Neuron (Nerve Cell): A neuron is a microscopic structure composed of three major parts, namely, the cell body, dendrites and axon. It is the structural and functional unit of the nervous system.
Cell Body (Soma or Cyton): Contains the cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules .Dendrites: Short fibres which branch repeatedly and project out of the cell body. They also contain Nissl’s granules. Dendrites transmit electrical impulses towards the cell body.Axon: A long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called a synaptic knob which possesses synaptic vesicles containing chemicals called neurotransmitters . The axon transmits nerve impulses away from the cell body to a synapse or to a neuromuscular junction.
Nerves: In the peripheral nervous system, the axons of neurons are bundled together to form a nerve. Nerves are covered by connective tissue sheaths.
Based on the number of axon and dendrites, the neurons are divided into four types:
Unipolar Neuron: Cell body with one axon only. Found usually in the embryonic stage.Bipolar Neuron: With one axon and one dendrite. Found in the retina of the eye and the olfactory epithelium.Multipolar Neuron: With one axon and two or more dendrites. Found in the cerebral cortex.Pseudounipolar Neuron: A single process arises from the cell body and then divides into an axon and a dendrite. Found in the dorsal root ganglia of the spinal cord.
Myelinated Nerve Fibre: The axon is enveloped with Schwann cells , which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier . Found in spinal and cranial nerves.Non-myelinated Nerve Fibre: The axon is enclosed by a Schwann cell that does not form a myelin sheath around the axon. Found in autonomous and the somatic neural systems.
A nerve impulse is a wave of electrochemical change that travels along a neuron.
In a resting neuron (a neuron not conducting any impulse), the axonal membrane is more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm.
Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of Na+.
In contrast, the fluid outside the axon contains a low concentration of K+ and a high concentration of Na+ and thus forms a concentration gradient.
This ionic gradient across the resting membrane is maintained by the active transport of ions by the sodium-potassium pump (Na+-K+ Pump) which transports 3 Na+ outwards for 2 K+ into the cell.
As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged. The electrical potential difference across the resting plasma membrane is called as the resting potential . The membrane is said to be polarized .
When a stimulus is applied, the membrane at the site becomes freely permeable to Na+.
The All-or-None Law
A nerve impulse is not graded. If a stimulus reaches the threshold, a full action potential is generated. If it doesn't, nothing happens. It's like pulling the trigger of a gun; the bullet fires with the same force regardless of how hard you pull, as long as you pull hard enough.
This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. This reversal of polarity is called depolarization .
The electrical potential difference across the plasma membrane at the site of the action potential is called the action potential , which is in fact termed as a nerve impulse .
At the peak of the action potential, the permeability to Na+ decreases, and the membrane becomes more permeable to K+.
K+ ions diffuse outwards, and the membrane potential returns towards the resting potential. This is called repolarization .
At sites immediately ahead, the axon membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from the site of the action potential to the next site. On the outer surface, a current flows from the next site to the site of the action potential to complete the circuit.
Hence, the polarity at the site is reversed, and an action potential is generated at the next site. Thus, the impulse (action potential) generated at one point arrives at the next point. This sequence is repeated along the length of the axon and consequently, the impulse is conducted.
Saltatory Conduction: In myelinated nerve fibres, the action potential jumps from one node of Ranvier to the next. This type of conduction is much faster than in non-myelinated fibres.
A nerve impulse is transmitted from one neuron to another through junctions called synapses.
Synapse: A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft .Types of Synapses:
Electrical Synapse: The membranes of pre- and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses. Transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse.Chemical Synapse: The membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft . Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.
When an action potential arrives at the axon terminal (synaptic knob), it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane.
This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.
Neurotransmitters: These are chemicals that transmit signals across a chemical synapse. Examples include Acetylcholine, Dopamine, Serotonin, GABA (Gamma-Aminobutyric Acid), etc.
The CNS includes the brain and the spinal cord and is the site of information processing and control.
Forebrain (Prosencephalon):
Cerebrum: Forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres . The hemispheres are connected by a tract of nerve fibres called corpus callosum . The layer of cells which covers the cerebral hemisphere is called the cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the grey matter due to its greyish appearance. The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions called as the association areas are responsible for complex functions like intersensory associations, memory and communication. The inner part of the cerebral hemisphere consists of fibres of the tracts covered with the myelin sheath, which constitute the white matter .Thalamus: A major coordinating centre for sensory and motor signaling.Hypothalamus: Lies at the base of the thalamus. It contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones.
Midbrain (Mesencephalon): Located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passes through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina .
Hindbrain (Rhombencephalon):
Pons: Consists of fibre tracts that interconnect different regions of the brain.Cerebellum: Has a very convoluted surface in order to provide the additional space for many more neurons. It is responsible for the regulation of balance and coordination of voluntary movements.Medulla Oblongata: Connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.
Structure: A long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. The spinal cord is covered by the same three meninges as the brain. The grey matter is in the centre (H-shaped) and the white matter is on the outside.Functions:
Conducts sensory and motor impulses to and from the brain.
