Microscopic Anatomy of the Skeletal Muscle

SKELETAL MUSCLE

Microscopic structure of Skeletal MusclesStructure

Description

Consists of hundreds of

thousands of muscle cells,

plus fascia, blood vessels

and nerve fibers

FASCICLE

Microscopic structure of Skeletal MusclesStructure

Description

FASCICLEDiscrete bundle of

muscle cells,

segregated from the

rest of the muscle by

the perimysiumFascicle

(wrapped by

perimysium)

Sarcolemma

Nucleus

(a) Segment of a muscle fiber (cell)

Muscle Cell

Microscopic structure of Skeletal MusclesStructure

Description

Long multinucleated

cells with a banded

(striated)

appearance.

Cell membrane =

sarcolemma

Cytoplasm =

sarcoplasm.

Myofibril

Microscopic structure of Skeletal MusclesStructure

Description

Specialized organelle

that contracts.

Occupy most of the

muscle cell volume.

Composed of

sarcomeres arranged

end to end.

Sarcomere

Microscopic structure of Skeletal MusclesStructure

Description

The contractile unit,

composed of

myofilaments called

actin and myosin.

Sarcomeres line up

end to end inside a

myofibril. Create

the striations.

Myofilaments: Actin and Myosin

Microscopic structure of Skeletal MusclesStructure

Description

Two types:

Actin: Thin contractile

filament. Contains

regulatory proteins that

controls myosin’s ability to

bind

Myosin: Thick contractile

filament.

Contains myosin heads

where the thin filament

attaches during

contraction.

Thick filament Thin filament

(d) Myofilament structure

(within one sarcomere)

Properties of Skeletal Muscle

Irritability – ability to receive and respond to a stimulus

Contractility – ability to shorten when an adequate stimulus is received

• Which Sarcomere is relaxed and which one is contracted?

• What is happening during contraction to the following?– Z line (disc)– I band– A band– Myosin (thick) filament– Actin (thin) filament– H - zone

Sliding Filament Theory When contraction occurs….

the thin filaments slide toward each other

the light-colored H-zone and I-bands (seen in the relaxed sarcomere) disappear

because the actin and myosin are completely overlapped

This happens because the myosin pulls the actintoward the middle. The z-lines become closer together.