Download Lecture 8: Skeletal Muscle Contraction

Lecture 8: Skeletal Muscle Contraction
Reading: OpenStax A&P Text Chapter 10.2-10.3
Silverthorn Chapter 12
Somatic Motor NS innervates skeletal muscle
1. One SM neuron connects CNS to target skeletal muscle.
2. SM neuron cell bodies are found in the ventral grey horn of the spinal cord.
3. After an action potential is initiated in a SM neuron, acetylcholine (ACh) is released into the synapse at the neuromuscular
junction (which includes the axon terminals and the motor end plates of the skeletal muscle cell).
4. ACh binds to nicotinic ACh receptors located on the motor end plate of skeletal muscles
A. Allow Na+ and K+ ions to pass, though Na passes faster b/c of a greater electrochemical gradient…stimulates AP in postsynaptic cell.
B. Broken down by acetylcholinesterase in the synapse…
Skeletal Muscle Cell
Skeletal muscles are organs consisting of many different tissue types. Each skeletal muscle has a hierarchical organization that must
be understood to truly appreciate it.
1. A myofiber (or muscle fiber) is another name for a skeletal muscle cell. It has some very unique structural characteristics that
distinguish it from other cells in the body.
A. Myofibers originated from myoblasts (baby muscle cells), which fuse together to form the myofiber.
B. Unique characters of myofibers (muscle cells)
i. Multiple nuclei: because baby muscle cells (myoblasts) FUSE to form grownup muscle cells (myofibers)
ii. Tons of mitochondria (why???)
iii. Packed full of cytoskeletal proteins called myofibrils
C. Special terms for muscle cell organelles
i. Sarcolemma: Cell membrane + an outer coat that helps make strong tendons.
ii. Sarcoplasm : Cytoplasm, which has a unique combo of ions to facilitate contraction
iii. Sarcoplasmic reticulum: Smooth ER
a. The SR stores Ca2+ ions…by actively pumping them from the sarcoplasm into its lumen
b. The SR wraps around each myofibril…
iv. T tubules = an extension of the sarcolemma that forms transverse tunnels into the cell
2. Myofibers are filled with protein bundles called myofibrils
A. Myofibrils are bundles of protein filaments called myofilaments.
B. Myofilaments are strings of proteins that enable muscle contraction. They come in two flavors: thick filaments and thin
filaments.
Thick filaments
Thick filaments are made primarily of the protein myosin.
1. Myosin is a contractile protein with binding sites for ATP and actin
2. When ATP binds to myosin, it “cocks” the myosin head…(energy from ATP us used to pull back the head…but ADP+P stay
attached to the myosin, keeping it in place)…this is a RESTING myosin…
Thin filaments
Thin filaments are made primarily of the protein actin
1. Actin has binding sites for myosin
2. Also contain tropomyosin, which covers actin’s myosin binding site (in the RESTING state)
3. Also contain troponin, which holds tropomyosin in place
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A. Myofilaments are organized into sarcomeres, which, when attached end to end, create the striations you see in skeletal
muscle tissue.
The Sarcomere
Myofilaments are organized, within a myofibril, in a very specific way.
1. First, there is a Z-disk that has thin filaments attached on either side. One sarcomere spans the full length between two Z-disks.
2. Then there is an M-line that has thick filaments attached on either side.
3. The thick filaments are found between the thin filaments.
4. Because of this arrangement, there is the illusion of stripes in the myofibril! Draw 3 sarcomeres in series and label the following
parts:
