In addition to its role in controlling touch, the somatosensory cortex is also involved in sensing temperature, pain, and body position and movement.

The somatosensory cortex is divided into 4 regions known as Brodmann’s areas 3a, 3b, 1, and 2. Area 3b is also known as the primary somatosensory cortex or S1 [1].

• The neurons in area 3b and 1 are involved processing tactile (touch) information.

• The neurons in the area 3a are involved in processing information associated with the stimulation of proprioceptors found in muscles, joints, and tendons.

Proprioceptors are receptors that detect the forces (in the form of sensations) that are exerted on muscles, tendons, and joints.

• The neurons in the area 2 are involved in sensing both tactile information and the stimulation of proprioceptors.

The somatosensory cortex receives tactile information from sensory neurons that are connected to sensory receptors found in the skin, muscle, tendons, and joints.

There are 3 types of sensory neurons that send tactile information to the somatosensory cortex:

• First-order neurons

These neurons have their cell body in the spinal cord with one axon extending to connect with proprioceptors (mechanoreceptors) and the other axon extending to connect with a second-order neuron through the synapse.

• Second-order neurons

These are generally sensory nerves such as cranial nerves or trigeminal ganglia (part of the trigeminal nerve) that have their cell body in the spinal cord or the brainstem.

• Third-order neurons

These neurons have their body in the ventral posterior nucleus of the thalamus and extend their axons to the somatosensory cortex.

The ventral posterior nucleus serves as a relay for the transmission of tactile information.

1. Where Does the Sense of Touch Takes Place?

The sense of touch takes place in the skin through sensory receptors found in the skin.

2. What Are the 4 Touch Receptors?

They are 4 types of touch receptors including Merkel’s Disks, Meissner’s Corpuscles, Ruffini Endings, and Pacinian Corpuscle [2].

Once activated by touch, the receptors convert the tactile information (mechanical stimuli) into action potential that is transmitted by neurotransmission through first-order, second-order, and third-order neurons

• Merkel’s Disks

Merkel’s Disks are receptors found in the basal layer of the epidermis of non-hairy skin (e.g., fingertips) and have the capacity to sense corners, curves, and edges.

• Meissner’s Corpuscles

These receptors are also known as Wagner-Meissner corpuscles or tactile corpuscles. They are found in the dermal papillae of the thick non-hairy skin (glabrous skin) such as the skin of the sole and palm.

They are neurons endings involved in sensing fine touch and low-frequency vibrations [3].

• Ruffini Endings

These receptors are found in hairy skin and are activated by the stretching of the skin.

• Pacinian Corpuscle

These receptors are also known as Vater-Pacini corpuscle and lamellar corpuscle. They are mechanoreceptors (mechanical sensations) found in hairy and non-hairy skins and involved in sensing vibrations.

Touch receptors are also classified based on their adaptability to dynamic changes in the skin.

Pacinian corpuscle receptors have very rapid adaptability, while Meissner’s Corpuscles have rapid adaptability and Ruffini corpuscle, Merkel disks have slow adaptability.

3. What Are the Different Types of Touch Senses?

The different types of touch senses are classified into light touch, pressure, pleasant touch, vibration, temperature, and pain.

This classification is based on the type of receptor involved and the intensity of their sensitivity

Light Touch

A light touch is also known as protective touch as it is immediately set something that brushes the skin such as tickling.

Pressure

These receptors are activated when there is pressure or squeeze of the skin.

Pleasant Touch

This type of touch differentiates between pleasant and unpleasant touch.

Vibration

This type of touch is associated with the sensing of low-frequency vibrations.

Temperature

This type of touch is associated with the sensing of cold, hot, or warm objects.

Pain

This is due to the stimulation of nerves ends by noxious stimuli such as prolonged pressure or chemicals.

4. Are There Disorders of the Sense of Touch?

Disorders of the sense of touch are associated with the loss of sensitivity, the intensity of the sensitivity, and the capacity of recognizing the objects being sensed by touch.

They are caused by lesions affecting the somatosensory pathways, neurological diseases such as dementia, and chronic diseases such as diabetes.

Below are some of the disorders associated with touch:

• Astereognosis

This disorder is characterized by the inability to identify objects by touch only. It is caused by lesions in the post-central gyrus [4].

• Hyperesthesia

Hyperesthesia is characterized by an increased sensitivity to stimuli associated with senses such as hearing, tasting, touch, smelling. It is caused by excessive stimulation of the nervous system such as overconsumption of caffeine.

• Allodynia

Allodynia is a pain that is caused by a stimulus that is not the one that initiated the pain, such as feeling pain after a gentle shake of hands or light touch on the back.

• Analgesia

Analgesia is characterized by the inability to feel the pain caused by a stimulus (cause) that should normally have caused pain.

• Paraesthesia

This disorder is characterized by an abnormal sensation of the skin such as numbness, tingling, burning, chilling.

• Hypoesthesia

This disorder is characterized by a decrease in the sensation of touch associated with nerve damage, ischemia (reduced blood supply to tissues).

• Hypoalgesia

Unlike analgesia that characterized by the inability to feel pain, hypoalgesia is associated with a reduced response to a painful stimulus.

• Hyperalgesia

Hyperalgesia is characterized by an abnormally increased sensitivity to pain due to the release of inflammatory hormone-like substances, known as prostaglandins, that increase the sensitivity of the nociceptors.

Hyperalgesia can be caused by fibromyalgia, diabetes, infection, trauma, postherpetic neuralgia, and complex regional pain syndrome.

Conclusion

The touch sensory system is very complex and sophisticated thanks to the presence of highly specialized touch receptors that have a wide range of sensitivity and intensity.

This system provides us with the capacity of sensing cold, warm, and hot objects but also shapes and curves, and vibrations’ frequencies.

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