Somatosensory system

Your body can feel many things.

You can feel a soft hug. You can feel something hot. You can feel a tiny tickle. This helps you know the world. It helps you stay safe.

Special parts in your skin feel touch. They feel things like pressure or shaking. Some parts feel if something is hot. Other parts feel if something hurts. These parts send signals to your brain. The brain then tells you what you feel. This helps you know where your body is. It is amazing how much you can feel!

Your body uses a special system to feel the world. This is called the somatosensory system. It helps you feel touch, heat, and pain. It also helps you know where your body parts are.

Your skin has many tiny parts called receptors. These receptors pick up different signals. Some feel light touch or shaking. Others feel pressure or skin stretch. Some receptors, called thermoreceptors, feel temperature. Other parts, called nociceptors, feel pain.

When a receptor is touched, it sends a signal. This signal travels through nerves to your spinal cord. From there, it moves up to your brain. The brain has a special area called the somatosensory cortex. This part of the brain acts like a map. It knows exactly where on your body you were touched.

Touch can also help you feel emotions. A kind touch can make you feel love or thanks. People who are deafblind use tactile signing to talk. They use touch to share words and feelings.

Your body has a special way of feeling the world around it. This is called the somatosensory system. It is a part of your sensory nervous system. This system helps you feel things from the outside, like a breeze. It also lets you feel things from the inside of your body. Most importantly, it helps you know your body's position and balance. This is called proprioception.

How does this system work? It starts with tiny parts in your skin called receptors. Some are mechanoreceptors that feel touch, pressure, or vibration. Others are thermoreceptors that feel temperature. There are also nociceptors that detect pain. When these receptors are touched, they send signals through nerves. These signals travel to your spinal cord. From there, they move up to your brain so you can feel them.

Scientists have studied how these signals travel for a long time. They found that the brain keeps a map of your body. This map is called a sensory homunculus. It is located in the somatosensory cortex. In this area, neurons that are close together represent parts of your skin that are close together. This helps your brain know exactly where a touch happened.

There are many different types of receptors in your skin. Merkel cell nerve endings help you feel shapes and edges. Tactile corpuscles let you feel light touch and vibrations. Pacinian corpuscles are very sensitive to quick vibrations. Bulbous corpuscles feel when your skin is being stretched. These receptors help you do things like read Braille.

Touch is about more than just feeling objects. It is also a way to share feelings. Humans can show emotions like love or sympathy through touch. This is called affective touch. People who are deafblind use a special way to talk called tactile signing. They use sign language and touch to communicate. This shows how important touch is for connecting with others.

The somatosensory system is a vital subset of the sensory nervous system. It allows the body to perceive external stimuli and internal sensations. This system also regulates body position and balance through a process called proprioception. Scientists view this system as a pathway between different sensory modalities. While much is known, researchers still debate the exact underlying mechanisms and how the system might impact bodily emotions.

Sensory perception begins with specialized receptors in the skin known as cutaneous receptors. These are divided into two main functional groups. The first group consists of mechanoreceptors, which detect physical changes like light touch, pressure, vibration, and tension. The second group involves nociception, which is the detection of pain and harmful temperature changes. Thermoreceptors specifically relay temperature information under normal circumstances. Nociceptors are specialized receptors that signal when a stimulus is considered noxious or harmful.

Mechanoreceptors in the skin are further classified by their response thresholds. Low-threshold mechanoreceptors respond to harmless stimuli. In glabrous, or hairless, skin, there are four specific types of these receptors. Merkel cell nerve endings are found in the basal epidermis and hair follicles. They react to low vibrations between 5 and 15 Hz and detect deep static touch like shapes and edges. Because they have small receptive fields, they provide extremely detailed information. They are especially prominent in the fingertips.

Other mechanoreceptors handle different types of physical feedback. Tactile corpuscles react to moderate vibrations between 10 and 50 Hz and light touch. They are located in the dermal papillae and are highly active in the lips and fingertips. These are essential for tasks like reading Braille. Pacinian corpuscles detect gross touch and distinguish between rough and soft substances. They are most sensitive to vibrations around 250 Hz. Because they react only to sudden stimuli, they quickly ignore constant pressures like the feeling of clothing.

Bulbous corpuscles are the fourth type of mechanoreceptor in hairless skin. They react slowly and respond to sustained skin stretch. These play a major role in the kinesthetic sense and help control finger position and movement. Interestingly, Merkel cells and bulbous cells are myelinated, while the other fast-response receptors are not. All these receptors are activated when pressure distorts their shape, causing an action potential. This electrical signal travels through peripheral sensory neurons to the spinal cord.

The pathway from the skin to the brain typically involves three specific neurons. The first-order neuron is a pseudounipolar neuron with its cell body in the dorsal root ganglion. It carries the signal from the receptor to the spinal cord or brainstem. The second-order neuron has its cell body in the spinal cord or brainstem. Its axons must cross over to the opposite side of the body, a process called decussation. Finally, the third-order neuron has its cell body in the ventral posterior nucleus of the thalamus. This neuron ends in the postcentral gyrus of the parietal lobe.

The brain processes this information in the primary somatosensory cortex, also known as S1. This area is located in the postcentral gyrus of the parietal lobe. S1 is divided into different Brodmann areas. Area BA3 receives the densest projections from the thalamus. BA3a helps sense the relative position of body parts and the effort used during movement. BA3b distributes information by sending texture data to BA1 and shape or size data to BA2. This organization ensures that the brain maintains a map of the body.

This cortical map is known as a sensory homunculus. In this map, neighboring neurons represent nearby locations on the skin. This topographic organization preserves the location of the physical stimulus. The secondary somatosensory cortex, or S2, is also involved in specific touch perception. Area S2 is linked with the amygdala and hippocampus to help encode and reinforce memories. Additionally, the insular cortex plays a role in bodily self-awareness and the perception of itch or local oxygen status.

Touch also serves a profound social and emotional purpose. Humans can communicate specific emotions through touch alone. These include feelings like love, gratitude, sympathy, fear, and disgust. This is known as affective touch, and it is coded differently than other sensory information. While the intensity of touch is processed in the primary somatosensory cortex, the pleasantness of social touch activates the anterior cingulate cortex. For people with deafblindness, tactile signing provides a vital method of communication using manual sign language.