Download Lecture 9: Skeletal Muscles

Lecture 9: Skeletal Muscles
Reading: Silverthorn Chapter 12
Types of muscle tissue
1. Skeletal muscle
2. Smooth muscle
3. Cardiac muscle
Organization of Skeletal Muscle Organs
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. Muscle organ (made up of a bundle of fascicles)
2. Fascicle (made up of a bundle of myofibers)
3. Myofiber (made up of a bundle of myofibrils)
A. A myofiber (or muscle fiber) is actually a single skeletal muscle cell
B. Myofibers originated from myoblasts, which fuse together to form the myofiber.
C. Unique characters of myofibers (muscle cells)
i. Multiple nuclei: because baby muscle cells (myoblasts) FUSE to form grown muscle cell (myofiber)
ii. Tons of mitochondria (why???)
iii. Packed full of cytoskeletal proteins called myofibrils
D. Special terms for muscle cell organelles
i. Sarcolemma: Cell membrane
ii. Sarcoplasm : Cytoplasm
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
4. Myofibril (made up of a bundle of myofilaments)
A. Myofibrils are bundles of proteins (myofilaments)
B. They show striations (or stripes) and are stacks of sarcomeres.
5. Myofilaments are strings of proteins that enable muscle contraction
Myofilaments
Bundles of myofilaments make up a myofibril. There are 2 kinds of myofilaments.
A. Thick filaments are made primarily of the protein myosin.
i. Myosin is a contractile protein with binding sites for ATP and actin
ii. 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…
B. Thin filaments are made primarily of the protein actin
i. Actin has binding sites for myosin
ii. Also contain tropomyosin, which covers actin’s myosin binding site (in the RESTING state)
iii. Also contain troponin, which holds tropomyosin in place
C. Myofilaments are organized into sarcomeres
Draw 3 sarcomeres here. Include the following parts:
A band
H zone
I band
Z line (disk)
Actin
Myosin
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Note: The Z line is made of actin… Source: http://www.pubmedcentral.nih.gov/picrender.
fcgi?artid=1170509&blobtype=pdf
The Muscle Contraction
1.
2.
3.
4.
Somatic motor neuron fires… (Remember the action potential graph...)
AP reaches synapse, opens voltage gated Ca2+ channels
Ca2+ rushes in, stimulating release of NT, in this case ACh
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.
2. What is required to make this happen?
Relate this fact to “rigor mortis”.
The motor unit
A motor unit is one somatic motor neuron and all the muscle fibers it innervates. A motor unit can range in size. For example, a motor
unit in the eye consists of just 3-5 myofibers/somatic motor neuron. A motor unit in the gastrocnemious, on the other hand, consists
of 2000 myofibers.
How does the motor unit relate to fine motor movements? Why do you play the piano
with your hands and not your feet?
Muscle fatigue
Physiological muscle fatigue occurs when something actively prevents the muscle from contracting. Some examples:
1. Lack of ATP (energy source)
A. Low Oxygen levels
B. Low glucose levels (glycogen)…
2. Accumulation of phosphates from ATP breakdown…then phosphates bind with Ca++ leading to low Ca levels…
3. Decreased pH (lactic acid buildup)
4. Usually psychological!!!
A. Feel tired and just don’t want to continue…REALLY???
B. Check out Sian Welch and Wendy Ingram at the Hawaii Ironman in 1997
http://www.youtube.com/watch?v=MTn1v5TGK_w
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The Patellar Ligament Reflex
mm Anatomy Link!
The patella (knee cap) is attached to the quadriceps group by the quadriceps tendon. This tendon extends
distally and covers the superficial surface of the patella, to become the patellar ligament (often referred to as
the patellar tendon). The patellar ligament’s distal attachment is the tibial tuberosity.
The patellar ligament reflex is often used to assess spinal cord function. Read through the steps of the test and label the various parts
of a reflex pathway.
1. Patellar ligament is tapped using a reflex hammer.
2. STRETCH RECEPTORS in the quadricpes group stimulate AP in AFFERENT NEURONS, sending nerve impulse to CNS.
3. Note: In a large area AROUND the stimulated stretch receptors, there are small EPSP (excitatory post synaptic potentials)
generated in the neurons, but these do not reach THRESHOLD, so they do not generate an action potential.
A. Afferent fibers pass through ventral rami and into spinal nerves L2, L3, and L4.
B. Cell body of the afferent neuron is found in the dorsal root ganglion.
C. Message passes through the dorsal gray horn.
4. The afferent neuron SPLITS, and synapses with TWO different somatic motor neurons in the in the ventral gray horn of the
spinal cord.
5. One synapses directly with a somatic neuron innervating the quad.
6. The other synapses with an INTERNEURON (in CNS) that creates an INHIBITORY POST SYNAPTIC POTENTIAL (IPSP)…
this one effectively INHIBITS the hamstring group!
7. Quad flexes, hamstring stays relaxed…
8. Outcome: KICK!
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External Brain 9: Skeletal Muscle
Study Guide Questions
1. Distinguish between the 3 types of muscle tissue.
2. Be able to label figure 12-3 from your text (pg 410) (or one similar to it...). You do NOT need to be able to label the flow
chart on the bottom of page 410!!! Use your lecture notes to GUIDE required structures.
3. How is a muscle cell unique from other cells in the body?
4. Know the special names for muscle cell structures, such as sarcoplasmic reticulum and sarcoplasm (etc).
5. Know what happens during contraction and relaxation in every part of the sarcomere.
6. Be able to describe the complete biochemical process of muscle contraction and relaxation.
7. What is rigor mortis? (Connect your answer to the previous question).
8. 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.
9. Understand the complete pathway in the patellar reflex.
10.Compare and contrast the action potentials in neurons and skeletal muscles.
11. What is a motor unit?
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