Module 1: Anatomy and Physiology

1.1 Macrostructure and Microstructure of Muscle

Each skeletal muscle is an organ that contains muscle tissue, connective tissue, nerves and blood vessels. This muscle is a bundle of bundles of muscle cells. This bundle of muscle cells is known as a fasciculus. A whole muscle is a bundle of fascicules. A muscle cell contains contractile proteins along with nuclei and mitochondria.

Each muscle is covered in connective tissue. Connective tissue covers the outside of muscles, fascicules of muscle cells and the muscle cells themselves.

Common Question:The connective tissues that surround muscle:

The epimysium surrounds the entire muscle belly The perimysium surrounds each fasiculus The endomysium surrounds each individual muscle cell (also known as a

muscle fiber)

Key Point:Many terms in anatomy have similar beginnings or endings of words. Connective tissue names end in “mysium” and structures within muscle often begin with “myo” or “sarco”.

Muscle cells are long (they run the entire length of muscle), cylindrical (approximately the diameter of human hair) and differ from other cells because they are multi-nucleated, and these nuclei are located on outside of the cell. Within the muscle cell is the sarcoplasm (the fluid inside the muscle cell), which contains myofibrils, proteins, stored glycogen and fat particles, enzymes and specialized organelles such a mitochondria and the sarcoplasmic reticulum.

The muscle cell is made up of hundreds of myofibrils. These myofibrils contain two different myofilaments, which are the contractile proteins of the muscle cell. These two myofilaments are actin and myosin.

Actin is the thinner of the two myofilaments and contains the proteins troponin and tropomyosin. Actin is attached directly to the Z lines on either end of each sarcomere.

Myosin is considered the thick filament. It has globular heads, called cross-bridges, that stick out at regular intervals. Myosin is anchored at the M bridge, located at the center of the H zone of each sarcomere.

The actin and myosin filaments attach and pull across each other to allow the muscle to contract. Six actin filaments surround each myosin filament and three myosin filaments surround each actin filament. This allows for myofilament interaction in every direction.

The sarcomere is the contractile, or functional unit of the muscle. It is defined as the distance between Z lines within the myofilament. These Z lines are repeated along the entire length of the muscle.

Must Know:Each sarcomere is divided into different areas that correspond with the location of actin and myosin:

The dark A band corresponds with the length of the myosin filaments (some overlap of actin filaments may occur within this band)

The H zone lies within the A band and is the area where only myosin filaments are present

The light I band is the area of the sarcomere where only actin filaments are present (the Z lines lie in the middle of the I band)

These alternating areas of light and dark are what give skeletal muscle its striated appearance

A motor unit is the motor nerve (also known as the motor neuron) and all of the muscle fibers that it innervates. It is important to understand that a motor unit can contain only a few muscle fibers (for very small, fine movements such as within the eye) or thousands of muscle fibers (for large, gross movements such as within the thigh), but a motor unit can only ever have one motor nerve.

1.2 Function of Muscle

Movement occurs by force being generated within the muscle cells. This force is transferred through the levels of muscle connective tissue (endomysium, perimysium, epimysium), which is continuous with the tendon. The tendon then pulls on the bones of the body, causing it to move.

Force is produced by the interaction of the contractile proteins, myosin and actin. The globular heads on myosin are able to bind to actin and ATP and contains an enzyme (myosin-ATPase) that can break down (through hydrolyzation) ATP to fuel muscle contraction. Actin has “active sites” for the myosin heads to attach. At rest these active sites are covered by tropomyosin. When a muscle is activated to contract, troponin will bind with calcium and pull tropomyosin off the active sites.

Must Know:Muscle contraction occurs through the sliding filament theory, which has five stages. Knowing the names of the five stages and what occurs in each stage is crucial.

