Muscle Physiology

SKELETAL MUSCLE

bundles called “fascicles”, fascicles have bundles called “fibers”, and fibers have muscle cells called “myofibrils”

Muscle strength based on the number of myofibrils and sarcomeres

BIG PARTS TO SMALL PARTS

Deep fascia: connective tissue that surrounds muscle groups with common actions

Epimysium “Above muscle”: sheet that separates individual muscles (ex. Italian sausage casing)

Perimysium “around muscle”: around fascicles and divide muscles into parts

Endomysium “Inside muscle”: separate fascicles into fibers

Sarcomeres “flesh part”: basic functional unit of muscle, contract for muscle contraction

• electrical impulses cause them to shorten

• 10K sarcomeres: 1 myofibril

• sarcolemma: specialized endomysium layer that surrounds myofibrils, connects to them through T tubules

  • separates sarcoplasm from interstitial fluids

• Sarcoplasmic reticulum: bunch up arounf the T tubule, web of myofibrils, T tubles pinch the SR

ZONES: A , Z disk/line, M line, density of filaments give striations (darker bands = myosin and actin) (lighter bands (actin OR myosin)

Conservability of Muscles

1. Contractility: shortening muscle cells

2. Excitability: responsiveness

3. Extensibility: lengthening of muscle cells

4. Elasticity: ability to bring muscle back to OG length

Sarcomere movement

myofilaments slide to shorten muscle (contraction), fast process

Muscle Synapses: chemical and electrical synapses (chemical ones cause electrical ones to be produced)

• chemical synapse: abundant, relies on neurotransmitters

• electrical synapses: rare, locked together with gap junctions

Plasma membranes in neurons and muscle cells are excitable

• can carry electrical currents

• resting potential -60mV and -90mV

Sarcomere Contraction

watch a video since you were late this day

Sarcomeres shorten and pull on the things next to them (ec. muscles, tendon)

• called “muscle tension”

muscles are not regulated by the body

VIDEO NOTES

Sarcomere regions:

A bands- dark bands

• H zone: in the A band, split by M line

I band- light bands

• Z disc

Myosin: thick filament, extend across A band, connect at M line

Actin: thin filament, extends across I band into A band, subunits have active sites where myosin can bind, tropomyosin (spiral strands) blocks these sites

elastic filaments containing titin

Myosin protein: globular ATP/actin binding sites, makes cross bridges

Troponin: has 3 peptides

1. binds actin

2. binds tropomyosin

3. binds calcium

Sarcoplasmic reticulum: tubules around a myofibril, regulate calcium (aids muscle contraction) through storage and secretion

• T-tubules: sit where A and I bands meet, help signals reach all portions of muscle cell

SLIDING FILAMENT MODEL

1. Nervous system stimulates muscle fibers

2. myosin heads (thick fil.) bind with actin filaments (thin fil.)

3. myosin head pulls actin filaments to center of sarcomere, shortening it

   1. Z discs move to M line

   2. I bands shorten

   3. H band disappears

   4. A bands get closer

NEUROMUSCULAR JUNCTION

Axon terminals: area where motor neuron meets the muscle

Synaptic Cleft: space where junctional folds form on postsynaptic membrane, accepts acetylcholine with receptors

Sodium Potassium pump\

PHASES OF TWITCH

1. Latent Twitch: beginning of contraction, from 0s to time contraction begins

2. Contraction Phase: tension increases, myosin-actin bridges form

3. Relaxation phase: Ca2+ levels drop, tension decreases, myosin-actin bridges detach

• Faster in an eye muscle than a calf muscle

TENSION TYPES

1. Peak Tension: most amount of tension, regulated by motor unit + cell bodies

2. Treppe: Muscles slowly increase tension over 30-50 muscle stimulations, doesn’t hit 0, “staircase” in German

3. Wave Summation: twitches do not cause relaxation and increase in intensity over time

4. Incomplete Tetanus: near peak tension, rapid cycles of relaxation and contraction, “holding up heavy objects”

5. Complete Tetanus: complete tension from constant stimulation, no relaxation bc bridge cannot relax; bacterial toxins that cause tetanus exhibits similar effects

Recuruitment: exciting other motot units, smooth and steady increase in tension, max #of motor units uased during tetanus, short time frame, make less than max tension, motoor units relax in rotation

CARDIAC MUSCLE

intercalated discs: contain gap junctions: a channel that conducts current between cells

desmosomes: anchors cells during contractions

SMOOTH MUSCLE

generally in arteries and GI tract

cells divde fast

Vasocosities

Phasic Contraction: rapid

Tonic Contraction: slow, controlled contraction and relaxation

Stress-Relaxation: mechanical stress causes contraction, immediately followed by relaxation

SPINAL CORD REFLEXES

Reflex: rapid, automatic responses to stimuli, involves dorsal root ganglion, ignore brain

Muscle stretch: one side stretches, other side is inhibited

flexor reflex

crossed extension reflex: simultaneous, opposing activity to maintain posture

Pain » dorsal root ganglia» spinal cord» Interneuron relays signals »effector muscles

SAME DAVE (Sensory/Dorsal, Afferent, Ventral, Efferent)

Main Classifications of Muscle Contraction

• isometric: muscle doesn’t change length and makes enough tension to remain still, many muscles do this (holding weight)

• Isotonic: change the length but keep constant tension (actually exercising)

  • concentric: muscles shorten and tension increases to exceed the force of gravity (to be able to lift weight and hold it)

  • eccentric: lengthening muscles and decreasing tension (to be able to lower the weight and hold it)

• Slow Oxidative Muscles: contract slowly, use aerobic repiration to make ATP, red, high fatigue resistance (endurance)

• Fast Oxidative Muscles: contract quickly, aerobic respiration to make ATP, pink, moderate fatigue resistance (no particular exercise)

• Fast Glycolytic Muscles: contract quickly, anaerobic glycolysis, white, poor fatigue resistance

• FO + FG5 v SO is determined by genes

• FO:FG ratio can change with exercise (makes FG act like FO)