Structure of a Single Muscle Fiber

Skeletal Muscle Fiber Structure

Key Points• An individual muscle cell is called a muscle fiber• A muscle fiber is enclosed by a plasma

membrane called the sarcolemma• The cytoplasm of a muscle fiber is called a

sarcoplasm• Within the sarcoplasm, the extensive T-tubules

allow transport of substances throughout the muscle fiber

• The sarcoplasmic reticulum (SR) stores calcium

The Myofibril

Key Points• Myofibrils are made up of sarcomeres• A sarcomere is composed of protein filaments

of myosin and actin• Interactions between the filaments is

responsible for muscle contraction• Myosin, the thick filament, is composed of two

protein strands, each folded into a globular head at one end

• The thin filament is composed of actin, tropomyosin, and troponin, with one end attached to a Z-disk

Alpha Motor Neurons

• One -motor neuron innervates many muscle fibers, collectively called the motor unit

• The action potential arrives at the dendrites and travels down the axon to the axon terminal

Events Leading toMuscle Fiber Contraction

1. A motor neuron releases acetylcholine (ACh) at the neuromuscular junction

2. ACh binds to receptors on the sarcolemma3. If enough ACh binds to receptors, an action potential

is transmitted the full length of the muscle fiber4. The action potential triggers the release of Ca2+ from

the sarcoplasmic reticulum5. Ca2+ binds to troponin on the actin filament, and the

troponin pulls tropomyosin off the active sites, allowing myosin heads to attach to the actin filament

Muscle Fiber Contraction

Key Points• Muscle contraction is initiated by an -motor

neuron action potential• The motor neuron releases ACh, which opens

up ion gates in the muscle cell membrane• Sodium enters the muscle cell depolarizing the

plasmalemma • The action potential travels throughout the

plasmalemma and through the T-tubules, which releases stored Ca2+ ions from the sarcoplasmic reticulum

(continued)

Muscle Fiber Contraction (continued)

Key Points• Ca2+ ions bind with troponin, lifting the

tropomyosin molecules off the active sites on the actin filament

• The myosin head binds to the active actin site• The myosin head binds ATP, and ATPase on the

myosin heads splits ATP into ADP and Pi, releasing energy to fuel the muscle contraction

• The myosin head tilts, pulling the thin filament past the thick filament—power stroke

• Muscle contraction ends when Ca2+ is actively pumped out of the sarcoplasm back to the sarcoplasmic reticulum

Muscle Biopsy

The muscle biopsy allows us to study muscle fibers and the effects of acute exercise and chronic training on muscle fiber composition– The skin is first anesthetized and then a small

incision is made through the skin – A hollow Bergstrom needle is inserted into the

muscle belly to take the sample– The sample is mounted, frozen, thinly sliced, and

examined under a microscope

Muscle Biopsy

Muscle Fiber Types

• Slow-twitch fibers, Type I (~50%), oxidative• Fast-twitch fibers, Type II

– Type IIa (25%), fast oxidative/glycolytic (FOG)– Type IIx in humans (~25%) ~ IIb in animals,

fast glycolytic (FG)– Type IIc (1-3%)

• The percentage of each fiber type is variable among muscles, among individuals, and with exercise training

Single Muscle Fiber Physiology

• Peak power is different between muscle fiber types

• All fiber types tend to reach their peak power at ~20% peak force

Muscle Fiber TypesKey Points• Skeletal muscle contains type I and type II fibers• Different fiber types have different myosin ATPase activities• Type II fibers have a more highly developed SR, delivering

more Ca2+

• Type II motor units are larger compared to type I motor units• Type II motor units have more muscle fibers to contract and

produce more force than type I motor units• The proportion of type I and type II fibers in an individual’s

arm and leg muscles are usually similar

(continued)

Muscle Fiber Types (continued)

Key Points• Type I fibers have higher aerobic endurance and are

well suited to low-intensity endurance activities• Type II fibers are better suited for anaerobic activity

– Type IIa fibers play a major role in high intensity exercise

– Type IIx fibers are activated when the force demanded of a muscle is high

Determination of Fiber Type

• Fiber type is genetically determined (twin studies)• Fiber type is determined by the -motor neuron that

innervates the muscle fibers• Endurance training, strength training, and muscular

inactivity may cause a shift in myosin isoforms– Exercise training ↓ type IIx and ↑ type IIa

• Aging may shift the relative distribution of type I and type II fibers – ↓ type II and ↑ type I

Motor Unit Recruitment

Principle of orderly recruitment

Motor units are activated on the basis of a fixed order– type I → type IIa → type IIx

Size Principle

Size principle

The order of motor unit recruitment is directly related to

the motor neuron size

Motor Unit Recruitment

Key Points• Motor units give an all-or-none response• Activating more motor units and thus more muscle

fibers produces more force• Motor units are recruited in an orderly way to generate

force or for long duration events– type I → type IIa → type IIx

Athletes and Fiber Type

Key Points• Muscle fiber composition differs in athletes by sport and

event• Speed and strength events are characterized by a

higher percentage of type II fibers• Endurance events are characterized by a higher

percentage of type I fibers

Types of Muscle Contraction

• Concentric contraction: Force is developed while the muscle is shortening

• Isometric contraction: Force is generated but the length of the muscle is unchanged

• Eccentric contraction: Force is generated while the muscle is lengthening

Muscle Force Generation

Key Points• 3 types of muscle contraction

– Concentric– Isometric– Eccentric

• Force production is increased by recruitment of more motor units and through increased frequency of stimulation

• Force production is maximized at the muscle’s optimal length

• Speed of contraction also affects the amount of force produced