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Muscle Response
How do muscles respond to
stimuli as an organ?
Muscle Tension vs. Load
 Muscle Tension: Force
exerted by a muscle on
an object.
 Muscle Load: Opposite
of tension, force exerted
on the muscle by the
weight of the object
being moved
Isometric vs. Isotonic Contractions
 Isometric Contraction:
 Muscle tension = Muscle load
 Load does not move
 Isotonic Contraction: Load moves
 Two Types
1. Concentric
 Muscle tension > Muscle load
 Muscle shortens
2. Eccentric
 Muscle tension < Muscle load
 Muscles lengthens
Motor Unit
 A motor neuron and all of the muscle fibers it
innervates
Motor Twitch
 Response of a motor unit to a single action
potential
 All-or-nothing contractions, no partial
The time course of the action
potential is indicated in A
Longer development of tension
of the twitch contraction is
shown in B.
Wave Summation
 describes response to successive action
potentials.




Frequency of stimuli increases
….so, muscle can’t fully relax between contractions
…and contraction force increases (wave
summation)
Twitches link together and fuse to become a
smooth contraction (tetanus)
Tetanus
No relaxation between action potentials
 Allows for smooth, continuous contraction:
(important in maintaining posture, sitting/standing upright)
 Degree of contraction/tetanus depends on:


Speed of stimulation
Number of muscle fibers activated. (recruitment)


100 to lift a pencil
1000’s to lift a barbell
 Order of recruitment controlled by size of fibers: small
motor units first.
Treppe
 A staircase pattern in strength of contraction.


Initial contractions weaker than response to
stimuli of same strength later
“Warm-up” period of muscles


Heat increases enzyme activity
Increases in Ca2+ availability in Sarcoplasmic
Reticulum
Muscle Tone
 Constant action potentials in different motor
units causing muscle to maintain a slight
contraction.

Muscle firm, ready to respond.
Energy for Contractions
 ATP

Little ATP stored in muscles.
 4-6 seconds worth
 Must be regenerated quickly.
Methods of regenerating ATP
 Methods of regenerating ATP:
 Creatine phosphate (for sudden, high demands of ATP)
 CP stored in muscle
 Directly makes ATP by phosphate transfer
 Glycolysis (splitting of sugar, prior to CR)
 Cellular Respiration (aerobic, mitochondrial)
 Anaerobic respiration – w/o enough O2, pyruvic acid
made in glycolysis is converted into lactic acid:
 to regenerate NAD for glycolysis to continue
 some ATP can therefore be formed through
glycolysis.
 Causes muscle soreness and fatigue
Sports Activities and Energy
 Sports involving burst of power: tennis,
soccer, sprints, diving, volleyball
 Rely on creatine phosphate and ATP
stores.
 Anaerobic respiration
 Sports involving endurance: cross-country,
basketball, swimming.
 aerobic respiration
 If demands are too great, switch to
anaerobic and muscle fatigue will set in.
Muscle Fatigue




Physiological inability to contract
Lactic Acid Build up
Not enough ATP to keep muscle working.
Cramps occur when no ATP available to
detach myosin heads
Oxygen Debt
 Amount of O2 needed to restore body back to
proper state after exercise.
 Lactic acid is converted back to pyruvic acid.
 New ATP and creatine phosphate made.
 Ion levels restored = Na+, K+, Ca2+