How Muscles Contract

Other Facts about Twitch Fibers

How Muscles Contract

Steps Continued:

• Humans have both types of fibers
• Distribution of fibers is genetically determined
• Neither type can be converted but capacity can be increased through intense exercise

• Ca2+ binds to troponin which is attached to actin.
• Ca2+ binding to troponin causes tropomyosin to move exposing attachment sites for myosin.
• Myosin heads bind to actin.
• ATP is released from myosin heads and heads bend toward center of sarcomere.
• Bending forces actin to slide over myosin.
• Acetylcholinesterase (enzyme breaks down acetylcholine) is released, Na+ channels close, and muscle contraction stops.

ATP and Muscle Contractions

• Energy for muscle contractions supplied by ATP
• Energy is released as ATP → ADP + P
• ATP is stored in myosin heads
• ATP help form cross-bridge formation between myosin and actin
• New ATP must bind to myosin before cross-bridge is released
• Rigor mortis: person dies and no ATP is available to release cross-bridges
• ATP is made in mitochondria from aerobic or anaerobic respiration.

During a muscle contraction:

Steps in a Muscle Contraction (Sliding Filament Theory)

• H zone and I band shorten
• A band stays the same

Terms

Fast Twitch Fibers

• Contract quickly
• Fatigue quickly
• Sprinters
• Use anaerobic respiration
• Energy from glycogen
• White meat

• Threshold: weakest stimulus needed to produce a response
• All or None Law: muscle contracts or doesn’t (no in between)
• Twitch: rapid contraction and relaxation of a muscle
• Tetanus: muscle remains contracted
• Isometric: amount of tension increases (weight)
• Isotonic: amount of repetitions increases
• Tone: constant tension over a long period of time

• An action potential travels down motor neuron to presynaptic terminal causing Ca2+ channels to open.
• Ca2+ causes synaptic vesicles to release acetylcholine into synaptic cleft.
• Acetylcholine binds to receptor sites on Na+ channels, Na+ channels open, and Na+ rushes into postsynaptic terminal (depolarization)
• Na+ causes sarcolemma and t-tubules to increase the permeability of sarcoplasmic reticulum which releases stored calcium.

Slow Twitch Fibers

• Contract slowly
• Fatigue slowly
• Long distance runners
• Use aerobic respiration
• Energy from fat
• Dark meat
• Red or dark because of myoglobin
• Myoglobin: helps O2 bind in muscle

designed by Péter Puklus for Prezi