Acts as a centre for reflex actions.
The PNS comprises all the nerves of the body associated with the CNS (brain and spinal cord).
Somatic Nervous System: Relays impulses from the CNS to skeletal muscles.Autonomic Nervous System (ANS): Transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. The ANS is further classified into the sympathetic and parasympathetic nervous systems.
The part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system. It is a part of the autonomic nervous system.
Sensory organs are specialized structures that detect environmental stimuli and convert them into neural signals through a process called transduction . The five primary senses work together with the nervous system to provide comprehensive information about our environment.
Adequate Stimulus : Each sense organ responds optimally to a specific type of energyThreshold : Minimum stimulus intensity required for detectionAdaptation : Decreased response to constant stimulationSensory Coding : How stimulus properties are encoded in neural activity
Eyelids (Palpebrae) : Protect the eye, contain meibomian glandsConjunctiva : Transparent membrane covering the sclera and inner eyelidsLacrimal Apparatus : Produces and drains tears
Lacrimal gland (tear production)
Lacrimal puncta, canaliculi, sac, and nasolacrimal duct (drainage)
Three Layers:
Fibrous Layer (Outer)
Sclera : White, tough connective tissue providing structureCornea : Transparent, avascular, highly innervated anterior portion
Epithelium, Bowman's layer, stroma, Descemet's membrane, endothelium
Vascular Layer (Middle/Uvea)
Choroid : Highly vascularized, provides nutrients to outer retinaCiliary Body : Contains ciliary muscle for accommodation
Ciliary processes secrete aqueous humor
Iris : Colored portion with central opening (pupil)
Sphincter pupillae (constricts pupil, parasympathetic)
Dilator pupillae (dilates pupil, sympathetic)
Neural Layer (Inner)
Retina : Light-sensitive tissue containing photoreceptors
Aqueous Humor : Clear fluid in anterior and posterior chambersLens : Transparent, biconvex structure for focusingVitreous Humor : Gel-like substance filling posterior chamberOptic Disc : Blind spot where optic nerve exitsMacula Lutea : Area of highest visual acuityFovea Centralis : Central depression in macula with only cones
Inner Limiting Membrane Nerve Fiber Layer : Axons of ganglion cellsGanglion Cell Layer : Cell bodies of ganglion cellsInner Plexiform Layer : Synapses between bipolar, amacrine, and ganglion cellsInner Nuclear Layer : Bipolar cells, horizontal cells, amacrine cells, Müller cellsOuter Plexiform Layer : Synapses between photoreceptors and bipolar/horizontal cellsOuter Nuclear Layer : Cell bodies of photoreceptorsExternal Limiting Membrane Photoreceptor Layer : Rods and conesRetinal Pigment Epithelium (RPE)
Rods (120 million per eye)
Structure : Outer segment with rhodopsin-containing discsFunction : Detect light/dark, responsible for scotopic (night) visionDistribution : Absent in fovea, highest density in peripheral retinaSensitivity : Extremely sensitive to light, saturate in bright light
Cones (6 million per eye)
Types : S-cones (short wavelength/blue), M-cones (medium/green), L-cones (long/red)Structure : Outer segment with photopsin-containing membrane foldsFunction : Color vision and high acuity photopic (day) visionDistribution : Concentrated in fovea and macula
Bipolar Cells : Transmit signals from photoreceptors to ganglion cellsHorizontal Cells : Lateral processing and contrast enhancementAmacrine Cells : Complex processing and motion detectionGanglion Cells : Generate action potentials, form optic nerveMüller Cells : Glial support cells spanning entire retinal thickness
Light absorption by rhodopsin (opsin + 11-cis retinal)Conformational change to all-trans retinalActivation of transducin (G-protein)Activation of phosphodiesteraseDecreased cGMP levelsClosure of Na+ channelsHyperpolarization of photoreceptorReduced neurotransmitter (glutamate) release
ON-pathway : Depolarizing bipolar cells (sign-conserving)OFF-pathway : Hyperpolarizing bipolar cells (sign-inverting)
1. External Environment → Cornea
Light rays enter the eye through the transparent cornea
Cornea provides ~43 diopters of refractive power (65% of total)Refraction occurs at air-cornea interface due to refractive index differenceLight rays begin converging toward the retina
2. Cornea → Anterior Chamber → Pupil
Light passes through aqueous humor in anterior chamber
Iris controls pupil size :
Bright light: Sphincter pupillae contracts → miosis (small pupil)
Dim light: Dilator pupillae contracts → mydriasis (large pupil)
Pupil diameter ranges from 1.5mm (bright) to 8mm (dark)
3. Pupil → Lens
Light enters the biconvex crystalline lens
Lens provides ~15-20 diopters (variable for accommodation)Accommodation mechanism :
