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C H A P T E R 1. Web site: mbl.tamu.edu. MUSCLES AND HOW THEY MOVE. Learning Objectives. w Learn the basic components of skeletal muscle, the muscle fiber, and the myofibril. w Note the cellular events leading to a basic muscle action.

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C h a p t e r 1

C H A P T E R 1

Web site: mbl.tamu.edu

MUSCLES

AND HOW THEY MOVE


C h a p t e r 1

Learning Objectives

w Learn the basic components of skeletal muscle, the muscle fiber, and the myofibril.

w Note the cellular events leading to a basic muscle action.

w Discover how muscle functions during exercise.

w Consider the differences in fiber types and their impact on physical performance.

w Learn how muscles generate force and movement by pulling on bones.


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Introduction

  • The muscular system is the anatomical system of a species that allows it to move. The muscular system in vertebrates is controlled through the nervous system, although some muscles (such as the cardiac muscle) can be completely autonomous.


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Types of Muscle Cells

  • Skeletal - striated

  • Voluntary muscle; controlled consciously

  • Over 660 throughout the body

  • Cardiac - striated

  • Involuntary; controlled unconsciously

  • Only in the heart

  • Smooth – non-striated

  • Involuntary muscle; controlled unconsciously

  • In the walls of blood vessels and internal organs

Each muscle tissue type with connective tissue wrappings, blood vessels,

nerve fibers = muscle as an organ


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Historical note: striated muscle cells

The father of microbiology

Antony van Leeuwenhoek (1632-1723) developed the microscope, and described the striped appearance of skeletal muscle.


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SKELETAL MUSCLE STRUCTURE

1. Around muscle body

1.

5.

2. A bundle of fibers wrapped in

a connective tissue sheath

3.

2.

3. Around muscle bundles

4.

4. Individual muscle cells

5. Around individual myofibers

 6. A basic rod-like unit of a muscle

6.

The average human skeletal muscle fiber is about 50 μm in diameter; the average human hair is about 150 μm in diameter.


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MUSCLE FIBER (Multinucleated)

An individual muscle cell

1. A muscle fiber is enclosed

by a plasma membrane called

the plasmalemma(sarcolemma)

1.

2. The cytoplasm of a muscle fiber

is called the sarcoplasm

3. The t-tubules allow transport of

substances and passage of action

potentials throughout the muscle fiber

2.

3.

4.

4. The sarcoplasmic reticulum stores and releases calcium during contraction


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Myofibrils

1. Myofibrilsare the contractile “threads” that run the length of the muscle fiber, with several hundred to several thousand within a single fiber

2. Myofibrils are made up of sarcomeres

in series, the smallest functional units

of a muscle

4

3. Thick filament: myosin, which has a

globular head at one end (cross-bridge)

3

4. Thin filament: composed of

actin, tropomyosin, and troponin

— is attached to the Z disks at the

ends of the sarcomeres

1

2


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Myofibrils

1. A band: The dark zone of a myofibril of striated muscle

2. I band: The light zone of a myofibril of striated muscle

3. H-zone: The light region in the center of A-band, visible only when myofibril is relaxed

4. M-line: The dark line in the middle

of H-zone

3

5. Z-line: I-bands are interrupted

by a dark stripe called Z-disk

5

4

2

1


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Electron Micrograph Showing Myofibrils and Sarcomeres

  • In the electron micrograph on the preceding slide, can you identify the followings?

  • Sarcomere A-bands

  • Sarcomere I-bands

  • Mitochondria

  • Lipid droplets

  • Z-lines

  • M-lines


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THICK FILAMENT

Cross-bridge (actin binding site; ATP binding site; ATPase)

Coiled coil rod holds

two heads together)


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THIN FILAMENT

(covers myosinbinding sites at rest)

(Ca2+ binding site)

(myosin binding site)


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THIN FILAMENT

Actin:

Acts with myosin to produce muscle

action

Troponin:

troponin I, troponin T, and troponin C

Integral to muscle contraction in 

skeletal and cardiac muscle

Tropomyosin:

A tube-shaped protein that twists

around the actin strands


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ARRANGEMENT OF FILAMENTS IN A SARCOMERE


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5. The Ca2+ binds to troponin on the thin filament, and the troponin pulls tropomyosin off the active sites, allowing myosin heads to attach to the actin filament.

(continued)

Excitation/Contraction Coupling

1. An α-motoneuron, with signals from the brain or spinal cord, releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction.

