Sensory Systems and Neural Control

Questions on Sensory Systems and Neural Control

Hair Cell Function and Receptor Potential

• Function: Hair cells are specialized receptor cells in the auditory and vestibular systems that transduce mechanical stimuli into electrical signals.
• Receptor Potential Generation:
  • Mechanical stimulation (e.g., sound waves or head movements) deflects the stereocilia (hair-like structures) on the hair cell.
  • Deflection opens mechanically gated ion channels (primarily K^{+} and Ca^{2+}) located on the stereocilia.
  • Influx of ions (especially K^{+} from the endolymph in the inner ear) causes depolarization of the hair cell.
  • Depolarization opens voltage-gated Ca^{2+} channels at the base of the hair cell.
  • Ca^{2+} influx triggers the release of neurotransmitter (usually glutamate) onto the afferent nerve fibers.
  • The neurotransmitter binds to receptors on the afferent neuron, generating a receptor potential (graded potential) that can lead to action potentials if the threshold is reached.

Vestibular Apparatus Divisions and Stimuli

• Divisions: The vestibular apparatus consists of:
  • Semicircular Canals (3): Detect angular acceleration (rotational movements) of the head.
  • Otolith Organs (2): Utricle and Saccule; detect linear acceleration and head tilt relative to gravity.
• Stimuli Registered:
  • Semicircular Canals: Angular acceleration in three orthogonal planes (horizontal, sagittal, and frontal).
  • Utricle: Linear acceleration in the horizontal plane and head tilt when head is upright.
  • Saccule: Linear acceleration in the vertical plane and head tilt when head is lying down.

Otolith Organs: Utricle and Saccule

• How they work:
  • Otolith organs contain hair cells embedded in a gelatinous layer.
  • The gelatinous layer is covered with otoconia (calcium carbonate crystals).
  • Linear acceleration or head tilt causes the otoconia to shift due to inertia, deflecting the stereocilia of the hair cells.
  • Deflection opens mechanically gated ion channels, leading to receptor potential generation (as described above).
• Difference between Utricle and Saccule:
  • Orientation: Utricle is oriented horizontally, and the Saccule is oriented vertically.
  • Sensitivity: Utricle is more sensitive to horizontal linear acceleration and head tilt when the head is upright. Saccule is more sensitive to vertical linear acceleration and head tilt when the head is lying down.
• Three-Dimensional Information:
  • The utricle and saccule are oriented at approximately 90 degrees to each other, allowing the brain to detect linear acceleration in two dimensions.
  • By combining information from both utricles and saccules (and the semicircular canals), the brain can infer three-dimensional head position and movement. Brain monitors the relative activity between the left and right utricle/saccule to encode a full 360 degrees of tilting in a given plane

Semicircular Canals

• How they work:
  • Each canal is a fluid-filled (endolymph) ring-like structure oriented in one of three orthogonal planes.
  • At the base of each canal is an ampulla containing the cupula, a gelatinous structure in which hair cell stereocilia are embedded.
  • Angular acceleration causes the endolymph to flow, pushing against the cupula and deflecting the stereocilia.
  • Deflection opens mechanically gated ion channels, leading to receptor potential generation.
• Insensitivity to Gravity:
  • The semicircular canals are insensitive to gravity because they only respond to changes in rotational movement (angular acceleration) with constant velocity the cupula returns to its resting position.
  • Unlike the otolith organs, the semicircular canals do not have otoconia, so gravity does not directly stimulate the hair cells.
• Difference between the three semicircular canals:
  • Each canal is oriented in a different plane (approximately orthogonal to each other): horizontal (or lateral), anterior (or superior), and posterior (or inferior).
  • This arrangement allows the detection of angular acceleration in all three dimensions.
• Difference between left and right inner ear canals:
  • The canals are arranged in pairs such that when one canal in a pair is stimulated, the corresponding canal on the opposite side is inhibited.
  • This push-pull arrangement allows for more precise detection of head rotation direction. For instance, the left horizontal canal is paired with the right horizontal canal, the left anterior canal with the right posterior canal, and the left posterior canal with the right anterior canal.

Vestibular Apparatus in Balance Control and Vestibulospinal Tract

• Situations for Balance Control:
  • Maintaining posture during standing, walking, and other movements.
  • Stabilizing the head during body movements. E.g. walking and running.
  • Making compensatory movements in response to sudden changes in support surface or external forces.
• Vestibulospinal Tract Function:
  • The vestibulospinal tract is a descending motor pathway that originates in the vestibular nuclei of the brainstem.
  • It projects to the spinal cord and influences motor neurons that control axial and proximal muscles (neck, back, and limb muscles).
  • Function include:
  • Maintaining upright posture by activating appropriate muscles to counteract gravity.
  • Making rapid adjustments to compensate for changes in balance.
  • Coordinating head and body movements.

