Have you ever wondered how humans are able to perceive pain? How do we realize the cold and the hot? What allows us to have this knowledge so important for our survival? In this article we will talk about the somatosensory system, responsible for making us perceive pain and temperature. It also allows us to give utility to the sense of touch and is responsible for proprioception, this perception of the position in which our body is located.
The somatosensory system is one of the most extensive systems in the human body. He is responsible for processing all sensory information (eg pain and temperature). It treats the soma, which is made up of the body, bones, muscles and viscera. But also the skin, since all of its receptors are distributed in our body. There are therefore two somatosensory systems:
- Skin somatosensory system : It is made up of the receptors in the skin, and is therefore peripheral – it is found throughout the body. It has kinesthetic receptors which inform about the position of the body and the movements. They are found in places such as joints and tendons
- Organic somatosensory system : it has receptors in the bones and viscera and is internal
The cutaneous somatosensory system: the key to understanding perception
In order to understand how humans perceive pain and temperature, it is important to understand how receptors in the skin work, including the most sensitive receptors capable of generating pain sensations.
The skin is the largest organ in the soma, and therefore the largest receptor. Along the skin there are a large number of receptors grouped differently, which allows us to define the sensitivity or one of the four sensations that we receive through it: pressure, vibration (touch), pain and temperature.
Therefore, through the receptors of the skin somatosensory system, we receive information from the environment in the form of pressure, touch, pain, cold and heat.
Does the down on the skin play a role in all of this?
We can distinguish between skin with hair and skin without hair. The skin with hair is the most common case. Hairless skin experiences a greater clustering of receptors. It is therefore more sensitive because it has more skin receptors.
The most sensitive sensory organs would be the lips, the external genitalia and the fingertips, since there is a greater density of receptors.
While it is not completely safe, it is believed that skin with hair is more sensitive to vibration or touch. Indeed, these two phenomena cause movements of the hairs.
What receptors do we have in the skin?
Skin receptors fall into two categories: free nerve endings and encapsulated receptors.
Free nerve endings are nerve extensions that reach the skin and are probably the simplest sensory receptors. They are dispersed throughout the skin and are the most sensitive receptors for the perception of pain. Although they are also used to perceive everything else, they are specialized in pain. There is specificity, but not exclusivity.
The transduction mechanism of these terminations is only the stretching of a specific part of them, which allows the sodium channels to be opened. It is at this moment that the depolarization of the membrane occurs, to reach the action potential. It occurs in cold by contraction and in heat by expansion.
Encapsulated receptors: everything they do inside their capsule
Encapsulated receptors are a type of skin receptor. They are called so because they are covered with a capsule. Some speak of four types, others of five. These receptors are classified as follows:
Pacini’s corpuscles: sensitive to pressure and touch
They are found in the subcutaneous tissues, especially present in hairless skin. Pacini’s corpuscles are densely clustered on the lips, mammary glands and external genitalia. They are particularly sensitive to pressure, vibration and, to a lesser extent, pain and temperature.
They are small, encapsulated receptors. They also have free nerve endings, but covered with connective tissue. They are found on hairy skin and they react to low frequency vibrations.
Meissner’s corpuscles and discriminating touch
They are in charge of the sensitivity of the discriminating touch. They are found in the hairless skin and fit into the papillae of the dermis.
The corpuscles of Krause
The Krause corpuscles are found only at the intersection of mucosa and dry skin. Their fibers are not myelinated and are extremely sensitive to pressure. Their pressure activation threshold is the lowest in the entire human body.
Merkel’s discs occupy a place similar to that of Meissner’s, in the papillae of the dermis. They slowly adapt to receptors and respond to a continuous change of stimuli, not directly – for example, the perception of temperature adaptation.
The perception of pain
Perceiving pain and temperature, in this specific case pain, is designed as an adaptive alert system. It allows us to avoid sources that produce damage to our body. However, this sensation can be influenced by emotional, psychological and social factors. But also by pharmaceutical products, the placebo effect or even hypnosis.
For this reason, it is a very subjective emotion, which suggests that there must be neural mechanisms that modify or interfere in the transmission of pain. It is not only based on skin receptors.
We can divide pain into two types:
- Avoidable pain, where the body’s best response is to eliminate the source of the pain, i.e. the behavior
- The inevitable pain, which exists at the peripheral level and at the central level. As the name suggests, there is no escaping it
At the peripheral level – where there is inevitable pain – it has been observed that molecular information related to pain circulates in the body. In pain, certain cells are damaged and secrete histamine and prostaglandin. The second has no effect per se, but histamine does, which lowers the pain threshold of the cells.
Prostaglandin makes damaged cells more sensitive to histamine and significantly lowers the pain threshold. These are pains in the broken tissues. Pharmacological mechanisms exist to block histamine (antihistamines) and prostaglandin (acetylsalicylic acid).
Can the pain be blocked? The thalamus has the solution
At the central level, pain studies have been carried out on the thalamus. Pain is adaptive, but if it is severe, it can block behavior. It is sometimes counterproductive. Some therefore ask themselves the question of the possibility of avoiding pain. Is it possible ? How can the thalamus be blocked?
Inhibition of pain is often referred to as analgesia, influenced by emotional and physiological factors. However, in people who have had a stroke it has been observed that injury or blockage of the posterior ventral nucleus of the thalamus usually results in loss of skin sensation. That is, superficial sensations of touch and pain.
Thus, lesions or blockages of the intralaminar nuclei eliminate deep pain but not skin sensitivity. The dorso-medial nuclei are connected to the limbic system and usually interfere with the emotional components of pain, eliminating them.
The perception of temperature
It is relative, because we have no receptors to perceive the temperature in an absolute way. We can only perceive sudden changes in temperature by moving our hand from a pot of very cold water to a pot of very hot water, for example.
There are two types of receptors, some for cold and others for heat, distributed heterogeneously in the skin. Cold receptors are closer to the epidermis, while heat receptors are found in a deeper area. They are the same receptors, they differ only in their location.
Transduction in these receptors occurs either by deformation of the membrane or cone of the receptor by expansion effect, or by contraction of the skin. This results in the opening of the membrane and sodium channels. If the receptors are densely clustered, the feeling of heat will be more intense. The nuclei associated with the difficulty of perceiving the cold and heat of the thalamus are the intralaminaria and, to a lesser extent, the ventricles.
It is therefore extremely interesting that the perception of pain is due, among other things, to small receptors in the skin and a high involvement of the thalamus. The same phenomenon occurs with temperature.
All of these functions seem to have developed in the pursuit of our survival. These tools we have are just a legacy of what our ancestors needed in the past, perhaps much more than we do today.