropoietin injection

hormone synthesized in the kidneys that acts on the bone marrow stimulating RBC formation -> inc O2 carrying capacity

blood doping

removing blood then freezing them -> transfused back days before competition (inc RBC count)