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Sensory ReceptorsSensory ReceptorsSensory Receptors

(some basic properties) Sensory Receptors

(some basic properties)

Organs which receive information from outside or

Organs which receive information from outside or within the body and send it to the central nervous system for processing

within the body and send it to the central nervous system for processing

Activated by various forms of energy:

light heat

Activated by various forms of energy:

light heat

Sensory Receptors(some basic properties)

Sensory Receptors(some basic properties)

Specialised nerve ending

light, heat, mechanical chemical etc.

light, heat, mechanical chemical etc.

Sensory receptors are either specialized endings of afferent neurones or separate cells that signal to the afferent neuroneSensory receptors are either specialized endings of afferent neurones or separate cells that signal to the afferent neurone

Specialised cells

Definition of a Transducer or TransductionConversion of one form of energy (e.g. heat, pressure, chemical) into another form: e.g. electrical energy or depolarisation

Definition of a Transducer or TransductionConversion of one form of energy (e.g. heat, pressure, chemical) into another form: e.g. electrical energy or depolarisation

Sensory Receptors Are Transducers Sensory Receptors Are Transducers

The depolarisation occurring at the sensory terminal is a). Graded – Non-propagating, confined to terminalb). Called the Receptor Potential (R.P.)

The depolarisation occurring at the sensory terminal is a). Graded – Non-propagating, confined to terminalb). Called the Receptor Potential (R.P.)

The Receptor Potential:If large enough will evoke all or nothing action potentials that propagate along the nerve fibre

The Receptor Potential:If large enough will evoke all or nothing action potentials that propagate along the nerve fibre

Variable stimulusintensity

Produces graded receptor potentials

The Receptor PotentialThe Receptor Potential

Adequate stimulus

10mV

Produces variable patterns of

action potentialsin the CNS

More Stimulus = More Transmitter Released

Intensity of Stimulus is Linearly EncodedIntensity of Stimulus is Linearly Encoded

The number of action potentials generated by a pressure-sensitive sensory afferent neuron is directly proportional to the stimulus intensity.

The number of action potentials generated by a pressure-sensitive sensory afferent neuron is directly proportional to the stimulus intensity.

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Receptor Potential: more or less proportional to stimulus

Receptor Potential: more or less proportional to stimulus

stimulus receptor potentialstimulus receptor potential

adaptationmV

Adaptation to stimulus depends on receptor typeAdaptation to stimulus depends on receptor type

adaptation

Who can guess how a membraneof a neurone could react topressure or temperature by achange in membrane potential?

The offresponse

Adaptation: A Mechanism Preventing Sensory Information Overload

Adaptation: A Mechanism Preventing Sensory Information Overload

Skin:Paccinian

Corpuscles pressure/touch

Skin:Paccinian

Corpuscles pressure/touch

Different receptors vary in their speed of adaptation to stimuliDifferent receptors vary in their speed of adaptation to stimuli

Muscle:stretch

receptors

Muscle:stretch

receptors

Specific types of sensory receptors for different mechanosensory stimulationSpecific types of sensory receptors for different mechanosensory stimulation

Receptor SpecificityReceptor Specificity

Mechanoreceptors PhotoreceptorsPhotoreceptors ChemoreceptorsChemoreceptors

Receptors have a differential sensitivity to different energies or stimuliReceptors have a differential sensitivity to different energies or stimuli

nerve ending

A receptor is specific to one energy (or stimulus), because it is most sensitive to that energy. Other energies may

stimulate but only at high intensities.

A receptor is specific to one energy (or stimulus), because it is most sensitive to that energy. Other energies may

stimulate but only at high intensities.

Sensory afferent (A) has a more spatially limited receptive field than sensory afferent

Sensory afferent (A) has a more spatially limited receptive field than sensory afferent

Sensory Processing:spatial discrimination

Sensory Processing:spatial discrimination

neuron (B).neuron (B).

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Lips Back

Sensory Processing:spatial discrimination – the paired point test

Sensory Processing:spatial discrimination – the paired point test

Overlapping stimulation between neighboring receptive fields

id l i f ti

Overlapping stimulation between neighboring receptive fields

id l i f ti

Sensory Processing:spatial discrimination

Sensory Processing:spatial discrimination

provides general information about the location of a stimulus. provides general information about the location of a stimulus.

Stimulus A occurs in an area of greater nerve ending density

Stimulus A occurs in an area of greater nerve ending density

Sensory Processing:density of sensory afferent terminals

Sensory Processing:density of sensory afferent terminals

Stimulus A will generatea greater number of action potentials than stimulus B.

Stimulus A will generatea greater number of action potentials than stimulus B.

Sensory Processing:descending modulation from higher centres

Sensory Processing:descending modulation from higher centres

Painful Condition

Descending inputPainful Condition& Fight or Flight

Context and Sensory PerceptionContext and Sensory Perception

Monty Python & the Holy Grail

Sensory Processing: sensory discriminationSensory Processing: sensory discrimination

Provide background information about touch and temperature

Deliver information about location-specific changes in touch and temperature in the periphery.

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Convergence of sensory pathways is one of the reasons for “referred” pain from visceral organs

Sensory Modality Representation is Location Specific in the Cerebral Cortex

Sensory Modality Representation is Location Specific in the Cerebral Cortex

Fine discrimination is highly representedFine discrimination is highly represented

Sensory Representation is CortexSensory Representation is Cortex

cortical homunculus

Summary:

1. Sensory receptors allow our body to interact with the external environment2. Sensory receptors in internal organs are essential for homeostasis and

also alert the body in case of some anomaly3. Some receptors adapt fast, others slow and some – almost not at all.4. The intensity of the stimulus is encoded by the frequency of the action

potentials.5. Most receptors detect the stimulus via a subset of highly specialised ion

channels which open (Na+) or close (K+) to evoke depolarisation of the membrane of the sensory ending in response to the stimulus.

6. Information sent from the receptors is called AFFERENT information. In the CNS it is usual that several afferent neurones contact the same postsynaptic cell. This is the convergence, which is also a reason for referred pain.

7. The organs with the highest tactile sensitivity have the largest representations in the “sensory homunculus”