A. Z-disk
B. M-line
C. A band
D. I band
E. H band
Perspective: Organization of Skeletal Muscle Organs
1. Muscle organ (made up of a bundle of fascicles)
2. Fascicle (made up of a bundle of myofibers)
3. Myofibers (made up of a bundle of myofibrils)
4. Myofibrils (made up of a bundle of myofilaments)
5. Myofilaments- thick and thin
The Muscle Contraction
1. Somatic motor neuron fires… (Remember the action potential graph...)
2. AP reaches synapse, opens voltage gated Ca2+ channels
3. Ca2+ rushes in, stimulating release of NT, in this case ACh
4. ACh is released at the synapse with muscle at the motor end plate (a folded area of sarcolemma containing nicotinic ACh
receptors)
5. ACh binds to receptors, which opens Na+ channels (K+ can get through these channels too…)
6. Na+ rushes in, depolarizing the muscle cell… (But K+ can get through too! So why does the cell depolarize???)
7. Depolarization travels through T tubules and hits sacs on the SR
8. Ca2+ channels in SR open; Ca2+ diffuses from the SR into sarcoplasm
9. Ca2+ binds with troponin,
10. Troponin moves tropomyosin, revealing myosin binding sites on actin!
11. Myosin binds to actin active site (forming a CROSS BRIDGE), kicking off ADP+P, and MOVES…
12. ATP must BIND to myosin again, to release actin and pull it back to its original position!
13. Contraction continues as long as ATP can release the myosin and get it ready to pull again (and as long as Ca2+ is
available)…11-12 “strokes” happen about 5 times/second
Relaxation…
Immediately upon stimulation and release of Ca2+ into the sarcoplasm, that Ca2+ is returned to the SR by active transport. In this
case, the active transporters are more attracted to Ca2+ than troponin is, so they steal the Ca2+ ions away from troponin. This causes
tropomyosin to fall back to cover the binding sites, no new myosins can bind. In addition, a few other things must happen.
1. Myosin must resume a “cocked” state. What is required to make this happen?
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Lab 8: Human Nerve Conduction
(iWorx HN-3)
Background
Each neuron in a nerve is capable of generating an action potential that can be propagated along its length without any loss of
amplitude or velocity. Neurons that have large diameters and are covered with an insulating myelin sheath, conduct impulses down
their length with the highest velocities. The somatic motor neurons that supply skeletal muscles are included in this category. Small
unmyelinated nerve fibers, like sensory fibers that originate in digestive organs, are the slowest neurons in the body.
Motor and sensory neurons can be bundled together to create a nerve. When a nerve that carries both SM and SS fibers is stimulated,
the nerve produces a compound action potential (CAP), which is the summation of all the action potentials from the neurons that are
activated by the stimulus. As the strength of the stimulus is increased, more motor units in the nerve are recruited and the amplitude of
the compound action potential is greater. The conduction velocity of the compound action potential is also related to the composition
of the nerve; nerves with a greater number of large myelinated neurons have higher relative conduction velocities.
Conduction velocities are often used to measure the relative health of the nervous system. Changes in the nerve conduction velocity
(NCV) of a patient can indicate the presence of a disease that affects the nervous system by damaging myelin sheaths, destroying
membranes and membrane transport, or constricting nerves. NCV tests are performed when the following diseases or conditions are
suspected: carpal tunnel syndrome, traumatic damage, poliomyelitis, diabetic neuropathy, chronic inflammatory polyneuropathy,
multiple sclerosis (MS), and Guillain-Barre syndrome.
In this experiment, you will stimulate the subject’s ulnar nerve and measure the response of a muscle innervated by the nerve to
indicate nerve activity. Three aspects of nerve activity will be studied in this experiment: the relationship between stimulus strength
and the amplitude of the nerve/muscle response; the latency of the nerve as a function of the polarity of the stimulus pulse; and the
nerve conduction velocity. To measure nerve conduction velocity, the nerve will be stimulated at two different points along its length.
Because the distance between the two stimulation points and time difference between muscle responses from those two positions can
be measured, the nerve conduction velocity can be calculated.