Near objects : Ciliary muscle contracts → lens becomes more spherical (higher power)Far objects : Ciliary muscle relaxes → lens flattens (lower power)
Presbyopia : Age-related lens hardening reduces accommodation
4. Lens → Vitreous Chamber
Light travels through vitreous humor (gel-like substance)
Maintains eye shape and provides structural supportTransparent medium allows unimpeded light passageAny floaters or opacities can cast shadows on retina
5. Vitreous → Retina
Light must pass through multiple retinal layers before reaching photoreceptors
Paradoxical arrangement : Light travels through neural layers firstMinimal light scattering due to transparent cellular organizationFoveal pit : Thinnest retinal area for optimal light access to cones
6. Retinal Processing
Photoreceptors (rods and cones) capture photons in outer segments Light passes through retinal layers in reverse order :
Inner limiting membrane
Nerve fiber layer
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
External limiting membrane
Photoreceptor layer (final destination)Retinal pigment epithelium (absorbs excess light)
Emmetropia (Normal Vision)
Parallel light rays from distant objects focus exactly on retinaTotal refractive power : ~60 dioptersAxial length : ~24mm correlates with refractive power
Refractive Errors
Myopia : Light focuses anterior to retina (eyeball too long)Hyperopia : Light focuses posterior to retina (eyeball too short)Astigmatism : Irregular curvature causes multiple focal points
Dark State (Depolarized)
High cGMP levels keep cation channels open
Continuous Na+/Ca2+ influx maintains depolarization (~-30mV)
Glutamate release onto bipolar cells
Light State (Hyperpolarized)
Photon absorption by rhodopsin/photopsins11-cis retinal → all-trans retinal conformational changeTransducin activation (G-protein cascade)Phosphodiesterase activation breaks down cGMPcGMP levels drop → cation channels closeHyperpolarization to ~-70mVReduced glutamate release
Myopia (Nearsightedness) : Eyeball too long, distant objects blurryHyperopia (Farsightedness) : Eyeball too short, near objects blurryAstigmatism : Irregular corneal curvature causing distorted visionPresbyopia : Age-related loss of accommodation
Macular Degeneration : Deterioration of central retinaDiabetic Retinopathy : Vascular damage from diabetesRetinal Detachment : Separation of neurosensory retina from RPERetinitis Pigmentosa : Progressive degeneration of photoreceptors
Glaucoma : Increased intraocular pressure damaging optic nerveCataracts : Lens opacity affecting vision clarityColor Blindness : Deficiency in cone photopigments
Auricle (Pinna) : Cartilaginous structure collecting sound wavesExternal Auditory Canal : S-shaped tube lined with ceruminous glandsTympanic Membrane : Thin membrane separating external and middle ear
Ossicles (Tiny Bones):
Malleus (Hammer) : Attached to tympanic membraneIncus (Anvil) : Intermediate boneStapes (Stirrup) : Attached to oval window
Other Structures:
Eustachian Tube : Connects middle ear to nasopharynxStapedius and Tensor Tympani Muscles : Protect inner ear from loud soundsMastoid Air Cells : Air-filled spaces in temporal bone
Bony Labyrinth:
Cochlea : Spiral structure for hearingVestibule : Central chamber containing saccule and utricleSemicircular Canals : Three perpendicular canals for rotational movement detection
Membranous Labyrinth:
Cochlear Duct (Scala Media) : Contains organ of CortiSaccule and Utricle : Detect linear acceleration and head positionSemicircular Ducts : Detect rotational movements
1. Sound Wave Generation → External Ear
Sound waves (pressure variations in air) enter the auricle (pinna) Pinna shape helps collect and funnel sound wavesSound localization : Pinna provides directional cues through filteringSound travels through external auditory canal (~2.5 cm length)
2. External Canal → Tympanic Membrane
External auditory canal acts as resonant tube Natural resonance around 3000 Hz amplifies important speech frequenciesCerumen (earwax) protects and lubricates canalSound waves strike tympanic membrane (eardrum)
Tympanic membrane vibration frequency matches sound wave frequency
3. Tympanic Membrane → Ossicular Chain (Middle Ear)
Mechanical Advantage System:
Malleus (Hammer)
Manubrium attached to tympanic membraneVibrations transfer to head of malleus
Lever arm provides mechanical advantage
Incus (Anvil)
Body articulates with malleus headLong process connects to stapesActs as intermediate link in ossicular chain
Stapes (Stirrup)
Footplate sits in oval window Smallest bone in human bodyPiston-like movement transfers energy to inner ear
Amplification Mechanisms:
Lever advantage : Malleus-incus lever ratio ~1.3:1Area advantage : Tympanic membrane area (~55 mm²) vs oval window (~3.2 mm²)Total amplification : ~20-fold increase in pressureImpedance matching : Converts low-pressure air waves to high-pressure fluid waves
