The document summarizes somatic sensations and the somatosensory system. It describes: 1) The general organization of the somatosensory system including receptors, pathways to the brain, and processing of sensations. Mechanoreceptors, nociceptors, and thermoreceptors are discussed. 2) Classification of somatic sensations based on function, stimulus, and location. Tactile sensations are subdivided based on the specific receptors. 3) Pathways for transmitting sensory information including the dorsal column-medial lemniscal system and anterolateral system. 4) Organization of the somatosensory cortex and somatosensory homunculus representation of the body.

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Somatic Sensations: I. General Organization,
the Tactile and Position Senses
Dr Ambrish Gupta MD

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Sensory system
• For awraeness of external and internal env.
Set up : Receptors >sense stimulus [Sensation or
esthesia]>Transduction > A.P in sensory neuron >
Sensory impulse >
sensory nerve >dorsal root of Spinal chord
>Ascending neurons > sensory areas in brain >
Processing of impulse > perception - [Good touch
/Bad touch]> change in behaviour
• Motor system > efferent pathways through descnding
pathways > ventral horn >spinal motor nerves

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Types of esthesia
• Anesthesia – no sensation
• Paresthesia – abnormal or altered sensation
• Kinesthesia – sensation of movement
• Algesia – Increased sensation
• Analgesia – decreased sensation

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Sensations 3 categories
• Somatic sensations : Somesthesia – Receptors
present body surface , muscles, tendons ,
bones , joints – Touch, pain, temperature,
vibrations, joint movement
• Visceral Sensations : in the wall of viscera or
in connective tissue of the viscera . Colick pain
, mesentric pain , Angina , migraine
• Special Sensations : Vision, Audition ,
olfaction , gustation, Vestibular sensation

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Aspects of Sensation
• Perception ; Interpretation by higher centres
recognition , comparision, discrimination , integration
. Modulation – control of sensation by accentuating or inhibiting
depending on afferent pathway, CNS areas , Psychology
. Modality- tactile sensation by Touch
. Intensity – Degree of perception – Touch is rapidly adaptive
while Pain is non Adaptive
. Affect – emotional component of sensation Positive / Negative
. Acuity – Precision of stimulus localisation – depend on Conc
of receptors, receptive field size , size of cortical area
representing

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Sensory Receptors
• Defined as Afferent nerve endings which are
Transducers which convert various forms
energy in env into action potential in neurons
• Sense organs – Tissue containing Receptors
and non neural structures
• Adequate stimulus- Form of energy to which
receptor is most sensitive ex – Light for Rods
and cones

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Classification of somatic sensations
[ Based on Functions ]
• Exteroreceptive – Present on skin and subcutaneous
Tissue. Cutaneous sense organs
• Interoceptors – detect chnge in internal env e.g
Baroreceptors – sense B. P , Osmoceptors change in
osmolality of tissue fluids
• Proprioceptors – detect kinesthesia . Position of body in
space dynamic or static
• Teleceptors – receive stimuli far away from the body – e.g
Auditory

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Classification of somatic sensations
[ Based on Adequate stimulus ]
Mechano receptive - stimulated by mechanical
displacement.
– Tactile: touch, pressure, vibration, tickle, itch
– Proprioceptive: static position, rate of change
Thermoreceptive.
– detect heat and cold.
Nociceptive.
– detect pain and any factor that damages tissue
– A. A delta – fast sharp pain
– B. C polymodal thermal Chemical
Chemoceptors- taste, olfactory
Photoceptors- photon sensitive – rods & cones
.

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Classification of somatic sensations
[ Based on Location ]
Superficial : skin touch , pressure
Deep : Muscle , tendons , bones
Visceral : Visceral pain receptors

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Mechanoceptors
• 1. Expanded endings – merckel dic , ruffini
• 2. Encapsulated endings – Pacinian corpuscles
, meisners corpuscles , Krause end bulbs
• 3. Naked Nerve Endings

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Tactile receptors – small field
Meissner corpuscles
• Location: non-hairy skin close to surface
(fingertips, lips, eyelids, nipples
and external genitalia).
• Function: motion detection, grip control
• Stimuli: skin motion, low frequency vibration
• Adaptation: rapid adaptation
• receptive field: 22 mm2
• type Aβ nerve fibers
Merkel discs
• Location: tip of epidermal ridges iggo dome
• Function: form and texture perception
• Stimuli: edges, points, corners, curvature
• Adaptation: slow adaptation
• receptive field: 9 mm2
• type Aβ nerve fibers
Meissner corpuscles Merkel discs

