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. They communicate with effectors either via the nervous system or the endocrine system. Ultimately this means that 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 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 carry nervous 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 from the lamellae as we will see below. In the resting state, there is a potential difference across the cell membrane generated by 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 a pressure stimulus 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. This creates a generator potential (the outside of the neurone is now becoming more negative than the inside).
- When and if the generator potential reaches the threshold level, an action potential is generated and continues along the length of the sensory neurone. A weak stimulus will not alter the potential difference enough to reach the threshold level.
There are many other types of receptors in the body. Here are just a few example:
- Photoreceptor – found in the retina and respond to light. We will look at these in more detail when we look (no pun intended) at the eye.
- Baroreceptors – found in the aorta and carotid arteries and respond to blood pressure.
- Chemoreceptors – found in the aorta, carotid arteries and the medulla oblongata (part of the brain) and respond to blood pH.
- Osmoreceptors – found in the hypothalamus and respond to water potential of the blood.
Not all receptors initiate a nervous response – some can initiate a hormonal response instead. We will look at the hormonal system (the endocrine system) in a separate series.
- 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.
- Receptors can communicate with effectors by the nervous system, endocrine system, or both.