psych b sensory system
TRANSCRIPT
Sensory Functions
• General SensationsMechanical – touch, pressure, vibration,
stretch Chemical
• Special SensationsVisionHearingTasteSmell
Receptor
Sensory modality
Sensory nerve
Central Connections
Ascending Sensory pathway
Sensory area in the brain
Touch stimulus
AFFERENT
• stimulation of a receptor usually produces only one sensationmodality specific
• But some receptors are stimulated by more than one sensory modalityeg. free nerve endings
Classification of receptors
• Mechanoreceptors
• Thermoreceptors
• Nociceptorspain
• Chemoreceptorstaste, smell, visceral
• Electromagnetic receptorsvisual
Guyton p.496
Mechanoreceptors
• Mainly cutaneousTouchPressureVibration
• Crude or Fine mechanosensations
• Others: auditory, vestibular, stretch
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
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
Mechanoreceptors
• Meissner’s corpusclesuperficial, sensitive to sideways
movements
• Krause’s endings
• Hair end organ
• Free nerve endings
Guyton p.496
Pacinian Corpuscle
Capsule
Nerve fibre
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
Transduction
Stimulus
Receptor potential(Generator potential)
Action potential
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
Action Potentials
Threshold
RestingMembranePotential
Resting
Receptor potentials
• mechanical transformation on the capsule
Physical Stimulus
Receptor potentials
• Opens up Na+ channels
• Na+ influx
Physical Stimulus
Receptor potentials
• Membrane inside the capsule: depolarisation
• thus receptor potential is generated
Physical Stimulus
local current
Physical Stimulus
Action Potentialsare generated
• Stimulus strength is coded as the frequency of AP
• Higher the stimulus more frequent are the APs
• Amplitude of AP is constant
Stimulus
Receptorpotentials
Action potentials
Adaptation• “getting used to”
• after a period of time sensory receptors adapt partially or completely
• different typesfast adapting receptorsslowly adapting receptors
Adaptation
• after a period of time sensory receptors adapt partially or completely
• different typesfast adapting receptorsslowly adapting receptors
Paciniancorpuscle
Musclespindle
Pain
Time
Imp
uls
es p
er s
eco
nd
Mechanism of adaptation
• In the Pacinian corpusclemechanical deformation is transmitted
throughout the capsule and pressure redistributes
Na+ channels inactivates after some time
Impulse
Stimulus
Redistribution of pressure inside the capsule
NoImpulse
Stimulus
• 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
Receptor
Sensory modality
Sensory nerve
Central Connections
Ascending Sensory pathway
Sensory area in the brain
Touch stimulus
Two ascending pathways
• Dorsal column - medial lemniscus pathway
fast pathway
• Spinothalamic pathwayslow pathway
These two pathways come together at the level of thalamus
Dorsal rootDorsal columns
Dorsal horn
Dorsal root ganglion
Spinothalamictracts
Posterior (dorsal)
Anterior (ventral)
Dorsal column pathwaySpinothalamic pathway
Lateral Spinothalamic tract
AnteriorSpinothalamic tract
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
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
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
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
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
dorsal column - medial lemniscus pathway
• spatial orientation in the thalamusmedial: upper part of the bodylateral: lower part of the body
Spinothalamic tracts
thalamus
thalamocortical tracts
sensory cortex
internal capsule
1st order neuron
2nd order neuron
3rd order neuron
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
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
spinothalamic pathway
• spatial orientation medial: upper part of the bodylateral: lower part of the body
Note the spatial orientation of fibres
Dorsal column pathwaySpinothalamic pathway
Lateral Spinothalamic tract
AnteriorSpinothalamic tract
thalamocortical tracts
• from the thalamus 3rd order neuron ascends up through the internal capsule
• up to the sensory cortex
• thalamocortical radiationtracts diverge
Sensory cortical areas
• parietal cortex
• a distinct spatial orientation exists
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
Sensory homunculus
Brodmann areas
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)
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
Abnormalities• Sensory loss
• Anaesthesiaabsence of sensation
• Paraesthesiaabnormal sensation
• HemianaesthesiaLoss of sensation of one half of the body
• Astereognosis
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