TYPES OF MUSCLE TISSUES - See Chapter 9, Mareib Human A and P,  p. 280.

1. A skeletal muscle (p. 282)  is a cylindrical or flat sheet of muscle fibers or cells.
It is wrapped and separated into muscle compartments by a layer of dense irregular
connective tissue called deep fascia. Under that is a layer of collagenous tissue called
epimysium which covers one muscle. The muscle is subdivided into fascicles by the
fibrous perimysium and each muscle fiber is surrounded by the thin fibrous endomysium.
At each end there is a cylindrical tendon or a sheet-like tendon (aponeurosis) that
attaches the muscle to bone. Taken all together, these energy storing, collagenous
tissues within and immediately outside the muscle are called series elastic elements.
2. Cardiac - heart muscle. Produces relatively strong contractions which are not fatigable.
They are under involuntary (autonomic nervous system) control. Nuclei are in center of
weaving cells, intercalated discs bind cells together with desmosomes and gap junction
tubules allow for ion currents to pass from one cell to another. Actin and myosin make
striations. Cardiac muscle has pacemaker cells which depolarize (fire) at a set rate
(autorhythmicity) which can be increased or decreased by the nervous system.

See  Click on Pumping Myocytes.



3. Smooth - produce weak, non fatigable contractions. Used for vasoconstriction and the
movement of food (peristalsis). Striations are not seen although actin and myosin are
present in a net pattern.

Motor units are groups of muscle fibers served a one motor neuron and its axon terminals.
There may be up to 2000 fibers in each motor neuron unit in the leg, but only 3 per unit in the
muscles of the larynx. Single unit smooth muscle has a few motor neurons serving many muscle cells.
In multi-unit smooth muscle, each neuron serves fewer muscle cells. Examples are the gut for single
unit and the iris for multi-unit. In which is the control of the nervous system more precise?
See pg. 296..
 

NERVOUS SYSTEM STRUCTURES WHICH INTERACT WITH MUSCLES

Nerve cells or neurons are classified in various ways. According to function, they can be
Sensory (Receptor), Inter neuron (Association or Connector) and Motor
(causing action, muscle contraction).

Neuron Structure - in the order of passage of a stimulus (see pg. 392):

1. dendrites (root-like) - pick up a stimulus and relay resulting electrical changes to
the cell body where the nucleus resides. If, such as in a single sensory neuron process,
a dendrite is sheathed by a many layers of cell membrane ( myelin) of a supporting
Schwann cells, it may be called a dendron.
2. cell body - impulses are passed by ion currents through the cell body to the axon.
The cell body may also receive a stimulus as does a dendrite.
3. axon - the single axon may be myelinated or unmyelinated. In gray matter all neuron
processes are unmyelinated. In white matter, most processes are myelinated. See pg. 393.
The axon terminates in a knob which communicates with another cell through an axon
terminal, a tiny space called a cleft, and a target cell membrane. The whole
complex is called a synapse. Where a muscle cell in involved, the knob/synaptic
cleft/target membrane is called a motor end plate). See pg. 288//289.

In 1786 Galvani discovered that frog nerves could be stimulated by electricity with muscle
contraction resulting. Hodgkin and Huxley won the 1963 Nobel Prize for describing the
electrical/ionic phenomena of the Action Potential or Wave of Depolarization which is
marked by the opening of Na⁺ gates in the membrane of the neuron cell body or process, causing
a flood of positive ions to go through the membrane to the inside where the charge was relatively
negative. We will describing the vents of muscle contraction with the axon membrane in its
charged or pre-fired state.

See  Click on pumping myocytes and then on ion channels.

The Neuro-Muscular Synapse (Motor End Plate) - See pg. 290..

The synaptic cleft is the tiny space between, for instance, an axon knob and the cell body or dendrite of
another neuron, or the cell body of a muscle cell. The synapse cannot be crossed by ions, so
another mechanism is involved which slows things down a bit.

An action potential, or wave of depolarization (See p.. 291), sweeps down an axon towards a knob (its like
the doors of RICH'S opening on their "One Day Sale"). Positive Sodium and Potassium ions rush
in until they reach the axon ending where Ca ²⁺ enter the knob and cause vesicles containing
neurotransmitters to be dumped into the synapse and bond on to receptors on the postsynaptic
membrane of the next neuron or muscle cell in line. Neurotransmitters may be
neuroexciters by causing depolarization (shortening the plank); an example is the Ach
(acetycholine) secreted by axon terminals (knobs) into a synapse with skeletal muscles (motor
end plate). Repeatedly stimulating muscle cells makes them go into a state of maximal
contraction called tetanus.

