Skeletal Muscle Histology Histology > Musculoskeletal System > Musculoskeletal System

SKELETAL MUSCLE HIERARCHY&NBSP;

• Skeletal muscles divide into fascicles.

• Fascicles are units of muscle cells (aka skeletal myocytes, skeletal muscle fibers).

• Skeletal muscle cells comprise myofibrils and other organelles (notably, mitochondria).

• Myofibrils comprise proteins, notably thick and thin myofilaments.

• Myofilaments arrange into functional contractile units, called sarcomeres.

CONNECTIVE TISSUE HIERARCHY

• Skeletal muscle is covered in epimysium.

• Fascicles are covered in perimysium.

• Muscle cells are covered in endomysium.

MUSCLE ORGANIZATION

Epimysium

• Envelopes the muscle (epi = upon, my = muscle).

• Dense irregular connective tissue.

Perimysium

• Divides the muscle into multiple wedges (peri = around); it covers each fascicle.

• Each fascicle comprises skeletal muscle cells (aka skeletal myocytes or muscle fibers).

Endomysium

- Surrounds the muscle cells.

1 / 8 - Loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.

HISTOLOGICAL SLIDE OF A MUSCLE

• Epimysium envelopes the muscle.

• Perimysium divides the muscle into fascicles.

• Endomysium lies within the muscle fascicle: it comprises a loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.

MUSCLE FASCICLE

Muscle cells

• They are multinucleated (meaning, each cell contains many nuclei).

- This reality reflects the actual process of muscle cell formation. Mature muscle cells form from fused myoblast cells (embryonic cells) and each myoblast contributes its nucleus to the adult muscle cell.

• Each skeletal myoctye comprises numerous myofibrils.Also show that each cell comprises numerous myofibrils.

Myofibrils contain Myofilaments

• Thick, myosin filaments

• Numerous thin, actin filaments.

- Thin filaments form a hexagonal shape around the thick filaments.

MUSCLE FASCICLE SLIDE

• Muscle cell is covered in endomysium.

• We see many cell nuclei.

• We see the dot-like myofibrils that constitute the muscle cell mileu.

Sarcolemma

• The plasma membrane of the muscle cell.

Sarcoplasm

• The muscle cell cytoplasm.

• Muscle cell nuclei lie within the periphery of the cell.

2 / 8 - During development, the nuclei transition from a central location to a peripheral one.

• Muscle cells comprise numerous mitochrondria.

Muscle fibers divide into 3 types based on their: myoglobin content and contraction speed, and, in related fashion, their number of mitochondria.

• Type 1 (slow, red)

• Type 2a (fast, intermediate)

• Type 2b (fast, white)

External lamina (sometimes referred to as the basal lamina)

• Lies external to the muscle cell.

• Within it, lie satellite cells, which are skeletal muscle stem cells: inactive myoblasts, lying in-wait: think: Army Reserves.

- Upon muscle injury, they enter mitosis, fuse with other satellite cells to form differentiated muscle fibers. Centrally located nuclei are a hallmark of regenerating muscle cells; whereas mature muscle cells contain peripherally located nuclei.

MYOFIBRIL HISTOLOGY: INTERNAL

Thick filaments.

• Form from myosin

• The A band refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light.

• H Zone is a zone of only thick filaments.

• M line bisects the A band.

Thin filaments

• Form from actin

• The I band is the region along the thin filaments (between the thick filaments).

• Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).

Z disks

• Transverse bands at the ends of the thin filaments.

3 / 8 ELECTRON MICROGRAPH OF A MUSCLE FIBER we see that the repeating light and dark bands of the sarcomere gives muscle fibers a striated appearance.

• Indicate the A Band, remember "A" for "dark", "anisotropic".

• Indicate the I Band, remember "I" for "light", "isotropic".

• Z disk bisects the I band.

• M line bisects A Bands; they contain myomesin and creatine kinase.

• Sarcomere comprises the area between the Z-disks.

• Triad at the A-I junction (which comprises T tubules and terminal cisternae of the sarcoplasmic reticulum).

Clinical Correlation: Muscle Atrophy (aka Muscle Wasting)

FULL-LENGTH TEXT

• Here we will learn the histology of muscle tissue.

To orient ourselves, let's draw a limb cross-section.

• Draw the bone.

• Then a few different muscle cross-sections.

• Next, let's draw a limb cross-section.

• Draw the bone.

• Then a few different muscles in cross-section.

• Show that a single muscle may have more originate from more than one head, so we don't later confuse these with muscle fascicles.

• Now, show that fascia envelopes the muscles.

