14.2 Mammalian Sense Organs

2026 📋 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature, and chemicals
  2. Identify, on a diagram, the structures of the eye: cornea, iris, pupil, lens, ciliary muscles, suspensory ligaments, retina, fovea, optic nerve, and blind spot
  3. Describe the function of each part of the eye listed above
  4. Explain the pupil reflex in terms of light intensity and the antagonistic action of circular and radial muscles in the iris
  5. Explain accommodation to view near and distant objects in terms of ciliary muscles, suspensory ligaments, lens shape, and refraction of light

📖 Main Content

https://cambridgeassistant.com

1. What Are Sense Organs?

A sense organ is a part of the body that contains a large number of receptor cells grouped together. Receptor cells are special cells that can detect a particular type of stimulus (a stimulus is anything in the environment that triggers a response).

Because many receptor cells are packed into one organ, sense organs are very sensitive — they can detect even very small changes in the environment.

Each sense organ responds to one specific type of stimulus. This means the eye responds to light but not to sound, and the ear responds to sound but not to light.

The five main types of stimuli and their sense organs:

StimulusSense Organ / Receptor Location
LightEyes
SoundEars
TouchSkin
TemperatureSkin
ChemicalsNose (smell) and tongue (taste)

💡 Think of it this way: A receptor cell is like a alarm sensor. Different alarms detect different things — a smoke alarm detects smoke, a motion sensor detects movement. Similarly, different receptor cells detect different stimuli.

2. Structure of the Eye

The eye is the sense organ for light. It contains receptor cells that respond to light. Before you learn the functions, you need to know where each structure is located. Study the diagram below carefully.

🔍 Diagram of the Eye (Cross-Section)

                        [FRONT OF EYE]

                          Cornea
                        (clear, curved,
                         outermost layer)
                              |
              Pupil  <----  Iris  (coloured ring)
           (the hole in      |
            the middle)      |
                           Lens
                        (sits behind iris,
                         held by ligaments)
                              |
              Ciliary muscles (ring of muscle around lens)
              Suspensory ligaments (fibres connecting
                                    ciliary muscle to lens)
                              |
                           Retina
                    (lines the back of the eye)
                              |
                      Fovea (central point
                             of retina)
                              |
                    Blind spot (where optic
                                nerve exits)
                              |
                         Optic nerve
                    (exits the back of the eye
                     and travels to the brain)

Note for exam: You must be able to label all of these on a diagram: cornea, iris, pupil, lens, ciliary muscles, suspensory ligaments, retina, fovea, optic nerve, blind spot.

Sign in to view full notes