Touch
Touch and pain are a part of the somatosensory system. The somatosensory system provides information from the body about touch, pressure, vibration, pain, temperature and proprioception (body-awareness/position).
This information can be discriminative or non-discriminative.
Discriminative: Information we can locate and identify, with fine features we can distinguish and discriminate. Eg. fine touch, pressure, vibration, position information, movement info)
Non-discriminative: The opposite of discriminative info. Eg. pain, temperature, crude affective forms of touch (ones that evoke emotions) like a hug.
Exteroception: Information about the external world
Mechanoreception: Pressure or touch (tactile sensitivity)
Thermoreception: Temperature (thermal sensitivity)
Nociception - noxious (damaging or potentially damaging) stimuli (noxious sensitivity)
Proprioception: Position and movement of the body in space (works with the balance system)
Introception: Information about internal organs
The somatic senses are detected by receptors (endings of nerve fibres). Unlike other sensory systems, the somatosensory system does not have specialised receptor cells but rather specialised endings/terminals of nerve fibres.
The nerve endings are specialised for specific stimuli through specific ion channels and accessory structures. This is why you don’t mix up pain and touch (or other sensations).
This means the specialised endings of nerve fibres (receptive zone) are specialised to respond to specific types of stimuli. This is due to the ion channels of the receptive zones of the terminals.
Different terminals have different specialised ion channels (which respond to different stimuli), allowing for different signals for different sensations.
The following are the types of ion channels in nerve endings for pain:
Nerve endings also specialise through accessory structures associated with the endings. In the image on the right, the nerve endings (eg. Meissner’s corpuscle, hair follicle, Pacinian corpuscle) have accessory structures at their ends. These accessory structures are formed by connective tissue and fluid.
Endings that don’t have accessory structures are known as free nerve endings (for the detection of temperature and pain)
We don’t need special accessory structures (like ears for hearing) because the skin is viscous and elastic and many nerve terminals also have connective tissue accessory structures associated with them, thus allowing to modify the way the stimulus energy arrives at the somatic receptor. Accessory structures can also modify the sensitivity of a receptor, becoming more sensitive from particular aspects of the stimulus.
Sensory receptors can be activated by either a direct pathway (mechanical displacement of a cell by the stimulus) or a indirect pathway (the stimulus activates molecules that act on the cell membrane)
Mechanorecptors and some nocireceptors use the direct pathway.
Thermoreceptors and most nocireceptors use the indirect pathway.
Both cases result in the opening of ion channels, allowing positive charge into the nerve terminal, depolarizing the membrane and changing the resting membrane potential.
Somatic nerve fibres can be classified by their axon diameters and myelin, as they affect the neuron’s conduction velocity.
This is seen in the table on the left.
The fastest conducting nerve fibres are those related to proprioception, allowing us to monitor second-by-second and if the terrain is even/uneven, and make needed adjustments. The slowest classes are those carrying pain and temperature. This is because, changes in them/occurence of events causes them, are not frequent and therefore the body doesn’t need to spend extra energy creating myelin or increasing axon diameter for these neurons.
Touch sensations are cutaneous discriminative mechanoreception (surface/touching you, specific pressure reception). The information is discriminative and from the skin, including touch, pressure, flutter and vibration.
There are many cutaneous mechanoreceptors, but not all are responsible for fine touch sensations.
The major ones in humans (for discriminative mechanorecption) are:
Meissner’s corpuscles: upper layers of skin
Merkel’s disks: upper layers of skin
Pacinian corpuscles: deep in the skin
Hair follicle receptors: In hairy skin, replace Meissner’s corpuscles. Nerve endings wrapped around the base of the hair follicle and are activated by the bending of the hair follicles.
All of these have things around their endings, called accessory sheaths. Accessory sheaths modify how energy from the stimulus acts on the nerve terminal underneath.
Touch receptors have ion channels activated by pressure/displacement of the membrane. This means they open if you cause displacement to the membrane where these channel are embedded. When they open, they allow for a in flow of Na+ and Ca++ ions, thus making the resting membrane potential more positive (depolization).
We have so many different touch receptors to signal different aspects of touch.
Each afferent fibre only receives input from 1 type of mechanreceptor (eg. only from the Merkel’s disks).
These nerve fibres are of the A beta class (middle speed). They are all sensitive to the mechanical indentation of the skin and all send information to the brain via the same type of nerve fibre.
Touch receptors are differentiated in terms of information provided because the receptors vary in adaptation rate and receptive field size. These properties influence the type of info they send to the brain.
Adaptation rate: the ability to tell if a stimulus is changing and how fast it changes
Receptive field size: Determines how precisely you can identify where the stimulus is happening on your skin/what the fine details are.
Fine touch has 3 major dimensions:
FORM: ability to identify the form and shape of objects solely based on touch (braille)
TEXTURE: Discriminate and feel the smoothness/roughness of an object
VIBRATION: Distinguishing between something fluttering on our skin to something vibrating.
Fluttering: frequencies < 40 Hz
Vibration: frequencies from 40-400 Hz
Touch sensitivity varies across the body, correlating with properties of the cutaneous mechanoreceptors responsible for these different sensations.
Touch sensitivity is best at our fingertips, tongue and lips.
The variation in tactile perceptions can be linked to the way we use these parts.
Our fingertips are used to explore the world, requiring the ability to feel small details and vibration, leading to a high sensitivity. The same logic applies to all the parts of the body. Lips/mouth for communication, backs are not used, toes to walk, etc.