NEUR2020 Lecture 6: Non-visual Sensory Systems Study Guide

University Acknowledgement and Course Introduction

• Acknowledgement of Country: The University of Queensland (UQ) acknowledges the Traditional Owners and their custodianship of the lands on which they meet. They pay respects to Ancestors and their descendants, recognizing their valuable contributions to Australian and global society.

• Course Details: NEUR2020 Neuroscience for Psychologists.

• Module: Lecture 6: Non-visual Sensory Systems.

• Lecturer: Phil Grove.

Models of Sensory Processing

• Former (Old) Model: Specifically structured as a strict hierarchy that was functionally homogeneous and serial. The flow of information was conceptualized as:

  • Receptors

  • Thalamus

  • Primary Sensory Cortex

  • Secondary Sensory Cortex

  • Association Cortex

  • (Minimal feedback considered).

• Current Model: Maintains a hierarchical structure but is characterized as functionally segregated and parallel. It emphasizes extensive feedback loops between different levels of the hierarchy.

Audition: Physical and Perceptual Dimensions of Sound

• Physical Stimulus: Sound waves are characterized by three primary physical dimensions that correspond to specific perceptual experiences:

  • Amplitude: Measured as the height of the wave. High amplitude sounds are perceived as having high loudness, while low amplitude sounds have low loudness.

  • Frequency: Measured in Hertz ($Hz$). High frequency (many cycles per unit time) corresponds to high pitch; low frequency corresponds to low pitch.

  • Complexity: Pure sine waves produce simple sounds, whereas complex waveforms (composed of multiple sine waves added together) produce rich sounds. The perceptual dimension for complexity is Timbre (e.g., the specific sound of a clarinet).

Anatomy of the Ear

• The Outer Ear:

  • Pinnae: These collect sound waves and channel them into the auditory canal. They protect the middle and inner ear.

  • Sound Localization: The folds of the pinnae selectively reflect various frequencies into the canal. As a sound source moves relative to the head, the frequency profile of these reflections changes, providing a cue for location.

• The Middle Ear:

  • Components: Includes the Tympanic Membrane (eardrum), Ossicles (hammer, anvil, stirrup), and the Oval Window.

  • Pressure Amplification: Vibrations from the large tympanic membrane are concentrated onto the much smaller area of the oval window. This produces a $17$-fold increase in pressure.

  • Lever Action: The ossicles act as a lever, amplifying vibrations by approximately $1.3$ times.

  • Total Amplification: The combined effect of area concentration and lever action results in a $22$-fold increase in the strength of vibrations reaching the inner ear.

• The Inner Ear:

  • Cochlea: A snail-shaped structure containing the sensory receptors. It includes the Organ of Corti, which sits on the basilar membrane.

  • Tectorial Membrane: Sits atop the hair cells. Movement of the basilar membrane causes a shear force against the hair cells relative to the tectorial membrane.

  • Hair Cells: The receptors of the auditory system. Upward and downward deflection of the membranes creates the neural signal Sent via the Auditory Nerve.

Frequency Coding in the Cochlea

• Direct Firing Rate: For frequencies between $0-100\,Hz$, coding is achieved through the direct firing rate of a neuron.

• Volley Principle: For frequencies between $100-5000\,Hz$, neurons fire in "volleys" or groups to keep pace with the sound frequency.

• Place Theory: For high frequencies between $5000-23000\,Hz$:

  • The basilar membrane is tuned such that different frequencies cause peak vibrations at different locations.

  • Base of the Basilar Membrane: Close to the stapes/oval window; tuned for high frequencies.

  • Apex of the Basilar Membrane: The far end (near the helicotrema); tuned for low frequencies.

Auditory Pathways

• Signal Flow:

  1. Cochlea and Auditory Nerve.

  2. Cochlear Nuclei (Hindbrain).

  3. Superior Olives (Hindbrain) - Crucial for localization.

  4. Lateral Lemniscus.

  5. Inferior Colliculus (Midbrain Tectum).

  6. Medial Geniculate Nucleus (MGN) of the Thalamus (Forebrain).

  7. Primary Auditory Cortex (located in the Lateral Fissure).

Sound Localization Mechanisms

• Interaural Intensity Differences (IID):

  • The head creates an "acoustic shadow," making sounds louder in the ear closer to the source.

  • This is more effective for high frequencies (e.g., $6000\,Hz$) than low frequencies (e.g., $200\,Hz$).

  • Neural Mechanism: The Lateral Superior Olive (LSO) processes intensity. A strong stimulus to the left ear excites the left LSO and inhibits the right LSO via the MNTB interneuron.

• Interaural Time Differences (ITD):

  • Calculates the difference in time it takes for a sound to reach one ear versus the other.

  • Neural Mechanism: The Medial Superior Olive (MSO) contains neurons that act as coincidence detectors. An MSO neuron fires most strongly when action potentials from both ears arrive simultaneously, based on varying path lengths from the cochlear nucleus.

