Neuroscience: Visual, Auditory, Olfactory, and Somatosensory Systems

Fusiform gyrus

Face processing area in the brain; infants show early biases to face-like patterns.

Experience-dependent plasticity

The principle that early visual input sculpts circuits; deprivation has lasting effects.

Critical periods

Sensitive windows for organizing excitation/inhibition across synapses.

Depth perception

Relies on binocular disparity, which involves slightly different images to each eye.

Strabismus

Condition where eyes do not align; leads to poor depth perception.

Monocular deprivation

Early deprivation of one eye leads to favoring of the stimulated eye and loss of representation of the deprived eye.

Binocular deprivation

Easier to recover from because neither eye dominates.

Activity-dependent competition

Mechanism that tunes excitatory/inhibitory synapses during critical periods.

Image disparity

Input used for depth perception, involving the difference between images seen by two eyes.

Disparity-selective neurons

Neurons that integrate signals from both eyes to infer distance.

Occlusion therapy

Treatment principle for strabismus that involves covering the stronger eye to promote use of the weaker eye.

Sound (Physical definition)

Pressure changes in a medium such as air or water.

Sound (Perceptual definition)

The subjective experience of hearing.

Amplitude

Corresponds to loudness; subjective and varies by individual.

Frequency (Hz)

Corresponds to pitch; higher frequency equals higher pitch.

Human hearing range

Approximately 15-20,000 Hz; high-frequency sensitivity declines with age or noise exposure.

Outer ear (pinna)

Funnels sound, aids localization, and protects with hair and earwax.

Middle ear

Contains tympanic membrane and ossicles that amplify and conduct sound to the oval window.

Inner ear (cochlea)

Snail-shaped structure filled with viscous fluid; involved in sound transduction.

Basilar membrane

Moves in response to sound, causing hair cells to bend and initiate transduction.

Frequency Theory

Theory stating that the basilar membrane oscillates with sound, causing the auditory nerve to fire at the same frequency.

Place Theory

Theory that specific places on the basilar membrane are tuned to specific frequencies.

Tonotopic map

Ordered frequency representation in the primary auditory cortex (A1).

Conductive (middle ear) deafness

ossicle damage; often improved with hearing aids (amplification).

Sensorineural/nerve (inner ear) deafness

cochlea/hair cells damage → frequency deficits; cochlear implants can help.

Tinnitus

phantom ringing from hair cell loss; neurons respond to lack of input.

Taste (Gustation)

Receptors in taste buds on papillae; thousands of buds.

Primary tastes

sweet, sour, bitter, salty, umami (MSG); evidence for fat and experiences like metallic, water.

Salty (Na⁺)

Na⁺ influx through channels.

Sour (acids)

closes K⁺ channels.

Sweet, Bitter, Umami

receptor‑mediated cascades.

Coding in taste

Brain compares response ratios across receptor populations (e.g., sour:sweet:bitter:salty...).

Signals in taste

via cranial nerves → thalamus → gustatory cortex (insula involvement).

PTC sensitivity

supertasters have more fungiform papillae; hormonal influences.

Smell (Olfaction)

Flavor = taste + smell; strong interactions with insula; endopiriform involvement.

Olfactory receptors

~350 olfactory receptor types in humans; located in olfactory mucosa at nasal cavity top.

Damage in olfaction

anosmia (general) or specific anosmias (particular chemicals).

Olfactory bulb glomeruli map

limited, chemical‑specific activation patterns; nearby chemicals → nearby areas.

Cortical targets in olfaction

piriform (odor identity), medial amygdala (social), entorhinal (memory).

Knockout Experiments

Eliminating K⁺ channels or receptors abolishes reception → demonstrates necessity.

Vomeronasal Organ (VNO)

Larger and behaviorally critical in many animals.

Male rat VNO removal

→ aberrant mating (e.g., mate indiscriminately; cannot locate females by odor).

Synesthesia

Cross‑modal perceptions (e.g., sounds evoke colors); illustrates atypical sensory coupling.

Mechanoreceptors

pressure/touch (e.g., Pacinian corpuscle—deep pressure, sodium permeability changes).

Thermoreceptors

temperature (greater sensitivity to cold; baseline body warmth).

Nociceptors

pain.

Chemoreceptors

chemical stimuli (e.g., capsaicin—activates nociceptors; "spicy").

Spinal Organization

Spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Dermatomes

skin territories served by spinal nerves.

Major Somatosensory Pathways

Fine touch & proprioception: Dorsal column-medial lemniscus; Crude touch: Anterior spinothalamic tract; Pain & temperature: Lateral spinothalamic tract.

Primary somatosensory cortex (S1)

in parietal lobe → conscious touch perception.

Lesions in somatosensation

astereognosis (can't recognize by touch); neglect syndrome (right parietal damage → ignore left space).

