A&P LECTURE TEST 3

Olfaction (Smell) Pathway

Odorant molecules dissolved in nasal mucus > Bind receptors on olfactory cilia > Olfactory receptor neurons (Cranial Nerve I) > Olfactory bulb (synapse at glomeruli with mitral/tufted cells) > Olfactory tract > Primary olfactory cortex (temporal lobe), amygdala, and hypothalamus.

Olfaction (Smell) Uniqueness

It is the only sensory modality that reaches the cerebral cortex directly without first synapsing in the thalamus.

Signal Generation & Odorant Processing

Mechanism:

1. Odorant binds GPCR

2. Activates GNAL protein > Stimulates adenylyl cyclase

3. ATP > cyclic AMP (cAMP)

4. cAMP opens cyclic nucleotide-gated Na+/Ca+ channels > Influx of Na+ and Ca+ depolarizes the cell, producing a receptor potential.

Distinguishing Smells:

The brain uses combinatorial coding. Each odorant activates a unique combination of receptor types, and each receptor can respond to multiple related odorants. The brain decodes these pattern overlays to recognize thousands of distinct smells using only $\sim 400$ functional receptor types.

Olfaction vs. Gustation Comparison: Receptor Type

+Olfaction: Primary sensory neurons (bipolar cells with cilia)

+Gustation: Epithelial cells with microvilli (renewed every 10-14 days)

Olfaction vs. Gustation Comparison: Target Pathway

+Olfaction: Directly to cortex (limbic/temporal), bypasses thalamus

+Gustation: Reaches gustatory cortex via thalamus

Olfaction vs. Gustation Comparison: Modality Range

+Olfaction: Thousands of distinct odors via combinatorial coding

+Gustation: 5 primary modalities (Sweet, Salty, Sour, Bitter, Umami)

Key Anatomy (Olfaction)-Olfactory Epithelium:

Located in the superior nasal cavity; contains olfactory receptor neurons, basal cells (stem cells), and supporting cells.

Key Anatomy (Olfaction)-Olfactory Epithelium:

-Olfactory Bulb:

Structure above the cribriform plate containing glomeruli and mitral cells where sensory axons synapse.

Key Anatomy (Olfaction)-Olfactory Epithelium:

-Pathways:

Axons pass through the cribriform plate via olfactory foramina to the bulb, then via the olfactory tract to the piriform/temporal cortex and limbic system.

Gustation (Taste) Pathway:

Taste receptor cell activation > Release of neurotransmitter > Sensory neurons of CN VII (Anterior 2/3 tongue), CN IX (Posterior 1/3 tongue), or CN X (Epiglottis/Pharynx) > Solitary nucleus of medulla oblongata > Thalamus (VPM) > Primary gustatory cortex (Insula).

Gustation (Taste) Cellular Modality Detection:

-Salty: Direct influx of $Na^+$ through epithelial sodium channels (ENaC) > Depolarization.

-Sour: Influx of H+ ions or blockage of K+ channels by H+ > Depolarization.

-Sweet, Bitter, Umami: G-protein coupled receptors (GPCRs / T1R and T2R families) > Activation of PLC beta 2 / IP3 second-messenger pathways > Intracellular Ca^2+ release > Depolarization and ATP release.

Multi-Sensory Perception & Discrimination; Olfactory Influence

Flavor perception is a composite of taste, retronasal olfaction, temperature, and texture (trigeminal input). Loss of smell severely diminishes the ability to appreciate flavor.

Multi-Sensory Perception & Discrimination; Discrimination Mechanisms

Gustation relies on labeled-line and population coding for five basic chemical classes, whereas olfaction relies on spatial activation patterns across hundreds of odorant receptor types.

Key Anatomy (Gustation)-Taste Buds

Key Anatomy (Gustation)-Papillae Types

+Vallate (Circumvallate): Large, inverted 'V' at back of tongue; contain many taste buds.

+Fungiform: Button-like, scattered on anterior 2/3; contain few taste buds.

+Foliate: Lateral folds of tongue; taste buds decline with age.

