ELECTROPHYSIOLOGY OF RETINA by dr anita teli

Views:
 
Category: Education
     
 

Presentation Description

No description available.

Comments

Presentation Transcript

ELECTROPHYSIOLOGY OF RETINA:

ELECTROPHYSIOLOGY OF RETINA Speaker:Dr Anita Teli pg Chairperson : Dr Anand R Dharwadkar Prof & Head Date & Time:23 rd jan 2010,3.30pm

Physiological Anatomy:

Physiological Anatomy Eye is a special organ of the sense of sight. The adult human eye ball is a hollow , spherical structure, 24mm diameter & situated in the orbital cavity.only 1/6 th of the eye ball is visible outside. The walls of the eye ball contain 3 principal layers, 1.Outer fibrous layer,sclera 2.Middle vascular layer,choroid 3.Inner nervous layer,the retina In addition,it also contains the lens, the aqueous humour & vitreous humour.

Layers of retina & photoreceptors(Rods & Cones):

Layers of retina & photoreceptors(Rods & Cones) The retina contains 10 layers except in optic disc & fovea centralis. Photoreceptors are placed outer most towards the choroid. The layers of retina from outwards to inwards are,

Layer 1: pigmented epithelium:

Layer 1: pigmented epithelium It contains melanin pigment which along with pigmented choroid absorbs extra amount of light, thus preventing the reflection of rays back through the retina. If this light gets refracted back, it would result in blurring of vision. It also has phagocytic function.

Layer 2: Rods and Cones layer- photoreceptors:

Layer 2: Rods and Cones layer- photoreceptors Each rod and cone is divided into outer segment,inner segment and a synaptic zone. The outer and inner segment form the layers of rods and cones. (i)The Outer segment a.These are modified cilia and are made up of flattened discs or saccules composed of membrane. In cones the saccules are formed by unfolding of the cell membrane but in rods, the discs are separated from the cell membrane.

PowerPoint Presentation:

b.The discs or saccules contains the photosensitive pigment(rhodopsin in rods and iodopsin in cones). The rods are extremely sensitive to light and are receptors for dimlight(scotopic) vision. While the cones are responsible for bright light(photopic) vision,high visual acuity and for color vision.

PowerPoint Presentation:

C. Rods are named for the thin , rod like appearance of there outer segments. Its outer segmets are being constantly renewed by formation of new discs at the inner edge of the segments and phagocytosis of old discs from the outer tip by cells of the pigment epithelium. In Retinitis Pigmentosa the phagocytic process is defective & a layer of debris accumulates between the receptor & the pigment epithelium finally producing blindness.

PowerPoint Presentation:

D. Cones generally have conical outer segment, its renewal is more diffused process & appears to occur at multiple sites in the outer segment.

PowerPoint Presentation:

(ii) The inner segment a. It is rich in mitochondria. cone’s inner segment is thick, oval in shape & is larger. b. Towadrs the inner side both rods & cones pierce the ‘extrenal limiting membrane’ ,then enlarge to form rod & cones nucleus respectively & finally enlarges into rod end bulb(knob like fashion) & cone plate respectively.

Layer 3:External lmiting membrane:

Layer 3:External lmiting membrane It is formed by the glial tissues, it is the continuation of internal limiting membrane & is pierced by the rods & cones.

Layer 4: Outer nuclear layer :

Layer 4: Outer nuclear layer It is formed by the nucleus of rods & cones.

Layer 5:Outer synaptic layer :

Layer 5:Outer synaptic layer It is formed by synapse between the ends of rods & cones with dendrites of ‘bipolar’ cells & ‘horizontal’ cell processes.

Layer 6: Inner nuclear layer :

Layer 6: Inner nuclear layer A. Bipolar cells; B. Horizontal cells-which connect one receptor cell to other receptor cell & C.Amacrine cells- Its processes make synaptic contacts with dendrites of both ganglion & bipolar cells, & connect ganglion cells to one another.

Layer 7:Inner synaptic layer :

Layer 7:Inner synaptic layer The synapse between the axons of bipolar cells with the dendrites of ganglion cells occur in this layer. It is the site of major processing of the visual image.

