Auditory Pathway - MBBS 1st Year

Pathway Diagram (from textbook)

Flowchart

SOUND STIMULUS (External Ear → Middle Ear → Inner Ear)
                        ↓
         ORGAN OF CORTI (Hair cells on Basilar Membrane)
         • Inner hair cells → main receptors
         • Tonotopic: apex = low freq | base = high freq
                        ↓
              1st ORDER NEURONS
         Cell body: Spiral Ganglion (in Canal of Rosenthal)
                        ↓
              COCHLEAR NERVE (CN VIII)
                        ↓
         Enters brainstem at medullopontine junction
                        ↓
              2nd ORDER NEURONS
     ┌─────────────────────────────────────┐
     ↓                                     ↓
Dorsal Cochlear Nucleus           Ventral Cochlear Nucleus
(in medulla)                   (Anteroventral + Posteroventral)
     ↓                                     ↓
Dorsal Acoustic Striae      Intermediate + Ventral Acoustic Striae
     ↓                                     ↓
     └──────────── Most fibers CROSS ──────┘
                   (Trapezoid Body - in caudal pons)
                        ↓
              SUPERIOR OLIVARY COMPLEX
         (First site of BINAURAL convergence)
         (Both ipsilateral + contralateral input)
                        ↓
              LATERAL LEMNISCUS
         (Runs up through pons and midbrain tegmentum)
                        ↓
              3rd ORDER NEURONS
         INFERIOR COLLICULUS (Midbrain tectum)
         • Central relay nucleus
         • ALL ascending auditory pathways end here
         • Auditory reflex center
                        ↓
         Brachium of inferior colliculus
                        ↓
              4th ORDER NEURONS
    MEDIAL GENICULATE BODY / NUCLEUS (MGB) - Thalamus
         • Thalamic auditory relay nucleus
         • Generates auditory radiations (geniculotemporal fibers)
                        ↓
         Auditory Radiations (in internal capsule)
                        ↓
         PRIMARY AUDITORY CORTEX
         Heschl's Gyrus (Transverse Temporal Gyrus)
         Brodmann Area 41 - Superior Temporal Gyrus
         + Auditory Association Cortex (BA 42)
         • High freq → medial | Low freq → lateral

Summary Table

High-Yield Points for Exams

1. Crossing (decussation): Most fibers cross at the trapezoid body in the caudal pons - so each auditory cortex receives input from BOTH ears (bilateral representation).
2. First binaural site: Superior olivary complex is where signals from both ears first converge - this is the key nucleus for sound localization.
3. Auditory reflex center: Inferior colliculus (midbrain) - mediates the acoustic startle reflex.
4. Thalamic relay: Medial geniculate body (NOT lateral geniculate, which is for vision).
5. Cortical area: Brodmann area 41 (primary) + BA 42 (association) on the superior temporal gyrus.
6. Tonotopy: Preserved throughout the entire pathway (cochlea → cortex).
7. Unilateral cortical lesion: Does NOT cause complete deafness because of bilateral representation - only causes subtle deficits in sound localization.
8. Commissure of Probst: Connects the two nuclei of the lateral lemniscus (an additional crossing point).