The sensory organs (eyes, ears, skin, nose and tongue) contain sensory receptors that allow the body to receive information about its environment. They react to a stimulus, which is a change in the environment that is perceptible by a sensory receptor. Examples of stimuli are touch, sound, taste, light, temperature, etc.
The Sensory Pathway
Nerve impulses always travel along the same sensory pathway, regardless of the stimulus that causes it. It all begins with the stimulus that is picked up by a sensory receptor. The stimulus is transformed into a nerve impulse, which then travels through a conductor (a sensory neuron, for example) to the cerebrum in the brain, where it is analyzed. The sensory receptors are scattered throughout the body to intercept mechanical, chemical, luminous or thermal messages from the environment (stimuli). These are mainly found in the sensory organs.
Here’s a table that summarizes the path of nerve impulses for each of the sensory receptors.
| Stimulus | Sensory organ containing receptors | Transformer | Signal | Conductor | Analyzer |
| Light | Eye | Retina | Nerve impulses | Optic nerve | Visual region of the cerebrum |
| Sound | Ear | Cochlea | Nerve impulses | Auditory nerve (or cochlear nerve) | Auditory region of the cerebrum |
| Pressure, pain and temperature |
Skin | Nerve endings |
Nerve impulses | Sensory nerves, spinal cord and brain stem |
Sensory region of the cerebrum |
| Odour | Nose | Olfactory epithelium | Nerve impulses | Olfactory nerve | Olfactory region of the cerebrum |
| Taste | Tongue | Taste buds | Nerve impulses | Cranial nerves and brain stem |
Gustatory region of the cerebrum |
Sensory receptors can be classified according to the type of stimulus they detect.
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Mechanoreceptors produce a nerve impulse when their tissue or adjacent tissue is deformed. The stimulus can be touch, vibration, pressure, stretching or itching.
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Thermoreceptors react when subjected to a change in temperature. For example, when the tongue reacts to hot food.
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Photoreceptors react to light, for example, the receptors in the retina of the eye.
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Chemoreceptors are stimulated by certain molecules. Odours and flavours, as well as changes in the chemical composition of the blood, are the three main stimuli linked to these receptors.
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Nociceptors react to pain and anything harmful to the organism. They transmit nerve impulses that the cerebrum interprets as pain so it can react quickly to avoid the dangerous situation.