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Skeletal Muscle Organization

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Notes

Skeletal Muscle Organization

Sections

skeletal muscle hierarchy

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.
Loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.

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.

Myofibrils contain Myofilaments

Thick, myosin filaments
Numerous thin, actin filaments.
- Thin filaments form a hexagonal shape around the thick filaments.

Sarcolemma

The plasma membrane of the muscle cell.

Sarcoplasm

The muscle cell cytoplasm.
Muscle cell nuclei lie within the periphery of the cell.
- 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.

Satellite cell biology:

Myostatin inhibits satellite cells and promotes protein degradation, which regulates the formation of mature skeletal muscle cells.

Testosterone, instead, encourages the synthesis of proteins, thus athletes abuse anabolic steroids that mimic testosterone to promote muscle growth.

myofibril histology: External

Sarcoplasmic reticulum (SR)

Form web-like rows.
Store calcium.
Are a key component to coupling muscle cell excitation to myofibril contraction.

Terminal cisternae (aka lateral cisternae) of the sarcoplasmic reticulum.

Flank the transverse tubules (T-Tubules)

Transverse tubules (T-Tubules)

Are tubular 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.

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.

Sarcomere

The contractile unit of the myofibril.
Comprises the area between the Z-disks.

special characteristics of skeletal muscle

Rich in glycosomes, which are used for energy creation.
Rich in myoglobin, which binds oxygen.
Sarcoplasmic reticulum (SR) instead of smooth endoplasmic reticulum.
Have specialized plasma membrane called sarcolemma.
Contain myofibrils, which are the contractile elements of muscle cells.

Clinical Correlations

Exam Findings

Proximal Muscle Weakness (ie, Shoulder/Hip Girdle Muscles)
Muscle Atrophy (aka Muscle Wasting)
Lumbar Lordosis & Calf Hypertrophy

Genetic Myopathies

Full-Length Text

Here we will learn how muscle tissue is organized, so we can understand its structure and ultimately its function.

To begin, start a table for skeletal muscle hierarchy.

First, denote that skeletal muscles divide into fascicles, which themselves, are units of muscle cells (aka 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.

Next, denote skeletal muscle connective tissue hierarchy:
- Skeletal muscle is covered in epimysium.
- Fascicles are covered in perimysium.
- Muscle cells are covered in endomysium.

Begin our diagram with a prototypical limb.

Draw a bone.

Show a muscle attach to the periosteum of the bone via a tendon (connective tissue mass).

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 than one head, so we don't later confuse these with muscle fascicles.

Also, indicate that each muscle generally has an antagonist.

Now, show that fascia envelopes the muscles.

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 dense irregular connective tissue, envelopes the muscle (epi = upon, my = muscle).

Now, show that perimysium divides the muscle into multiple wedges (peri = around); it covers each fascicle.

Show a representative neurovascular bundle within the perimysium.

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

Show that endomysium (endo = within) surrounds it.
- It comprises a loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.

Fill the fascicles with muscle cells and neurovascular structures.

Now, draw a magnified view of a fascicle.

Draw muscle cells.

Show that 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.

Now, show that each skeletal myoctye comprises numerous myofibrils.

Show a representative myofibril, which each comprise numerous myofilaments: thick, myosin filaments surrounded by numerous thin, actin filaments.
- We see that the thin filaments form a hexagonal shape around the thick filaments.

Now, 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 cell.

During development, the nuclei transition from a central location to a peripheral one.

Next, 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 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.

As a corollary, write that:
- Myostatin inhibits satellite cells and promotes protein degradation, which regulates the formation of mature skeletal muscle cells.
- Testosterone, instead, encourages the synthesis of proteins, thus athletes abuse anabolic steroids that mimic testosterone to promote muscle growth.

Now, along one myofibril, draw web-like longitudinal rows of 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.

And show that they flank transverse tubules (T-Tubules); they are tubular 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.

Next, let's skip to a 2-dimensional view of the myofibril (we'll return to the 3-D view later).

Draw a myofibril.

Then, draw a series of of short, thick filaments: the myosin, thick filaments.

Show that thin filaments interweave with them on both sides.

Now, at the ends of the thin filaments, draw transverse bands – the Z disks.

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

Then, 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.

Next, show that the H Zone comprises the region between the thin filaments, which only contain the thick filaments.

Show the M line, which bisects the A band (and H Zone).

Now, in order to show the I band, draw another sarcomere next to this one.

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).
- Elsewhere, we see that the repeating light and dark bands gives muscle fibers a striated appearance.

Return to the 3-D view, and now include a Z-disk, which transects an I Band, flanked by A Bands.

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.

Finally, let's consolidate some of these points with the following special characteristics – skeletal muscle is:

Rich in glycosomes, which are used for energy creation.

Rich in myoglobin, which binds oxygen.

Sarcoplasmic reticulum (SR) instead of smooth endoplasmic reticulum.

Have specialized plasma membrane called sarcolemma.

Contain myofibrils, which are the contractile elements of muscle cells.