PHGY 215 – FINAL 2026 FINAL PAPER
COMPLETE QUESTIONS AND ANSWERS FULL
SOLUTION
◉ describe the sliding filament mechanism in skeletal muscle
contraction.
Answer: -cross bridge formation forms the basis of the sliding
filament mechanism
-thin filaments move inwards over thick filaments, z lines move
closer together
--> all sarcomeres shorten to the same degree
-neither the length of the thin filaments or thick filaments changes,
just the degree of overlap
-the power stroke is the interaction between myosin and actin that
leads to shortening of the sarcomere
◉ what is the primary storage site for calcium in skeletal muscle
cells?.
Answer: sarcoplasmic reticulum (SR)
,◉ describe the steps of the cross-bridge cycle in skeletal muscle.
Answer: 1. binding:
-The ATPase site on myosin head binds ATP, and it splits it into ADP
and inorganic phosphate (Pi)
-Stored energy is released and transferred to the myosin cross-
bridge
2. power stroke:
-In the presence of Ca2+, the troponin-tropomyosin complex
exposes the actin molecules and the cross-bridge can bind with the
actin molecule
-Myosin head bends, pulling thin myofilament inward
-During the power stroke, the Pi is released and when the power
stroke is complete, the ADP is also released leaving the ATPase site
empty yet the cross-bridge is still bound to the actin
3. detachment:
-Cross-bridge detaches at end of power stroke and returns to
original conformation, caused by binding of a new ATP
--> cycle repeats!
◉ define excitation-contraction coupling in skeletal muscles.
,Answer: The process of converting an electrical signal into an actual
contraction
-at the motor end plate, Ach is released into the neuromuscular
junction where it causes permeability changes and initiates an
action potential
-sarcoplasmic reticulum & t-tubules help transmit this signal to the
muscle fibres
◉ describe motor unit recruitment and how it influences muscle
contraction.
Answer: -once a motor neuron enters a muscle, it branches out and
innervates multiple muscle fibres; when that motor neuron is
activated, it causes all of its muscle fibres to contract --> this is called
a motor unit
-the muscle fibres of a motor unit are spread throughout the entire
muscle so that the activation of only one motor unit causes only a
weak contraction
-more motor units = stronger contraction
◉ describe muscle twitches (the basic unit of muscle contraction).
Answer: -influenced by frequency of stimulation & motor unit
recruitment
, 1. If a muscle fibre is restimulated AFTER it has completely relaxed,
the second twitch is the same magnitude as the first
2. If a muscle fibre is restimulated BEFORE it has completely relaxed,
the second twitch is added on to the first twitch, resulting in twitch
summation
3. If a muscle fibre is stimulated so rapidly that the twitches overlap,
tetanic contraction occurs
-these contractions can be unfused, where the muscle fibres do not
fully relax before the next stimulus; or fused, where there is no
relaxation of the muscle fibres between stimuli
-a fused tetanic contraction is the strongest single-unit twitch in
muscle contraction - known as tetanus
◉ sustained contraction: explain what would occur if you held a 10
pound weight with a straight arm for 5 minutes.
Answer: -when holding a heavy load, motor units are stimulated by
motor neurons at very high frequency
-each stimulus causes a twitch
-with no periods of relaxation, the twitches would overlap, causing
fused tetanus
-this maximal contraction would occur until fatigue; the weight
would either be dropped or the arm position would change in order
to recruit other muscles
COMPLETE QUESTIONS AND ANSWERS FULL
SOLUTION
◉ describe the sliding filament mechanism in skeletal muscle
contraction.
Answer: -cross bridge formation forms the basis of the sliding
filament mechanism
-thin filaments move inwards over thick filaments, z lines move
closer together
--> all sarcomeres shorten to the same degree
-neither the length of the thin filaments or thick filaments changes,
just the degree of overlap
-the power stroke is the interaction between myosin and actin that
leads to shortening of the sarcomere
◉ what is the primary storage site for calcium in skeletal muscle
cells?.
Answer: sarcoplasmic reticulum (SR)
,◉ describe the steps of the cross-bridge cycle in skeletal muscle.
Answer: 1. binding:
-The ATPase site on myosin head binds ATP, and it splits it into ADP
and inorganic phosphate (Pi)
-Stored energy is released and transferred to the myosin cross-
bridge
2. power stroke:
-In the presence of Ca2+, the troponin-tropomyosin complex
exposes the actin molecules and the cross-bridge can bind with the
actin molecule
-Myosin head bends, pulling thin myofilament inward
-During the power stroke, the Pi is released and when the power
stroke is complete, the ADP is also released leaving the ATPase site
empty yet the cross-bridge is still bound to the actin
3. detachment:
-Cross-bridge detaches at end of power stroke and returns to
original conformation, caused by binding of a new ATP
--> cycle repeats!
◉ define excitation-contraction coupling in skeletal muscles.
,Answer: The process of converting an electrical signal into an actual
contraction
-at the motor end plate, Ach is released into the neuromuscular
junction where it causes permeability changes and initiates an
action potential
-sarcoplasmic reticulum & t-tubules help transmit this signal to the
muscle fibres
◉ describe motor unit recruitment and how it influences muscle
contraction.
Answer: -once a motor neuron enters a muscle, it branches out and
innervates multiple muscle fibres; when that motor neuron is
activated, it causes all of its muscle fibres to contract --> this is called
a motor unit
-the muscle fibres of a motor unit are spread throughout the entire
muscle so that the activation of only one motor unit causes only a
weak contraction
-more motor units = stronger contraction
◉ describe muscle twitches (the basic unit of muscle contraction).
Answer: -influenced by frequency of stimulation & motor unit
recruitment
, 1. If a muscle fibre is restimulated AFTER it has completely relaxed,
the second twitch is the same magnitude as the first
2. If a muscle fibre is restimulated BEFORE it has completely relaxed,
the second twitch is added on to the first twitch, resulting in twitch
summation
3. If a muscle fibre is stimulated so rapidly that the twitches overlap,
tetanic contraction occurs
-these contractions can be unfused, where the muscle fibres do not
fully relax before the next stimulus; or fused, where there is no
relaxation of the muscle fibres between stimuli
-a fused tetanic contraction is the strongest single-unit twitch in
muscle contraction - known as tetanus
◉ sustained contraction: explain what would occur if you held a 10
pound weight with a straight arm for 5 minutes.
Answer: -when holding a heavy load, motor units are stimulated by
motor neurons at very high frequency
-each stimulus causes a twitch
-with no periods of relaxation, the twitches would overlap, causing
fused tetanus
-this maximal contraction would occur until fatigue; the weight
would either be dropped or the arm position would change in order
to recruit other muscles
