VISUAL PATHWAY

GUEST AUTHOR

SWALEHA AKHTAR

AJMAL KHAN TIBBIYA COLLEGE

ALIGARH

INDIA

INTRODUCTION

The visual pathway is the part of the central nervous system (CNS) responsible for processing visual details and several photo response functions. Non-image forming visual functions, independent of visual perception includes pupillary light reflex and circadian photoentrainment. With the development of binocular stereoscopic vision in humans, it has become possible to adapt more efficiently to the surrounding environment. This “visual perception” is vital for our daily tasks and any deterioration in this phenomenon can affect the quality of life of the individual. Glaucoma is a condition which causes progressive loss of retinal function. This directly influences the psycho-physical condition of the individual.

This blog post focuses on how visual perception is achieved by the propagation of visual impulses from the retinal ganglion cells (RGCs) to the visual cortex. The visual pathway is important in understanding the anatomical relations in image acquisition and processing. The visual cortex corresponds to approximately 55% of the entire cortical area of the primate brain. Thus, visual stimuli are directly or indirectly responsible for more than half of all information stored in the brain.

RETINA

The cornea and lens act as a compound lens and focus an inverted image of the object onto the retina.

The RGCs constitute the first neurons of the visual pathway.

They are located in the innermost layers of the retina.

Thus, light needs to cross all the layers to reach the photoreceptors without interference from blood vessels or other structures of the retina.

There are 2 types of photoreceptors viz. rods (130 million) and cones (7 million).

Rods are present in periphery and used to see in low levels of light (scotopic vision).

Cones (3 types, depending on wavelength they absorb viz. blue, green and red) are present in centre (fovea). They are responsible for vision in bright light (photopic vision).

Five types of ganglion cells have been identified in the retina:

• 1.       M cells, large center-surround receptive fields sensitive to depth.
• 2.       P cells, smaller center-surround receptive fields sensitive to color and shape.
• 3.       K cells, with very large center-only receptive fields that are sensitive to color.
• 4.       Intrinsically photosensitive cell population.
• 5.       Cell population used for eye movements.

The photoreceptors contain a photopigment (protein) composed of opsin (a membrane protein) and 11-cis-retinal (a chromophore).

A photon has the capability of producing conformational changes (hyper-polarization) in cis-retinal (bent form), converting it to trans-retinal (straight form). This bleaching of pigment leads to a cascade of chemical reactions that convert electromagnetic energy into an electrical stimulus (signal transduction pathway).

This electrical stimulus is propagated to other retinal layers via neurotransmitters.

From the photoreceptors the impulse travels to the bipolar cells and then onto the RGCs.

Other neurons in the retina (e.g. horizontal and amacrine cells) transmit information laterally (from one neuron to the other side by side in the same layer). This results in more complex receptive fields.

The axons of the RGCs form the retinal nerve fiber layer and travel towards the optic nerve head (ONH).

Blood supply:

Outer 1/3^rd of retina: Posterior ciliary arteries

Inner 2/3^rd of retina: Central retinal artery (a branch of ophthalmic artery)

OPTIC NERVE

It contains approximately 1 million axons (nearly 40% of all axons in the CNS).

It is formed of 4 main regions:

1.       Nerve fiber layer

2.       Prelaminar region

3.       Lamina cribrosa

4.       Retrolaminar region

The optic nerve exits the eye through the lamina cribrosa.

The optic nerve in the retrobulbar area:

·         Gets invested with meninges (pia- and dura-mater).

·         Myelinated by oligodendrocytes.

The optic nerve consists of the following parts:

• 1.       Intra-ocular (scleral) portion (3-4 mm) [This mainly forms the ONH and is supplied by the arterial circle of Zinn-Haller (composed of anastomotic branches of the posterior ciliary arteries), the pial arteriolar plexus and peripapillary choroid].
• 2.       Intra-orbital portion (25 mm)
• 3.       Intracanalicular portion (6-7 mm; within the optic canal and lesser wing of sphenoid bone)
• 4.       Intracranial portion (18-20 mm)

Blood supply: Intra-cranial and intra-canalicular parts by superior hypophyseal artery (branch of internal carotid artery). Intra-orbital and intra-ocular parts are supplied predominantly from the ophthalmic artery and the Circle of Willis.

OPTIC CHIASM

The optic nerves travel backwards to cross at the optic chiasm.