On the other hand, some neurotransmitters may be inhibitors by causing hyperpolarization
by raising the resting voltage or by lowering the threshold - both lengthen the plank!). That is
called neuroinhibition, examples are GABA, glycine, and Ach acting on heart muscle. After an
impulse is transmitted across the synapse, the neurotransmitter Ach is cleaned up by the enzyme
Ach-esterase.

Myasthenia gravis is an autoimmune disease characterized by muscle weakness, in which
antibodies attack the postsynaptic membrane receptors for Ach. Treatments include methods to
increase Ach in the synapse (see Ach-ase inhibitors below). If not treated, death will eventually
result from respiratory muscle paralysis.

Ach stimulates skeletal muscle but inhibits cardiac muscle. How so? See the heart and autonomic
nervous system chapters.

Many poisons are Ach-esterase inhibitors , which allow neurotransmitters to stay in the
synapse, therefore, the next cell (muscle fiber) continues to depolarize and paralysis results. Many
insecticides operate this way.

MUSCLE CELL DEPOLARIZATION - See p. 287-295.

When Ach binds to its postsynaptic membrane receptors on the muscle cell, the Na + gates open
and a wave of depolarization sweep down the outer membrane or sarcolemma of the muscle cell,
down into the T or transverse tubules and into the interior of the cell where the
sarcoplasmic reticulum and the contractile units, the sarcomeres, are located. When the
wave of Na⁺ depolarization reaches the vicinity of the sarcoplasmic reticulum calcium ion
storage sacs, the calcium gates in the S.R. are induced to open, releasing Ca⁺⁺ ions into the
sarcomere. Before the actins and myosins can ratchet, the myosin cross bridges must attach to
the actins. They are blocked by the protein troponin - a protein which is like a gourd on a rope
called tropomyosin. Ca⁺⁺ ions remove the troponin block by changing the shape of the
troponin, then the tropomyosin/troponin complex moves over so that the myosin cross bridges
can attach to the globular g actin cogs.

ATP is needed to power the ratcheting action, although only 20% goes to move the actin, the
balance of 80% becomes heat. Explain shivering.

The sequence of events are: (1.) A new ATP breaks the myosin cross bridge bonds, (2.) the
cross bridge (head) recocks, (3.) the cross bridge attaches to the actin, and (4.) it ratchets.
Remember the mantra: detach, recock, attach, ratchet. 

Now, explain rigor mortis!

Why cant muscle cells contract if they are stretched 165% of their length?  See p. 305.

What is RICE therapy for muscle injuries?

Which muscles are the typical ones for arm, hip and thigh injections?

Shinsplints is a term for inflammations of the tibial periosteum of tendinitis associated with
tibialis posterior or anterior. It usually results from running on hard surfaces.

MUSCLE TENSION - See p. 300.

A whole muscle which is composed of many motor units can produce differing amounts of tension
and force - this is called graded strength. See previous section on motor units.

1. Twitch - if a muscle is provided with a single stimulus which exceeds the threshold voltage,
the muscle will twitch. The periods of importance are: (1) the latent period of 4 msec
during which the fibers are depolarizing and calcium ions are being released into the
sarcomeres, (2) the contraction period of 25 msec where myosins and actins are
ratcheting, and (3) the relaxation period of 20 msec where the calcium ions go
back into the S.R.s and the sarcolemma repolarizes. See pg. 294//297.

1. Wave summation - if we "piggy-back" a second stimulus on top of the first before the relaxation period is complete, the muscle will contract with more force. This is due to an increase in background heat generated by the contractions and the increased availability of calcium ions.See pg. 295.

1. Tetanus - if we apply many (1-5/sec) stimuli just after relaxation or piggy-backed repetitively, the muscle contraction increases with each stimulus ( stair-case or treppe phenomenon). This is called an incomplete tetanus. The muscle will contract maximally. If 20 stimuli/sec. are applied to the muscle, the staircase fuses into a smooth curve of maximal force called complete tetanus. See p. 297.

