muscle twitches Flashcards
muscle mechanics principles
same principles apply to contraction of a single muscle fiber and a whole muscle
Contraction produces tension, the force exerted on the load or object to be moved
Isometric contraction
no shortening; muscle tension increases but does not exceed the load
Isotonic contraction
muscle shortens because muscle tension exceeds the load
Motor unit
a motor neuron and all (a least four but up to several hundred) muscle fibers it supplies
smaller vs larger motor units
Small motor units in muscles that control fine movements (fingers, eyes)
Large motor units in large weight-bearing muscles (thighs, hips)
muscle twitch
Response of a muscle to a single, brief threshold stimulus
myogram
a graph of twitch tension development
three phases of a isometric twitch
latent period
period of contraction
period of relaxtion
latent period of twitch
events of excitation-contraction coupling
period of contraction of twitch
cross bridge formation; tension increases
period of relaxtion of twitch
Ca2+ reentry into the SR; tension declines to zero
Different strength and duration of twitches are due to
variations in metabolic properties and enzymes between muscles
motor unit in a muscle usually contract…
asynchronously (some turn off while others turn on) helps prevent fatigue unless load too big
Graded Muscle Responses
Variations in the degree of muscle contraction
Required for proper control of skeletal movement
graded muscle responses are graded by
Changing the frequency of stimulation
Changing the strength of the stimulus
A single stimulus results in
a single contractile response—a muscle twitch
Response to Change in Stimulus Frequency
Increase frequency of stimulus (muscle does not have time to completely relax between stimuli)
If another stimulus is applied before the muscle relaxes completely
then more tension results. This is temporal (or wave) summation and results in unfused (or incomplete) tetanus. (low stimulation frequency)
If stimuli are given quickly enough
fused (complete) tetany results
high stimulation frequency
Threshold stimulus
stimulus strength at which the first observable muscle contraction occurs
as stimulus strength is increased above threshold
Muscle contracts more vigorously
Contraction force is precisely controlled by
recruitment (multiple motor unit summation), which brings more and more muscle fibers into action
Size principle
motor units with larger and larger fibers are recruited as stimulus intensity increases
what helps prevent fatigue
fibers are contracting asynchronously
recruitment principle
Can control strength of contraction to meet the needs of the moment
muscle tone
Constant, slightly contracted state of all muscles
Keeps muscles firm, healthy, and ready to respond
muscle tone due to
spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles
Isotonic contractions two types
concentric contractions or eccentric contractions
Concentric contractions
the muscle shortens and does work
eccentric contractions
the muscle contracts as it lengthens
isotonic contractions
Muscle changes in length and moves the load
Isometric Contractions
The load is greater than the tension the muscle is able to develop
Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens
what is the only source used directly for contractile activities
ATP
For contraction, Available stores of ATP are depleted in
4-6 seconds
ATP is regenerated by three ways
Direct phosphorylation of ADP by creatine phosphate (CP) (1st 14-16 seconds – turbo boost) Anaerobic pathway (glycolysis) (30-40 seconds) Aerobic respiration (95%)
Anaerobic Pathway At 70% of maximum contractile activity
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
Anaerobic Pathway lactic acid
Diffuses into the bloodstream
Used as fuel by the liver, kidneys, and heart
Converted back into pyruvic acid by the liver
Aerobic Pathway
Produces 95% of ATP during rest and light to moderate exercise
Aerobic Pathway fuels
stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids
short duration exercise 6 seconds
ATP stored in muscles is used first
short duration exercise 10 seconds
ATP is formed from creatine phosphate and ADP
short duration exercise 30-40 seconds to end of exercise
glycogen stored in muscles is broken down to glucose, which is oxidized to generate ATP
prolonged duration exercise hours
ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway. this pathway uses oxygen released from myoglobin or delivered in the blood by hemoglobin. when it ends, the oxygen deficit is paid back
muscle fatigue
Physiological inability to contract
muscle fatigue two types
nervous fatigue and metabolic fatigue
Nervous fatigue
Nerve ion imbalance can’t send same intensity of signal anymore
Metabolic fatigue
Either lack of fuel (ATP – very rare) or some metabolic waste product (lactic acidocis) interferes with calcium function in sarcomere
Extra O2 needed after exercise for
Replenishment of (Oxygen reserves
Glycogen stores
ATP and CP reserves) and Conversion of lactic acid to pyruvic acid, glucose, and glycogen
40% of the energy released in muscle activity is
useful for work
how much energy in muscle activity is given off as heat
60%
Dangerous heat levels are prevented by
radiation of heat from the skin and sweating