Stage 1: Resting Phase Little calcium is present in the myofibril (it is stored in sarcoplasmic

reticulum)

Therefore, very few of the myosin cross-bridges are bound to actin No tension within muscle

Stage 2: Excitation-Contraction Coupling Phase The muscle cell is activated (more about this later) Calcium is released from the sarcoplasmic reticulum into the sarcoplasm Calcium binds to troponin Troponin shifts its position, pulling tropomyosin away from the actin active

site, exposing the active site The ATPase portion of the myosin head hydrolyzes ATP, thereby making

energy available The head of the myosin binds to the active site on actin

Stage 3: Contraction Phase The head of the myosin tilts (called the powerstroke) and pulls the actin

myofibril across the myosin myofibril A new ATP binds to the emptied ATP binding site on the myosin head,

causing the release of the actin-myosin bond

Stage 4: Recharge Phase Repeat of Excitation-Contraction Coupling phase and contraction phase

occurso Measurable shortening of the muscle will only occur if these events

repeat many, many times throughout the muscle fibers This will continue to occur as long as

o Calcium is availableo ATP is availableo ATPase is available

Stage 5: Relaxation Phase If the cell is no longer activated, everything returns to its resting state: Calcium is released from its binding site on troponin and taken back up into

the sarcoplasmic reticulum Myosin is released from actin when the new ATP binds to the myosin head Troponin shifts back to resting position Tropomyosin shifts back to resting position, covering the active sites on actin

There are three types of muscle action:Concentric- the amount of tension generated within the muscle is greater than the external resistance and the muscle shortensEccentric- the amount of tension generated within the muscle is less than the external resistance and the muscle lengthensIsometric- the amount of tension generated within the muscle is equal to the external resistance and the muscle remains the same length

Common Question:What is occurring to the different areas of the sarcomere during one of the three muscle actions.During concentric contractions: distance between Z lines decreases, length of I band and H zone decreases and eventually disappears.During isometric contractions: everything stays the sameDuring eccentric contractions: distance between Z line increases, as does length of I band and H zoneLength of A band always stays the same length in any type of muscle contraction because it is the length of the myosin myofilament, which does not change length

Muscles are activated by the central nervous system (which includes the brain and spinal cord), the control center of the body. The central nervous system sends electrical signals for the muscle to contract via motor nerves, which extend from the spinal cord to the muscle fibers. This electrical signal is called an action potential.

At the neuromuscular junction, the point where the motor nerve meets the muscle fiber, the neurotransmitter acetylcholine is used to transfer the action potential across the junction. Acetylcholine travels along the sarcolemma (membrane of the muscle fiber) to the transverse tubules, where it can enter the sarcoplasmic reticulum and lead to muscle contraction via the sliding filament theory.

Key Point: Motor units abide to the “all of none law”, which states that when a motor unit is stimulated to contract, either every muscle fiber will contract, or none of them will.

1.3 Factors Affecting Muscle Force Production

Key Point:Muscle force is directly related to the number of actin-myosin cross-bridges that can occur.

The more motor units that are recruited, the more force that can be increased. This can be trained and is the root of the majority of strength improvements.

When an action potential (also known as a twitch) occurs, a small amount of force is produced. If another occurs before the muscle relaxes, the force generated by this second twitch will build upon the first. If this continues to occur, twitches will continue to build on each other until they fuse (known as tetanus). Therefore increasing the rate of motor unit activation will also increase force production. This is another neural adaptation that occurs due to training, the nervous system will adapt to fire motor units at a higher rate earlier in the muscle contraction (this is increased rate of force production).

Another adaptation that can lead to increased force production is to improve the synchronicity of motor unit firing. For every movement there are an agonist muscle (muscle directly responsible for movement), antagonist muscle(s) (muscles that oppose the movement), and synergist muscle(s) (muscles that either produce force to aid in movement or stabilize other joints to make movement efficient). With training, more force can be produced by the agonist and synergist muscles either through increased motor unit recruitment or increase rate of motor unit recruitment (or both) and decreased recruitment, or inhibition, of antagonist muscle(s).

