Auditory

graph TD
    A["Auricle of the external ear<br>collects sound waves"] --> B["Sound waves enter<br>external acoustic meatus"];
    B --> C["Sound waves cause the<br>**tympanic membrane**<br>to vibrate"];
    C --> D["Malleus, incus, and stapes<br>transmit vibrations"];
    D --> E["Oscillation of the **stapes**<br>at the **oval window**"];
    E --> F["Fluid waves develop<br>within the **cochlea**"];
    F --> G["Fluid waves carried by<br>**peri-lymph** in cochlea"];
    G --> H1["High-frequency (short)<br>waves carried at the<br>**base** of the cochlea"];
    G --> H2["Low-frequency (long)<br>waves carried to the<br>**apex** of the cochlea"];
    H1 & H2 --> I["Energy of fluid waves<br>transmitted across<br>**scala media**"];
    I --> J["Fluid waves stimulate<br>sensory epithelium in the<br>**organ of Corti**"];
    J --> K["Transduction of mechanical<br>energy into<br>**chemoelectrical<br>potentials**"];
    I --> L["Fluid waves cross scala<br>media to reach<br>**scala tympani**"];
    L --> M["Fluid waves descend to<br>reach the **round window**"];
    M --> N["Fluid wave causes the<br>**round window membrane**<br>to move in and out"];

    K --> A1;

    subgraph HAIR CELLS
        A1["As basilar membrane<br>vibrates"]
        A1 --> B1["The stereocilia bend"]
        B1 --> C1["The tip link part opens<br>up ion channels (K+)"]
        C1 --> D1["K+ influx into cell"]
        Z1["Outer hair cells<br>excited by motor fibres"]--> E1
        D1 --> E1["Cell depolarisation occurs."]
        
        E1 --> F1["Ca++ channels open"]
        F1 --> G1["Influx of Ca++ causes<br>neurotransmitter to be<br>released."]

        E1 --- H1a["The cell becomes more<br>more + or less -"]
        H1a --> J1["The protein in the cell<br>is less repellent againts<br>each other"]
        J1 --> K1["The height of the outer<br>hair cell shortens"]
        K1 --> L1["This pulls the tectorial<br>membrane anchored to<br>the stereocilia down"]
        L1 --> M1["The tectorial membrane<br>increases contact to the<br>inner hair cells"]
        M1 --> N1["Better stimulation of<br>inner hair cell"]
    end

• The mechanical events involved in the transduction of sound waves into chemoelectrical potentials occur as follows:
• Auricle of the external ear collects sound waves that enter the external acoustic meatus
• Sound waves cause the tympanic membrane to vibrate.
• Malleus, incus, and stapes transmit vibrations
• Oscillation of the stapes at the oval window causes fluid waves to develop within the cochlea.
• Fluid waves are carried in the cochlea by peri-lymph.
• High-frequency (short) waves are carried at the base of the cochlea.
• Low-frequency (long) waves are carried to the apex of the cochlea.
• Energy of the fluid waves are transmitted across the scala media from the scala vestibuli to the scala tympani.
• Fluid waves stimulate the sensory epithelium in the organ of Corti, which transduces the mechanical energy of fluid waves into chemoelectrical potentials.
• After crossing the scala media to reach the scala tympani, the fluid waves descend to reach the round window.
• The fluid wave causes the round window membrane to move in and out.

• Two features of the middle ear enhance the efficiency of the energy transfer from air to fluid:
• Because the area of the oval window is substantially smaller than the area of the tympanic membrane, the vibratory force created by sound waves at the tympanic membrane is greatly magnified at the fluid interface of the oval window;
• Because fluid has a higher acoustic impedance than air, the direct transfer of sound waves to the fluid-filled cochlea would result in a substantial loss of energy.
• The transfer of sound energy to the fluid-filled cochlea via the tympanic membrane and ossicles ameliorates this potential loss of energy.

• Question 1: Where are the 1st order neurons located in the primary auditory pathway?
A. Cochlear nuclei
B. Spiral ganglion
C. Medial geniculate body
D. Superior olivary nucleus
E. Inferior colliculus
Answer: B. Spiral ganglion. The 1st order neurons are located in the spiral ganglion. They are bipolar neurons and their peripheral processes reach the hair cells in the spiral organ of Corti, which is the end organ for hearing.

• Question 2: What is the role of the neurons in the dorsal cochlear nucleus?
A. They project axons to the ipsilateral superior olivary nucleus.
B. They project axons across the midline in the dorsal acoustic stria.
C. They receive fibres from both cochleae.
D. They calculate the time difference in arrival of inputs from the right and left cochleae.
E. They project axons to the contralateral superior olivary nucleus via the trapezoid body.
Answer: B. They project axons across the midline in the dorsal acoustic stria. Neurons in the dorsal cochlear nucleus project axons across the midline in the dorsal acoustic stria. These axons then ascend in the contralateral lateral lemniscus to terminate in the inferior colliculus.

• Question 3: Where is the medial geniculate body located?
A. Anterior thalamus
B. Posterior thalamus
C. Medulla
D. Midbrain
E. Pons
Answer: B. Posterior thalamus. The medial geniculate body, where the 3rd order neurons arise, is located in the posterior thalamus.

• Question 4: What is the location of the primary auditory cortex?
A. Inferior temporal gyrus
B. Middle temporal gyrus
C. Superior temporal gyrus, along the lateral (sylvian) fissure: Heschl’s gyrus
D. Precentral gyrus
E. Postcentral gyrus
Answer: C. Superior temporal gyrus, along the lateral (sylvian) fissure: Heschl’s gyrus. The primary auditory cortex is located in the superior temporal gyrus, along the lateral (sylvian) fissure: Heschl’s gyrus.

• Question 5: What happens when there is a lesion in the central auditory pathway?
A. It will result in ipsilateral hearing loss.
B. It will result in complete deafness in either ear.
C. It will result in diminution in hearing on both sides but more so on the contralateral side.
D. It will result in complete deafness in both ears.
E. It will not affect hearing.
Answer: C. It will result in diminution in hearing on both sides but more so on the contralateral side. Lesions of the central auditory pathway, i.e., lateral lemniscus, medial geniculate body, or auditory cortex, will result in diminution in hearing on both sides but more so on the contralateral side. This is because auditory sensation is represented bilaterally in the cortex (but predominantly on the contralateral side).