psych b sensory system
TRANSCRIPT
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Sensory Functions
• General SensationsMechanical – touch, pressure, vibration,
stretch Chemical
• Special SensationsVisionHearingTasteSmell
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Receptor
Sensory modality
Sensory nerve
Central Connections
Ascending Sensory pathway
Sensory area in the brain
Touch stimulus
AFFERENT
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• stimulation of a receptor usually produces only one sensationmodality specific
• But some receptors are stimulated by more than one sensory modalityeg. free nerve endings
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Classification of receptors
• Mechanoreceptors
• Thermoreceptors
• Nociceptorspain
• Chemoreceptorstaste, smell, visceral
• Electromagnetic receptorsvisual
Guyton p.496
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Mechanoreceptors
• Mainly cutaneousTouchPressureVibration
• Crude or Fine mechanosensations
• Others: auditory, vestibular, stretch
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Mechanoreceptors
• Pacinian corpuscle
• Meissner’s corpuscle
• Krause’s corpuscle
• Ruffini’s end organ
• Merkel’s disc
• Hair end organ
• Free nerve endings
Guyton p.496
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Mechanoreceptors
• Pacinian corpuscledeep, pressure sensitive, fast adapting
• Ruffini’s end organdeep, tension sensitive, slow adapting
• Merkel’s discsuperficial, sensitive to deformation of
skin, slowly adapting
Guyton p.496
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Mechanoreceptors
• Meissner’s corpusclesuperficial, sensitive to sideways
movements
• Krause’s endings
• Hair end organ
• Free nerve endings
Guyton p.496
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Pacinian Corpuscle
Capsule
Nerve fibre
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What happens inside a receptor?
• TRANSDUCTIONStimulus energy is converted to action
potentials Inside the nervous system signals are always action
potentialsLanguage of the nervous system contains only 1
word: action potentials
• At the brain opposite happens in order to feel the sensationPERCEPTION
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Transduction
Stimulus
Receptor potential(Generator potential)
Action potential
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Receptor potentials
• are graded
• do not follow all-or-none law
• amplitude depends on the strength of the stimulus
• when reaches the threshold: triggers the action potentials
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Action Potentials
Threshold
RestingMembranePotential
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Resting
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Receptor potentials
• mechanical transformation on the capsule
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Physical Stimulus
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Receptor potentials
• Opens up Na+ channels
• Na+ influx
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Physical Stimulus
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Receptor potentials
• Membrane inside the capsule: depolarisation
• thus receptor potential is generated
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Physical Stimulus
local current
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Physical Stimulus
Action Potentialsare generated
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• Stimulus strength is coded as the frequency of AP
• Higher the stimulus more frequent are the APs
• Amplitude of AP is constant
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Stimulus
Receptorpotentials
Action potentials
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Adaptation• “getting used to”
• after a period of time sensory receptors adapt partially or completely
• different typesfast adapting receptorsslowly adapting receptors
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Adaptation
• after a period of time sensory receptors adapt partially or completely
• different typesfast adapting receptorsslowly adapting receptors
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Paciniancorpuscle
Musclespindle
Pain
Time
Imp
uls
es p
er s
eco
nd
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Mechanism of adaptation
• In the Pacinian corpusclemechanical deformation is transmitted
throughout the capsule and pressure redistributes
Na+ channels inactivates after some time
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Impulse
Stimulus
Redistribution of pressure inside the capsule
NoImpulse
Stimulus
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• Rapidly adapting receptorsphasic or rate or movement receptors
detect changes in stimulus strengtheg. Pacinian corpuscle, hair end-organ
• Slowly adapting receptorstonic receptors
detect continuous stimulus strengtheg. muscle spindles, Golgi tendon organ,
baroreceptors, Ruffini endings and Merkel’s discs, pain receptors
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Receptor
Sensory modality
Sensory nerve
Central Connections
Ascending Sensory pathway
Sensory area in the brain
Touch stimulus
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Two ascending pathways
