Test: Sensory System: The Eye - 1 - JAMB MCQ

The term aperture represents a hole through which light enters the eye. It is known as pupil. Its size can be changed by iris. Iris is a thin circular structure in the eye. It controls the diameter and size of the pupil (aperture of the eye) and thus regulates the amount of light entering the pupil and reaching the retina.
So, the correct answer is 'Iris'.

Cones are the secondary photoreceptors that are stimulated by bright light and are responsible for color perception. The presence of slightly different photopigment in different cones allows them to absorb a particular wavelength more strongly than others. Rods are repolarized by dim light and are responsible for night vision. Choroid lacks rods and cones. The correct answer is A.

The retina makes up the inner layer and occupies only the posterior two-third of the eye. The retina consists of several layers of cells, including the rods and cones, the sensory cells, that respond to light. The tips of the rods and cones are embedded in a pigmented layer of cells on the back of the retina. The pigment helps prevent light from scattering in the back of the eye.

All photoreceptors share a common visual pigment: a mixture of a vitamin A-based chromophore (retinal) and a seven-transmembrane-helix apoprotein (opsin). Together, they form rhodopsin. The pigments belong to a special class of receptors called as G protein-coupled receptors, characterised by signaling through heterotrimeric G proteins. 
In vertebrates, the rhodopsin is thermally unstable, breaking down with light into the opsin and chromophore; rhodopsins are maintained by an enzymatic process. Invertebrate rhodopsins are stable, although renewal also must take place there.

Pupil is sensitive to light and control the amount of light entering the eye. Pupil contracts when bright light falls on the eye so that less light can enter the eye.

The retina is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision, while cones support the daytime vision and the perception of colour. The third type of photoreceptor, the intrinsically photosensitive ganglion cell, is important for reflexive responses to bright daylight. Neural signals from the rods and cones undergo processing by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells, whose axons form the optic nerve. 

The change in the shape of the lens is controlled by the cilliary muscles inside the eye. Changes in contraction of the ciliary muscles alter the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation.

Rhodopsin serves as a visual purple pigment of rod cells and is responsible for night vision. Rhodopsin deficiency causes night blindness. Myopia is caused by elongation of eyeball while shortening of eyeball causes hypermetropia. The cornea is the transparent anterior window of an outer layer of the eyeball and does not have rods. Thus, the correct answer is option D.

Myopia also known as nearsightedness or short-sightedness is a condition of the eye where light focuses in front, instead of on the retina. People having myopia can see close objects clearly, but objects farther away appear blurred. It occurs if the eyeball is too long or the cornea is too curved. So, the correct answer is option B.

The vitreous cavity is located behind the lens i.e., the posterior cavity and in front of the retina. It is filled with a gel-like fluid, called as the vitreous humour.

• Adjusting to light conditions: When the eye is exposed to bright light, the iris contracts, making the pupil smaller to reduce the amount of light entering the eye. In dim light, the iris relaxes, allowing the pupil to become larger and let in more light.
• Regulating the focus: By controlling the size of the pupil, the iris also plays a role in regulating the focus of the lens. A smaller pupil provides a greater depth of field, while a larger pupil allows more light to enter, which can be helpful in low-light situations.
• Protecting the retina: By controlling the amount of light that enters the eye, the iris helps to protect the light-sensitive retina at the back of the eye from damage caused by excessively bright light.

• The eyes are located in two sockets present in the skull known as orbits.
• The orbits are bony cavities in the front part of the skull in which the eye and its appendages are accommodated and protected.

• The wall of the eyeball has three layers.
• The outermost layer is known as the sclera.
• The middle layer is known as the choroid and it is bluish in color.
• The innermost layer is known as the retina.

The yellow spot or macula is an oval yellow spot near the centre of the retina of the human eye. It has a size of 1.5 millimeters. It is specialized for seeing things with highest clearness or visual acuity. It is the area of best vision, where maximum amount of rod cells and cone cells are present.

Neuroectoderm gives rise to the following compartments of the eye: retina, epithelial lining of ciliary body and iris, optic nerves. Surface ectoderm produces the following parts: lens, corneal epithelium, skin of eyelid.

Presbyopia occurs due to decrease in the power of accommodation of the eye usually because of ageing which causes gradual weakening of the ciliary muscles and diminishes flexibility of the eye lens.

Ciliary muscles are found in:

A: Junction of choroids and iris in eye ball

Explanation:

• Ciliary muscles are a group of smooth muscles located in the ciliary body of the eye.
• The ciliary body is a part of the middle layer of the eye, called the uvea, and is situated between the choroid and the iris.
• The primary function of the ciliary muscles is to control the shape of the lens in the eye, allowing for the adjustment of focus on objects at varying distances (accommodation).
• When the ciliary muscles contract, the lens becomes more rounded, allowing for near vision. When the muscles relax, the lens flattens, allowing for distance vision.

Myopia is a defect of vision in which far objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye’s lens too strong. Myopia can be corrected by wearing glasses with concave lenses these help to focus the image on the retina.

Answer: A. Hypermetropia

Explanation:

• A convex lens is a converging lens that brings light rays together at a focal point.
• Hypermetropia, also known as farsightedness or long-sightedness, is a vision condition in which a person can see distant objects clearly but has difficulty focusing on near objects.
• This condition occurs when the eyeball is too short, or the cornea or lens is not curved enough, causing light to focus behind the retina instead of directly on it.
• A convex lens is used to correct hypermetropia by shifting the focus of light rays onto the retina, allowing the person to see near objects more clearly.
• Convex lenses are often prescribed as eyeglasses or contact lenses for those with hypermetropia to improve their near vision.

Answer: C. Only rods in retina

Explanation:

• Owls have a higher proportion of rods in their retina compared to cones
• Rods are photoreceptor cells that are more sensitive to low light conditions, allowing owls to see well in the dark
• Cones, on the other hand, are responsible for color vision and function better in bright light conditions
• Having more rods in their retina enables owls to move freely during the night and effectively hunt for prey in low light environments

Colour blindness and haemophilia are both X-linked recessive disorders. 

Red-green colour blindness, a very common trait in humans affects between 7% and 10% of men and 0.49% to 1% of women. 

Hemophilia A, a blood clotting disorder is caused by a mutation of the factor VIII gene and hemophilia B, also known as 'Christmas disease', is a blood clotting disorder, caused by a mutation of the factor IX gene. 

The light is mapped as an image along the surface of the retina by activating a series of light-sensitive cells which are known as rod cells and cone cells. These photoreceptor cells convert the light into electrical impulses, which are transmitted to the brain via nerve fibres. The neurons of the retina are arranged in 3 main layers separated by 2 intermediate layers whose main purpose is to make connections among the various neurons. The deepest layer of neurons processes the light first. These neurons are the photoreceptors, the only cells in the retina that can convert light into nerve impulses. The photoreceptor layer then transmits these impulses to the bipolar neurons in the second layer and on to the ganglion neurons in the third layer. It is only the axons of these ganglion neurons, that exit the eye and carry the nerve impulses to the first visual relay in the brain.

• The middle layer of the eye is known as the choroid layer.
• The choroid id thicker in the anterior portion of the eye and forms the ciliary body.
• The ciliary body continues forward to form the iris.