13 October 2008

Chapter 9: Muscle physiologyLab this week: Sensory Physiology and HearingAll instructions provided during lab (no advance prep)Short preferable for testing cutaneous receptors of calf.

Test # 2 Monday 20 October

Fig. 09.01

Striated (with sarcomeres)

Cylindrical/polygonal

branched fusiformFusion of myoblasts produce multinucleated skeletal myofibers

Fig. 09.02

Myofilaments

Fig. 09.03

dArk band and lIght band

Fig. 09.05

Fig. 09.09

Fig. 09.15

1 AP in motor axon leads to 1 AP in myofiber because end plate potential (EPP) is always suprathreshold

Neuromuscular junctionmyoneural junction

Fig. 09.11aAction potentials are conducted into T-tubles

Fig. 09.11b

Fig. 09.12Ca++ sequestration is slower than Ca++ release from SR

3 Roles of ATP

Preview: one mechanism of fatigue is AP conduction failure in t-tubules at high fAP

Sequence of Events Leading to Contraction of Skeletal Muscle

• Action potential in motor axon

• Release of Acetylcholine into synaptic cleft of NMJ

• ACh binds to nAChRs in sarcolemma

• Increase permeability to Na+ and K+ via nAChR (inotropic)

• Sarcolemma depolarizes (End Plate Potential =EPP)

• Threshold exceeded....AP generated and conducted across the sarcolemma

• AP conducted into Transverse tubules

• Depolarization detected by DHP (dihydropyridine) receptors in T-tubules

• Conformational change in DHP receptors leads to opening of Ca++ channels in sarcoplasmic reticulum

• Diffusion of Ca++ out of SR into cytoplasm– Ca++ constantly pumped back into SR by primary active transport

pumps in the SR membrane– Ca++ in SR stored in combination with Calsequestrin

• Ca++ binds to troponin• Tropomyosin shifts to expose binding sites of F-actin• Myosin heads (which have already hydrolyzed ATP) bind to sites on F-

actin• Powerstroke of myosin crossbridge• IF Ca++ AND ATP REMAIN AVAILABLE

– CROSSBRIDGE RELEASES ADP AND FRESH ATP BINDS, PERMITTING RELEASE AND RE-FORMATION OF NEW CROSSBRIDGE

• IF Ca++ unavailable (returned to sarcoplasmic reticulum)– tropomyosin covers F-actin binding sites– myosin head units unable to attach to actin......end of contraction.

Types of contractions

a) Isometricb) Isotonic

i) Concentricii) Eccentric

Examples: lifting a barbell