• Dorsal column - medial lemniscus pathway
fast pathway
• Spinothalamic pathwayslow pathway
These two pathways come together at the level of thalamus
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Dorsal rootDorsal columns
Dorsal horn
Dorsal root ganglion
Spinothalamictracts
Posterior (dorsal)
Anterior (ventral)
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Dorsal column pathwaySpinothalamic pathway
Lateral Spinothalamic tract
AnteriorSpinothalamic tract
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Dorsal column pathway Spinothalamic pathway
• touch: fine degree
• highly localised touch sensations
• vibratory sensations
• sensations signalling movement
• position sense
• pressure: fine degree
• Pain
• Thermal sensations
• Crude touch & pressure
• crude localising sensations
• tickle & itch
• sexual sensations
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Dorsal column nuclei(cuneate & gracile nucleus)
Dorsal column
Medial lemniscus
thalamus
thalamocortical tracts
sensory cortex
internal capsule
1st order neuron
2nd order neuron
3rd order neuron
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Dorsal root ganglion
Dorsal root
Dorsal column
Dorsal column nuclei(cuneate & gracile nucleus)
Med
ulla cross over
Medial lemniscus
thalamus
thalamocortical tracts
sensory cortex
1st order neuron
2nd order neuron
3rd orderneuron
Dorsal column medial lemniscus pathwayDorsal column medial lemniscus pathway
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dorsal column - medial lemniscus pathway
• after entering the spinal cordlateral branch: participates in spinal cord
reflexesmedial branch: turns upwards
• forms the dorsal columns
• spatial orientation: medial: lower parts of the bodylateral: upper part of the body
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dorsal column - medial lemniscus pathway
• synapse in the dorsal column nucleinucleus cuneatus & nucleus gracilus
• 2nd order neuron cross over to the opposite side and ascends upwards as medial lemniscus
• as this travels along the brain stem fibres from head and neck are joined (trigeminal)
• ends in the thalamus (ventrobasal complex) ventral posterolateral nuclei
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dorsal column - medial lemniscus pathway
• spatial orientation in the thalamusmedial: upper part of the bodylateral: lower part of the body
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Spinothalamic tracts
thalamus
thalamocortical tracts
sensory cortex
internal capsule
1st order neuron
2nd order neuron
3rd order neuron
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Dorsal root ganglion
Dorsal root
cross over
Spinothalamic tracts
thalamus
thalamocortical tracts
sensory cortex
1st order neuron
2nd order neuron
3rd orderneuron
Spinothalamic pathwaySpinothalamic pathway
Dorsal horn
anterior lateral
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spinothalamic pathway
• after entering the spinal cordsynapse in the dorsal horn
• cross over to the opposite side
• divide in to two tractslateral spinothalamic tract:
pain and temperature
anterior spinothalamic tractcrude touch
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spinothalamic pathway
• spatial orientation medial: upper part of the bodylateral: lower part of the body
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Note the spatial orientation of fibres
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Dorsal column pathwaySpinothalamic pathway
Lateral Spinothalamic tract
AnteriorSpinothalamic tract
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thalamocortical tracts
• from the thalamus 3rd order neuron ascends up through the internal capsule
• up to the sensory cortex
• thalamocortical radiationtracts diverge
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Sensory cortical areas
• parietal cortex
• a distinct spatial orientation exists
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Sensory cortex• different areas of the body are represented
in different cortical areas in the sensory cortex
• sensory homunculussomatotopic representation not proportionate distorted mapupside down map
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Sensory homunculus
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Brodmann areas
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Sensory cortical areas
• primary somatosensory cortex (SI)postcentral gyrus(Brodmann areas 3a, 3b, 1, 2)
• secondary somatosensory cortex (SII)parietal cortex behind postcentral gyrus(Brodmann areas 5, 7)
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somatosensory cortex
Functions
• Localisation of somatic sensations
• to judge critical degree of pressure
• identify objects by their weight, shape, form - stereognosis
• to judge texture of materials
• localisation of pain & temperature
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Abnormalities• Sensory loss
• Anaesthesiaabsence of sensation
• Paraesthesiaabnormal sensation
• HemianaesthesiaLoss of sensation of one half of the body
• Astereognosis
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Localisation of the abnormality• Peripheral nerve
part of a limb is affected
• Rootsdermatomal pattern of sensory loss
• spinal corda sensory level
• internal capsuleone half of the body