Auditory Cortex and Streams

• Primary Auditory Cortex (A1): Organized tonotopically, meaning its layout corresponds to the frequency tuning of the cochlea (from apex to base).

• Secondary Auditory Cortex (Belt Areas): Surrounds the primary cortex.

• Functional Streams:

  • Anterior Auditory Pathway: Projects toward the prefrontal cortex; involved in identifying "What" the sound is.

  • Posterior Auditory Pathway: Projects toward the parietal cortex; involved in identifying "Where" the sound is (localization).

Hearing Loss and Treatment

• Intensity Benchmarks ($dB SPL$):

  • Threshold of Hearing: $0\,dB$.

  • Quiet Whisper ($5$ feet): $20\,dB$.

  • Average Office: $50\,dB$.

  • Average Conversation: $60\,dB$.

  • Average Factory: $80\,dB$.

  • Threshold of Pain: $120\,dB$.

  • Colt .45 Pistol ($25$ feet): $140\,dB$.

• Types of Deafness:

  1. Conduction Deafness: Damage to the tympanic membrane or ossicles (e.g., fused ossicles). Does not involve the nervous system.

  2. Sensorineural Deafness: Permanent damage where auditory nerve fibers are not stimulated. Caused by trauma, infection, or toxins (e.g., Streptomycin is ototoxic and can destroy all hair cells). Can be treated with Cochlear Implants, which bypass damaged hair cells using a microphone and electrical signals.

  3. Central Deafness: Caused by brain lesions (e.g., stroke) in the temporal lobes. Can lead to specialized deficits like loss of language processing (left lobe) or sound discrimination (right lobe).

The Somatosensory System

• Three Sub-systems:

  • Exteroceptive System: Senses external stimuli on the skin (Mechanical/touch, Thermal/temperature, Nociceptive/pain).

  • Proprioceptive System: Senses limb position (joint angles), body posture, and vestibular balance.

  • Interoceptive System: Senses internal body conditions (e.g., blood pressure, internal temperature).

• Cutaneous Receptors:

  • Free Nerve Endings: Sensitive to temperature and pain.

  • Pacinian Corpuscles: Adapt quickly; respond to sudden displacements but not constant pressure.

  • Merkel’s Disks & Ruffini Endings: Adapt slowly; respond to gradual pressure or skin stretching.

Somatosensory Pathways and Cortex

• Dorsal Column Medial-Lemniscus: Primarily carries touch and proprioception. Sensory neurons enter the dorsal root, ascend ipsilaterally, synapse and decussate (cross over) at the dorsal column nuclei in the hindbrain, then travel via the medial lemniscus to the Thalamus (Ventral Posterior Nucleus) and Primary Somatosensory Cortex.

• Anterolateral System: Primarily carries pain and temperature. It involves three tracts that decussate immediately upon entering the spinal cord:

  • Spinothalamic Tract: Projects to the Thalamus.

  • Spinotectal Tract: Projects to the tectum.

  • Spinoreticular Tract: Projects to the reticular formation.

• Cortical Organization:

  • Somatotopic Organization: The Somatosensory Homunculus (Postcentral Gyrus) shows that larger cortical areas are dedicated to sensitive parts of the body (e.g., hands, lips).

  • Columns: Arranged in functional columns where neurons respond to the same body part and same stimulus type (e.g., heat).

  • Streams: A dorsal stream (Posterior Parietal) for multisensory integration and a ventral stream (Secondary somatosensory cortex/S2) for shape identification.

Pain Perception and Control

• Anterior Cingulate Cortex: Likely mediates the emotional response to pain rather than the sensation itself. Prefrontal lobotomies reduce emotional pain response without changing the pain threshold.

• Descending Control: The Periaqueductal Grey (PAG) in the midbrain has analgesic effects. It contains receptors for opiate drugs and uses endorphins to modulate pain.

Chemical Senses: Olfaction and Gustation

• Olfaction (Smell):

  • Responds to air-born chemicals.

  • Pathway: Olfactory receptors → Olfactory bulb → Amygdala/Piriform Cortex (Limbic system for emotional response) → Medial dorsal thalamus → Orbitofrontal cortex (Conscious perception).

  • COVID-19: Targets Sustentacular Cells, which maintain the saltwater balance and clear waste on the olfactory membrane.

• Gustation (Taste):

  • Responds to chemicals in solution via taste buds (~$100$ receptors each) located on Papillae.

  • Five Primary Tastes: Bitter, Sweet, Umami (Type 1 receptors), Sour (Type 2), and Salty (Type 3).

  • Pathways: Facilitated by the Facial (VII), Glossopharyngeal (IX), and Vagus (X) nerves. All terminate at the Solitary Nucleus in the Medulla, then move to the Ventral Posterior Thalamus, and finally to the Primary Gustatory Cortex (Insula).

Deficits in Chemical Senses

• Anosomia: Inability to smell. Often caused by head trauma shearing the olfactory receptor axons.

• Ageusia: Inability to taste. Extremely rare due to the multiple/diffuse pathways (nerves VII, IX, and X) providing redundancy.