Modalities of Sensations PDF

Presented by Dr: Karima El-Sayed Ismail Assistant professor of medical physiology S.C.U 2024-2025  DEFINITION AND TYPES OF SENSATIONS  Somatosensory system is defined as the sensory system associated with different parts of the body.  Sensations are of two ty...

Presented by Dr: Karima El-Sayed Ismail Assistant professor of medical physiology S.C.U 2024-2025  DEFINITION AND TYPES OF SENSATIONS  Somatosensory system is defined as the sensory system associated with different parts of the body.  Sensations are of two types: 1. Somatic sensations 2. Special sensations  Somatic sensations  Somatic sensations are the sensations arising from skin, muscles, tendons and joints. These sensations have specific receptors, which respond to a particular type of stimulus.  TYPES OF SOMATIC SENSATIONS Generally, somatic sensations are classified into three types: 1. Epicritic sensations 2. Protopathic sensations 3. Deep sensations Epicritic Sensations (Highly Myelinated, Large , Rapid, Fine)  Epicritic sensations are the mild or light sensations.  Such sensations are perceived more accurately. Epicritic sensations are: 1. Fine touch 2. Tactile localization 3. Tactile discrimination 4. Temperature sensation with finer range between 25°C and 40°C. Fine touch Tactile localization Tactile discrimination  Protopathic Sensations (Low Myelin, Small > Slow, Coarse and Crude)  Protopathic sensations are the crude sensations. These sensations are primitive type of sensations. Protopathic sensations are: 1. Pressure sensation 2. Pain sensation 3. Temperature sensation with a wider range, i.e. above 40°C and below 25°C.  Deep Sensations  Deep sensations are sensations arising from deeper structures beneath the skin and visceral organs. Deep sensations are: i. Sensation of vibration or pallesthesia, which is the combination of touch and pressure sensation ii. Kinesthetic sensation or kinesthesia: Sensation of position and movements of different parts of the body. This sensation arises from the proprioceptors present in muscles, tendons, joints and ligaments. Proprioceptors are the receptors, which give response during various movements of a joint.  Sensory unit refers to a single sensory axon and all of its peripheral branches.  Receptive field of a sensory unit is the distribution from which a stimulus produces a response in that unit  Tactile localization: refers to the ability of the brain to identify the location of the stimulus accurately.  Tactile discrimination: refers to the ability of the brain to identify the number of the stimuli accurately.  Both , depends on the size of the receptive field of the sensory unit.  The smaller the receptive field of the sensory unit, the greater its Tactile localization and discrimination.  Thermoreceptors  Temperature receptors, or thermoreceptors, are free nerve endings located in: 1. the dermis 2. skeletal muscles 3. the liver 4. the hypothalamus  Thermoreceptors -- Ending of unmyelinated C-fibers -- Spontaneous firing at low-frequency (skin temperature of 34oC) Two Types: 1. Cold receptors – fire when Temp. decreases from 34oC, maximal firing at 25oC 2. Warmth receptors – fire when Temp. increases from 34oC, maximal firing at 45oC Hot sensation – noxious stimulus detected by nociceptors, not thermal receptors Cold receptors are three or four times more numerous than warm receptors.  Pain  Primarily a protective mechanism meant to bring a conscious awareness that tissue damage is occurring or is about to occur  Storage of painful experiences in memory helps us avoid potentially harmful events in future  Sensation of pain is accompanied by motivated behavioral responses and emotional reactions  Subjective perception can be influenced by other past or present experiences  Pain receptors or nociceptors are always free nerve endings on skin & internal organs  Stimulations  Mechanical  Thermal (above 450C)  Chemical (Bradykinin, 5-HT, histamine, K+ ions, proteolytic enzymes, Ach, acids, prostaglandins etc)  Types of pain  Fast (acute) pain – caused by mechanical & thermal stimulus  Slow (chronic) pain – caused mainly by chemical stimulus, but mechanical & thermal stimulus may also cause slow pain  Mechanical sensations  Mechanoreceptors are sensitive to stimuli that distort their cell membranes, which contain mechanically regulated ion channels whose gates open or close in response to: 1. stretching 2. compression 3. twisting  There are three classes of mechanoreceptors: 1. Tactile receptors 2. Baroreceptors 3. Proprioceptors 1- Tactile receptors provide the sensations of touch, pressure, and vibration. 2- Baroreceptors detect pressure changes in the walls of blood vessels and in portions of the digestive, reproductive, and urinary tracts. 3- Proprioceptors monitor the positions of joints and muscles. They are the most structurally and functionally complex of the general sensory receptors.  Fine touch and pressure receptors are extremely sensitive and have a relatively narrow receptive field. They provide detailed information about a source of stimulation, including: 1. its exact location 2. shape 3. size 4. texture  Crude touch and pressure receptors have relatively large receptive fields, provide poor localization, and give little information about the stimulus  Touch and pressure are sensed by four types of mechanoreceptors : 1- Meissner’s corpuscles are dendrites encapsulated in connective tissue and respond to changes in texture and slow vibrations. 2- Merkel cells are expanded dendritic endings, and they respond to sustained pressure and touch. 3- Ruffini corpuscles are enlarged dendritic endings with elongated capsules, and they respond to sustained pressure. 4- Pacinian corpuscles consist of unmyelinated dendritic endings of a sensory nerve fiber, 2 μm in diameter, encapsulated by concentric lamellae  GENERATOR POTENTIALS  When a small amount of pressure is applied to the Pacinian corpuscle, a non-propagated depolarizing potential is recorded. This is called the generator potential or receptor potential  As the pressure is increased, the magnitude of the receptor potential is increased.  