2.87 understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity

Notes


When the lens shape is changed to adjust to varying distances of objects, this is known as accommodation and is how we focus light.
- The ciliary muscles are attached to the suspensory ligaments which in turn are connected to the lens (ciliary body -> suspensory ligaments -> lens)
- The lens, when no force is exerted on it, is naturally a rounded, disc-like shape
- The fovea is where the light is focused, it is a small point on the retina

Near objects
- When objects are near, the light from objects are more spread out (not very parallel)
- To focus this light onto the fovea, the lens must be more rounded to bend the light more
- The ciliary muscles contract and the suspensory ligaments go slack, since the lens is naturally spherical, light is bent through it's spherical shape

Far objects
- When objects are far, the rays of light are more parallel to each other
- To focus this light onto the fovea, the lens must be as little distorted as possible, so that the light is not bent much
- The ciliary muscles relax and the suspensory ligaments are pulled which in turn pulls the lens into a flatter disc shape

Process of pupil reflex
- When light enters your eye, it hits photoreceptors on your retina.
- These convert light energy to electrical impulses which are sent along sensory neurones in the optic nerve
- This information is unconsciously interpreted by the brain and as such it is known as a reflex; the motor neurones send information to the effector (iris)
- The effector muscles react
- Response is a change in pupil size, allowing more or less light in

Stimulus (light intensity) -> Receptor (retina) -> Sensory neurones -> Unconscious part of brain -> Motor neurones -> Effector (iris) -> Response (pupil)

-> The iris consists of radial muscles arranged like the ciliary muscles, when they contract the radius decreases, the surface area of the iris increases (the contracted muscles take up more space when arranged in a circle) and less light is let into the eye
-> Bright light: Radial muscles contract (these run through the diameter of the iris)
-> Dim light: Circular muscles contract (these run along the circumference of the iris)