Motor Units, Muscle Contraction, and ATP Flashcards
Whole muscle contraction
- muscle tension: the force exerted on an object by contracting a muscle
- load: the opposing force exerted on the muscle by the weight of the object to be moved
- the principles that apply to contraction of a single fiber apply to contraction of whole muscles
Motor unit
1 motor neuron and all the muscle fibers it innervates
- each skeletal muscle is served by at least 1 motor nerve; a nerve that contains the axons of 100s of motor neurons
- as an axon enters a muscle it branches into terminals; each terminal forms a neuromuscular junction with a single muscle fiber
- when a motor neuron fires, all the muscle fibers it innervates will contract
Motor units continued
- # of muscle fibers per motor unit may be as high as several hundred or as low as four
- muscles that exert fine control have small motor units
- muscle that create large, less precise movements have larger motor units
- muscle fibers within a particular motor unit are spread throughout the muscle - not clustered together
- stimulation of a single motor unit causes a weak but uniform contraction throughout the muscle
Muscle twitch
the simplest form of contraction - a muscle fiber’s response to a single action potential
- muscle fiber contracts quickly than relaxes
Myogram
graphical recording of muscle activity
3 Phases of Muscle twitch
- latent period
- period of contraction
- period of relaxation
latent period
1st few milliseconds following stimulation; excitation-contraction coupling is occurring; cross bridges begin to cycle, but muscle tension is not yet measurable
period of contraction
cross bridges are active; myogram tracing rises to a peak; period lasts 10-100ms
period of relaxation
final phase lasting 10-100ms; ca 2+ is being pumped back into the SR; # of active cross bridges is declining; muscle tension declines to 0
the muscle twitch
- all muscles contract faster than they relax
- some twitches are rapid and brief (extraocular muscles)
- some twitches are slow and long (gastrocnemius and soleus)
graded muscle responses
- normal muscle contractions are smooth
- strength of muscle contraction varies by need - graded muscle responses
responses are graded by- changing frequency of stimulation
- changing strength of stimulation
changes in stimulus frequency
- a single stimulus results in a single contractile response (muscle twitch)
- wave temporal summation occurs when a second stimuli occurs before the first relaxation period is completed, which increases the strength of the contraction
frequency of stimuli continues to increase
- relaxation between twitches gets shorter
- concentration of ca2+ in the cytosol becomes greater
- degree of summation becomes greater
unfused (incomplete) tetanus
sustained, quivering contraction
fused (complete) tetanus
contractions fuse into 1 smooth, sustained contraction plateau
-prolonged muscle contractions lead to muscle fatigue
recruitment
also called multiple motor unit summation; stimuli of increasing voltage are delivered, and more muscle fibers are called into play (this controls the force of the contraction more precisely)
3 types of stimuli involved in recruitment
- subthreshold stiumulus: stimulus is not strong enough, no contractions are seen
- threshold stimulus: stimulus is strong enough to cause 1st observable contraction
- maximal stimulus: strongest stimulus that increases contractile force - all motor units are recruited
size principle in recruitment
- motor units w smallest muscle fibers are recruited first - they’re controlled by smallest + most excitable neurons
- motor units with larger muscle fibers are recruited next and contractile strength increases
- largest motor units, containing large & coarse muscle fibers are controlled by largest and least excitable (highest threshold) neurons - these units are only activated when the most powerful contraction is necessary
muscle tone
the constant, slightly contracted state of all muscles
- due to spinal reflexes - groups of motor units are alternately activated in response to input from stretch receptors in the muscles
- keep muscles firm, healthy, ready to respond
* hypotonia - low muscle tone
* hypertonia - high muscle tone
isotonic muscle contractions
muscle changes in length and moves load
- once enough tension is generated to move the load, tension remains relatively constant
- can be concentric or eccentric
concentric contractions
muscle shortens and does work
- ex. biceps brachii contraction to pick up a book
eccentric contractions
muscle lengthens and generates force - 50% stronger contractions than concentric
- ex. quadriceps contractions while walking downstairs
isometric contractions
muscle tension develops, but the load is not moved - load is greater than the tension the muscle can develop
- ex. attempting to lift a piano w one hand
