Physiology 1: Excitation And Contraction Of Muscle Chp6

Front 1

structures of muscle

Back 1

sarcomere (smallest fx unit) --> myofibril --> muscle fiber --> fiber bundle --> muscle

Front 2

histology of muscle

Back 2

-subdivided by z plates into functional subunits called sarcomeres
- max length of muscle fiber is 30cm which may contain 100K sarcomeres

Front 3

zones of sarcomere

Back 3

-I band: region of actin filaments on both sides of the Z plates
- A band: region in which actin and myosin filaments overlap
- H zone: region which consists solely of mysosin filaments
- z plate: in middle of actin filaments, between two= sarcomere
-

Front 4

filament sliding/ shortening

Back 4

- one filament sliding shortens a sarcomere by approximately 1%
- muscle fiber shortens by 1%
- to achieve a 50% (max) shortening of an entire muscle, the cycle has to be repeated 50x

Front 5

contraction

Back 5

1. each of 2 heads of myosin binds ATP: head/neck angle 90
2. binding of Ca to troponin terminates the inhibitory effect on tropomyosin and the myosin head binds to actin
3. ATPase splits the ATP, Mg required
4. dissociation of the ADP tips the myosin heads from 50-45 degrees
5. without ATP the binding is very strong, eg rigor mortis
6. Binding to ATP again lifts the head off and turns it back to the 90 degree position

Front 6

sarcotubular system of muscle fiber

Back 6

-sarcolemma: cell membrane
- T system: transverse tubuli
- sarcoplasmic reticulum: longitudinal tubuli, resevoir for Ca
-triad: 2 longitudinal + 1 transverse
- blindly ending longitudinal tubular system has no open connection to the extracellular space
- l-system is the intracellular reservoir for Ca

Front 7

AP's: cardiac vs. skeletal

Back 7

-AP of skeletal and nerve much shorter: 1-2 vs 200-400 msec
- latency period (mechanical response to electrical stimulation) of skeletal shorter: 1 vs 100 msec
- duration of contraction of skeletal shorter: 10 vs 200-300 msec

Front 8

Calcium and cardiac contraction

Back 8

- Ca responsible for the long duration of cardiac APs
- mediator b/w electrical stimulation and contraction: coupling
- Ca released from: 1. longitudinal tubular( s. reti=intracellular)
2. transverse tubuli (T system=extracellular): without this, depolarization w/o contraction
- increases 100 fold in <100msec
- Ca blood level vital for strength of contraction (normal 8-13mg/100ml)

Front 9

Ca distribution in resting muscle cell

Back 9

-during the resting state, the membrane is polarized (-70mV inside)
-and- intracellular [Ca] low (0.01micromol) compared with extracellular [Ca] of 1-10micromol

Front 10

delay b/w stim and contraction of skeletal m

Back 10

-resting: high [ECF Ca], l system
- depolarization: Ca influx from ECF and release from l-system
-repolarization: Ca resorption into l-system and transport to ECS
- resulting contraction is delayed and lasts as long as the [ICF Ca] is high

Front 11

skeletal m. Ca dependency

Back 11

Ca release from the L-system is induced by depolarization of the T-system

Front 12

cardiac m. Ca dependency

Back 12

Ca release from the L-system is induced by incoming Ca from the ECF (Ca induced Ca release)

Front 13

Motor unit

Back 13

-motor endplate: synapse connecting m. neuron and m. fiber
- motor unit: one neuron and all the skeletal m. it innervates = fx unit of skeletal m
- # of innervated muscle fibers of one motor unity varies from 5-1000

Front 14

types of motor units

Back 14

1. fast twitch: engaged in rapid movements, predominate in white muscles (eg gastrocnemius)
2. slow twitch: specialized in maintaining posture, predominate in red muscles (eg soleus)

Front 15

functional synctium

Back 15

-all heart and muscle cells electrically connected
-depolarization spreads until all cells in synctium are depolarized
- intercalated discs connect ends of neighboring muscle fiber branches
- gap junctions (joint ion channels) are open pores connecting the cytosols of neighboring cells, allowing APs to "jump"
-

Front 16

smooth muscle contraction

Back 16

- perform contraction with actin and myosin filaments without structured arrangement of sarcomeres
- stimulated by nervous signals, hormones, stretch, etc.
-triggered by influx of Ca from interstitium
-no tubular (l or t) system
- calmodulin plays a similar regulatory role as troponin in striated m.
-diffuse junctions of autonomic neurons

Front 17

types of muscle contraction

Back 17

1. isometric: length is constant while tension changes
2. isotonic: length changes while tension is constant
3. axotonic contraction: both length and tension change

Front 18

force of contraction and m. length

Back 18

-max tension when all myosin heads are involved in contraction
- overlap of actin filaments or free myosin heads decrease tension

Front 19

tension of skeletal muscle

Back 19

-active: determined by the # of actin-myosin crossbridges and varies with fiber length
-passive: developed when the muscle is stretched at rest. Determined by elastic components (sarcolemma, tendons, CT, vessels, nerves)
- total= active + passive
- skeletal can be stretched more than cardiac

Front 20

all or none rule

Back 20

-each AP causes a max release of Ca which in turn produces a max single twitch of striated muscle fiber

Front 21

muscular force

Back 21

determined by:
1. number of motor units activated
2. frequency of APs: increasing frequency removal of intracellular Ca is stinted and [Ca] increases, providing more myosin head binding

Front 22

summation of single twitches

Back 22

- when the AP superimposes on another, the result is a further shortening of the muscle fiber
-muscle fiber has no time to relax

-mechanical summation

Front 23

tetanus

Back 23

If the intervals of the APs becomes less than 1/3 of the time required for a single twitch ( slow m.= 20 Hz, fast m.= 60-100Hz) the twitches fuse together and cause maximum contraction
=tetanus

Front 24

Contracture

Back 24

-Prolonged muscle shorterning by:
1. sustained local depolarization: eg. high [Kextra]
2. pharmacologically: induced release of Ca inside the cell, eg caffeine

Front 25

response to increasing strength of electrical stimulation

Back 25

1. cardiac: all or none response
-Stannius ligature between sinus venosus and R atrium)
2. Skeletal: graded response

Front 26

response to increasing frequency of APs

Back 26

1. cardiac: no tetanus at high rate (50Hz): Stannius ligature between sinus venosus and R atrium
2. skeletal: tetanus (summation/fuse of muscle twitches)

Front 27

prevention of tetanus in cardiac m.

Back 27

-absolute RP: no superceded excitation possible
- relative RP: only very strong stimuli trigger an AP
-signal strength required to initiate a superceded AP is highest in the beginning of the RRP (9x) and decreases to normal strength towards the end of the RRP (1x)

Front 28

duration of contraction of cardiac m

Back 28

-begins a few msec after the AP begin
- ends a few msec after AP ends
-is a fx of duration of AP
- atria: ca 200 msec
-ventricles: ca 300msec