One basic idea behind every sense
Your brain sits in the dark inside your skull. It never touches light, sound, scent or heat directly. Everything it knows about the world arrives as electrical and chemical signals in nerves, and the job of the senses is to produce those signals.
The key step is called transduction: a specialized cell, a receptor, responds to one kind of physical or chemical change and converts it into a nerve signal. Each sense uses different receptors tuned to different kinds of energy, but the overall pattern is the same. A stimulus arrives, receptors respond, nerves carry the signal inward, and the brain interprets it.
The traditional list of five senses (sight, hearing, smell, taste and touch) comes from ancient times and is a useful starting point. It is not a complete count. People also sense balance, body position, temperature, pain and internal conditions such as hunger or a full bladder. Scientists do not all agree on exactly how many senses there are, because it depends on how a sense is defined.
Sight
The eye focuses light onto the retina, a layer of tissue at the back of the eye. There, rod and cone cells contain light-sensitive molecules that change shape when they absorb light, which starts a signal. Rods work well in dim light, and cones give color vision in brighter light. The optic nerve carries the signals to the brain, which assembles them into a picture. The details are covered in the cluster article on how vision works, and the physics behind it is part of the broader science of light.
Hearing
Sound is a pressure wave in air. The outer ear funnels it to the eardrum, three tiny bones pass the vibration along, and it reaches the fluid-filled cochlea in the inner ear. Inside, hair cells bend as the fluid moves, and that bending triggers nerve signals. Sound waves with different pitches stimulate different regions of the cochlea, which is one way the brain tells a low note from a high one.
Smell
Smell is a chemical sense. Molecules drift into the nose and bind to receptor proteins on nerve cells high in the nasal cavity. Humans have hundreds of different kinds of smell receptor, and a given odor typically activates a combination of them. The brain reads that combination as a pattern, much as a chord is more than one note. Smell signals travel to brain regions that are closely connected with emotion and memory formation, which is a common explanation for why a scent can bring back a scene from years ago.
Taste
Taste buds, mostly on the tongue, detect chemicals dissolved in saliva. The commonly recognized basic tastes are sweet, sour, salty, bitter and umami (a savory taste). Much of what people call flavor actually comes from smell: air carries food molecules from the back of the mouth up to the nose. That is why food seems bland during a bad cold. Texture, temperature and even appearance also shape the experience of flavor.
Touch and the body senses
The skin contains several kinds of receptors. Some respond to light pressure, some to vibration, some to stretch, some to warmth or cold, and others to potentially damaging stimuli, which is the basis of pain. Muscles, tendons and joints carry their own receptors that report where your limbs are without looking, a sense called proprioception. The inner ear also holds structures that detect head movement and gravity, giving a sense of balance.
Pain is worth a special note. It is not simply a measurement of tissue damage. The brain shapes how much pain is felt, and factors such as attention, expectation and stress response can change it. For that reason pain is understood as an experience built by the nervous system, not a direct readout.
How the brain builds the picture
Signals from the senses travel along nerves as brief electrical impulses, a process described in more detail in the explanation of neuron signals. Most pathways pass through a relay station deep in the brain called the thalamus before reaching specialized areas of the cortex, the brain’s outer layer. Different regions handle vision, hearing, touch and so on, and other regions combine information from several senses at once.
Perception is not a passive recording. The brain fills gaps, uses past experience to make guesses, and sometimes gets things wrong. Optical illusions are a familiar example: the signals from the eyes are accurate, but the interpretation is fooled. This active, constructive quality is one of many brain facts that makes the nervous system hard to compare with a camera or microphone, and it features in broader questions about the human mind.
Adaptation: why constant stimuli fade
Most senses respond strongly to change and less to steady input. This is called sensory adaptation. When you first put on a sweater you feel it clearly, but a few minutes later you hardly notice it. The smell of a room fades after you have been in it a while, and your eyes adjust when you move from bright sunlight into a dim hallway, a process that can take many minutes for full effect. Adaptation likely helps the nervous system avoid being flooded by unchanging information so that new or changing events stand out. It is also one reason the senses do not work like measuring instruments: they are tuned to report what is important, not simply what is there.
How the senses work together
The senses rarely operate alone. Flavor combines taste, smell, touch and temperature. Balance depends on the inner ear, vision and body-position receptors together, which is why closing your eyes makes standing on one foot harder. In conversation, the brain uses the movement of a speaker’s lips as well as the sound, and when the two disagree, people can hear a sound that matches neither, a well-known illusion. Such cases suggest that the brain combines the senses into a single best estimate of what is happening.
Senses differ between people and species
Sensitivity varies. Some people are color-blind, which usually means one or more cone types work differently. Hearing generally declines with age, particularly for high pitches. Smell and taste can be reduced by illness or injury. Other animals sense things humans cannot, such as ultraviolet light in some insects and birds or very high-pitched sounds in bats and many rodents. Every species’ senses are shaped by what its environment and way of life require.
A note on health
This article is general educational information, not medical advice. If you notice changes in your vision, hearing, smell, taste or sensation, individual cases differ, and a qualified health professional is the right person to consult.