Muscle Histology and Physiology

Lecture 6

functions of the muscular system

1. movement of the body

2. maintenance of posture

3. respiration

4. heat production

5. constriction of organs and vessels

6. contraction of the heart

muscle qualities

1. excitability

2. contractility

3. extensibility

4. elasticity

skeletal muscle tissue

voluntary motion, posture

smooth muscle tissue

all involuntary actions outside of heart (includes BLOOD VESSELS)

cardiac muscle tissue

provides pumping of blood through circulatory system

skeletal muscles are innervated by…

motor neurons

whole muscles are made up of bundles of…

fascicles

fascicles are made up of bundles of…

myofibers

myofibers are made up of bundles of…

myofibrils

myofibrils contain...

myofilaments

myofibers are surrounded by…

endomysium

myofilaments contain thousands of…

sarcomeres

A bands

entirety of myosin and any overlapping actin

H zone

region in A band where actin and myosin do NOT overlap (only myosin)

M line

middle of H zone and A band

I bands

contains actin, but no myosin

Z-disk

protein that attaches actin myofilaments

titin filaments

elastic chains of amino acids that provide elasticity

during contraction, what disappears in the sarcomere?

H zone and I band

what part of the sarcomere never changes in length?

A band

what is the arrangement of the sarcomere? (shape)

hexagonal

myosin proteins contain…

two heavy and four light chains

tropomyosin

a long protein that winds along the groove of the F actin double helix

troponin

protein composed of 3 subunits (one binds to actin, one binds tropomyosin, one binds to calcium ions)

muscle fiber contraction steps

1. stimulation of neuromuscular junction

2. muscle fiber depolarization

3. excitation-contraction coupling

4. cross bridge formation and cycling

neurotransmitter that activates skeletal muscles

acetylcholine

effects of acetylcholine binding to ligand-gated ion channels

sodium in, potassium out (3:2 ratio)

enzyme that degrades acetylcholine

acetylcholinesterase

Myasthenia Gravis

autoimmune disease, ACh receptors are attacked by one’s own antibodies (fatigue, muscle weakness, eyelids drooping)

resting potential of a muscle cell

-90 mV

muscle cell threshold potential

-70 mV

after a muscle cell has picked up an action potential, it travels into…

T tubules

where is Ca2+ held in a muscle cell?

terminal cistern of the sarcoplasmic reticulum

what does calcium bind to?

troponin

cross bridge formation

energized myosin head attaches to an actin myofilament

the power (working) stroke

ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state

cross bridge detachment

after ATP attaches to myosin, the link between myosin and actin weakens and the myosin head detaches

cocking of the myosin head

as ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy

Duchenne Muscular Dystrophy (DMD)

sarcolemma tears easily, allowing entry of excess calcium which damages contractile fibers

Rigor Mortis

calcium not pumped back into sarcoplasmic reticulum; myosin binds to actin and stays there (no ATP)

muscle tone

always contracted state, maintains overall readiness

isometric contraction

muscle does not shorten but contracts (isoMetric DOESN’T Move)

isotonic contraction

changes length because muscle tension exceeds load

concentric contractions

muscle overcomes a load and the muscle becomes shorter (raising)

eccentric contractions

muscle lengthens while contractile tension remains (lowering)

muscle twitch

contraction of a single muscle fiber in response to a single action potential from one motor neuron

phases of muscle twitch

1. latent period

2. contraction

3. relaxation

latent period

excitation-contraction coupling; no muscle tension seen

contraction

cross bridge formation, tension increases

relaxation

Ca2+ reentry into SR, tension returns to 0

treppe

increased strength of stimulation while allowing for full relaxation

temporal summation

additional stimuli delivered before relaxation is complete, wave summation

unfused tetanus

higher stimulation frequency, muscle quivering

fused tetanus

even higher stimulus frequencies, muscle fatigue

recruitment

stimulus is sent to more muscle fibers according to the size principle

size principle

motor units with smallest muscle fibers are recruited first, larger and larger fibers are recruited as stimulus intensity increases

slow oxidative fibers

aerobic, fatigue-resistant, low-intensity endurance activities

fast oxidative fibers

aerobic, medium-intensity activities

fast glycolytic fibers

anaerobic, short powerful movements

hypertrophy

muscle growth, from exercise

resistance exercise

weight-lifting: increase in muscle size, increase in myofibrils, increase in nuclei and mitochondria

aerobic exercise

endurance: increase circulation, number of mitochondria, convert fast glycolytic fibers into fast oxidative fibers that resist fatigue

atrophy

muscle loss, from lack of exercise

pathways for ATP production

1. creatine phosphorylation

2. anaerobic pathway

3. aerobic pathway

creatine phosphorylation

forms a new ATP molecule for immediate energy; can sustain short energy burst for about 15 seconds

aerobic respiration

glycolysis → Krebs cycle → oxidative phosphorylation
*32 ATP for one glucose

anaerobic respiration

pyruvate converted into lactic acid; yields 5% of ATP production, but quickly (30-40 seconds)

fatigue

physiological inability to contract despite continued stimulation

fatigue is caused by…

1. ion imbalances

2. decreased ATP

recovery

post exercise requires oxygen levels to rise to normal

oxygen debt

during exercise, body is trying to catch up to O2 levels necessary; post workout, need to breath heavily until O2 levels are normalized

smooth muscle fiber shape

spindle-shaped

longitudinal layer

SMOOTH muscle, contraction causes organ to shorten (move forward)

circular layer

SMOOTH muscle, contraction causes lumen of organ to constrict (no backward movement)

visceral or unitary

SMOOTH muscle, cells in sheets function as a unit; waves of contraction

multiunit

SMOOTH muscle, cells act as independent units; sheets, bundles, single cells

smooth muscle neuromuscular junctions

varicosities

smooth muscle T tubules

caveolae

smooth muscle connected via

gap junctions

smooth muscle calcium binds to

calmodulin

activated calmodulin activates…

myosin light chain kinase

cardiac muscle is controlled

autorhythmically

cardiac muscle electrical signal is made by

sinoatrial node

cardiac muscle is connected by

gap junctions