OVERVIEW

Muscle type

1. Skeletal muscle – striated, voluntary control, long, cylindrical, multinucleated

2. Cardiac muscle – striated, involuntary control, branched, short, uni or binucleated

3. Smooth muscle – not striated, involuntary control, short, spindle shaped

Muscle function

1. Movement – movement; blood through heart; food, urine, others by smooth muscle

2. Postural maintenance – constant adjustments being made by skeletal muscle

3. Joint stability – as mentioned in previous chapter

4. Heat generation – skeletal muscle most important in this respect

Functional characteristics

1. Excitability (irritability) – the ability to receive and respond to stimuli

2. Contractility – the ability to shorten forcibly

3. Extendibility – muscle can stretch beyond resting size when relaxed

4. Elasticity – the ability to return to its resting length after being stretched

SKELETAL MUSCLE ANATOMY

Gross anatomy: connective tissue wrappings

1. Endomysium – fine layer of areolar connective tissue; surrounds each muscle fiber

2. Perimysium – collagenic sheath which surrounds bundles of muscle fibers

-a. Fascicles – name of the bundles of muscle fibers surrounded by the perimysium

3. Epimysium – course sheet of fibrous connective tissue; surrounds the entire muscle

4. Deep fascia – fibrous connective tissue; binds separate muscles into functional groups

Gross anatomy: attachments

1. Direct – epimysium to the bone (periosteum) or cartilage (perichondrium)

2. Indirect – more frequent method; epimysium continues beyond the muscle it covers

-a. Tendon – when this continuation is cord like

-b. Aponeurosis – when this continuation is sheet like

Microscopic anatomy: general

1. Muscle fiber – cell, very thick and very long (30 cm)

2. Sarcolemma – special name given to the plasma membrane of muscle fibers

3. Sarcoplasm – name given to the cytoplasm of muscle fibers

-a. Myoglobin – oxygen binding molecule similar to hemoglobin

-b. Glycogen – to provide all the energy needed

Microscopic anatomy: myofibril

1. Myofibrils – cylindrical structures, make up muscle fibers; ability to contract

2. Bands – the different bands of the myofibrils line up; muscle fiber as a whole, striated

3. A band – the darker bands

-a. H zone – middle portion of the A band which is slightly lighter then the rest

-b. M line – slightly darker line which runs in the middle of the H zone

4. I band –the lighter bands

-a. Z disc – slightly darker disc seen in the middle of the I bands

5. Sarcomere – the functional unit; the part of the myofibril between two Z discs

6. Myofilaments – two types; make up sarcomere

Microscopic anatomy: myofilament

1. Thick filaments – also the darker of the two filaments

-a. Myosin – hundreds form a single thick filament

-b. Head – each myosin; two globular ends; binds to actin during contraction

-c. ATPase – head of each myosin; it provides the energy for the contraction

-d. A band – the darker band is the result of arrangement of these thick filaments

-e. H zone – part of A band not overlapped by thin filaments

-f. M line – slightly darker because of fine strands; hold adjacent thick filaments together

2. Thin filaments – the lighter thinner microfilament

-a. Actin – G actin; looks like a twisted string of pearls

-b. Myosin binding site – found on each molecule of G actin

-c. Tropomyosin – two strands; spiral along the light filament block myosin binding sites

-d. Troponin molecules (complex) – holds the tropomyosin in place when muscle fiber is

-e. Z disc – coin shaped protein sheet, anchors thin filaments together, joins sarcomeres

Microscopic anatomy: sarcoplasmic reticulum and T tubules

1. Sarcoplasmic reticulum – smooth ER; surrounds a myofibril with interwoven tubules

2. Terminal cisternae – perpendicular chambers (pair) where the I and A bands meet

-a. Calcium ion – stores calcium and releases it when the muscle cell is stimulated

3. T (transverse) tubules – extensions of sarcolemma; through the cytoplasm; extracellular

-a. Impulse – stimulated and impulse runs along the sarcolemma and down the T tubules

