Fans, for the past two weeks you have been reading about a bad break I got. Yet today I consider myself the luckiest man on the face of the earth.,,,I might have been given a bad break, but I've got an awful lot to live for.
-Lou Gehrig, 1939



Audesirk, Audesirk & Byers Part of Chapter 39

Today's musical selection
Little Eva Let's turkey trot
Happy Thanksgiving

How muscle works molecularly has been a real success story in cell-molecular biology.

Cell structure

Figure 39-2
Skeletal ("striated" = striped) muscle cell ("fiber" = cell) 10- 100 microns [micro, 10 to the minus 6. meters] (huge) and long (from tendon to tendon)
There are smaller units within fiber called "myofibrils" (1-2 microns in cross section)
Thus 1000-2000 myofibrils/fiber

Figure 39-3
Sarcomeres are units along the length of myofibrils
Interestingly, the striped (striated) pattern of myofibrils is in register for all the myofibrils in the fiber giving the whole muscle fiber a striated appearance.
Within the myofibrils are the filaments
Actin - G (globular) polymerizes to F (filamentous) actin - the thin filament
Myosin - (2 heavy chains and 4 light chains) - the thick filament
I-band (isotropic - light), A-band (anisotropic, dark) based on actin and myosin, see figure

here is a picture from our histology course, but watch out because the arrows for A, I, and H do not point accurately

Muscle proteins

Figure 39-3 (again)
Z disc where actins are joined in the middle of the actins
M line in the middle of the myosin
A (anisotropic)= where myosin is
I (isotropic) where actin is but not myosin
H (helle) (lighter) where there is myosin but not actin

Involvement of ATP

Interestingly ATP binding unhooks myosin from actin. This can be remembered by thinking about rigor mortis - a "stiff" in a detective show - has been dead long enough so that ATP has run out and actin and myosin are locked together. ATP -> ADP and a phosphate added to the myosin and this is like the rower back-stroking to get ready to take another power stroke. When the phosphate gets kicked off of the myosin, the myosin and actin bind, followed by the power stroke

Involvement of Ca2+

Ca2+ ions are released to make muscle contract (explained later)
tropomyosin on actin
troponin has a Ca2+ binding site like calmodulin
Ca2+ binding to troponin pulls tropomyosin off of actin's binding sites for myosin

The neuromuscular junction

Figure 39-5
here is a similar picture from our histology course of the neuromuscular junction
Action potential from nerve opens channels (nicotinic acetylcholine receptors) at "synapse" called the neuromuscular junction. (Notice that the nerve branches.)
This is a big "synapse" and it works.
Here is a transmission electron micrograph of a portion of a neuromuscular junction. Note the folds, increasing the area on the muscle cell. Note the space with electron density in the cleft. Note the numerous vesicles.

The spinal motor neuron

Lou Gehrig's farewell speech (YouTube
Amyotropic Lateral Sclerosis (Lou Gehrig's disease affects spinal motor neurons)
some cases familial led to identification on chromosome 21
coper/zinc Super Oxide Dismutase (SOD1) reduces oxygen radicals
some famous baseball personalities, Lou Gehrig set consecutive games record until broken by Cal Ripken Jr.

Figure 39-5
(relative to the aforementioned "nerve branches") Motor units
(how many muscle cells per motor neuron)13 eye, 1730 calf

The muscle cell's action potential

Figure 39-3
Then action potential goes down muscle cell. But cell is too big. So transverse tubules (T tubules) get action potential into cell at numerous locations (for each sarcomere and for each myofibril). Proximity with a specialized smooth endoplasmic reticulum called the sarcoplasmic reticulum causes release of Ca2+.

In summary:

ACh to synapse Ecxitation to spike
Final common pathway - motor neuron carries integrated information from nervous system
action potential in membrane and t-tubules, t=transverse
Ca++ release from sarcoplasmic reticulum (ER)

Types of skeletal muscle:

Difference obvious in turkeys
Fast twitch, strong, anaerobic, white meat
Slow twitch, enduring, aerobic, dark meat
capillaries (hemoglobin), myoglobin, cytochromes in mitochondria
can alter with training

Smooth and Cardiac muscle

Table 39-1
smooth muscle - arterioles, gut, uterus - involontary, autonomic
actin and myosin are arranged differently (striations helped in sliding filament theory)
Heart muscle cells branch and come together and are joiined at intercalated discs with gap junctions that spread the electrical signal from cell to cell.
cardiac muscle - automatic action potentials
here is a picture from our histology course of heart muscle cells joined at intercalated discs

Questions used in 2007 & 2008 related to this outline

Amyotropic lateral sclerosis is a disease that
(a) scars central nervous system myelin.
*(b) damages spinal motor neurons.
(c) is treated by Prozac.
(d) decreases transmission by dopamine.
(e) is caused by cocaine addiction.

