NOTES: The Muscular System (Ch 8, part 3) Muscular Responses

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

MUSCULAR RESPONSES If a muscle is stimulated to threshold potential a TWITCH CONTRACTION (single contraction) will 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 disease caused by a Bacteria (aka “lockjaw”)!

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

Isotonic (same tension) Contraction Types of Contraction: Isotonic (same tension) Contraction Occurs when the tone or tension in a muscle stays the same, but the length of a muscle changes 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 Ex: attempting to lift an immovable object, holding a weight at arm’s length; maintaining your posture

ISOTONIC ISOMETRIC

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 unit:1 motor neuron & all the muscle fibers it gives a message to, will also respond in an all-or-none manner All muscle fibers of a motor unit contract together 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)

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 Engage more motor units to generate more strength Different muscles can also generate different degrees of strength so we can match the force of movement to specific demands

Oxygen Supply and Cellular Respiration ● Aerobic respiration requires oxygen ● Cellular Respiration Equation: glucose + 02 CO2 + H20 + ATP/Heat!!!

Oxygen Supply and Cellular Respiration ● Red blood cells carry oxygen to body cells (oxygen binds to HEMOGLOBIN proteins inside RBCs) ● MYOGLOBIN : protein in muscle cells that temporarily binds & stores oxygen When the O2 supply in a cell decreases rapidly, as during exercise, it can be quickly resupplied by myoglobin Similar to hemoglobin (also contains iron, gives muscle a reddish color) Cells with a lot of myoglobin, called red fibers (“red meat”) White fibers have less myoglobin (“white meat”)

**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 (IMMEDIATE BURN!) ● OXYGEN DEBT is the amount of oxygen required to convert accumulated lactic acid back to glucose and to restore supplies of ATP **the metabolic capacity of a muscle may change with training!**

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

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

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

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

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

Genetics may affect the amount of slow and fast twitch fibers you have

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 # of cells) More mitochondria More myofilaments and myofibrils More glycogen stored in muscles More connective tissue between muscles