Skeletal Muscle Physiology Notes

Skeletal Muscle Physiology

Module 2 delves into skeletal muscle physiology.
Content is divided into mini-lectures; taking breaks between sections is recommended.
The module aims to provide a comprehensive understanding of:
- Basic and specific components of skeletal muscle and its substructures.
- The relationship between muscle fiber types and sports performance.
- Methods for determining fiber types in experimental studies.
- Characteristics of different muscle fiber types.

Understanding muscle cell development is crucial for understanding mature muscle fibers.
Figure 1.1: Depicts muscle cell development.
- (A) Primitive cells differentiate into myoblasts.
- (B) Myoblasts fuse to form primary myotubes.
  - Prefix "myo" relates to muscle.
- (C-F) Secondary myotubes arise beneath the basal lamina of primary myotubes.
  - Myoblasts fuse radially and longitudinally.
  - Forms muscle fibre beneath the basal lamina.
  - Some unfused myoblasts remain as satellite cells (important for later discussion).
- (G) As muscle matures, primary and secondary myotubes separate.
  - Each contains myonuclei (nuclei of muscle fibers).
  - Satellite cells are present.
  - Forms a mature fiber.
- (H) Muscle fibers grow and arrange as tightly packed polygonal cells, characteristic of mature adult muscle.

Muscles vary in mass, length, fiber length, pennation angle, cross-sectional area, and lean-fat to lean-mass ratio.
Example: Psoas vs. Gluteus Medius
- Gluteus medius has:
  - Higher mass.
  - Shorter muscle length.
  - Shorter fiber length.
  - Greater pennation angle.
  - Suggests capability for greater force production.

Muscle contraction relies on nervous system innervation.
Neuromuscular physiology is covered in detail in Module 3.
Process: Nerve action potential travels -
- Neuromuscular junction.
- Excitation-contraction coupling occurs.
- Muscle fiber contracts.

Circulation: Oxygen supply and removal of metabolic byproducts.
Energy generation: Mitochondrial content (aerobic capacity; Type 1 fiber).
Cytoskeleton: Provides structural support.
Extracellular matrix: Provides protection.
Satellite cell activation: Regeneration after injury (oxidative stress, inflammation).
Hypertrophy/Atrophy: Changes due to disuse or clinical conditions.
Fibrosis.

Skeletal muscle attaches to bone via tendons.
Fascia covers bundles of muscle fibers.
Bundles are organized into:
- Epimysium
- Perimysium: Smaller iteration of the fascia.
Individual muscle fiber (cell)
- Nucleus (oriented towards outside).
- Sarcolemma.
- Myofibrils.
- Sarcoplasm.
- Striations.
- Myosin and actin filaments (important for muscle contraction).

T-tubules and sarcoplasmic reticulum are critical for muscle contraction.
Nerve action potential interacts with the sarcolemma, T-tubules, & sarcoplasmic reticulum which = Calcium release.

Each myofibril within a muscle fiber contains:
- Light I band.
- Dark A band.
- Sarcomere: Region where actin and myosin interact.
  - Thick filaments: Myosin.
  - Thin filaments: Actin.
- M line: Middle of the sarcomere.
- Z line: End of each sarcomere.
Contraction Process:
- Cross-bridges form between actin and myosin.
- I band shortens, A band stays the same.
- H zone (containing only myosin at rest) condenses.
- I band closes in on itself.
3D Structure:
- Myosin heads are oriented around the myosin structure.
- Actin is also a 3D structure.
Table 1.2: Shows the size of various components.

Connective tissue accounts for passive tension during lengthening.
- In conjunction with titin (a giant protein).
Differs between muscles due to size and protein structure.
Functional role in force production.

Triad: One T-tubule and two terminal cisternae.
Sarcoplasmic Reticulum: Terminal cisternae + longitudinal portion.
- T = T-tubule (runs perpendicular to muscle fiber membrane).
- Terminal cisternae.
- longitudinal portion.
Sarcoplasmic reticulum:
- Interconnected network of channels running parallel to the fibers.
- Supplies calcium with terminal cisternae at either end storing the calcium.
- Bigger in type 2 fibers.
Transverse tubule (T-tubule):
- Runs perpendicular to muscle fibers.
- Resides in between the sarcoplasmic reticulum.
Each sarcomere contains two triads in each Z line region.
Figure 1.11: Scanning electron micrograph of the T-tubule system and sarcoplasmic reticulum.

Essential Features:
- Reside inside the basal lamina of the fiber.
- Separated from the fiber by the sarcolemma.
- Has its own nucleus surrounded by cytoplasm.
- Play a role in skeletal muscle regeneration.
Satellite Cell Activation:
- Quiescent state (non-activated).
- Activated by muscle injury, inflammation, reactive oxygen species, etc.
- PAX7 gene is a key factor in this activation.
- Some become self-renewing and return to the quiescent state.
- Others proliferate and differentiate into different muscle fibers.
Satellite cells with activation and PAX7 genes form into myoblasts, myocytes, myotubes, and mature muscle fibers.
PAX7: Required for satellite cells to generate committed myogenic progenitors.

Muscle fibers secrete myokines (exokines during exercise).
Myokines have different impacts:
- Inflammatory (e.g., interleukin 6).
- Myostatin, IGF-1, interleukin-15, etc.
Interleukin 6: Inhibits self-renewal.
IGF-1 and others: Facilitate activation and proliferation and facilitate fusion and differentiation of satellite cells.