When muscle is not activated, what prevents myosin from binding to actin?
*(a) tropomyosin
(b) lactic acid
(c) acetylcholine
(d) dopamine
(e) transverse tubules.

In familial cases of Lou Gehrig's disease, what enzyme is missing?
(a) an enzyme that makes cGMP after the action of NO (nitric oxide)
(b) cyclooxygenase (COX)
*(c) super oxide dismutase (SOD)
(d) acetyl CoA
(e) trypsin

What chemical is released from the sarcoplasmic reticulum to make muscles contract?
(a) ATP
*(b) calcium ions
(c) troponin
(d) acetylcholine
(e) nitric oxide

What structural feature gets the action potential into the muscle cell at numerous locations?
(a) seminiferous tubules
(b) tropomyosin
(c) sarcoplasmic reticulum
*(d) transverse tubules
(e) Fallopian tubules

Why does rigor mortis occur?
(a) Sodium goes out of the cell.
(b) Myofibrils cease to form into motor units.
*(c) ATP runs out.
(d) The input to striated muscle from the autonomic nervous system stops.
(e) Actin runs out.

The calcium sequestering cistern in skeletal muscle is the
A) lysosome.
B) rough endoplasmic reticulum.
*C) sarcoplasmic reticulum.
D) microsomal fraction.
E) synaptic vesicle.

In skeletal muscle, where is the action potential initiated?
*A) the neuromuscular junction (synapse)
B) the dendrite
C) the gap junction
D) the cilium
E) the basal lamina

What is "dark meat?"
A) skeletal muscle damaged by Lou Gehrig's disease
B) the kind of muscle that has intercalated disks
C) liver
*D) striated muscle with aerobic metabolism
E) smooth muscle

What keeps the sliding filaments from binding with each other if calcium ions are not present?
*A) tropomyosin
C) superoxide dismutase (SOD)
D) acetylcholine
E) myosin

How is smooth muscle different from skeletal (striated) muscle?
A) Smooth muscle is dark meat and skeletal muscle is white meat.
B) Smooth muscle has big cells while skeletal muscle cells are spindle shaped.
C) Smooth muscle is anaerobic while skeletal muscle uses lactose.
D) Smooth muscle has actin while skeletal muscle has myosin.
*E) Smooth muscle is innervated by the autonomic nervous system while skeletal muscle is innervated by spinal motor neurons.

Which is the largest?
A) thick filament
B) myofibril
C) muscle fiber
D) muscle cell
*E) motor unit

The characteristic of white meat (that distinguishes it from dark meat) is
A) the intercalated disk that connects cells end to end.
B) innervation by the sympathetic and parasympathetic nervous systems.
*C) anaerobic glycolysis.
D) complete oxidation of glucose.
E) rigor mortis.

From one z-line to the next z-line is
A) one troponin.
B) one T (transverse) tubule.
C) one globular actin.
*D) one sarcomere.
E) one thin filament.

Deficiency of (what?) leads to rigor mortis?
B) sarcoplasmic reticulum
C) the z line
D) hemoglobin
E) calcium

Questions used in 2002 relating to this outline (and other outlines)

Rigor mortis is caused by
(a) too much acetylcholine.
(b) too much calcium.
(c) a lack of action potentials.
(d) tetanus.
*(e) lack of ATP.

Which would be the smallest?
*(a) G-actin
(b) sarcomere
(c) myofibril
(d) A-band
(e) muscle fiber

The Ca2+ -binding protein in muscle is
(a) myosin.
(b) actin.
(c) myoglobin.
*(d) troponin.
(e) cytochrome.

The muscle protein that forms thick filaments is
(a) actin.
*(b) myosin.
(c) sarcoplasmic reticulum.
(d) troponin.
(e) tropomyosin.

When muscle is not activated by spinal motorneurons, what prevents myosin from binding to actin?
(a) calcium
(b) lactic acid
*(c) tropomyosin
(d) tetany
(e) action potentials.

What is the source of the Ca2+ that binds to troponin?
(a) neuromuscular junction
(b) parathyroid
(c) synaptic vesicle
(d) parasympathetic nervous system
*(e) sarcoplasmic reticulum

A motor unit is
(a) all the myofibrils in the muscle fiber.
(b) the combination of actin, myosin, tropomyosin and troponin.
(c) all the microfilaments within a sarcomere.
*(d) all the muscle cells innervated by one spinal motor neuron.
(e) the cell junctions of cardiac muscle cells.

This page was last updated 8/17/09

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