+Filiform: Cover anterior 2/3; no taste buds (provide mechanical friction).

Vision- Light Refraction

Light bends when moving between media of different densities.

Cornea: Provides fixed primary refractive power ($\sim 2/3$ of total power).

Lens: Provides variable refractive power ($\sim 1/3$ of total power) for dynamic focusing.

Vision- Light Accommodation

+ Distant Vision: Ciliary muscle relaxes > Suspensory ligaments (zonules) tighten > Lens becomes flattened.

+ Near Vision: Ciliary muscle contracts > Suspensory ligaments slacken > Lens becomes rounded/convex (higher refractive power).

Focal Deficits & Structures; Myopia (Nearsightedness):

Eyeball is too long or lens too strong. Focal point falls in front of retina. Corrected with concave lenses.

Focal Deficits & Structures; Hyperopia (Farsightedness):

Eyeball is too short or lens too weak. Focal point falls behind retina. Corrected with convex lenses.

Structure-Function Relationships: Cornea

Transparent avascular collagen matrix; lets light in and bends it

Structure-Function Relationships: Lens

Flexible crystal-protein structure; adjusts focal point.

Structure-Function Relationships: Retina

Neural layer housing photoreceptors; converts light into electrical signals.

Vision - Photoreceptors & Processing

Rods vs. Cones ; Vision Type

+ Rods: Scotopic (Night / Low light)

+ Cones: Photopic (Daylight / Color)

Vision - Photoreceptors & Processing

Rods vs. Cones ; Sensitivity

+ Rods: High sensitivity, low acuity

+ Cones: Low sensitivity, high acuity

Vision - Photoreceptors & Processing

Rods vs. Cones ; Distribution

+ Rods: Peripheral retina

+ Cones: Concentrated in fovea centralis

Vision - Photoreceptors & Processing

Rods vs. Cones ; Photopigments

+ Rods: Rhodopsin

+ Cones: Cone opsins (Red, Green, Blue)

Phototransduction Mechanics

In the Dark:

Retinal is in the 11-cis form.

High intracellular cGMP levels keep cGMP-gated Na+/Ca^2+ channels open ("dark current").

Photoreceptor stays depolarized (~ -40mV) and continuously releases glutamate (inhibiting bipolar cells).

Phototransduction Mechanics

In the Light:

Light converts 11-cis retinal to all-trans retinal, activating opsin (bleaching).

Opsin activates the G-protein transducin, which activates phosphodiesterase (PDE).

PDE breaks down cGMP > Na+ channels close.

Photoreceptor hyperpolarizes (~ -70 mV) and stops releasing glutamate, disinhibiting/stimulating downstream bipolar cells.

Phototransduction Mechanics

Vitamin A

Essential precursor needed to regenerate 11-cis retinal. Deficiency leads to night blindness (nyctalopia).

color vision

Produced by differential stimulation of three cone types (S/Blue, M/Green, L/Red).

color blindness

Genetic deficiency/absence of one or more cone photopigments (most commonly X-linked red-green color blindness).

visual pathway

Retina > Optic nerve (CN II) > Optic chiasm (decussation of nasal retinal fibers) > Optic tract > Lateral Geniculate Nucleus (LGN) of thalamus >Optic radiations > Primary Visual Cortex (Occipital lobe).

depth perception (steropsis)

Achieved by overlapping visual fields from both eyes (binocular vision) processed together in the visual cortex.

Vision - Clinical Connections - Blind Spot

+ Structure Affected : Optic Disc

+ Pathophysiology & Functional Impact : Area lacking photoreceptors where optic nerve axons exit; creates a natural gap in visual field

Vision - Clinical Connections - Cataracts

+ Structure Affected : Lens

+ Pathophysiology & Functional Impact : Crystallin proteins denature/clump, turning lens opaque and scattering light.

Vision - Clinical Connections - Glaucoma

+ Structure Affected : Anterior Chamber/Optic Nerve

+ Pathophysiology & Functional Impact : Scleral venous sinus blocked > Increased aqueous humor volume > Elevated IOP > Retinal ganglion cell damage and tunnel vision.

