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Sensory System Sensory system By: Dr.Fareeda Banu A.B Associate professor Dept of Physiology USM-KLE IMP OBJECTIVES Regarding the components of sensory system and their function Define and classify sensory receptors and state their properties Explain the generation of receptor potential and its role in the stimulation of the afferent nerve Describe the ascending sensory pathways that sub serve perceived sensations and state the functions of collaterals from sensory pathway Introduction The nervous system is unique in the vast complexity of thought processes and control actions it can perform. It receives each minute literally millions of bits of information from the different sensory nerves and sensory organs and then integrates all these to determine responses to be made by the body. The central nervous system contains more than 100 billion neurons. Which are the basic functional unit of CNS. 1) Definition: Conscious awareness of a particular feeling caused by stimulation of certain type of receptor by its adequate stimulus 2) Classifications: Sensations General/ Special Emotional Somesthetic Introduction 1. Special senses: Include 5 important sensations- Vision, smell, taste, hearing and equilibrium. 2. Somaesthetic sense: Depending on point of origin are classified into three types: a. Cutaneous sensation: arising from surface of body; like touch, pressure, pain and temperature. b. Visceral sensations: arises from the deep viscera, i.e. from internal organs like liver , pancreas etc. c. Proprioceptive and Kinesthetic sensations: arising from muscles, tendons and joints. 3. Emotional sense: E.g. anxiety, fear, sadness Classification of Sensations by Structural Complexity Somatic (= general) Special senses senses 1. Vision 1. Touch 2. Hearing 2. Temperature 3. Taste 3. Nociception (pain) 4. Smell 4. Itch 5. Equilibrium 5. Proprioception 6. Stereognosis • Defn… Sensations arise from somatic structures of the body i.e. skin and deep tissues e.g. skin and muscles 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. Somato sensory axis of the Nervous system Sensory system transmits sensory information from the receptors of the entire body surface and from some deep structures. This information enters the central nervous system through peripheral nerves and is conducted immediately to multiple sensory areas in The spinal cord at all levels The reticular substance of the medulla, Pons and mesencephalon of the brain; The cerebellum The thalamus and Sensory Areas of the cerebral cortex. Somato sensory axis Components of Sensory System The sensory division of the human nervous system includes following components: 1. Sensory receptors: These are specialized neurons that transduce stimulus energy into neural signals. 2. Afferent neurons: these carry sensory impulses to the sensory cortex and constitute the neural pathway, which consists of: • First-order neurons • Second-order neurons and • Third-order neurons 3. Sensory cortex: Includes the sensory areas of cortex; formed by the 4th order neurons. It is responsible for the conscious perception of the stimulus i.e. Sensation. Sensory Pathway Stimulus Sensory receptor (= transducer) Afferent sensory neurons Spinal cord CNS (sensory cortex) Integration, perception Sensory Receptors Defn: These are specialized neuronal cells that receive stimuli from the external or internal environment and transduce these signals into nerve impulses. A stimulus is a change of environment of sufficient intensity to evoke a response in an organism; such as touch, sound, light, pain, cold, and warmth etc. The sensory receptors transduce sensory stimuli into nerve signals (Aps) that are then conveyed to and processed in the central nervous system (through ascending tracts). Types of Sensory Receptors… Receptors can be grouped according to the stimulus source 1. Exteroceptors: These are close to the body surface and are specialized to detect sensory information from the external environment (such as visual, olfactory, gustatory, auditory, and tactile stimuli). These are further classified as: • Telereceptors (G. tele, “distant”), include receptors that respond to distant stimuli (Eg: Smell, vision and hearing), and do not require direct physical contact with the stimulus in order to be stimulated; • Contact receptors, which transmit tactile, pressure, pain, or thermal stimuli, require direct contact of the stimulus with the body. Receptors according to the stimulus source: 2. Proprioceptors: Proprioceptors transmit sensory information from muscles, tendons, and joints about the position of a body part, such as a limb in space and allow fine control of skeletal movements. 