Today we are beginning a new series about nervous communication (the type that happens in animals, not making a speech in front of the class). We will cover the nervous system in a lot of detail over the next few articles, but today we are going to start by looking at receptors.
What are receptors?
Receptors detect stimuli. Each type of receptor only detects a specific stimulus, for example mechanoreceptors only respond to mechanical stimuli (e.g. pressure). Receptors can either be specialised cells, or proteins embedded into the cell-surface membrane. Through communication with other cells, the body can produce a response to a stimulus. Each type of stimulus has a different form of energy e.g. a light stimulus has light energy. A receptor converts this stimulus energy into electrical energy. Something that converts energy from one form to another is called a transducer. We’re getting a bit close to physics here… let’s swiftly move on.
Pacinian corpuscles
Pacinian corpuscles are a type of mechanoreceptor which are found in the skin. They’re actually pretty big – around 1mm in length. They respond to mechanical stimuli such as pressure and vibration. A Pacinian corpuscle has concentric layers of connective tissue called lamellae. The end of a sensory neurone is found in the centre (sensory neurones transmit nerve impulses from receptors to the central nervous system). The cell-surface membrane of the sensory neurone contains stretch-mediated sodium ion channels. These open in response to pressure on the lamellae as we will see below. In the resting state, there is a potential difference across the cell-surface membrane generated by movement of ions. The inside of the neurone is more negatively charged than the outside. This is called resting potential. All this will make a little more sense when we look at action potentials.
When pressure is applied to the Pacinian corpuscle, the following process happens:
- The lamellae are deformed and press on the sensory neurone ending.
- The cell-surface membrane of the sensory neurone stretches, and the stretch-mediated sodium ion channels are deformed.
- The stretch-mediated sodium ion channels open and sodium ions diffuse into the sensory neurone. If the threshold is reached, the membrane depolarises and a generator potential is established (the outside of the neurone is now more negative than the inside). A weak stimulus may not open enough stretch-mediated sodium ions channels for the threshold to be reached.
- An action potential continues along the length of the sensory neurone.
Other receptors
There are many other types of receptors in the body. Here are just a few examples:
- Photoreceptors – found in the retina and detect light.
- Baroreceptors – found in the aorta and carotid arteries and detect blood pressure.
- Chemoreceptors – found in the aorta, carotid arteries and the medulla oblongata (part of the brain) and detect blood pH.
- Osmoreceptors – found in the hypothalamus and detect the water potential of the blood.
Summary
- Receptors are transducers – they convert a specific stimulus energy into electrical energy.
- Pacinian corpuscles are mechanoreceptors found in the skin. The sensory neurone ending has stretch-mediated sodium ion channels in the cell-surface membrane which open when under pressure.