Vision - Clinical Connections - Conjunctivitis

+ Structure Affected : Conjunctiva

+ Pathophysiology & Functional Impact : Inflammation of conjunctival membrane > Dilation of blood vessels ("pink eye"), tearing, irritation.

Vision - Clinical Connections - Mascular Degeneration

+ Structure Affected : Macula Lutea / Fovea

+ Pathophysiology & Functional Impact : Loss of central photoreceptors > Distortion and loss of central, high-acuity vision.

Vision - Clinical Connections - Astigmatism

+ Structure Affected : Cornea or Lens

+ Pathophysiology & Functional Impact : Aspherical curvature of cornea/lens causing uneven light refraction across different axes.

Hearing > Sound Transmission & Cochlear Mechanics - PATHWAY

Sound waves > Auricle > External acoustic meatus > Tympanic membrane vibrates > Auditory ossicles (Malleus > Incus > Stapes) amplify force > Oval window moves > Pressure waves in perilymph (Scala vestibuli) > Vestibular membrane shifts > Endolymph waves in Cochlear duct > Basilar membrane flexes > Hair cell stereocilia bend against Tectorial membrane > Hair cell depolarizes > Release of neurotransmitter > Cochlear branch of CN VIII.

Hearing > Sound Transmission & Cochlear Mechanics - AMPLIFICATION

Achieved via the area difference between the large tympanic membrane and small oval window, plus the lever mechanics of ossicles.

Hearing > Sound Transmission & Cochlear Mechanics - PITCH & LOUDNESS

+ Pitch (Frequency): Encoded by place theory on the basilar membrane. High frequencies flex the narrow, stiff base near the oval window; low frequencies flex the wide, flexible apex.

+ Loudness (Amplitude): Encoded by the degree of deflection of hair cells and higher action potential firing rates.

Base (Narrow/Stiff)

[High Frequency / Pitch)

Apex (Wide/Flexible)

[Low Frequency/Pitch]

Key Structures & Pathway: Hearing - Basilar Membrane

Supports the organ of Corti; vibrates variably based on frequency.

Key Structures & Pathway: Hearing - Tectorial Membrane

Gelatinous structure resting over hair cells; shearing force bends stereocilia.

Key Structures & Pathway: Hearing - Hair Cells

Gelatinous structure resting over hair cells; shearing force bends stereocilia.

Key Structures & Pathway: Hearing - Fluids

Perilymph (scala vestibuli/tympani, high Na+ vs. Endolymph (scala media, high K+).

Key Structures & Pathway: Hearing - Auditory Pathway

CN VIII > Cochlear nuclei (Medulla) > Superior olivary complex > Inferior colliculus (Midbrain) > Medial Geniculate Nucleus (MGN) of thalamus > Primary Auditory Cortex (Temporal lobe).

Dynamic vs. Static Equilibrium

Semicircular Canals (Rotational / Dynamic):

Three fluid-filled loops with dilated ends (ampullae). Rotational movement causes endolymph movement in the ampulla, displacing the gelatinous cupula and bending embedded hair cells.

Dynamic vs. Static Equilibrium

Utricle & Saccule (Gravitational / Linear Acceleration):

Contain maculae with hair cells embedded in a gelatinous otolithic membrane weighted by calcium carbonate crystals (otoliths).

Utricle

Responds to horizontal movement and head tilt.

Saccule:

Responds to vertical movement.

Equilibrium Pathway

Vestibular hair cells > Vestibular nerve (CN VIII) > Vestibular nuclei (Pons/Medulla) OR directly to Cerebellum.

Equilibrium Integration

Vestibular nuclei integrate input from vestibular receptors, visual system, and somatic proprioceptors to coordinate head movements via CN III, IV, VI (vestibulo-ocular reflex) and posture via vestibulospinal tracts.

Age-Related Hearing Changes- Presbycusis

Age-related sensorineural hearing loss.

