Y2 s1 sensory system

Sensory receptors, sensory pathways &

sensory cortex

Prof. Vajira Weerasinghe

Professor of Physiology

www.slideshare.net/vajira54

Sensory functions

• Humans do not have receptor for every possible stimuli

• There are different sensory modalities that human brain can perceive

• Arrival of information is sensation

• Awareness of a sensation is perception

Sensory Functions

• General SensationsPhysical (touch, pressure, vibration, stretch)TemperaturePain Chemical

• Special SensationsVisionHearingTasteSmell

Modality specificity

• Stimulation of a receptor usually produces only one sensationmodality specific

• But some receptors are stimulated by more than one sensory modality (polymodal)eg. free nerve endings

Sensory pathway

• Once a receptor is stimulated

impulse travels through a particular pathway

known as sensory pathway or ascending pathway

up to the brain

Receptor

Sensory modality

Sensory nerve

Central Connections

Ascending Sensory pathway

Sensory area in the brain

Touch stimulus

AFFERENT

Sensory pathway

Receptors

• Receptor cells are specific cells that are sensitive to different forms of energy from the environment

• These cells contain membrane receptors coupled to ion channels

• They transform the stimulus into electrical signals

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, proprioceptors

Cutaneous mechanoreceptors

• Pacinian corpuscle

• Meissner’s corpuscle

• Krause’s corpuscle

• Ruffini’s end organ

• Merkel’s disc

• Hair end organ

• Free nerve endings

Mechanoreceptors

• Pacinian corpuscledeep, pressure sensitive, fast adapting, large receptive field

• Meissner’s corpusclesuperficial, sensitive to touch, small receptive field

• Ruffini’s end organdeep, tension sensitive, slow adapting, large receptive field

• Merkel’s discsuperficial, touch, pressure and texture sensitive, slowly

adapting, small receptive field

• Krause’s endingsvibration sensitive

Mechanoreceptors

• Hair end organ

• Free nerve endingsCrude mechanosensations(Pain, temperature)

Pacinian corpuscles

looks like onion, large receptive field, rapidly adapting

Hair follicle receptor

nerve endings around root of hair in hairy skin, small receptive field, either slowly or rapidly adapting

Ruffini's ending

looks like small Pacinian, large receptive fields, slowly adapting

Merkel's diskssmall arrays of small disks which may have synapses to nerve endings, small receptive fields, slowly adapting

Meissner's corpuscles

hang under ridges of glabrous skin, small receptive fields, rapidly adapting

Krause end bulbs

look like knotted balls of string in skin in border between dry skin and mucous membrane in mouth, genitals, anus

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 potentials

Language of the nervous system contains only 1 word: action potentials

• At the brain opposite happens in order to feel the sensationPERCEPTION

Receptor potentials

• When a stimulus activate a receptor initially a “receptor potential” is generated

• This is also called “generator potential”

• This is a graded potential

• It does not follow “all-or-none law”

• Its amplitude depends on the strength of the stimulus

• When it reaches the threshold it triggers an “action potential”

Transduction

Stimulus

Receptor potential(Generator potential)

Action potential

Action Potentials

Threshold

RestingMembranePotential

-70

- 55

+30

StimulusReceptor potential

Coding of sensory stimuli

• 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

Sensory coding

• A receptor must convey the type of information it is sending the kind of receptor activated determined the signal recognition by the brain

• It must convey the intensity of the stimulus the stronger the signals, the more frequent will be the APs

• It must send information about the location and receptive field, characteristic of the receptor

Transduction in different receptors

• Different receptors have different ion channels

• Their opening causes receptor potential

Receptor potential generation in the Pacinian corpuscle

Pacinian corpuscle

Resting

Physical Stimulus

Physical stimulus causing mechanical deformation on the capsule

Physical Stimulus

Mechanical deformation is transmitted to the inside

Opens up mechanosensitive Na+ channel

Causes depolarisation and thus receptor potential

Physical Stimulus

local current

Current flow through a local circuit

Physical Stimulus

Action Potentialsare generated

Opening of voltage gated Na+ channels causes generation of action potentials

Adaptation

• “getting used to”

• after a period of time sensory receptors adapt partially or completely

• different typesRapidly 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 redistributesNa+ 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

Classification of receptors

• Mechanoreceptors Cutaneous (touch, pressure, vibration) eg. Pacinian, Meissner’s corpuscle, free

nerve endings Proprioceptors (joint position receptors) eg. Muscle stretch receptors, tendon

organs Baroreceptors Auditory/vestibular hair cells

• Chemoreceptors Taste buds and smell receptors Visceral chemoreceptors sensitive to Pco2, pH, osmolality etc

