Chp 6

Muscular System

6-1. Roles of muscles

a- Roles

b- Structure:

6-2. Muscle contraction

a- Sliding filament mechanism:

b- Pathology

c- Sources of energy

d- Oxygen debt

e- Muscle fatigue

f- Types of muscle contraction

6-3. Muscle activity

a1- Twitch

a2- Threshold

a3- All-or-none effect

a4- Wave summation and tetanus

a5- Recruitment

a6- Effect of exercise on muscles

6-4. Cardiac and smooth muscles

6-5. Disorders of the muscular system

6-1. Muscle properties

a- Roles:

- Produce movement or tension: Body has about 600 muscles. Muscles are attached to bones by tendons.

- Generate heat: help control body temperature. Shivering produces extra heat.

- Muscle cells are excitable, can contract and relax

http://www.bmb.psu.edu/courses/bisci004a/muscle/b4muscle.htm

b- Structure:

Muscles are made of bundles of fibers, the fascicles, enclosed into a fibrous membrane. The fascicles are made from many muscle fibers. Muscle fibers run from one end of the muscle to the other. Within the muscle fiber are many myofibrils, responsible for the mechanism of contraction.

Nerves, carrying the fibers from the motor neurons, connect to the muscles. The junction between motor neuron and muscle fiber is called the neuromuscular junction (NMJ). The contact between muscle and nerve is also called a synapse.

A motor unit is one motor neuron and all the muscle fibers attached to this motor neuron. When the motor neuron fires an impulse, all the muscle fibers have to contract

The myofibrils are formed by myosin and actin fibers aligned in a distinct pattern. The myosin is a large fiber with enlargement called head which will bind with actin. The actin fiber is shaped like a string with a smaller string, the tropomyosin wrapped around it. A protein, troponin, attached to the tropomyosin is a binding site for calcium. When not bound to calcium, troponin prevents myosin from attaching to the actin. The actin filament will bind to the myosin during contraction.

Myosin and actin form a repeating pattern, the sarcomere.

6-2. Muscle contraction

a- Sliding filament mechanism:

Contraction

1- The nerve impulse coming from the motor neuron reaches the synapse.

2- The impulse triggers the release of the neurotransmitter, acetylcholine, in the

synapse.

3- The acetylcholine binds to receptors on the surface of the muscle cell and

triggers an impulse along the membrane of the muscle cell and also deep inside

these cells.

4- Calcium ions, normally present in special vesicles, move toward the myofibrils

and bind to troponin.

5- Troponin moves out, allowing actin and myosin to react.

6- The thin actin filaments slides inside the myosin filaments, shortening the I band (the A band remains unchanged). This is repeated for each sarcomere, along the muscle fiber. This causes a shortening of the entire muscle.

Relaxation

7- An enzyme present in the synaptic cleft, acetylcholinesterase, destroys Ach shortly after it is bound to the receptors. The membrane, thus, no longer, respond to impulse.

7- Ca⁺⁺ goes off troponin and is transported by into its storage vesicles (using ATP). Therefore, tropomyosin moves back in place. Actin and myosin are no longer linked. The sarcomere slides back into resting position. The I band increases in width. The muscle relaxes.

b- Pathology

Rigor mortis: After death, calcium is released into the sarcoplasm, thus lead to binding on troponin and muscle contraction. Yet, since no oxygen is being taken in, ATP cannot be regenerated. When ATP supplies from the cell are exhausted, Ca⁺⁺ can not be brought back into the sarcoplasmic reticulum ---> the muscle remains in a state of contraction (rigor mortis). The muscle will relax several hours/days later, due to degradation of the proteins, among them actin and myosin.

On rigor mortis:

http://www.nurseminerva.co.uk/dying.htm

Insecticide poisoning: The active ingredient in some insecticide is an inhibitor of the enzyme, acetylcholinesterase. Since acetylcholine in the synapse is no longer degraded, the muscles remain in a state of constant stimulation, thus contraction. The insects (and sometimes humans) will die due to lack of breathing (the breathing muscles are paralyzed in a state of contraction).

Curare use: Curare blocks the acetylcholine receptors on the muscle membrane. Since acetylcholine can not bind to them, the muscles can not contract, resulting in flaccid paralysis. Curare is sometimes used during surgery, to immobilize a person and prevent spontaneous breathing (the patient must therefore be placed on respirator).

Myasthenia gravis: An autoimmune disease where the Ach receptors are destroyed.

Botulism: The toxin prevents release of Ach at the synapse.

http://www.tjclarkinc.com/bacterial_diseases/tetanus_and_botulism.htm

http://www.drlera.com/bacterial_diseases/botulism.htm

http://microvet.arizona.edu/Courses/MIC420/lecture_notes/clostridia/clostridia_neurotox/movie/botulinum_movie.html

Animation on mechanism of toxicity of botulism

http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/bacpath/botox.html

On the toxicity of the tetanus toxin:

http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/bacpath/tetexo.html

Toxins of the neuromuscular junction:

http://members.aol.com/Bio50/LecNotes/lecnot12b.html

c- Sources of energy

Muscle contraction requires ATP, yet, there is only a small amount of ATP in the muscle fibers. ATP is generated from other metabolic systems:

c1- Creatine phosphate has a high energy level and can generate ATP:

But this source of ATP is quickly exhausted and the muscle must turn to glucose metabolism.

c2- Aerobic system

If muscle activity lasts too long, stores of energy will be depleted and must be regenerated by aerobic metabolism or cellular respiration. Glucose, fatty acids will be burned in mitochondria with oxygen coming from hemoglobin from the blood.