Age-Related Hearing Changes- Structural Changes

Loss/damage of hair cells at the basal end of the cochlea, stiffening of the basilar membrane, and degeneration of auditory pathway neurons.

Age-Related Hearing Changes- Functional Effect

Progressive loss of the ability to hear high-frequency sounds, reduced ability to speech-discriminate in noisy environments.

Somatic Neuron Pathway

Single motor neuron extending directly from CNS to skeletal muscle target.

Autonomic Neuron Pathway

Two-neuron chain (Preganglionic neuron in CNS > Autonomic ganglion > Postganglionic neuron > Target organ).

ANS vs. Somatic Nervous System- Why 2 Neurons?

Allows divergence, signal integration, and fine-tuning across complex, widely distributed visceral organs.

ANS vs. Somatic Nervouse System - Targets & Control

>Somatic: Skeletal muscle; voluntary control.

>Autonomic: Smooth muscle, cardiac muscle, glands, adipose tissue; involuntary control.

Sympathetic Division Architecture- Organization

Thoracolumbar outflow (T1-L2). Preganglionic soma lie in the lateral gray horns of the spinal cord.

Sympathetic Division Architecture- Ganglia

Located near the spinal cord in the sympathetic chain (paravertebral) ganglia or prevertebral (collateral) ganglia.

Sympathetic Division Architecture- Widespread Effects

Caused by high preganglionic divergence (one preganglionic fiber synapses with many postganglionic fibers) and systemic release of hormones via the adrenal medulla.

Sympathetic Division Architecture- Adrenal Medulla

Modified sympathetic ganglion where preganglionic neurons directly synapse on chromaffin cells, releasing Epinephrine (80%) and Norepinephrine (20%) into the bloodstream. Effects last longer because circulating hormones take time to be degraded by the liver and kidneys.

Sympathetic Neurotransmitters & Receptors

[CNS] ---Acetylcholine (ACh)---> (Nicotinic Receptor) [Ganglion] ---Norepinephrine (NE)---> (Alpha/Beta Receptors) [Target Organ]

Sympathetic Neurotransmitters

Preganglionic: Acetylcholine (ACh).

Postganglionic: Norepinephrine (NE) (except sweat glands, which use ACh).

Sympathetic Adrenergic Receptors -

> α1 Receptors: Excitatory; causes smooth muscle contraction/vasoconstriction in blood vessels of skin/viscera.

> α2 Receptors: Inhibitory; presynaptic autoreceptors that reduce further NE release.

Sympathetic Adrenergic Receptors P2-

Excitatory; located mainly in the heart (increases heart rate and force of contraction).

> β2 Receptors: Inhibitory; causes smooth muscle relaxation/bronchodilation and vasodilation in skeletal muscle vessels.

> β3 Receptors: Found in adipose tissue; stimulates lipolysis.

Parasympathetic Division Architecture - Organization

Craniosacral outflow. Brainstem nuclei (CN III, VII, IX, X) and sacral spinal cord ($S2-S4$).

Parasympathetic Division Architecture - Ganglia

Terminal or intramural ganglia located very close to or inside target tissues.

Parasympathetic Division Architecture - Localized Effects

Preganglionic fibers exhibit minimal divergence (synapsing with 1 or 2 postganglionic neurons), and postganglionic fibers are extremely short, acting directly on a localized target tissue.

Parasympathetic Neurotransmitters & Receptors -

[CNS] ---Acetylcholine (ACh)---> (Nicotinic Receptor) [Ganglion] ---Acetylcholine (ACh)---> (Muscarinic Receptor) [Target Organ]

Parasympathetic Neurotransmitter -

ACh used at both preganglionic and postganglionic synapses.

Parasympathetic Receptor Types -

+ Nicotinic Receptors: Ligand-gated ion channels located on all postganglionic autonomic soma; always excitatory.

+ Muscarinic Receptors: G-protein coupled receptors on parasympathetic target organs; can be excitatory (e.g., GI tract stimulation) or inhibitory (e.g., slowing heart rate via $M_2$ receptors).

