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