Today's lab uses human subjects. The classic animal lab is the "frog nerve-muscle" preparation, one of the labs at University of Missouri - Columbia. Electrodes in the gastrocnemius muscle (right) stimulate the muscle directly while a cuff on the sciatic nerve (left) stimulate the muscle via the nerve. One tendon is fixed down while the other is tethered to the force transducer. The force transducer can be calibrated by using a weight. In another lab, smooth muscle from rabbit gut was studied. A chamber, an air tube, and a transducer are used for the gut preparation. The demonstration involved manipulations like atropine to block motility. Contractions were monitored on a computer.

Slide 2 (See Figure 12-1 and Table 12-3 in your text).
1.) Muscle Generates force and motion.
2.) There are three types of muscle:
A.) Skeletal Muscle is striated and arranged in sarcomeres.
-It is under voluntary control (Ca++ and troponin).
-Skeletal muscle is attached to bone and serves to function with a few sphincters (the external sphincter on the bladder, which controlled by somatic motor neurons).
-Skeletal muscle cells are multinucleated and have large, cylindrical fibers.
B.) Cardiac Muscle is striated and arranged in sarcomeres.
-It is under involuntary control (Ca++ and troponin).
-Cardiac muscle is the tissue of the heart.
-Cardiac muscle cells are uninucleated and have short, branching fibers (Example: Purkinje fibers that rapidly conduct electrical signals at the base of the heart).
C.) Smooth Muscle is non-striated and arranged in long bundles.
-It is under involuntary control (Ca++ and Calmodulin.Remember from Cell??)
-Smooth muscle comprises the walls of hollow organs; present in some sphincters (the internal sphincter on the bladder, which is actually a continuation of the bladder wall. Also, it is normally contracted).
-Smooth muscle cells are uninucleated and are arranged in small, spindle-like fibers.

Slide 3 (See Figure 12-2)
Skeletal muscle can be arranged into flexors and extensors.
-Flexors pull the center of two bones closer together, like the biceps brachii muscle bringing the humerus and ulna closer together.
-Extensors push the center of two bones farther apart, like the triceps brachii muscle pushing the humerus and ulna farther apart.

Slide 4 (See Figure 12-3 a)
This figure shows the macro-organization of skeletal muscle.
-Skeletal muscle (seen here attached to a tendon) is comprised of muscle fasicles.
-Muscle fasicles
are comprised of muscle fibers.
-Muscle fibers
are comprised of myofibrils.

Slide 5
(See Figure 12-3 b)
This figure shows the micro-organization of the muscle fiber.
-The sarcolemma is the cellular membrane for muscle fiber.
-The sarcoplasm is the cytoplasm of the muscle fiber.
Again, myofibrils comprise the basis of each muscle fiber.
-The sarcoplasmic reticulum wraps around each myofibril. Terminal cisternae are located at each end of the sarcoplasmic reticulum. The sarcoplasmic reticulum release calcium ions, while the terminal cisternae concentrate the calcium ions. The t-tubules (transverse tubules) are a continuation of the fiber membrane (see the holes in the sarcolemma??).

Slide 6 (See Figure 12-3 c)
This figure shows the composition of a myofibril.
-H Zone=myosin only.
-M Line=the attachment site for the thick filaments.
-A Band=entire length of thick filament (notice that this includes actin).
-I Band=actin only.
-Z Disk=attachment site for thin filaments (has protein).
Therefore, one sarcomere is found between two Z Disks.
Titin and Nebulin help stabilize the filament structure and return the sarcomere to it's resting state.
Troponin and Tropomyosin regulate sarcomere function.

H Zone and I Band shorten, while the A Band remains constant.

Slide 7 (See Figure 12-5)
This figure shows the two and three dimensional arrangement of a sarcomere.
Again, I Band has thin filaments.
H Zone has thick filaments only.
M Line has thick filaments and attachment proteins.
A Band has thick and thin filaments.

Slide 8 (See Figure 12-9)
This figure explains the events of a contractile cycle.
1.) The Rigor State is the state where no nucleotide (ADP or ATP) is bound to the myosin head. The myosin heads are tightly bound to G-actin. [The rigor state is actually the state of "rigor mortis", which is when the body has no ATP available to complete the contractile cycle. A body with rigor mortis is therefore very stiff.]
2.) ATP binds to the myosin head and myosin detaches from G-actin (change in the binding affinity.
3.) ATP is hydrolyzed to ADP and Pi (inorganic phosphate).
4.) Myosin weakly attaches to G-actin because energy is released from the hydrolysis of ATP. ADP and inorganic phosphate are still bound to myosin, which provides the potential energy to drive a power stroke.
5.) Pi releases from myosin and actin is pushed towards the center of the sarcomere. This is Cross-bridge tilting because the myosin and actin binding angle shifts from 90 degrees to 45 degrees.
6.) ADP is released and myosin is again tightly bound in the rigor state.

