Human Anatomy and Physiology

Biology 231

Human Anatomy and Physiology

Chapter 10 Lecture Outline

Muscle Tissue – converts chemical energy to mechanical energy (contraction)

3 types of muscle:

skeletal – attaches to and moves skeleton

striated, voluntary control

cardiac – heart

striated, involuntary control (autorhythmicity)

smooth – walls of hollow organs and vessels

also associated with hair and inner eye

non-striated, involuntary control

Functions of Muscle

body movement

stabilizing body position

controlling movement of contents in hollow organs and vessels

generating heat – thermogenesis

Properties of Muscle

electrical excitability – responds to stimulus by producing an electrical signal

(action potential) which can travel (propagate) along the plasma

membrane

contractility – contracts forcefully when stimulated by an action potential

Anatomy of Skeletal Muscle

muscle fiber (cell) – individual cell; long, multinucleate

endomysium – sheath of elastic connective tissue around individual fibers;

contains capillaries and nerve endings supplying muscle fibers

fascicle – bundle of muscle fibers (10-100)

perimysium – dense irregular connective tissue sheath around fascicle

contains blood vessels and nerves

muscle – bundle of fascicles; function together

epimysium – dense connective tissue sheath around entire muscle

tendon – dense regular connective tissue continuous with all 3 sheaths

attaches muscle to periosteum of bone

aponeurosis – broad, flat tendon sheet

Nerve and Blood Supply to Muscle:

somatic motor neurons – stimulate muscle to contract

one neuron sends branches to multiple muscle fibers

neuromuscular junction – site of communication between motor neuron and

muscle fiber

plentiful capillary supply – supply nutrients and oxygen, remove heat and waste

Anatomy of a Muscle Fiber – formed by fusion of 100+ embryonic myoblast cells

muscle fibers cannot divide – number set at birth

sarcolemma – plasma membrane of muscle cell

T tubules – invaginations of sarcolemma into center of muscle fiber

open to interstitial space – full of interstitial fluid

sarcoplasm – cytoplasm of muscle cell

glycogen – storage form of glucose

myoglobin – red protein, binds oxygen

lots of mitochondria – ATP for contraction

sarcoplasmic reticulum – network of sacs and tubules

terminal cisterns – dilated sacs on either side of T tubules (triad)

stores calcium ions for muscle contractions

myofibrils – contractile protein fibers; have visible striations (stripes)

sarcomere – basic contractile unit of myofibril

thin filaments and thick filaments overlap to create striations

A band – appears dark

middle, thick filaments + overlap with thin

M line – center of A band; anchors thick filaments

I band – appears light

thin filaments only (ends of 2 sarcomeres)

Z lines – center of I band; separates sarcomeres

satellite cells – a few myoblasts remaining in adult muscle; help repair

damaged muscle

Muscle Proteins

3 kinds:

contractile proteins – cause sarcomere to shorten

myosin – about 300 form thick filaments

actin – thin filaments; have binding sites for myosin

regulatory proteins – switch contraction on and off

tropmyosin and troponin – form strands that cover myosin-binding sites

on thin filaments

structural proteins – align and stabilize myofibrils; give elasticity and

extensibility

titin – large protein; anchors thick filament to Z line

gives sarcomeres a degree of elasticity

Contraction and Relaxation of Skeletal Muscle

Sliding Filament Mechanism – thick and thin filaments slide over each other;

myosin heads attach to thin filaments and pull them closer to M line;

sarcomere shortens, length of filaments doesn’t change

Steps of Contraction

sarcoplasmic reticulum releases calcium ions

calcium ions bind to troponin on thin filament

frees myosin-binding sites on actin

Contraction Cycle:

1) myosin heads are energized – break down ATP and use energy to

become “cocked”

