cc 46-sensory receptors, neuronal circuits for processing information

Chapter(46:(((Sensory(Receptors,(Neuronal(Circuits(for(Processing(

Information(

Guyton'and'Hall,'Textbook'of'Medical'Physiology,'12th'edition'

Types&of&Sensory&Receptors&and&Their&S3muli&

• !!!Mechanoreceptors-!detect'mechanical'compression''or'stretching'

• !!!Thermoreceptors-!detect'changes'in'temperature'

• !!!Nociceptors-!pain'receptors'(damage'to'tissues)'

• !!!Electromagnetic!receptors-!detect'light'on'the'retina'

• !!!Chemoreceptors-!detect'taste,'smell,'oxygen'level,''osmolality,'etc.'

'(See'Table'46.1'in'the'text)'


• !!!Differential!Sensitivity!of!Receptors!

a.  Each'receptor'type'is'highly'sensitive'to'one'type''of'stimulus'for'which'it'is'designed'

'b.  NonIresponsiveness'to'other'types'of'sensory'

'stimuli''c.  Pain'receptors'do'not'respond'to'usual'touch'or''

'pressure'but'will'become'active'when'the'stimuli''become'severe'enough'to'damage'the'tissues''


• !!!Modality!of!Sensation-!The!“Labeled!Line”!Principle-!!the'specificity'of'nerve'fibers'for'transmitting'only'one'!modality'of'sensation'

a.  Each'receptor'type'is'highly'sensitive'to'one'type''of'stimulus'for'which'it'is'designed'

'b.  NonIresponsiveness'to'other'types'of'sensory'

'stimuli''c.  Pain'receptors'do'not'respond'to'usual'touch'or''

'pressure'but'will'become'active'when'the'stimuli''become'severe'enough'to'damage'the'tissues''

Fig.'46.1'''Several'types'of'somatic'sensory'nerve'endings'

Transduc3on&of&Sensory&S3muli&into&Nerve&Impulses&

• !!!Local!Electrical!Currents!at!Nerve!Endings—!!Receptor!Potentials!

!a.  Mechanisms'of'receptor'potentials'

1.  Mechanical'deformation'of'the'receptor'which''stretches'the'membrane'and'opens'channels'

2.  Application'of'a'chemical'to'the'membrane'3.  Change'of'the'temperature'of'the'membrane'4.  Effects'of'electromagnetic'radiation'


• !!!Local!Electrical!Currents!at!Nerve!Endings—!!Receptor!Potentials!

!b. 'Maximum'receptor'potential'amplitude'(100'mV)'

c.  Relation'of'the'receptor'potential'to'APs'–the'more''the'receptor'potential'rises'above'the'threshold'''level,'the'greater'becomes'the'AP'frequency'


Fig.'46.2''Typical'relation'between'receptor'potential'and'action'potentials'when'the''''''''''''''''''receptor'potential'rises'above'threshold'


• !!!Receptor!Potential!of!the!Pacinian!Corpuscle!

Fig.'46.3'''Excitation'of'a'sensory'nerve'fiber'by'a'receptor'potential''''''''''''''''''''produced'in'a'Pacinian'corpuscle'


• !!!Relation!Between!Stimulus!Intensity!and!the!!Receptor!Potential!

Fig.'46.4'''Relation'of'amplitude'of'receptor'potential'to'strength'''''''''''''''''''of'a'mechanical'stimulus'applied'to'a'Pacinian'corpuscle'


• !!!Adaptation!of!Receptors!

Fig.'46.5'''Adaptation'of'different'types'of'receptors,'showing'rapid'''''''''''''''''''adaptation'of'some'receptors'and'slow'adaptation'of'others'


• !!!Mechanism!of!Receptor!Adaptation-!different'for'''each'type'of'receptor'

'a.  In'the'mechanoreceptor'the'initial'compression'

'causes'the'receptor'potential'which'disappears'''within'a'fraction'of'a'second'even'though'the'''compression'continues'

'b.  AccommodationI'slower'adaptation'and'occurs'in'

'the'nerve'fiber'itself;'the'tip'of'the'nerve'gradually''becomes'“accommodated”'to'the'stimulus''


