Cutaneous sensation

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Introduction : 

Introduction A sensation is the conscious or subconscious awareness of changes in the external or internal conditions of the body. For a sensation to arise, four events typically occur: a stimulus capable of activating specific sensory neurons must occur a sensory receptor or sense organ responds to the stimulus and transduce (convert) it into a nerve impulse nerve impulses are conducted to the brain a region of the brain receive and integrate the nerve impulses, producing sensation

Adaptation of Receptors : 

Adaptation of Receptors When a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreases to very few or often to none at all Receptors are of two types according to the rate of adaptation Slow adapting receptors Rapid adaptation receptors

Types of sensations : 

Types of sensations Sensations can be grouped into two classes: General sensations Somatic sensation(touch, pressure, vibration, warm, cold, pain, and proprioceptive sensations) Visceral sensations 2. Special sensations ( taste, vision, hearing, and equilibrium)

Cutaneous sensations : 

Cutaneous sensations The dermis of the skin contains sensory receptors for various senses Touch Pressure Vibration Pain Temperature (warmth and cold)

Touch : 

Touch Touch sensation generally results from stimulation of tactile receptors in the skin or in tissues immediately beneath the skin. Two types 1- Crude (rough) touch: It is not sharply localized Its receptors are hair end organs and free nerve endings It is transmitted by the ventral spinothalamic tract Its afferent fibers are of group Aδ. It is tested by touching the skin with a piece of cotton while the eyes are closed.

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2. Fine touch:- It is sharply localized Its receptors are mainly Meissner's corpuscles and Merkel's discs It is transmitted by gracile & cuneate tracts and its afferent fibers are of type Aβ It can be tested by two methods

1-Tactile localization (or topognosis) : 

1-Tactile localization (or topognosis) It is the ability of the person with closed eyes to localize the stimulated area of the skin It is tested by touching the skin lightly by a marker, then the person is asked to mark the stimulated area. The closer these two points to each other, the more accurate is the localization

2-Tactile Discrimination : 

2-Tactile Discrimination Ability of the person with closed eyes to differentiate between two stimuli of touch , applied simultaneously to the skin, as two separate points of touch. There must be a threshold or minimal distance between the two stimuli. This distance is small in finger tips (2-3 mm) but it may reach 60-70 mm in the back. It is tested by touching the skin with the two blunt points of a compass, while the individual is closing the eyes and then he is asked whether he was touched by one or two points. We change the distance between the points of the compass to determine the minimal distance in different parts of the body (finger tips, lips, back and thigh).

2-Tactile Discrimination : 

2-Tactile Discrimination

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Both Tactile Localization and Tactile Discrimination are more accurate with: Large number of receptors in the stimulated area. Large number of afferents arising from the stimulated area.

Touch Receptors : 

Touch Receptors Touch sensation is mediated through a no. of receptors Free nerve endings Meissner’s corpuscle Merkel’s disc Hair end organ Ruffini’s end organs Pacinian Corpuscles

Free Nerve endings : 

Free Nerve endings Found everywhere in the skin and tissues Can detect touch and pressure

Meissner’ Corpuscles : 

Meissner’ Corpuscles Elongated encapsulated nerve ending of a large (type Aβ) myelinated sensory nerve fibers Inside the capsulation are many branching terminal nerve filaments Present in the non hairy parts of the skin (Glabrious skin) and are particularly abundant in the finger tips, lips, and other areas of the skin Adapt in a fraction of a second after they get stimulated Sensitive to movement of objects over the surface of the skin as well as to low frequency vibration

Merkel’s Disc : 

Merkel’s Disc Present in both hairy and non- hairy part of the skin Transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts only slowly. Responsible for giving steady-state signals that allow one to determine continuous touch of objects against the skin

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Merkel’s discs are often grouped together in a receptor organ called the Iggo dome receptor, which projects upward against the underside of the epithelium of the skin Causes the epithelium at this point to protrude outward, thus creating a dome and constituting an extremely sensitive receptor. Entire group of Merkel’s discs is innervated by a single large myelinated nerve fiber (type Aβ) Along with the Meissner’s corpuscles, they play extremely important roles in localizing touch sensations to specific surface areas of the body and in determining the texture of what is felt.