2. ACh crosses the junction and binds to receptors on the plasmalemma (sarcolemma).

3. This initiates an action potential.

4. The action potential travels along the plasmalemma and down the t-tubules to the SR, which releases calcium ions (Ca2+).


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Excitation/Contraction Coupling

6. Once a strong binding state is established with actin, the myosin head tilts, pulling the thin (actin) filament toward the center of the sarcomere (power stroke).

7. The myosin head binds to ATP, an ATPase found on the head splits ATP into ADP and Pi, releasing energy.

ATP ADP + Pi + work + heat

myosin ATPase

8. Muscle action continues as long as calcium is elevated, and ends when calcium is actively pumped out of the sarcoplasm back into the sarcoplasmic reticulum by a calcium ATPase.


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Thought Question

What is the “basic molecule” of exercise physiology? Why?


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MOTOR UNIT

CNS (brain, spinal cord)


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EVENTS LEADING TO MUSCLE ACTION


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Sliding Filament Theory

w When myosin cross-bridges are activated, they bind strongly with actin, resulting in a conformational change in the cross-bridge.

w This change in the cross-bridge causes the myosin head to tilt toward the arm of the cross-bridge and pull the thin filament toward the M-line in the center of the sarcomere.

w The tilt of the myosin head is known as a power stroke.

w The pulling of the actin filament past the myosin results in muscle shortening and generation of muscle force.


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CONTRACTING MUSCLE FIBER


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Thought Question

  • What tools are available to the Exercise Physiologist to study skeletal muscle function?

    • animal models

    • cell culture

    • muscle biopsy


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Muscle Biopsy

  • An important advancement in the field of exercise physiology was the implementation of the skeletal muscle biopsy.

  • It enabled scientists to have a ‘snapshot’ look at the cellular structure, morphology, metabolism, and physiology of skeletal muscle.


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Muscle Biopsy

  • There are a variety of tools available to the exercise physiologist if tissue biopsies are obtained:

    • Assessments of muscle fiber morphology

    • Assessments of metabolism

    • Assessments of turnover

    • DNA expression

    • Contractile/functional properties

    • Hormone function


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Historical note: Needle Biopsy

Eric Haltman (1925-2011) applied muscle needle biopsy at the Karolinska Institute, from the late 1960s through 1970s in Sweden.


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Muscle Biopsy

wArea near biopsy is shaved and cleansed with povidone iodine ointment.


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Muscle Biopsy

wLydocaine is injected below the skin


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Muscle Biopsy

  • The needle is inserted into the ‘belly’ of the muscle, and with suction applied, the biopsy is removed via a sharp-edged trocar located within the biopsy needle. The biopsy needle is in the muscle for ~1-2 seconds until the procedure is completed.

  • A small incision is made at the biopsy site using a sterile scalpel.


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Muscle Biopsy

wAfter the biopsy is obtained, the sample is removed from the needle, and placed in the appropriate environment for experiments or analyses.


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Muscle Biopsy

wPressure is applied to the biopsy site to ensure a minimum of bleeding, and dressed with a closure and pressure bandage.


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Muscle Biopsy

wSample is mounted, frozen, thinly sliced, and examined under a microscope.

wSamples may also be used for a variety of in vitro analyses, such as muscle protein synthesis.

wAllows study of muscle fibers and the effects of acute exercise and exercise training on fiber composition/metabolism.


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Type I (ST) muscle fibers (high concentration in deep antigravity muscles, e.g., soleus)

w High aerobic (oxidative) capacity and fatigue resistance

1. High mitochondrial density

2. High capillary density: high blood flow

3. High myoglobin: high oxygen storage and transfer

w Low anaerobic (glycolytic) capacity

w Relatively slow contractile speed

1. low myosin ATPase activity

2. Relatively little SR (slow Ca2+

release and uptake)

w < 300 fibers per motor neuron

(relatively small motor units)


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Type IIa (FTa) Muscle Fibers

w Moderate aerobic (oxidative) capacity and fatigue resistance

w High anaerobic (glycolytic) capacity

1. High glycolytic enzymes, thus, high lactic acid production

w Fast contractile speed

1. High myosin ATPase activity

2. High density of SR (fast Ca2+

release and uptake)

w Relatively large motor units (>300 muscle fibers per motor neuron)


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Type IIb (FTb) Muscle Fibers

w Low aerobic (oxidative) capacity and low fatigue resistance

w High anaerobic (glycolytic) capacity; high lactate production when active

w Fast contractile speed (high myosin ATPase and SR density)

w Relatively large motor units (>300 muscle fibers per motor neuron)


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Thought Question

Why are slow twitch muscle fibers dark in color?

