Biology (0610) | Sense organs

Welcome to the World of Perception: Your Sense Organs!

Hello Biologists! In the previous chapter, we learned how the nervous system coordinates our actions using electrical impulses. But how does your body know what's happening outside (or inside)? That's where your sense organs come in!

Sense organs are your body's reporters. They take information from the environment (like light, sound, or temperature) and turn it into signals your brain can understand. This chapter focuses exclusively on the most complex of these: the human eye.

Let's dive in and see how we turn light into sight!

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1. Defining Sense Organs (Core Content)

What is a Sense Organ?

A sense organ is simply a group of specialised cells, called receptor cells, that are sensitive to specific types of stimuli.

A stimulus is any detectable change in the internal or external environment that causes a response.

The syllabus requires you to know that sense organs respond to specific stimuli, including:

• Light (Eyes)
• Sound
• Touch
• Temperature
• Chemicals (Taste and Smell)

Fun Fact: Your skin contains receptor cells for touch, temperature, and pain, making it one of your largest sense organs!

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2. The Structure and Function of the Human Eye (Core Content)

The eye is often compared to a sophisticated camera. Its job is to collect light and focus it precisely onto the back surface, where receptor cells detect it.

Key Parts and Their Roles (Core 14.2.3)

Don't worry if this seems like a lot of terms! We will break down the function of each crucial part required by the syllabus:

Part 1: The Light Entry Point

• Cornea: This is the transparent, protective outer layer at the front of the eye.
  Function: It performs the majority of the refraction (bending) of light, starting the focusing process.
• Pupil: This is the central hole in the iris.
  Function: It is the aperture (opening) through which light enters the inner part of the eye.

Part 2: Controlling the Light (The Iris)

• Iris: This is the coloured part of the eye, which is a muscular diaphragm.
  Function: It controls the diameter of the pupil, therefore controlling how much light enters the eye.

Part 3: Final Focusing

• Lens: A transparent, flexible structure behind the iris.
  Function: It focuses light rays precisely onto the retina. Its shape can change to focus on near or distant objects (a process called accommodation).

Part 4: The Receptor Layer

• Retina: The light-sensitive layer at the back of the eye. Think of it as the film in a camera.
  Function: It contains the light receptors (rods and cones). Some of these receptors are sensitive to light of different colours.

Part 5: Transmission to the Brain

• Optic Nerve: A bundle of sensory neurones leading away from the retina.
  Function: It carries electrical impulses from the retina to the brain, where the image is processed.
• Blind Spot: The point on the retina where the optic nerve leaves the eye.
  Function: Since there are no light receptor cells here, any image that falls on this spot cannot be detected—hence the name, blind spot.

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3. The Pupil Reflex (Core & Extended Content)

The pupil reflex is an automatic, involuntary response (a type of reflex action) that protects the sensitive retina from damage due to excessively bright light, and helps the eye see better in dim conditions.

The Process in Core (Core 14.2.4)

The pupil size changes based on light intensity:

1. In Bright Light: The pupil gets smaller (constricts) to reduce the amount of light entering, protecting the retina.
2. In Dim Light: The pupil gets larger (dilates) to allow more light to enter, helping maximise vision.

The Mechanism: Antagonistic Muscles (Supplement 14.2.5)

The iris contains two sets of muscles that work antagonistically (opposite to each other) to control the pupil diameter:

• Circular Muscles (arranged in rings, like a sphincter)
• Radial Muscles (arranged like spokes radiating outwards)

Scenario 1: Bright Light

1. The bright light stimulates receptors in the retina.
2. Impulses travel to the brain, leading to a response.
3. The circular muscles contract.
4. The radial muscles relax.
5. Result: The pupil constricts (gets smaller).

Memory Trick: When the circular muscles contract, they make the circle of the iris smaller, pulling the pupil in.

Scenario 2: Dim Light

1. The dim light causes a response via the nervous system.
2. The radial muscles contract.
3. The circular muscles relax.
4. Result: The pupil dilates (gets larger).

Did you know? The pupil reflex is a key sign doctors use to check if someone's brain stem is functioning normally!

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4. Focusing on Objects: Accommodation (Extended Content Only)

Accommodation is the process by which the eye changes the shape of the lens to focus objects at different distances onto the retina. This is essential for maintaining a sharp image.

This process involves the ciliary muscles (a ring of muscle surrounding the lens) and the suspensory ligaments (fibres connecting the ciliary body to the lens).

Focusing on a Distant Object (Relaxed Eye)

When you look at a distant mountain, your eye needs less light refraction (bending):

1. Ciliary muscles: Relax (The ring of muscle widens).
2. Suspensory ligaments: Become taut (tight), pulling on the lens.
3. Lens Shape: The lens is pulled thin and flat (less convex).
4. Refraction: Light is refracted less, focusing the distant image onto the retina.

Focusing on a Near Object (Strained Eye)

When you read a book close up, your eye needs maximum light refraction:

1. Ciliary muscles: Contract (The ring of muscle shrinks).
2. Suspensory ligaments: Become slack (loose), removing the tension on the lens.
3. Lens Shape: The natural elasticity of the lens causes it to spring back and become thicker and more convex (bulgier).
4. Refraction: Light is refracted more, focusing the near image onto the retina.

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5. Detailed Vision: Receptors in the Retina (Extended Content Only)

The retina contains two types of light receptor cells, named after their shape:

A. Rods

• Function: Rods are highly sensitive to low light levels. They are responsible for night vision (seeing in black and white or shades of grey).
• Distribution: They are distributed across the entire retina, but are absent in the fovea (see below).
• Colour Vision: Rods are sensitive to light of all colours, meaning they do not provide colour vision.

B. Cones

• Function: Cones are much less sensitive than rods and require bright light to function. They are responsible for colour vision.
• Types: There are three different kinds of cones, each absorbing light of a different colour (red, green, or blue).
• Distribution: Cones are concentrated in the centre of the retina, especially in the fovea.

The Fovea (Yellow Spot) (Supplement 14.2.9)

• Position: A small pit in the centre of the retina.
• Composition: It contains the highest concentration of cones and almost no rods.
• Function: Because of this high cone concentration, the fovea provides the sharpest, clearest colour vision (the region we use for reading or looking directly at something).