Flashcards in Ch 12 part 2 Deck (22):
If tension tension never overcomes the weight of the load, still generates heat
If the tension > load, muscle shortens, allowing it to perform external work (e.g. moving the load).
contraction --> tension
-Can be studied in vitro where one end of the muscle is fixed and the other is movable
-Electrical stimulations are applied, and contractions recorded & displayed as currents
Force vs. Velocity
For muscles to contract : F(muscle) > F(load)
-As F(load) ↑ -->V (Shortening) ↓
Muscle Twitch and Summation
1. Single action potential generated by a motor neuron stimulates an all-or-none twitch of a muscle fiber. (single muscle twitch generated)
2. Latent period – time between stimulus and contraction (Ca2+ release --> binding to troponin --> crossbridge formation)
3. If second stimulus occurs BEFORE the muscle completely relaxes from first twitch, second twitch will summate with the first one.
--> Reason: Ca2+ from first twitch has not all been taken back up by s.r., so it ADDS to the total Ca2+ released by second stimulation = summated twitch/contraction (increased rate/freq of stimulation = increased strength)
As increase the frequency of stimulations, will increase the tension up to a peak plateau, beyond which the muscle is UNABLE to respond any further.
-->muscle stuck in a contraction
Maximum tension generated when muscle is 100-120% of its resting length—above that range and tension decreases (due to fewer interactions between myosin & actin)- optimal cross bridge formation
—> below that range and tension decreases (due to fiber getting shorter & thicker, generating increased fluid pressure, increased distance between actin and myosin, scrunch muscle up = unable to contract)
Under normal conditions this optimum state is maintained by NEURAL reflexes.
Individual muscle fibers respond in an all-or-none fashion. Yet, our muscles are capable of smooth, graded movements, allowing variations in effort and fine motor control
One motor neuron + All the muscle fibers it innervates
--> A muscle may have many motor units of different types ("a family").
--> one neuron does NOT innervate the entire muscle
-this is how all-or-none twitches of single muscle fibers are integrated into smooth, graded movements of a whole muscle
-strength of contraction = # of motor units recruited
-Single motor neuron makes synaptic contact with a number of muscle fibers— this is the basic unit of motor organization.
-The number varies from one muscle to another and from one motor neuron to another (e.g. a single motor neuron may contact 10-20 muscle fibers or > 1000)
-However, a single muscle fiber normally receives synaptic input from only one motor neuron.
--> When a motor neuron fires, all the muscle cells in that neuron’s motor unit will contract together—fundamental unit of contraction of the whole muscle is not the contraction of a single muscle fiber, but the contraction produced by all the muscle cells in a motor unit.
Increasing motor units activated...
By variation in total number of motor neurons activated (and hence, the total number of motor units contracting)
--As ↑ number of motor units activated --> ↑ strength of contraction
As action potential frequency increases...
By variation in the frequency of action potentials in the motor neuron of a single motor unit.
--As ↑ rate of firing within a motor unit --> ↑ strength of [up to the point of tetany]
Fine muscle control
Fine muscle control requires smaller motor units (fewer muscle fibers per motor neuron
--Eye muscles: ~20 muscle fibers/motor unit
--Larger, stronger muscles may have 1000s of myofibers/motor unit
Thus, control vs strength are tradeoffs
Summary: Muscle strength is determined by...
1. Frequency of stimulation
2. Thickness of each muscle fiber (e.g. via strength training—protein synthesis of contractile/regulatory components for new myofibrils)
3. Initial length of the fiber at rest
4. Number of fibers recruited to contract (concept of the motor unit)
--> more units required to lift arm quickly versus lifting it slowly
slow twitch muscles
steady contractions, e.g. for standing upright (postural muscles)--rich capillary supply, more mitochondria (high oxidative capacity), more myoglobin (aka red fibers)
-more active aerobically
-use these primarily with low/moderate exercise
fast twitch muscles
rapid contraction, e.g. running, jumping; fastest are ocular muscles (control eye movements)—fatigue faster, fewer capillaries, fewer mitochondria (lower oxidative capacity), less myoglobin (aka white fibers), have more glycogen stores
-more active anaerobically
-much faster return to baseline
Time delay between muscle fiber action potential and peak muscle tension varies across muscle fibers
ATP (Energy) Reserves
1. Ready Reserve (phosphagens): pools in muscle cells, constantly replenished
-->store phosphate in creatine forming phosphocreatine
-->only lasts a few seconds
2. Long-Term: glycogen, triacylglycerol, protein
-->lasts minutes to months
Muscle at rest:
ATP from metabolism + creatine -- (add kinase) --> ADP + phosphocreatine
Muscle at work:
Phosphocreatine + ADP -- (add kinase) --> creatine + ATP
Source of energy for muscle during exercise...
Source of energy for muscle contraction depends upon 1) duration and 2) intensity of effort
1. Use up phosphagen supplies (
What is ATP used for?
1. Myosin ATPase (contraction)
2. Ca ATPase (relaxation)
3. Na-K-ATPase (restores ions that cross cell membrane during action potential to their original compartments)
Which fuels create the most energy?
1. FA oxidation: 20.4 millimoles ATP/g/min
2. Glucose oxidation: 30
3. Glucose fermentation: 60
4. P-creatine/ATP hydrolysis: 96-360