Auditory and Somatosensory Systems
Auditory System
Sound Waves
Sound waves consist of air compression and expansion.
The auditory system processes information about three key elements:
Frequency
Measured in Hz or cycles per second (e.g., the inverse of wavelength).
Perceived as pitch.
Higher frequency corresponds to higher pitch.
Amplitudes
Defined by the size of the wave.
Perceived as loudness.
Location of Source of Sound
Example: A tuning fork producing a "pure" tone (a single frequency).
Understanding Pitch and Frequency
Pitch refers to the rate of vibrations and can be described as deep or shrill.
Frequency is a physical property referring to the number of vibrations (or cycles) per second of a sound wave, measured in Hertz (Hz).
Generally, higher frequencies lead to higher perceived pitches; lower frequencies correspond to lower pitches.
However, pitch perception is influenced by factors such as:
Harmonic content
Sound intensity
Auditory system processing
Most sounds are complex mixtures of frequencies.
Sound Wave Transformation in Auditory System
Outer Ear
Components:
Pinna: Collects sound and directs it to the ear canal.
Ear Canal: Funnels sound to the tympanic membrane (eardrum).
Functions of Outer Ear:
Collecting sound and funneling it into the ear canal.
Spectral shaping: Alters certain frequencies, enhancing localization cues (especially for vertical sound source location).
Modest gain: Provides a slight increase (a few dB) in sound energy at frequencies around 2-5 kHz, aiding in speech perception.
Middle Ear
Components:
Tympanic membrane (eardrum)
Ossicles: Small bones named malleus, incus, and stapes.
Functioning:
Vibrations from the tympanic membrane are transmitted and amplified by the ossicles.
The pressure changes from the larger diameter tympanic membrane to the smaller diameter oval window increase pressure.
Inner Ear
Cochlea: Converts sound vibrations into neural signals.
Vestibular system: Maintains balance.
Fluids in cochlea:
Perilymph: Found in scala vestibuli and scala tympani.
Endolymph: Found in scala media.
Auditory Transduction
Occurs in the organ of Corti.
Vibrations of the basilar membrane deflect stereocilia of hair cells against the tectorial membrane, opening mechanically gated ion channels.
This causes a $K^+$ influx from the endolymph, depolarizing inner hair cells, and resulting in $Ca^{2+}$-mediated neurotransmitter release to auditory nerve fibers, signaling to the brain.
Frequency Coding
Hair cells signal frequency with great fidelity up to around 1000 Hz using "phase locking."
For intermediate frequencies (up to 4000 Hz), the brain employs population coding: it assesses the combined activity of active axons to decode frequency.
For high frequencies (5000-20000 Hz), labelled lines inform the brain that use only location along the basilar membrane (frequencies below 200 Hz cannot be coded).
Two main methods of coding frequency:
Location along the basilar membrane (cannot code below 200 Hz)
Firing Rate of afferent axons (applies to low and medium frequencies; high frequencies rely solely on location).
Limitations of Phase Locking
Cannot code very high frequencies because neurons cannot fire fast enough (refractory period of ~1-2 ms) and spike timing may become imprecise (temporal jitter).
As an example, for a 5000 Hz tone, cycles occur every 0.2 ms.
Inner vs. Outer Hair Cells
Inner Hair Cells (IHCs):
Responsible for signaling about sound waves.
Outer Hair Cells (OHCs):
Act as active amplifiers in the cochlea, changing length in response to sound (via the motor protein prestin), increasing the vibration of the basilar membrane.
This action enhances sensitivity and frequency selectivity, critical for detecting faint sounds and distinguishing similar pitches.
Auditory Processing
Involves Top-Down Modulation:
OHCs receive descending input from the brain affecting basilar membrane responsiveness.
This modulation likely sharpens frequency sensitivity, responsiveness to low-amplitude sounds, and input based on attention.
Middle ear ossicles have muscles controlling their tension.
Pathway to the Brain
Spiral Ganglion Cells - located in the organ of Corti; provide afferent axons receiving signals from hair cells.
