copyright 2010, john wiley & sons, inc. chapter 8 the muscular system

Copyright 2010, John Wiley & Sons, Inc.

Chapter 8

The Muscular System


Muscle Types Skeletal Smooth Cardiac

Similarities All muscle cells are elongated = muscle fibers Muscle contraction depends on 2 kinds of

myofilaments (actin & myosin) Cell membrane of a muscle cell =

sarcolemma – cytoplasm is called sarcoplasm

Chapter 8 Muscular System


Types of Muscle and Function Skeletal - 40–50% of total body weight- voluntary

movement of bone & body parts Stabilizing body positions

Cardiac - involuntary Heart only Develops pressure for arterial blood flow

Smooth - grouped in walls of hollow organs Sphincters regulate flow in tubes Maintain diameter of tubes Move material in GI tract and reproductive organs


Functional Characteristics of Muscle Excitability – to receive & respond to stimuli Contractility – shorten forcibly when

stimulated Extensibility – stretched or extended Elasticity – to bounce back to original length


Skeletal Muscle Tissue Muscle includes: muscle fibers, connective

tissue, nerves & blood vessels


Connective Tissue Coverings Endomysium – thin delicate layer of CT that

wraps each muscle fiber Fascicles – many muscle fibers bundled together

into groups Each is wrapped in a 2nd layer of CT made of collagen –

perimysium Skeletal muscle – many fascicles bundled

together Each is covered by a 3rd layer of dense fibrous CT –

epimysium Deep Fascia – each skeletal muscle is then

covered by a 4th , very tough fibrous layer of CT May extend past the length of the muscle (tendon) and

attach that muscle to a bone, cartilage or muscle


Structure of a Skeletal Muscle

9-4

• Deep fascia• epimysium• perimysium• fascicle• endomysium

Outside to Inside


Muscle Histology Sarcoplasm contains myoglobin

Red pigmented protein related to Hemoglobin that carries oxygen


Skeletal Muscle Fibers 2 types of protein filaments

Thick – protein myosin Thin – protein actin

Striations are caused by the arrangement of thick & thin filaments A-Band – dark area – overlapping of thick & thin I-Band – light area – thin filaments only

Length of each myofibril is divided into sarcomeres Sarcomeres meet one another at an area called –

Z-line


Sarcomere

9-6

•


Muscle Histology


Sarcomere


Thick filaments – myosin Rod-like tail (axis) terminates in 2 globular heads or

cross bridges Cross bridges interact with active sites on thin

filamentsThin filaments – actin Coiled helical structure (resembles twisted strands of

pearls) Tropomyosin – rod-shaped protein spiraling around

actin backbone to stabilize it Troponin – complex polypeptides

One binds to actin One binds to tropomyosin One binds to calcium ions

Both help control actin’s interaction with myosin during contraction


Myofilaments

9-7


Sarcomere


Functional Structure Tropomyosin blocks myosin binding site

when muscle is at rest


Within sarcoplasm – 2 specialized membranous organelles

Sarcoplasmic reticulum (SR) Network of membranous channels that surrounds

each myofibril & runs parallel to it Same as ER in other cells SR has high concentrations of Ca ions compared to

the sarcoplasm (maintained by active transport – calcium pump)

When stimulated by muscle impulse, membranes become more permeable to Ca ions and Ca diffuses out of SR & into sarcoplasm


Within sarcoplasm – 2 specialized membranous organellesTransverse tubules (TT) Set of membranous channels that extends into

the sarcoplasm as invaginations continuous with muscle cell membrane (sarcolemma)

TT’s are filled with extracellular fluid & extend deep into the cell

Each TT runs between 2 enlarged portions of SR – cisternae Form a triad near the region where actin &

myosin overlap


Skeletal Muscle Contraction

Neuromuscular Junction (NMJ) – site where a motor nerve fiber & a skeletal muscle fiber meet (synapse or synaptic cleft)

Fibers must first be stimulated by a motor neuron for a skeletal muscle to contract

