Muscles and Neurons (Extended Response)

Question 1

Explain the events that occur during a nerve impulse and how the impulse is propagated along an axon.
(7 marks)

Answer 1

-nerve impulses are action potentials propagated along axons of neurons
- resting potential is −70 mV
OR
relatively negative inside in comparison to the outside
-Na+/K+ pumps maintain/re-establish «the resting potential»
-more sodium ions outside than inside «when at the resting potential»
OR
more potassium ions inside than outside «when at the resting potential»
-action potential stimulates «wave of» depolarization along the membrane/axon
-«when neuron is stimulated» if threshold potential is reached Na+ channels open
-sodium ions diffuse/move in
-«Na+ move in» causing depolarization / inside of the neuron becomes more positively charged than the outside of the neuron
-potassium ion channels open
OR
potassium ions diffuse/move out
-«K+ move out» causing repolarization
k. local currents
OR
description of Na+ ion diffusion between depolarized region and next region of axon to depolarize
-myelination increases propagation speed/allows saltatory conduction

Question 2

Outline how neurons generate a resting potential.
(4 marks)

Answer 2

-sodium /Na+ out and potassium /K+ in
OR
sodium/Na+ concentration higher outside and potassium/K+ higher inside
-Sodium potassium pump: three Na+ pumped for every two K+ (hence negative inside)
OR
inside of axon holds negative ions/Cl- ions/negatively charged proteins/organic anions (hence negative inside)
-by active transport / using ATP
-inside (of axon/neuron) is negative in comparison to outside
OR
electrochemical concentration/charge difference (across the membrane) is the resting potential
-resting potential is –70 mV

Question 3

Explain the process of muscle contraction. (7 marks)

Answer 3

-muscle fibre contains many myofibrils
-made up of sarcomeres/labelled diagram showing one or more sarcomeres
-nerve impulse/signal from (motor) neuron causes release of calcium (ions)/Ca2+
-calcium (ions)/Ca2+ released from sarcoplasmic reticulum
-calcium (ions)/Ca2+ link to troponin
-tropomyosin moves to expose actin binding sites
-ATP hydrolysis/ATP to ADP and phosphate/release of phosphate from ATP
-ATP binds to myosin heads and causes them to cock/change angle/release/gain energy
-myosin heads bind/form cross-bridges to actin
-myosin heads push on actin and then detach from actin
-actin moved towards centre of sarcomere/sliding of filaments
-sarcomere becomes shorter (resulting in muscle contraction)

Question 4

Coughing to clear the airways is accomplished by muscle contractions. Explain muscle contraction. (8 marks)

Answer 4

-myofibrils «in muscle fibers/cells»
-sarcomeres «are the repeating units in muscle/myofibrils»
-sarcomeres arranged end to end / sarcomeres shorten during muscle contraction
-actin and myosin/overlapping protein filaments/diagram to show sarcomere with actin and myosin overlapping
-dark and light bands «in sarcomeres»/diagram to show this/light bands narrower when muscle is contracted
-thick filament is myosin and thin filament is actin/diagram to show this
-nerve impulses stimulate contraction/cause depolarization of sarcolemma/T-tubules/trigger release of calcium from sarcoplasmic reticulum
-calcium ions released from sarcoplasmic reticulum/bind to troponin
-troponin causes tropomyosin to move/exposes binding sites on actin
-myosin «heads» form cross bridges with/bind to actin
-myosin heads move/change angle/swivel/cock / myosin heads cause the power stroke
-myosin filaments pull actin towards center of sarcomere/more overlap between actin and myosin/Z-lines move closer
-ATP is used «to provide energy»/cause cross-bridges to break/cause movement of myosin heads/cause filaments to slide/cause muscle contraction
-intercostal /abdominal /diaphragm muscles contract «to cough»