Microscopic Anatomy and Representative Muscle Groups Study Notes

Microscopic Anatomy and Organization of Muscle Tissue

• Cell Comparison:

  • Skeletal Muscle: Long, cylindrical, striated, and multinucleate cells with peripheral nuclei; voluntary control; attached to bones.

  • Smooth Muscle: Spindle-shaped, non-striated, single central nucleus; involuntary control; located in walls of hollow organs like the gut and blood vessels.

• Organizational Hierarchy (Outside to Inside):

  • Muscle (Organ): Covered by dense irregular connective tissue called Epimysium.

  • Fascicle: A bundle of muscle cells surrounded by Perimysium.

  • Muscle Fibre (Cell): An elongated cell surrounded by a plasma membrane called the Sarcolemma and a wispy connective tissue layer called Endomysium.

  • Myofibril: Rod-like contractile elements within the fibre made of actin and myosin filaments.

• Excitation-Contraction Coupling:

  1. Nerve impulse triggers Acetylcholine (ACh) release at the neuromuscular junction.

  2. ACh binds receptors on the motor end plate, triggering a muscle action potential.

  3. The potential travels down T tubules, opening $Ca^{2+}$ release channels in the Sarcoplasmic Reticulum (SR).

  4. $Ca^{2+}$ floods the sarcoplasm, binding troponin to expose myosin-binding sites on actin.

  5. Myosin heads use $ATP$ to pull thin filaments during the cross-bridge cycle.

Muscle Groups and Nerve Supply

• Scalp and Face (Facial Nerve, $CN VII$):

  • Epicranius: Consists of the Frontal belly (raises eyebrows) and Occipital belly (pulls scalp posteriorly).

  • Expression Muscles: Includes Orbicularis oculi (closes eyes), Orbicularis oris (purses lips), Zygomaticus (smiling), and Buccinator (cheek compression for chewing/whistling).

• Mastication (Trigeminal Nerve, $CN V_3$):

  • Masseter and Temporalis elevate the mandible; Medial and Lateral pterygoids assist in grinding.

• Neck and Back:

  • Sternocleidomastoid (SCM): Flexes or rotates the head; supplied by the Accessory nerve ($CN XI$).

  • Erector Spinae: Three columns lateral to medial are Iliocostalis, Longissimus, and Spinalis.

• Shoulder and Rotator Cuff ($SITS$):

  • Subscapularis: Medial rotation; Subscapular nerve.

  • Supraspinatus: Abduction initiation (15^\regular{\text{o}}) and Infraspinatus: Lateral rotation; both supplied by the Suprascapular nerve.

  • Teres minor: Lateral rotation; Axillary nerve.

• Lower Limb:

  • Gluteals: Gluteus maximus (extension; Inferior gluteal nerve); Gluteus medius and minimus (abduction/stability; Superior gluteal nerve).

  • Quadriceps: Four muscles (Rectus femoris, Vastus lateralis, medialis, intermedius) that extend the knee; Femoral nerve.

  • Hamstrings: Biceps femoris, Semitendinosus, Semimembranosus; flex the knee and extend the hip; Sciatic nerve.

  • Calf: Gastrocnemius and Soleus plantarflex the ankle via the Achilles tendon; Tibial nerve.

Trunk, Breathing, and the Inguinal Canal

• Abdominal Wall: Layered from outside to inside as External oblique, Internal oblique, and Transversus abdominis. The Rectus abdominis is a vertical strap within a sheath.

• Breathing:

  • Inspiration: Diaphragm (primary) and External intercostals elevate ribs.

  • Expiration: Internal intercostals and abdominal muscles assist in forced expiration.

• Inguinal Canal:

  • Dimensions: A $4\,cm$ long oblique passage superior to the Inguinal ligament.

  • Structure: The entrance is the Deep (internal) inguinal ring (transversalis fascia); the exit is the Superficial (external) ring (external oblique aponeurosis).

  • Boundaries: Anterior wall (External oblique aponeurosis), Floor (Inguinal ligament reinforced medially by the Lacunar ligament), Roof (Internal oblique and Transversus abdominis).

  • Contents: Spermatic cord in males, Round ligament of the uterus in females, and the Ilioinguinal nerve in both.

Questions & Discussion

• Question: Identify the layer of connective tissue surrounding the entire skeletal muscle organ.

• Answer: Epimysium.

• Question: Which two rotator cuff muscles are supplied by the Suprascapular nerve?

• Answer: Supraspinatus and Infraspinatus.

• Question: True or False: The nerve supply to the Gluteus medius is the Inferior gluteal nerve?

• Answer: False; it is supplied by the Superior gluteal nerve.

• Question: Identify the three bones covered by the Temporalis muscle.

• Answer: Temporal, frontal, and parietal bones.

• Scenario: A patient requires a rotator cuff repair following a motorcycle accident. Identify the four muscles of this group.

• Response: Subscapularis, Supraspinatus, Infraspinatus, and Teres minor.

• Question: Explain why inguinal hernias are more common in males.

• Answer: This is due to the passage of the Spermatic cord through the inguinal canal, creating a relative weakness.