2) formation of cross bridges - energized myosin head binds to actin

at myosin-binding site

3) power stroke - myosin head pivots and releases ADP

pulls thin filament closer to M line

4) cross bridges detatch – occurs when ATP binds to myosin head

myosin heads “walk” up thin filament as long as ATP and calcium ions

are sufficient

pulls thin filaments towards M line, sarcomere shortens

300 myosin molecules/thick filament

Length-Tension Relationship – forcefulness of contraction depends on length of

sarcomere before contraction

optimal zone of fiber overlap = maximum tension

reduced overlap – fewer myosin heads can bind

increased overlap – fiber orientation disrupted, less binding

Excitation of Skeletal Muscle – electrical signal from nervous system

initiates contraction of sarcomeres; voluntary control

Neuromuscular Junction (NMJ) – site of communication (synapse) between

somatic motor neuron and muscle fiber

synaptic cleft – small gap between the neuron and muscle fiber

somatic motor neuron – axon branches end at synaptic terminals

synaptic vesicles – contain acetylcholine (ACh)

neurotransmitter – chemical released by neuron in synaptic cleft;

diffuses across cleft and binds to receptor on sarcolemma of muscle fiber and initiates a response

muscle fiber

motor end plate – ACh receptors form ligand-gated ion channels

channels open when Ach binds to them

Steps of Excitation

1) Release of ACh – electrical impulse in neuron causes exocytosis of

synaptic vesicles; ACh released in synaptic cleft

2) Activation of ACh receptors – ACh diffuses to motor end plate;

ACh binds to receptors; ligand-gated channels open to small

cations (mainly Na+ due to Na+/K+ pump)

3) Production of muscle action potential – Na+ flows into muscle fiber

near sarcolemma; electrical charge in cell becomes more positive;

change in charge opens voltage-gated Na+ channels in sarcolemma;

wave of electrical current travels (propagates) along the sarcolemma and T tubules

4) Termination of ACh activity – ACh broken down rapidly by

acetylcholinesterase (AChE) in synaptic cleft

by-products taken up by neuron to be recycled

NMJ located near center of muscle fiber – action potential propagates

towards both ends rapidly

botulism – toxin prevents release of ACh (Botox)

curare – blocks ACh receptors

neostigmine – anticholinesterase agent (antidote for curare)

Excitation-Contraction CouplinG – action potential triggers

contraction of muscle

1) Action potential travels down T tubules

2) Calcium channels in SR membrane open – triggered by action

potential; calcium ions diffuse out of SR

3) Calcium ions bind troponin on actin filaments

4) Myosin-binding sites are exposed – contraction begins

Ca ions pumped back into SR by active transport pumps using ATP

Rigor Mortis – begins 3-4 hours after death, lasts about 24 hours

no ATP synthesis

calcium ions leak out of SR – myosin heads bind and can’t detatch

ends when lysosomal enzymes digest proteins

A single nerve impulse causes a single action potential in each muscle

fiber it synapses with.

The action potential is always the same size (all-or-none) and causes

minimal contraction of the muscle fibers.

twitch – brief contraction due to a single action potential

Amount of contraction (tension) in a muscle fiber depends mainly on

frequency of nerve stimulations arriving and availability of ATP

Muscle Fiber Metabolism

Sources of ATP:

free ATP – few seconds of contraction

creatine phosphate – stores high energy phosphate groups from ATP; passes

phosphates to ADP as it accumulates during contraction (100m dash)

anaerobic cellular respiration (no oxygen used)

glycolysis – glucose is broken down into 2 pyruvic acids with a net

gain of 2 ATP/glucose molecule (400 meter dash)

(pyruvic acid converted to lactic acid, diffuses into blood, liver converts

back to glucose)

aerobic cellular respiration (oxygen required)

pyruvic acid from glycolysis enters mitochondria – completely broken

down to carbon dioxide and water

produces 36 ATP/glucose molecule

produces 95% of ATP; used in prolonged activities

(can also use lipids from adipose cells and amino acids from proteins)

Sources of Glucose:

breakdown of glycogen stores in muscle fibers

facilitated diffusion into muscle fiber from bloodstream

Sources of Oxygen:

release from myoglobin in muscle fibers

diffusion from blood capillaries

Muscle Performance

motor unit – a somatic motor neuron and all of the muscle fibers it

stimulates (avg. 150)

fine movements – small motor units (2-20 fibers)

large, powerful movements – large motor units (2-3 thousand fibers)

strength of contraction depends on:

size of motor unit and frequency of stimulation

number of motor units stimulated

Myogram – record of muscle contraction

stimulus – nerve impulse resulting in an action potential

latent period – delay before contraction begins calcium ions released, elastic components stretch

contraction phase – sarcomeres shorten

relaxation phase – calcium pumped into SR, sarcomeres relax

wave summation – repeated stimulations before relaxation is complete

causes stronger contraction

unfused tetanus – sustained, wavering contraction

fused tetanus – sustained, maximal contraction

occurs when no relaxation is allowed between stimulations

(treppe – “warming up” effect)