Parasymp vs. Symp Structure & Functional Comparison : Origin

Symp: Thoracolumbar (T1-L2)

Parasymp: Craniosacral (CN III, VII, IX, X; S2-S4)

Parasymp vs. Symp Structure & Functional Comparison : FIber Lengths

Symp: Short preganglionic, long postganglionic

Parasymp: Long preganglionic, short postganglionic

Parasymp vs. Symp Structure & Functional Comparison : Ganglia Location

Symp: Close to spinal cord (Chain/Collateral)

Parasymp: In or near target organs (Terminal/Intramural)

Parasymp vs. Symp Structure & Functional Comparison : Divergence

Symp: High (widespread activation)

Parasymp: Low (localized activation)

Parasymp vs. Symp Structure & Functional Comparison : General Role

Symp: Fight or Flight

Parasymp: Rest and Digest

Parasymp vs. Symp Structure & Functional Comparison : Heart Rate

Symp: Increases

Parasymp: Decreases

Parasymp vs. Symp Structure & Functional Comparison : Digestion

Symp: Decreases motility/secretions

Parasymp: Increases motility/secretions

Parasymp vs. Symp Structure & Functional Comparison : Respiration

Symp: Bronchodilation (opens airways)

Parasymp: Bronchoconstriction (constricts airways)

Dual Innervation

Target organs receive inputs from both sympathetic and parasympathetic divisions, providing fine-tuned antagonistic control.

Autonomic Tone

Background rate of autonomic neuron activity maintained even in resting conditions. Allows a single division to either increase or decrease an organ's activity level (e.g., sympathetic tone regulates blood vessel diameter).

Control of the Heart

Parasympathetic (Vagus nerve): Decreases heart rate via ACh at m₂ receptors. Dominate at rest (vagal tone).

Sympathetic: Increases heart rate and contractility via NE at β1 receptors during exercise or stress.

Higher Control of the ANS: Hypothalamus

The chief integration center of the ANS. Its anterior region regulates parasympathetic activity, and its posterior region regulates sympathetic activity.

Higher Control of the ANS: Brainstem

Contains autonomic centers (cardiovascular, vasomotor, respiratory, swallowing) that directly control visceral functions through preganglionic output.

Higher Control of the ANS: Higher Centers

Limbic system and cerebral cortex modulate autonomic responses based on emotional states and cognitive appraisal (e.g., fear triggering sympathetic fight-or-flight).

Memory Types

+ Short-Term (Working) Memory: Temporary retention of information (seconds to minutes); limited capacity.

+ Long-Term Memory: Stable storage of information over days to years; virtually unlimited capacity.

Memory Consolidation

Process of converting short-term memory into long-term memory through structural synaptic changes (Long-Term Potentiation / LTP).

Memory Key Brain Regions - Hippocampus

Essential for converting short-term declarative memories into long-term memories (consolidation).

Memory Key Brain Regions - Amygdala

Processes emotional valence attached to memories (especially fear).

Memory Key Brain Regions - Cerebral Cortex

Stores long-term declarative memories across functional areas.

Memory Engram

Physical or biochemical change in neural pathways corresponding to a stored memory.

Sleep & Consciousness: Deep Sleep (NREM)

Divided into stages 1-3; characterized by slow, high-amplitude EEG delta waves, reduced metabolic rate, decreased blood pressure, and physical recovery.

Sleep & Consciousness: REM Sleep

Rapid Eye Movement sleep; characterized by high metabolic activity, brain waves resembling waking state, dreaming, active muscle inhibition (atonia), and irregular heart/respiratory rates.

Sleep & Consciousness: Reticular Activating System (RAS)

Network in the brainstem that sends continuous activating signals to the cortex to maintain alertness and wakefulness. Inhibition of the RAS induces sleep.

ANS Role in Memory & Sleep : Stress & Memory

Moderate sympathetic activation (via epinephrine/cortisol) stimulates the amygdala and enhances memory consolidation for emotionally charged events. Extremely high or prolonged stress impairs hippocampal function and retrieval.