Motor Unit – 1 motor neuron & many skeletal muscle fibers The # of muscle fibers in a motor unit varies from 10 -

hundreds Motor End Plate – specific part of a skeletal muscle

fiber’s sarcolemma directly beneath the NMJ Neurotransmitter – chemical substance released from

a motor end fiber, causing stimulation of the sarcolemma of muscle fiber – acetylcholine (ACh)

Synaptic Cleft - small space between neuron & muscle


1

Axon terminal

Axon terminal

Axon collateral ofsomatic motor neuron

Sarcolemma

Myofibril

ACh is releasedfrom synaptic vesicle

Junctional fold

Synaptic vesiclecontainingacetylcholine(ACh)

Sarcolemma

Synaptic cleft(space)

Motor end plate


(a) Neuromuscular junction

(b) Enlarged view of the neuromuscular junction

(c) Binding of acetylcholine to ACh receptors in the motor end plate

Synapticend bulb

Synapticend bulb

Neuromuscularjunction (NMJ)

Synaptic end bulb

Motor end plate

Nerve impulse

11

Axon terminal

Axon terminal


Sarcolemma

Myofibril


ACh binds to Achreceptor

Junctional fold


Sarcolemma


Motor end plate





Synapticend bulb

Synapticend bulb


Synaptic end bulb

Motor end plate

Nerve impulse

Na+

1

22

1

Axon terminal

Axon terminal


Sarcolemma

Myofibril


ACh binds to Achreceptor

Junctional fold


Sarcolemma


Motor end plate





Synapticend bulb

Synapticend bulb


Synaptic end bulb

Motor end plate

Nerve impulse

Muscle action potential is produced

Na+

1

2

3

2


Stimulus for contractionBegins – motor impulse reaches the end of the motor

nerve fiber/ending – membrane depolarized (-70mV to -55mV)

Calcium ions rush into motor nerve fiber Neurotransmitter (acetylcholine) released in to NMJ

(exocytosis)Acetylcholine diffuses across the NMJ &

stimulates/depolarizes the motor end-plate (sarcolemma) -100mV to -70mV

The muscle impulse travels over the surface of the skeletal muscle fiber & deep into the muscle fiber by means of the TT

calcium moves from sarcoplasm reticulum into sarcoplasm


Contraction Cycle Myosin binds to actin & releases phosphate

(forming crossbridges) Crossbridge swivels releasing ADP and

shortening sarcomere (power stroke) ATP binds to Myosin → release of myosin

from actin ATP broken down to ADP & P → activates

myosin head to bind and start again Repeats as long as Ca2+ concentration is

high


Contraction Cycle


Changes in muscle during contraction H zones and the I bands narrow Regions of overlap widen Z lines move closer together Shortening the sarcomere

Sliding Filament Theory


Cross-bridge CyclingWhen calcium ions are present, myosin binding sites on

actin are exposed Cross-bridge attaches

ATP breakdown provides E to “cock” myosin head “Cocked” myosin attaches to exposed actin binding

site Cross-bridge (myosin head) springs from cocked

position and pulls on actin filament Cross-bridges break

ATP binds to cross-bridge Myosin heads are released from actin

*As long as Ca ions and ATP are present, this walking continues until muscle fiber is fully contracted

9-13


Relaxation

9-14

• acetylcholinesterase – enzyme present in NMJ•breaks down acetylcholine, so the motor end-plate is no longer stimulated •muscle impulse stops•calcium moves back from sarcoplasm into sarcoplasmic reticulum•myosin and actin binding sites are broken•Muscle fiber relaxes


ATP stored in skeletal muscle lasts only about six seconds

ATP must be regenerated continuously if contraction is to continue

3 pathways in which ATP is regenerated Coupled reaction with Creatine Phosphate (CP) Anaerobic Cellular Respiration Aerobic Cellular Respiration

