A motor unit consists of one

motor neuron and all the muscle

fibers it stimulates. This

figure shows three motor units.

Motor units from different muscles will have different numbers of

muscle fibers.• Large muscles (e.g., quadriceps femoris)

may have 300-400 muscle fibers per motor unit.

• Small muscles (e.g. external eye muscles) may have 1 muscle fiber per motor unit.

• How does the precision of control we have over a muscle relate to the number of muscle fibers in its motor units?

Fig. 6.17Striated and

smooth muscle.

Mature muscle fibers are very long and result from the fusion of many cells.

• Now go to your notes for some introductory information about the structure of muscle fibers.

Muscle fibers contain myofibrils, which are bundles

of myofilaments. Each myofibril is surrounded by the

sarcoplasmic reticulum.

Fig. 6.28 also shows this arrangement.

A myofibril consists of a bundle of thick and thin

myofilaments. This bundle is composed of repeating units called sarcomeres. Each

myofibril is surrounded by the sarcoplasmic reticulum.

Each sarcomere is bounded by Z lines and consists of overlapping thin and thick

myofilaments. The thin myofilaments are attached to the Z line and the thick myofilaments

are anchored to the Z line by elastic myofilaments.

Fig. 6.19 shows the

sarcomere in the context of

an EM photograph

Review of levels of organization of skeletal muscle.

Structure of the thin filament.

Structure of the thick filament.

Fig. 6.18 shows the structure of

both myofilaments.

Muscle fibers shorten as each sarcomere shortens. Look at the distance between the Z

lines.