Layer 8: Ganglion cell layer :

Layer 8: Ganglion cell layer It is a single layer of cell containing round cells.

Layer 9:Optic nerve :

Layer 9:Optic nerve It is formed by joining the axons of ganglion cells:here all the axons run parallel.

Layer 10: Internal limiting membrane:

Layer 10: Internal limiting membrane It separates the retina from the vitreous humour. It is formed by the glial tissues.

Genesis of electrical activity of retina:

Genesis of electrical activity of retina General Photo receptor potentials Responses of bipolar, amacrine and horizontal cells Response pattern of ganglion cells Responses of neurons in the lateral geniculate bodies and visual cortex Electroretinogram(ERG)

General :

General 1.Visual images have different characteristics, such as color, form, depth, movement & texture; each processed simultaneously by a separate channel in the visual system,called parallel processing of visual informations. 2. The processing of visual information in the retina involves the generation of electrical activity at 3 places, a. First activity is generated by the action of light on the photoreceptors b. Second in the bipolar cells & c. Thirdly in the ganglion cells

PowerPoint Presentation:

3.The electrical activity in the bipolar cells and ganglion cells is altered by the horizontal cells and amacrine cells respectively. 4.The potential changes that initiate action potential in the retina are generated by the action of light on photo-sensitive pigment in the rods and cones. When light is absorbed by these pigments, they trigger a sequence of events that initiates neural activity(phototransduction).

PowerPoint Presentation:

5. Sequence of events in phototransduction in photoreceptors: Light rays structural changes in photo pigments in rods and cones activates transducin a G-protein activates phosphodiesterase cGMP 5’-GMP

Photo Receptor potentials:

Photo Receptor potentials In the dark, Na+ channels in the outer segments of the photoreceptors are kept open by cGMP so there is a steady current flow from the inner to the outer segment of the rods and probably of the cones. 2. The steady current also flows in the synaptic ending of the photoreceptors. Light by decreasing intra-cellular cGMP closes these Na+ channels that decreases the current flow; and the resulting hyperpolarization generates local graded potentials (generator potential) in the synaptic terminal of the photoreceptor.

PowerPoint Presentation:

3. Mechanism of development of hyperpolarization: Exact mechanism is not known; probably straightening of retinene(metarhodopsin II) decreases Na+ permeability in the outer segment due to release of a substance that blocks the Na+ channels in the membrane of the saccules.

PowerPoint Presentation:

4. The rods and cones (RMP-40 mV) do not generate action potential ; their response to adequate stimulus is hyperpolarization rather than depolarization . The cones receptors potential has a sharp ‘onset’ and a sharp ‘offset’ , where as the rod receptor potential has a sharp ‘onset’ and a slow ‘offset’.

PowerPoint Presentation:

5. Rod responses are proportionate to stimulus intensity at levels of illumination that are below the threshold for cones; while cone responses are proportionate to stimulus intensity at high levels of illumination.

C . Responses to bipolar, amacrine and horizontal cells:

C . Responses to bipolar, amacrine and horizontal cells 1.The bipolar cells do not generate action potentials , instead they generate relatively steady hyperpolarizing or depolarizing potential upto 10 mV . (i) In some cells, hyperpolarizing potential are produced by a spot light, where as depolarizing potentials by an annulus of light around the centre. (ii) The receptive fields of the bipolar cells is organised into central and peripheral portions which generate opposite reactions. (iii) If the periphery and the centre are stimulated at the same time , the activities tend to cancel each other.

PowerPoint Presentation:

2. The horizontal cells produce only graded hyperpolarizing and depolarizing responses. They appear to play a role in colour coding and also increase retinal sensitivity by improving contrast.

PowerPoint Presentation:

3.The amacrine cells produce transient depolarizing potentials and spikes at the onset and offset of visual stimulus . These are the first cells in the visual pathway capable of generating impluses which are initiated during depolarizing.