c3- Glycogen-lactic acid system

When ATP and creatine reserves are depleted, glucose will be used to generate ATP. If no oxygen is present (anaerobic conditions), glucose will be converted in pyruvic acid, and under anaerobic conditions in lactic acid with production of 2 ATP. Lactic acid can be used by muscle, heart, liver. Glucose can be formed from reserve of glycogen in the liver and muscle. Eventually, glucose and glycogen stores must be restored. Glycogen and lactic acid provide energy for short period of time.

d- Oxygen debt

The oxygen used during strenuous exercise must be eventually replaced. Heavy breathing will occur for a while after exercise. Oxygen must be used to 1) convert lactic acid back into pyruvic acid and glucose; 2) reestablish glycogen stores; 3) restock creatine phosphate and ATP and 4) replace the oxygen on the myoglobin.

e- Muscle fatigue

Occurs when the muscle can not synthesize enough ATP to sustain its contraction. Factors involved in muscle fatigue are lack of oxygen, depletion of glucose, glycogen, creatine phosphate, depletion of acetylcholine at the synapse, and especially build up of lactic acid.

f- Types of muscle activity

Isotonic contraction: contraction with movement

Isometric contraction: contraction without movement (tension present)

6-3. Muscle activity

a1- Twitch

Contraction resulting from all the fibers innervated by a single motor neuron. A motor neuron will innervate few muscle fibers if the movement produced is fine (ex: thumb) and a large number of fibers for gross movement (ex: back).

A myogram can record a twitch. There is a brief delay between the stimulation and the beginning of contraction, called the latent period. It corresponds to the time it takes for the nerve impulse to reach deep inside the muscle cell. In the second phase, the contraction phase, the muscle contracts. Myosin heads bind to actin and slide along it. It lasts 10-100 msec. The third phase or relaxation period lasts slightly longer than the contraction period. It corresponds to the calcium ions being shipped back into the storage vesicles. Shortly after initial stimulation, the muscle fiber can not contract. It is the refractory period, lasting a short time in this muscle and is due to the depolarized state of the muscle membrane.

a2- Threshold

An action potential must have a minimal strength in order to trigger contraction of the muscle fibers. This minimal value is the threshold.

a3- All-or-none effect

If the strength of an action potential arriving at the synapse is adequate (above threshold), all the muscle fibers innervated by the motor neuron will contract to the fullest.

a4- Wave summation and tetanus

In wave summation, if a second stimulus comes before the relaxation phase is complete, the contraction of other muscle fibers will be added to that of the first one, thus increasing the overall contraction strength of the muscle. If a muscle is rapidly stimulated before relaxation can take place, the muscle remains in a state of constant contraction or tetanus.

a5- Recruitment

During stimulation, various motor units are being recruited. They do not contract in unison. The greater the stimulation, the more motor units are recruited, the stronger the contraction. During a lengthy contraction, recruited motor units alternate, to avoid fatigue.

.

http://homepages.ius.edu/KEDMONDS/muscles.htm

http://www.mrs.umn.edu/~goochv/HAP/lectures/muscle/muscle.html

Notes:

http://highered.mcgraw-hill.com/sites/0070272352/student_view0/chapter12/chapter_summary.html

http://www.shef.ac.uk/uni/projects/mc/intro2p2.html

a6- Effect of exercise on muscles

Two types of exercise: strength (resistance) training and aerobic (endurance) training.

- Strength training: uses resistance to make muscle work harder. Short, intense à weight training à promote increase in size of fast twitch myofibrils, rich in glycogen, creatine phosphate

- Aerobic training: ex: running, bycicling. Promotes development of more muscle fibers rich in mitochondria (slow twitch fibers), with more blood vessels. Improves cardiovascular and respiratory system. Less intense than strength training but more prolonged in time. Best is to exercise in order to increase heart rate to target rate for 20 minutes three times per week and to practice also strength training.

6-4. Cardiac and smooth muscles

Cardiac muscle is striated like skeletal muscle but the cells are not fused and the cell wall is visible as intercalated disc. Cardiac muscles are involuntary and can contract without external stimulation. They have a network of self-depolarizing cells, triggering contraction. They respond to nerve impulses, hormonal stimulation (acetylcholine and epinephrine).

Smooth muscles are also involuntary, slow, have no striations because the myofibrils are not organized like in the other 2 muscles. Smooth muscle is present is the wall of many viscera (blood vessels, intestines, uterus, iris, bronchioles..). Some fibers can self stimulate and thus trigger a wavelike motion called peristalsis. Theses muscles respond to nerve, hormones, chemical conditions (pH...)

6-5. Disorders of the muscular system

- Muscular dystrophy: muscle proteins are being destroyed, leading to muscle wasting and death due to cardiac and respiratory failure

- Tetanus: disease caused by a bacteria which growth in deep, anoxic wound. The bacteria secretes a toxin in the wound which travel along the nerve, back to the brain. There, it will trigger a severe contraction of most of the body muscles. The person will die of hyperthermia or injury (due to contraction)

- Muscle cramps: due to ion imbalance in the muscle cell environment, mostly potassium. Increased blood circulation decrease cramping.

- Pulled muscle: torn muscle fibers. very painful. If a muscle is severly torn off, muscle cells can not divide thus won't regenerate if there is muscle damage. They can grow in size under the influence of exercise.

- Fasciitis: inflammation of the fascia, the membrane surrounding the muscle. Painful, slow to heal.

- Aging: after the 30's, muscle mass is slowly replaced by fat. A principal cause is a decrease in the level in physical activity in older age.