Know your Neurobiology: Tactile System
The rich tactile world we inhabit is accessed through our skin. Our skin is our largest organ and as such has a wide-ranging impact on our development, our behaviours and our understanding of the social and physical world we inhabit.
We can chunk its role into three main functions. To protect, regulate, and discriminate.
These distinct functions are possible due to the array of different Receptor cells, Neuronal pathways and structures in the brain and spinal cord to which our tactile information travels.
- Receptor cells are distributed with differing densities across our two types of skin; hairy skin, covering most of our body, and glabrous skin found on palms, lips, oral cavity, tongue, souls of feet, genitals. The two types of skin each have distinct, highly specialised sense organs particular to the skin type.
- Neuronal pathways take information form the skin to the central nervous system (spinal cord and brain) for processing. We have 3 types.
- C-Fibres: Thin, old, slow, unmyelinated fibres. Information involved in our protection social touch, pain, itch, warmth, and cold travel along these fibres.
- Alpha-B fibres: Thick, new, fast, myelinated fibres. Information for discriminating tactile information, e.g. texture, shape travel along these.
- Alpha-D fibres: Lightly myelinated, medium speed fibres can also transportpain and temperature information involved in the physiological regulation of the body’s homeostasis.
- CNS Structures are where tactile experiences are processed and used to build our brain’s connectivity and functioning, our understanding of ourselves, others and the environments we inhabit. The tactile sensations which we experience early in life when our brain is still building its foundational links and learning to interpret the world, can have the greatest impact on the type of brain we will build, how we view and respond to ourselves and the world around us.
Receptor Cells, Nerve fibres, CNS Structures
We have 3 major types of receptors: mechano, thermo and nociceptors
1. Mechanoreceptors: detect light touch, vibration, pressure, and texture We have anestimated 17,000 mechanoreceptors in our hand, rivalling the eye for sensitivity.
There are 6 types of Mechanoreceptors:
Merkel’s disks and Meissner’s corpuscles. These are very sensitive, densely packed and found close to the surface in glabrous skin e.g., lips, hands and oral structures, making them highly responsive to touch. Glabrous skin to provide detailed information about a source of touch stimulation, including its precise location, shape, size, texture and movement.
Information is sent via the fast thickly myelinated Alpha-B fibres into the spine and then up via the Dorsal Column Medial Lemniscus pathway to the Thalamus, a central junction in the brain. Tactile information from our head and neck makes its way into the brain via the Trigeminal nerve and onto the Thalamus. From the Thalamus, information heads to the Somatosensory Cortex at the top of our brain circulating here and beyond for detailed perception of our tactile experience. Fast information flow allows for discrimination between subtly different touch experiences and gives us the tactile information to learn: where is it? what is it? what do I do with it ?
Ruffini’s Corpuscles and Pacinian Corpuscles are found deeper in the skin. These feel sensations such as vibrations, rotational movement of limbs, and the stretching of skin. This greatly aids ability to understand our body and do physical activities such as running and dancing. These also feed heavily myelinated fibres that head along the Dorsal Column Medial Lemniscus pathway to the Somatosensory Cortex.
Hair Follicle Receptors are another type of mechanoreceptor found in hairy skin. There are 3 different types of hair follicles. We have about the same number of hairs as apes! But they are much smaller and have little pigment. Hairs have a role in our protection, and in regulating temperature and fluid levels. Hair follicles receptors have Alpha-B, Alpha-D and C fibre tracts taking a rich variety of tactile information to the CNS.
C-Tactile receptors are a distinct type of unmyelinated, mechanoreceptors existing only in hairy skin. This distinct unmyelinated system is poor at encoding discriminative aspects of touch but is well suited to encoding slow, caress-typetouch, fundamental to our social interactions, underpinning bonding and relationships.
We are unable to survive on our own and we survive as a species because of our social interactions. It is now thought that innovation of the C-Tactile receptors through gentle skin-to-skin interactions provides the sensory underpinning to our surviving and thriving.
While discriminative touch gives us the factual, cognitive aspects of tactileinformation, C-tactile information gives us the foundations for emotional safety, (protection) trust and social interaction.
The unmyelinated C-Tactile fibres link to the brain along the Spinothalamic tract. In this system, the tactile information moves to the Thalamus and then on to the Insula Cortex rather than the Somatosensory Cortex. The Insula, an old cortical structure hidden deep in the brain, combines our visceral (interoceptive) sensory information with tactile information. It gives us the how do I feel about this, do I feel OK? (regulation)
When C-Tactile receptors are stimulated, not only is information sent to the Insula, the seat of our self-perception, social interaction and empathy, but we also release oxytocin, our hormone of bonding. Oxytocin also impacts the Insula’s development. So, by the means of positive slow gentle touch we have a two fold impact on this structure and it’s impact on our wellbeing. With increased social motivation, enhanced emotional empathy plus reduced physiological and behavioural reactivity to stressors, including pain.
The course the C-Tactile information takes, landing in the Insula Cortex and linking with our homeostasis (regulation) means that some now consider this sensory information is part of the Interoceptive sense.
2. Thermoreceptors: Register hot and cold.
This information travels on C-Fibres and Alpha-D fibres regulating, and protecting us
3. Nociceptors: Register pain, these are essential for our protection. which there are three million pain receptors throughout the body, found in skin, muscles, bones, blood vessels, and some organs.
Temperature and pain are other forms of touch that protect us. Because of it’s importance there are a number of pathways for pain to make its way to different structures in the brain. Both pain and temperature have the ability to travel via the C-fibres to structures in the Brainstem, the Limbic system and to the Insula Cortex. They also travel via another type of fibre the Alpha-D fibres that head to the Somatosensory Cortex to give precise information about location.
Pain and temperature can therefore be processed as a discriminative, protective, and emotionally regulatory experience.
The two main tactile systems of Discrimination and Protection need to work together to give us the full tactile picture of ourselves and the world around. This enables us to feel Regulated, to feel OK so that I can go out there and explore.
We will discuss pain and hypersensitivities in more detail in future blogs.