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Chapter 9

Muscle Structure & Physiology

Lecture 16

Visual Anatomy & PhysiologyFirst Edition

Martini & Ober

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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– Fixed in place without movement– Maintain posture in space

• Purposeful movement– Perform tasks consciously, purposefully

• Regulate internal organ movement and volume (mostly involuntary)

• Guard entrances/exits (digestive/urinary)

• Generation of heat (thermogenesis)

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Characteristics of All Muscle Tissue

• Contractile– Ability to shorten with force– 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

• epimysium (around muscle)

• perimysium (around fascicles)

• endomysium (around fibers, or cells)

Alphabetical order 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 (Cell)

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

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

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• triad

• cisternae of sarcoplasmic reticulum (2)• transverse tubule

• myofibril (1-2 µm diam.)

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Structure of the Sarcomere

• I band• A band• H zone• Z line• M line

The sarcomere is the contractile unit of skeletal (and cardiac) muscle

(~ 2µm long)

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

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Structure of the Sarcomere

‘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?)

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Sliding Filament Theory

Theory used to explain these observations is called the sliding filament theory

…

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

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MyofilamentsThick Filaments

• composed of myosin• cross-bridges

Thin Filaments• composed of actin• associated with troponin and tropomyosin

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

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Mechanism of Sarcomere Contraction

When you think myosin, think mover:

1. Bind

2. Move3. Detach4. 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. Move3. Detach

4. Reset

What would happen if ATP was not present?

Cycle repeats about 5 times/secEach power stroke shortens sarcomere by about 1%So, each second the sarcomere shortens by about 5%

…

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

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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

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

19Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Summary of Skeletal Muscle Contraction

Contraction Relaxation

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Modes of ATP Synthesis During Exercise

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

(ATP and CP)

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

1) Creatine phosphate

2) Glycolysis (30 sec – 2 min)

3) Aerobic respiration

• stores energy that quickly converts ADP to ATP

• CP + ATP provide about 10-15 seconds of energy

myoglobin stores extra oxygen

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

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Oxygen Debt

• when oxygen is not available

• glycolysis continues

• pyruvic acid converted to lactic acid (WHY?)

• liver converts lactic acid to glucose

(The Cori Cycle)

Oxygen debt – amount of extra oxygen needed by liver to convert lactic acid to glucose, resynthesize creatine-P, 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

• Commonly caused by • decreased blood flow• ion imbalances• accumulation of lactic acid• relative decrease in ATP availability• decrease in stored ACh

• Cramp – sustained, involuntary contraction

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Length-Tension RelationshipFigure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Maximum tension in striated muscle can only be generated when there is optimal 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

Recording a Muscle Contraction (myogram)

• latent period• period of contraction• period of relaxation• refractory period• all-or-none response

A single twitch

An individual muscle fiber (cell) is either “on” or “off” and produces maximum tension at that resting length for a given frequency of stimulation

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Treppe, Wave Summation, and Tetanus

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

Wave (Temporal) SummationTreppe

(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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Treppe, Wave Summation, and Tetanus

• Treppe, Wave Summation, and Tetanus – all involve increases in maximum tension generated in

a muscle fiber after 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 tetanus – partial relaxation between stimuli

• Complete tetanus – NO relaxation between stimuli

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Motor Unit

• single motor neuron plus all muscle fibers controlled by that motor neuron

** Contraction in a single muscle fiber is an “all or none” phenomenon (but remember that the tension will not always be maximal)

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

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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 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 FibersSlow Oxidative

(SO)

(GO REDSOX!)

Fast Oxidative-Glycolytic (FOG)

Fast Glycolytic (FG)

Alternate name

Slow-TwitchType I

Fast-TwitchType II-A

Fast-Twitch Type II-B

Myoglobin (color) +++ (red) ++ (pink-red) + (white)

Metabolism

Oxidative(aerobic)

Oxidative and Glycolytic

Glycolytic (anaerobic)

StrengthSmall 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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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

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

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Types of Smooth Muscle

Single-unit 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• smooth muscle slower to contract and relax• smooth muscle more resistant to fatigue

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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

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

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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

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

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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)