Tactile discrimination is the ability to differentiate information through the sense of touch. The somatosensory system is the nervous system pathway that is responsible for this essential survival ability used in adaptation. There are various types of tactile discrimination. One of the most well known and most researched is two-point discrimination, the ability to differentiate between two different tactile stimuli which are relatively close together. Other types of discrimination like graphesthesia and spatial discrimination also exist but are not as extensively researched. Tactile discrimination is something that can be stronger or weaker in different people and two major conditions, chronic pain and blindness, can affect it greatly. Blindness increases tactile discrimination abilities which is extremely helpful for tasks like reading braille. In contrast, chronic pain conditions, like arthritis, decrease a person's tactile discrimination. One other major application of tactile discrimination is in new prosthetics and robotics which attempt to mimic the abilities of the human hand. In this case tactile sensors function similarly to mechanoreceptors in a human hand to differentiate tactile stimuli.
Pathways
Somatosensory system The somatosensory system includes multiple types of sensations from the body. This includes light, touch, pain, pressure, temperature, and joint /muscle sense. Each of these are categorized in three different areas: discriminative touch, pain and temperature, and proprioception. Discriminative touch includes touch, pressure, being able to recognize vibrations, etc. Pain and temperature includes the perception of pain/ amounts of pain and the severity of temperatures. The pain and temperature category of sensations also includes itching and tickling. Proprioception includes receptors for everything that occurs below the surface of the skin. This includes sensations on various muscles, joints, and tendons. Each of these three categories have their own types of pathways and receptors. These pathways target the cerebellum in the brain. This section of the brain tracks what the muscles are doing at all times so any potential damage to this area can greatly affect one's senses.
Within each Somatosensory pathway there are three types of neurons: the pseudounipolar neuron, secondary afferent neurons, and tertiary afferent neurons. There are also slowly adapting receptors that signify the receptors that sense the indents made on the skin. Rapidly adapting receptors are also present in this system. An example of a slowly adapting receptor in use is when a person breaks his/her arm, the arm is immobilized until it is healed. He/she does not want to forget that it is broken and do something that could potentially worsen the damage in the arm. An example of a rapid adapting receptor in use is putting on clothes. Initially you will feel the clothes being worn, but after a while you forget you are wearing clothes. It is not at the forefront of the brain to focus on the feeling of the clothes on your body; however, if you were to concentrate on that feeling, you could instantly feel the contact between your skin and the clothing being worn.
Discriminative touch system The discriminative touch system deals with everything from the toes to the neck through the spinal cord. The sensation experienced enters the periphery by axons. More specifically, the sensory axons. This signal passes through axon to axon from the distal to proximal process. The proximal end of the specific axon leads into the spinal cord on the dorsal half. This then moves towards the brain. These axons that are leading the signal towards the spinal cord to the brain are classified as primary afferents. This makes sense as afferent is defined conducting toward something. These neurons are sending signals towards the brain. Those that receive the neuron synapses are classified as secondary afferents. These neurons go to the thalamus and then synapsed towards another set of neurons that move towards the cerebral cortex.
Types of receptors There are many types of receptors in the somatosensory pathway including:
Peripheral Mechanoreceptors - Activation of these receptors is the initial step of recognizing a stimulation. An indentation, as stated before, becomes an electrical signal in the peripheral process of a primary afferent neuron. This creates a depolarization across the membrane of the neuron and this leads to an action potential that goes to the cerebellum of the brain to initiate an action. Merkel's Disks - Located on the upper part of the dermis. Slow adapting receptors. Found on the fingertips as well as the eyelids. Meissner's Corpuscles - Located also on the upper part of the dermis. Found on the hairless skin including the lips as well as the eyelids. These are rapid adapting receptors. Thermoreceptors - These are receptors that are able to detect heat. There are actually 2 types of these receptors in mammals. One that can identify heat higher than body temperature and vice versa!
Types of tactile discrimination
Stereognosis Stereognosis (Tactile Gnosis) is defined as the ability to tell the difference and identify objects via touch in the absence of visual or auditory contact. The subject will need to be able to recognize temperature, spatial properties, texture, and size to reach an accurate conclusion to what the object is. This type of tactile discrimination will give an indication of the status of the Parietal lobe of the brain. When conducting this test, common objects that the subject is familiar with are used in order to ensure an accurate reading, and consistency amongst multiple tests with multiple different subjects. By utilizing this form of tactile discrimination, practitioners will be able to detect and track the presence or effects of Neurodegenerative Diseases such as Alzheimer's disease due to Astereognosis, which is the failure to recognize objects via touch without visual recognition.
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