4. Oval Window → Inner Ear Fluids
Perilymph Movement (Scala Vestibuli and Tympani)
Stapes footplate movement creates pressure waves in perilymphPerilymph composition : Similar to extracellular fluid (high Na+, low K+)Wave propagation : From base to apex of cochleaFrequency-dependent travel : High frequencies travel short distances, low frequencies travel further
Endolymph (Scala Media)
Unique composition : High K+ (~150 mM), low Na+ (~1 mM)Endocochlear potential : +80mV relative to perilymphMaintained by : Stria vascularis (ion pumps and channels)Critical for : Hair cell depolarization
5. Cochlear Mechanics
Basilar Membrane Properties
Base (near oval window) :
Narrow (~0.1 mm) and stiff
High-frequency response (20,000 Hz)Hair cells have short stereocilia
Apex (helicotrema) :
Wide (~0.5 mm) and flexible
Low-frequency response (20 Hz)Hair cells have long stereocilia
Traveling Wave Theory (Georg von Békésy)
Progressive wave travels from base to apexPeak amplitude occurs at frequency-specific locationSharp tuning enhanced by outer hair cell active mechanismsEnvelope peak determines pitch perception
6. Hair Cell Activation
Mechanical Transduction Process:
Basilar membrane displacement causes relative motionTectorial membrane shears against hair cell stereociliaStereocilia deflection opens mechanotransduction channelsK+ influx from endolymph causes depolarizationCa2+ influx triggers neurotransmitter releaseSpiral ganglion activation generates action potentials
Hair Cell Types:
Inner Hair Cells (IHC) - Primary Sensors
Number : ~3,500 per cochleaFunction : Main sensory transductionInnervation : 95% of auditory nerve fibersArrangement : Single row along cochleaNeurotransmitter : Glutamate
Outer Hair Cells (OHC) - Amplifiers
Number : ~12,000 per cochlea (3 rows)Function : Active amplification and tuningMechanism : Prestin-mediated electromotilityEffect : 40-60 dB amplificationDamage : Causes hearing loss and reduced frequency selectivity
7. Neural Encoding
Frequency Coding:
Place theory : Different frequencies activate different cochlear locationsTemporal theory : Phase-locking to stimulus waveform (up to ~1000 Hz)Combination : Both mechanisms work together
Intensity Coding:
Rate coding : Higher intensity → higher firing ratesPopulation coding : More neurons recruitedDynamic range : ~120 dB range compressed into neural firing rates
8. Central Auditory Processing
Auditory Pathway Sequence:
Spiral ganglion → Cochlear nerve (CN VIII)Cochlear nuclei (ventral and dorsal)Superior olivary complex (sound localization)Inferior colliculus (midbrain integration)Medial geniculate nucleus (thalamic relay)Primary auditory cortex (temporal lobe)
Sound Localization Mechanisms:
Interaural time differences (ITD): Sound arrival time differencesInteraural level differences (ILD): Sound intensity differencesHead-related transfer function : Pinna and head filtering effects
Acoustic Reflex
Stapedius muscle : Contracts to loud sounds (>70-90 dB)Tensor tympani : Additional protectionFunction : Reduces transmission by ~20 dBLatency : 40-160 milliseconds (too slow for sudden loud sounds)
Eustachian Tube
Pressure equalization : Connects middle ear to nasopharynxOpens during : Swallowing, yawning, sneezingDysfunction : Can cause conductive hearing loss
Location : Spiral organ within cochlear duct
Key Structures:
Basilar Membrane : Supporting structure with tonotopic organizationHair Cells : Mechanoreceptors with stereocilia
Inner Hair Cells (IHC) : ~3,500 cells, primary sensory receptorsOuter Hair Cells (OHC) : ~12,000 cells, amplify and fine-tune response
Tectorial Membrane : Overlying structure that contacts hair cell stereociliaSupporting Cells : Pillar cells, Deiters' cells, Hensen's cells
Sound waves cause basilar membrane displacementStereocilia deflection opens mechanically-gated K+ channelsK+ influx (from K+-rich endolymph) causes depolarizationCa2+ influx triggers neurotransmitter releaseSpiral ganglion neurons generate action potentials
Place Theory : Different frequencies activate different regions of basilar membraneBase : High frequencies (20,000 Hz)Apex : Low frequencies (20 Hz)Temporal Coding : Phase-locking for frequencies below 1,000 Hz
Saccule and Utricle:
Macula : Sensory epithelium with hair cellsOtolithic Membrane : Contains calcium carbonate crystals (otoconia)Function : Detect linear acceleration and head position relative to gravity
Three Canals : Anterior, posterior, lateral (horizontal)
Ampulla : Enlarged region containing crista ampullarisCupula : Gelatinous structure deflected by endolymph movementFunction : Detect rotational movements
Type I : Flask-shaped, surrounded by calyx endingType II : Cylindrical, contacted by bouton endingsKinocilium : Single true cilium (absent in cochlear hair cells)Stereocilia : Actin-filled projections arranged by height