The receptor therefore converts mechanical energy into an electrical response, the magnitude of which is proportional to the intensity of the stimulus.  Thus, the responses are described as graded potentials rather than all-or-none as is the case for an action potential  SOMATOSENSORY PATHWAYS  Nervous pathways of sensations are called the sensory pathways. These pathways carry the impulses from receptors in different parts of the body to centers in brain. Sensory pathways are of two types: 1. Pathways of somatosensory system, convey the information from sensory receptors in skin, skeletal muscles and joints. Pathways of this system are constituted by somatic nerve fibers called somatic afferent nerve fibers. 2. Pathways of viscerosensory system, convey the information from receptors of the viscera. Pathways of this system are constituted by visceral or autonomic fibers.  The ascending pathways from sensory receptors to the cortex are different for the various sensations (2 pathways) 1. Ascending sensory pathway that mediates touch, vibratory sense, and proprioception ( dorsal column medial lemniscal pathway ) 2. Ascending sensory pathway that mediates pain and temperature (ventrolateral spinothalamic pathway ).  The posterior column pathway carries sensations of highly localized (“fine”) touch, pressure, vibration, and proprioception.  This pathway, also known as the dorsal column/medial lemniscus, begins at a peripheral receptor and ends at the primary sensory cortex of the cerebral hemispheres.  The axons of the first-order neurons reach the CNS within the dorsal roots of spinal nerves and the sensory roots of cranial nerves.  The axons ascending within the posterior column are organized according to the region innervated. a. Axons carrying sensations from the inferior half of the body ascend within the fasciculus gracilis and synapse in the nucleus gracilis of the medulla oblongata. b. Axons carrying sensations from the superior half of the trunk, upper limbs, and neck ascend in the fasciculus cuneatus and synapse in the nucleus cuneatus.  Axons of the second-order neurons of the nucleus gracilis and nucleus cuneatus ascend to the thalamus.  As they ascend, these axons cross over to the opposite side of the brain stem. This crossing of an axon is called decussation.  Once on the opposite side of the brain, the axons enter a tract called the medial lemniscus.  As it ascends, the medial lemniscus runs alongside a smaller tract that carries sensory information from the face, relayed from the sensory nuclei of the trigeminal nerve.  The axons in these tracts synapse on third-order neurons in one of the ventral posterior lateral nuclei of the thalamus. These nuclei sort the arriving information according to: 1. the nature of the stimulus 2. the region of the body involved  Processing in the thalamus determines whether you perceive a given sensation as fine touch, as pressure, or vibration.  then from contralateral thalamic ventral posterior lateral (VPL) to then to the primary somatosensory cortex.  Dorsal column pathway : Sensory fibers ascend ipsilaterally via the spinal dorsal columns to medullary gracilus and cuneate nuclei; then cross the midline and ascend in the medial lemniscus to the contralateral thalamic ventral posterior lateral (VPL) and then to the primary somatosensory cortex. The Dorsal column medial lemniscal system First order neurons – goes through dorsal column – synapse in nucleus of medulla – second order neuron decussate in medulla – goes through medial lemniscus of brain stem – synapse in thalamus – third order neuron projects through internal capsule to somatosensory cortex  The Anterolateral Pathway  The anterolateral pathway provides conscious sensations of poorly localized (“crude”) touch, pressure, pain, and temperature.  In this pathway, the axons of the first-order sensory neurons enter the spinal cord and synapse on second-order neurons within the posterior gray horns.  The axons of these interneurons cross to the opposite side of the spinal cord before ascending. This pathway includes relatively small tracts that deliver sensations to reflex centers in the brain stem as well as larger tracts that carry sensations destined for the cerebral cortex.  The lateral spinothalamic tracts carry pain and temperature sensations.  These tracts end at third-order neurons in the ventral nucleus group of the thalamus.  After the sensations have been sorted and processed, they are relayed to the primary sensory cortex. Most somatic sensory information is relayed to the thalamus for processing. A small fraction of the arriving information is projected to the cerebral cortex and reaches our awareness. The Anterolateral system First order neuron synapse in dorsal horn – second order neuron decussate to lateral or anterior columns – synapse mainly in thalamus or synapse in reticular formation & then goes to thalamus – from thalamus goes to somatosensory cortex through internal capsule Somatosensory area I  Somatosensory area I & II Somatosensory area I is more prominent than somatosensory area II Brodmann’s areas 3, 1 & 2 constitute somatosensory area I, Brodmann’s areas 40 constitute somatosensory area II and areas 5 & 7 constitute somatosensory association area Representation of body in somatosensory area  Somatic senses are represented contralaterally, except for few fibers from face.  Larger areas for lips, thumb, & face, smaller areas for other body parts like trunk & lower parts  41490_big  Sensory homunculus: This model shows what a man's body would look like if each part grew in proportion to the area of the cortex of the brain concerned with its sensory perception. When damage of somatosensory area occurs  Discrete localization of different sensation is lost. But crude localization is possible with intact brain stem & thalamus.  Loss of judgment for critical degrees of pressure  Stereognosis (ability to judge shapes & forms of objects) is lost  Unable to judge the texture of materials