D.Response pattern of ganglion cells :

D.Response pattern of ganglion cells These cells generate action potential which is transmitted along their axons to the lateral geniculate bodies. They discharge steadily at slow rate even in the absence of input from the rods & cones called ‘Resting discharge’. There are 2 types of ganglion cells : (i) on-centre cells (ii) off-centre cells

On centre cells :

On centre cells They respond with increased discharge when small circular beam of light shone on their receptive field,called on discharge, while illumination of surrounding zone inhibits discharge but a burst of impulse follows when the light is switched off,called off response.

Off centre cells :

Off centre cells In these cells light falling on the centre of their receptive field causes of response & on the surrounding zone produces on discharge.

PowerPoint Presentation:

3. The form of inhibition in which activation of a particular neural unit is associated with the inhibition of the activity of surrounding units is called ‘lateral or afferent inhibition’. 4. Different types of ganglion cells have been found to have different functional properties. a. large ganglion cells(y cells): concerned with movements & stereopsis b. small ganglion cells(x cells): are concerned with colour vision.

Neurotransmitters within retina:

Neurotransmitters within retina 1. cones releases glutamic acid a. It has inhibitory effect on bipolar cells produced by opening k+ channels & closing Na+ channels; both being operated via G- protien; b. It has excitatory effect on bipolar cells produced by opening k+ channels &/or Na+ channels; both being operated via G- protien;

PowerPoint Presentation:

2. Horizontal cell release GABA. It produces both its inhibitory effect & its excitatory effect by depolarizing cones. 3. Amacrine cells release A-ch to produce the effect.

E. Responses of neurons in lateral geniculate bodies & visual cortex:

E. Responses of neurons in lateral geniculate bodies & visual cortex Like the ganglion cells, neurons in LGB & in layer 4 of the visual cortex respond to stimuli in their ‘receptive’ fields with on centres or off centres. The neurons are classified into 2 main groups; a. simple cells ; They respond best to linear stimulus such as bars of light, lines or edges, but only when they have particular position. b. complex cells ; They also require a particular position of a linear stimulus but are less dependent upon the location of a stimiulus in the visual field than the simple cells.

PowerPoint Presentation:

3. Lateral geniculate bodies(LGB); a.carry signals for detection of movements & flickers, & signals for colour vision, texture, shape, & fine depth vision. b. Its function is to separate & relay these different kinds of informations from the retina to different cortical zones.

PowerPoint Presentation:

4. In the visual cortex there are many neurons associated with each fibre projected from the LBG. a. most neurons in one subdivision of the visual cortex are responsive only to stimuli oriented in a particular direction in the visual field. This is important in the detailed description of the form of an object. b. some neurons in another subdivision are most responsive to movements of an object across the visual field. c. some neurons respond best to color & some to depth perception.

F. Electroretinogram (ERG) :

F. Electroretinogram (ERG) At rest, the potential difference between the front & back of the eye is 6mV. When light falls on the eye it produces series of potential changes which can be recorded by placing one electrode on the cornea & other indifferent electrode on in the mouth or forehead. The record of this sequence is known as electroretinogram(ERG).

PowerPoint Presentation:

The sequence of potential changes which occur are; i. when the light stimulus is turned ‘on’ it produces, ‘A’, ‘B’, ‘C’ waves; while when the stimulus is turned ‘off’ , a small negative ‘off’ deflection is produced(off response) a. ‘A’ wave is the first sharp negative deflection due to rods & cones potential. b. ‘B’ wave is a positive wave which follows the ‘A’ wave & results from activity in the bipolar cells or glial cells. c. ‘C’ wave is due to activity in the pigmented epithelium of the retina.

PowerPoint Presentation:

ii. When the light stimulus is turned ‘off’ , a small negative ‘off’ deflection is produced, called ‘off’ response. a. They appear as a ‘slow’ decay in predominant rod retinae, called Remnant Negativity; & appear as a ‘faster’ decay from retinae containing only cones.

PowerPoint Presentation:

Uses of ERG: a. Helpful in the diagnosis of diseases in which visualization of the retina is difficult because the ocular fluids are cloudy. b. Helpful in congenital retinal dystrophies in which the retina appears normal by ophthalmoscopy.

References:

References Text book of Physiology by A k Jain. Text book of Physiology by GUYTON Review of medical Physiology by Ganong.

PowerPoint Presentation:

THANK YOU

authorStream Live Help