Anatomy & Physiology



Anatomy & Physiology

Muscular System

I. Muscles overview

A. Types of Muscles

1. Cardiac – only in heart; striated; involuntary

2. Smooth – in walls of hollow organs; non-striated; involuntary

3. Skeletal – attached to and covers the bones of the skeleton; striated; voluntary

B. Muscle Functions

1. Produce movement

2. Maintain posture

3. Strengthen & stabilize joints

4. Guard entrances and exits of the digestive & urinary tracts

5. Generate heat

C. Muscle Characteristics

1. irritability – ability to respond to a stimulus (usually from an associated nerve impulse)

2. contractility – ability to shorten & produce movement

3. extensibility – ability to stretch when relaxed

4. elasticity – ability to return to a normal length after being stretched

D. Muscle Information

1. Skeletal muscles account for 40% of the body weight.

2. There are over 600 muscles in the human body

II. Skeletal Muscle

A. Gross Anatomy

1. Attachments

a. insertion – bone or tissue that moves as a result of contraction

b. origin – bone or tissue that does not move

c. tendon – fibrous connective tissue (FCT) that anchors the muscle to a bone.

d. Aponeurosis – broad, flat sheet of FCT that connects a muscle to a bone or other tissue.

2. Connective tissue coverings of a muscle

a. Endomysium – FCT that surrounds each individual muscle fiber (cell).

b. Perimysium – FCT that surround bundles of muscle fibers called Fascicles.

c. Epimysium – FCT that surrounds an entire muscle.

3. Muscle Fiber Anatomy

a. Each muscle cell is called a muscle fiber. Muscle cells are very long (up to 12 inches) and have multinuclei in order to control them.

b. The outer cell membrane is called the Sarcolemma.

c. The cytoplasm is called the Sarcoplasm

d. The ER is called Sarcoplasmic Reticulum and contains extra Calcium.

e. Myoglobin is a red pigment molecule that contains extra oxygen within the muscle cells.

f. Each muscle fiber has hundreds of Myofibrils

g. Myofibrils are composed of fibrous proteins called Myofilaments.

h. There are two types of myofilaments

1) Actin – thin, white filaments

2) Myosin – thick, red filaments

i. The myofilaments form repeating light and dark bands (which give skeletal muscles their characteristic striated appearance)

j. A single light-dark-light segment forms the Functional Unit of the muscle, which is called a Sarcomere. It is at the microscopic sarcomeres that muscle contraction occurs.

4. Sarcomere

5. Sarcomere contracted

a. I band disappears

b. A band remains unchanged

c. H zone disappears

d. Z lines drawn closer together

B. Sliding Filament Theory of skeletal muscle contraction

1. Thick myosin filaments are twisted together and have globular heads (proteins) projecting from the surface. The heads are also called cross bridges.

2. Thin actin filaments contain active sites to which the heads attach during contraction.

3. During contraction the cross bridges of the myosin bind to the active sites on the actin.

4. The myosin heads pivots and bends as it pulls the actin, thus sliding the actin toward the M line.

5. The pivoting and bending of myosin heads requires ATP energy.

6. The myosin detaches from the actin, resets itself and then binds to a new active site farther down the actin filament and pulls again.

7. When the muscle is resting, the proteins troponin and tropomyosin cover the active sites on actin and prevent myosin cross bridges from binding.

8. Upon the signal to contract, the Sarcoplasmic Reticulum releases calcium. The calcium binds to troponin, which causes the troponin & tropomyosin to uncover the active site thus allowing contraction to occur.

C. Neuromuscular Junction (NMJ)

1. Each muscle fiber (cell) is connected to the axon of a motor neuron.

2. The site where the nerve fiber and muscle fiber join is called the neuromuscular junction.

3. The Sarcolemma of the muscle is specialized at the NMJ and is called the Motor End Plate.

4. When the signal to contract reaches the end of the axon, the neuron releases Acetylcholine (ACh) which is a neurotransmitter.

5. The ACh binds to receptors on the motor end plate and stimulates an electrical reaction (depolarization) to occur across the sarcolemma of the muscle fiber.

6. The depolarization causes the SR to release its calcium and thus start the contraction.

7. After depolarization, the ACh is broken down by Acetylcholinesterase (AChE).

8. A Motor Unit consists of the Motor Neuron and all the Muscle Fibers that it stimulates.

D. Contraction Physiology

1. Muscle response

a. All-or-None Law – A muscle fiber will contract fully or not at all..

b. Threshold stimulus – the weakest signal (from the neuron) that will cause contraction in the muscle fiber.

c. Myogram – graphic recording of mechanical contractile activity.

2. Twitch – the response to a single threshold stimulus. Three phases

a. Latent period (.01 sec.)

b. Contractile period (.05 sec.)

c. Relaxation period (.05 sec.)

3. Tetanus - sustained signals to contract, thus not giving the muscle time to relax. Sustained muscle contractions eventually uses up all the ATP in the muscle. The muscle therefore is unable to contract and its tension level drops to zero (Muscle Fatigue).