If the nervous system fires rapid stimuli at small patches of heart muscle,  cardiac muscle fibers may twitch in randomly - a
phenomenon called fibrillation which results in death (due to poor blood flow) without CPR
and electroshock intervening. Because cardiac muscle has a long refractory time period during
which it cannot be induced to contract again, a state of constant contraction resulting in fatigue
called tetanus (see below) cannot occur.

MUSCLE METABOLISM - See pg. 300.

Glucose is metabolized just outside and inside the mitochondria to make ATP. There
are two ways to make ATP from the energy stored in the chemical bonds of glucose and
there is an supercharger system providing for a reserve of ATP.

1. Anaerobic - just outside the mitochondrion, each glucose molecule is broken down

to yield a net of  2 ATP of a possible 36. The product lactic acid is produced. This
process is wasteful of energy but it is quick! There is another maximum net yield of
34 ATP's that could be made from each glucose - a lot of energy stored in the
chemical bonds of lactic acid. Lactic acid accumulations make muscles sore and
if the pH drops too far, retards breathing (small decreases of .05 pH points would
cause increases of breathing rate). However, the lactic acid is taken by the blood
to the liver where it can be reformed into glucose. Also, it can be changed back
to the crossroads molecule pyruvic acid and entered into the aerobic pathway.
The more lactic acid you accumulate the greater your oxygen debt which must
be repaid by increased breathing. The conversion of glucose into 2 ATP's and
pyruvic acid is called glycolysis. The conversion of pyruvic acid and 2H to lactic
acid is called muscle cell fermentation.

2. Phosphagen system - reserves of high energy phosphate bonds are made when

creatine phosphate (CP) is formed. The high energy phosphate is transferred to ADP
to make an ATP which then is used to ratchet actins in sarcomere contraction. The
creatine phosphate and glycogen reserves are good for about 40-45 seconds of maximal
muscle activity such as in sprinting the 400 m dash in the Olympics. More specifically,
there are 6 sec. worth of stored ATP, 30 sec. worth of creatine phosphate, the balance
coming from anerobic fermentation that supplements energy production when breathing
cannot keep up with oxygen needs for aerobic cellular respiration. . You know that the
latter has happened because when you go anerobic, you have to stop and breathe hard
to catch your breath - you have generated an oxygen debt that has to be repaid.
3. Aerobic - remember that anaerobic metabolism leads to aerobic. Deep inside
the mitochondrion, oxygen is used to fully metabolize glucose,
C₆H₁₂0₆ + 60₂ + 36 ADP + 36 P ----> 36 ATP's + 6CO₂ +  6H₂O.


Define aerobic exercise.

Weight lifters that take creatine can develop more power but the creatine does
not increase muscle size. Why?
 

Muscle fatigue occurs when muscle strength fails as first glycogen and creatine phosphate
deplete, then ATP runs out, Ca⁺⁺ release from the SR declines, and lactic acid builds up.

"Hitting the wall" is a phrase that distance runners use to describe the tremendous feelings
of fatigue and pain encountered when the liver and muscle cells stop supplying glucose that
comes from the breakdown of glycogen starch. The body then switches to fat and protein
catabolism to make "new glucose" and other products that can be used in cellular metabolism to
make the ATP needed to keep going. Natural pain-killing opiates (endorphins) are released in the
Central Nervous System at this time, resulting in "runner's high."

A isotonic contraction happens if muscle fibers shorten as the myosin cross-bridges move
the actins. Isometric contractions occur when muscle tension increases but muscle length
does not shorten, the myosin cross-bridges are "spinning their wheels" against the actins.
Aerobic-isotonic contractions are said to be better for your overall bone, muscle and joint
health. That's why swimming and "power walking" (with arm weights) are good exercises. If
you want to just increase muscle size, isometric contractions are best.

What happens to muscles when they are not used as in quadriplegia or they are not innervated or
used as in polio? Muscles literally waste away (atrophy) and are replaced by fat.
Exercise causes muscle fibers to increase in size; this is a process called hypertrophy.
See pg. 307

MUSCLE FIBER TYPES - See p.309, Table 9.3.

At either extreme, white muscle cells have few mitochondria, few blood vessels, and no
oxygen-binding myoglobin. Red muscle cells have lots of mitochondria,
blood vessels and myoglobin. See specific types below.

