Special sensory organs, Summaries of Anatomy

Download Special sensory organs and more Summaries Anatomy in PDF only on Docsity!

Click here to join join our telegram

channel

Special senses 10 marks Describe the visual pathway & the effect of lesions at various levels with the suitable diagram The visual pathway consists of

1. Retina
2. Optic nerve
3. Optic chiasma
4. Optic tract
5. Lateral geniculate nucleus
6. Optic radiation(geniculo-calcarine tract)
7. Visual cortex

Site of lesion Condition Diagram 1 Right optic nerve Right eye anopia 2 Optic chiasma Bitemporal hemianopia 3 Lateral Fibers Binasal hemianopia

4 Right optic

Tract Left homonymous Hemianopia

5 Rjght optic

radiation Left homonymous Hemianopia

6 Inferolateral

fibers of optic radiation Left homonymous Superior Quadranopia

7 Superomedial

Fibers of Optic radiation Left homonymous Inferior Quadranopia

8 Inferolateral

fibers of optic radiation in calcarine fissure Left homonymous Superior Quadranopia With macular Sparing

9 Superomedial

fibers of optic radiation in calcarine fissure Left homonymous Inferior Quadranopia With macular Sparing (^10) Visual cortex Left homonymous Hemianopia With macular Sparing

5 marks

1. Describe the circulation & functions of aqueous humour. Aqueous humour ❖ Homogenous fluid that fills the anterior & Posterior chambers ❖ pH 7.1-7. ❖ Refractive index 1. ❖ Composition – Less glucose & more Lactic Acid than plasma with high ascorbic acid Formation of Aqueous Humour: ❖ Formed by the ciliary processes- ❖ Mechanism: 1. Active secretion 2. Ultra-filtration ❖ Rate of formation: 2 - 3 cu.mm per minute Circulation of Aqueous Humour: ❖ Aqueous humor circulates within the eye ❖ Formed by the ciliary processes ❖ Secreted into posterior chamber ❖ Passes between ligaments of lens ❖ Passes through pupil into Anterior chamber ❖ Flows into angle between cornea & iris ❖ Flows through trabeculae ❖ Flows into canal of Schelmn & extra ocular veins ❖ Re-enters blood circulation Functions Of Aqueous humour:  Provides nutrition to cornea & lens (avascular structures)  Maintains IOP (Intra ocular pressure)  Maintains shape of eyeball  Acts as refractive medium

2. Describe the mechanism of accommodation for near vision - It is the ability of the eye to see distant and near objects clearly. This involves the process of adjusting the shape of the lens so that the external image falls exactly on the retina. Accommodation of the Lens for near vision - Ciliary muscles contract - Ciliary body pulls forward and inward - Tension on suspensory ligaments of lens is decreased - Lens becomes thicker (rounder) due to its elasticity - Pupils constricts Near point:

• It is the nearest point to the eyes at which an object can be brought into clear focus by accommodation.
• At age 10: Near point – 9 cm
• At age 60: Near point – 83 cm
• The near point recedes with age. Near response

1. Convergence of eye ball
2. Constriction of pupil
3. Curvature (anterior) change in lens

3. Briefly describe the mechanism of dark adaptation Adaptation to dark (Scotopic vision) On entering dark room from bright area, initially the vision is poor, later it improves.This decline in visual threshold is called dark adaptation. Time duration for dark adaptation depends

1. Intensity of light
2. Duration of exposure
3. Vit A Content Two phases
4. Adaptation of the cones (5min)
5. Adaptation of rods (20min) Changes in the eye during dark adaptation
   1. Pupils dilate
   2. Sensitivity of the photoreceptors to light increases
   3. Resynthesis of photo pigments
   4. Decrease in visual acuity
   5. Vision changes from cone to rods (photopic to scotopic). This is called PURKINJE SHIFT. Visual Purple//Rhodopsin Cycle Rhodopsin (11 cis retinal+ opsin ) Pre-lumi rhodopsin Lumi rhodopsin Meta rhodopsin I Opsin Meta rhodopsin II

Retinal isomerase 11 cis retinal All transretinal Isomerase 11 cis retinol All transretinol (Vitamin A) Light Dark

4. Write short notes on colour vision - A sensation evoked by different wavelengths of light. - Function of cones. Physiological Basis of colour vision

• Three different types of cones
• Three types of pigments (the opsin protein part differs from rhodopsin),
• Each pigment has maximum absorption at different wavelengths
• blue-absorbing cones – cyanopsin pigment (max absorption at 445nm)
• green-absorbing cones – Iodopsin pigment (max absorption at 535 nm)
• red-absorbing cones – porphyropsin pigment (max absorption at 570 nm) Primary colours - Red - Green - Blue Theories of colour vision
• Young – Helmholtz theory
• Granit modulator & dominator theory
• Hering opponent colour theory
• Land’s retinex theory Young – Helmholtz theory
• Trichromatic theory
• Red , green , blue – 3 primary colours
• The 3 types of cones have 3 different pigments
• Each pigment is maximally sensitive to one primary colour
• But also responds to other 2 primary colours
• Sensations of various colours are due to stimulation of different receptors at

different intensities.