Slide 9 (See Figure 12-10)
This figure shows how contraction is actually regulated.
-Tropomyosin is a long protein that wraps around the actin filament. Tropomyosin blocks the binding site on actin so that the myosin heads cannot complete a contractile cycle.
-Troponin is bound to tropomyosin and regulates its position.
-Actually, when tropomyosin is turned "off", the myosin heads are still weakly bound to actin. The weak binding is not enough to complete the power stroke.
-When Cytosolic Calcium increases from Excitation-Contraction Coupling, it binds to troponin. Troponin moves tropomyosin and muscle contraction can take place.

(1) An action potential in the spinal motor neuron goes to the neuromuscular junctions of all the muscle fibers (cells) in the motor unit (all the striated muscle cells innervated by one spinal motor neuron.
(2) Vesicles of acetylcholine are released and bind to nicotinic acetylcholine receptors (channels) on the end plate.

Excitation Contraction Coupling (Figure 12-11)
1. Initiated by Acetylcholine from the somatic motor neuron onto the motor end plate
2. This opens Na and K channels in the end plate and creates a positive charge on the muscle fiber due to more Na being pumped in that K out because the electrochemical driving force is greater for Na than for K
3. This creates an "end-plate" potential which then initiates a muscle action potential
4. The action potential goes down the t-tubules on the surface of the muscle fiber by the opening of voltage gated Na channels
12-11 b
5. Eventually the action potential causes the dihydropyrididne (DHP) receptor to open a mechanically linked Ca release channels on the sarcoplasmic reticulum. The release channels are called ryanodine receptors.
6. The stored Ca then moves into the cytoplasm (which is down its chemical gradient)
Free cystolic Ca levels are typically very low, but after an action potential they typically increase about 100 times their initial amount.
7. Because the cystolic Ca is so high, Ca can bind to troponin which allows tropomyosin to moves to its "on" position and contraction occurs the way that was just described.. using the power strokes to lift the myosin heads off of the actin filament and reach towards the z disk to contract the muscle.
8. Relaxation begins when the sacroplasmic reticulum pumps Ca back into the system using a Ca ATPase. When the cystolic Ca levels decrease Ca loses its binding affinity for troponin and tropomyosin slides back into place and the fiber relaxes.

Twitch (Figure 12-12)
1. The timing of electrical and mechanical events is seen here.
2. It is easy to see that the somatic motor neuron (although it operates in the same way by moving an action potential by sodium voltage gated channels) actually moves much faster than the muscle action potential. Also you can see that the nerve cell action potential creates a single muscle action potential which leads to one muscle contraction which is called a twitch.
3. This short delay here is called the latent period (time between muscle action potential and muscle contraction)
4. The speed of muscle twitches depends on the particular fiber

Muscle Fiber Types (Table 12-2)
1. Muscle fibers are divided groups based upon their speed of contraction, and their resistance to fatigue.
2. There are 3 types of fibers a) Slow Twitch Oxidative (Red Muscle) b) Fast Twitch Oxidative (Red Muscle) c) Fast Twitch Glycolytic (White Muscle).
3. The term fast twitch means that they develop tension 2 to 3 times faster than slow twitch muscles. However fast twitch muscle contractions do not last as long as slow twitch. Actually slow twitch muscles may last up to 10 times as long.
4. Fast twitch muscles fatigue quicker than slow twitch muscles due to the fact that fast twitch muscles rely on ATP and anaerobic glycolosis, which leads to a back up of lactic acid. Lactic acid contributes to acidosis thus leading to fatigue.
Slow twitch rely on oxidative phosphorylation for production of ATP. They also have more mitochondria and more blood vessels to bring in oxygen.
5. Slow twitch have the red color because of the fact that there are more blood vessels and more myoglobin (which gives the red color) also they are of a smaller diameter so it takes less distance for blood to get to the mitochondria. Fast twitch muscles are white because there is less blood flow and less myoglobin and they have a larger diameter.
6. Fast Twitch oxidative Red Muscle-smaller that gylcolytic, contain some myoglobin, and use a combo of oxidative and glycolytic metabolism to make ATP. Fast twitch oxidative are more fatigue resistant than fast twitch glycolytic.