muscle fatigue – inability to contract forcefully after prolonged activity

reduced calcium release from SR and ACh from NMJ

depletion of oxygen, glycogen, and nutrients

build-up of lactic acid and ADP

damage to muscle fibers

oxygen debt and recovery – increased use of oxygen after exercise

resynthesis of glycogen, creatine phosphate and ATP

reoxidizing myoglobin

increased body temperature, heart and respiratory rates

tissue repair

delayed onset muscle soreness – 12-48 hrs after exercise

microscopic damage – torn sarcolemmas, myofibrils, and

Z discs

blood proteins seen when muscle is damaged – myoglobin,

creatine kinase (CK)

motor unit recruitment – motor units within one muscle alternately contract

and relax

delays muscle fatigue, smoothes motion

muscle tone – small degree of tension maintained when not using muscle by

alternating activity of small groups of motor units;

regulated by involuntary nerve functions

flaccid muscle – loss of nerve stimulation

Types of Muscle Contractions

Isotonic Contractions – change length of muscles to move body parts

concentric contractions – muscle shortens

eccentric contractions – muscle lengthens

Isometric Contractions – create tension equal to stretching force on muscle;

maintain posture and stabilize joints

no movement occurs

Types of Skeletal Muscle Fibers

vary in content of myoglobin, capillaries, mitochondria and glycogen

variable speed of contraction cycle due to differences in ATPase

variable sources of ATP

1) Slow Fibers

smallest diameter – least powerful

dark red - lots of myoglobin and capillaries = lots of oxygen

many mitochondria – ATP mainly from aerobic respiration

slow twitch - slow ATPase (slow contraction cycle)

slow but very resistant to fatigue – posture and endurance activities

2) Fast Fibers

largest diameter – most powerful

white - little myoglobin and few capillaries = little oxygen

few mitochondria, lots of glycogen – ATP from glycolysis

fast-twitch - fast ATPase

fast and strong but fatigue rapidly – strength and speed activities

3) Intermediate Fibers

intermediate diameter

pink - little myoglobin but more capillaries

mitochondria and glycogen – ATP from aerobic and anaerobic processes

fast-twitch - fast ATPase

intermediate properties – faster and more fatigue resistant, but less

strength and endurance

Distribution of Fibers – most muscles have all 3 types

postural muscles (back and neck) – high in slow fibers

shoulders and arms – high in fast fibers

legs (postural and active) – high in slow and intermediate fibers

motor units are composed of 1 fiber type

recruitment order – slow, intermediate, fast

ratio of fast and slow twitch fibers is genetic

Regeneration of Skeletal Muscle

muscle fibers don’t divide

satellite cells regenerate fibers to a small degree

muscle growth is due to hypertrophy (increased cell size due to more thick

and thin filaments)

significant muscle damage results in fibrosis

Cardiac Muscle - heart

similar arrangement of myofibrils – striated

shorter, branched muscle fibers – usually one nucleus

intercalated discs – connect muscle fibers by desmosomes

have gapjunctions – allow ions to diffuse between cells

muscle action potential spreads rapidly throughout cardiac muscle fibers

autorhythmic muscle fibers (pacemaker) – contracts and relaxes 75 times/min

while resting; involuntary contractions

mainly aerobic respiration – large numbers of mitochondria

(during exercise can use lactic acid to form ATP)

prolonged contraction due to calcium channels in sarcolemma

little regenerative ability; hypertrophy due to aerobic exercise

Smooth Muscle

walls of blood vessels and hollow organs, arector pili muscles, inner eye

spindle-shaped, one nucleus

thick and thin filaments have no regular arrangement (no striations)

contractile fibers anchored to dense bodies by intermediate filaments;

contraction draws dense bodies closer together

can stretch a lot and maintain contractile function

involuntary stimulation – respond to stretching, chemicals, hormones,

autonomic nerve impulses

gap junctions connect muscle fibers – contract in unison

contractions develop slowly and last longer than in skeletal muscle

calcium ions enter mainly from interstitial fluid (little SR)

maintains smooth muscle tone

more regenerative capacity than skeletal or cardiac muscle

pericytes – stem cells around capillaries and venules

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