Not all adaptation occurs through conscious muscle recruitment, as two main reflexes directly relate to muscle force production. The stretch reflex, detected by muscle spindles (lie in parallel to muscle fibers and are also known as extrafusal fibers) causes an excitatory signal that increases agonist force production. Muscle spindles sense muscle length and change in length and will send a signal to contract a muscle if stretched too far or too fast in order to protect it from damage. The effects of the muscle spindle can be enhanced through training the stretch-shortening cycle using methods such as plyometrics.

The golgi tendon organ is another sensory organ that affects force production. This is found within the musculotendinous junction and causes inhibition of the agonist muscle if tension developed within it is too high. This greatly decreases the amount of external resistance that can be overcome. One of the first adaptations of resistance training is to increase the threshold of tension that causes the Golgi tendon organ to send the inhibitory signal.

The length of the muscle plays a role in the amount of force that can be produced. Muscles can produce the most force at resting length because this length allows the optimal position of the actin and myosin myofilaments to form cross-bridges. This potential is decreased if the muscle is shortened or lengthened. This is why flexibility and posture is important for force production; it allows the muscles to be at their correct resting length.

Increasing muscle cross-sectional area (also known as muscle size) will increase the potential for enhanced force production (neural adaptations must also occur). Muscle size increases occur due to more myofilaments. This increased number of contractile protein means more actin-myosin cross-bridges.

Muscles that are pre-stretch prior to contraction result in increased force production. This is due to three factors: pre-stretching allows for actin-myosin bonds to already be created as an eccentric contraction will occur during this pre-stretch, elastic energy will be created (muscle has elastic properties) and the muscle spindle will be activated. Only the stretch-shortening cycle factor can be improved through training.

The type of muscle fiber being recruited affects the amount of force being produced. Type II or fast twitch fibers will produce greater amounts of force than type I, or slow-twitch fibers.

Must Know:The difference in characteristics of the two main muscle fiber types:

Type II fibers are larger and have larger motor neurons Type II fibers produce more force and power Type II fibers conduct nerve impulses, contract and relax faster Type I fibers have higher endurance, fatigue resistance, aerobic enzyme

content, capillary density, myoglobin content and mitochondrial size and density

Type II fibers have higher anaerobic enzyme contentSometimes Type II fibers are divided into Type IIa and Type IIx, Type IIx are larger, faster and produce more force.

The size principle states that motor units are recruited smallest (type I fibers) to larger (type II fibers). However, this can be bypassed as a result of extensive training, it is a later stage neural adaptation.

The force-velocity relationship (or curve) states that concentric contractions can produce less force than isometric contractions, which produce less force than eccentric contractions. Fast concentric contractions produce less force than slower concentric contractions. Fast eccentric contractions produce more force than slower eccentric contractions. Training can increase the amount of force that can be produced, but only at the velocity that training occurs at. Therefore training at multiple velocities will increase force production throughout the velocity spectrum.

Muscles with more pennation can produce more force per area of muscle. This is because more pennation results in more sarcomeres in parallel, which means more actin-myosin cross-bridges per area of muscle. Increasing cross-sectional area of pennated muscle can result in greater angles of pennation (more sarcomeres in parallel), but this will not increase pennation of muscle with straight muscle fiber orientations.

1.4 Respiratory Structure and Function

Every powerstroke during muscle contraction requires ATP (the energy currency of the body). The body has a small supply of readily available ATP, the rest is supplied through anaerobic and aerobic metabolism. Aerobic metabolism makes up the majority of our energy supply and requires oxygen to produce ATP from carbohydrates, proteins or fats.