The stimuli that stimulate the nerve and evoke the response recorded from the muscle will be generated by the built-in stimulator of
the BOX, which is programmed to provide mild electrical shocks to the nerve through the skin. The electrical stimulation of the nerves
in the forearm is safe; but, standard safety precautions need to be observed. People with poor cardiac function, pacemakers, or any
other condition that can be aggravated by electrical stimulation should not volunteer to be a subject in this experiment.
Equipment Required
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Laptop computer
The BOX (aka IXTA data acquisition unit) with USB cable and power supply
Electrode reader (aka iWire-B3G EMG/ECG/EEG cable)
Green, black and red electrode lead wires
Black and red stimulating lead wires
7 disposable EMG electrodes/group
Measuring tape
Setting up the BOX (Marlena)
1. Do not turn the box on at any time during setup.
2. Place the BOX on the bench, close to the computer.
3. Use the USB cable to connect the computer to the USB port on the rear panel of the BOX.
4. Plug the power supply for the BOX into the electrical outlet and connect the other end to the labeled socket on the back
of the BOX. (Make sure the box is powered OFF.)
5. Connect the red, green, and black electrode lead wires to Channel 1 on the electrode reader (iWire-B3G).
6. Attach the electrode reader to the iWire 1 input on the front of the BOX.
7. Connect the color-coded stimulator lead wires to the High Voltage Current Stimulator (top right red area on the front of the
BOX). Make sure you push the safety connector of each lead wire into the appropriate socket as far as possible.
8. See Figure 1 for details.
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Start the Software (Students)
1. Turn on your computer.
2. Once your computer is on, switch on the BOX (switch is on the back).
3. Click on the LabScribe2 shortcut on the computer’s desktop to open the program. The LabScribe Main window will appear as
the program opens.
4. An information box should pop up indicating the IX-TA hardware was found. Click “OK.”
5. On the Main window, pull down the Settings menu and select Load Group.
6. Open the “Complete Settings” folder.
7. Locate the folder that contains the settings group IPLMv4Complete.iwxgrp. (It is found at the bottom of the Complete Settings
folder. Select this group and click “open.”)
8. Pull down the Settings menu again. This time, hover over the Human Nerve option and select the “HumanNerveConductionLS2” option. This sets up the correct experiment. Close out the PDF file that opens.
9. Now LabScribe is configured with the HumanNerveConduction-LS2settings and it is ready to collect data.
Setup the Stimulator (so you can shock the subject)
10. Go to Edit/Preferences and select the Stimulator tab.
A. In the drop down menu in the upper left corner, change S1 to HVS (high-voltage stimulator).
B. Change all other settings so that they match the values shown in the table below.
Note: many settings will already be set appropriately.
Prepping the Subject
1. The subject should remove all jewelry from his/her right hand and wrist.
2. Clean the areas where the electrodes will be attached with an alcohol swab. Abrade
the skin in those areas.
3. Obtain 7 electrodes and attach them (one at a time) in the following positions:
A. on the medial edge of the fifth phalanx of the right hand, so the electrode button/
snap is just above the proximal knuckle (attach the red (+) recording lead wire to
this electrode)
B. on the medial edge of the palm, so the electrode button/snap is halfway between
the base of the little finger and the wrist crease (attach the black recording (-)
lead wire to this electrode).
C. on the medial edge of the wrist at the crease (attach the green ground lead wire to
this electrode).
D. on the medial edge of the forearm, so the electrode button/snap is 5 centimeters
proximal to the wrist crease (attach the black (-) stimulating wire to this
electrode).
E. immediately proximal to the previous electrode (attach the red (+) stimulating
wire to this electrode).
F. on the medial edge of the forearm 20 centimeters proximal to the wrist crease
(blank electrode).
G. immediately proximal to the previous electrode (blank electrode).
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4. Snap the three color-coded electrode cables onto the disposable electrodes, so that:
A. the red “+1” lead is attached to the electrode (A) closest to the pinkie.
B. the black “-1” lead is attached to the electrode (B) on the center of the side of the hand.
C. the green “C” lead (C) is attached to the electrode near the wrist.
5. Warning: Before connecting the stimulating electrodes to the subject, check the Stimulator Control Panel to make sure
the amplitude value is set to zero (0). Make sure subject is not wearing jewelry.
