Muscle Contraction

Microscopic Anatomy• Sarcolemma= plasma membrane of the muscle cell• Filled with myofibrils (look like tubes)

• Sarcoplasmic Reticulum= organelle that holds Calcium ions

Myofibrils

• Are made up of units called sarcomeres. (from Z to Z)

Myofibrils• Composed of myofilaments: actin (thin) and myosin (thick). • I band= area with actin..thin bands protein• A band= area that contains actin and myosin bands alternating

which is why they appear darker

I bands and A bands• Appear as light and dark striations…makes skeletal

muscles look striated• H-zone= area where only myosin is located in

relaxed muscle

Relaxed muscle shows H zone (bare zone)

Contraction

• When nerve impulse reaches a muscle it triggers the sarcoplasmic reticulum to release large amounts of Ca 2+

• Calcium binds to actin on regulatory proteins and causes those proteins to change shape and move revealing myosin binding sites.

Contraction continued

• Myosin then binds to those sites on the actin. • The myosin pulls on the actin and pulls actin in

toward the H-zone. • This happens multiple times until the muscle

is fully contracted

Myosin Heads Move..attach to actin and move again….

Figure 11.13

2- Muscle contraction: Cell events

Synaptic events• The AP reaches the

axonal bulb• Voltage-gated

calcium channels open

• The influx of Ca in the bulb activates enzymes the vesicles containing the neurotransmitter molecule dock and release the neurotransmitter in the synapse

• The neurotransmitter for skeletal muscles is always acetylcholine

• The receptors on the muscle fiber are cholinergic receptors

2- The Mechanism of Force Generation in Muscle

Figure 12.7