4. Triads – three membranous structures; terminal cisterna, T tubule, terminal cisterna

MUSCLE FIBER PHYSIOLOGY

Nerve-muscle relationship

1. Somatic motor fibers – axons in nerves; bodies in brain or spinal cord

2. Motor unit – one nerve fiber and all the muscle fibers stimulated by it

3. Synapse – the functional connection between nerve and its target cell

4. Neuromuscular junction – when target cell is muscle fiber; synapse called

5. Axonal terminal – axon branches with each branch terminating in an axonal terminal

-a. Neuromuscular junction – several axonal terminals to the same muscle fiber

6. Synaptic cleft – glycoprotein filled space between axonal terminal and muscle fiber

7. Motor end plate – highly folded depression in the muscle fiber found

8. Acetylcholine receptors – millions of receptors found on the folds of motor end plate

9. Nerve stimulus – results in

-a. Nerve impulse – travels down the membrane of the axon

-b. Calcium channels – voltage regulated; impulse reaches; open and calcium floods in

-c. Calcium influx – as a result of calcium channels opening

-d. Synaptic vesicles – contained in the axonal terminal

-e. Acetylcholine – in the vesicles

-f. Exocytosis – calcium; acetylcholine containing vesicles fuse with plasma membrane

Electrically excitable cells

1. Depolarization – chemically activated sodium channels opened; become less negative

2. Action potential – great enough; voltage dependent sodium channels open; propagating

3. Repolarization – immediately follows depolarization wave

-a. Potassium ion channels – open, potassium moves out; resting membrane potential

4. Refractory period – time it takes for repolarization, muscle fiber cannot be stimulated

5. Na+,K+ ATPase – restore normal ionic conditions; sodium out, potassium in

6. All-or-none response – action potential propagated along entire fiber or not at all

7. Acetylcholine esterase – enzyme, sarcolemma quickly destroys the acetylcholine

Excitation

1. Acetylcholine – travels across the synaptic cleft to the motor end plate

2. Acetylcholine receptors – acetylcholine attach; chemically regulated Na⁺ channels

-a. Ligand – gated ion channel – chemically gated ion channels

3. Action potential – moves out from motor end plate

Excitation-contraction coupling

1. Action potential – from motor end plate along the sarcolemma and down the T tubules

2. Calcium ions – are released from the sarcoplasmic reticulum

3. Low calcium concentration – results in

-a. Troponin – conformation holds the tropomyosin in place

-b. Tropomyosin – blocks myosin attachment on G actin

4. High calcium concentration – results in

-a. Troponin – conformation moves tropomyosin out of its myosin binding site

-b. Tropomyosin – moves out of myosin binding site

-c. Myosin binding site – no longer blocked

5. Cross bridge attachment – myosin binds to its binding site on the actin molecule

6. Power stroke – after it attaches, it pivots to its low energy, bent configuration

-a. Thin filament – pulled toward the center of the sarcomere

-b. ADP – released

-c. Inorganic phosphate – also released

7. Cross bridge detachment – ATP binds myosin head; myosin looses attachment, myosin

8. Myosin ATPase – head, ATP hydrolyzed ADP, inorganic phosphate; energy released

–a. Myosin head – to its higher energy, straightened configuration

9. Sliding filament theory – thin filaments slide over thick filaments; more overlap

Sliding filament theory

1. Mechanism (theory) – thin filaments slide over thick filaments; more overlap

2. Hugh Huxley – proposed this mechanism 1954

Relaxation

1. Nerve stimulus – stops; acetylcholine not released

2. Acetylcholine esterase – breaks down acetylcholine

3. Active calcium pump – pumped out of sarcoplasm

4. Troponin – changes conformation

5. Tropomyosin – blocks binding site

WHOLE MUSCLE PHYSIOLOGY

Muscle twitch

1. Myogram – a diagram representing the contraction of a muscle

2. Muscle twitch – response of a motor unit to a single action potential

3. Phases – all muscle twitches have three phases

-a. Latency period – from stimulation and actual contraction; excitation-contraction

