Mandible

Lateral (External) View

Medial (Internal) View

Overall Structure

• A body — formed by the fusion of right and left halves at the mandibular symphysis anteriorly (marked by a small vertical ridge externally)
• Two rami — quadrangular vertical plates posteriorly

Body of the Mandible

External (Lateral) Surface

• Mental protuberance — midline chin prominence where the two halves meet
• Mental tubercles — small bumps just lateral to the mental protuberance
• Mental foramen — opens midway between the upper and lower borders, usually below the 2nd premolar; transmits the mental nerve and vessels (terminal branches of the inferior alveolar nerve/vessels)
• Oblique line — ridge running from the front of the ramus onto the body; attachment for muscles that depress the lower lip

Internal (Medial) Surface

• Superior & inferior mental spines (genial spines) — two pairs of small bony projections posterior to the symphysis:
  • Superior pair → attachment for genioglossus (tongue muscle)
  • Inferior pair → attachment for geniohyoid (connects mandible to hyoid)
• Mylohyoid line — oblique ridge running posterosuperiorly from near the midline to just below the last molar; origin of the mylohyoid muscle (floor of mouth)
• Sublingual fossa — shallow depression above the anterior 1/3 of the mylohyoid line; lodges the sublingual salivary gland
• Submandibular fossa — depression below the posterior 2/3 of the mylohyoid line; lodges the submandibular salivary gland
• Shallow groove for the lingual nerve — between the last molar and the mylohyoid line
• Retromolar triangle — small triangular depression behind the last molar; attachment site for the pterygomandibular raphe

Ramus of the Mandible

Medial Surface

• Mandibular foramen — large opening for the inferior alveolar nerve and vessels entering the mandibular canal
• Lingula — small bony spine overhanging the mandibular foramen anteriorly; attachment for the sphenomandibular ligament

Superior Border

Key Foramina & Their Contents

Muscle Attachments (Summary)

Clinical Relevance

• Inferior alveolar nerve block: Local anaesthetic is injected near the mandibular foramen, on the medial surface of the ramus, to anaesthetise the teeth, lower lip, and chin on that side.
• Mental foramen: The mental nerve exits here; the foramen migrates posteriorly with age (from the deciduous incisor region toward the premolar region).
• Edentulous mandible: Loss of teeth causes resorption of the alveolar part, reducing overall mandibular height and potentially exposing the mental foramen to the upper border — relevant in denture fitting.
• Mandible fractures: Common at the condylar neck (most frequent), angle, parasymphysis/canine region; often bilateral because force is transmitted around the arch.
• Mandible dislocation: The condyle moves anterior to the articular eminence; associated with wide opening, trauma, or yawning.

Smooth Muscle & Cardiac Muscle

SMOOTH MUSCLE

Histology & General Characteristics

Location & Function

Types of Smooth Muscle

Excitation–Contraction Coupling

1. Depolarization opens sarcolemmal voltage-gated Ca²⁺ channels → Ca²⁺ enters from ECF
2. Entered Ca²⁺ triggers Ca²⁺-induced Ca²⁺ release (CICR) from SR via ryanodine receptor (RYR2)
3. Two additional routes raise [Ca²⁺]:
   • Ligand-gated Ca²⁺ channels — opened by hormones/neurotransmitters via G proteins
   • IP₃-gated SR channels — hormones → G protein → PLC → PIP₂ → IP₃ → SR Ca²⁺ release
4. Ca²⁺ binds calmodulin (4 ions cooperatively)
5. Ca²⁺-calmodulin activates MLCK, which phosphorylates myosin light chain
6. Phosphorylated myosin has increased ATPase activity → binds actin → cross-bridge cycling → tension
7. Ca²⁺-calmodulin also phosphorylates calponin and caldesmon (thin-filament proteins), relieving their inhibition of myosin ATPase

• Hyperpolarization (closes voltage-gated Ca²⁺ channels)
• cAMP/cGMP-mediated Ca²⁺ channel inhibition
• Reduced IP₃ production
• Increased SR Ca²⁺-ATPase activity
• Myosin-light-chain phosphatase dephosphorylates myosin light chain → cross-bridges stop

Ca²⁺ Sources (Comparison)

CARDIAC MUSCLE

Histology & Ultrastructure

Intercalated Discs — Structure & Function

• Desmosomes — mechanical adhesion, resist contractile forces
• Fascia adherens — anchors actin filaments; provides strong intercellular adhesion

• Gap junctions — ionic continuity between cells; act as "electrical synapses"; permit rapid impulse conduction so many adjacent cells contract as a functional unit (functional syncytium)

Excitation–Contraction Coupling in Cardiac Muscle

• Depolarization → voltage-gated Cav1.2 channels open → Ca²⁺ enters from ECF
• Entered Ca²⁺ triggers CICR via RYR2 on SR → large amplified Ca²⁺ release
• Ca²⁺ binds troponin C (same as skeletal muscle) → tropomyosin moves → cross-bridge cycling
• T-tubules are located at Z lines and form dyads (one T-tubule + one SR terminal cisterna), vs. triads in skeletal muscle

Special Properties

• Autorhythmicity: Contractions are intrinsic and spontaneous; individual cardiomyocytes in culture continue to beat. Pacemaker nodes regulate and coordinate rhythm. Autonomic innervation modulates rate (sympathetic ↑, parasympathetic ↓) but does not initiate contraction.
• All-or-none contraction: Individual cardiomyocytes contract all-or-none; the gap junctions ensure coordinated contraction of the entire heart wall.
• Helical fiber arrangement: Cells branch and interweave in spiraling layers, producing an efficient wringing contraction that empties the ventricles.

Ventricular vs. Atrial Differences

Regenerative Capacity

Side-by-Side Comparison

Viva Questions: Smooth Muscle & Cardiac Muscle

SMOOTH MUSCLE

• Skeletal muscle: Ca²⁺ binds troponin C → removes tropomyosin inhibition → cross-bridge cycling
• Smooth muscle: No troponin. Ca²⁺ binds calmodulin → Ca²⁺-calmodulin activates myosin-light-chain kinase (MLCK) → MLCK phosphorylates myosin light chain → increases myosin ATPase → cross-bridge cycling

1. Voltage-gated Ca²⁺ channels (sarcolemma) — opened by depolarization
2. Ligand-gated Ca²⁺ channels (sarcolemma) — opened by hormones/neurotransmitters via G proteins
3. IP₃-gated SR channels — hormone → G protein → PLC → PIP₂ → IP₃ → Ca²⁺ release from SR

Key point: Unlike skeletal muscle (SR only), smooth muscle can raise [Ca²⁺] from both ECF and SR.

• Hyperpolarization → closes voltage-gated Ca²⁺ channels
• cAMP/cGMP → inhibit Ca²⁺ channels or activate SR Ca²⁺-ATPase
• Reduced IP₃ production → less SR Ca²⁺ release
• Myosin-light-chain phosphatase → dephosphorylates myosin light chain → stops cross-bridge cycling

CARDIAC MUSCLE

Atrial cardiomyocytes have absent or very small T-tubules.

• Autorhythmicity: The heart's rhythm is intrinsic — pacemaker nodes (SA node, AV node) generate spontaneous action potentials independently. Cardiac muscle cells in culture continue to beat.
• Autonomic control modulates but does not initiate the rhythm:
  • Sympathetic → ↑ heart rate and contractility
  • Parasympathetic → ↓ heart rate

HIGH-YIELD COMPARISON QUESTION