Energy Sources for Contraction


Energy Sources for Contraction

9-15

• creatine phosphate – stores energy that quickly converts ADP to ATP

CP + ADP creatine + ATP


Production of ATP for Muscle Contraction


Glycolysis Break down glucose to 2 pyruvates 2 ATPs If insufficient oxygen, pyruvate → lactic acid Anaerobic Phase

occur in cytoplasm of cell Get about 30 – 40 seconds more activity

maximally


Oxygen Supply and Cellular Respiration

9-16

• Aerobic Phase• if oxygen present• citric acid cycle and electron transport chain• occurs in mitochondria• produces CO2 & most ATP • myoglobin stores extra oxygen


Production of ATP for Muscle Contraction


Muscle Fatigue Inability to contract forcefully after prolonged

activity No O2 is available in muscle cells to complete

aerobic respiration Pyruvic acid is converted to lactic acid Muscle fatigue & soreness

Results form a relative deficit of ATP and/or accumulation of lactic acid (decreases pH)


Fatigue Limiting factors can include:

Ca2+

Creatine Phosphate Oxygen Build up of acid Neuronal failure

cramp – sustained, involuntary contraction


Oxygen Debt

9-17

– amount of oxygen needed by liver to convert lactic acid to glucose


Heat Production

9-19

• Almost half of E released during muscle contraction is lost to heat - maintain body temperature


Muscle Tone Even at rest some motor neuron activity

occurs = Muscle Tone If nerves are cut fiber becomes flaccid (very

limp) very important in maintaining posture


Control of Muscle Contraction Single action potential (AP) → twitch

single contraction that lasts a fraction of a second, followed by relaxation

Total tension of fiber depends on frequency of APs (number/second) Require wave summation Maximum = tetanus

Total tension of muscle depends on number of fibers contracting in unison Increasing numbers = Motor unit recruitment


Muscular Responses

9-20

Threshold Stimulus• minimal strength required to cause contraction

Most muscle fiber contraction is “all or nothing”


Recording a Muscle Contraction Myogram – recording of a muscle contraction

latent period – delay between stimulation & contraction

refractory period – muscle fiber must return to its resting state (-100mV) before it can be stimulated

again all-or-none response

If a muscle fiber is brought to threshold or above, it responds with a complete twitch

If the stimulus is sub-threshold, the muscle fiber will not respond


Myogram


Staircase Effect (treppe)A muscle fiber that has been inactive can be subjected

to a series of stimuli & The fiber undergoes a series of twitches with

relaxation between & The strength of each successive contraction

increases slightlyPhenomenon is small & brief & involves excess

calcium in sarcoplasm


Summation

9-21

•When several stimuli are delivered in succession to a muscle fiber, it cannot completely relax between contractions•individual twitches combine and muscle contraction becomes sustained•When resulting sustained contraction lacks even slight relaxation, it is called tetanic contractions


Types of Contractions

9-24

• concentric – shortening contraction• eccentric – lengthening contraction

• isometric – muscle contracts but does not change length – attachments do not move, tensing a muscle


Fiber Types Slow oxidative (SO)- small diameter and red

Large amounts of myoglobin and mitochondria ATP production primarily oxidative Fatigue resistant

Fast oxidative glycolytic (FOG) Large diameter = many myofibrils Many mitochondria and high glycolytic capacity

Fast glycolytic fibers (FG) White, fast & powerful and fast fatiguing For strong, short term use


Recruitment Recruited in order: SO → FOG → FG


Effects of Exercise

SO/FG fiber ratio genetically determined High FG → sprinters High SO → marathoners

Endurance exercise gives FG → FOG Increased diameter and numbers of

mitochondria Strength exercise increases size and

strength of FG fibers


Fast and Slow Twitch Muscle Fibers

9-25

Slow-twitch fibers (type I)• always oxidative• resistant to fatigue• red fibers • oxygen containing pigment - myoglobin• good blood supply• contain many mitochondriaFast-twitch glycolytic fibers (type II)• white fibers (less myoglobin)• fewer mitochondria• contain extensive sarcoplasmic reticulum to store and reabsorb calcium• poorer blood supply• contract rapidly, but fatigue easily due to lactic acid accumulation

