2011A12 Compare and contrast a single twitch and a tetanic contraction in a skeletal muscle fibre.

 

List:

·       Skeletal muscle

·       Graphs: potential difference and force of contraction

·       Single twitch vs tetanic contraction (note most marks here)

 

Skeletal muscle:

Sarcomere

Excitation and contraction

·  Action potential at motor nerve terminal

·  Depolarisation of post-junctional membrane

·  Activation of L-Ca2+ channels

·  Ca2+ influx

·  Ca2+ triggered Ca2+ release from sarcoplasmic reticular L-Ca2+ channel

·  Ca2+ binds troponin C

·  Troponin removes tropomyosin from actin

·  Actin-myosin cross-bridge cycling occur

 

Graphs:

Potential difference vs time

Note short refractory period (~3msec) cf. cardiac muscle (~250msec)

 

Force vs time

 

Single twitch vs tetanic contraction:

 

Single twitch

Tetanic contraction

Definition

·  Single contraction then relaxation

Continuous fused contraction

Cause of onset

·  Single nerve action potential

·  Repeated nerve action potential and cross-bridge cycling

·  Time between APs shorter than time to peak force

·  -Slow twitch 100ms duration hence >>10Hz

·  -Fast twitch 10ms duration hence >>100Hz

Cause of offset

·  ACh metabolism and reuptake

·  Ca2+ reuptake and extrusion

·  Cessation of nerve action potential

·  Then ACh metabolism and uptake

·  Ca2+ reuptake and extrusion

Duration

·  Same each time

·  10-100msec

·  (Fast -> slow fibre)

·  Variable.

·  Until fatigue or voluntary cessation

Force developed

·  Same each time

·  Less

·  4x ↑tension possible

Inertia to be overcome

·  More

·  Less (only present at the start)

Energy consumed overall

·  Less overall

·  More per time

·  More overall

·  Less per time

Effect of depolarizing NMBD

·  ↓Height

·  No fade

Effect of non-depolaring NMBD

·  ↓Height

·  Fade

Sensitivity for detecting residual blockade

·  Low

·  Very high