Visual sensory system investigation. Anatomy of the eye: Three coats of the eye (Fig.81.): a. Outer fibrous layer (cornea and sclera); b. Middle vascular layer or uvea (choroid, iris, and ciliary body); c. Inner nervous layer (retina). The eye consists of anterior, posterior, and vitreous chambers. The anterior and posterior chambers of eye are filled with aqueous humour. It is produced by the ciliary body and enters the posterior chamber and then anterior chamber through pupil. It is reabsorbed to canal of Schlemm and venous plexus. Aqueous humour supplies nutrition to cornea and lens and maintains intraocular pressure. Clinical correlation The normal intraocular pressure is 10-18 mm Hg. An increase in intraocular pressure due to obstruction of the outflow of aqueous humour is glaucoma (the peripheral vision is affected first). Fig.81. Anatomy of the eye. Eye Optics: The optical apparatus includes the cornea, the aqueous humour, the lens and the vitreous humour (Fig.82.). The light rays are bent and refracted to a point behind lens (principal focus). The distance between the principal focus and lens is the principal focus distance. Fig.82. Optical components of the eye. The total refractive power of the eye is 60 dioptres (D) at rest; most of refraction occurs as light passes through the cornea. Dioptre is equal to the reciprocal of the focal distance in metres. The total refractive power of the lens can be changed from 13 D to 26 D. Accommodation involves lens accommodation, pupil accommodation and convergence. 1. Lens accommodation is the process by which the curvature of the lens is increased: a. when the lens is flat – far vision; b. when lens is rounded – near vision. The lens is held by circular lens suspensory ligament (zonular fibres) which is attached to ciliary body. Ciliary muscle contracts when the gaze is directed at near object. It relaxes zonular fibres and lens becomes more convex. The increasing of the lens curvature leads to the greater refractive power. 2. Pupil accommodation. The iris contains muscles that constrict or dilate the pupil. The pupil constricts (miosis) in the light to prevent the excessive entry of light. The pupil dilates (mydriasis) in the dark to allow more light to enter the eye. The image is inverted and reversed with respect to the object. However, the mind perceives objects in the upright position despite the upside-down orientation on the retina because the brain is trained to consider an inverted image as the normal. Clinical correlation Common refractive defects (Fig.83.) Fig.83. Defects of refractions and their correction. 1. Presbyopia – is reduced ability to accommodate for near vision with age; it is caused by declining flexibility of the lens. 2. Astigmatism – is inability to focus light rays that enter the eye on different planes due to uneven curvature of the cornea; corrected with cylindrical lenses; 3. Hyperopia or farsightedness – is inability to see the near objects clearly, the retina lies in front of the focal point of the lens due to shortening of axis of the eye; corrected with convex lenses, which cause light rays to converge. 4. Myopia or nearsightedness – is inability to see the distant objects clearly, the light rays are focused in front of the retina due to lengthening of axis of the eye; corrected with concave lenses, which cause light rays to diverge. Retina – is the light-sensitive portion of the eye, containing the cones, which are responsible for colour vision, and the rods, which are mainly responsible for vision in the dark. Human eye can detect light rays of 370-740 nm. Cells of retina (Fig.84.): 1. a three-neuron chain provides direct flow of visual information: a. photoreceptors: sensory transducers; two main types - rods and cones; b. bipolar cells: interneurons between photoreceptor and ganglion cells; c. ganglion cells: their axons form the optic nerve (generate action potentials): on-centre/off-surround cells and off-centre/on-surround cells. 2. lateral interactions (e.g. lateral inhibition) are mediated by: a. horizontal cells: between photoreceptors and bipolar cells; b. amacrine cells: between bipolar cells and ganglion cells. Fig.84. Retina. Each eye contains about 120 million rods and 6 million cones. Macula lutea is small area in the retina which contains only cones. It is responsible for acute and detailed vision. Centre of macula is fovea centralis, region of the greatest visual acuity. There are no cones or rods in the optic disc (blind spot). Visual pigments – these are light-sensitive chemicals that decompose on exposure to light and excite nerve fibres leading from eye. Rods contain rhodopsin (consists of opsin and 11-cis-retinal). Phototransduction (Fig.85.): 1. In the dark: the photoreceptors are depolarized (approximately -40 mV). a. high cGMP level leads to opening of cation (Na+/Ca2+) channels; b. cations influx (dark current) keeps the rod in a depolarized state. 