2. Interoceptors detect sensory information concerning the status of the body’s internal environment such as stretch, blood pressure, pH, oxygen or carbon dioxide concentration, and osmolarity. Types of Sensory Receptors… Classification according to modality: Receptors are further classified into the following three categories according to the modality to which they respond: 1. Nociceptors (pain receptors), which detect damage occurring in the tissues, whether physical damage or chemical damage. 2. Thermoreceptors, which detect changes in temperature, with some receptors detecting cold and others warmth. 3. Mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor Nociceptors These are rapidly adapting receptors that are sensitive to noxious or painful stimuli. They are located at the free nerve endings of type Aδ or C fibers,. These are further classified into three types. 1. Mechanosensitive nociceptors (of Aδ fibers), which are sensitive to intense mechanical stimulation (such as pinching with pliers) or injury to tissues. 2. Temperature-sensitive (thermosensitive) nociceptors (of Aδ fibers), which are sensitive to intense heat and cold. 3. Polymodal nociceptors (of C fibers), which are sensitive to noxious stimuli that are mechanical, thermal, or chemical in nature. Although most nociceptors are sensitive to one particular type of painful stimulus, some may respond to two or more types. THERMORECEPTORS These are sensitive to warmth or cold and are slowly adapting receptors which are further classified into three types: 1. Cold receptors, which consist of free nerve endings of lightly myelinated Aδ fibers. 2. Warmth receptors, which consist of the free nerve endings of unmyelinated C fibers that respond to increases in temperature. 3. Temperature-sensitive nociceptors that are sensitive to excessive heat or cold. Mechanoreceptor Mechanoreceptors are activated following physical deformation of the skin, muscles, tendons, ligaments, and joint capsules in which they reside. These may be classified as non-encapsulated or encapsulated depending on whether a structural device encloses its peripheral nerve ending component. Meissner’s corpuscles are encapsulated and respond to changes in texture and slow vibrations. Ruffini’s end organs are enlarged dendritic endings with elongated capsules, and they respond to sustained pressure. Pacinian corpuscles are encapsulated by concentric lamellae of connective tissue that give the organ the appearance of a cocktail onion. Theses receptors respond to deep pressure and fast vibration. The sensory nerves from these mechanoreceptors are large myelinated Aα and Aβ fibers whose conduction velocities range from ∼70–120 to ∼40–75 m/s, respectively. Properties of Receptors 1. Specificity of response: 2. Effect of strength of Stimulus 3. Effect of velocity of stimulus 4. Projection. 5. Adaptation Properties of Receptors 1. Specificity of response: Each receptor is easily stimulated by only one type of appropriate adequate specific stimulus. This specificity is called law of adequate stimulus. Eg: Rods and cones – Light 2. Effect of strength of Stimulus: Receptor potential amplitude depends on the strength of stimulus. Greater the strength of the stimulus larger will be the amplitude of receptor potential. The magnitude of sensation felt is directly proportional to the log of intensity of stimulus which is known as Weber Fechner law. Properties of Receptors 3. Effect of velocity of stimulus: The magnitude of the receptor potential rises with rate of change of stimulus application. It also applies to removal of stimulus. Eg: off response 4. Projection: When any part of sensory path of a particular sense organ is stimulated, conscious sensation produced is referred to the location of the receptors, no matter how or where along the pathway the activity is originated. This principle is called law of projection. Eg- Phantom limb Properties of Receptors 5. Adaptation: Another characteristic of all sensory receptors is that they adapt either partially or completely to any constant stimulus after a period of time. That is, 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. This property is called adaptation. Eg: the pacinian corpuscle adapts very rapidly, hair receptors adapt within a second or so, and some joint capsule and muscle spindle receptors adapt slowly. 5. Adaptation: Depending on the rate of adaptation the receptors are of two types: I. Tonic Receptors: These are slow and incompletely adapting
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