A&P Exam 2 - Four Major Phases of Muscle Contraction and Relaxation

Why doesn't the thin filament slide back when it is released?

A single power stroke by all of the myosin heads in a muscle fiber would shorten the fiber by ~1%, but a fiber may shorten by as much as 40% The cycle must be repeated many times by each myosin head Each head carries out ~5 strokes per second Each stroke consumes one ATP molecule

Four major phases of muscle contraction and relaxation: EXCITATION STEP 4

ACh receptors are ligand-gated ion channels Step 4: ACh (the ligand) binds to the receptor ↓ Receptor changes shape and opens an ion channel ↓ Na+ diffuses in and K+ diffuses out ↓ RMP quickly goes from -90 mV to +75 mV and back to ~ -90mV. This is called an end-plate potential (EPP)

CONTRACTION

Contraction = the step in which the muscle fiber develops tension and may shorten The sliding filament theory of Jean Hanson and Hugh Huxley

Four major phases of muscle contraction and relaxation

Excitation Excitation-contraction coupling Contraction Relaxation

EXCITATION

Excitation = the process in which action potentials in the nerve fiber lead to action potentials in the muscle fiber

EXCITATION-CONTRACTION COUPLING

Excitation-contraction coupling = events that link the action potential on the sarcolemma to activation of the myofilaments

Four major phases of muscle contraction and relaxation: EXCITATION STEP 1

Step 1: A nerve signal arrives at the synaptic knob ↓ Voltage-gated calcium channels open ↓ Calcium ions enter the synaptic knob

Four major phases of muscle contraction and relaxation: EXCITATION STEP 2

Step 2: Calcium ions ↓ Exocytosis of synaptic vesicles ↓ Release of ACh into the synaptic cleft

Four major phases of muscle contraction and relaxation: EXCITATION STEP 3

Step 3: ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma

RELAXATION

Relaxation: after the muscle has contracted, it relaxes and returns to its resting length

Four major phases of muscle contraction and relaxation: EXCITATION STEP 5

Step 5: End plate potential (EPP) ↓ Voltage-gated ion channels on the sarcolemma open ↓ Some are specific for Na+ and allow Na+ to enter the cell Some are specific for K+ and allow K+ to leave the cell ↓ These ion movements → an action potential The muscle fiber is now excited

Two forces that return the muscle to its resting length

Series-elastic components stretch the muscle Antagonistic muscles lengthen the relaxed muscle

Four major phases of muscle contraction and relaxation: CONTRACTION STEP 11

Step 11: The cocked myosin binds to an active site on the thin filament

Four major phases of muscle contraction and relaxation: CONTRACTION STEP 12

Step 12: The myosin releases the ADP and phosphate molecules, and flexes into a bent, low-E position ↓ Pulls the thin filament with it ↓ Power stroke

Four major phases of muscle contraction and relaxation: CONTRACTION STEP 13

Step 13: Myosin binds another ATP molecule ↓ Releases the actin and is ready to repeat the whole process again Hydrolysis of the ATP and recocking = the recovery stroke

Four major phases of muscle contraction and relaxation: RELAXATION STEP 14

Step 14: Nerve signals stop arriving at the neuromuscular junction ↓ Synaptic knob stops releasing ACh

Four major phases of muscle contraction and relaxation: RELAXATION STEP 15

Step 15: ACh dissociates from its receptor ↓ AChE breaks it down into fragments that can't stimulate the muscle ↓ Synaptic knob reabsorbs fragments for recycling ↓ With no new ACh released, stimulation of the muscle fiber ceases

Four major phases of muscle contraction and relaxation: RELAXATION STEP 16

Step 16: Active transport pumps in the SR begin to pump Ca2+ from the cytosol into the cisternae ↓ The Ca binds to a protein, calsequestrin, for storage until the muscle is stimulated again NOTE: active transport requires ATP, so ATP is needed for muscle relaxation as well as for contraction

Four major phases of muscle contraction and relaxation: RELAXATION STEP 17

Step 17: Calcium ions dissociate from troponin ↓ Ca ions are pumped into the SR

Four major phases of muscle contraction and relaxation: RELAXATION STEP 18

Step 18: Tropomysin moves back into position where it blocks the active sites on the actin filament ↓ Myosin can't bind to actin ↓ The muscle fiber can't maintain tension

Four major phases of muscle contraction and relaxation: EXCITATION-CONTRACTION COUPLING STEP 6

Step 6: A wave of action potentials spreads from the motor end plate in all directions When they reach the T tubules, they continue down them into the sarcoplasm

Four major phases of muscle contraction and relaxation: EXCITATION-CONTRACTION COUPLING STEP 7

Step 7: Action potential opens voltage-gated ion channels in the T tubules, that are physically linked to calcium channels in the terminal cisternae of the sarcoplasmic reticulum (SR) ↓ Gates in the SR open ↓ Calcium ions diffuse out of the SR, down their concentration gradient, into the cytosol

Four major phases of muscle contraction and relaxation: EXCITATION-CONTRACTION COUPLING STEP 8

Step 8: Calcium ions bind to the troponin of the thin filaments

Four major phases of muscle contraction and relaxation: EXCITATION-CONTRACTION COUPLING STEP 9

Step 9: The troponin-tropomysin complex changes shape and shifts to a new position ↓ Exposure of the active sites on the actin filaments, making them available for binding to myosin heads

Four major phases of muscle contraction and relaxation: CONTRACTION STEP 10

The myosin heads must have an ATP molecule attached to initiate the contraction process Step 10: Myosin ATPase hydrolyzes ATP ↓ The energy released activates the myosin head, which "cocks" into an extended, high-E position, while it continues to bind the ADP and phosphate group