Vestibuloocular Reflex (VOR)

• Function:
  • The VOR stabilizes gaze during head movements by producing compensatory eye movements in the opposite direction of the head movement.
  • This ensures that the visual image remains stable on the retina, preventing blurry vision during head motion.
• Reflex Pathway Location:
  • The VOR pathway is located in the brainstem.
  • The pathway involves the vestibular nuclei, the oculomotor nuclei (which control eye muscles), and the medial longitudinal fasciculus (MLF), which connects these nuclei.

Auditory System Questions

Organ of Corti

• Description:
  • The organ of Corti is the sensory transducer of the auditory system, located within the cochlea of the inner ear.
  • It sits on the basilar membrane and contains:
    • Hair Cells: Inner and outer hair cells, which are the receptor cells.
    • Supporting Cells: Provide structural support and maintain the environment of the hair cells.
    • Tectorial Membrane: An overlying structure that contacts the stereocilia of the outer hair cells.

Sound Stimulation of Hair Cells and Traveling Wave

• How sound stimulates hair cells:
  • Sound waves enter the ear and cause the tympanic membrane (eardrum) to vibrate.
  • Vibrations are transmitted through the ossicles (malleus, incus, and stapes) to the oval window of the cochlea.
  • Movement of the oval window creates pressure waves in the perilymph fluid within the cochlea.
  • These pressure waves cause the basilar membrane to vibrate.
  • Vibration of the basilar membrane deflects the stereocilia of the inner hair cells, leading to receptor potential generation.
• Traveling Wave:
  • The vibration of the basilar membrane is not uniform; instead, it forms a traveling wave that propagates from the base of the cochlea towards the apex.
  • The amplitude and location of the peak of the traveling wave depend on the frequency of the sound.
  • High-frequency sounds cause the peak to occur near the base of the cochlea, while low-frequency sounds cause the peak to occur near the apex.

Frequency Identification and Coding in Cochlear Nerve

• Frequency Identification in Basilar Membrane:
  • The basilar membrane is tonotopically organized: different locations along its length respond best to different frequencies. The base responds best to high frequencies, whereas the apex responds best to low frequencies.
• Coding in Cochlear Nerve:
  • Inner hair cells synapse with afferent fibers of the cochlear nerve.
  • When a particular location on the basilar membrane vibrates, it stimulates the hair cells in that region, causing them to release neurotransmitter.
  • The afferent fibers of the cochlear nerve fire action potentials at a rate proportional to the level of stimulation of the hair cells.
  • The frequency of the sound is coded by the location of the activated hair cells on the basilar membrane and the pattern of activity in the cochlear nerve fibers.

Outer Hair Cells Function

• Function:
  • Outer hair cells (OHCs) act as cochlear amplifiers, enhancing the sensitivity and frequency selectivity of the inner hair cells (IHCs).
  • They exhibit electromotility: they change their length in response to changes in membrane potential.
• How they work:
  • When the basilar membrane vibrates, OHCs respond by contracting and expanding.
  • This movement amplifies the vibration of the basilar membrane, increasing the stimulation of the IHCs.
  • OHCs also sharpen the tuning of the basilar membrane, making it more sensitive to specific frequencies.
  • The OHCs are innervated by efferent fibers from the brainstem, which can modulate their activity and thus influence the sensitivity of the cochlea.

Tonotopic Organization of Primary Auditory Cortex

• Meaning:
  • Tonotopic organization means that the primary auditory cortex (A1) is organized according to frequency.
  • Neurons in A1 are arranged in a spatial map, with neurons that respond best to low frequencies located in one region and neurons that respond best to high frequencies located in another region.
  • This organization reflects the tonotopic organization of the basilar membrane in the cochlea.

Auditory System Strategies for Sound Direction

• Strategies:
  • Interaural Time Difference (ITD): The difference in the time it takes for a sound to reach each ear.
  • ITD is used to localize low-frequency sounds. Neurons in the medial superior olive (MSO) are sensitive to ITDs.
  • Interaural Level Difference (ILD): The difference in the intensity (loudness) of a sound at each ear.
  • ILD is used to localize high-frequency sounds. The head casts an acoustic shadow, reducing the intensity of high-frequency sounds at the ear opposite the sound source. Neurons in the lateral superior olive (LSO) are sensitive to ILDs.
  • Head-Related Transfer Function (HRTF): The filtering and reflection of sound waves by the head, pinna (outer ear), and torso. This information is used to localize sounds in elevation (vertical plane) and to resolve front-back ambiguities.