Anatomical relations of the optic chiasm:

• ·         Anteriorly:
• ·         Posteriorly: Infundibulum.
• ·         Inferiorly: Sella turcica, pituitary gland and cavernous sinus.
• ·         Superiorly: Hypothalamus.

In the chiasm, the nerve fibers originating in the nasal retina decussate to the opposite side and join the temporal retinal fibers of the fellow eye.

Blood supply: Anastomotic arteries from the Circle of Willis.

OPTIC TRACT

The axons originating in the RGCs travel through the optic chiasm onto the optic tract to synapse with neurons in the Lateral Geniculate Nucleus (LGN).

Blood supply: From anastomotic branches of the posterior communicating and anterior choroidal artery (branch of internal carotid artery).

LATERAL GENICULATE NUCLEUS

The axons of the optic tract synapse with neurons in the LGN, which form the second neurons of the visual pathway.

These neurons in the LGN are distributed in 6 layers.

RGC axons from the ipsilateral eye (temporal retina) synapse in layer 2,3,5.

RGC axons from contralateral eye (nasal retina) synapse in layers 1,4,6.

There are 2 types of neurons in LGN:

• 1.       Large neurons, forming the “magnocellular layers” (located in layers 1 & 2)
• 2.       Small neurons, forming the “parvocellular layer” (located in layers 3,4,5 & 6)
• 3.       “Koniocellular layer” is irregularly distributed between the magnocellular and parvocellular layers.

In the LGN the central portion (hilum) receives macular fibers, while the lateral and medial horns receive fibers from the inferior and superior retina respectively.

Blood supply: Branches of internal carotid artery (mainly anterior choroidal artery) and posterior cerebral arteries (2-3 posterior choroidal arteries).

SUPERIOR COLLICULI, PRETECTAL NUCLEI AND SUPRACHIASMATIC NUCLEUS

Some of the fibers from the optic tract connect to midbrain nuclei (related to autonomic functions).

Superior colliculi are responsible for: Coordinating eye and head movements to sudden visual and other sensory stimuli and saccadic gaze. Also receive input from other sensory organs and visual cortex.

Pretectal nuclei receive afferent input from RGCs, which travel in dual connections to Edinger-Westphal nucleus.

Parasympathetic fibers from the Edinger-Westphal nuclei travel through the oculomotor nerves to the ciliary ganglion and control pupillary size and consensual reflex.

Some RGCs contain melanopsin and these axons travel to the suprachiasmatic nucleus (at the base of anterior hypothalamus). This centre is sensitive to changes in ambient light and sends fibers to the pineal gland. It regulates physiologic functions related to circadian rhythms.

OPTIC RADIATIONS

The second neuron axons from the LGN reach the visual cortex via the optic radiations.

These fibers initially project anteriorly and then posteriorly towards the occipital lobe.

Blood supply:

• ·         Anterior portion: Branches of Circle of Willis and middle cerebral artery.
• ·         Distal portion: Anastomotic branches of the posterior cerebral artery.

VISUAL CORTEX

Axons from the 6 layers of the LGN travel along the optic radiations to synapse in the primary visual cortex (called V1).

Axons from the parvocellular layers of the LGN synapse at layer IV-C-β.

Axons from magnocellular layers synapse at layer IV-C-α.

The vertical meridian of the visual field is represented medially within the calcarine lips.

The horizontal meridian of the visual field is represented deep within the calcarine fissure.

The macula (central visual field) is represented in the posterior pole of calcarine cortex.

The macular representation is greatly magnified in the visual cortex retinotopic map.

Blood supply: Posterior cerebral arteries and branches.

Occipital lobe: Has dual blood supply to the area corresponding to central vision. These include: anastomoses between branches of the posterior cerebral artery and branches of the middle cerebral artery.

FEEDBACK MECHANISMS AND THE HIGHER-ORDER VISUAL CORTEX

Stimuli from the retina reach the visual cortex, where they are regulated and processed before being finally perceived as an image.

Up-down connections between the thalamic and higher-order cortical levels provide an accurate perceptual interpretation of the visual stimuli.

Some of these connections include those between V1 and LGN, as well as those between different mesencephalic nuclei.

From V1 the information travels to extrastriate areas responsible for different features of vision, such as color, motion, depth, contrast and memory.

From V4 and V5 the information is conducted and/or stored in different areas which are related with other functions (e.g somatosensory, speech and hearing), motor activity and emotions.