6. Connect the 2 stimulating electrodes as follows:
A. Connect the black (-) stimulating lead to electrode (D).
B. Connect the red (+) stimulating lead to the electrode (E).
C. The “blank” electrodes (F and G) will be used when the nerve conduction velocity is determined.
WARNING - The Stimulator should only be used for the method of application for which the Stimulator is intended as shown
in the directions below.
Figure 1: Equipment needed to stimulate the ulnar nerve and record the impulses from the abductor muscle of the little finger.
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Exercise 1: Stimulus Strength and Muscle Response
Aim: To determine the effect of stimulus strength on the response of the innervated muscle.
1. Ask the subject to place his or her right hand on the bench with the palm down. Tell the subject to relax. Note: The subject
should make sure to relax his/her forearm and hand completely. Any tensing of the muscles will interfere with the recording.
2. Set the Amplitude to zero in the Stimulator Control Panel. CLICK APPLY after every time you CHANGE the output amplitude!
3. Click Record button on the LabScribe Main window. LabScribe will record a single sweep with a display time of 50
milliseconds. Since the output amplitude is set to zero, there should be no stimulus and no response from the abductor muscle.
4. Increase the output amplitude to 0.5 mA. Click the Record button again and record another single sweep. Click the AutoScale
button for the Muscle channel to improve the display of the muscle’s response. Note: You many need to continue to increase the
amplitude until a response is generated.
5. Continue to increase the output amplitude step wise by 0.5 mA and record the response until the muscle impulse reaches a max.
6. Instruct the subject to wiggle their fingers and hand after every 5-6 stimuli
7. For each increase to stimulus intensity, record the muscle response by dragging one red cursor to the baseline of the muscle
response and the other red cursor to the peak of the muscle response. Record the V2-V1 value for each sweep in Table 1.
Data
Table 1.
TRIAL
Stimulus
intensity
(mA)
Muscle
response
V2-V1
(mV)
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
Subthresh?
Suprathr?
Submax?
Suprama?
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1. Graph the amplitude of the muscle response (y-axis) as a function of the stimulus amplitude (x-axis).
Questions
* Does the amplitude of the action potential in each fiber in the ulnar nerve increase or do the
numbers of nerve fibers in the nerve that respond increase with increased stimulus strength?
* Does the amplitude of the muscle response increase because the response of each muscle fiber
increases or the number of muscle fibers responding increases?
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Exercise 2: Effect of Stimulus Polarity
Aim: To determine if the polarity of the stimulus pulse delivered to the nerve affects the latency and amplitude of the muscle response.
1. Ask the subject to place his or her right hand on the bench with the palm down. Tell the subject to relax.
2. Set the Amplitude to the value that delivers the maximal muscle response as determined during Exercise 1.
3. Click Record button on the LabScribe Main window to record a single sweep at this stimulus strength.
4. Reverse the polarity of the stimulus pulse delivered to the ulnar nerve by reversing the positions of the stimulating leads on the
subject’s forearm. Switch the stimulating leads by moving the red (+) stimulating lead to where the black (-) stimulating lead
was snapped, and move the black (-) stimulating lead to where the red (+) lead was snapped.
5. Click Record button on the LabScribe Main window to record a single sweep at this stimulus polarity using the same stimulus
amplitude.
Data
Table 2.
TRIAL Regular polarity
(1)
Reversed polarity
(1)
Regular polarity
(2)
Reversed polarity
(2)
Stimulus
intensity
(mA)
Muscle
response
V2-V1
(mV)
Muscle
latency
(T2-T1)
Questions
* How does the amplitude and the latency of the muscle response change when the stimulus
polarity is reversed?
