The Neuromuscular System (Part 7) – Introduction to somatosensory output through skin receptors.

Often forgotten in the process of understanding human movement, are the processes that may lead to a rapid reflex or a coordinated plan of action! The brain and spinal cord tend to be the middle person in many of these situations, so the focus of today’s article are somatosensory receptors, particularly within the skin.

The output of somatosensory receptors typically falls in three different neural circuits:

1. Local reflexes – generally involves quick contractions of a localised region through specific spinal cord interneurons in the dorsal root area e.g. bicep contraction once fingertip recognises a hot surface.

Localised reflexes tend to occur at the spinal segment of the muscles required to contract only.

2. Coordinated movement –  involves complex and synchronised contractions in various regions of the body in response to proprioceptive and kinesthetic feedback. This also occurs through spinal cord interneurons but now at various segments of the spine (see my spinal cord anatomy article if unsure).

3. Messages from skin receptors – information sent to the brain allowing you to be conscious of such information e.g surroundings, temperature, balance etc. This either travels through the lemniscal or spinothalamic pathway and eventually reaches the thalamus at the contralateral side.

The skin is one of the largest organs of the body (in terms of area) and serves as a protective barrier between you and the world through keeping out bacteria, dirt, maintaining temperatures within your body while adapting to the outside world. However, we all know that the skin must operate on a much more complex set of functions and explanations, because hey, when is anatomy and physiology ever just easy right??

The skin is compartmentalised into 2 (sometimes you may see 3) layers; the epidermis, the dermis (and the hypodermis).

• The epidermis (epi – above/on top, dermis – skin) is essentially multiple layers of dead skin cells forming the overall protective layer. Here, you will not find many nerve endings or receptors, which means you’re less likely to respond to pain, pressure or touch at this particular level of the skin. It’s like you being annoyed with a slight bit of protruding skin and pulling it off while being ever so careful, but to your surprise: this time it’s fine! This may be due to you tearing only a few layers of the epidermis while more cells are dying and migrating outwards to replace them. Who would have thought: we literally are dead on the outside!
• The dermis is a collection of living skin cells and the largest layer, sitting beneath the epidermis while above a layer of subcutaneous tissue and the hypodermis. It is the home to various receptors and glands whilst providing strength and flexibility through it’s fibrous and elastic properties.

Now while we’ve spent a great deal of time focusing on the motor cortex and motor neurons initiating movement, the focus here is the somatosensory cortex and how somatosensory neurons detect and process external stimuli. So let us begin with sensory neuron anatomy.

The morphology of these neurons differ in comparison to the typical motor neuron, in that they are classified as pseudounipolar. The only problem is: you probably have no idea how that was supposed to help. Let’s take a look at the diagram below

You can see that  a single axon leaves the cell body but now bifurcates (separates into two pathways) towards a) the dorsal root ganglion and b) the dermis. The pathway of the axon towards the dorsal root ganglion is what we could consider the typical neural synaptic pathways whereby information is communicated via spinal interneurons towards other spinal cord regions (or further up towards the brain) OR initiates a stretch reflex response. However the axons that terminate at the dermis are of four different types due to the tips being specialised touch receptors, also known as mechanoreceptors. These are the:

1. Merkel disks – These are the pressure receptors that sit on the border between the dermis and epidermis (and sometimes may extend into the epidermis itself). They are disk shaped and ultimately aid in detecting how much pressure is being applied upon the skin.
2. Meissner’s corpuscles – Also pressure receptors but have receptive fields of increased sensitivity. As a result, corpuscles found within the border between or the epidermis are shallow, while those deep within the dermis have relatively receptive fields. These receptors are also more specialised towards recognising shearing forces.
3. Ruffini corpuscles – These are large receptors that respond to stretch and play a role in keeping the skin intact against tears.
4. Pacinian corpuscles – The most important of them all; this is due to a) these being the fastest reacting receptors with large receptive fields and b) their fundamental role in muscle reflex actions.

Note: Pacinian and Ruffini corpuscles are found in muscles, tendons and ligaments also. Here they are known as proprioceptors where their main role is to mediate a sense of position and movement. They allow us to understand how much muscle force/stretch is being applied or the position of a joint.

So how do mechanoreceptors work?

At a typical synapse, we would expect voltage gated ligand channels at the post synaptic membrane to respond to neurotransmitter release from the pre-synaptic axon terminal (click here if you need to briefly revise this process). However, in the case of these touch receptors, they respond (and are activated) in response to mechanical stretch/tension. We can summarise this in 3 simple steps:

1. Stretching of a skin cell membrane cause the generation of action potentials in axonal endings, which lead to the transmission of information to dorsal root ganglion. (Note how the neural information is travelling away and TOWARDS the cell body this time).
2. Once the information reaches the spinal cord, the somatosensory neuron will synapse with spinal interneurons.
3. The interneuron will either communicate this information with motor neurons to create a rapid reflex action and/or will synapse with other ascending neurons to higher segments of the spinal cord or the brain itself.

Finally, it is worth remembering that just like with motor units of various muscles, sensory receptors vary with each region of skin e.g. the fingertips will have a higher density of receptors at the surface in comparison to the skin on your leg. Cortical processes tend to represent receptor density in a given area, so as with the motor homonculus, the somatosensory homonculus dictates areas of the brain devoted to a skin region and will show proportionally larger cortical areas in the cortex, in relation to receptor density.

That concludes my summary of somatosensory output and skin receptors for today! Stay tuned for the following articles/topics within the NM System series:

• Understanding proprioceptors
• Introduction to Electrophysiology and single cell recordings.
• Neurological adaptations to training.

Once again, if you found this useful in anyway, please feel free to drop a comment, give the article and share to all your friends and family. Feel free to connect with me also via Twitter  @Nasiruddin4595 or Instagram @Physcombat