Sensory System Physiology III PDF
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Horus University
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This document details the sensory system. It explores various types of receptors, their characteristics, and classifications. The document also describes the mechanisms of transduction and transmission of sensory information, alongside a discussion of how the brain differentiates different sensory modalities, intensity, and location of stimuli.
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Chapter 1 Sensations Unit I: Sensory System Nervous System The major functions of the nervous system are to detect, analyze, and transmit information. Information is gathered by sensory systems, integrated by th...
Chapter 1 Sensations Unit I: Sensory System Nervous System The major functions of the nervous system are to detect, analyze, and transmit information. Information is gathered by sensory systems, integrated by the brain, and used to generate signals to motor and autonomic pathways for control of movement and of visceral and endocrine functions. I) Anatomical organization of the nervous system: 1-Central nervous system,which includes: a) Brain. b) Spinal cord. 2-Peripheral nervous system which includes: - 12 pairs of cranial nerves and 31 pairs of spinal nerves. Figure 1:human brain 1 Chapter 1 Sensations II- Functional organization of the nervous system: 1-Sensory nervous system: -Detects changes in internal and external environments and informs the CNS about them. 2-Motor nervous system: -Initiates and controls the motor activities of skeletal muscles, plain muscles, cardiac muscles and even glands. 3-Intellectual nervous system: - Provides basis for consciousness, memory, learning, thoughts, emotions and behavior. Sensory System -It is the division of the nervous system concerned with the detection of the stimuli (as regard nature, locality and intensity) and informs the specialized centers in CNS to adopt adequate responses. Components of the sensory nervous system: a) Sensory receptors. Then, b) Afferent nerve fibers, that carry impulses to, c) Specific areas in the brain & spinal cord (sensory centers). CNS Afferent nerve fibers Center Receptor Figure 2:sensory system 2 Chapter 1 Sensations Sensory Receptors Def: -Receptors are specialized microscopic structures located at the peripheral terminations of the afferent nerves. Functions of the receptors: -They detect the stimuli and convert these stimuli into nerve impulses which are then conducted along the afferent nerves to the sensory centers in central nervous system i.e. the receptors act as "detectors and transducers". Figure 3: tactile receptors Physiological classification of receptors: 1) According to the location of the receptor: -They are classified into: (i) External receptors:on the body surface mainly in skin. (ii) Internal receptors: in deeper structures & viscera. 3 Chapter 1 Sensations 2) According to nature of the stimulus: -They are classified into 5 major classes: 1) Mechanoreceptors: - They are stimulated by the mechanical forms of energy. they include: a- Touch receptors in the skin. b-Baroreceptors "in the aortic arch & carotid sinus" that detect the changes in arterial blood pressure. c-Proprioceptors "in the muscles " e.g. muscle spindle & golgi tendon organs. d-Pressure receptors e.g. pacinian corpuscles. e- Sound receptors. f- Vestibular receptors that detect the changes in the equilibrium. g-Stretch receptors as in alveoli, urinary bladder & right atrium. 2) Chemoreceptors: they respond to the chemical stimuli e.g. a- Peripheral chemoreceptors in the aortic and carotid bodies. b-Central chemoreceptors in the brain stem. c- Taste & smell receptors. d-Hypothalamic glucoreceptors & osmoreceptors. 3) Thermal receptors: - they respond to changes in temperature. - They include cold receptors which respond to the drop of temperature & warm receptors which respond to the increase in temperature. 4) Pain receptors (nociceptors): - they respond to the injurious or painful stimuli. 5) Photorceptors or electromagneticreceptors: they respond to the electromagnetic waves of light in the retina. 4 Chapter 1 Sensations Properties of Receptors 1. Specificity (Adequate stimulus). 2. Excitability (Receptor potential). 3. Rate of discharge from the receptors (detection of the stimulus intensity). 4. Adaptation of the receptors. 1) Specificity The receptors show a high degree of specificity as Muller's law applies: Muller's law: “Each type of receptors is highly sensitive to one type of stimuli called the adequate stimulus and its stimulation gives rise to one type of sensation whatever the way of stimulation”. Example: Retinal receptors are highly sensitive to the light waves. Auditory receptors are highly sensitive to the sound waves…etc. However, The receptors may respond to stimuli other than their specific or adequate ones, provided that these stimuli are very strong; but still the response is the same modality to which the receptor is specialized e.g retinal receptors are normally stimulated by the light waves and give rise to sense of vision. But if heavy mechanical stimuli applied to the eye as in heavy blow, the retinal receptors can be stimulated and eye sees flashes of light. 