- takes place primarily to uphold posture or keep joints stationary
- ex. wall sits and planks
- cross bridges are formed and generate force, but they do not slide the thin filaments
ATP
as a muscle contracts, atp supplies energy to:
- move / detach cross bridges
- operate the calcium pump in the SR
- operate the na+ - k+ pump in the plasma membrane
muscles store 4-6 seconds worth of atp
- atp is the only source of energy for contractile activities - must be regenerated quickly
3 mechanisms of atp regeneration
- direct phosphorylation of ADP by creatine phosphate (cp)
- anaerobic pathway: glycolysis and lactic acid formation
- aerobic respiration
creatine phosphate (CP)
unique high energy molecule stored in muscle fibers
- donates a phosphate to ADP to instantly form ATP
creatine kinase
enzyme that carries out the transfer of phosphate
direct phosphorylation of ADP by CP
muscle fibers have enough atp + cp reserves to power the muscle cell for 15 seconds
- cp reserves are replenished during rest or inactivity
CP+ADP–> creatine + ATP
glycolysis - sugar splitting
1st step in glucose breakdown
- process does NOT require oxygen
- glucose is broken into 2 pyruvic acid molecules
- 2 ATPs are generated for each glucose broken down
anaerobic pathway
ATP can be generated by breaking down glucose from the blood or the glycogen stored in the muscle
- glycolysis
- normally, pyruvic acid enters the mitochondria to start the aerobic respiration phase
- during high intensity activity, oxygen isn’t available because contracting muscles compress blood vessels and impair oxygen delivery
- in absence of oxygen, pyruvic acid is converted to lactic acid
anaerobic pathway part 2
lactic acid diffuses into the bloodstream
- its used as fuel by liver, kidneys, heart
- its converted back to pyruvic acid or glucose by the liver
- lactic acid is responsible for post-activity muscle soreness
- anaerobic respiration harvests only about 5% as much atp from each glucose molecule as the aerobic pathway, but it produces atp about 2.5x faster
- glycolysis can provide most the atp needed for 30-40 seconds of the strenuous muscle activity
- together, stored atp + cp and glycolysis can support about a minute of strenuous activity
aerobic respiration
- during rest and light-moderate exercise, 95% of the atp used for muscle activity comes from aerobic respiration
- aerobic respiration is slower than the anaerobic pathway, but creates more atp (up to 32 molecules)
- aerobic respiration starts with glycolysis and then moves into multiple reactions that take place within the mitochondria
- glucose + O2 –> CO2 + H2O + ATP
- aerobic respiration requires consistent O2 and glucose
- the CO2 produced diffuses out of the muscle and into blood - will be removed by lungs
aerobic respiration part 2
- as exercise begins, muscle glycogen provides most of the fuel
- shortly after bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are major fuel sources
- after abt 30 mins, fatty acids become the major energy fuels
- aerobic respiration is slow but provides a high yield of atp
energy systems for exercise
- when atp demands are within the capacity of the aerobic pathway, light to moderate activity can continue for several hours
- when atp demands exceed the ability to carry out the necessary reactions fast enough, anaerobic pathways contribute more
aerobic endurance
the length of time a muscle can continue to contract using aerobic pathways
anaerobic endurance
the point at which muscle metabolism converts to anaerobic glycolysis
energy systems used during exercise part 2
- exercises that require a surge of power but last only a few seconds rely entirely on atp and cp stores
- exercises requiring slightly longer bursts of activity are fueled almost entirely by anaerobic glycolysis
- prolonged endurance activities depend mainly on aerobic respiration - levels of atp and cp don’t change much - pay as you go
muscle fatigue
- fatigue: the physiological inability to contract despite continued stimulation
- prevents total depletion of atp
- possible causes
- ionic imbalances that disrupt membrane potentials
- increased organic phosphate (Pi) from CP and ATP breakdown may interfere with calcium release from the SR
- decreased atp and increased mg2+ act on voltage-sensitive proteins in the t tubule and decrease ca2+ release from the SR
- decreased glycogen
- lack of atp and rise of lactic acid are rarely reasons for muscle fatigue
excess post exercise oxygen consumption
for a muscle to return to its pre-exercise state:
- oxygen must be replenished
- accumulated lactic acid must be reconverted to pyruvic acids
- glycogen stores must be replaced
- atp and creatine phosphate reserves must be resynthesized
excess postexercise oxygen consumption (EPOC)
the extra amount of oxygen that the body must consume for these restorative processes - formerly called the oxygen debt
