Unit V: Movement
Muscle Contraction - Part II
Chapter 9 – pg 293-307
Review
1. What are the four stages of muscular activity?
2. Before a muscle fiber can contract, ATP must bind to: a.) a Z disc,
b.) the myosin head, c.) sarcomere, d.) motor end plate
3. Before a muscle fiber can contract, Ca2+ must bind to:
a.) calsequestrin, b.) the myosin head, c.) tropomysin,
d.) troponin, e.) G-actin
4. The __________ portion of the thin filament houses the active
site to make muscles contract.
5. Whereas the ______________ is the portion of the thin filament
that changes shape and allows the muscles to relax.
Neural Control
A skeletal muscle fiber
contracts when stimulated
by a motor neuron
Muscle Activity
The action potential causes the
release of ACh which leads to the
production of an action potential
in the sarcolemma.
Release of calcium ions from the
sarcoplasmic reticulum.
Excitation
Calcium
release
triggers
Contraction cycle begins.
The sarcomeres shorten, pulling
the ends of the muscle fiber closer
together.
Thick-thin
filament interaction
Muscle fiber
contraction
leads to
Skeletal muscle produces tension
on the tendons at either end.
Tension
production
Muscle Twitch in Frogs
• Threshold =
– Twitch: cycle of contraction and
relaxation at threshold (lasting less than
1/10 second)
• Phases of a twitch contraction
– latent period (2 msec delay)
– contraction phase
– relaxation phase
•
Myogram
Contraction Strength of Twitches
Varies with:
•Muscle temperature
•Muscle pH
•Hydration
•Concentration of Ca+
•Stretch of muscle
•Intensity
•Frequency
Intensity:
• Multiple motor unit summation (recruitment)
– lift a glass of milk versus a whole gallon of milk
Contraction Strength of Twitches
Twitch
Muscle twitches
(a)Low
Stimuli
Complete tetanus
Fatigue
High
Highest-Tetanus
Frequency:
• Higher frequency = stronger contractions
– sustained fluttering contractions
• Maximum frequency stimulation (40-50 stimuli/second)
– Tetanus
Contraction Strength
Asynchronous motor unit summation during a sustained contraction
Tension
Tension in tendon
Motor unit 1
Motor unit 2
Motor unit 3
Time
Sarcomere Length vs. Tension
With complete
overlap,
tension equals
zero.
Tension (% of maximum)
Decrease in the
resting sarcomere
length reduces
tension.
Optimal range =
optimal tension
Increase in sarcomere
length, reduced tension
Normal
range
When overlap equals zero,
fibers cannot produce
tension.
Decreased length Increased sarcomere length
Optimal resting length:
75 to 130% of the optimal length.
Isometric and Isotonic Contractions
• Isometric muscle contraction
– prelude to muscle movement
• Isotonic muscle contraction
– Concentric
6 kg
6 kg
Tendon
2 kg
2 kg
–
Eccentric
6 kg
6 kg
Muscle Contraction Phases
Immediate Energy Needs
• Phosphagen system
– myokinase
– creatine kinase
• Short, intense exercise (100 m
dash)
• Result is power enough for 1
minute brisk walk or 6 seconds of
sprinting
Energy Needs
Short-term
• Glycogen-lactic acid system
– produces ATP for 30-40 seconds of maximum activity
• playing basketball or running around baseball diamonds
Long-term
• Aerobic respiration
–Produces 36 ATPs/glucose molecule
0
10 seconds
40 seconds
Duration of exercise
Repayment of
oxygen debt
Mode of ATP synthesis
Aerobic respiration
using oxygen from
myoglobin
Phosphagen
system
Glycogen–lactic acid
System (anaerobic
fermentation)
Aerobic respiration
supported by
cardiopulmonary
function
Muscle Fatigue
• Progressive weakness from use
– ATP synthesis declines
– Na+ and K+ pumps slow
– lactic acid inhibits enzyme function
– accumulation of extracellular K+
– motor nerve fibers use up their ACh
Oxygen Debt
• Difference between resting rate of O2 consumption and
elevated rate following exercise.
• Purposes for extra oxygen
– replace oxygen reserves
– replenishing the phosphagen system
– oxidizing lactic acid
– serving the elevated metabolic rate
Slow- and Fast-Twitch Fibers
• Slow oxidative, slow-twitch fibers, red
– adapted for aerobic respiration and resistant to fatigue
– postural muscles of the back (100msec/twitch)
• Fast glycolytic, fast-twitch fibers, white
– adapted for anaerobic fermentation
– sarcoplasmic reticulum releases calcium quickly so
contractions are quicker (7.5 msec/twitch)
– extrinsic eye muscles, gastrocnemius and biceps
brachii
• Proportions genetically determined
Cardiac Muscle
• Autorhythmic due to pacemaker cells
• aerobic respiration
– resistant to fatigue
– very vulnerable to interruptions in oxygen supply
Types of Smooth Muscle
Functional categories:
• Multiunit smooth muscle
– terminal nerve branches synapse on myocytes
– large arteries, iris, arrector pili muscles
– independent contraction
(a) Multiunit
• Single-unit smooth muscle
smooth muscle
– blood vessel walls, digestive, respiratory, urinary, and
reproductive tracts
Autonomic
nerve fibers
– coupled by gap junctions
– large number of cells contract as a unit
Gap junctions
Autonomic
nerve fibers
Synapses
Smooth Muscle Contraction
•
•
Ca2+ binds to calmodulin
• activates myosin lightchain kinase
• which activates myosin
ATPase
• power stroke occurs
Thin filaments pull on plasma
membrane
– shortens the entire cell in a
twisting fashion
Repsonses to Stretch
• Ex. – esophagus distended by food
brings on peristalsis
• Stress-relaxation response
– Ex. - urinary bladder
The process of peristalsis
Bolus of food
arrives in
digestive
system.
Food
bolus
Toward
anus
Longitudinal
muscle
Circular muscle
Circular muscles
contract behind
bolus.
Longitudinal
muscles ahead
of bolus
contract.
Contraction in
circular muscle
layer forces
bolus forward.