Physiology of CNS Sensory System By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology
Coding of Sensory Informations
Type of sensation Site of sensation Intensity of sensation Stimuli 4
Coding of sensory information Def • It is the ability of the CNS to discriminate (recognize) or identify the; 1. Type of stimulus (modality discrimination) 2. Site of stimulus (locality discrimination) 3. Strength of stimulus (intensity discrimination) • Although, all stimuli reach the brain in the same manner i. e. action potential 5
Modality Discrimination • Mechanism: explained by 2 mechanisms; 1. At the level of receptor: explained by Muller law of specific nerve energy 2. At the central level: explained by Labelled line principle • Each sensation has specific receptor, pathway and center in the cerebral cortex • Stimulation of receptor, pathway, and center gives specific sensation whatever may be the method of stimulation 6
Cold sensation At central level Cold stimulus At receptor level 7
Pain sensation Painful stimulus 8
Locality Discrimination Def. • Is the ability of brain to accurately determine the site of stimulus Mechanism: law of projection • depends upon the topographic area of representation • Each part in the body surface is represented by a certain area in the sensory cortex • stimulation of certain areas in the cortex leads to a sensation felt in the site or in the body surface 9
Intensity Discrimination Mechanism: • Depends upon the frequency of discharge from the receptors along the afferent nerves • ↑ stimulus strength →↑ frequency of AP due to; a) ↑ Frequency of AP from the receptor (Weber Fechner law) b) Number of receptors stimulated (recruitment of receptors) 12
Recruitment of Receptors
Recruitment of Receptors
Sensations 1) Def. , • Conscious awareness of a particular feeling caused by stimulation of certain type of receptor by its adequate stimulus 2) Classifications: Sensations General Special Emotional
Sensations 1) Emotional sensations: • E. g. anxiety, fear, sadness 2) Special sensations: • E. g. anxiety, fear, sadness 3) General sensations: • Arise from receptors distributed allover the body • Are classified into; a) Somatic sensations: from somatic structures e. g. skin b) Visceral sensations: from viscera c) Organic senses: e. g. thirst, hunger and sexual desire
Somatic Sensations Def, • Sensations arise from somatic structures of the body i. e. skin and deep tissues e. g. sk ms Types: 1. Mechanoceptive sensations: include a. Tactile: e. g. touch, pressure and vibration b. Proprioceptive e. g. sense of position and movement 2. Pain (nociceptive) sensation. 3. Thermal sensation; cold and warm.
Touch Sensation Def. , • Feeling produced by application of light mechanical pressure to the skin Types: • They include 2 types : 1. Crude touch 2. Fine touch
Crude Touch Def. , • Poorly localized touch sensation produced by touching the skin with diffuse ill defined object e. g. a piece of cotton or the touch of clothes.
Crude Touch Receptors: 1. Free nerve endings 2. Hair end organs Afferents: a. A- nerve fibers (5 -30 m. /s) b. C-non myelinated nerve fibers (0. 5 -2 m/s)
Crude Touch Pathway: Ventral Spinothalamic tract (part of anterolateral • a. b. c. system) Characterized by Slow rate of transmission Poor spatial arrangement of its fibers Inaccurate information about site and size of crude touch
Crude Touch PVNT Sensory Radiations Spinal Leminiscus Lamina IV, III A delta and C Receptors Free nerve endings Hair end organs Ventral spinothalamic tract
Fine Touch Def. , • Highly localized touch sensation produced by application of a well localized object to the skin e. g. a tip of a pencil or a head of a pin or teeth of a comb.
Fine Touch Receptors: 1) Meissner's corpuscles (rapidly adapting). 2) Merkel's discs (slowly adapting) • These receptors are numerous in the distal than the proximal parts of the body (more in tips of fingers – tip of the nose – lobule of ear – and nipples of the breast).
Fine Touch Afferents: • A-beta rapidly conducting nerve fibers (30 -70 meters/second Pathway : • Dorsal column – medial lemniscal system (Gracile & Cuneate tracts) • Characterized by a. Fast rate of transmission b. Good spatial arrangement of its fibers c. Accurate information about site and size of fine touch
Fine Touch PVNT Sensory Radiations Gracile and Cuneate Nuclei A beta Receptors Medial Leminiscus Gracile and Cuneate tracts
Fine Touch Types: • It includes : 1. Tactile localization: • It is the ability to localize exactly the point of touch with eyes closed 2. Tactile (2 point) discrimination : • It is the ability to feel 2 points of touch with eyes closed as 2 separate points of touch provided that the distance between the 2 points is greater than minimal or threshold stimulus • It is tested by the compass
Fine Touch Types: Minimal distance: • It is the distance between 2 points of fine touch below it the 2 points are felt as a single point whereas at or above it the 2 points are felt as separate points. • It is very short at tip of tongue (1 mm), finger tips (3 mm) , lips (2 mm) and nipples of the breast. • It is long in the back of trunk (70 mm)
Fine Touch Types: • Tactile localization and discrimination are more accurate on the extremities than on the proximal parts due to: 1. Great number of receptors 2. Great number of afferents 3. Little convergence of afferents centrally 4. Wide area of cortical representation →good analysis and interpretation.
Fine Touch Types: 3. Stereognosis: • It is the ability to recognize a familiar object with eyes closed e. g. recognition of a key, a pen or a coin. 4. Texture of materials: • Ability of the person with his eyes closed to recognize the nature of an object e. g. glasses and wood.
Fine Touch N. B. Gracile and cuneate tracts carry the following sensations: 1. Fine touch 2. Vibration 3. Pressure 4. Proprioceptive sensations