• !!!Tonic!Receptors!(Slow!Adapting)''

a.  Continue'to'transmit'impulses'as'long'as'the'stimulus''is'present;'include'the'following:'

'1.  Macula'receptors'in'the'vestibular'apparatus'2.  Pain'receptors'3.  Baroreceptors'of'the'arterial'tree'4.  Chemoreceptors'of'the'carotid'and'aortic'bodies'


• !!!Phasic!Receptors!(Rapidly!Adapting)-!also!called!!“rate!receptors,”!and!“movement!receptors”'

'a.  Stimulated'only'when'the'stimulus'strength'changes'

b.  React'strongly'while'a'change'is'actually'taking'place'

c.  Cannot'be'used'to'transmit'a'continuous'signal'


• !!!Importance!of!Phasic!Receptors-!have'a'predictive'''function'

'• !!!General!Classification!of!Nerve!Fibers!

a.  Type'AI'large'and'medium'sized'myelinated'fibers''of'spinal'nerves'(alpha,'beta,'gamma,'delta)'

'b.  Type'CI'small,'unmyelinated'fibers'that'conduct'at'

'low'velocities'


Fig.'46.6''Physiologic'classification'and''''''''''''''''''functions'of'nerve'fibers'

Transmission&of&Signals&of&Different&Intensity&in&Nerve&Tracts&

• !!!Spatial!Summation-!increased'signal'strength'by'''using'progressively'larger'numbers'of'fibers;''stronger'signals'spread'to'more'and'more'''fibers'(Fig.'46.7)'

'• !!!Temporal!Summation-!increased'signal'strength'

'by'increasing'the'frequency'of'nerve'impulses''in'each'fiber'(Fig.'46.8)'

Fig.'46.7''Pattern'of'stimulation'of'pain'fibers'in''''''''''''''''''a'nerve'leading'from'an'area'of'skin'''''''''''''''''''pricked'by'a'pin'(spatial'summation)'

Fig.'46.8''Translation'of'signal'strength'into'a''''''''''''''''''frequency'modulated'series'of'nerve''''''''''''''''''impulses'(temporal'summation)'

Transmission&and&Processing&of&Signals&in&Neuronal&Pools&

• !!!Relaying!of!Signals!Through!Neuronal!Pools-!!organization'of'neurons'for'relaying'signals'

Fig.'46.9''Basic'organization'of'a''''''''''''''''''neuronal'pool'

The'neuron'area'stimulated'by'each'incoming'nerve'fiber'is'the'stimulatory'field'


• !!!Threshold!and!Sub-threshold!Stimuli!

a.  The'discharge'of'a'single'excitatory'presynaptic'!terminal'almost'never'causes'an'action '''potential'in'a'postsynaptic'neuron'''

''''''b. 'Instead,'large'numbers''of'input'terminals'must''discharge'on'the'same'neuron'either'at'the'same'''time'or'in'rapid'succession'to'cause'excitation'


• !!!Threshold!and!Sub-threshold!Stimuli!!

Fig.'46.10''“Discharge”'and'“Facilitated”'zones'of'a'neuronal'pool'


• !!!Inhibition!of!a!Neuronal!Pool!

a.  Some'incoming'fibers'inhibit'neurons,'rather'than''excite'them'

b.  This'is'the'opposite'of'“facilitation”'and'is'called''the'“inhibitory'zone”'

'!


• !!!Divergence!of!Signals!

a.  AmplifyingIan'input'signal'spreads'to'an'increasing'number'of'neurons'as'it'passes'through'successive'orders'of'neurons'in'its'path'

b. 'Divergence'in'multiple'tractsI'the'signal'is'transmitted'into'two'directions'from'the'pool;'information'transmitted'up'the'dorsal'column'from'the'spinal'cord'takes'two'courses''(a)'into'the'cerebellum,'and'(2)'on'through'the'lower'regions'of'the'brain'to'the'thalamus'and'cerebral'cortex'

'!


• !!!Divergence!of!Signals!!