5 Chapter 1 Sensations 2) Excitability “Receptor Potential” Def: Excitability of receptors is the ability of the receptors to respond to their adequate stimuli. Mechanism of stimulation of the receptors and generation of action potential in the nerve fibers: -Most of the body receptors on adequate stimulation show depolarization through the increased permeability of their membranes to Na+. -Only the visual receptors (Rods & Cones) when stimulated adequately, they show hyperpolarization. Receptor Potential:is the potential changes that occur in the receptors on adequate stimulation, usually in the form of depolarizations. Mechanism: -It is best studied in pacinian corpuscles (mechano-receptors present in the skin, deep tissues involved in the sensations of touch, pressure and vibrations). Pacinian corpuscles preferred because they are: i) Easily stimulated by microglass rods under microscope. ii) Large in size and easily dissected. -Pacinian corpuscle is a vesico-elastic structure consists of several concentric layers of connective tissues like an onion surrounding a central nerve terminal. The ending of the sensory nerve is not myelinated, but the first node of Ranvier is also located inside the capsule (figure 4). Concentric lamellae First node of Ranvier Nerve terminal Figure 4:Pacinian corpuscle 6 Chapter 1 Sensations -When the pressure is applied to the corpuscle it compresses the concentric lamellae deformation of the nerve terminal opening of Na+ channels increasing Na+ influx partial state of depolarization in the nerve terminal known as "Receptor Potential". -The maximal amplitude of receptor potential around 100 mv that occurs when maximal opening of Na+ channels in the receptor membrane. -When the receptor potential reaches certain threshold it generates local electrotonic current which flows to the first node of Ranvier gradual depolarization till the firing level is reached & action potential is generated (figure 5). Pressure stimulus Receptor potential Local electric circuit Figure 5 -Action potential is then propagated along the afferent nerve to CNS. -The amplitude of electrotonic current is determined by the amplitude of the receptor potential which by its turn depends upon the intensity of the stimulus (figure 6). Pressure stimulus Receptor potential Figure 6 7 Chapter 1 Sensations -As long as the amplitude of the electrotonic current is strong, the 1 st node of Ranvier can be depolarized in short time & this increases the rate of discharge. N.B.: The nerve terminal is termed the Transducer region and the 1st node of Ranvier is called the Spike Generator region. Properties of the receptor potential: 1. Graded amplitude:i.e. its amplitude is directly related to the intensity of the stimulus i.e. does not obey the all or none rule. 2. Can be summated:multiple receptor potentials can be summated with each other. 3. Passive localized spread. 4. Its has a short duration.. 5. It can initiate repeated action potentials at the 1st node of Ranvier. 6. It is not blocked by local anaesthetics. 3) Rate of Discharge from Receptors -The relation between the intensity of the stimulus and the rate of discharge from the receptor is explained by, Weber- Fechner 's law: Weber- Fechner 's Law: The frequency of impulses discharged from the receptors through afferent nerves is directly proportionate with the logarithm intensity of the stimulus. Logarithmic relation means: -to increase the rate of discharge twice, you must increase the intensity of the stimulus 100 times, for example 100 times increase in intensity of the stimulus is expressed by increasing the rate of discharge twice only. Thus, the receptors compress wide range of stimuli into narrow range of discharge. -This is called “compression function of receptors”. 8 Chapter 1 Sensations 4) Adaptation of Sensory Receptors Def: -Means decline "decrease" in the rate of discharge from the receptors inspite of continous constant stimulation. -According to the speed of decline "Rate of adaptation" in the rate of discharge from receptors we can classify the receptors into 2 main categories (figure 7): A) Rapidly adapting receptors (phasic receptors). B) Slowly adapting receptors (tonic receptors). Slowly adapting receptors Rate receptor of potential discharge Rapidly adapting receptors receptor potential 2 3 4 1 Figure: 7 Time in seconds receptor 1- Rapidly adapting receptors 2-potential Slowly adapting R. = phasic receptors = tonic receptors - they first discharge at high rate, - they continue to discharge