Cochlear Nucleus - first relay of the auditory nerve; extracts timing, intensity, and frequency cues and sends information to higher brain centers.
Superior Olivary Complex - first region to receive input from both ears; critical for sound localization.
Lateral Lemniscus - relays sound information to the inferior colliculus and processes timing and intensity cues for sound localization.
Inferior Colliculus - integrates inputs from lower brainstem centers; crucial for sound localization and processing complex auditory patterns.
Medial Geniculate Nucleus (MGN) - thalamic relay that processes sound features and sends them to the primary auditory cortex.
Primary Auditory Cortex - responsible for conscious perception of sound, including processing of pitch, loudness, and spatial location.
Auditory Information Processing in the Brain
The auditory processing exhibits tonotopic organization.
The auditory system diverges between:
“What is it?” pathway (dorsal cochlear nucleus) - focusing on analysis of sound characteristics.
“Where is it?” pathway (ventral cochlear nuclei) - focusing on sound localization.
Sound Localization Mechanisms
Localizing sound relies on:
Interaural Timing Differences - the time difference each ear receives the sound.
Interaural Level Differences (ILDs) - the intensity difference due to head sound shadowing, most effective for high-frequency sounds (>2000 Hz).
Medial Superior Olive - detects interaural time differences for low-frequency sounds.
The Pinna modifies frequency amplitude depending on vertical sound location.
Properties of sound localization are learned through integration of auditory and visual cues.
Inferior Colliculus Functions
Combines both pathways for higher-level analysis, responding to fundamental auditory features and specific sound patterns.
Auditory Features in Anterior and Posterior Belt Regions
Anterior Belt responds to auditory features primarily.
Posterior Belt is more responsive to spatial location with no specific topographic map.
Somatosensory System Overview
Somatic sensation allows perception of touch, pain, and temperature.
Mediated by Mechanoreceptors sensitive to environmental stimuli, different from other sensory systems.
Types of Skin and Mechanoreceptors
Glabrous (Hairless) Skin
Locations: palms, soles, lips, and genitals.
Hairy Skin
Covers most of the body (arms, legs, back, face).
Main Types of Mechanoreceptors
Merkel Cells/Discs - steady pressure and texture.
Meissner’s Corpuscles - light touch and low-frequency vibrations.
Ruffini Endings - skin stretch detection.
Pacinian Corpuscles - deep pressure and high-frequency vibration.
Hair Follicle Receptors - detect slight hair displacements contributing to touch sensitivity.
Proprioceptive Receptors and Their Roles
Muscle Spindles: detect muscle length and stretch.
Golgi Tendon Organs: sense muscle tension/force.
Joint Receptors: sense joint angle and movement.
Pain and Thermoreception
Nociceptors serve to alert the body of potential tissue damage through pain response.
Thermoreceptors detect changes in temperature, with extremes activating pain pathways.
Somatosensory System Processing
Dorsal Column Pathway - touch and proprioception reach the brain executed through the dorsal columns.
Spinothalamic Pathway - pain and temperature sensations ascend through this route.
Sensory neurons in the skin also have free nerve endings crucial for pain detection.
Receptive Fields and Resolution in Touch
Receptive field size varies by location and affects two-point discrimination.
Types of touch receptors (Merkel, Meissner’s, Pacinian, Ruffini) differ in:
Speed of adaptation (fast vs. slow adapting).
Size of receptive fields.
Sensitivity to specific sensory stimuli.
Central Somatosensory Pathways
Information is somatotopically organized throughout S1 (primary somatosensory cortex), involving different areas for processing different types of information.
The posterior parietal cortex integrates somatic sensation, visual stimuli, and movement planning.
Pain System Overview
Pain systems are distinct from other somatosensory systems; pain is more than a heightened response from non-pain receptors.
First Pain: mediated by A fibers, quick response.
Second Pain: mediated by C fibers, slower response.
Sensory-discriminative vs. affective-motivational pain aspects define the experience of pain and associated responses.
