1. NOTES: The Muscular System (Ch 8, part 4)
Muscular Responses

2. MUSCULAR RESPONSES
THRESHOLD STIMULUS: the minimal stimulus required to cause a muscular contraction

3. MUSCULAR RESPONSES ALL-OR-NONE RESPONSE: -If a skeletal muscle
fiber contracts at all,
it will contract
completely
*Conditions may affect
the force of the cell’s contraction
i.e. temperature
-Motor units (all muscle fibers innervated by the same motor neuron) also respond in an all-or-none manner
One motor neuron may stimulate a few muscle fibers or hundreds at a time
The fewer the # of fibers supplied, the more precise the movements will be (ex. Hand)

5. Graded Strength Principle
Skeletal muscles as a whole do not contract with the all-or-none principal like individual muscle cells
Muscles contract with varying degrees of strength at different times
Different muscles can also generate different degrees of strength so we can match the force of movement to specific demands

6. MUSCULAR RESPONSES
If a muscle is stimulated to threshold potential a TWITCH CONTRACTION would occur
SUMMATION:
-A rapid series of stimuli may produce summation of TWITCHES (short contractions)
-Forceful, sustained contraction without relaxation is a tetanic contraction (TETANUS)
*NOT to be confused with
the condition caused by a
bacteria!

8. MUSCULAR RESPONSES
*Even when a muscle appears to be at rest, its fibers remain partially contracted…this is known as “muscle tone,” or TONUS

9. MUSCULAR RESPONSES Isotonic (same tension) Contraction
Occurs when the tone or tension in a muscle stays the same, but the length of a muscle change
Little resistance is placed on the muscle so the sarcomeres shorten
Ex: lifting exercises, doing a sit up,
swinging a bat
Isometric (same length) Contraction
The muscle length remains the same, but the tension increases
Does not produce movement
Myosin cross-bridges are not strong enough to move the load
Ex: attempting to lift an immovable object, holding a weight at arm’s length; maintaining your posture

10. ISOTONIC
ISOMETRIC

11. Oxygen Supply and Cellular Respiration
● Aerobic respiration requires oxygen
● Red blood cells carry oxygen to body cells (oxygen binds to HEMOGLOBIN in the RBCs)
● MYOGLOBIN in muscle cells temporarily stores oxygen

12. **the metabolic capacity of a muscle may change with training!**
Oxygen Debt
● During rest or moderate exercise, muscles receive enough oxygen to respire aerobically
● During strenuous exercise, oxygen deficiency may cause LACTIC ACID to accumulate within the cells
● OXYGEN DEBT is the amount of oxygen required to convert accumulated lactic acid to glucose and to restore supplies of ATP
**the metabolic capacity of a muscle may change with training!**

14. No O2
With oxygen: Glucose  Pyruvate  ATP + CO2 +H2O
Without oxygen: Glucose  Pyruvate  Lactic Acid

15. Muscle Fatigue: ● A fatigued muscle loses its ability to contract
● Muscle fatigue is usually due to accumulated lactic acid

16. Fiber Types
The ability of a muscle to contract and to continue to contract without fatigue depends on fiber types
Our muscle fibers can be classified as slow twitch fibers or fast twitch fibers

17. Slow twitch
More efficient at using oxygen to generate ATP over a long period of time
Contracts more slowly than a fast twitch fiber
Used in endurance sports

18. Fast twitch Generates short bursts of strength and speed
Fatigues more quickly
Uses stored glycogen instead of blood delivered glucose
Has fewer mitochondria & less myoglobin than slow twitch

19. Genetics may affect the amount of slow and fast twitch fibers you have
You may be genetically built to excel at some sports!

21. Muscle Adaptations to Exercise
With exercise, muscle will increase in size and strength as well as efficiency
Effects of Aerobic (Endurance) Exercise
Increased #’s of capillaries in and around muscles
Increased #’s of mitochondria
More myoglobin
Effects of Resistance Exercise
Hypertrophy (increase in size) and hyperplasia ?? (increase in #)
More mitochondria
More myofilaments and myofibrils
More glycogen stored in muscles
More connective tissue between muscles

23. Factors that contribute to graded strength:
Metabolic conditions of individual muscle fibers
Some fibers can maintain a high level of ATP
Number of fibers contracting simultaneously
Depends on the # of motor units recruited
More motor units are recruited with more stimulation
Length-Tension Relationship
A muscle that begins it contraction from a short initial length will not generate much strength because the sarcomeres are already compressed
A muscle that begins its contraction from an overstretched initial length will not develop much tension because the myofilaments are too far away
i.e. biceps brachii
Amount of load
Heavier the load, the stronger the contraction (to a point)
Occurs because of a stretch reflex