CNS 3 Receptors

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Physiology and classification of sensory receptors

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Dr. Amit Upadhyah Sensory Receptors

Sensation:

Sensation A stimulus is a change in environment of sufficient intensity to evoke a response. Sensation: The basic recognition of a stimulus Also K/a Esthesia Absence of sensation:Anesthesia Abnormal sensation:Parasthesia Movement sensation: Kinesthesia Perception: Appreciation and interpretation of sensation

Dimensions of Sensation:

Dimensions of Sensation Modality: Type or quality of sensation Intensity: Degree of perception Affect: Emotional component Acuity: Precision of stimulus localization

Sensation:

Sensation Each of the principle types sensation; touch, pain, sight, sound, is called a modality of sensation . Perception is determined by the characteristics of the receptor and the central connections of the axon connected to the receptor. Specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle .

Receptors:

Receptors Receptors are specialized cells that receive stimuli from the external or internal environment & transduce these signals into neural signals. Function:- Acts as a transducer Converts the various forms of energies into electrical energy (AP)

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Each type of receptor is activated by only one type of environmental energy. Coding of Stimulus Intensity & Duration Specific sensory receptors are associated with specific CNS sensory pathways.

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Receptor ?

What factors generate receptor potentials?:

What factors generate receptor potentials? Mechanical deformation - stretches the membrane and opens ion channels . Application of chemicals - also opens ion channels . Change in temperature - alters membrane permeability. Electromagnetic radiation - changes membrane permeability to ions.

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- APs occur when receptor potential rises above threshold - ↑ stimulus intensity causes ↑ receptor potential, which ↑ AP frequency.

Structural Classification:

• Free nerve endings - bare dendrites e.g. pain, temperature, tickle, itch & light touch • Encapsulated nerve endings - dendrites enclosed in connective tissue capsule e.g. pressure, vibration & deep touch • Separate sensory cells - specialized cells that respond to stimuli e.g. vision, taste, hearing, balance Structural Classification

Classification by Location :

Classification by Location Exteroceptors – near surface of body, receive external stimuli – hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature Interoceptors – monitors internal environment (viscera) – not conscious except for pain or pressure -- Baroreceptors , Osmoreceptors Proprioceptors – muscle, tendon, joint & internal ear – senses body position & movement Telereceptors - Sensory perception at a distance e.g. vision, hearing,smell

Classification by Stimuli Detected :

Classification by Stimuli Detected Mechanoreceptors – detect pressure or stretch – touch, pressure, vibration, hearing, proprioception , equilibrium & blood pressure Thermoreceptors detect temperature Nociceptors detect damage to tissues – pain Photoreceptors detect light Chemoreceptors detect molecules – taste, smell & changes in body fluid chemistry Osmoreceptor detect increased osmolality

Somatosensory nerve endings:

Somatosensory nerve endings

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Location of receptors

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Cutaneous receptors (Tactile) Smooth skin (glaborous) Hairy skin Subcutis Dermis Epidermis Free nerve endings Merkel disks detect steady pressure & are slowly adapting Free nerve endings around hair root can be either rapid or slowly adapting - depends on hair type Meissner’s corpuscles detect flutter & are rapidly adapting Pacinian corpuscles detect vibration & are very rapidly adapting Ruffini corpuscles detect steady pressure at higher threshold & are slowly adapting Free nerve endings in the skin are modality specific and can detect either pain or touch or pressure or temperature The receptor location and its associated structure can alter the stimulus and influence the response

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Somatosensory receptors

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Principles of Transduction Environmental stimulus becomes encoded as a sequence of nerve impulses in an afferent nerve. First stage- generation of a graded receptor potential. The magnitude of the stimulus is related to that of the receptor potential which is related to either a) the sequence or frequency of action potentials generated or b) modulated release of transmitter from the receptor cell generating a sequence of action potentials in a second order neuron.

Receptor potential:

Receptor potential Pacinian corpuscle

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Adequate Stimulus Activated only by specific mode and strength of stimulus Usually sodium, remember direction of the flux is determined by the gradient The graded potential can either be depolarising or hyperpolarising Alternatively - modulated release of transmitter from receptor cell Generation of graded or action potentials in second order neurones Can allow more local integration (eg retina of the eye)

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Inhibitory interneurons give rise to lateral inhibition - refines input Axon projections to third-order sensory neurones Second-order sensory neurons with convergent excitatory inputs Axonal branches give rise to divergent outputs - diffuses input Axons of primary sensory neurons Sensory units with overlapping receptive fields. Field size and receptor density equates to sensory discrimination. Integration of sensory input Stimulus Simple Processing? T ransduction

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A dequate stimulus Sensory Receptor Primary Afferent Neuron Synapse 2nd Order Neuron graded receptor potential threshold Above threshold--AP frequency coded action potentials conducted down primary afferent neurone synaptic integration AP cause NT release & generate graded potentials (EPSPs) in 2nd order neuron reduced frequency of AP conducted down 2nd order neuron Transduction & Coding related to stimulus intensity and duration transduction and generation of graded receptor potential EPSPs

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Sensory Coding for Intensity & Duration A mplitude 40mv D uration 4ms amplitude 65mv duration 7ms small amount transmitter released large amount transmitter released E xceeds threshold Generates AP AP conducted down sensory axon generates higher frequency of AP for longer period more APs conducted down sensory axon recording arrangement from sensory unit

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Stimulus Property Mechanism of Coding Type of Stimulus (stimulus modality) Distinguished by the type of receptor activated and the specifi c pathway over which this information is transmitted to a particular area of the cerebral cortex Location of Stimulus Distinguished by the location of the activated receptive fi eld and the pathway that is subsequently activated to transmit this information to the area of the somatosensory cortex representing that particular location Intensity of Stimulus (stimulus strength) Distinguished by the frequency of action potentials initiated in an activated afferent neuron and the number of receptors (and afferent neurons) activated

Properties of Receptor:

Properties of Receptor 1. Specificity or differential sensitivity:- Every receptor is highly specific or sensitive to one type of sensory stimulus. and having lowest threshold for that. Almost Insensitive to normal strength of any other type of stimulus e.g. Rods and cones easily respond to light but only at very high threshold to pressure

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Electrical properties- receptor potential. 3. Sensory transduction:- Receptors are biological transducers, which convert stimulus energy into action potential or impulse (electrical energy). Convert one type of energy (mechanical/chemical / thermal / photic / other) into electrical energy.