There are five steps for oxygen to be utilized to generate ATP:1. Intake of air into the body, completed by the respiratory system2. Gas exchange between blood and outside air, occurs in the alveoli3. Delivery of oxygen to the working muscle fibers, completed by the

cardiovascular system4. Uptake of oxygen into the working muscle fibers, occurs through the

capillaries5. Conversion of oxygen and energy substrates to ATP, occurs in the

mitochondria of the muscle fibers

Air is breathed in through the nasal cavity and mouth, down through the pharynx, larynx and trachea before entering the bronchi within the lunges. The bronchi further branches into bronchioles before finally reaching the alveoli.

The amount of air that does not reach the alveoli (and therefore does not participate in gas exchange) is known as the anatomical dead space. Tidal volume is the lung volume representing the normal volume of air displaced between normal inspiration and expiration when extra effort is not applied. Minute ventilation is the volume of air inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs in one minute.

Common Question:What muscles are used for respiration?

During inspiration the diaphragm is used. When breathing becomes forced (such as during exercise), the external intercostal, anterior serrate, scalene and sternocleidomastoid muscles are activated as well.

At rest expiration is a passive process that occurs due to differences in air pressure between the lungs and outside air. When expiration is forced the internal intercostal, oblique and rectus abdominus muscles are recruited.

Gas exchange occurs in the alveoli due to diffusion (moving a substance from a high concentration to a low concentration). Oxygen moves from the lungs into the blood and carbon dioxide moves from the blood into the lungs.

1.5 Cardiovascular Structure and Function

The cardiovascular system is the delivery system of the body. Oxygen, carbohydrate, nutrients and hormones are all shuttled throughout the body through the cardiovascular system.Key Point:Blood travels back to the heart through veins, blood travels away from the heart through arteries.

There are two circulatory systems: pulmonary and systemic. In pulmonary circulation deoxygenated blood will be pumped out of the right ventricle into the pulmonary artery and to the lungs, where gas exchange will occur and oxygenated blood will travel back to the left atria via the pulmonary vein. In systemic circulation oxygenated blood will be pumped out of the left ventricle into the arteries toward the rest of the body, nutrient and gas exchange will occur at the capillaries and deoxygenated blood will come back to the right atria via the veins.

Common Question:The ranking of areas of the cardiovascular system from most oxygenated to least is as follows:

1. Pulmonary vein2. Left side of the heart3. Systemic arteries4. Capillaries

5. Systemic veins6. Right side of the heart7. Pulmonary artery

Contraction of the heart is not voluntary, but like activating muscular tissue, it begins with an electrical signal. This signal begins in the sinoatrial (SA) node and causes the atria to contract. The signal is then delayed in the atrioventricular (AV) node too allow as much blood to transfer from the atria to the ventricles as possible. The signal then continues down the Purkinje fibers to contract the ventricles.

The electrical current of the heart can be measured with an electrocardiogram. The different waves that can be read on this electrocardiogram correspond with different activities of the heart. The P wave is the depolarization (contraction) of the atria, the QRS complex is the depolarization (contraction) of the ventricles and the T wave is the repolarization of the ventricles. A repolarization of the atria occurs as well but it is a very small electrical signal that is masked by the QRS complex.

When the heart is contracting this is known as systole. When the heart is relaxed, it is known as diastole. When measuring blood pressure the larger number (systolic blood pressure) is the pressure on the blood vessels when the heart is contracting and the smaller number (diastolic blood pressure) is the pressure on the blood vessels when the heart is relaxed.

Module 1 Practice Questions

1. Which of the following joint speeds of a concentric contraction will all the largest

amount of force production?

A. 10 degrees/second

B. 30 degrees/second

C. 90 degrees/second

D. 180 degrees/second

2. Which of the following muscular connective tissues surround each individual

muscle cell?

A. epimysium

B. perimysium

C. endomysium

D. sarcoplasmic reticulum

3. Which of the following does not change during an isometric contraction?

A. Distance between the Z lines

B. A Band

C. I Band

D. All of the above

4. Which of the following structures is responsible for the bringing oxygen into the

working tissues?