Conductive : Problem in external/middle ear (cerumen, otitis media, otosclerosis)Sensorineural : Inner ear or neural damage (presbycusis, noise-induced, ototoxicity)Mixed : Combined conductive and sensorineural components
Benign Paroxysmal Positional Vertigo (BPPV) : Displaced otoconiaMénière's Disease : Excess endolymph causing vertigo, hearing loss, tinnitusVestibular Neuritis : Inflammation of vestibular nerveLabyrinthitis : Inflammation affecting both hearing and balance
Nasal Bones : Form bridge of noseCartilages : Lateral, septal, and alar cartilages provide shapeNostrils (Nares) : External openings
Nasal Septum : Divides cavity into left and right sidesConchae (Turbinates) : Three bony projections creating air turbulence
Superior, middle, and inferior conchae
Meati : Spaces beneath each concha
Location : Superior portion of nasal cavity and upper septumArea : ~5 cm² each side in humansOlfactory Mucosa : Specialized pseudostratified epithelium
Olfactory Receptor Neurons (ORNs)
Bipolar neurons with apical dendrites and basal axonsOlfactory Cilia : 10-30 non-motile cilia containing odorant receptorsLifespan : 30-60 days, continuously replaced
Supporting (Sustentacular) Cells
Columnar cells providing structural supportSecretions : Mucus and detoxifying enzymesInsulation : Electrical isolation between ORNs
Basal Cells
Stem cells for ORN replacementTypes : Horizontal basal cells (quiescent) and globose basal cells (active)
Microvillar Cells
Chemoreceptive cells of unknown functionSparse distribution throughout epithelium
Serous glands in lamina propriaFunction : Secrete mucus containing odorant-binding proteinsDucts : Open onto epithelial surface
G-protein coupled receptors (GPCRs) ~400 functional genes in humans (largest gene family)One receptor type per neuron (monogenic expression)Broad tuning : Each receptor responds to multiple odorants
Odorant binding to receptor proteinG-protein activation (Golf - olfactory specific)Adenylyl cyclase activation (type III)cAMP increase Cyclic nucleotide-gated channel opening Na+ and Ca2+ influx Depolarization and action potential generationCa2+-activated Cl- channels amplify response
Receptor desensitization cAMP phosphodiesterase activation Ca2+/calmodulin-dependent adaptation
Glomeruli : Spherical neuropil regions (~2,000 in humans)
Convergence: ~25,000 ORNs of same type → 25 mitral/tufted cells
Mitral Cells : Primary output neuronsTufted Cells : Secondary output neuronsPeriglomerular Cells : Local interneuronsGranule Cells : Lateral inhibition and contrast enhancement
Piriform Cortex : Primary olfactory processingAmygdala : Emotional associationsEntorhinal Cortex : Memory formationOrbitofrontal Cortex : Conscious perception and integration
Vomeronasal Organ (Jacobson's Organ) : Detects pheromonesAccessory Olfactory Bulb : Processes vomeronasal signalsFunction : Important in many mammals, minimal in humans
Complete loss of smellCauses : Upper respiratory infections, head trauma, neurodegenerative diseasesCongenital : Kallmann syndrome (with hypogonadism)
Reduced ability to smellAge-related : Gradual decline with agingTemporary : During respiratory infections
Distorted smell perceptionPost-viral : Common after COVID-19Quality changes : Pleasant odors become unpleasant
Phantom odors without external stimulusCauses : Seizures, migraines, psychiatric conditions
Intrinsic Muscles : Change tongue shape (superior/inferior longitudinal, transverse, vertical)Extrinsic Muscles : Change tongue position (genioglossus, hyoglossus, styloglossus, palatoglossus)Innervation :
Motor: Hypoglossal nerve (CN XII)
Sensory: Trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX)
Fungiform Papillae
Location : Anterior 2/3 of tongueNumber : ~200-400Taste Buds : 3-5 per papillaShape : Mushroom-like
Circumvallate Papillae
Location : V-shaped row at tongue baseNumber : 7-12Taste Buds : 100-300 per papilla (50% of all taste buds)von Ebner's Glands : Serous glands washing taste pores
Foliate Papillae
Location : Lateral tongue edgesStructure : Parallel foldsTaste Buds : Well-developed in children, regress with age
Filiform Papillae
Location : Entire tongue surfaceFunction : Mechanical (no taste buds)Structure : Keratinized, conical projections
Location : Within papillae epitheliumShape : Barrel-shaped, 50-100 μm tallTaste Pore : Apical opening to oral cavityNumber : ~10,000 in young adults (decreases with age)
Type I Cells (Dark Cells)
Function : Support and taste bud maintenanceFeatures : Dense cytoplasm, wrap around other cellsPercentage : ~50-60% of taste bud cells
Type II Cells (Light Cells)
Function : Sweet, bitter, and umami detectionFeatures : G-protein coupled receptorsPercentage : ~20-30% of taste bud cells
Type III Cells (Intermediate Cells)
Function : Sour detection, some saltFeatures : Voltage-gated ion channels, synapses with nervesPercentage : ~15-20% of taste bud cells
Type IV Cells (Basal Cells)