at a lower then the rate decreases rapidly to rate as long as the stimulus is applied. zero after 1 or 2 seconds in spite of continous stimulation. Examples: -mostly tactile: - ms. Spindle and Golgi tendon organ i- Pacinian corpuscle - pain receptors ii -Meissner 's corpuscle - cold receptors iii -hair follicle receptors iv - some joint receptors 9 Chapter 1 Sensations Pattern of discharge: - they discharge only at: -they discharge continuously to CNS i- onset (start) of stimulation as long as the stimulus is applied. ii- termination of stimulation iii- change in rate of stimulation so they are called phasic receptors So they are called tonic receptors Physiological significance: CNS need continuous impulses from these CNS ese need continuous impulses from these ese receptors CNS only needs to receive receptors because they serve vital impulse at the start, termination & functions: change in rate of stimulation & doesn't for example: pain & cold receptors need continuous impulses as continuity inform CNS about harmful stimuli & of discharge from these receptors isn’t and initiate protective reflexes. necessary. 10 Chapter 1 Sensations Coding of sensory information Def: - It is the ability of higher centers of the brain to identify: a) Type of the stimulus (modality discrimination) b) Intensity of the stimulus (intensity discrimination) c) Locality of stimulus (locality discrimination) although action potential is the same in all nerves. 1- Modality discrimination - It's the ability of the CNS to discriminate the type of a sensation (e.g touch, pain,sound, …) it depends on: 1- Peripheral mechanism: - Occurs at the level of receptors and depends on muller's law. - According to muller's law:each type of receptor gives one type of sensation when stimulated by its adequate stimulus. e.g stimulation of retinal receptors gives rise to light sensation. - So that specificity of receptors represents the first step in the coding of different modalties (types) of stimuli. 2- Central mechanism: -Depends on“ labelled line principle” which is an extension to Muller 's law. Labelled line principle states that: ''There's a specific anatomical connections (pathways) for each sensation which begins at a specific receptor then the a specific tract to terminate at a specific center (area) in the cerebral cortex''. - Stimulation of this pathway at any point gives rise to a specific sensation e.g stimulation of the visual pathway at any point gives rise to visual sensation. -This means that there are specific pathways for transmission of different modalities and that each pathway carries only one sensory modality. 11 Chapter 1 Sensations 2- Intensity Discrimination - It's the ability of the CNS to discriminate the intensity of a sensation, it depends on: 1- Peripheral mechanism: which depends on: a- Rate of impulse discharge from each receptor: the higher the rate of discharge, the stronger is the stimulus. b- The number of stimulated receptors (recruitement of receptors): the more the number of stimulated receptors, the stronger is the stimulus. 2- Central mechanism: - Depends on the number of afferents reaching CNS: the more the number of afferents reaching CNS the stronger is the stimulus. 3- Locality Discrimination - It's the ability of CNS to discriminate the site of a sensation,it depends on: Somatotopic map: -According to which each area in the body e.g hand or leg is represented in a specific area in the cerebral cortex with accurate anatomical pathways from the receptor to the center in the cerebral cortex. - So when the impulse reaches specific area in the cerebral cortex , the sensation isn't felt in the cortical neuron but it is referred (projected) to its original site in the body this principle is called law of projection. Phantom Limb: - in amputee, a severe pain is felt at the site of amputated (non existing) limb this is called phantom limb phenomenon. - Phantom sensations may also occur after removal of body parts other than limbs for example; Phantom breast, teeth pain and phantom eye syndrome. 12 Chapter 1 Sensations Pain Sensation Def: -Unpleasant sensory and emotional experience associated with actual tissue damage. -Pain receptors are stimulated whenever the tissues are damaged. -In spite of being unpleasant, it is a protective sense, where it directs the person to get rid of the injurious agents. Pain receptors: 1. They are free nerve endings. 2. They are highly specific i.e. respond only to tissue damage. The specificity may be to the nature of the injurious agents, so the pain receptors subtypes include: (i) Mechanical pain receptors respond only to the mechanical trauma e.g. heavy pressure. (ii) Chemical pain receptors respond to tissue damage produced by chemical agents e.g. concentrated H2SO4 or HCl in the esophagus. (iii) Thermal pain receptors respond to tissue damage produced by thermal stimuli (temperatures >45oC &