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09 SciBio

GEP 1Topic 8 Lesson 1: EYE

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Accomodation-Distant Object

Parallel light rays coming from a distantobject are refracted as they enter the cornea.

Light rays pass through the pupil and are

refracted further by the lens which does thefine focusing.

Ciliary muscles relax, suspensory ligamentsare pulled taut, pulling on edge of lens.

The elastic lens becomes thinner, less convexand image is focused on retina.

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Accomodation-Nearby Object

If the eye focuses onto a near object, theciliary muscles will contract.

This will cause the suspensory ligaments to

slacken releasing their pull on the elastic lens.The elastic lens becomes thicker and moreconvex.

Diverging rays from the near object will be

refracted by the cornea and then be brought

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Additional Questions

What is meant by the term pupilreflex?

Pupil reflex is a cranial reflex

Involuntary action where the pupilconstricts in response to a stimulus ofincreased light intensity

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Pupil Reflex action

Cranial Reflex

Receptor : retina

Sensory neurons: optic nerve --> brain

Control centre: brain

Effector neurons: brain -->optic nerve

Effector: Iris Muscles

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Pupil Reflex

The size of the pupil is controlled by 2 sets ofinvoluntarymuscles in the iris the circular& radial muscles

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Pupil Reflex- BrightLight

Sudden bright light will be detected by thephotoreceptors of the eye.

Electrical signals/Nervous impulses will besent via the sensory neurones in the opticnerve to the brain.

Nervous Impulses will then be sent to themotor neurones to the iris muscles.

This causes the circular muscles of the iris tocontract, and the radial muscles to relax.

This causes the pupil to constrict and reducethe amount of light entering the eye.

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Question 1a

i) by means of the letter X, the region whenstimulated by light, results in no nerve impulsesbeing transmitted by L;

X= Blind Spotii) by means of the letterY, the region of clearestvision.

Y= yellow spot/fovea centralis [1]

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Question 1

b) What is the function of structure A?

allow light to enter [1] (reject control amount oflight to enter)

c) Explain the statement Structure B has aprotective function.

decrease in pupil size [1]; to prevent retinadamage due to strong light intensity [1] or radialmuscles relax, circular muscles contract, pupilconstricts/becomes smaller [1]

t

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uest onii) Describe how the appearance of the lens in(di) is brought about.

ciliary muscles contract [1]; suspensory ligamentslacken [1]; lens become thicker and moreconvex.

Q ti 2

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Question 2

0 2 4 6 8

Time (seconds)

Maximum curvature

Increase

in curvature

Minimum curvature

Q ti 2

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Question 2

0 2 4 6 8

Time (seconds)

Maximum curvature

Increase

in curvature

Minimum curvature

Q ti 2b

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Question 2bDescribe the changes that take place inside thestudents eye at time 6s.

Impulses are sent to the ciliary muscle whichcontracts, causing the suspensory ligaments toslacken; This releases the pull on the elastic lens

and causes it to become thicker and moreconvex; The diverging rays from a near object isfocused clearly onto the retina;

0 2 4 6 8

Time (seconds)

Maximum curvature

Increase

in curvature

Minimum curvature

t

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uest on(i) respond to bright light,(ii) is affected if a person is colour-blind, and(iii) changes shape to focus on near and distant

objects

The following explanation is not necessary foranswering this question. It is written to help youunderstand.

Iris responds to bright light. Pupil will constrict, This iscaused by the contraction of the circular muscles andrelaxation of the radial muscles in the iris.

The cone cells (one of the photoreceptors on the

retina) is for colour vision. These cone cells aremainly distributed in the fovea.

The lens changes its convexity and thickness whenfocusing on distant and near objects.

(iii) lens/ cornea,

conjunctiva

(ii) retina/ fovea

Fig. 3.1

(i) iris

Q ti 3b

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Question 3b(b) Explain the similarities between a pupil reflex actionand a knee jerk reaction. [4]

Both reflexes are involuntary because they do not involveconscious decision by the brain; (the reflex centre for thepupil reflex is the optic centre in the brain, and the spinalreflex is the spinal cord.)

Both reflexes are initiated by sensory receptors /organs:(pupil reflex is the retina; spinal reflex is the stretchreceptor at the upper thigh.)

Both reflexes are fast acting/immediate as the distancetraveled by the impulses from the sensory organ to thereflex centre and back to the effector is short;

Both reflexes are innate, no prior experience is needed to

shape the response; and The response for both reflexesis alwa s the same as it involves the same reflex arc and

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