The Sights and Sounds of Life: What are Sensory Neurons?

Jun 20, 2016 | Disease Models

What goes into enjoying a cup of hot coffee? There is the distinct fruity, flowery, nutty aroma. Hearing the crunch of when you plunge a spoon into the ground beans. The soothing sound of water being poured. The feeling of warm pressure as you grasp the handle and the side of the mug. And finally the complex taste of the coffee as you enjoy the feeling of heat, comfort and rejuvenation that comes in waves with each sip…

There is a lot at play helping you to enjoy your coffee. Today we’re looking at a key player in the process, your sensory neurons.

What are sensory neurons?

In short, sensory neurons are nerve cells that transmit information gained by our senses to the CNS (Central Nervous System).

While there is much debate about how many senses we have, scientists agree we have at least five basic senses to perceive the world around us: vision, olfaction, taste, audition, and touch . It is also certain there are far more than the basic five, however, scientific investigations are still in progress. We can attribute each of these senses to the hard work of our body’s neural circuitries. This intricate system converts peripheral sensory inputs into neural messages or impulses. These impulses travel to the central nervous system for interpretation.

How do we sense things?

Sensory neurons are connected to the CNS and allow the brain to know what is happening in our environment. Most are unipolar meaning they have dendrites at both ends connected by a long axon and a cell body in the middle.

Although each sensory neuron is distinct, they all follow the same process and rely on sensory nerve endings called sensory receptors to sense stimuli. This is how it happens:

The jargon: an external stimulus is detected and triggers a process known as sensory transduction. Sensory receptors are activated by the stimuli. The stimuli triggers excitatory or inhibitory responses in the neurons. And, their signals are carried by secondary relay neurons to the CNS for processing.

For example: touching your cup of coffee triggers your body’s sensory receptors to send a message to the CNS letting it know it is HOT! Depending on what you touched, a different combination of sensory receptors will activate and send a signal. This allows you to get a different sensation for different things and is why petting a puppy feels different to putting your hand into your cup of coffee which in turn feels different to being upside down.

Let’s break this down further:

The Two Steps to Sensory Transduction:

Registering a stimulus or stimuli.
Creating a unique nerve signal and sending it to a specific part of the CNS.

What are the different types of sensory neurons? Or rather….

This question is actually best answered by answering three others ones.

Question 1: where are the endings of the sensory neuron located?

Question 2: what morphological characteristics do the neuron receptors have?

Question 3: what are the different types of sensory neuron receptors?

What are the different types of sensory neuron receptors?

You can classify cells based on their morphology, location or by what kind of stimulus they respond to. It is common to group them into 5 classes: mechanoreceptors, thermoreceptors, nociceptors, electromagnetic receptors and chemoreceptors.

Mechanoreceptors:

Sense physical deformation like pressure, touch, stretch, motion and sound. While there are some obvious cases when these are used like enabling you to feel yourself moving around some are less obvious but are very cool like baroreceptors in your blood vessels which can detect and warn the brain of blood pressure changes! Proprioceptors which tell us the positions of our various body parts are also in this class. This class also warns you when you need to pee!

Chemoreceptors or chemosensors:

Sense the overall solute concentration of a solution and the individual concentration of special molecules or ions with it. Examples of these include osmoreceptors which actually detect the water concentration in the blood and warn the brain when we need to drink more!

Chemoreceptors can be subdivided into two main types: direct and distant.

Direct chemoreceptors interact directly with the stimulus e.g. the taste chemoreceptors on your taste buds bind chemical compounds allowing you to taste your coffee.
Distance or distant chemoreceptors allows us to sense something from a distance e.g. when olfactory receptors are bound by odor molecules allowing you to smell your delicious coffee.

Thermoreceptors:

Located in the skin and respond to heat and cold to assist in body temperature awareness. These guys will fire off warning if you miss your cup and pour hot water on your hand instead and trigger motor neurons to get you to move.

Nociceptors or pain receptors:

Detect pain and can become more sensitive to it in response to prostaglandin release. Nociceptors can be subdivided into two main types: somatic (more peripheral: skin, muscles, bones and joints) and visceral (internal organs and their supporting tissues).

Electromagnetic receptors:

Detect electromagnetic energy e.g. visible light, electricity and magnetism. This includes your eyes which contain photoreceptors called rods and cones which work together so you can see the wonderful things of the world (like your coffee!).

There are so many different types of sensory receptors especially when you look outside of the human body and into other animals. Some of them are super cool like infrared receptors that allow snakes to see the temperature of animals around them, just like infrared goggles!

So next time you enjoy a cup of joe, remember all the little neurons working hard to make sure you enjoy every gulp.