-b. Contraction period – from beginning of contraction to peak of tension development

-c. Relaxation period – removal of calcium from the sarcoplasm; muscle tension to zero

4. Differences – in twitch duration myofibril metabolism

-a. Extrinsic eye muscles – fast twitch; precise movements

-b. Calf muscles – slow twitch; less precise movements

5. Refractory period – period of lost excitability; has already been stimulated

-a. Skeletal muscle – short refractory period, about 5 milliseconds

-b. Cardiac muscle – long refractory period, about 300 milliseconds

Graded muscle response

1. Graded response – cause smooth contractions; strength vary

2. Wave (temporal) summation – vary frequency of stimuli; smoothes contraction

-a. Wave summation – second stimulus, stronger; does not have time to relax completely

-b. Unfused (incomplete) tetanus – time between stimuli, shorter; less relaxation between

-c. Fused (complete) tetanus – muscle relaxation disappear; contractions fuse; sustained

3. Motor unit recruitment – varying number of motor units stimulated

-a. Stimulus strength – increased voltage of increased number of motor units

-b. Motor units – typically do not contract in unison

Treppe: the staircase effect

1. Treppe – first contraction is weakest; subsequent stimuli result in stronger contractions

2. Calcium availability – its concentration in the sarcoplasm becomes greater and greater

3. Heat – generated by muscle work results in enzyme systems becoming more efficient

Muscle tone

1. Muscle tone – the slightly contracted state of relaxed muscle; result of

2. Spinal reflexes – causes muscle tone

3. Motor units – alternatively stimulated

4. Joint stability – results from it

4. Posture – also the result of muscle tone

Isotonic and isometric contractions

1. Isotonic contractions – the tension generated changes the length of the muscle

-a. Concentric – muscle shortens as it contracts

-b. Eccentric – muscle lengthens as it contracts

2. Isometric contractions – no change in length; maintain posture or joint stability

MUSCLE METABOLISMS

Providing energy

1. ATP – cross bridge movement, detachment, ionic pumps; 4 to 6 seconds worth stored

2. Creatine phosphate – transfers phosphate group to ADP to generate new ATP; 15 sec.

3. Anaerobic glycolysis – 1st part cell respiration; oxygen not needed; 2 ATP ; 60 sec.

-a. Lactic acid – from pyruvic acid; into blood; liver, back to pyruvic acid, glucose

4. Aerobic respiration – oxygen; 36 ATP produced; sustained exercise for hours

5. Sports activities – duration ATP sources vary

-a. Aerobic endurance – time using aerobic sources of ATP; long time, less strength

-b. Anaerobic endurance – time using anaerobic sources of ATP; less time, more strength

Muscle fatigue

1. Muscle fatigue – production of ATP no longer keep up with demand; do not contract

2. Contractures – cross bridges no longer able to detach; continual contraction or cramp

3. Lactic acid – local build up causes pH to decrease and muscles begin to ache

4. Ionic imbalances – inactive ATP pump; potassium lost, sodium enters; unresponsive

Oxygen debt

1. Restoration – rest; glucose, oxygen, ATP, glycogen, creatine phosphate restored

2. Oxygen debt – the amount of oxygen for these processes to bring back to resting state

Heat production

1. Source – 40% efficient; converted to movement; rest converted to heat

2. Metabolism activities – speed up; too much denature

3. Homeostasis – keep temperature within limits

-a. Dermal capillaries – are opened to cool body off

-b. Shivering – involuntary muscle contractions build up heat in response to cold

FORCE, VELOCITY, AND DURATION OF CONTRACTION

Force

1. Number of muscle fibers stimulated – more motor units stimulated, greater the force

2. Relative size – the larger the muscle the more tension it can generate

3. Series elastic elements – connective tissue covering and tendons; must be pulled taut

4. Degree of muscle stretch – optimal about 120% of their resting length

-a. Too little – overlap of actin and myosin lessened distance when crossbridging occurs