Hair End Organ : 

Hair End Organ Each hair and its basal nerve fiber together are called the hair end-organ as slight movement of any hair on the body stimulates a nerve fiber entwining its base. Adapts readily and, like Meissner’s corpuscles, detects mainly Movement of objects on the surface of the body and Initial contact with the body

Ruffini’s End Organ : 

Ruffini’s End Organ Multibranched, encapsulated endings located in the deeper layers of the skin and also in still deeper internal tissues Adapt very slowly and, therefore, are important for signaling continuous states of deformation of the tissues, such as heavy prolonged touch and pressure signals They are also found in joint capsules and help to signal the degree of joint rotation

Pacinian Corpuscles : 

Pacinian Corpuscles Lie both immediately beneath the skin and deep in the fascial tissues of the body Stimulated only by rapid local compression of the tissues Adapt in a few hundredths of a second Important for detecting tissue vibration or other rapid changes in the mechanical state of the tissues

Neural pathways : 

Neural pathways All specialized sensory receptors transmit their signals in type Aβ nerve fibers that have transmission velocities ranging from 30 to 70 m/sec Free nerve ending tactile receptors transmit signals mainly by way of the small type Aδ myelinated fibers that conduct at velocities of only 5 to 30 m/sec

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All sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves From the entry point into the cord, the sensory signals are carried through the dorsal column–medial lemniscal system to the brain

Dorsal Column- Medial Lemniscal Pathways : 

Dorsal Column- Medial Lemniscal Pathways Dorsal column–medial lemniscal system carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord Then, after the signals synapse and cross to the opposite side in the medulla, they continue upward through the brain stem to the thalamus by way of the medial lemniscus The dorsal column–medial lemniscal system is composed of large, myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec

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(Gracile and cuneate nuclei)


PRESSURE SENSATION Pressure is a maintained touch Enables the person to discriminate between different weights Pressure sensation is transmitted by gracile and cuneate tracts via A afferents

Types of Pressure sensations : 

Types of Pressure sensations Deep pressure sensation: Its receptors are pacinian corpuscles It is important for discrimination of various weights, by putting them on a supported hand (on the table) while the eyes are closed. Muscle Tension sensation: Its receptors are Golgi tendon organs. It is important for discrimination of various weights by lifting them from the ground while the eyes are closed. This is determined by the force of muscular discrimination contraction needed for lifting them.

Vibration : 

Vibration It is a rapidly repetitive sensation Perceived by Meissener's corpuscles: respond to vibrations up to 80 cycles /sec Pacinian Corpuscles: respond to vibrations from 80 to 500 cycles/ sec

Pathway : 

Pathway It is transmitted by A fibers then via gracile and cuneate tracts It is tested by putting the base of a vibrating tuning fork on a bony prominence, while the patient is asked if he feels vibrations or not

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Bone only magnifies vibration, but it does not contain receptors for vibrations Depression of this sense occurs early in diseases affecting the posterior column of the spinal cord (gracile & cuneate tracts) as uncontrolled diabetes & pernicious anemia.

Pain : 

Pain Pain occurs whenever any tissues are being damaged, and it causes the individual to react to remove the pain stimulus Pain has been classified into two major types: fast pain or acute pain and slow pain or chronic pain Fast pain is felt within about 0.1 second after a pain stimulus is applied and transmitted by small Aδ fibers This type of pain is felt when a needle is stuck into the skin, when the skin is cut with a knife, or when the skin is acutely burned

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Slow pain begins only after 1 second or more and then increases slowly over many seconds and sometimes even minutes It is transmitted by C-fibers This type of pain is usually associated with tissue destruction. It can lead to prolonged, unbearable suffering

Nociceptors : 

Nociceptors Pain receptors or nociceptors are widely distributed in the body Pain receptors adapt very little and sometimes not at all Under some conditions, excitation of pain fibers becomes progressively greater, especially so for slow-aching-nauseous pain, as the pain stimulus continues Importance of this failure of pain receptors to adapt is that it allows the pain to keep the person apprised of a tissue-damaging stimulus as long as it persists.

Types of nociceptors : 

Types of nociceptors Mechanical nociceptors respond to strong pressure (eg, from a sharp object) Thermal nociceptors are activated by skin temperatures above 45 °C or by severe cold Chemically sensitive nociceptors respond to various agents like bradykinin, histamine, high acidity, and environmental irritants Polymodal nociceptors respond to combinations of these stimuli

Stimulus for pain : 

Stimulus for pain Pain can be elicited by multiple types of stimuli. They are classified as mechanical thermal and chemical pain stimuli (bradykinin, serotonin, histamine)

Mechanism of Pain sensation : 

Mechanism of Pain sensation

Neural Pathway : 

Neural Pathway Even though all pain receptors are free nerve endings, these endings use two separate paths for transmitting pain signals into the central nervous system, via Anterolateral pathway The two pathways mainly correspond to the two types of pain— a fast-sharp pain pathway and, a slow-chronic pain pathway

Transmission of Pain Signals into Spinal Cord : 

Transmission of Pain Signals into Spinal Cord

Transmission of Pain Signals into Brain : 

