Physiology
Medical physiology for MBBS: cell and body fluids, blood, nerve and muscle, and the cardiovascular, respiratory, gastrointestinal, renal, endocrine, reproductive and nervous systems, special senses and integrative physiology.
12chapters144questions84High-Yield
Definition
Image formation in the eye occurs by refraction of light through the cornea and lens to form a real, inverted image on the retina.
Refractive Media (front to back)
- Cornea — main refraction (~40 D)
- Aqueous humour
- Lens — adjustable (~15–30 D)
- Vitreous humour
Image Formation
- Light refracted → focused on the retina
- Image is real, inverted and smaller
- The brain re-inverts the perception
Emmetropia (normal eye)
- Parallel rays focus on the retina (relaxed)
- Total refractive power ~60 D
The cornea and lens bend light to focus a sharp image on the retina. Structure Power / role Cornea ~40 D (fixed) Lens ~15–30 D (variable) Retina Image formed Applied
- Refractive errors (myopia, hypermetropia)
- Cataract — lens opacity
🔑KEY POINTS TO REMEMBER- Image on retina is real, inverted, smaller.
- Cornea = main refraction (~40 D); lens = adjustable.
- Total power ~60 D (emmetropia).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The retina is the light-sensitive inner layer of the eye; phototransduction converts light into electrical signals in the photoreceptors.
Cells / Layers
- Photoreceptors — rods & cones
- Bipolar cells
- Ganglion cells → optic nerve
- Pigment epithelium
Phototransduction
- Light → rhodopsin (rods) breaks down
- ↓ cGMP → Na⁺ channels close
- Photoreceptor hyperpolarises
- Signal → bipolar → ganglion → optic nerve
Special Areas
- Fovea/macula — cones, sharpest vision
- Optic disc — blind spot (no receptors)
Light bleaches rhodopsin, hyperpolarising the receptor and generating a signal. Cell Function Rods Dim light, black & white Cones Bright light, colour Ganglion Forms optic nerve Applied
- Vitamin A deficiency → night blindness
- Retinal detachment
🔑KEY POINTS TO REMEMBER- Retina: rods/cones → bipolar → ganglion → optic nerve.
- Phototransduction: light → rhodopsin → hyperpolarisation.
- Fovea = sharpest vision; optic disc = blind spot.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Hearing is the perception of sound; sound is conducted through the outer/middle ear and transduced in the cochlea.
Conduction of Sound
- Outer ear → tympanic membrane vibrates
- Ossicles (malleus, incus, stapes) amplify
- Stapes → oval window → cochlear fluid
Transduction (Cochlea)
- Fluid wave → basilar membrane moves
- Hair cells (organ of Corti) bent
- → receptor potential → auditory nerve
Pitch & Loudness
- Pitch — place on basilar membrane (base = high, apex = low)
- Loudness — amplitude of vibration
Sound is amplified by the ossicles and transduced by cochlear hair cells. Part Role Ossicles Amplify sound Cochlea Transduction Hair cells Receptors Applied
- Conductive vs sensorineural deafness
- Presbycusis (age-related hearing loss)
🔑KEY POINTS TO REMEMBER- Sound: eardrum → ossicles → oval window → cochlea → hair cells → nerve.
- Pitch = place on basilar membrane; loudness = amplitude.
- Deafness: conductive vs sensorineural.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The vestibular apparatus in the inner ear detects head position and movement, maintaining balance and equilibrium.
Components
- Semicircular canals — rotational (angular) acceleration
- Utricle & saccule — linear acceleration + head position
- Hair cells as receptors
Mechanism
- Head movement → endolymph moves
- Bends hair cells (cupula / otoliths)
- → vestibular nerve → brainstem & cerebellum
Functions
- Static & dynamic equilibrium
- Coordinates eye movements (vestibulo-ocular reflex)
- Maintains posture
Head motion moves endolymph, bending hair cells that signal balance. Structure Detects Semicircular canals Rotation (angular) Utricle / saccule Linear / gravity Applied
- Vertigo, motion sickness
- Ménière’s disease
🔑KEY POINTS TO REMEMBER- Semicircular canals = rotation; utricle/saccule = linear/gravity.
- Hair cells → vestibular nerve → balance.
- Dysfunction → vertigo.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Taste (gustation) and smell (olfaction) are chemical senses that detect dissolved and airborne chemicals respectively.
Taste
- Five primary tastes: sweet, sour, salty, bitter, umami
- Taste buds (tongue) → cranial nerves VII, IX, X
- → thalamus → cortex
Smell
- Olfactory receptors in the nasal epithelium
- Olfactory nerve (I) → olfactory bulb
- → cortex (does NOT relay in the thalamus first)
- Highly sensitive; adapts quickly
Chemicals excite taste or smell receptors, whose signals reach the cortex. Sense Receptor Nerve Taste Taste buds VII, IX, X Smell Olfactory epithelium I Applied
- Anosmia (loss of smell)
- Taste and smell combine as ‘flavour’
🔑KEY POINTS TO REMEMBER- Taste: 5 primaries; nerves VII, IX, X → thalamus → cortex.
- Smell: olfactory nerve (I) → bulb → cortex (bypasses thalamus).
- Both are chemical senses.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Rods and cones are the two photoreceptors of the retina, differing in function.
Rods
- Dim-light (scotopic) vision
- Black & white only
- Contain rhodopsin
- More numerous; peripheral
Cones
- Bright-light (photopic) vision
- Colour vision (three types)
- High visual acuity
- Concentrated in the fovea
Rods handle dim light; cones give bright-light and colour vision. Feature Rods Cones Light Dim Bright Colour No Yes Acuity Low High Location Periphery Fovea Pigment Rhodopsin Photopsins Applied
- Vitamin A deficiency → night blindness (rods)
- Colour blindness (cones)
🔑KEY POINTS TO REMEMBER- Rods: dim light, B/W, rhodopsin, periphery.