4. Muscle Tone – the constant contraction of some muscle fibers in a muscle at all times. Muscle tone does not produce movement, but keeps muscles firm & healthy & helps maintain posture.

5. Isotonic contraction – contraction that shortens the length of the muscle thus causing movement.

6. Isometric contractions – contraction in which the muscle neither shortens of lengthens.

E. Energy for Contraction

1. ATP is the energy needed for muscle contraction. Your muscles have a limited store of ATP.

2. As ATP is used up it is converted to ADP.

ATP ADP + P + Energy

3. The muscles also store Creatin Phosphate (CP) that can bind with ADP to produce more ATP.

ADP + CP ATP + creatin

4. When the muscles work vigorously and burn energy, they begin to breakdown glucose to produce ATP.

5. The first step in the breakdown of glucose is called Glycolysis. This is an anaerobic process that splits glucose into two pyruvic acids and produces 2 ATP molecules.

Glucose 2 Pyruvic Acid + 2 ATP

6. Normally, pyruvic acid will then enter the mitochondria and go through the Kreb’s Cycle, which is an aerobic method of breaking down the pyruvic acid into 34 ATP.

7. As the muscle cell continues to burn energy, it begins to run out of oxygen and the circulatory system cannot supply the oxygen to the muscle cells fast enough to continue aerobic respiration.

8. When your muscles run out of oxygen, they begin to break down pyruvic acid using anaerobic respiration called Lactic Acid Fermentation.

9. Lactic acid Fermentation breaks down pyruvic acid into 2 ATP and lactic acid (which gives muscles that burning sensation after hard exercise.)

F. Oxygen Debt

1. Oxygen debt is the extra amount of oxygen that must be taken into the body after vigorous exercise to restore the anaerobic processes (CP, & lactic acid).

2. Basically, oxygen is needed to convert creatin back into CP& lactic acid back into pyruvate.

3. For this reason, your body breaths heavily after working out in order to supply extra oxygen to the cells until the oxygen debt is paid back.

G. Muscle Fatigue

1. Muscle Fatigue is the physiological inability to contract.

2. Muscle fatigue is caused by:

a. lack of ATP

b. blood supply interruption (heart attack)

c. exhaustion of ACh

d. build up of lactic acid

H. Muscle Mechanics

1. Muscles are attached to bones in a lever system.

2. The lever is a rigid bar that moves on a fixed point or Fulcrum. The resistance is called the Load, and the work to move the load is called the Effort.

3. There are three categories of levers

a. First (1st) Class Lever – the Fulcrum is located between the Load and the Effort.

b. Second (2nd) Class Lever -the Fulcrum is at one end and the Effort at the other end of the lever with the Load in the middle.

c. Third (3rd) Class Lever – the Effort is applied at a point between the Fulcrum and the Load.

4. First class levers are at a mechanical advantage and are also called Power Levers. They provide the greatest amount of work per effort applied. These levers are predominately found in insects and account for their ability to lift many times their body weight.

5. Third class levers are at a mechanical disadvantage but are also called Speed Levers. They allow the load to be moved rapidly though a large distance. These levers are predominately found in humans and other mammals.

G. Muscle Arrangements in the Body

1. Because muscles can ONLY “pull”, muscles are arranged in the body in opposing pairs. Therefore, whatever action one muscle group causes you to “do”, the opposing group causes you to “undo”.

2. Agonist – prime mover

3. Antagonist – muscle that is opposite the agonist and reverses a particular movement.

4. Synergist – assist the agonist in producing a desired movement.

III. Disorders of the Muscular System

A. Muscular Dystrophy (MD) – group of inherited muscle-destroying diseases that appear during childhood. The muscle fibers slowly degenerate and atrophy (reduction in size and strength).

1. Duchenne’s MD – most common type. Sex-linked. Carried by female, but expressed in males. Appears in boys from 2-6 years of age. Die by early 20’s due to respiratory failure.

B. Myasthenia Gravis – autoimmune disease where the immune system destroys motor end plates thus preventing ACh from signaling muscle contraction. A common symptom is droopy eyelids, although other effects include weakness of arm, head, and chest muscles.

C. Poliomyelitis – viral disease that destroys motor neurons. The affected muscles atrophy. Sever cases cause death due to respiratory failure. There is no cure for polio, however there is a vaccination. The virus is still in the environment – so get kids immunized.

D. Botulism – a bacterial toxin that prevents the release of ACh. Botulism is usually linked to improper canning or storage followed by failure to adequately cook the food. In sever cases respiratory failure may cause death (10%).

E. Tetanus – bacterial infection that causes sustained contraction of some muscles. Usually begin in the neck and jaw (Lockjaw). Can be fatal due to resp. failure.

F. Cramp – sustained contraction of a muscle usually at night after strenuous exercise. Due to a low blood sugar, dehydration, or low electrolyte levels.

G. Strain – pulled muscle.

H. RICE -Rest, Ice, Compression, & Elevation. Standard treatment of excessively pulled muscle, ligament or tendons.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download