Muscle fibers and exercise

Skeletal muscles have two types of muscle fibers: fast-twitch and slow-twitch. Anaerobic exercise uses fast-twitch fibers. Such exercise includes activities that are fleeting and require brief high-energy expenditure. Weightlifting, sprinting, and push-ups are examples of anaerobic exercise. Because all cells require oxygen to produce energy, anaerobic exercise depletes oxygen reserves in the muscle cells quickly. The result is an oxygen debt. To repay the debt, humans breathe deeply and rapidly, which restores the oxygen level. Anaerobic exercise creates excess lactic acid (a waste product). By increasing oxygen intake, the liver cells can convert the excess lactic acid into glucose, the primary food molecule used in cellular metabolism.

Aerobic exercise uses slow-twitch muscle fibers. Such exercise includes activities that are prolonged and require constant energy. Long distance running and cycling are examples of aerobic exercise. In aerobic exercise, the muscle cell requires the same amount of oxygen that the body supplies. The oxygen debt is slashed and lactic acid is not formed.

Answer the questions

How many skeletal muscles do people have?

What similarity is there between cardiac muscle and skeletal muscles?

What similarity is there between cardiac muscle and smooth muscles?

What is epimysium?

What are the two types of protein in myofilaments?

Describe movements of a knee.

What is the similarity and difference between tendons and ligaments.

What is percentage of muscle weight in body weight?

Is cardiac muscle voluntary or involuntary?

What is the difference between smooth muscles and other types of muscles?

What is the structure of a muscle fiber?

When does muscle contraction occur?

What attach most skeletal muscles to bones?

Myasthenia gravis

Myasthenia gravis is a neuromuscular disorder characterized by variable weakness of voluntary muscles, which often improves with rest and worsens with activity. The condition is caused by an abnormal immune response.

CAUSES, INCIDENCE, AND RISK FACTORS

In myasthenia gravis, weakness occurs when the nerve impulse to initiate or sustain movement does not adequately reach the muscle cells. This is caused when immune cells target and attack the body's own cells (an autoimmune response). This immune response produces antibodies that attach to affected areas, preventing muscle cells from receiving chemical messages (neurotransmitters) from the nerve cell.

The cause of autoimmune disorders such as myasthenia gravis is unknown. In some cases, myasthenia gravis may be associated with tumors of the thymus (an organ of the immune system). Patients with myasthenia gavis have a higher risk of having other autoimmune disorders like thyrotoxicosis, rheumatoid arthritis, and systemic lupus erythematosus.

Myasthenia gravis affects about 3 of every 10,000 people and can affect people at any age. It is most common in young women and older men.

SYMPTOMS

1. Vision changes:

2. Double vision

3. Difficulty maintaining steady gaze

4. Eyelid drooping

5. Patients with generalized disease may also have:

6. Swallowing difficulty, frequent gagging or choking

7. Weakness or paralysis (may worsen with exertion later in the day)

8. Muscles that function best after rest

9. Drooping head

10. Difficulty climbing stairs

11. Difficulty lifting objects

12. Need to use hands to rise from sitting positions

13. Difficulty talking

14. Difficulty chewing

Additional symptoms that may be associated with this disease:

1. Hoarseness or changing voice

2. Fatigue

3. Facial paralysis

4. Drooling

5. Breathing difficulty

SIGNS AND TESTS

Examination may be normal or may show muscle weakness that progressively worsens as the muscle is used. In many patients the eye muscles are affected first. Reflexes and sensation are normal. Weakness may affect the arms, legs, breathing or swallowing muscles and any other muscle group.

Standard EMG results are usually normal. Repetitive stimulation (type of nerve conduction studies) are more sensitive.

Single-fiber EMG is even more sensitive.

Acetylcholine receptor antibodies may be present in the blood.

A Tensilon test is positive in some cases but must be interpreted carefully by an experienced doctor. Baseline muscle strength is evaluated. After Tensilon (edrophonium, a medication that blocks the action of the enzyme that breaks down the transmitter acetylcholine) is given, muscle function may improve.

TREATMENT

There is no known cure for myasthenia gravis. However, treatment may result in prolonged periods of remission.

Lifestyle adjustments may enable continuation of many activities. Activity should be planned to allow scheduled rest periods. An eye patch may be recommended if double vision is bothersome. Stress and excessive heat exposure should be avoided because they can worsen symptoms.

Some medications, such as neostigmine or pyridostigmine, improve the communication between the nerve and the muscle. Prednisone and other medications that suppress the immune response (such as azathioprine or cyclosporine) may be used if symptoms are severe and there is inadequate response to other medications.

Plasmapheresis, a technique in which blood plasma containing antibodies against the body is removed from the body and replaced with fluids (donated antibody-free plasma or other intravenous fluids), may reduce symptoms temporarily and is often used to optimize conditions before surgery.

Surgical removal of the thymus (thymectomy) may result in permanent remission or less need for medicines.

There are several medications that may make symptoms worse and should be avoided. Therefore, it is always important to check with your doctor about the safety of a medication before taking it.

Crisis situations, where muscle weakness involves the breathing muscles, may occur. These attacks seldom last longer than a few weeks. Hospitalization and assistance with breathing may be required during these attacks. Often plasmapheresis is used to help end the crisis.

EXPECTATIONS (PROGNOSIS)

There is no cure, but long-term remission is possible. There may be minimal restriction on activity in many cases. Patients that only have eye symptoms (ocular myasthenia gravis), may progress to have generalized myasthenia over time.

Pregnancy is possible for a woman with myasthenia gravis but should be closely supervised. The baby may be temporarily weak and require medications for a few weeks after birth but usually does not develop the disorder.

COMPLICATIONS

1. Restrictions on lifestyle (possible)

2. Side effects of medications (see the specific medication)

3. Complications of surgery

4. Myasthenic crisis (breathing difficulty), may be life threatening