ST fibers use primarily oxidative respiration, while FT fibers use primarily anaerobic respiration. Since the ST fibers require more O2, they have more capillaries nearby and have more myoglobin inside the cells. The capillaries and myoglobin are what makes ST fibers a darker color, usually reddish.


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SLOW- AND FAST-TWITCH FIBERS

Type I

Type IIa

Type IIb


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Muscle fiber types

wSlow twitch (type I) - #1 below

wFast twitcha (type IIa) - #2 below

wFast twitchb (type IIb) - #3 below

Myosin

ATPase (Shortening Velocity)

Succinate Dehydrogenase Activity (Mitochondrial

Density: Oxidative Capacity)

Enzyme Histochemistry


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Force Production in the Fiber Types

The difference in force development between type I and type II motor units is due to the larger number of muscle fibers per motor unit and the larger diameter of the type II fibers.


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PEAK POWER GENERATED BY FIBERS

Power = force x distance

time

or

Power = work/time

or

Power = force x velocity

For fibers of the same size,

force production is about

the same in type I and II

fibers; however, shortening

velocity is much higher in

type II fibers


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What Determines Fiber Type?

wThe α-motoneuron of the motor unit controls the properties of the muscle fibers

wDemonstrated by:

wCross-innervation of slow and fast muscles

McComas, Skeletal

Muscle, Human Kinetics,

1996,

from Buller et al., 1960.


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What Determines Fiber Type?

w Genes determine which motor neurons innervate individual muscle fibers; generally, motor units are made up of homogenous fiber composition .

w Both resistance and endurance training and muscular inactivity may result in small changes in the percentages of type I and II fibers.

w There is often an increase in the percentage of ST fibers with aging (Why? A decline in FT fibers).


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The All-Or-None-Response

wFibers in a motor unit are recruited when an ɑ-motoneuron carries an action potential (impulse) from the central nervous system.

wAll muscle fibers in the motor unit are recruited simultaneously.

wMore force is produced by activating more motor units.


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Orderly Recruitment of Muscle Fibers

  • Principle of orderly recruitment states that motor units are activated in a fixed order

  • For the production of a given force, the same motor units are usually recruited each time and in a same order.

  • Size principle states that the order of recruitment is directly related to the motor neuron size.

  • Slow-twitch fibers, which have smaller motor neurons, are recruited before fast-twitch fibers.


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RAMPLIKE RECRUITMENT OF FIBERS


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Functional Classification of Muscles

Agonists—prime movers; responsible for the movement

Antagonists—oppose the agonists to prevent overstretching of them

Synergists—assist the agonists and sometimes fine-tune the direction of movement


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TYPES OF MUSCLE ACTION

Force > load

Force = load

Force < load


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Neural Factors Influencing Force

Number of motor units activated (recruitment)

Rate of stimulation of the motor unit (rate coding)

Synchronized motor unit firing


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Frequency of action potentials (rate coding)

Tension (force) produced by one motor unit


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Factors Influencing Force Generation

Number of motor units activated

Rate of stimulation of the motor unit (rate coding)

Type of motor units activated (FT or ST)

Muscle size

Initial muscle length

Joint angle

Speed of muscle action (shortening or lengthening)


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Force-Velocity Relationship


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Muscle protein homeostasis…

  • Balance of muscle protein synthesis (RPS) and degradation (RPD)

    • RPS = RPD = stable muscle mass

    • RPS > RPD = muscle hypertrophy

    • RPS < RPD = muscle atrophy

  • Muscle is in a constant state of turnover, and therefore ‘plastic’.

    • The term muscle plasticity refers to the muscle’s ability to increase (hypertrophy), decrease (atrophy) or maintain stable muscle mass in response to changes in muscle/external environment.


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    Other Studies using Biopsied Muscle…

    • Muscle Biology Laboratory Studies…

      • Muscle protein synthesis and degradation (using radioactive and stable isotopic methodologies)

        • Signaling mechanisms for muscle growth

        • Signaling mechanisms for muscle breakdown

      • Gene Expression – mRNA, specific proteins

      • Muscle glucose metabolism

        • Glycogen content

        • Glucose disposal/transport

    • Experimental paradigms include aging, obesity and microgravity in rats and humans.


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