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Physiology of Touch: Receptors and Pathways, Animation

Tactile receptors: Rapidly-adapting (Miessner and Pacinian Corpuscles ) and slowly-adapting (Mekel disks and Ruffini); two-point discrimination test; touch sensory pathways. Part of the sensation and perception series – special sense organs and general senses.
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The sense of touch, or tactile sense, is the perception of objects that come into contact with the skin.
Basically, mechanical stimuli such as pressure, stretch, vibration or movement, cause physical deformation in tactile receptors, which are essentially nerve endings of sensory neurons. The deformation typically leads to opening of ion channels, allowing ions to flow in or out of the cells, resulting in cellular depolarization. If the stimulus is strong enough, action potentials are generated and sent to the brain.
There are several types of tactile receptors, located in different layers of the skin and deeper tissues.
Anatomically, they can be encapsulated or unencapsulated, meaning they are wrapped or not wrapped in connective tissue, respectively.
Functionally, they can be rapidly-adapting, or slowly-adapting:
– Rapidly-adapting receptors generate action potentials when first stimulated, then quickly adapt and reduce or stop generating signals even if the stimulus continues. They respond to changes and therefore detect movements and sequence of events, rather than static objects.
– Slowly-adapting receptors, on the other hand, keep generating signals for a longer time. They carry messages about steady pressure on the skin and sense object’s texture, edges and shapes.
Touch receptors also differ in their sensitivity – some respond to light touch, others sense only heavy pressures. A sensory neuron receives signals from an area called its receptive field. Any touch within a receptive field stimulates one sensory neuron, transmitting one single signal to the brain. Being touched at 2 points within a receptive field would feel like a single touch. On the skin of the back, 2 points of contact several centimeters apart may feel like 1 touch, while on a finger, 2 contact points just 2 millimeters apart can be felt separately. Receptors with small receptive fields are more sensitive, they respond to lower frequencies of vibration; receptors with large receptive fields typically respond to higher frequencies of vibration.
A touch sensory pathway involves 3 neurons:
– Touch receptors are first-order neurons. Their axons form sensory fibers that enter the spinal cord via the dorsal root of spinal nerve.
– Inside the cord, first-order neurons synapse with second-order neurons either near the point of entry, or in the medulla of the brainstem. Second-order neurons then cross over to the other side of the cord, before ascending to the thalamus. This is how sensory information from the left side of the body is transmitted to the right side of the brain, and vice versa.
– Third-order neurons conduct the information from the thalamus to the sensory cortex.
Sensory neurons from the face and head follow several cranial nerves, mainly the trigeminal nerve, to the brainstem, where they synapse with second-order neurons, which decussate and continue to the thalamus.
The sensory cortex is spatially organized so that its adjacent areas represent neighboring regions of the body. The size of the cortical area representing a certain body region is proportional to the number of sensory receptors it has. Thus, sensitive regions with high density of receptors, such as fingers or face, have a larger cortical representation than the less sensitive trunk.

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