bio211 lecture 16 -...

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Chapter 9

Muscles and Muscle Tissue

Lecture 16

Marieb’s Human

Anatomy and Physiology

Marieb w Hoehn

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Lecture Overview

• Types, characteristics, functions of muscle

• Structure of skeletal muscle

• Mechanism of skeletal muscle fiber

contraction

• Energetics of skeletal muscle contraction

• Skeletal muscle performance

• Types of skeletal muscle contractions

• Comparison of skeletal muscle with

smooth muscle and cardiac muscle

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Muscular System

Review - Three Types of Muscle Tissues

Skeletal Muscle • usually attached

to bones

• under conscious

control (voluntary)

• striated

• multinucleated

Smooth Muscle • walls of most viscera,

blood vessels, skin

• not under conscious

control

• not striated

Cardiac Muscle • wall of heart

• not under

conscious control

• striated

• branched

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Functions of Muscle

• Provide stability and postural tone (skeletal)

– Fixed in place without movement

– Maintain posture in space

• Purposeful movement (skeletal)

– Perform tasks consciously, purposefully

• Regulate internal organ movement and

volume (mostly involuntary - smooth)

• Guard entrances/exits (digestive/urinary –

skeletal and smooth)

• Generation of heat (thermogenesis - skeletal)

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Characteristics of All Muscle Tissue

• Contractile

– Ability to shorten (if possible) with force;

exerts tension

– CANNOT forcibly lengthen

• Extensible (able to be stretched)

• Elastic (returns to resting length)

• Excitable (can respond electrical impulses)

• Conductive (transmits electrical impulses)

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Structure of a Skeletal Muscle –

Gross/Histological Level

• epimysium

(around muscle)

• perimysium

(around fascicles)

• endomysium

(around fibers, or

cells)

Alphabetical order of MUSCLE from largest to smallest: fascicle, fiber, fibril, and filament

Figure from: Hole’s Human A&P, 12th edition, 2010

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Skeletal Muscle Fiber (Cellular level)

Sarcoplasmic reticulum is like the ER of other cells; but it contains [Ca2+ ]

Transverse or T-tubules contain extracellular fluid ( [Na+], [K+])

Fully differentiated, specialized cell – its structures are given special names

Figure from: Saladin, Anatomy &

Physiology, McGraw Hill, 2007

• sarcolemma (plasma membrane)

• sarcoplasm (cytoplasm)

• sarcoplasmic reticulum (ER)

• transverse tubule (T-tubule)

• triad • cisternae of sarcoplasmic

reticulum (2)

• transverse, or T-tubule

• myofibril (1-2 µm diam.)

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Structure of the Sarcomere (Histological Level)

• I band

• A band

• H zone

• Z line

• M line

The sarcomere is the contractile unit of skeletal (and cardiac) muscle (~ 2µm long)

Figures From:

Marieb & Hoehn,

Human Anatomy &

Physiology, 9th ed.,

Pearson, 2013

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Structure of the Sarcomere

(Histological/Molecular Level)

‘A’ in A band

stands for

Anisotropic

(dArk)

‘I’ in I band

stands for

Isotropic (LIght)

Zones of non-overlap: I band (thin filaments), and H zone (thick filaments)

A sarcomere runs from Z line (disk) to Z line (disk) (From ‘Z’ to shining ‘Z’!)

Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

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Preview of Skeletal

Muscle Contraction

Major steps:

1. Motor neuron firing

2. Depolarization (excitation)

of muscle cell

3. Release of Ca2+ from

sarcoplasmic reticulum

4. Shortening of sarcomeres

5. Shortening of muscle/CTs

and tension produced

Figure from: Martini, Anatomy & Physiology,

Prentice Hall, 2001 Physiology here we come!!

T Tubule

Sarcoplasmic

reticulum

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Grasping Physiological Concepts

• The steps in a physiological process give you the

‘when’, i.e. tell you when things happen and/or the

order in which they happen.

• For each step in a process, you should MUST ask

yourself the following questions - and be sure you get

answers!

– How? (How does it happen?)

– Why? (Why it happens and/or why it’s important?)

– What? (What happens?)

See Figures 9.7 and 9.8 in your textbook for excellent

overall summaries of the muscle contraction process

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Sliding Filament Theory

Theory used to

explain these

observations is

called the sliding

filament theory

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Myofilaments (Molecular Level)

Thin Filaments

• composed of

actin

• associated

with troponin

and

tropomyosin

Thick Filaments

• composed of

myosin

• cross-bridges

Figures From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson, 2013

The Sarcomere as a 3D Object…

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https://www.youtube.com/watch?v=-pg09F5V63U

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Mechanism of Sarcomere Contraction

When you

think myosin,

think mover:

1. Bind

2. Move

3. Detach

4. Reset

Ca2+ troponin

myosin actin

Figure from: Hole’s

Human A&P, 12th

edition, 2010




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Mechanism of Sarcomere Contraction

1. Bind

2. Move 3. Detach

4. Reset

What would

happen if ATP was

not present?