1. Slow twitch/oxidative/fatigue resistant ( ST, Type I, Red Fibers) - these are small, red fibers with
the oxygen binding molecule myoglobin and therefore use aerobic glucose
metabolism. Produce slow, fatigue resistant contractions necessary for posture or walking.
Marathon runners have a larger proportion of these and type IIA below.
2. Fast twitch (10x vs. the slow twitch) /oxidative and glycolytic/fatigue resistant
(FT-A, Type IIA, Intermediate Fibers) - aerobic, good for long sprints or distance runners, have less blood flow
and myoglobin than do the red fibers but more than white. CP and glycogen stores are
moderately high and fiber size is large. Sprinters and power lifters have a larger proportion
of these fibers in their leg and arm muscles.
3. Fast twitch/glycolytic/fatiguable (FT-B, Type IIB, White Fibers) - use anaerobic metabolism, have
low myoglobin and mitochondria, low blood vascularization, called white fibers, produce
strong but fatigable contractions as in sprinting up to 100 m or 10 seconds. CP and
glycogen stores are moderately high.  Low amounts of aerobic enzymes, high amounts of
glycolytic enzymes (anerobic) and fiber size is large.

The effect of anerobic, short-duration weight lifting exercises are largely cosmetic, but they increase
strength.

Long sprints or distance running is more aerobic, endurance is increased as muscles acquire
more circulation, mitochondria and enzymes; thus, making the white cells "pink" or
fast twitch/fatigue resistant, see above.

Why do sprinters have bulging quads, hamstrings and gastrocs while distance runners have longer,
slimmer muscles?

A simple test: genetics has dictated our proportions of ST and FT fibers. Determine your one
repetition maximum weight that can be lifted, e.g., a bar bell in a forarm flexion or "curl."
Take 80% of that weight. Then when rested, see how many repetitions you can do. If less
than 7, you have a higher proportion of FT fibers, if more than 12 repetitions are performed,
there is a higher proportion of ST fibers in the muscle group being tested.

Recruitment

Recruitment of muscle fibers occurs as the nervous system fires more of them. In order to
delay fatigue, the nervous system can rotate the activated fibers, instead of firing the same ones
continuously. Small motor units with ST fibers are recruited first and will be the only ones
recuited if the exercise impact  is low. As exercise intensity increases, FT-A fiber units are next,
following by FT-B as required.

Selected Diseases

Muscle cramps occur if K⁺ ions are depleted. The process of repolarization of the muscle
fiber is affected. If cell don't repolarize completely, they stay in a somewhat depolarized state.
This results in spasmodic contractions or cramps. Eat a banana!

Muscular dystrophy of the Duchienne type is hereditary due to a mutated gene which fails to
make the dystrophin protein that helps strengthen the outer membrane. The sarcolemma tears as
a result.. Harmless viruses can carry good dystrophin genes back into the affected cells
(called genetic engineering). 

Eosinophilic myalgia is a muscle weakening allergic-like autoimmune attack on muscles. It
was apparently caused by a contaminant in health foodstore-bought tryptophan - an amino
acid used to enhance the serotonin secretions which help induce sleep in the brain.  Physicians no
longer advise their patients to take this for insomnia.

Anabolic Steroid Abuse - One reason why guys have more muscle and larger bones is the
effects of increased testosterone and growth hormone during development. The natural secretion
of testosterone and the production of sperms are indirectly controlled by the
hypothalamus/pituitary gland. If one takes too much testosterone, the secretion of pituitary
gonadotropin hormones decrease and sterility may result. Excessive testosterone also causes
liver problems like gout. It is a mitogen for mutated cells, making cancers grow and spread
rapidly. Behaviorally, "steroid rage" and impulsive behavior are seen.

Aging is characterized by decreased in muscle size, increased muscle fat and decreased muscle
protein. These effects are caused by decreased growth hormone secretions controlled by the
hypothalamus/pituitary, and decreased testosterone.

Study Questions

1. Draw the graphs of the mucle twitch, wave summation, incomplete tetanus and complete
tetanus. Compare the rate of stimulation in each.

2. Explain a wave of depolarization traveling down an axon and how it causes a muscle
fiber to contract.

3. Compare white, red and intermediate fibers as to anatomy and metabolism.
 
 
 

Email:jaliff @ gpc.edu