Electrodiagnostic testing Retinal Electrophysiology Brodmann Areas 17, 18, 19 and 20 Sympathetic and parasympathetic nerves

1. GROUP V
 PINON KHAEMBA
 MARY MUTHONI
 FRIDAH ONCHABA
 JENTRIX MUKONZA
 JOY BASIL
 MARION KEMBOI

2. ELECTRODIAGNOSTIC
TESTING OF THE VISUAL
SYSTEM

3. ELECTRODIAGNOSTIC TESTING
• Visual system uses electric impulses to transmit data from photoreceptors to other
neurons and eventually to the brain
• One way to study and assess visual system is to measure its electrical activity
• The VER and ERG tests measure gross electrical potentials of the visual system
• They provide information about the visual system and can help diagnose certain
diseases

4. ELECTRODIAGNOSTIC TESTING
Types
• Electrooculogram (EOG)
• Electroretinogram(ERG)
• Visually Evoked Response (VER) or (VEP)

5. ELECTROOCULOGRAM(EOG)
• Measures the difference in electrical between the front and back
of the eye
• It is approx. 6mV but varies
Procedure
i. Electrodes placed on the skin near the inner and outer canthi
ii. Subject looks right and left while the change in electrical
potential is recorded
iii. EOG value is the difference in the electrical charge between
the front(+) and back(-) of the eye
• EOG is not used much anymore but has largely been supplanted
by the ERG

6. ELECTROOCULOGRAM(EOG)
• Some retinal diseases can be diagnosed by EOG and not ERG
E.g.
Stargardt’s disease (fundus flavimaculatus)
Butterfly shaped dystrophy
Best’s disease (vitelliform dystrophy)

7. ELECTRORETINOGRAM (ERG)
• Measures the response of the entire exposed retina to light
• Measured voltage is only about 1m V
Types of ERG procedures
 The standard full field ERG floods the entire retina with a single flash
 The flicker ERG exposes the retina to a flickering light
 The focal ERG exposes a limited area of the retina
 The multifocal ERG tests a large number of discrete locations in the
retina using a honey comb –like target
 The pattern ERG projects a small checkerboard pattern onto the
retina

8. VISUALLY EVOKED RESPONSE(VER)
• Also known as VEP or VECP
• Records the very small change in voltage at the visual
cortex in response to a light stimulus
• The primary test for diseases that affect the central
vision
Uses of VER
Optic nerve disease
Objectively test VA or contrast sensitivity in non-
responsive patients. Sometimes used to test infant
vision
Diagnosis of malingering or hysteria

9. SUMMARY

10. RETINAL
ELECTROPHYSIOLOGY

11. RETINAL ELECTROPHYSIOLOGY
• Involves the study of the electrical response of individual retinal neurons
• Much knowledge comes for lab research using animals such as mudpuppies , cats or
monkeys
TECHNIQUES
• Extracellular recording
• Intracellular recording

12. EXTRACELLULAR RECORDING
• Anesthetize the lab animal
• Insert an electrode into the retina near the cell you wish to study
• Stimulate the cell by shining light on that part of the retina and record the electrical
response
• Used to study neurons that respond with action potentials – amacrine or ganglion
cells

13. INTRACELLULAR RECORDING
• Insert a microelectrode into the neuron
• Can record from cells that respond with graded potentials
• Include photoreceptors , horizontal cells , bipolar cells
• After recording a dye can be injected into the neuron so the cell can later be
identified and studied histologically

14. GANGLION CELLS
Electrical response
• They respond to stimulation with action potentials
• Described as a sudden burst, or spike-like burst of electrical activity
• Spontaneous activity/maintained discharge – resting level of firing
• Ganglion cells were designed to use action potentials , rather than the
graded potential seen in the photoreceptors since action potentials can
be relayed across long distances
• May respond with excitation or inhibition
• Excitation – the action potential frequency increases
• Inhibition – the rate of firing decreases below the maintained discharge

15. GANGLION CELLS
Spatial summation
• if a small spot is projected onto the center of this receptive field , it will cause excitation
• If the spot size is gradually increased, the frequency of the action potential will increase
Spatial antagonism(lateral inhibition)
• If a spot size is further increased, it will begin to fall onto the surrounding inhibitory
region. The rate of neuronal firing will decrease
• Spatial antagonism- center and surround regions that show opposite responses to
stimulus

16. GANGLION CELLS
Contrast Vs illumination
• When light illuminates the entire receptive field, response is the same as if there
was no light at all
• Ganglion cells do not respond well to diffuse illumination of the visual field
• Instead they respond most strongly to stimuli that create a pattern with contrast
within their receptive fields
• This shows that the visual system has been designed to respond more strongly
rather than to diffuse illumination

17. GANGLION CELLS
Ganglion cell types by response
• The excitation response differ depending on whether it is sustained or transient type cell
• Sustained- excitation continues as long as the light is on
• Transient – responds briefly only when the light is turned on or off
• On center – respond with excitation when the receptive field center is illuminated but
with inhabitation when the annular surround is illuminated
• Off-center- inhibition when center is illuminated but excitation when the surround is
illuminated

