Skeletal Muscle

Question 1

myology

Answer 1

• Scientific study of muscle
• Muscle cells (fibers) are the only cells in the body that have the property of contractility, which allows them to shorten and
develop tension.

Question 2

skeletal muscle

Answer 2

• Attaches to and moves the skeleton.
• The contractile molecules are very organized giving skeletal muscle a striated pattern, hence the name, “striated muscle”.
• It is under voluntary control.
• Skeletal muscle comprises about 36 percent of the total body weight in women and 42 percent in men.
• 75 percent of skeletal muscle is water, 20 percent is protein, and the remainder consists of inorganic salts, pigments, fats, and carbohydrates.

Question 3

smooth muscle

Answer 3

• Found in the walls of hollow organs and blood vessels.
• The contractile molecules are not aligned in a set pattern like skeletal muscle, hence the name smooth muscle.
• This muscle is under involuntary control.

Question 4

cardiac muscle

Answer 4

• The contractile tissue of the heart wall.
• Has characteristics of both smooth muscle and skeletal muscle.
• Contractile molecules are organized very much like skeletal muscle.
• Under involuntary control.

Question 5

syncytium

Answer 5

Facilitating ejection of blood.

Question 6

6 Criteria Used To Name Muscles

Answer 6

1. Shape - deltoid (triangular), trapezius, rhomboid, latissimus (wide)
2. Action - various muscle names include the terms flexor, extensor, adductor, or pronator.
3. Location - tibialis anterior, intercostals, pectoralis major
4. Divisions - triceps brachii, quadriceps femoris
5. Size relationships - gluteus maximus, gluteus medius, gluteus minimus. Several names include the terms “brevis” (short), and “longus” (long).
6. Direction of fibers - transversus (across), rectus (straight)

Question 7

10 Functions of Skeletal Muscle

Answer 7

1. Skeletal muscles are responsible for all locomotion and manipulation.
2. Maintain posture and body position by creating tension in skeletal muscles resisting the effects of gravitational forces.
3. Breathing – the diaphragm is a skeletal muscle
4. Skeletal muscles stabilize and strengthen the joints of the skeleton
5. Support soft tissues (organs) and protect internal tissues from injury.
6. Generate heat to maintain body temperature.
7. Guard entrances and exits within the body - skeletal muscle sphincters encircled the openings of the urinary and digestive tracts, and provide voluntary control over swallowing, defecation, and urination.
8. Chewing food and talking.
9. Provide nutrient reserves – when the diet contains inadequate calories, muscle protein can be broken down to provide an energy source.
10. Acts as an endocrine organ - contracting skeletal muscles, produce a variety of signalling protein molecules called “myokines” that are secreted into the circulation and have a wide range of beneficial, hormone-like effects throughout the body.

Question 8

Skeletal Muscle Architecture

Answer 8

• Fasciculi (bundles of muscle fibers) may run parallel to long axis of muscle (greater range of motion, less strength).
• Or insert diagonally (pennate) into a tendon running the length of the muscle (smaller range of motion, greater strength).

Question 9

5 Skeletal Muscle Architectures

Answer 9

1. Unipennate
   • All fasciculi insert on one side of a tendon (semimembranosus)
2. Bipennate
   • Fasciculi insert on both sides of tendon (rectus femoris)
3. Multipennate
   • Convergence of several tendons (deltoid)
4. Longitudinal (strap)
   • Fasciculi run parallel to the long axis of the muscle (sartorius, rectus abdominus)
5. Radiate (convergent)
   • Fibers fan out from a single attachment (pectoralis major)

Question 10

Prime mover [Muscle Actions 1/3]

Answer 10

• A muscle whose contraction is primarily responsible for a particular movement.

Question 11

Antagonist [Muscle Actions 2/3]

Answer 11

• Muscles that oppose one another upon contraction, located on opposite sides of a joint.

Question 12

Fixators/stabilizers [Muscle Actions 3/3]

Answer 12

• Muscles that immobilize a bone or joint near the origin of the prime mover so that the prime mover can act more efficiently.

Question 13

Origin [Gross Anatomy 1/3]

Answer 13

• Less movable end of a muscle, usually proximal.

Question 14

Belly [Gross Anatomy 2/3]

Answer 14

• Widest portion of a muscle, between its origin and insertion.

Question 15

Insertion [Gross Anatomy 3/3]

Answer 15

• More movable end of a muscle, usually distal.

Question 16

Connective Tissue

Answer 16

• The three layers of connective tissue, endomysium, perimysium, and epimysium, surrounding muscle fibers, bundles of muscle fibers (fasciculi), and whole muscle, respectively.
• Serves to maintain intramuscular pressure, thereby augmenting force production.
• Tendons are extensions of connective tissue membranes beyond the end of the muscle, and is much stronger than muscle fibers.
• Tendons transmit the force of contractile tissue to bone.

Question 17

Microanatomy of Skeletal Muscle

Answer 17

• Muscle cell/fiber consists of many myofibrils.
• Each myofibril consists of a large array of contractile proteins arranged repeatedly in series - sacromere. (resulting in striated pattern)

Question 18

Blood Supply

Answer 18

• Muscle tissue can increase its metabolic rate 100x, and requires an abundant blood supply.

• 3-4 capillaries surrounding each muscle fiber of a sedentary person.
• Training can induce capillary angiogenesis, resulting in up to 7 capillaries per muscle fiber.
• As muscle force increases, so does intramuscular pressure, and can exceed that of blood pressure and restrict blood flow within the muscle.
• Occur at about 15-20 % of maximum muscle force.
• Completely halt blood flow at about 50 % of maximum force.
• Rhythmic contraction and relaxation during exercise like running and cycling facilitate the maintenance of blood flow.