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When same strength of stimulus is constantly applied to the receptor, frequency of action potential declines over time. Rate of adaptation varies from receptor to receptor. On this basis receptor may be Tonic receptors Phasic receptors. Stimulus Slowly adapting Rapidly adapting Time (s) 0 1 2 3 4 4. Adaptation of Receptors

Slowly Adapting (Tonic) Receptors:

Slowly Adapting (Tonic) Receptors Continue to transmit impulses to brain for long periods of time while stimulus is present. Keep brain apprised of the status of the body with respect to its surroundings. Will adapt to extinction as long as stimulus is present, but may take hours or days. E.g : Muscle spindle, Golgi tendon apparatus, Ruffini endings, Merkel discs, Macula, pain, temperature, chemo- and baroreceptors .

Rapidly Adapting (Phasic) Receptors:

Rapidly Adapting ( Phasic ) Receptors Respond only when change is taking place . Rate and strength of response is related to rate and intensity of stimulus. Important for predicting future position or condition of body . Very important for balance and movement . Types of rapidly adapting receptors: pacinian corpuscle, Meissner’s corpuscle, semicircular canals .

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Tonic and Phasic Receptors Amplitude Sensitive - Slowly adapting R a p R= response, p = position, t = time Stimulus p t Velocity Sensitive - Rapidly adapting R a dp/dt R a d 2 p/dt 2 Acceleration Sensitive - Rapidly adapting

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5. Recruitment :- As the strength of stimulus is increased, more and more receptors are recruited. S timulus tends to spread over large area. Increased strength of stimulus recruits high threshold receptors also. Strong stimulus results increase firing rate in the same sensory unit &/or stimulation of more sensory units. Interpreted in the brain as increased in the intensity of sensation.

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Intensity coding by recruitment

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6. Muller’s law of specific nerve energy:- No matter where or how a sensory pathway is stimulated along its course from receptor to sensory cortex or highest center, that sensation felt for which receptors are specialized depending upon area of the cerebral cortex stimulated. Specific cortical areas are for specific sensation and that’s why sensation felt depends upon area of the cortex stimulated.

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7 Labeled Line Principle:- Specific nerve fibers transmit only one specific modality of sensation. 8 Law of Projection:- No matter where or how a sensory pathway is stimulated along its course from receptor to sensory cortex or highest center, the conscious sensation produce is referred or projected to the location of the receptor.

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9. Lateral inhibition or Gating theory of Inhibition:- In a pathway centrally placed nerve fibers inhibit peripheral nerve fibers. Helps in localization of stimulus and in increasing contrast of stimulus. At the spinal level one pathway of sensory impulse may inhibit other pathway (that is transmitting other sensory modality) through collaterals and involvement of inhibitory interneurons. Example- pain pathway can be inhibited by touch pathway.

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10. Discrimination of strength of stimulus:- As strength of stimulus is increased number of receptors or afferent nerves stimulated and frequency of action potential is increased. Weber Fechner Law:- Magnitude of the sensation felt is proportionate to the log of the intensity of the stimulus. S = k log I + C S = magnitude of the sensation I = intensity of stimulus’ K & C are constant.

Types of Cutaneous receptors:

Types of Cutaneous receptors

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Meissner’s corpuscles:- Detect fine touch and low frequency vibrations. Present on glabrous(non hairy) skin,close to surface (fingertips, lips, eyelids, nipples and external genitalia). Supplied by A β nerve fibers ,adaptation is fast Function: motion detection, grip control Nerve ending is branched to form a complex structure inside corpuscle Single layered capsule Stimuli: skin motion, low frequency vibration Receptive field: 22 mm 2

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2. Merkel’s discs:- Detect continuous touch and feel of shape and texture Supplied by A-delta and type C nerve fibers and adaptation is slow. Function: form and texture perception Present in the hairy part of body & also where Meissner’s corpuscles are present. (like finger tips, tip of epidermal ridges ). Most superficial Flattened termination of afferent axons Stimuli: edges, points, corners, curvature Receptive field: 9 mm2 Iggo - dome receptor

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3. Pacinian corpuscles (deep):- Detect deep pressure and high frequency vibration. Supplied by A-beta nerve fibers and adaptation is fastest Large, onion shaped, having many layers and encapsulated. Found in large numbers in skin, subcutaneous tissues and in tendons and joints.

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4. Ruffini’s end organs (deep):- Detect continuous or sustained deep pressure Supplied by A-delta and type C nerve fibers and adaptation is slow. Expanded encapsulated endings of myelinated A delta or unmyelinated type C. Smallest mechanoreceptors

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5. Hair end organs:- Detect movement of objects on the surface of the body. Supplied by A-beta nerve fibers and adaptation is fast. 6. Free nerve endings:- Detect nociceptive stimuli (pain sensation). Supplied by A-delta and type C nerve fibers and adaptation is slow.

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