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Adequate Stimulus for Sensory Receptors

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson will describe the basics of how your sensory receptors communicate with your brain. Readers will discover which types of stimuli trigger different sensory neurons and what the adequate stimulus for a particular sensory neuron is.

What Is the Adequate Stimulus?

Why do we only hear with our ears? Why can't we see with our skin? The answer to these questions is found in the type of sensory receptors present in different parts of our body. Sensory receptors are a type of cell that senses what's happening in our environment. Each type of sensory receptor is different and responds to different environmental cues. There are sensory receptors that respond to sound in our ears and others that respond to light in our eyes. The type of stimulus a sensory receptor responds to is called the adequate stimulus. Each sensory receptor has just one or a few types of stimuli it will respond to. Before we get into examples of each type of sensory receptor, let's review how they get information to the brain.

How Do Sensory Organs Communicate with the Brain?

When a sensory receptor is triggered by the adequate stimulus, it sends a message to a brain cell called a neuron. Neurons are distributed throughout our body and relay messages to and from the brain. When the sensory receptor sends a signal to the neuron, the signal is then sent to our brain where we interpret the stimulus as sound, sight, touch, or other information from our environment.

Examples of Sensory Receptors

Sensory receptors are located throughout our body. Think of all the places you can feel something from your environment. Probably your entire body, right? You can even feel things internally. Think about if you have a stomach ache, achy muscles, or even a serious condition like appendicitis. In this lesson we'll examine three examples of sensory receptors: hair cells, photoreceptors, and nociceptors.

Hair Cells

Hair cells are sensory receptors in your ears and only respond to sound. Other stimuli, like light, isn't part of the adequate stimulus for hair cells. Sound is caused by vibrations in the air. The vibrations carry through the small bones in your ears until they reach the tympanic membrane, which encloses the fluid in your ears. The fluid vibrates according to the vibrations in the air in a space called the cochlea, which contains the hair cells. When the hair cells move, they let certain ions in and out of the cells. When the ions move they cause an electric impulse, or signal, in the hair cells which sends a message to the neuron.

Hair cell diagram
hair cell diagram

Only certain pitches of sound, or frequencies, cause this to happen. Human hair cells can only sense vibrations in the air up to about 20,000 Hz. Any frequency greater than that can't be heard. Bats, however, are able to hear frequencies up to 200,000 Hz, ten times greater than the sensitivity of human hearing.

Photoreceptors

Your retina is the tissue in the back of the eye that allows for sight. The sensory receptors inside the eye are photoreceptors, or rods and cones. Rods are responsible for black and white vision and are more active at night during low levels of light. Your cones are for color vision and are used during the day when light is ample. The adequate stimulus for both rods and cones is light.

However, just as our hair cells have a range of sound frequencies that they can detect, our eyes only have a certain frequency of light that we can detect. Electromagnetic radiation from the sun, or what we know as light, ranges from gamma rays, with a very high frequency and are very damaging to our body, to microwaves, which have a very low frequency. Visible light is a tiny slice of the spectrum and is the only light we can see. Other organisms such as honey bees can actually see ultraviolet radiation, the type that gives us a tan from the sun.

Visible light is only a small portion of the electromagnetic spectrum
visible light spectrum

When light comes into contact with photoreceptors, the photoreceptors send an impulse to neurons called bipolar cells. These cells in turn network with other neurons in the retina called amacrine cells, horizontal cells, and ganglion cells. The ganglion cells send messages through the optic nerve in the back of the eye to the brain, where it interprets the light as vision.

A micrograph with color labeling of cells in the retina
retina cells

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