Question 19

sarcomere

Answer 19

• Each repeated array of contractile proteins is called a sarcomere.

Question 20

Two major contractile proteins of the sarcomere.

Answer 20

• Actin (thin filament) and myosin (thick filament).
• Myosin has cross-bridges extending from its thick central core.
• The protruding cross-bridges on myosin attach to actin.

Question 21

the sliding filament theory

Answer 21

• When the muscle is activated, the protruding cross-bridges on myosin attach to actin.
• With the aid of ATP, the cross-bridge microstructure can “rotate” thus causing the thin actin filament to “slide” over myosin.
• This causes the sarcomere to shorten.

Question 22

The Motor Unit

Answer 22

• Functional unit of a muscle, consisting of a motor neuron and all the muscle fibers that motor neuron innervates.
• The cell body of a motor neuron is located in the spinal cord.
• The axon of that motor neuron extends from the spinal cord to the target muscle, and separates many times (bifurcates) to innervate all the muscle fibers of that particular motor unit.
• On average motor unit the motor neuron will innervate about 200 muscles fibers.
• The range is from two to three muscle fibers per motor unit for muscles capable of very fine movements to 2000 fibers per motor unit for large muscles that perform only gross movements.

Question 23

all-or-none law

Answer 23

• If the cell body of the motor neuron receives a strong enough stimulus, an action potential is generated.
• This action potential travels along the axon and all its bifurcations to stimulate each and every muscle fiber in that particular motor unit.

Question 24

3 Types of Motor Units (classified on the basis of speed of contraction, and metabolic characteristics)

Answer 24

• Slow twitch oxidative (SO) – aka “type I”
• Fast twitch oxidative-glycolytic (FOG) – aka “type IIa”
• Fast twitch glycolytic (FG) – aka “type IIx” (or type IIb – found in rodents)
• All of the muscle fibers have identical contractile and metabolic properties.
• Slow twitch fibers and fast twitch fibers can’t be inter-converted by physical training. However, FOG and FG fibers can be inter-converted by physical training.

Question 25

2 Methods of Force Control of a Motor Unit.

Answer 25

1. Multiple Motor Unit Summation. A large muscle may contain up to 2,000 motor units. A skeletal muscle can increase force production by activating more motor units. A low force requires the activation of a small number of motor units while a higher force requirement progressively enlists more motor units.
2. Frequency or Wave Summation.
   • If a single action potential travels down a motor neuron axon, the motor unit response is a twitch.
   • If many action potentials travel down the axon at a rate faster than the twitch response time of the motor unit, then the mechanical force response will summate.

Question 26

size principle of motor unit recruitment

Answer 26

• As the muscle force requirement increases, motor units with progressively larger axons are recruited.
• Slow twitch motor units with the lowest activation threshold are selectively recruited during light to moderate effort.
• More rapid, powerful movements progressively activate FOG motor units and then FG motor units, until all the motor units of all types are activated.

Question 27

Muscle Length-Tension Relationship

Answer 27

• An isolated muscle can exert its maximal force or tension while in a resting stretched position. As the muscle shortens, less tension can be exerted. (also applies to sacromeres)
• Within the range of sarcomere lengths there is an optimal length at which provides for the greatest possible active force production.
• Muscles must be stretched from their resting length for most effective action. (Example - flexing the knee, hip, and ankle joints before jumping.)

Question 28

Muscle Force-Velocity and Power-Velocity Relations

Answer 28

• Faster a muscle shortens the less force it produces.
• Power is the product of force times velocity.
• The graph of the relation of power with velocity shaped like an upside down ‘U’, hence the name the “inverted U hypothesis.” This occurs because, when velocity is zero, power must be zero.
• The peak torque generated by a muscle decreases with increasing velocities of movement.
• Maximum power output occurs at approximately one half of maximum velocity and one third of maximum concentric force.

Question 29

isometric contraction

Answer 29

• When the velocity of shortening is zero (i.e., no change in muscle length).

Question 30

eccentric contraction

Answer 30

• When velocity is negative, and the muscle is lengthening when it is activated.

(e.g. When you slowly lower the barbell after doing a biceps curl. You are still using your biceps, but your muscle is lengthening. In this condition it is possible for the biceps to generate more force than at the opposite speed while shortening.)

Question 31

Angle of Muscle Pull

Answer 31

• Muscles act on the bones about the joints to form a lever system.

• When a muscle is pulling at an angle of 90 degrees to a bone, all of the muscle contractile force is acting to rotate the bone around the joint.
• At angles greater than 90 degrees, the magnitude of the rotational component of the muscle pull force decreases while the magnitude of the stabilizing component of the force increases.

Question 32

three factors that affect the expression of strength by a muscle

Answer 32

1. The initial length of the muscle fibers.
2. Speed of shortening.
3. The angle of pull of the muscle on the bony skeleton.

Question 33

sarcopenia “poverty of flesh”

Answer 33

• Age-related loss of skeletal muscle mass, strength, and function

Sarcopenia is caused by various factors:
• Neural Apoptosis – neuronal atrophy in areas of the brain responsible for motor control.
• Imbalance of muscle protein synthesis/degradation
• Decline in mitochondrial function in skeletal muscle
• Physical inactivity – “If you don’t use it, you lose it!”
• Hormonal imbalance – decreased levels of testosterone, estrogen, growth hormone, etc
• Food intake – inadequate protein intake, Vitamin D intake, etc.

Question 34

Physical characteristics of old age

Answer 34

• A significant decline in skeletal muscle mass, strength, and power
• Decreased Physical Activity
• Decreased Food Intake