Cycle repeats about 5 times/sec

Each power stroke shortens sarcomere by about 1%

So, each second the sarcomere shortens by about 5%

…


See Textbook Figure 9.12 (Focus – Cross Bridge Cycle)

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Neuromuscular Junction

• site where axon and

muscle fiber

communicate

• motor neuron

• motor end plate

• synaptic cleft

• synaptic vesicles

• neurotransmitters

The

neurotransmitter for

initiating skeletal

muscle contraction is

acetylcholine (ACh)

Figures from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

SR

Ca2+

Ca2+

Ca2+ Ca2+ Ca2+

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Stimulus for Contraction: Depolarization

• nerve impulse causes release of

acetylcholine (ACh) from

synaptic vesicles

• ACh binds to acetylcholine

receptors on motor end plate

• generates a muscle impulse

• muscle impulse eventually

reaches sarcoplasmic reticulum

(via T tubules) and Ca2+ is

released

• acetylcholine is destroyed by

the enzyme acetylcholinesterase

(AChE)

Linking of nerve stimulation with muscle contraction is called

excitation-contraction coupling (See Fig 9.11 in textbook)

Figure from:

Martini,

Anatomy &

Physiology,

Prentice Hall,

2001

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Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Summary of Skeletal Muscle Contraction

Contraction Relaxation

See Textbook Figure 9.12 (Focus – Cross Bridge Cycle)

- Bind (Ca, myosin)

- Move

- Detach

- Reset

5. Contraction

Cycle begins

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Modes of ATP Synthesis During Exercise

Continual shift from one energy source to another

rather than an abrupt change

Muscle stores enough ATP for about 4-6 seconds worth of contraction, but is

the only energy source used directly by muscle. So, how is energy provided for

prolonged contraction?


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Energy Sources for Contraction


myoglobin stores extra oxygen so it can rapidly supply

muscle when needed

(Creatine-P)

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Oxygen Debt (Excess Post Exercise O2 Consumption – EPOC)

• when oxygen is not available

• glycolysis continues

• pyruvic acid converted to

lactic acid (WHY?)

• liver converts lactic acid to

glucose

(The Cori Cycle)

EPOC - amount of extra oxygen needed by liver to convert lactic

acid to glucose, resynthesize creatine-P, make new glycogen, and

replace O2 removed from myoglobin.

Figure from: Hole’s

Human A&P, 12th edition,

2010

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Muscle Fatigue

• Inability to maintain force of contraction although

muscle is receiving stimulus to contract

• Commonly caused by

• decreased blood flow

• ion imbalances

• accumulation of lactic acid

• relative (not total) decrease in ATP availability

• decrease in stored ACh

• Cramp – sustained, involuntary contraction

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Length-Tension Relationship

Maximum tension in striated muscle can only be generated when there

is optimal (80-100%) overlap between myosin and actin filaments


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Muscular Responses

Threshold Stimulus

• minimal strength required to cause contraction in an

isolated muscle fiber

Record of a Muscle

Contraction = myogram

• latent period

• period of contraction

• period of relaxation

• refractory period

• all-or-none response

An individual muscle fiber (cell) is either “on” or “off” and

produces maximum tension at that resting length for a given

frequency of stimulation

Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson, 2013

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Treppe, Wave Summation, and Tetanus

• Treppe, Wave Summation, and Tetanus

– all involve increases in tension generated in a muscle fiber

after more frequent re-stimulation

• The difference among them is WHEN the muscle

fiber receives the second, and subsequent,

stimulations:

– Treppe – stimulation immediately AFTER a muscle cell

has relaxed completely.

– Wave Summation – Stimulation BEFORE a muscle fiber is

relaxed completely

• Incomplete (unfused) tetanus – partial relaxation between stimuli

• Complete (fused) tetanus – NO relaxation between stimuli

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Treppe, Wave Summation, and Tetanus


Wave (Temporal)

Summation Treppe

(10-20/sec)

Incomplete

Tetanus

(20-30/sec)

Complete

Tetanus

(>50/sec)