* What causes the difference to occur?
Exercise 3: Conduction Velocity
Aim: To measure the conduction velocity of the ulnar nerve by measuring the time required for the nerve impulse to travel from the
site of the nerve stimulation to the site of the muscle response.
1. Ask the subject to place his or her right hand on the bench with the palm down. Tell the subject to relax.
2. Return the stimulating electrodes to the original positions from Exercise 1 shown in Figure 1.
3. Set the Amplitude to the value that delivered a maximal muscle response.
4. Click the Record button on the LabScribe Main window to record a single sweep at this stimulus strength.
5. Record the sweep number (shown at the bottom of the screen) here: ____
6. Move the red (+) stimulating lead from Electrode E to Electrode G, and the black (-) stimulating lead from Electrode D to
Electrode F.
7. Click Record button on the LabScribe Main window to record a single sweep with the stimulus delivered at this location on the
forearm.
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8. Record the sweep number (shown at the bottom of the screen) here: ____
Note: If you do not get a muscle response with the stimulating electrodes placed farther from the wrist, move the black (-) stimulus
lead to Electrode E and the red (+) stimulus lead to Electrode F. You may need to increase your stimulator amplitude. Be sure to use
the same stimulus amplitude for both stimulus sites.
9. Use a measuring tape, measure the actual distance between the two electrodes used as the (-) stimulating electrodes (the distance
between the two electrodes that you attached the BLACK wires to!).
10. Record the distance (in cm) here: ________. How many meters is that? ________.
Data
Table 3.
TRIAL Electrode position Electrode position
(1)
(2)
Stimulus
intensity
(mA)
Muscle
latency
(T2-T1)
Muscle
latency
(T2-T1)
1. Click the Analysis icon in the LabScribe toolbar to view the recorded sweeps. From the Sweeps list, select the sweeps recorded
while the stimulating electrodes were in different positions (you wrote this down above).
2. Click Add Function, then Select General and T2-T1.
3. Click on the drop-down menu just to the right of where T2-T1 appeared and select Hide Average. The two traces are now
superimposed on each other.
4. Drag each of the red cursors to the peaks of the muscle responses, and read the value for T2-T1. The value for T2-T1 displayed
in the table on the Analysis window is the time it took the action potential in the ulnar nerve to travel the distance between the
two positions of the negative stimulating electrode.
5. Convert your distance from cm to m (100cm = 1m)
6. Calculate the conduction velocity (in m/sec) by dividing the distance (in m) between the two positions of the negative
stimulating electrode by the time (T2-T1) between the peaks of the muscle responses from those two positions.
A. CONDUCTION VELOCITY: ______________
7. Example: 0.15m between positions of (-) electrode / 0.002ms = 75 m/s
8. The subject may now remove the electrodes from their forearm and hand.
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Ext. Brain 8: Skeletal Muscle Contraction
Study Guide
1. Be able to describe and contrast all efferent pathways.
2. Describe the organization of skeletal muscle organs.
3. Compare a regular cell to a skeletal muscle cell.
4. Draw a picture thick and thin myofilaments. Include all their parts.
5. Draw 3 sarcomeres in series. Include the following parts:
a. A band
b. H zone
c. I band
d. M line
e. Z disk
f. Actin
g. Myosin
6. How is a muscle cell unique from other cells in the body?
7. Know the special names for muscle cell structures, such as sarcoplasmic reticulum and sarcoplasm (etc).
8. Know what happens during contraction and relaxation in every part of the sarcomere.
9. Be able to describe the complete biochemical mechanism of muscle contraction and relaxation.
10.What is rigor mortis? (Connect your answer to the previous question).
11. Be able to apply your understanding of a muscle contraction to various conditions and diseases. Even if you don’t know any
details about a condition, you should be able to come up with a viable hypothesis regarding a potential cause for a disease
or reasons why a treatment might work.
12.What is a motor unit?
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