hello and welcome to another complete Cambridge IGCSE biology lesson where you'll learn absolutely everything you need to know on topic 14.2 sense organs as always we'll be following the Cambridge syllabus exactly and we'll cover absolutely everything you need to know for your final exam the topic 14.2 you need to describe the term sense organ identify the structures of the eye and describe their functions and explain the pupil reflex for extended you need to know about the muscles that control the diameter of the pupil the two different types of light sensitive cells and a part of the retina called the fovea so sense organs are groups of receptor cells that respond to specific stimuli or changes in the environment such as light sound touch temperature and chemicals one specific sense organ is the eye which will be the focal point of this lesson we'll begin with the main structures of the eye and their functions the cornea is a tough transparent curved layer located at the front of the eye it protects the internal structures of the eye and refracts or bends light to help focus it behind the cornea is the iris a colored ring of muscles that controls the size of the pupil the pupil is the circular opening at the center of the iris that lets light into the eye behind the iris is the lens a clear flexible structure that refracts light to focus it onto the retina it's held in place by ligaments and is capable of changing shape when focusing on near and distant objects we'll return to this later on the lining on the inside surface at the back of the eye is called the retina the retina contains millions of light sensitive receptor cells some of which are sensitive to light of different colors While others form images in Shades of Gray when light hits these cells they send electrical impulses through the optic nerve to the brain resulting in the sensation of sight finally the blind spot is a small area on the retina directly in front of the optic nerve it contains no light sensitive cells hence creating a blind spot in our vision next you need to understand the pupil reflex so the diameter of the pupil changes when exposed to light of different intensities when the eye is exposed to Bright Light the muscles of the iris respond by constricting or reducing the diameter of the pupil this means that less light is able to enter the eye which protects the light sensitive cells of the retina from damage in dim light the muscles of the iris dilate or increase the diameter of the pupil this allows as much light as possible to reach the retina improving the clarity of the image this is known as the pupil reflex as you cannot control it consciously okay so that's everything for core so we'll move on now to the extended content beginning with some additional information on the pupil reflex the diameter of the pupil is altered by the actions of the antagonistic muscles in the iris which are the circular muscles and the radial muscles antagonistic muscles are pairs of muscles that oppose each other in their actions as one muscle contracts the other relaxes to allow the movement to occur when light intensity is high the circular muscle contracts and the radial muscle relaxes this constricts the pupil and reduces the intensity of light entering the eye in low light intensities the radial muscle contracts and the circular muscle relaxes it dilates or enlarges the pupil allowing more light to enter the eye's ability to adjust its focus on objects that are near or far is known as accommodation the lens is held in place by suspensory ligaments that are connected to a circular band of muscle called the ciliary muscle when focusing on a distant object the celery muscle relaxes and widens pulling on the suspensory ligaments this stretches and flattens the flexible lens reducing the angle of refraction the eye is now accommodated for distant Vision when focusing on a near object the celery muscle contracts which constricts the circle of muscle and reduces tension on the suspensory ligaments and lens the lens returns to its relaxed convex shape increasing the angle of refraction and focusing light onto the retina there are two types of light sensitive cells in the retina rods and cones the cones enable us to distinguish colors in bright light while the rods are sensitive to low intensities of light and play an important role in night vision the images we form at night appear in Shades of Gray as the intensity of the light is insufficient to stimulate the cone cells there are thought to be three different types of cone cell one type responds best to red light one to green and one to Blue Rod cells are found all over the retina except at the blind spot and fovea while most of the cone cells are concentrated at the fovea the fovea is a small depression at the center of the retina that's responsible for central vision it's the point at which visual Acuity or the ability to identify detail and color is at its highest when you study something closely like a single letter of a word you're making its image fall on the fovea the high concentration of cone cells there means that you see the letter in fine detail by comparison light that lands on the rest of the retina produces an image that's much less defined this is called peripheral vision well done you've discovered absolutely everything you need to know on topic 14.2 sense organs if you enjoyed this video I'd really appreciate your subscription and I'll see you next time topic 14.3 hormones