• Today
– Role of calcium
– Muscle fiber membrane potential & contraction
– Neural control of muscle
Role of calcium
Tropomyosin
Troponin complex
•Troponin and Tropomyosin bind to actin
block the actin – myosin binding sites
•Troponin is a calcium binding protein
• When Troponin binds calcium it moves
Tropomyosin away from the actin-myosin
binding site
Ca
Ca
Where does Calcium come from?
• Intracellular storage called Sarcoplasmic
Reticulum
• Surround each myofibril of the whole muscle
• Contains high concentration of calcium
• Transverse Tubules connects plasma membrane
to deep inside muscle
Myofibril
Transverse tubules
Sarcoplasmic Reticulum
Transverse tubules
• So far:
– Actin and myosin will bind to each other
– Troponin / tropomyosin inhibit this
– Calcium removes inhibition
What controls muscle calcium?
• What else do we know?
– Neurons initiate muscle contraction at NMJs
by generating postsynaptic potentials
(some muscle fibers have APs)
• Maybe muscle membrane potential is
important
Excitation-Contraction coupling:1
Stimulate nerve
Vm
Tension
Force Transducer
Muscle fiber
‘twitch’
Tension
Muscle AP
Vm
Time
Excitation-Contraction coupling:2
Vm
Tension
Muscle fiber
Force Transducer
Vary [K+] outside
1.0
Tension
Conclusion:
Muscle contraction
occurs with Vm
depolarization
0
-70
-60
-50
Vm (mV)
-40
-30
Why T-tubules important?
Stimulate near T-tubule see contraction
of adjoining sarcomeres
No contractions
T-tubule
‘Local stimulation’
Motor nerve
Membrane depolarization or APs
carried deep into the muscle by Ttubules
T-tubule
+
Neurotransmitter
receptors
SR
Text Fig 10-21
Myofibril
Transverse tubules
Sarcoplasmic Reticulum
Transverse tubules
SR
Ryanodine Receptor
T-tubule My
SR
myoplasm
Dihydropyridine
receptor
Ca++
Ca++
Ca++
SR
Ca++
pump
Myoplasm
(intracellular)
_
+
_
+_ +
_+
_
_ + +
_ + _
+
_
_ + _+ _+ +
T-tubule
(extracellular)
Summary of events
1. Synaptic Depolarization of the plasma
membrane is carried into the muscle by
T-Tubules
2. Conformational change of
dihydropyridine receptor directly opens
the ryanodine receptor calcium channel
3. Calcium flows into myoplasm where it
binds troponin
4. Calcium pumped back into SR
• Neural Control of Muscle
– Voluntary
– Reflex
Neural control of muscle contraction
Motor Pool: all of the motor neurons that
innervate a single muscle
Motor Unit: single motor neuron and all the
muscle fibers it innervates
– a few fibers  1000s of fibers
• Size of the motor units determines
precision of movement
Fingers have small motor units, legs have big
motor units
• Recruitment of twitch fibers
• Smallest motor units to a single muscle are
recruited first
• Why?
Allow smooth generation of movement
First
Second
Third
Individual myofibrils
Motor neurons
Whole muscle
Little force
More force
1+2+3 = maximum force
Even more force
Reflex control of muscle
contraction
Two sensory receptors
1. Muscle Spindle
•
Monitors muscle length
2. Golgi Tendon Organ
•
Monitors muscle tension
Muscle Spindle
 Motor
neurons
Group I and II
Sensory fibers
Muscle Spindle
Intrafusal
Muscle fibers
Extrafusal
Muscle fibers
 Motor
neurons
Muscle spindle
nerve
Extrafusal
muscle
fibers
•  motor neurons innervate extrafusal
muscle fibers and cause the muscle to
contract
•  motot neurons innervate only the
intrafusal muscle fibers and cause them to
contract
• The sensory endings in the muscle spindle
are activated by muscle lengthening
Isolated muscle
Spinal cord
Ia sensory
neuron
AP
Muscle stretch
AP
Motor
neurons
APs in
sensory
APs in
motor
Muscle length
Longer 
Effect of muscle spindle
•
When muscle stretches, spindle stretches
1. Increase APs in 1a sensory neuron
2. Increase APs in motor neuron
3. Muscle contracts and returns to original
length (almost)
• When muscle contracts, spindle shortens
– Might expect activity of spindle to decrease
• BUT
– To maintain sensitivity of the spindle, the
intrafusal fibers also contract
– Controlled by  motor neurons
Muscle Spindle
 Motor
neurons
 Motor
neurons
Group I and II
Sensory fibers
Extrafusal
Muscle fibers
Intrafusal
Muscle fibers
 Motor
neuron
stimulate
Ia sensory
neuron
APs in sensory -  Motor neuron only
shorter
record
Muscle
length
longer
 Motor
neuron
stimulate
Ia sensory
neuron
APs in sensory -  Motor neuron only
shorter
record
 Motor
neuron
Muscle
length
longer
APs in sensory -  and  Motor neurons
Muscle Spindle  motor neurons
• permit muscle spindle to function at all
muscle lengths
• Maintains sensitivity of the spindle
Spinal cord
Ia sensory
neuron
Inhibitory interneuron
 Motor neurons
Muscle spindle
Golgi Tendon Organ
• Operates like muscle spindle, but monitors
muscle tension (force)
• Negative feedback because they inhibit
the muscle they are located in
Golgi Tendon Organ
Very little at rest
Increased APs during contraction
APs from GTO
shorter
Muscle
length
longer
Spinal cord
Inhibitory
interneuron
sensory
neuron
 Motor neurons
Golgi tendon organ
Muscle
Passive Stretch
Active Contraction
Spindle Response
Tendon Organ Response
Increase APs
Decrease APs
No change
Increase APs
Summary
• Muscle Spindles
– Monitor muscle length
– When activated cause contraction
• Golgi Tendon Organ
– Monitor muscle tension
– When activated reduce contraction
Whole muscle physiology
• Types of skeletal muscle fibers
• Neural control of muscle contraction
• Production of force
Classification of muscle fiber types
1. Electrical properties of muscle
membrane – does muscle have APs?
2. Maximal rate of contraction (Vmax)
•
determined by myosin ATPase activity
3. Density of SR calcium pumps
4. Density of mitochondria and blood supply
Vertebrate Skeletal Muscle Fiber Types:
1. Tonic
2. Twitch (or Phasic)
a. Slow oxidative (Type I)
b. Fast oxidative (Type IIa)
c. Fast glycolytic (Type IIb)
• Tonic fibers
– Very slow contractions
– Do not produce APs  do not twitch
– Postural muscles
Twitch muscles
• Slow oxidative (Type I)
– Contract slowly
– Resist fatigue
– Postural
• Fast Oxidative
– High rate of contraction
– Moderately resistant to fatigue
– Rapid, repetitive motion (flight muscles migratory
birds)
• Fast Glycolytic
– Rapid contraction
– Rapid fatigue
Non-twitch fibers
• Many arthropods (crayfish, insects) do not
have muscle APs
• Rather they have graded synaptic
potentials
• Calcium released from SR in graded
manner
• Degree of contraction depends on
summation and facilitation of neural input
Tension
Muscle AP
Vm
Time
Tension
Summation of Synaptic Potential
Vm
Time
Non-twitch fibers
• Graded potential  graded contraction
• Even large motor units can have precise
contraction
Force is proportional to crosssectional area