Function : Stem cells for taste bud renewalTurnover : Taste cells replaced every 7-10 days
Sweet
Receptors : T1R2+T1R3 heterodimerLigands : Sugars, artificial sweetenersTransduction : G-protein (gustducin) → PLC → IP3 → Ca2+ release → depolarization
Umami (Savory)
Receptors : T1R1+T1R3 heterodimerLigands : L-glutamate, nucleotides (IMP, GMP)Transduction : Similar to sweet pathway
Bitter
Receptors : T2R family (~25 different receptors)Ligands : Alkaloids, toxins, bitter compoundsTransduction : G-protein pathway → Ca2+ releaseFunction : Protective against potentially harmful substances
Sour
Stimulus : H+ ions (acid)Mechanisms :
Direct H+ entry through ion channels
H+ blockade of K+ channels
Cells : Primarily Type III cells
Salt
Primary : Na+ through epithelial sodium channels (ENaC)Secondary : Other mechanisms for different saltsCells : Type III cells and possibly others
Type II Cells : Release ATP as neurotransmitterType III Cells : Form conventional synapses, release serotonin and GABAPurinergic Signaling : ATP activates P2X receptors on nerve fibers
Facial Nerve (CN VII) : Anterior 2/3 via chorda tympaniGlossopharyngeal Nerve (CN IX) : Posterior 1/3 and circumvallate papillaeVagus Nerve (CN X) : Few taste buds in pharynx and epiglottis
Medulla : Nucleus tractus solitarius (gustatory nucleus)Pons : Parabrachial nucleusThalamus : Ventral posterior medial nucleusCortex : Primary gustatory cortex (insula and frontal operculum)
Complete loss of tasteRare condition Causes : Severe nerve damage, certain medications
Reduced taste sensitivityCommon with aging Causes : Medications, dry mouth, zinc deficiency
Altered taste perceptionMetallic or bitter taste commonCauses : Medications, chemotherapy, infections
Chronic burning sensationOften affects taste perceptionMultifactorial etiology
Layers (superficial to deep):
Stratum Corneum : Dead, keratinized cellsStratum Lucidum : Clear layer (thick skin only)Stratum Granulosum : Keratohyalin granulesStratum Spinosum : Desmosomes connect cellsStratum Basale : Mitotic activity, melanocytes
Papillary Layer : Loose connective tissue with dermal papillaeReticular Layer : Dense irregular connective tissueContents : Blood vessels, nerves, hair follicles, glands
Adipose tissue and loose connective tissueInsulation and energy storageAnchors skin to underlying structures
SA-I: Merkel's Discs
Location : Epidermis and hair folliclesReceptive Field : Small (2-4 mm diameter)Function : Fine spatial discrimination, texture, Braille readingInnervation : Aβ fibersAdaptation : Slowly adapting to sustained pressure
SA-II: Ruffini Endings
Location : Deep dermis and subcutaneous tissueReceptive Field : Large and diffuseFunction : Skin stretch, hand conformation, finger positionInnervation : Aβ fibersSensitivity : Lateral skin stretch and joint movement
RA-I: Meissner's Corpuscles
Location : Dermal papillae (especially fingertips, lips)Receptive Field : Small (2-4 mm diameter)Function : Light touch, texture, slip detectionInnervation : Aβ fibersFrequency : Optimal response 20-50 Hz
RA-II: Pacinian Corpuscles
Location : Deep dermis, subcutaneous tissue, fasciaReceptive Field : Large (can cover entire hand)Function : Vibration detection (200-300 Hz), tool useStructure : Onion-like lamellated capsuleInnervation : Single Aβ fiber
Nerve Fibers : Aδ and C fibersTemperature Range : 15-35°CPeak Sensitivity : ~25°CAdaptation : Slow adaptation to constant temperature
Nerve Fibers : C fibersTemperature Range : 30-45°CPeak Sensitivity : ~40°CDistribution : Less numerous than cold receptors
TRP Channels : Transient receptor potential ion channelsTRPM8 : Cold and menthol receptorTRPV1 : Heat and capsaicin receptor (>43°C)TRPV2, TRPV3, TRPV4 : Various temperature ranges
Aδ Nociceptors
Myelinated fibers : Fast conduction (5-30 m/s)Pain Type : Sharp, pricking, well-localizedActivation : Mechanical and thermal stimuliFunction : First pain, protective withdrawal
C Nociceptors
Unmyelinated fibers : Slow conduction (0.5-2 m/s)Pain Type : Burning, aching, poorly localizedActivation : Mechanical, thermal, chemical stimuliFunction : Second pain, tissue damage signaling
Mechanical Nociceptors
High-threshold mechanoreceptors Activation : Intense pressure, pinprick, crushing
Thermal Nociceptors
Heat : >45°C (TRPV1, TRPV2)Cold : <15°C (TRPA1)
Polymodal Nociceptors
Multiple stimuli : Mechanical, thermal, chemicalMost common type of nociceptorC fibers predominantly
Prostaglandins : Sensitize nociceptorsSubstance P : Neurotransmitter and inflammatory mediatorBradykinin : Potent pain mediatorHistamine : Itch and inflammation
Function : Fine touch, vibration, proprioception
First-order neurons : Dorsal root ganglia → dorsal columnsSecond-order neurons : Nucleus gracilis/cuneatus → medial lemniscusThird-order neurons : VPL thalamus → primary somatosensory cortex
Function : Pain, temperature, crude touch