Hyperalgesia and Allodynia
Hyperalgesia: heightened pain in response to painful stimuli.
Allodynia: pain in response to normally non-painful stimuli.
Central Modulation of Pain
Top-down inputs impact pain perception significantly, highlighting the complexity of pain processing in the nervous system.
Endogenous Opioid System
The analgesic effects of opioids are mediated by an endogenous opioid-peptide system, where major families include:
Enkephalins
Endorphins
Dynorphins
Motor System Overview
The motor system interacts with cognitive centers for conscious processing and decision-making.
The Cerebellum detects discrepancies between intended and actual movements, aiding correction.
The Basal Ganglia help in suppressing unwanted movements and initiating movements.
Types of Muscle Movements and Lower Motor Neurons
Lower Motor Neurons innervate no muscle fiber alone but groups of them. Each muscle fiber is innervated by one motor neuron, while each motor neuron can innervate multiple fibers, termed a motor unit.
Motor Neuron Pool: All motor neurons for a given muscle.
Neuromuscular Junctions and Muscle Fiber Contraction
Muscle fibers have membranes known as sarcolemma, where Acetylcholine (ACh) acts as the neurotransmitter.
The transmission is reliable due to the quantity of nicotinic ACh receptors, leading to muscular contraction.
Muscle Contraction Mechanism
Troponin blocks actin and myosin interactions.
$Ca^{2+}$ from the sarcoplasmic reticulum binds troponin, revealing the myosin-binding site.
Myosin heads bind actin, leading to filament sliding, powered by $ATP$.
Lower Motor Neurons and Reflexes
Lower motor neurons in cords constitute basic reflexes like the Stretch Reflex (myotatic reflex), triggered by muscle spindles detecting stretch.
Reciprocal Inhibition: Activation of a muscle (agonist) inhibits its opposing muscle (antagonist).
Reflex Pathways
Flexor Reflex: protective reflex causing limb withdrawal from noxious stimuli.
Crossed-Extensor Reflex: extends opposite limbs to maintain balance when one limb withdraws.
Central Pattern Generators (CPGs)
CPGs produce rhythmic motor activities without sensory input or conscious control, typically more prominent in quadrupeds.
Amyotrophic Lateral Sclerosis (ALS)
Degeneration of motor neurons and motor cortex neurons, leading to muscle atrophy and potential cognitive effects.
Most cases are sporadic, while some hereditary forms involve mutations in the SOD1 gene.
Lower Motor Syndrome
Damage to lower motor neurons leads to paralysis (paresis) without reflexes and observable atrophy.
Difference between Lower Motor Syndrome and ALS is that LMS has various potential causes, while ALS is a specific disease resulting in both upper and lower motor neuron degeneration.
Neuromuscular Disorders
Myasthenia Gravis: characterized by variable weakness of voluntary muscles due to an abnormal immune response and improvement with rest.
Upper Motor Motor System Functions
Modulates and controls activities in lower circuits and motor neuron pools.
Two main descending tracts:
Corticobulbar Tract: influences cranial nerve motor nuclei controlling facial and neck muscles.
Corticospinal Tract: carries voluntary motor commands from the cortex to the spinal cord, responsible for limb and trunk control.
Corticospinal Tracts
Lateral Corticospinal Tract:
Originates from primary motor cortex and descends crossing at the pyramidal decussation. It controls fine movements of distal limbs.
Medial (Ventral) Corticospinal Tract:
Mostly uncrossed fibers contributing to control of proximal muscles for posture and balance.
Brain Stem Pathways
Vestibulospinal Tract: integrates head position adjustments with responses of proximal muscles.
Reticulospinal Tracts: facilitate/postpone extensor/flexor responses for posture and locomotion.
Cerebellar Involvement
The reticular formation within the brainstem integrates sensory inputs with top-down control, influencing automatic movements for stability and coordination.
Conclusion on Motor Control
Involves extensive interactions between the motor cortex, brainstem, and spinal cord with critical feedback and fine-tuning from sensory and cortical systems to enable adaptive motor control in humans.