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

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

Potential difference vs time

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

Force vs time

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