18. GANGLION CELLS
• In the cat retina, X cells or Beta
cells show the following
features
Sustained response
Small receptive fields
Prevalent in the fovea
Specialize in fine detail
Associated with the
parvocellular system
• The Y-cells or Alpha cells show:
Transient response
Large receptive fields
Found mostly in the periphery
Specialize in motion detection
Associated with the
magnocellular system

19. GANGLION CELLS
• Can be classified into
X- type on-center
X-type off-center
Y-type on-center
Y- type off –center
• Summary of 2 major ganglion cell classification
Parvo :X-cells, Beta cells, P cells, midget cells
Margno : Y-cells, Alpha cells, M cells, Parasol cells

20. PHOTORECEPTORS AND
PHOTOTRANSDUCTION
• Photoreceptors do not respond with action potentials but with graded potentials
• When stimulated by light, the photoreceptors hyperpolarizes
Situation prior to exposure to light
• The membrane contains tiny pores that allow slow passive diffusion of some Na+ back
into the outer segment
• The diffusion of Na+ maintains a slight negative charge inside the cell
Situation after exposure to light
• Phototransduction begins with the absorption of a photon and ends with
hyperpolarization within the outer segment

21. PHOTORECEPTORS AND
PHOTOTRANSDUCTION
Process of phototransduction
A rhodopsin molecule is composed of an optically inert opsin and a light absorbing
chromophore portion. The chromophore consists of 11-cis retinal, which absorbs a
photon and undergoes a chemical change to 11-trans retinal
This activates a chemical called transducin that causes the release of enzymes
(phosphodiesterase)
The enzymes cleaves small clusters of cGMP. In the resting state, cGMP clusters
stand guard at the pores and keep them open so NA+ can diffuse back into the cells
When the cGMP clusters are broken up, the Na+ gates slam shut. Na+ can no
longer diffuse in and the outer segment rapidly increases its negative charge – it
hyperpolarizes

22. OTHER RETINAL NEURONS
Horizontal cells
• Contribute to spatial summation by synapsing with large numbers of photoreceptors
• When stimulated , their electrical response is similar to photoreceptors - the
hyperpolarize
• Sign conserving synapse – horizontal cell- photoreceptor synapse
Bipolar cells
There are atleast 9 different types of bipolar cells

23. OTHER RETINAL NEURONS
Amacrine cells
• Spatial antagonism
• Some respond with sustained action potentials
• Some with transient action potentials. Two kinds
Brief increase in action potentials when the light turns on
Brief increase in action potential when the light turns off
• Transient response may contribute to motion perception

24. BRODMANN AREAS
AREAS 17, 18,19 and 21

25. BRODMANN AREA
• Originally defined and numbered into 52 regions by
the German anatomist Korbinian Brodmann
• Is a region of the cerebral cortex in the human or
other primate brain defined by its cytoarchitecture
or histological structure and organization of cells
• Many of the areas Brodmann defined based solely on
their neuronal organization have since been
correlated closely to diverse cortical function

26. BRODMANN AREA 17- THE PRIMARY
VISUAL CORTEX
• Also known as visual area 1
• The part of the visual cortex that receives the sensory inputs
from the thalamus
• Located in and around the calcarine fissure in the occipital lobe
• Highly specialized for processing information about static and
moving objects and is excellent in pattern recognition
• It receives , integrates and processes visual information that is
relayed from the retinas

28. BRODMANN AREA 18
• A part of the occipital cortex in the human brain
• It accounts for the bulk of the volume of the occipital lobe
• It is known as a visual association area
• It is responsible for interpretation of images

29. BRODMANN AREA 19
• Part of the occipital lobe cortex in the human brain
• Along with area 18, it comprises the wextrastriate cortex
• Extraztriate cortex is a visual association area with feature –extracting, shape
recognition, attentional and multimodal integrating functions
• Contains regions of the visual areas designated V3, V3, V5 and V6
• It has been noted to receive inputs from the retina via the superior colliculus and
pulvinar and may contribute to the phenomenon of blind sight

30. BRODMANN AREA 21- MIDDLE TEMPORAL
GYRUS
• It is a gyrus in the brain on the temporal lobe
• It has been connected with processes as different as contemplating as
contemplating distance, recognition of known faces and accessing word meaning
while reading

31. THE VISUAL PATHWAY

32. THE VISUAL PATHWAY
• Consists of the series of cells and
synapses that carry visual
information from the environment to
the brain for processing
• It includes
Retina
Optic nerve
Optic chiasma
Optic tract
Lateral geniculate nucleus (LGN))
Optic radiations
Striate cortex

33. THE VISUAL PATHWAY
• The first cell in the pathway , the photoreceptor , converts light into a neuronal
signal that is passed to the bipolar cell and amacrine cell and then to the ganglion
cell; all these lie within the retina
• The axons of the ganglion cells exit the retina via the optic nerve, with the nasal
fibers from each eye crossing in the optic chiasm and terminating in the opposite
side of the brain
• The optic tract carries these fibers from the chiasm to the LGN, where the next
synapse occurs