• Thermoreceptors Cold and hot receptors

• Nociceptors (pain receptors)

• Other receptors: Visual (rods and cones): electromagnetic

Two ascending pathways

• Dorsal column - medial lemniscus pathwayfast 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 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


• 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


• spatial orientation in the thalamusmedial: upper part of the bodylateral: lower part of the body

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

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

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

Representation

•upside down•distorted

concept of homunculus

Map

Sensory homunculus

Brodmann areas

Sensory cortical areas

• Primary somatosensory cortex (SI)postcentral gyrus(Brodmann areas 3a, 3b, 1, 2)

• Secondary somatosensory cortex and Somatosensory association cortex Posterior parietal areas(Brodmann areas 5, 7)

Somatosensory cortex

•FunctionsTo localise somatic sensationsTo judge critical degree of pressureTo identify objects by their weight,

shape, form - stereognosisTo judge texture of materialsTo localise pain & temperature

Somatosensory cortex

• Damage to the sensory cortex results in decreased sensory thresholdsinability to discriminate the properties of

tactile stimuliInability to identify objects by touch

(astereognosis)

Secondary somatosensory cortex and Somatosensory association cortex

• Located directly posterior to the sensory cortex in the superior parietal lobes

• Consists of areas 5 and 7

• Receives synthesized connections from the primary and secondary sensory cortices

• Neurons respond to several types of inputs and are involved in complex associations

Secondary somatosensory cortex and Somatosensory association cortex

• Damage can cause Tactile agnosia

inability to recognize objects even though the objects can be felt

Spatial neglectThis typically happens with non-dominant hemisphere

lesions Neglect can be so severe that the individual even denies

that their left side belongs to them

Receptive fields

• The receptor area which when stimulated results in a response of a particular sensory neuron

• Receptive fields of adjacent neurons overlap

Two-Point Discrimination

• Whether a stimulus feels like one sensation or two distinct sensations depends on the size of the receptive fields of the sensory receptors

• Different areas of the body have sensory receptors with different sized receptive fields

• Smaller receptive fields result in greater sensitivity

• Fingers are more sensitive than backs

Lateral Inhibition

• The capacity of an excited neuron to reduce the activity of its neighbors

• When the skin is touched by an object several sensory neurons in the skin next to one another are stimulated neurons that are firing suppress the stimulation of neighbouring neurons only the neurons that are most stimulated and least inhibited will fire so the firing pattern tends to concentrate at stimulus peaks

• Lateral inhibition increases the contrast and sharpness

• Weaker signals get weaker, stronger signals get stronger

• It is preset in the retina

Lateral inhibition improves 2-point discrimination

Sensory abnormalities• Various types of sensory abnormalities can

occur when the sensory pathways are damaged

• Sensory loss, altered sensations or pain could occur as a result

• In addition, motor pathways could also be affected resulting in motor weakness

Types of sensory abnormalities• Sensory loss

• Anaesthesiaabsence of sensation

• Paraesthesia (numbness or pins-needles-sensation)altered sensation

• Neuropathic pain

• HemianaesthesiaLoss of sensation of one half of the body

• Astereognosis

• Spatial neglect

Localisation of the abnormality• Peripheral nerve

innervated area affected

• Rootsdermatomal pattern of sensory loss

• Spinal corda sensory level

• Internal capsuleone half of the body

• Cortical areasOther features

Examples of sensory lesions or sensory disorders

• Carpal tunnel syndrome Median nerve lesion at the wristNumbness of thumb, index and middle

fingersPain in the handPain could radiate

upwards


• PolyneuropathyAll sensory nerves of

both upper and lower limbs are degenerated

Numbness of hands and feet

Glove and stocking type of sensory loss

Diabetic or nutritional neuropathy


• Cervical radiculopathyCervical root lesion

Compression of nerve root as it comes out through intervertebral foramina

Numbness and sensory loss of relevant dermatomes

Commonly affected are C56 dermatomes


• Spinal cord lesion (cervical myelopathy)

• Damage to the spinal cord

• Sensory loss or numbness below the level of the spinal cord lesion

• eg. Sensory loss at T10


• Sensory stroke Internal capsule lesion Numbness and sensory loss of one side of the body


• Dorsal column disease (eg. Diabetes, tabes dorsalis)

• Dorsal column pathways are affected

• Vibration, proprioception affected early in disease process


• Syringomyelia Spinal cord central canal lesion

Dissociated sensory loss

Temperature and pain sensations affected in early in disease process

Touch and dorsal column functions not affected

Documents

Y2 s1 sensory system