- Cones: bright light, colour, high acuity, fovea.
- Night blindness (rods) vs colour blindness (cones).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Accommodation is the eye’s ability to focus on near objects by increasing the curvature (power) of the lens.
Mechanism
- Near object → ciliary muscle contracts
- Suspensory ligaments slacken
- Lens becomes more convex → ↑ power
- Accompanied by pupil constriction + convergence (near reflex)
Contracting the ciliary muscle rounds the lens to focus near objects. Object Ciliary muscle Lens Near Contracts More convex Far Relaxes Flatter Applied
- Presbyopia — loss of accommodation with age
- Near reflex = accommodation + convergence + miosis
🔑KEY POINTS TO REMEMBER- Accommodation focuses near objects by rounding the lens.
- Ciliary muscle contracts → ligaments slacken → lens convex.
- Presbyopia = age-related loss.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Refractive errors occur when light does not focus on the retina; the common types are myopia and hypermetropia.
Myopia (short-sight)
- Image focuses in front of the retina
- Eyeball too long / lens too strong
- Distant objects blurred
- Corrected by a concave (−) lens
Hypermetropia (long-sight)
- Image focuses behind the retina
- Eyeball too short / lens too weak
- Near objects blurred
- Corrected by a convex (+) lens
Each refractive error shifts the focus, corrected by the matching lens. Error Focus Correction Myopia Front of retina Concave (−) Hypermetropia Behind retina Convex (+) Astigmatism Irregular Cylindrical Applied
- Astigmatism — cylindrical lens
- Presbyopia — age-related near-vision loss
🔑KEY POINTS TO REMEMBER- Myopia: focus in front → concave lens.
- Hypermetropia: focus behind → convex lens.
- Astigmatism → cylindrical lens.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The visual pathway carries impulses from retina to visual cortex; lesions at different points cause characteristic field defects.
Pathway
- Retina → optic nerve
- Optic chiasma (nasal fibres cross)
- Optic tract → lateral geniculate body (thalamus)
- Optic radiation → visual cortex (occipital)
Lesions
- Optic nerve → blindness of that eye
- Optic chiasma → bitemporal hemianopia
- Optic tract → homonymous hemianopia
Nasal fibres cross at the chiasma; the pathway ends in the occipital cortex. Lesion site Field defect Optic nerve Blind eye Optic chiasma Bitemporal hemianopia Optic tract Homonymous hemianopia Applied
- Pituitary tumour → bitemporal hemianopia
- Stroke → homonymous field defects
🔑KEY POINTS TO REMEMBER- Pathway: retina → optic nerve → chiasma → tract → LGB → cortex.
- Chiasma lesion → bitemporal hemianopia.
- Tract lesion → homonymous hemianopia.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Deafness (hearing loss) is of two main types: conductive (outer/middle ear) and sensorineural (cochlea/nerve).
Conductive Deafness
- Problem in the outer or middle ear
- Sound not conducted to the cochlea
- Causes: wax, otitis media, otosclerosis
Sensorineural Deafness
- Problem in the cochlea or auditory nerve
- Causes: presbycusis, noise, ototoxic drugs, congenital
Where the fault lies — conducting apparatus vs cochlea/nerve — defines the type. Test Conductive Sensorineural Rinne BC > AC (negative) AC > BC (positive) Weber To affected ear To normal ear Applied
- Rinne & Weber tuning-fork tests
- Hearing aids, cochlear implants
🔑KEY POINTS TO REMEMBER- Conductive = outer/middle ear (wax, otitis, otosclerosis).
- Sensorineural = cochlea/nerve (presbycusis, noise, drugs).
- Rinne & Weber distinguish them.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Colour vision is the ability to distinguish colours, explained by the trichromatic theory (three cone types).
Trichromatic Theory
- Three cone types: red, green, blue
- Each senses a range of wavelengths
- Brain mixes the signals → all colours
Colour Blindness
- Usually X-linked recessive (mostly males)
- Red-green commonest (protanopia, deuteranopia)
- Tested by Ishihara charts
Three cone types and the brain’s mixing of their signals produce colour vision. Cone Wavelength Red (L) Long Green (M) Medium Blue (S) Short Applied
- Colour blindness — Ishihara test
- Opponent-process theory (complementary colours)
🔑KEY POINTS TO REMEMBER- Trichromatic theory: red, green, blue cones.
- Brain mixes signals for all colours.
- Colour blindness = X-linked, red-green commonest.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The pupillary light reflex is constriction of the pupil in response to light, protecting the retina and regulating light entry.
Pathway
- Light → retina → optic nerve (afferent)
- Midbrain (Edinger–Westphal nucleus)
- Oculomotor nerve (parasympathetic, efferent)
- Sphincter pupillae contracts → pupil constricts
Features
- Direct (same eye) + consensual (other eye)
- Bilateral because of fibre crossing
Light in one eye reflexly constricts both pupils via the midbrain. Limb Nerve Afferent Optic (II) Efferent Oculomotor (III) Applied
- Absent reflex → nerve or midbrain lesion
- Argyll Robertson pupil (neurosyphilis)
🔑KEY POINTS TO REMEMBER- Light reflex: optic nerve (afferent) → midbrain → oculomotor (efferent).
- Direct + consensual constriction.
- Loss localises nerve/midbrain lesions.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).