A. alveoli

B. blood

C. mitochondrion

D. capillary

5. Which structure of the heart is responsible for sending blood toward the lungs?

A. left ventricle

B. left atrium

C. pulmonary veins

D. right ventricle

6. During which stage of the sliding filament theory does calcium release from

troponin and is “pumped” back into the sarcoplasmic reticulum?

A. Resting Phase

B. Relaxation Phase

C. Excitation-Contraction Coupling Phase

D. Contraction Phase

7. The primary muscle of respiration is which of the following?

a. internal intercostals

b. rectus abdominus

c. diaphragm

d. external intercostals

8. Where in the body is the partial pressure of oxygen the highest?

A. the lungs

B. the blood

C. the capillaries

D. the working muscle cells

9. Which of the following represents the depolarization of the ventricles on an

electrocardiogram?

A. p wave

B. t wave

C. qrs complex

D. it isn’t seen, it’s masked by the qrs complex

10. Energy is produced through aerobic metabolism in which structure of a muscle

cell?

A. sarcoplasmic reticulum

B. sarcolemma

C. mitochondria

D. myofibril

11. An athlete is trying to deadlift 100lbs more than he has ever done before,

halfway up, he suddenly drops the bar back to the floor. Which sensory structure is

most likely to be involved in not being able to perform this action?

A. muscle spindle

B. intrafusal muscle fiber

C. Golgi tendon organ

D, pacinian corpuscle

12. Which of the following structures initiates the electrical stimulation of the

muscles of the heart?

A. SA Node

B. Purkinje fibers

C. AV Node

D. Internodal pathways

13. Which of the following will lengthen the I-bands of the sarcomeres within the

pectoralis major muscle?

A. downward movement phase of the bench press

B. upward movement phase of the upright row

C. downward movement phase of the triceps pushdown

D. upward movement phase of the bent-over row

14. Where does the left atria receive blood from?

A. systemic circulation

B. pulmonary vein

C. superior and inferior vena cava

D. aorta

15. Which structure contains calcium when a muscle is at rest?

a. t tubules

b. sarcoplasmic reticulum

c. mitochondrion

d. nucleus

16. What is the name of the place where the nerve meets the muscle, and what is the

name of the neurotransmitter that passes between them?

A. cardiovascular junction, calcium

B. cardiovascular junction, acetylcholine

C. neuromuscular junction, calcium

D. neuromuscular junction, acetylcholine

17. A motor unit is defined as which of the following?

a. the motor control area of the brain

b. the motor neuron and the muscle fibers it innervates

c. all of the motor neurons innervating a specific muscle

d. the area where the contractile proteins meet

18. Which of the following expands during an eccentric contraction?

I. Distance between Z lines

II. I-Band

III. M-Line

IV. H-Zone

A. II and III only

B. I and IV only

C. I, II and III only

D. I, II and IV only

19. Which of the following is a primary neural adaptation that occurs with beginning

a strength training program?

A. increased motor unit recruitment

B. increased muscle size

C. recruiting type II motor units at the same time as type I motor units

D. improving the stretch reflex

20. Which of the following occurs during the recharge phase?

A. calcium binds to tropomyosin

B. calcium is pumped back into the sarcoplasmic reticulum

C. measureable shortening of the muscle occurs

D. an action potential occurs

Module 1 Practice Question Answers

1. A (Module 1.3)

2. C (Module 1.1)

3. D (Module 1.2)

4. D (Module 1.4)

5. D (Module 1.5)

6. B (Module 1.2)

7. C (Module 1.4)

8. A (Module 1.4)

9. C (Module 1.5)

10. C (Module 1.4)

11. C (Module 1.2)

12. A (Module 1.5)

13. A (Module 1.2)

14. B (Module 1.5)

15. B (Module 1.1)

16. D (Module 1.2)

17. B (Module 1.1)

18. D (Module 1.2)

19. A (Module 1.3)

20. C (Module 1.2)