Little/no

relaxation

period

Tetany is a sustained contraction of skeletal muscle

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Motor Unit

• single motor neuron plus all muscle fibers controlled by

that motor neuron

Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson, 2013

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Recruitment of Motor Units

• recruitment - increase in the number of motor

units activated to perform a task

• whole muscle composed of many motor units

• as intensity of stimulation increases,

recruitment of motor units continues, from

smallest to largest, until all motor units are

activated

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Sustained Contractions

• smaller motor units recruited first

• larger motor units recruited later

• produces smooth movements

• muscle tone – continuous state of partial contraction

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Types of Contractions

• isotonic – muscle contracts and

changes length

• concentric – shortening contraction

• isometric – muscle “contracts” but does not change length

• eccentric – lengthening

contraction

Figure from:

Hole’s Human

A&P, 12th edition,

2010

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Types of Skeletal Muscle Fibers

Slow Oxidative

(SO)

(REDSOX)

Fast Oxidative-

Glycolytic (FOG)

Fast Glycolytic

(FG)

Alternate name

Slow-Twitch

Type I

Fast-Twitch

Type II-A

Fast-Twitch

Type II-B

Myoglobin (color)

+++ (red)

++ (pink-red)

+ (white)

Metabolism

Oxidative

(aerobic)

Oxidative and

Glycolytic

Glycolytic

(anaerobic)

Strength

Small diameter,

least powerful

Intermediate

diameter/strength

Greatest diameter,

most powerful

Fatigue resistance High Moderate Low

Capillary blood

supply

Dense

Intermediate

Sparse

All fibers in any given motor unit are of the same type

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Types of Skeletal Muscle Fibers

All fibers in any given motor unit are of the same type

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Smooth Muscle Fibers

Compared to skeletal muscle fibers

• shorter

• single nucleus

• elongated with tapering ends

• myofilaments organized

differently

• no sarcomeres, so no striations

• lack transverse tubules

• sarcoplasmic reticula not well

developed

• exhibit stress-relaxation

response (adapt to new stretch

state and relax)

Figure from: Martini, Anatomy &

Physiology, Prentice Hall, 2001

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Types of Smooth Muscle

Single-unit (unitary)

smooth muscle

• visceral smooth muscle

• sheets of muscle fibers

that function as a group,

i.e., a single unit

• fibers held together by

gap junctions

• exhibit rhythmicity

• exhibit peristalsis

• walls of most hollow

organs, blood vessels,

respiratory/urinary/

reproductive tracts

Multiunit Smooth Muscle

• fibers function

separately, i.e., as

multiple independent

units

• muscles of eye,

piloerector muscles,

walls of large blood

vessels

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Smooth Muscle Contraction

• Resembles skeletal muscle contraction

• interaction between actin and myosin

• both use calcium and ATP

• both depend on impulses

• Different from skeletal muscle contraction

• smooth muscle lacks troponin

• smooth muscle depends on calmodulin

• two neurotransmitters affect smooth muscle

• acetylcholine and norepinephrine

• hormones affect smooth muscle

• have gap junctions

• stretching can trigger smooth muscle contraction (but briefly,

then relaxation again occurs)

• smooth muscle slower to contract and relax

• smooth muscle more resistant to fatigue

• smooth muscle can undergo hyperplasia, e.g., uterus

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Cardiac Muscle

• only in the heart

• muscle fibers joined together by

intercalated discs

• fibers branch

• network of fibers contracts as a

unit (gap junctions)

• self-exciting and rhythmic

• longer refractory period than

skeletal muscle (slower contract.)

• cannot be tetanized

• fatigue resistant

• has sarcomeres


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Review

• Three types of muscle tissue

– Skeletal

– Cardiac

– Smooth

• Muscle tissue is…

– Contractile

– Extensible

– Elastic

– Conductive

– Excitable

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Review

• Functions of muscle tissue

– Provide stability and postural tone

– Purposeful movement

– Regulate internal organ movement and volume

– Guard entrances/exits

– Generation of heat

• Muscle fiber anatomy

– Actin filaments, tropomyosin, troponin

– Myosin filaments

– Sarcomere

– Bands and zones

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Review

• Muscle contraction

– Sliding filament theory

– Contraction cycle (Bind, Move, Detach, Release)

– Role of ATP, creatine

– Metabolic requirements of skeletal muscle

– Stimulation at neuromuscular junction

• Muscular responses

– Threshold stimulus

– Twitch – latent period, refractory period

– All or none response

– Treppe, Wave summation, and tetanus

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Review

• Muscular responses

– Recruitment

– Muscle tone

– Types of muscle contractions

• Isometric

• Isotonic

• Concentric

• Eccentric

• Fast and slow twitch muscle fibers

– Slow Oxidative (Type I) (think: REDSOX)

– Fast Oxidative-glycolytic (Type II-A)

– Fast Glycolytic (Type II-B)

bio211 lecture 16 -...

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