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Tactile receptors – large field
Pacinian corpuscle. (~ 1mm) Largest Onion like 20-
70 lamillae
• Location: dermis and deeper tissues
• Function: perception of distant events
through transmitted vibrations;
tool use
• Stimuli: vibration, fine touch, pressure
(250 Hz is optimal)
• Adaptation: very rapid adaptation
• receptive field: entire finger or hand
• type Aβ nerve fibers
Ruffini corpuscle[ Smallest ]
• Location: dermis
• Function: tangential force; hand shape;
motion detection
• Stimuli: skin stretch
• Adaptation: slow adaptation
• receptive field: 60 mm2
• type Aβ nerve fibers
Pacinian corpuscles
Fig. 48-3, Guyton and
Hall Textbook of Medical
Physiology, 13th edition

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• Krause end Bulb:
- Rapidly adapting Mechanoceptor in dermis
-Encapsulating the branched unmylienated
endings of Abeta nerves
- Sense touch and pressure
Golgi Mazzoni Corpuscle – 10-15 lamellae
- Presnt in tendons and muscles
- Sense of position

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Hair Follicle Endings
• Afferent fibre ending in hair follicles
• By bending a hair without touch can cause
sensation
- Simple Hair Follicle – without erectile tissue ,
unmylienated nerves . Rapidly adaptive
- Sinus Hair follicle – Large hairs with erectile
tissue – both slow and rapid adaptaion –
vibrissae or tactile hairs
- Nonsinus Hair follicle – Spray like terminals –
in face slow adaptive

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Free nerve endings
Free nerve endings (Myelinated).
• Location: surface of body and elsewhere
• Function/stimuli: pain, temperature
• Adaptation: slow adaptation
• type Aδ nerve fibers
Free nerve endings (unmyelinated).
• Location: surface of body and elsewhere
• Function/stimuli: pain, temperature, itch
• Adaptation: slow adaptation
• type C
Fig. 48-13, Guyton and Hall Textbook
of Medical Physiology, 13th edition

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Pathways for the Transmission of
Sensory Information
• Almost all sensory information enters spinal cord through dorsal roots
of spinal nerves.
• Two pathways for sensory afferents.
– Anterolateral system
– Dorsal column-medial lemniscal system
Anterolateral
system
Dorsal column-medial
lemniscal system
Purves. Neuroscience. 5th ed, 2012.
Figure 9.1B

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3 sets of neurons
• 1st Order : Primary Afferent receptor > Dorsal
root Ganglion DRG >dorsal Horns
• 2nd Order : Dorsal Horns > contralateral [ At
level of spinal chord / medulla ] Thalamus
• 3rd Order : SP nuclei in thlamus [ MAIN
SENSORY RELAY CENTRE ]> sensory
cortex
• 4th Order: sensory cortex > cortical sensation
association area

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Dorsal Column-medial lemniscal System
• Contains large myelinated nerve
fibers (30-110 m/sec).
• Three neurons to sensory cortex /
decussates in medulla oblongata
• High degree of spatial orientation
maintained throughout the tract .
• Transmits information rapidly and
with a high degree of spatial fidelity
(ie. discrete types of mechanoreceptor
information).
• Transmits Fine touch, vibration,
position, fine pressure, Tactile
localisation , discrimination and
stereognosis Figure 48-3

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DORSAL COLUMN PATHWAYS
[ Tract of Goll & Burdach]
• 1st Order :Lower extrimity > medial gracile fasciculus
+ upper extremity > lateral Cuneate fasciculus >
nucleus gracilis / Nucleus cuneatus .[Dorsal Column
nuclei in medulla ]
• 2nd Order : DCN> fibres cross midline to opp side
[decussate]> ascend in medial leminiscus >
contralateral Thalamic nuclei
• 3rd Order VPL nucleus in thalamus > project to
somtosensory cortex postcentral gyrus [Below
sindoor area ] area 3,1,2. lower limbs & genitals in
medial central aprt and upperlimb and fingers
laterally

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Dorsal Column nuclei
in Medulla Oblongata
• Somatotropic Organization – Face hands placed
laterally and trunk & hind limbs medially . 2 zones
• 1. Cluster – cells in cluster from where 2nd order
neurones decussates to opposite side and inform
contralateral thalamus and cortex
• 2. Noncluster reigon – more superior part of
thalamus . Inputs from desecnding motor tracts .
Project to nonthalamic araes like cerebellum, tectum,
pretectum , inferior olive , red nucleus .
Function :direct sensory input to motor areas of brain
for immediate regulation of movements

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The Anterolateral System
• Contains smaller myelinated and
unmyelinated fibers for slow
transmission (0.5-40 m/sec).
• three neurons to sensory cortex /
decussates in spinal cord
• low degree of spatial orientation.
• transmits a broad spectrum of
modalities.
• pain, thermal sensations, crude
touch and pressure, tickle and itch,
sexual sensations.
Figure 47-13