Processing of colour perception

• Analysis of colour occurs in the retina
• Information is then passed on to the brain for interpretation.
• Centre of fovea is blue blind.
• Blue cones are absent here.
• Retina , lateral geniculate nucleus , visual cortex all have a combined role in

perception of colour.

Colored light strikes the retina ↓ Depending on the color mixture cone will respond ↓ Response is in the form of local potentials ↓ LP transmitted in bipolar cells ↓ Ganglion cells activated ↓ Signals from the 3 cones are processed in the ganglion cell ↓ Reach the layers of LGN ↓ Processed in LGN

Error Defect Cause Feature Correction Myopia Long distant objects not clear Longer eye ball / high refractive power of lens Light rays are focused in front of retina Biconcave lens Hypermetropia Short distant objects not clear Shorter eye ball / Low refractive power of lens Light rays are focused behind the retina Biconvex lens Presbyopia Short distant objects not clear Loss of elasticity & plasticity of lens and also decrease in power of ciliary muscle due to aging Decrease in the power of accommodation of eye Biconvex lens Astigmatism Blurring of vision Ununiform curvature of the cornea Light is focussed at multiple points on retina Cylindrical lens Aniso metria Difference in the refractive power between the two eyes Congenital Eye with high refractive power – Dominant eye Eye with less refractive power – Suppressed eye Correction of each eye separately with appropriate lenses

6. Describe the functions of middle ear. Components of middle ear: 1. Three small bones (ossicles): 1)Malleus 2)Incus 3)Stapes 2. Two small muscles: 1)Tensor tympani 2)Stapedius muscle Functions of middle ear 1. Tympanic Reflex: - When loud sounds are transmitted through the ossicular system (Malleus, Incus, stapes) into the CNS, a reflex occurs to cause contraction of both Stapedius and tensor tympani muscles. This is called tympanic reflex or attenuation reflex - The contraction of tensor tympani muscles pulls the handle of the malleus inward, while the stapedius muscle contraction pulls the stapes outward - These two forces oppose each other and this causes rigidity of the entire ossicular system which greatly reduces the transmission of low frequency sounds. Significance of tympanic reflex to protect the cochlea from damaging vibrations caused by excessive loud sound i.e. low frequency sounds. 2. Impedance Matching:- - Whenever sound wave travels from a thinner medium to denser medium, some

amount of sound energy is lost at the interphase of two medium.

• This happens in ear also. When sound travels from air filled middle ear into

denser fluid medium of inner ear, there is a loss of sound energy at oval window

• Middle ear compensates this by increasing the sound energy level by several times at oval window.
• Middle ear achieves this by three mechanism which are combinely referred as impedance matching. The mechanism are:- 1. Area difference
• As the area of the tympanic membrane is large than the area of the oval window, the forces collected over the tympanic membrane are concentrated on a smaller area of oval window.
• This increases the pressure at the oval window by 17 times. 2. Lever action of the middle ear bones. - The arm of incus is shorter than that of malleus and this produces a lever action. - This increases the force by 1.32 times and decreases the velocity at the stapes. Aphakia Diplopia & Astigmatism due to absence of lens Removal of lens following cataract surgery / dislocation of lens Complete loss of accommodation (hypermetropic) Wearing spectacles with power of + 11 diopters/IOL implantation

8. Trace the pathway for hearing I order neuron : From the bases of the hair cells → cell bodies form the spiral ganglion around the modiolus → axons form the cochlear nerve →joins with the vestibular nerve to form the vestibulocochlear nerve → end in cochlear nuclei II order neuron : From cochlear nuclei → ascend to the nearby superior olivary nucleus (of both sides) → then ascend in the lateral lemniscus → end in inferior colliculi of midbrain III order neuron: From inferior colliculi to medial geniculate bodies of thalamus IV order neurons: complete the pathway from thalamus to primary auditory complex

9. Trace the olfactory pathway receptor cell axon ↓ pierce the cribriform plate of ethmoid ↓ enters olfactory bulb ↓ synapse with dendrites of mitral cells to form olfactory glomeruli ↓ axons of mitral cells pass posteriorly through olfactory stria ↓ olfactory cortex (anterior olfactory nucleus, olfactory tubercle, prepyriform cortex, amygdala, entorhinal cortex ↓ From the olfactory cortex signals reach ↓

• Orbito frontal cortex
• Hypothalamus
• Hippocampus Thus olfactory impulses are projected both to
• Neocortex Perception & discrimination of odours
• Limbic system Emotional, motivational, behavioral & physiological effects of odours Olfactory receptors Olfactory neuron Olfactory bulb Cribriform plate of ethmoid bone bone Glomerulus Olfactory straie

12. Name the primary taste sensation. How are they distributed on the tongue? Outline the basic taste modalities & explain the mechanism of taste sensation BASIC TASTE SENSATIONS

PRODUCED BY MECHANISM OF

STIMULATION

PART OF TONGUE MOST

SENSITIVE

Sweet Sugars, glycols & aldehydes. ↑ cAMP→↓K+ conductance Tip Bitter Alkaloids ↑ IP 3 → ↑Ca++ release Back Sour H+^ ions Blocking K+^ channels Posterior ½ of lateral Salt Anions of ionised salts Na+^ ion permeability Anterior ½ of lateral Umami Monosodium Glutamate

Primary taste sensations

**1. Sweet

1. Salt
2. Sour
3. Bitter
4. Umami**