The role of a sensory neuron is to detect change and then inform other neurons. They convert environmental stimuli into nerve signals.
Sensory
Neurons
The image to the right is a sensory neuron. Immediately you should be able to pick some key differences from the sensory neuron compared to the motor or interneuron.
- There are no dendrites at the top. Instead we have receptors.
- The nucleus is located away from the receptors and dose not have dendrites protruding from it.
Sensory neurons are located all throughout the body. They can detect changes within the body (such as glucose levels) or outside the body (temperature). Any change from light to sound or even chemical can be detected by a specific sensory neuron.
The image on the left illustrates a few of the different types of sensory neurons in your body.
Sensory Nerve cells can be located in muscle and joints, taste buds, olfactory receptors (nose), auditory receptors (ear) and Photoreceptors (eye).
Photoreceptors
As the name might suggest these receptors detect changes in light.
They can vary greatly between organisms. From simple light sensitive cells in worms, to the complex eyes of a cuttlefish. (Click here for more information on the cuttlefish)
In most vertebrates it is our vision which gives us the most information about our environment.
Types of receptors:
Chemoreceptors
Chemoreceptors are stimulated by specific chemicals in the external or internal environment.
External chemicals are detected by receptors in the nose and mouth. They detect changes in taste and smell.
Animals use Chemoreceptors to find food, avoid predators and find potential mates (Click here for more information on pheromones).
Mechanoreceptors
Mechanoreceptors are receptors are activated when tissue is stretched (tendons), compressed (skin), distorted or vibrated (inner ear). We can further divide Mechanoreceptors into sub categories
Thermo receptors
Thermoreceptors detect changes in temperature. They are located on the skin and in the brain.
Receptors on the skin detect temperature changes, where as those in the brain detect internal temperature changes.
Some receptors within the brain can detect changes as small as 0.01 degrees Celsius.
Internal receptors are found in various organs throughout the body (I.e. Hypothalamus and bladder).
Internal receptors detect changes within the body. For example:
Your body does not measure the amount of oxygen in your system, it actually measures the amount of Carbon dioxide. Carbon dioxide receptors are located all throughout major arteries.
When these receptors detect an increase in Carbon Dioxide they cause an increase in heart rate and breathing.
Touch receptors
Touch receptors are located near the surface of the skin and respond to very little force. As you can see by animation to the right.
This receptor might be activated if someone brushes past you.
Pressure receptors
Pressure receptors are located deep in the skin and respond to pressure and force. Their shape changes as pressure is added.
As you can see by animation to the left.
This might be activated
if someone pokes you.
Balance receptors
Mechanoreceptors which are sensitive to vibration are located within the ear. Your balance and equilibrium is based on receptors effected by fluid in the semicircular canals of the inner ear.
Stretch receptors
These receptors relay information to the nervous system about how stretched our muscles are. Everything from Posture to the dilation of our blood vessels is detected by these neurons.
Our stretch receptors tell us when our lungs are fully inflated, causing us to stop breathing and start breathing out.
Photoreceptors contain pigments which absorb a specific forms of wavelengths.
As with plants, the receptors in our eyes, absorb only a limited section of the light spectrum.
Photoreceptors can only absorb light from infrared through out Ultra Violet.
With in our eye we have Rods and Cons. The picture too the right is a close up of the photoreceptors in our eyes. You can see that the rods and cons get their names due too their rod and cone structure, respectively.
Within your eye there are roughly 120 million Rods. These rods are extremely sensitive too light, in fact they are 1000 x more sensitive than Cones. As such they are very good at detecting the presence of light.
Unlike cones, they can not detect colour. They are responsible for black and white vision.
Cones are extremely important for site. There are roughly 6 million cones in your eye and are responsible for detecting colour.
We breakup these cones into 3 different categories. Red cones (60%), Green cones (30%) and Blue cones (10%).
Another key difference between Rods and Cones is their recovery time. By this I mean the time it takes for a neuron to be able to fire again.
Rods are much slower than Cones, meaning they take longer to recover. You would have experienced this when you go from a light to dark room and your vision is impaired. The reason there is a long delay is because your Rods have to be reset.
Click here for more information on the eye
