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Mechanoreceptors: a type of sensory receptors which transduce stimuli into signals through mechanically gated ligand ion channels.
Mechanoreceptors only respond to mechanical pressure caused by physical force. An example of this would be the pressure of your shoe against the sole of your foot when walking.
Generator potential is caused by depolarisation across the membrane that is typically produced in response to a stimulated sensory receptor. It is a graded potential, meaning that the changes in membrane potential can vary in size, rather than being all-or-none like action potentials are.
An overview of receptors
Before we dive into the details of Pacinian corpuscles, it is important to discuss what a receptor is.
A receptor is a cell or group that receives information from stimuli.
The stimulus may be external change, such as a decrease in the temperature outside, or an internal change such as a lack of food. The identification of these changes by receptors is called sensory reception. The brain then receives this information and processes it. This is called sensory perception.
Receptors are, therefore, essential in the body as they facilitate communication between the brain and different parts of the body, helping us adjust to external and internal environmental conditions. Receptors are a special class of proteins, so they are also referred to as receptor proteins.
When your fingers touch a piece of paper, the stimuli, in this case, would be the mechanical pressure caused by the paper pressing against your fingertip. The Pacinian corpuscles would transduce this pressure into a generator potential. This nervous impulse would then be sent to the central nervous system, allowing us to 'feel' the paper.
Where is the Pacinian corpuscle located?
Pacinian corpuscles are located all around the body. One key area is deep inside the skin, in the hypodermis layer. This layer is below the dermis and consists mainly of fat.
Pacinian corpuscles are encapsulated sensory nerve endings that act as pressure and vibration receptors.
In particular, Pacinian corpuscles in the skin are most abundant on the fingers, the soles of the feet, and the external genitalia, which is why these areas are so sensitive to touch. They are also commonly found in joints, ligaments, and tendons. These tissues are essential for movement - joints are where bones meet, ligaments connect bones, and tendons connect bones to muscles. Therefore, having Pacinian corpuscles is useful as they allow the organism to know which joints are changing direction.
Fig. 1 - The different types of skin sensory receptors
The only one you need to remember is the Pacinian Corpuscle (Figure 2), but the rest are good to be aware of to understand all the different changes our skin is sensitive to.
What is the structure of a Pacinian corpuscle?
The structure of Pacinian Corpuscles is quite complex - it consists of layers of connective tissue separated by a gel. These layers are called lamellae. This layered structure resembles that of an onion when sliced vertically.
At the centre of these layers of tissue is the ending of a single sensory neurone's axon. The sensory neurone ending has a particular sodium channel called a stretch-mediated sodium channel. These channels are called 'stretch-mediated' because their permeability to sodium changes when they are deformed, for example, by stretching. This is explained in more detail below.
Fig. 2 - Structure of the Pacinian Corpuscle
How does a Pacinian corpuscle perform its function?
As mentioned above, the Pacinian corpuscle responds to mechanical pressure, its stimulus. How does the Pacinian corpuscle transduce this mechanical energy into a nerve impulse that the brain can understand? This has to do with sodium ions.
Resting state
In the normal state of the Pacinian corpuscle, i.e. when no mechanical pressure is applied, we say that it is in its 'resting state'. During this state, the stretch-mediated sodium channels of the connective tissue membrane are too narrow, so sodium ions cannot pass through them. We refer to this as the resting membrane potential in the Pacinian corpuscle. See StudySmarter's other article on Action Potential for more information about what a resting membrane potential means.
Application of pressure
When pressure is applied to the Pacinian corpuscle, the membrane becomes stretched as it is deformed.
As the sodium channels in the membrane are stretch-mediated, the sodium channels will now widen. This will allow sodium ions to diffuse into the neurone.
Due to their positive charge, this influx of sodium ions will depolarise the membrane (i.e. make it less negative).
This depolarisation continues until a threshold is reached, triggering a generator potential to be produced.
The generator potential will then create an action potential (nerve impulse). This action potential passes along the neurone and then to the central nervous system via other neurones.
Directly after the activation, the sodium channels do not open in response to a new signal - they are inactivated. This is what causes the refractory period of the neurone. Remember that the refractory period is where the nerve cannot fire another action potential. This only lasts for a very brief time, normally around 1 millisecond.
Pacinian Corpuscle - Key takeaways
A receptor is a cell or group of cells that receive information from stimuli such as a change in temperature. Receptors are specific and work by acting as transducers.
A key example of a receptor is the Pacinian corpuscle, which is a mechanoreceptor (detects changes in mechanical pressure). Other examples include chemoreceptors and photoreceptors.
Pacinian corpuscles are encapsulated sensory nerve endings that act as pressure and vibration receptors. Pacinian corpuscles are located in the skin (particularly the fingers, soles of the feet, and external genitalia) and in joints, ligaments, and tendons.
The structure of a Pacinian corpuscle consists of a single sensory neurone ending surrounded by connective tissue, separated by a gel. Stretch-mediated sodium channels are embedded in this membrane.
In its resting state, a Pacinian corpuscle does not send out nerve impulses as the stretch-mediated sodium channels are too narrow, so sodium ions cannot enter to depolarise the membrane. When pressure is applied to the Pacinian corpuscle, the membrane is stretched, causing the sodium channels to open. The influx of sodium ions will depolarise the membrane, leading to a generator potential and an action potential, which passes to the central nervous system.
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Frequently Asked Questions about Pacinian Corpuscle
What is the importance of the Pacinian corpuscle?
Pacinian corpuscles allow us to distinguish between different levels of pressure that we touch as they respond differently to different levels of pressure.
Why is the Pacinian corpuscle described as a transducer?
A transducer is simply something that converts energy from one form to another. So, because the Pacinian corpuscle converts mechanical energy into a nervous impulse, we can describe it as a transducer.
Which layer of skin contains Pacinian corpuscles?
The hypodermis contains the Pacinian corpuscle. This is found deep below the skin below the dermis.
What are Pacinian corpuscles?
Pacinian corpuscles serve as mechanoreceptors in the body, sensitive to vibrations and pressure and are critical for proprioception.
Which type of sensation can Pacinian corpuscles detect?
They detect mechanical energy in the form of pressure and movement, so are very important for distinguishing touch.
Where is the Pacinian corpuscle located?
Pacinian corpuscles are located in subcutaneous tissue as well as deep in the interosseous membranes and mesenteries of the gut.
Why is the Pacinian corpuscle described as a transducer?
The Pacinian corpuscle can be considered a biological transducer. When a pressure stimulus is applied to the corpuscle, the lamellae are compressed and exert pressure on the sensory neuron. The cell surface membranes of neuronal tips become deformed and more permeable to sodium ions (Na+).
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