Transmission of Pain Signals into Brain

Referred Pain : 

Referred Pain It is that type of pain which is felt away from the area of its origin. It usually associated with visceral and deep somatic pain. It is referred according to the dermatomal rule i.e. it is referred to the dermatome (area of the skin) supplied by the same dorsal roots through which impulses from the diseased viscera reach the CNS

Dermatomes : 


Mechanism of Referred pain : 

Mechanism of Referred pain

Analgesia System : 

Analgesia System Capability of the brain itself to suppress input of pain signals to the nervous system by activating a pain control system, called an analgesia system The analgesia system consists of three major components: The periaqueductal gray and periventricular areas the raphe magnus nucleus a pain inhibitory complex

Opiate system of the brain : 

Opiate system of the brain Morphine is a powerful pain-killer (analgesic) found in opium. Acts by combining with CNS receptors called opioid receptors These receptors are concerned with modulation of pain as well as other functions A search was made for morphine-like substances in the brain itself, and the major compounds with morphine like properties (i.e.opioids) have been isolated from the brain, the enkephalins (most extensively distributed) , the B-endorphins (most potent) and dynorphins (similar to encephalin but less extensively distributed)

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Fine details of the brain’s opiate system are not understood, Activation of the analgesia system by nervous signals entering the periaqueductal gray and periventricular areas, or inactivation of pain pathways by morphine-like drugs, can almost totally suppress many pain signals entering through the peripheral nerves

Hyperalgesia : 

Hyperalgesia A pain nervous pathway sometimes becomes excessively excitable; this gives rise to hyperalgesia, which means hypersensitivity to pain Possible causes of hyperalgesia are excessive sensitivity of the pain receptors themselves, which is called primary hyperalgesia, and facilitation of sensory transmission, which is called secondary hyperalgesia

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An example of primary hyperalgesia is the extreme sensitivity of sunburned skin, which results from sensitization of the skin pain endings by local tissue products from the burn— histamine, prostaglandins etc. Secondary hyperalgesia frequently results from lesions in the spinal cord or the thalamus

Itching : 

Itching The purpose of the itch sensation is presumably to call attention to mild surface stimuli such as a flea crawling on the skin or a fly about to bite, and the elicited signals then activate the scratch reflex or other maneuvers that rid the host of the irritant Itch can be relieved by scratching if this removes the irritant or if the scratch is strong enough to elicit pain The pain signals are believed to suppress the itch signals in the cord by lateral inhibition

Thermal sensation : 

Thermal sensation The human being can perceive different gradations of cold and heat, from freezing cold to cold to cool to indifferent to warm to hot to burning hot. Thermal gradations are discriminated by at least three types of sensory receptors: cold receptors warmth receptors, and pain receptors

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The pain receptors are stimulated only by extreme degrees of heat or cold and, therefore, are responsible, along with the cold and warmth receptors, for “freezing cold” and “burning hot” sensations The cold and warmth receptors are located immediately under the skin at discrete separated spots

Stimulation of Thermal Receptors : 

Stimulation of Thermal Receptors

Stimulatory Effects of Rising and Falling Temperature : 

Stimulatory Effects of Rising and Falling Temperature Thermal senses respond markedly to changes in temperature, in addition to being able to respond to steady states of temperature. This means that when the temperature of the skin is actively falling, a person feels much colder than when the temperature remains cold at the same level. Conversely, if the temperature is actively rising, the person feels much warmer than he or she would at the same temperature if it were constant. The response to changes in temperature explains the extreme degree of heat one feels on first entering a tub of hot water and the extreme degree of cold felt on going from a heated room to the out-of-doors on a cold day.

Mechanism of Stimulation ofThermal Receptors : 

Mechanism of Stimulation ofThermal Receptors Cold and warmth receptors are stimulated by changes in their metabolic rates Temperature alters the rate of intracellular chemical reactions more than twofold for each 10°C change Thermal detection probably results not from direct physical effects of heat or cold on the nerve endings but from chemical stimulation of the endings as modified by temperature

Transmission of Thermal Signals : 

Transmission of Thermal Signals Thermal signals are transmitted in pathways parallel to those for pain signals On entering the spinal cord, the signals travel for a few segments upward or downward in the tract of Lissauer and then terminate mainly in laminae I, II, and III of the dorsal horns—the same as for pain. After a small amount of processing by one or more cord neurons, the signals enter long, ascending thermal fibers that cross to the opposite anterolateral sensory tract and terminate inboth (1) the reticular areas of the brain stem and (2)the ventrobasal complex of the thalamus A few thermal signals are also relayed to the cerebral somatic sensory cortex from the ventrobasal complex

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