First-order neurons : Dorsal root ganglia → dorsal hornSecond-order neurons : Decussate → lateral/anterior spinothalamic tractThird-order neurons : VPL thalamus → somatosensory cortex
Primary (S1) : Postcentral gyrus, somatotopic organizationSecondary (S2) : Posterior parietal cortex, bilateral representationHomunculus : Disproportionate representation based on sensitivity
Allodynia : Pain from normally non-painful stimuliHyperalgesia : Increased pain from painful stimuliHypoesthesia : Decreased touch sensationAnesthesia : Complete loss of sensation
Peripheral neuropathy : Diabetes, chemotherapyCentral pain : Stroke, spinal cord injuryComplex regional pain syndrome : Chronic pain with autonomic changes
Thermal hyperalgesia : Increased pain from temperatureCold allodynia : Pain from mild coolingThermal hypoesthesia : Reduced temperature sensation
Receptor Level : Transduction of stimuliCircuit Level : Local processing and feature detectionPerceptual Level : Integration and interpretation
Parallel Processing : Multiple pathways process different aspects simultaneouslyHierarchical Organization : Simple to complex feature detectionPlasticity : Experience-dependent changes in neural responsesAttention : Top-down modulation of sensory processing
Medulla
Gustatory nucleus (taste)
Cochlear nuclei (hearing)
Vestibular nuclei (balance)
Pons
Superior olivary complex (sound localization)
Trigeminal nuclei (facial sensation)
Midbrain
Superior colliculus (visual reflexes, eye movements)
Inferior colliculus (auditory processing)
VPL : Somatosensory (body)VPM : Somatosensory (face), tasteLGN : Visual processingMGN : Auditory processingNote : Olfaction bypasses thalamus
Primary Sensory Areas
V1 : Primary visual cortex (occipital lobe)A1 : Primary auditory cortex (temporal lobe)S1 : Primary somatosensory cortex (parietal lobe)Gustatory : Insula and frontal operculumOlfactory : Piriform cortex
Association Areas
Visual : V2, V3, V4, V5/MT (motion)Auditory : Secondary auditory areasMultisensory : Superior temporal sulcus, posterior parietal cortex
Convergence zones : Areas receiving multiple sensory inputsTemporal binding : Synchronization of neural activityCross-modal plasticity : One sense compensating for another
Selective attention : Focusing on relevant sensory informationSensory gating : Filtering of irrelevant stimuliBinding problem : Integration of features into unified percepts
Sensory memory : Brief retention of sensory informationPerceptual learning : Improvement with experienceCross-modal associations : Linking different sensory modalities
Vision
Visual acuity charts (Snellen, LogMAR)
Visual field testing (perimetry)
Color vision testing (Ishihara plates)
Ophthalmoscopy and slit-lamp examination
Hearing
Audiometry (pure tone, speech)
Tympanometry (middle ear function)
Otoacoustic emissions (cochlear function)
Vestibular testing (electronystagmography, rotary chair)
Smell and Taste
Scratch-and-sniff tests (University of Pennsylvania Smell Identification Test)
Taste threshold testing
Electrogustometry
Touch and Pain
Two-point discrimination
Vibration testing (tuning fork, vibrometer)
Temperature discrimination
Pain threshold assessment
Quantitative sensory testing (QST)
MRI : Detailed soft tissue imaging, functional MRI for brain activityCT : Bone and dense tissue visualizationUltrasound : Real-time imaging of eye structuresOCT (Optical Coherence Tomography): High-resolution retinal imagingPET/SPECT : Metabolic and functional brain imaging
Presbyopia : Loss of accommodation starting ~40 yearsCataract formation : Lens opacity increases with ageMacular degeneration : Leading cause of blindness in elderlyGlaucoma risk : Increases significantly with agePupil size : Becomes smaller (senile miosis)
Presbycusis : Age-related hearing loss, especially high frequenciesHair cell loss : Progressive loss of cochlear hair cellsCentral processing : Reduced speech discrimination in noiseTinnitus : More common with aging
Olfactory decline : Significant reduction after age 60Taste bud reduction : Gradual loss throughout lifeMedication effects : Polypharmacy affects taste/smellDry mouth : Reduced saliva affects taste perception
Mechanoreceptor loss : Reduced density with agingVibration sensitivity : Decreased, especially in feetPain threshold : May increase with ageTemperature sensitivity : Reduced discrimination ability
Color Blindness
X-linked : Red-green color blindness (8% males, 0.5% females)Types : Protanomaly/Protanopia (L-cone), Deuteranomaly/Deuteranopia (M-cone)Rare : Tritanomaly/Tritanopia (S-cone), complete achromatopsia
Inherited Retinal Diseases
Retinitis pigmentosa : Progressive rod-cone dystrophyLeber congenital amaurosis : Severe early-onset blindnessStargardt disease : Juvenile macular degenerationUsher syndrome : Combined hearing and vision loss
Congenital Hearing Loss