4 / 8 • Specify both the superficial and deep fascia.

• Show a neurovascular bundle (artery, vein, and nerve) lie within the deep fascia (and as we'll see they enter the muscle).

• Then, cover the limb in skin.

Now, let's address the muscle's organization, itself.

• Show that epimysium, a type of dense irregular connective tissue, envelopes the muscle. (epi = upon, my = muscle).

• Now, show that perimysium (peri = around) divides the muscle into multiple wedges of muscle cells.

• Now, draw a muscle cell within one of the fascicles.

• Show that endomysium (endo = within) surrounds it.

• Fill the fascicles with muscle cells and neurovascular structures.

Let's look at this on a Histological slide of a muscle.

• Show that epimysium envelopes the muscle.

• Show that perimysium divides the muscle into fascicles.

• Show that endomysium lies within the muscle fascicle: it comprises a loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.

• Now, draw a magnified view of a fascicle.

• Draw the muscle cells.

• Show that muscle cells are multinucleated; each cell contains many nuclei, which reflects the process of muscle cell

5 / 8 formation.

- Mature muscle cells form from fused myoblast cells (embryonic cells); each myoblast cell contributes its nucleus to the adult cell.

- Also show that each cell comprises numerous myofibrils.

• Show a representative myofibril, which each comprise numerous myofilaments: fewer thick, myosin filaments and more numerous thin, actin filaments, which form a hexagonal shape around the thick filaments.

Next, let's look at a slide of a muscle fascicle.

• Encircle the muscle cell, which is covered in endomysium.

• Indicate one of the cell's many nuclei.

• Try to see the dot-like myofibrils that constitute the muscle cell milieu.

Next, let's draw a muscle cell in a cylindrical, 3-dimensional view.

• Internal to the muscle cell, draw 3 representative myofibrils.

• Indicate that the muscle cell is bordered by the sarcolemma: its plasma membrane.

• And is filled with sarcoplasm: its cytoplasm.

• Show a muscle cell nucleus within the periphery of the muscle cell.

• And draw a representative mitochondrion.

- Muscle cells comprise numerous mitochrondria.

• As we learn elsewhere, muscle fibers divide into 3 types: Type 1 (slow, red), 2a (fast, intermediate), and 2b (fast, white) based on their: myoglobin content and contraction speed, and, in related fashion, their number of mitochondria.

• External to the muscle cell, draw the external lamina (it is sometimes referred to as the basal lamina).

Within it, draw a satellite cell, which are skeletal muscle stem cells: inactive myoblasts, lying in-wait: think: Army

6 / 8 Reserves.

- Upon muscle injury, they enter mitosis, fuse with other satellite cells to form differentiated muscle fibers.

- As you might predict, centrally located nuclei are a hallmark of regenerating muscle cells; whereas mature muscle cells contain peripherally located nuclei.

• Now, along one myofibril, draw web-like sarcoplasmic reticulum (SR), which stores calcium and is a key component to coupling muscle cell excitation to myofibril contraction.

• Then draw bands of transversely-oriented terminal cisternae of the sarcoplasmic reticulum, which flank transverse tubules (T-Tubules): invaginations of sarcolemma.

- Elsewhere, we see that T-tubules and terminal cisternae connect the terminal synapse firing (its depolarization) to the sarcoplasmic reticulum, again, ultimately coupling muscle cell excitation and myofibril contraction.

On the next myofibril, show its constituents.

• Indicate that the A band refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light. draw a series of of short, thick filaments: the myosin, thick filaments.

• Show that the I band is the region along the thin filaments, between the thick filaments.

- Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).

• Now, show that transverse bands: the Z disks bisect the I bands.

- Indicate that a sarcomere, which is the contractile unit of the myofibril, comprises the area between the Z-disks.

Now, on the next myofibril, once again draw the sarcoplasmic reticulum and terminal cisternae with intervening T- tubules and specify that they occur as Triads at the A-I junctions.

Lastly, let's look at an electron micrograph of a muscle fiber: we see that the repeating light and dark bands of the sarcomere gives muscle fibers a striated appearance.

• First, indicate the A Band, remember "A" for "dark", "anisotropic".

• Then, indicate the I Band, remember "I" for "light", "isotropic".

• Show that a Z disk bisects the I band.

• Show that an M line bisects the A Band; they contain myomesin and creatine kinase.

7 / 8 • Indicate that a sarcomere is the unit that spans the area between adjacent Z disks.

• Finally, show the Triad at the A-I junction (which comprises T tubules and terminal cisternae of the sarcoplasmic reticulum).