Types of Pain (Scholz & Woolf)

Nociceptive: signals impending/actual tissue damage (heat, chemicals, severe pressure, cold).

Nociceptive Pain

Signals impending/actual tissue damage (heat, chemicals, severe pressure, cold).

Inflammatory Pain

From tissue/joint damage or tumors.

Neuropathic Pain

From CNS or nerve damage (e.g., stroke, carpal tunnel).

Mild Pain

Primarily glutamate.

Strong Pain

Add Substance P release in spinal cord.

Thicker/Myelinated Fibers

Faster, sharp pain.

Thin/Unmyelinated Fibers

Slower, dull/aching pain.

Gate Control Theory

Non‑painful input (touch/pressure) can close the gate on pain transmission.

Analgesia

Reduced pain via stimulation of adjacent areas (e.g., TENS, acupuncture).

Capsaicin Overuse

Depletes Substance P; explains icy‑hot style effects.

Descending Control

Periaqueductal gray (PAG) activation.

Stress and Placebo Effects

Often endorphin mediated.

Naloxone

Blocks opioid receptors → reverses endorphin‑based analgesia; Substance P rises; pain perceived.

Skeletal Muscle

Voluntary; on bones.

Smooth Muscle

Involuntary; gut, vessels.

Cardiac Muscle

Involuntary; heart.

Motor Units

A motor axon can innervate >1 muscle fiber (eye: ~1:1-1:3; arm: 1:>100).

Neuromuscular Junction (NMJ)

Releases acetylcholine (ACh); AChE breaks it down to acetate + choline.

Myasthenia Gravis

ACh receptors blocked by antibodies → muscle fatigue; treatments target AChE or immune modulation.

Slow-Twitch Fibers

Aerobic, resist fatigue; support sustained/non‑strenuous activities.

Fast-Twitch Fibers

Anaerobic, fatigue quickly; for quick, powerful moves.

Muscle Spindle

Detects stretch → stretch reflex (e.g., knee‑jerk).

Golgi Tendon Organ

Senses tension → inhibits contraction if too strong.

Central Pattern Generators (CPGs)

Spinal circuits generating coordinated rhythms (e.g., walking)—do not require brain input to operate basic rhythm.

Primary Motor Cortex (M1)

In precentral gyrus: fine, complex, contralateral control with somatotopic map.

Dorsolateral Corticospinal Tract

Mostly contralateral, distal/peripheral control.

Ventromedial Corticospinal Tract

Bilateral control of axial/whole‑body movements (e.g., posture, walking).

Cerebellum

Roles: balance, posture, coordination; aim & timing, sequential movement execution, smooth pursuit, control of trajectory/velocity/acceleration.

Ataxia

Slowed, poorly timed sequences; deficits associated with cerebellum damage.

Striatum

Composed of caudate and putamen; involved in initiation of movement and learning movement patterns.

Globus pallidus dysfunction

Leads to loss of appropriate inhibition of thalamus, resulting in inappropriate motor outputs.

Huntington's Disease (HD)

A genetic disorder characterized by chorea, cognitive decline, and degeneration of basal ganglia.

Onset of Huntington's Disease

Typically occurs in the 30s to 40s.

Motor symptoms of Huntington's Disease

Includes chorea, which starts small and grows, eventually leading to immobility.

Cognitive/psychiatric symptoms of Huntington's Disease

Includes dementia, depression, anxiety, OCD, and psychosis.

Neuropathology of Huntington's Disease

Involves degeneration of basal ganglia with early loss of inhibitory components leading to excess movement.

Genetics of Huntington's Disease

Autosomal dominant inheritance with expanded CAG trinucleotide repeats on chromosome 4.

Parkinson's Disease (PD)

A neurodegenerative disorder characterized by bradykinesia, resting tremor, and rigidity.

Symptoms of Parkinson's Disease

Includes bradykinesia, resting tremor, rigidity, shuffling gait, and postural instability.

Neuropathology of Parkinson's Disease

Loss of dopaminergic neurons in substantia nigra leading to reduced dopamine in striatum.

Etiology of Parkinson's Disease

Involves genetic susceptibility and environmental factors such as toxins and lifestyle.

L-DOPA

A dopamine precursor used as symptomatic treatment for Parkinson's Disease, effectiveness declines over time.

Fetal tissue transplantation

A historical treatment for Parkinson's Disease with mixed and ethical issues.

Deep Brain Stimulation (DBS)

Involves implanted electrodes to modulate circuits in the brain for treating movement disorders.

Monocular early deprivation

Results in dominance of the open eye and poor function in the deprived eye due to competitive plasticity.

Conductive hearing loss

Occurs in the middle ear, characterized by poor conduction and amplification, often remedied by hearing aids.

Sensorineural hearing loss

Occurs in the inner ear or nerve, characterized by frequency-specific loss, often remedied by cochlear implants.