2. In the light: the photoreceptors are hyperpolarized (about -65 mV). a. absorption of a photon of light by rhodopsin causes change in configuration from 11-cis-retinal to 11-trans isomer and generates metarhodopsin II; b. ↓cGMP level and closure of the cation-selective channels lead to hyperpolarization. In the dark In the light activation of guanylate isomerisation of rhodopsin cyclase ↓ ↓ activation of transducin synthesis of cGMP ↓ ↓ activation of opening of cation phosphodiesterase (Na+/Ca2+) channels ↓ ↓ breakdown of cGMP cations flow into outer ↓ segments (dark current) closure of cation channels ↓ ↓ depolarization of rod hyperpolarization of rod membrane membrane ↓ ↓ glutamate release at decreased glutamate synapses release ↓ ↓ hyperpolarization of depolarization of bipolar bipolar cell by opening Cl- cell and release of channels glutamate ↓ ↓ inhibition of ganglion cells generation of AP in ↓ ganglion cells no signal in optic nerve ↓ fibre signal in optic nerve fibre Fig.85. Phototransduction. Rods and cones make different contributions to visual activity: 1. scotopic vision: very low level of illumination (only rods are activated); 2. mesopic vision: low level of illumination (both rods and cones are activated); 3. photopic vision: moderate and high levels of illumination (only cones are activated). Interpretation of colour in the nervous system Colour vision is based on three types of cones (Fig.86.): 1. short-wavelength (S) or blue cones (peak sensitivity at 420-440 nm); 2. medium-wavelength (M) or green cones (peak sensitivity at 531-555 nm); 3. long-wavelength (L) or red cones (peak sensitivity at 558-580 nm). Fig.86. Stimulation colour-sensitive cones by monochromatic lights. Clinical correlation Night blindness (nyctalopia) is caused by significant deficiency of dietary vitamin A (also leads to xerophthalmia). Colour blindness is genetic disorder and more common in males. Individuals with normal colour vision are called trichromats. Dichromats are individuals with only two cone systems; they may have protanopia, deuteranopia, or tritanopia. Protanopia – loss of red cones. Deuteranopia – loss of green cones. Tritanopia – loss of blue cones. The visual pathways After nerve impulses leave the retina they pass backward through the optic nerves (Fig.87.): 1. Ganglion cell axons leave the retina at the optic disk and form the optic nerve. 2. About half of the optic nerve fibres cross the midline in the optic chiasm and project to the contralateral hemisphere; the other half stay ipsilateral; 3. Ganglion cell axons from the ipsilateral temporal hemiretina and contralateral nasal hemiretina form the optic tract. 4. axons exit the optic tract and terminate in the diencephalon and midbrain: a. diencephalon: - lateral geniculate nucleus (LGN) in the thalamus, relays visual signals through optic radiation to the primary visual cortex; - suprachiasmatic nucleus of the hypothalamus, responsible for endogenous circadian rhythms; b. midbrain: - superior colliculus, involved in coordinating orientating responses to a visual stimulus; - pretectum, involved in pupillary light reflex. 5. the principle projection to the visual parts of the cerebral cortex originates in the LGN and terminates in primary visual cortex V1, Brodmann’s Area 17 (also called the “striate cortex”), which is located in the banks of the calcarine fissure of the occipital lobe. Fig.87. The visual pathways. The visual field (Fig.88.) is the part of the surroundings from which the eyes can perceive light. It is what each eye sees. We divide the retina and the visual field in two halves: the temporal and the nasal parts. Clinical correlation Lesions of the optic pathways: a. lesion of the optic nerve results in blindness in the ipsilateral eye; b. lesion of the optic chiasm results in loss of vision in temporal fields (bitemporal heteronymous hemianopsia); c. lesion of the optic tract results in loss of vision in the contralateral visual field (homonymous hemianopsia); d. lesion of the primary visual cortex results in contralateral homonymous hemianopsia with macular sparing (loss of peripheral vision with intact macular vision). Fig.88. Optic lesions. The central parts of the visual fields of the eyes coincide (binocular vision). Coordination of eye movements is very important for binocular vision. Eye movements is mediated by six muscles which are innervated by CN III (oculomotor), IV (trochlear), and VI (abducens): medial, lateral, superior, inferior, rectus musles, superior and inferior oblique muscles. Types of eye movements: 1. Saccades (rapid movements when gaze fixes on object for a long time). 2. Smooth pursuit movements (when eyes follow moving objects). 3. Convergence movements (help to focus on near objects). 4. Vestibular movements (maintain visual fixation). Control questions: 1. Eye optic system characteristics. 2. Eye refraction and accommodation mechanism. 3. Pupil retraction, its mechanism and role. 4. Retina structure and its separate layers function. 5. Eye light sensitivity and adaptation. 6. Coloured vision. 7. Binocular vision. .Auditory and vestibular sensory systems investigation. The outer, middle ear, and cochlea of the inner ear detect sounds. The utricle, saccule, and semicircular canals maintain balance and equilibrium.