Fast-twitch fatigue-resistant fibers (type IIb)• intermediate fibers• oxidative• intermediate amount of myoglobin• pink to red in color


Cardiac Muscle• Self-exciting tissue (pacemaker) - auto-

rhythmicity Function as a “syncytium” (all or nothing) rhythmic contractions (60-100 beats/minute) longer refractory period than skeletal muscle

Striated, branched short fibers with single, central nucleus in each fiber Intercalated discs (thickened cell membranes) Gap junctions that allow spread of action

potentials


Cardiac Muscle Pumps blood to

Lungs for oxygenation Body for distribution of oxygen & nutrients


Cardiac Muscle


Smooth Muscle Involuntary Found in internal organs such as stomach,

bladder, walls of arteries Structure

Tapered cells each with single nucleus Filaments not regular so tissue does not appear

striated


Smooth Muscle Fibers

9-26

Compared to skeletal muscle fibers• shorter• lack transverse tubules• sarcoplasmic reticula are reduced• Contractions are slow & sustained• troponin is absent


Smooth Muscle Types

Visceral (single unit) Form sheets and are auto-rhythmic Contract as a unit

Multi-unit type Each has own nerve and can contract independently

Graded contractions and slow responses Often triggered by autonomic nerves Modulated chemically, by nerves, or by

mechanical events (stretching)


Types of Smooth Muscle

9-27

Visceral Smooth Muscle•fibers held together by gap junctions•exhibit peristalsis – wave-like motion that helps push substances through passageways•2 layers of muscle surround passageway•Inner circular layer•Outer longitudinal layer


Multiunit Smooth Muscle irises of eye walls of blood vessels contraction is rapid & vigorous (similar to

skeletal muscle tissue)


Smooth Muscle ContractionResembles skeletal muscle contraction• interaction between actin and myosin• both use calcium and ATP• both depend on impulses

9-28


Different from skeletal muscle contraction Calmodulin binds to calcium ions

(no troponin) activates the contraction mechanism

most calcium diffuses from extracellular fluid (reduced SR)

two neurotransmitters acetlycholine and norepinephrine

more resistant to fatigue


Smooth Muscle


Life-Span Changes

9-65

• myoglobin, ATP, and creatine phosphate decline in forties• by age 80, half of muscle mass has atrophied – replaced by connective & adipose tissue•Reflexes are reduced• exercise helps to maintain muscle mass and function


Homeostatic Imbalances/Disorders Poliomyelitis Myasthenia Gravis Duchenne Muscular Dystrophy Rigor Mortis Botulism TMJ Parkinson’s Disease


Clinical Application

9-66

Myasthenia Gravis• autoimmune disorder• receptors for acetylcholine on muscle cells are attacked• weak and easily fatigued muscles result• difficulty swallowing and chewing• ventilator needed if respiratory muscles are affected• treatments include• drugs that boost acetylcholine• removing thymus gland• immunosuppressant drugs• antibodies


Movement Muscles move one bone relative to another

around one or more joint(s) Origin → most stationary end

Location where the tendon attaches Insertion → most mobile end

Location where tendon inserts


Movement Generally arranged in opposing pairs

Flexors - extensors; abductors - adductors The major actor: prime mover/agonist Muscle with opposite effect: antagonist Synergists - help prime mover Fixators - stabilize origin of prime mover


Basis of Muscle Names: Table 8.2 Direction of fibers relative to body axes

Examples: lateralis, medialis (medius), intermedius, rectus

Size of muscle Examples: alba, brevis, longus, magnus, vastus

Shape of muscle Examples: deltoid, orbicularis, serratus,

trapezius


Basis of Muscle Names: Table 8.2 Action of muscle

Examples: abductor, adductor, flexor, extensor Number of tendons (heads) of origin

Examples: biceps, triceps, quadriceps Location of muscle

Examples: abdominus, brachialis, cleido, oculo-, uro-,

copyright 2010, john wiley & sons, inc. chapter 8 the muscular system

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