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Anterolateral System
• 1. Anterior Spinothalamic tract – Crude Touch
• 2. Lateral Soinothalamic Tract – Pain , Temperature
• 1st Order Neuron – afferent from nocio, thermo,
mechanoceptors >DRG>Dorsal horns
• 2nd Order –cell bodies Dorsal Horns in Laminae 1,11
and V > axon cross the midline in the same segment >
ascend up in the opp side of anterolateral funiculus >
Thalamus .fibres of crude touch anteriorly- Ventral
STT. Pain temp fibres laterally Lat STT
• 3rd Order – VPL, Midline and Intralaminar Nuclei
>area 3, 1 2 of sensory cortex

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Clinical Significance of dorsal
column sensation
• Thermoanaesthesia
• Hypesthesia
• Complete Anesthesia
• Dissociated Anesthesia
• Hemianesthesia
• Hyperesthesia
• Alloesthesia
• Parasthesia
• Agraphesthesia
• Astereognosis
• Tabes Dorsalis
• Romberg sign sensory ataxia

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Dermatomes
Dermatome – area of skin supplied
by sensory neurons that arise
from a spinal nerve ganglion.
Clinical significance
• Localizing cord lesion
• Viruses such as varicella zoster
hibernate in ganglia causing rash in
associated dermatome.
• Referred pain (discussed later)
Figure 48-14
p. 22,
Gray’s
Atlas of
Anatomy,
2nd edition

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Years or decades after a chickenpox infection, the virus
(varicella zoster virus) may break out of nerve cell bodies and
travel down nerve axons to cause viral infection of skin
associated with nerve.
The rash occurs in the dermatome of the infected nerve cell.
Shingles follows the dermatome

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The Somatosensory Cortex
• Located in the postcentral gyrus.
• Highly organized distinct spatial
orientation.
• Each side of cortex receives
information from opposite side of
body.
• Unequal representation of the body.
– lips have greatest area of
representation followed by face
and thumb.
– trunk and lower body have least
area of representation.
Figure 48-6

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Figure 47-7
Sensory homunculus
Homunculus – (latin) little human

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Brodmann areas
Somatosensory area 1 (primary somatosensory area)
Brodmann’s areas: 1, 2, 3
Somatosensory association area
Brodmann’s areas: 5,7
Cytoarchitectural organization of neurons (cell size, packing density,
lamination) that Brodmann observed in cerebral cortex using Nissl stain
(1909).
Figure 48-5 Figure 48-6

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Lesions of somatosensory cortex
• Destruction of somatosensory area I results in:
– loss of vibration, fine touch, and proprioception.
– discrete localization ability.
– inability to judge the degree of pressure.
– inability to determine the weight of an object.
– inability to judge texture.
Also:
– Hemineglect (unilateral neglect, hemispatial
neglect or spatial neglect): patients are unaware
of items to one side of space.
– Astereognosis: inability to recognize objects by
touch
– Agraphesthesia: a disorientation of the skins
sensation across its space (e.g., hard to identify
a number or letter traced on the hand)
Figure 47-7

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Somatosensory association area
• Areas 5 and 7 in parietal area.
• Receives input from somatosensory
cortex, ventrobasal nuclei of thalamus,
visual and auditory cortex.
• Function is to decipher complex sensory
associations.
• Loss of these areas
• inability to recognize complex
objects
• neglect of contralateral world and
even refusal to acknowledge
ownership of contralateral body.
Figure 47-6
Figure 48-5

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Incoming signals
Diffuse lower input
Related brain areas
To brainstem and cord
To thalamus
Figure 48-8
Structure of Cerebral Cortex

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Cellular Organization of the Cortex
• Six separate layers of neurons with layer I near the surface of
the cortex and layer VI deep within the cortex.
• Incoming signals enter layer IV and spread both up and down.
• Layers I and II receive diffuse input from lower brain centers.
• Layers II and III neurons send axons to closely related
portion of the cortex presumably for communicating between
similar areas.
• Layer V and VI send axons to more distant parts of the
nervous system, layer V to the brainstem and spinal cord, layer
VI to the thalamus.

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Cellular Organization of the Cortex
(cont’d)
• Within the layers the neurons are also arranged in
columns.
• Each column serves a specific sensory modality
(i.e., stretch, pressure, touch).
• Different columns interspersed among each other.
– interaction of the columns occurs at different cortical
levels which allows the beginning of the analysis of the
meaning of the sensory signals.

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Transmission of single-point stimulus
on skin to cerebral cortex
Figure 48-9

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Lateral inhibition improves two point
discrimination
• Lateral inhibition is the capacity
of an excited neuron to reduce
activity of neighboring neurons; it
improves degree of contrast
– Occurs at every synaptic level
(for dorsal column system):
dorsal column nuclei,
ventrobasal nuclei of
thalamus, cortex
Lateral
inhibition
present
Figure 48-10