34. THE VISUAL PATHWAY
• The fibers leave the LGN as the optic radiations that terminate in the
visual cortex of the occipital lobe
• From various points in this pathway , information about the visual
environment is transferred to related neurologic centers and to visual
association areas

35. VISUAL PATHWAY
LATERAL GENICULATE BODY
• Also known as lateral geniculate nucleus or lateral geniculate complex
• it is a relay center in the thalamus for the visual pathway
• It receives a major sensory input from the retina
• It is the main central connection for the optic nerve to the occipital lobe- primary cortex
• Each LGN has 6 layers of neurons alternating with optic fibers
• The LGN is a small , ovoid, ventral projection at the termination of the optic tract on
each side of the brain

36. LATERAL GENICULATE BODY
• The LGN receives information directly from the ascending retinal ganglion cells via
the optic tract and for the reticular activating system
Structure
Layer 1,2
• Large cells,- magnocellular pathways
• Inputs from Y-ganglion cells
• Very rapid conduction
• Color blind system

37. LATERAL GENICULATE BODY
Layer 3-6
• Parvocellular
• Input from X-ganglion cells
• Color vision
• Moderate velocity

38. LATERAL GENICULATE BODY
Functions in visual perception
• A signal is provided to control the vergence of the two eyes so they converge at the
principal plane of interest in object space
• A signal is provided to control the focus of the eyes based on the calculated distance
to the principal plane of interest
• Computations are achieved to determine the position of every major element in
object space relative to the principle plane

39. MEDIAL GENICULATE BODY
• Also known as medial geniculate nucleus
• Made up of a number of sub-nuclei that are distinguished by their neuronal
morphology and density ,afferent and efferent connections, and by the coding
properties
• It is thought to influence the direction and maintenance of attention
Function
• Responsible for relaying frequency , intensity and binaural information to the cortex

40. PARASYMPATHETIC NERVES

41. PARASYMPATHETIC NERVES
• One of the 2 divisions of the ANS
• Responsible of stimulation of “rest-and-digest” or “feed and breed” activities that
occur when the body is at rest
• Nerve fibers arise from the CNS
• Specific nerves include several cranial nerves,

42. PARASYMPATHETIC NERVES
CRANIAL NERVES
• The oculomotor nerve is responsible for a number of
parasympathetic functions related to the eye
• The oculomotor PNS fibers originate from the Edinger-
Westphal nucleus in the CNS and travel through the
superior orbital fissure in the ciliary ganglion
• From which they leave via short ciliary nerve fibers
• The short ciliary nerves innervate the orbit to control the
ciliary muscle and the iris sphincter muscle , responsible
for miosis or constriction of the pupil

43. PARASYMPATHETIC NERVES
Receptors
• Uses Ach as its neurotransmitter
• Acts on muscarinic and nicotinic receptors

44. SYMPATHETIC NERVES

45. SYMPATHETIC NERVES
• Its primary process is to stimulate the body’s fight-flight-or-
freeze response
• Described as being antagonistic to the parasympathetic
nervous system
• Sympathetic fibers innervating the eye separate from the
carotid plexus within the cavernous sinus
• Sympathetic fibers from the superior cervical ganglion
innervate blood vessels, sweat glands, and 4 eye muscles: the
dilator pupillae, the superior tarsal muscle, the inferior tarsal
muscle and the orbitalis

46. SYMPATHETIC NERVES
• The dilator papillae dilates the pupil- agonistic to the sphincter
papillae. Pupil is therefore under dual control of sympathetic
and parasympathetic nerves
• The superior tarsal muscle elevates the upper eyelid. The levator
palpebrae superioris which is innervated by a branch of the
oculomotor , also elevates the eyelids
• Eyelid elevation is therefore under both voluntary and
involuntary control
• The other two eye muscles with sympathetic innervation are
vestigial in humans

47. SYMPATHETIC NERVES
Sympathetic innervation
• Involves a 3 neuron pathway
First order neurons- located in the dorsal root ganglia
Second- order neurons – send their axons to the thalamus
Third – order neurons – are in the ventral nuclear group in the thalamus and fibres
from these ascend to the postcentral gyrus. Axons from the 3rd neuron then project
from the thalamus to the primary somatosensory cortex of the cerebrum.

48. SYMPATHETIC NERVES
The dilation response is the widening of the pupil and may caused by adrenaline ,
anticholinergic agents or drugs such as cocaine. Dilation of the pupil occurs when the
smooth cells of the radial muscle , controlled by the sympathetic nervous system ,
contract

49. REFERENCES
• Adler FH and Hart WH. Adler’s physiology of the eye : clinical application. 9th
edition , Mosby, USA. 1992
• Schwartz SH . Visual perception- A clinical orientation, 3rd edition. Appleton
&Lange, Stamford, Connecticut, 2004
• Mather, George. "The Visual Cortex"
(http://www.lifesci.sussex.ac.uk/home/George_Mather/Linked%20Pages/Physiol/Cort
ex.html). School of Life Sciences: University of Sussex. University of Sussex.
Retrieved 6 March 2017.