In previous years, in this lab, we stimulated the forearm of subjects with small electrical pulses. Phan and Nicole got this series from Matt M in response to increasing stimulation at low frequency
Here is the result from that from Joel and his lab partners for tetanus using the force transducer

Tetnus? (12-17)
1. 'A' shows that muscle relaxes completely between twitches in single twitch stimulation when muscles have time to relax
2. B shows that if two stimuli happen close to one another, then the muscle contraction is additive. Unlike in the nerve system, where if an action potential is going, there is a refactory period where the nerve will not re-excite.
3. C shows that eventually muscles get to a point where the stimuli no longer add up. That is called maximum tension. C demonstrates unfused, or imcomplete tetnus because the muscle is given a brief relaxation.
4. D is called compete tetanus and shows that the stimulation rate is so fast that the muscle cannot relax. As you can see eventually the muscle fatigues even though the stimuli are still coming.

Today's Lab = Knee Jerk Reaction Figure 13.7
1. Knee Jerk Reflex- Stimulus is the hammer hitting the pattelar tendon.
The tap stretches the muscle fibers on the front of the thigh. Stretching sends an action potential through to the spinal cord. (Afferent path). There are then two different Efferent paths away from the integrating center in the spinal cord. One goes through a somatic motor neuron to the quadriceps and tells the quadriceps to contract. The other goes to the Hamstring and tells the hamstring to relax. The response of the first efferent path is that the quadriceps swing the lower leg forward, and the response to the second efferent path is that the hamstring allows the quadriceps to swing the foot forward by relaxing.

Motor units (Fig. 12-18)
1.There could be more than one type and numerous motor neurons attaching to one muscle fiber. This allows the muscle to do a variety of muscle contractions and strengths of contractions.
2. This slide demonstrates the way a muscle contracts. First, it starts in the integrating center (spinal cord brain) and then an efferent stimulus is sent to your muscles via these motor neurons and lastly your muscle contracts by excitation coupling.


(1) You should become versatile with the use of the electrophysiological computer accessories
(2) You will achieve the above in the context of learning muscle physiology.

Today's lab:
(1) This will be our first, after the tutorial, use of the LabScribe for recording electrical signals
(2) We will generate strong responses with the hand dynamometer feeding into iWorx.
(3) We will record reflexes, and the plethysmograph will show us the stimulus timing.

Quiz and midterm questions from 2005 - 2006 relating to this lecture, interactive physiology and lab.

List the three types of muscle found in the body.
A.) Skeletal
B.) Smooth
C.) Heart

What are bundles of skeletal muscle called?
Muscle Fassicle

A.) blocks the binding sites on myosin, preventing actin cross-bridge attachment at high Ca++ levels.
B.) blocks the binding sites on actin, preventing myosin cross-bridge attachment at low Ca++ levels.
C.) blocks the binding sites on actin, preventing myosin cross-bridge attachment at high Ca++ levels.
D.) blocks the binding sites on myosin, preventing actin cross-bridge attachment at low Ca++ levels.
E.) assists the binding of actin to myosin at low Ca++ levels.

The M lines serve as the attachment site for __________, while the Z disks serve as the attachment sites for ___________.
Myosin; actin

Name the state when no nucleotide is bound to the myosin head.
The Rigor State

Skeletal muscles are commonly defined as flexors and extensors. The biceps would be a __________, while the triceps would be a __________.
Flexor; extensor

What is the regulatory protein that helps to determine the position of tropomyosin?

What determines the speed of relaxation of a muscle fiber?
The rate that the Ca++ ATPase pumps Ca++ into the sarcoplasmic reticulum

How large is the increase in Ca2+ into the cytosol after it is released from the sarcoplasmic reticulum.

Name four differences between fast twitch glycolytic muscle and slow twitch oxidative muscle.
See 12.2 in your book

What is happening during the "latent" period?
The Ca++ is being released into the cytosol

What is the stimulation of additional motor units for increased speed and contraction called?

What is the difference in desired function between large motor units and small motor units?
Fine motor skills versus gross movement

Skeletal muscles are commonly arranged into flexors and extensors. The quadriceps would be a _flexor_, and the hamstring would be a_extensor_.

__Tropomyosin__ blocks the binding site on actin so that the myosin heads cannot complete a contractile cycle.

Calcium has to bind to troponin/tropomyosin before muscle contraction can occur.