Connexin mutations : Gap junction proteins, most common genetic causeWaardenburg syndrome : Hearing loss with pigmentation changesPendred syndrome : Hearing loss with thyroid enlargementAlport syndrome : Hearing loss with kidney disease
Congenital Anosmia
Kallmann syndrome : Anosmia with hypogonadotropic hypogonadismCHARGE syndrome : Multiple congenital anomalies including anosmia
Diabetic retinopathy : Leading cause of blindness in adultsNeuropathy : Affects touch, pain, and temperature sensationTaste changes : Delayed wound healing affects taste bud regeneration
Hypertensive retinopathy : Retinal vascular changesStroke risk : Can affect any sensory processing area
Multiple Sclerosis
Optic neuritis : Inflammation of optic nerveSensory symptoms : Numbness, tingling, altered sensationsCentral processing : Demyelination affects sensory pathways
Sjögren's Syndrome
Dry eyes : Reduced tear productionDry mouth : Affects taste perceptionNeuropathy : Peripheral sensory involvement
Viral Infections
COVID-19 : Anosmia and dysgeusia common symptomsHerpes zoster : Can affect trigeminal nerve (facial sensation)Cytomegalovirus : Can cause retinitis in immunocompromised
Bacterial Infections
Otitis media : Middle ear infection affecting hearingMeningitis : Can damage cranial nerves
Aminoglycosides : Damage to hair cellsLoop diuretics : High-dose can cause hearing lossChemotherapy : Cisplatin, carboplatin cause hearing lossAspirin : High doses can cause tinnitus and hearing loss
Chloroquine/Hydroxychloroquine : Retinal toxicityCorticosteroids : Can increase intraocular pressureDigitalis : Can cause color vision changes (yellow tinting)
ACE inhibitors : Metallic taste, altered smellAntibiotics : Metallic taste commonChemotherapy : Severe dysgeusiaAntihistamines : Dry mouth affects taste
Mechanism : Damage to cochlear hair cellsPrevention : Hearing protection, noise reductionIndustries : Construction, manufacturing, military, musicCharacteristics : Initially affects 4000 Hz frequency
Solvents : Can affect multiple sensory systemsHeavy metals : Lead, mercury can cause neuropathyIndustrial chemicals : Various effects on sensory organs
Ionizing radiation : Cataract formation, retinal damageUV radiation : Photokeratitis, macular damageLaser exposure : Retinal burns
Central Retinal Artery Occlusion
Presentation : Sudden, painless, severe vision lossAppearance : Cherry-red spot on maculaTreatment : Ocular massage, hyperbaric oxygen (limited window)
Acute Angle-Closure Glaucoma
Symptoms : Severe eye pain, headache, nausea, halos around lightsSigns : Red eye, fixed mid-dilated pupil, corneal edemaTreatment : Immediate pressure reduction, laser iridotomy
Retinal Detachment
Symptoms : Flashing lights, floaters, curtain-like vision lossTypes : Rhegmatogenous, tractional, exudativeTreatment : Urgent surgical repair
Sudden Sensorineural Hearing Loss
Definition : >30 dB loss in <72 hoursTreatment : High-dose corticosteroids within 2 weeksPrognosis : 1/3 recover completely, 1/3 partially, 1/3 no recovery
Acoustic Trauma
Causes : Explosive noise, gunshots, loud musicPrevention : Immediate removal from noise sourceTreatment : Corticosteroids may help if given early
Low vision aids : Magnifiers, telescopes, electronic devicesOrientation and mobility : White cane training, guide dogsBraille : Tactile reading systemTechnology : Screen readers, voice recognition
Hearing aids : Amplification for mild-moderate lossCochlear implants : Electronic hearing for severe-profound lossAssistive listening devices : FM systems, infrared systemsCommunication : Sign language, lip reading
Vestibular rehabilitation : Exercise-based therapyCanalith repositioning : Treatment for BPPVBalance training : Fall prevention programs
Leber congenital amaurosis : FDA-approved gene therapy (Luxturna)Inherited hearing loss : Gene therapy trials ongoingStem cell therapy : Potential for regenerating sensory cells
Artificial retina : Electronic implants for blindnessCochlear implants : Continued improvements in technologyTactile substitution : Converting visual/auditory to tactile signals
Cross-modal plasticity : How senses compensate for each otherPerceptual learning : Training to improve sensory functionBrain stimulation : Enhancing sensory processing
Transduction mechanisms are specific to each sensory modality but follow similar principlesSpecialized cells in each organ convert environmental energy into neural signalsCentral processing involves multiple levels from brainstem to cortexIntegration of sensory information creates our perception of the environmentClinical assessment requires understanding of normal function and common pathologiesAge-related changes affect all sensory systems but to varying degreesSystemic diseases often have sensory manifestations that aid in diagnosisPrevention and early intervention are crucial for maintaining sensory function