-b. Too much – not good; no overlapping; contraction could not take place

Velocity and duration of contraction

1. Load – the greater the load the less the velocity and duration of skeletal muscle

2. Muscle fiber type – slow and fast pertain to the type of ATPase in the myosin head

-a. Slow oxidative fibers – aerobic;↑ mito., myoglobin, blood supply; endurance, posture

-b. Fast oxidative fibers – aerobic;↑ mito., myoglobin, blood supply; walking

-c, Fast glycolytic fibers – anaerobic; ↓ mito., myoglobin, blood; high glycogen; power

Exercise

1. Aerobic (endurance) exercise – such as biking

-a. Benefits – increases capillaries, myoglobin, and mitochondria; good for other systems

2. Resistance (anaerobic) exercise – such as weight lifting

-b. Benefits – increase the size of muscle fibers (hypertrophy), mostly the fast glycolytic

3. Cross training – use of both aerobic and anaerobic exercises is best

SMOOTH MUSCLE

Arrangement of smooth muscle cells

1. Sheets – typical arrangement of muscle fibers around hollow organs

2. Longitudinal layer – along the long axis of organ; contractions dilate and shorten

3. Circular layer – around the lumen of the organ; contractions constrict and lengthen

Microscopic anatomy

1. General – spindle shaped cells with single central nucleus

2. Innervation – lacks highly structured neuromuscular junctions

-a. Varicosities – many bulbous swellings of the terminal endings of the innervating axon

-b. Diffuse junctions – wide synaptic cleft; neurotransmitters released in general area

3. Calcium source – differs from skeletal muscle in that much of it is extracellular

-a. Caveoli – infoldings of plasma membrane; high in calcium ions

-b. Sarcoplasmic reticulum – some from these; most from above

4. Intermediate filaments – noncontractile; attached to dense bodies

5. Dense bodies – attached to sarcolemma; anchor thin filaments

6. Myofilaments – not arranged into myofibrils; spiral down long axis; diffuse

-a. Thick filament – less than skeletal; actin binding heads along entire length

-b. Thin filament – attached to dense bodies (act like Z disc); no troponin complex

Contraction

1. Calcium – stimulation enters smooth muscle cell, mostly from the extracellular space

2. Calmodulin – is activated by calcium; activates kinase on myosin

3. Kinase – myosin, transfers P from ATP to the myosin react with action thin filament

4. Relaxation – when cytoplasmic calcium levels fall

Special characteristics

1. Duration – slow and sustained taking about 30 times longer to contract and relax

2. ATP efficient – due to extremely slow ATPase (myosin light chain kinase)

3. Aerobic respiration – low energy requirements; most ATP comes from this

4. Regulation – action potential generated like skeletal muscle

-a. Autonomic control – unconscious control; not somatic control

-b. Neurotransmitter receptors – those which cause and those which inhibit contraction

-c. Other stimuli – hormones, carbon dioxide, low pH; some depolarize spontaneously

5. Other special features – include

-a. Response to stretch – while they will contract more powerfully to increased stretch

-b. Hyperplasia – some capable of increases size; others in number; puberty, pregnancy

Types

1. Single unit smooth muscle – viscera, stimulated as a group to contract rhythmically

-a. Gap junctions – between cells; action potential propagated from one cell to another

2. Multiunit smooth muscle – large arteries, pupil, arrector pilli; own stimulating neuron

REGENERATION AND DEVELOPMENT

Regeneration

1. Skeletal muscle – can not divide

-a. Hypertrophy – increase in size and not number

-b. Hyperplasia – not an increase in number

-c. Satellite cells – fuse with existing fibers; repair damage

-d. Fibrosis – mostly replaced by scar tissue

2. Cardiac muscle – maybe some limited regeneration; not of much importance

-a. Hypertrophy – hearts can increase in size

3. Smooth muscle – more so than others; less than most tissue

-a. Pericytes – stem cells from which new smooth muscle develops

Development

1. Mesoderm – most develops from this

2. Somites – most of skeletal muscle except head and limbs