Name the state during the contractile cycle when no nucleotide is bound to the myosin head.
The Rigor State

When inorganic phosphate is released from myosin,
a.) Cross bridge tilting occurs because the myosin-actin binding angle shifts from 90 degrees to 45 degrees.
b.) Myosin is pushed towards the center of the sarcomere.
c.) Actin is pushed towards the center of the sarcomere.
d.) Two of the above.

Matching--- There may be more than one correct answer.

1.) H Zone ___ B,G A.) Actin Only
2.) M Line ___ D B.) Myosin Only
3.) A Band ___ E,F, H C.) Attachment site for thin filaments
4.) I Band ___ A,G D.) Attachment site for thick filaments
5.) Z Disk ___ C E.) The entire length of thick filaments
F.) Actin and Myosin
G.) These bands shorten
H.) This band is constant

Muscle fibers are comprised of _myofibrils_.

The cellular membrane of a muscle fiber is the__sarcolemma__, while the cytoplasm of a muscle fiber is the__sarcoplasm__.

Name one of the two filament proteins that stabilize the filament structure and help return the sarcomere back to its resting state.
Titin or Nebulin

For most people, although the force transducer worked for the muscle lab, the hand dynanometer did not. Why not?

we could not stimulate merve and get a full grip, so we measured twitches

Depletion of what chemical after death causes rigor mortis?


What initiates EC coupling in muscle cells?
Acetylcholine from the somatic motor neuron onto the motor endplate

_End plate potential__________ initiates muscle action potential.

Which action potential moves faster, muscle or neural?

What color are fast twitch muscles?

What type of metabolism do fast-twitch muscles utilize?

Put the three different muscle types into the correct order from easiest fatigued to most fatigue resistant.
Fast twitch (glycoltyic) Fast Twitch Oxidative Slow Twitch

What does lactic acid contribute to?

What happens when two muscle contractions happen close to each other? How is this different from what happens in the nervous system?
The sum together (summation). Summation does not occur in the nervous system because of the mandatory refractory period.

What determines the twitch speed of muscles?
Fiber type

Describe the difference between complete tetanus and incomplete tetanus?
Complete tetanus there is no relaxation between twitches. In incomplete tetanus there is some relaxation inbetween contractions.

In the knee jerk reaction, we are hitting the __patellar_____________ tendon.

In the knee jerk reaction, what are the efferent paths and what is the afferent path?
Afferent- stretch of tendon sensory neuron to spinal cord
Efferent - 1. Somatic motor neuron to quadriceps muscle to contract
- 2. Interneuron inhibiting somatic motor neuron to hamstring to relax

What is recruitment?
Stimulation of additional motor units for increased speed and contraction

Similar to the situation for breakdown of polysaccharides and proteins, what type of reaction takes off a phosphate from ATP to convert it to ADP.


Not counting the neuromuscular junction, how many synapses are present in the knee jerk reflex?


The Z disc is a connection point for which protein?


Which protein does Ca2+ bind to, thus exposing the binding site on actin?


In the myofibril, what is the contractile unit that extends from Z disc to Z disc?


Will a flexor bring the centers of the connected bones together or push them apart?

bring them together

Why did we measure the weights of books and place them on the grip ball?

to calibrate the computer

When an inorganic phosphate is released from the myosin binding site, what ensues?

power stroke

Of the 3 muscle fiber types, which would a person use to exert all of their energy in doing
one very heavy bench press at the gym?

fast twitch glycolytic

What is the name of the protein that gives muscles their dark red color?

myoglobin (also hemoglobin and cytochrome)

In the lab, how long did it say that synaptic transmission takes?

1/2 msec

When a muscle goes into tetany, what is happening electrically?

actin potentials are coming one after another so soon that twitches summate

Name the receptor in the sarcoplasmic reticulum that is mechanically linked the DHP


What is the stimulus that activates the DHP receptor?

action potential

During contraction, will actin slide toward or away from the center of the sarcomere?

toward the center

A motor neuron and all of the muscle cells that are stimulated by that motor neuron is
referred to as a(n) ____________________.

motor unit

When muscle relaxation begins, what type of protein/enzyme pumps Ca2+ back into the
Sarcoplasmic Reticulum?

Ca2+ ATPase

Acknowledgements - Thanks to Chris Hawkins and Katie Richards, 2005 TAs, for part of this lecture.

This page was last updated 1/25/08

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