Fig.'46.11''Divergence'in'neuronal'pathways.''A:''Divergence'within'a'pathway'to'cause''''''''''''''''''amplification'of'the'signal,'B:''Divergence'into'multiple'tracts'to'transmit'the''''''''''''''''''signal'to'separate'areas.'


• !!!Convergence!of!Signals-!signals'from'multiple''inputs'uniting'to'excite'a'single'neuron'

a.  Convergence'from'a'single'sourceI'multiple'terminals'from'a'single'incoming'fiber'tract'terminate'on'the'same'neuron'

b.  Convergence'from'input'signals'(excitatory'or'inhibitory)'from'multiple'sources'

'Allows'the'summation'of'information'from'different'sources'

!


• !!!Convergence!of!Signals-!signals'from'multiple''inputs'uniting'to'excite'a'single'neuron'

!

Fig.'46.12''Convergence'of'multiple'input'fibers'onto'a'single'neuron'


• !!!Neuronal!Circuit!with!both!Excitatory!and!!Inhibitory!Output!Signals'

a.  Sometimes'an'incoming'signal'causes'an'excitatory'signal'going'in'one'direction'and'an'inhibitory'signal'going'elsewhere'

b.  Reciprocal'inhibition'circuit'in'some'reflexes'


• !!!Neuronal!Circuit!with!both!Excitatory!and!!Inhibitory!Output!Signals'

Fig.'46.13''Inhibitory'circuit.''Neuron'2'is'an'inhibitory'neuron'


• !!!Prolongation!of!a!Signal!by!a!Neuronal!Pool-!“Afterdischarge”!

a.  Synaptic'AfterdischargeI'when'excitatory'synapses'discharge'on'a'dendrite'or'on'the'soma,'a'postsynaptic'electrical'potential'develops'in'the'neuron'and'lasts'for'msec'

b.  As'long'as'the'potential'lasts,'it'will'continue'to'excite'the'neuron,'causing'it'to''transmit'a'continuous'train'or'output'impulses'


• !!!Reverberatory!(Oscillatory)!Circuit!

a.  Caused'by'positive'feedback'within'the'neuronal'circuit'that'feeds'back'to'reIexcite'the'input'of'the'same'circuit'

'


• !!!Reverberatory!(Oscillatory)!Circuit!

Fig.'46.14''Reverberatory'circuits'of''''''''''''''''''''increasing'complexity'


• !!!Characteristics!of!Signal!Propagation!from!a!!!Reverberatory!Circuit!

Fig.'46.15''Typical'pattern'of'the'output'signal'from'a'reverberatory'circuit'''''''''''''''''''after'a'single'input'stimulus'showing'the'effects'of''''''''''''''''''''facilitation'and'inhibition'


• !!!Continuous!Signal!Output!from!Some!Neuronal!Circuits!

a.  Continuous'discharge'caused'by'intrinsic'neuronal'excitability'

b.  Continuous'signals'emitted'from'reverberating'circuits'


• !!!Continuous!Signal!Output!from!Some!Neuronal!Circuits!

Fig.'46.16''Continuous'output'from'either'a'reverberating'circuit'or'a'pool'of''''''''''''''''''''intrinsic'discharging'neurons.''Also'showing'the'effects'of'excitatory''''''''''''''''''''or'inhibitory'input'signals'


• !!!Rhythmical!Signal!Output!

Fig.'46.17''Rhythmical'output'of'summated'nerve'impulses'

Instability&and&Stability&of&Neuronal&Circuits&

• !!!Inhibitory!Circuits!as!a!Mechanism!for!Stabilizing!!Nervous!System!Function!

• !!!Synaptic!Fatigue!as!a!Means!of!Stabilizing!the!Nervous!!System!

• !!!Automatic!Short-Term!Adjustment!of!Pathway!!Sensitivity!by!the!Fatigue!Mechanism!

• !!!Long-Term!Changes!in!Synaptic!Sensitivity!by!Automatic!

!Down!Regulation!or!Up!Regulation!of!Synaptic!!Receptors!

Fig.'46.19''Successive'flexor'reflexes'showing'fatigue'of'conduction'through'the'reflex'pathway'

cc 46-sensory receptors, neuronal circuits for processing information

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