Somatic Sensory System
Sensation arising from skin muscle
joints
Allow you to survive in your
environment and make appropriate
motor responses
Somatic Sensation
• The ability to feel your physical
environment, to ache, feel temperature,
pain, to know where your body position is
• Pressure, position of joints/muscles,
temperature, distension of bladder, stomach
• Over-stimulation of body can be damaging
Sensory Receptors
• Spread throughout body
• Senses 4 types of information aka sensory
modality
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–
–
–
Touch, pressure, vibration
Body position, propioceptive receptors
Temperature
Pain-Nociception
Encoded Information
•
•
•
•
Intensity
Duration
Position
Direction
Propioception
• Sensory Information regarding joint and
limb position
• Allow you to know your body position
without looking in the mirror
• Muscle spindles- intrafusal fibers and
• Golgi Tendon organs are specialized
structures that are innervated by DRGNs
and send information to the spinal cord
Touch
• Skin: largest sensory organ
– Epidermis (outer layer) & dermis (inner layer)
• Sensitive to raised dot: 0.006mmH x
0.04mmW; braille dot is 167 times bigger
• Skin receptors
– Hairy & glabrous (hairless)
12.1
Mechanoreceptors
• Found in skin for sensing contact with
physical world
• Found in bladder, blood vessels, heart
digestive organs and teeth to sense pressure
• Mechanoreceptors are innervated by
myelinated axons
• Axons have mechanosensitive ion channels
gated by stretch & tension changes
Types of Mechanoreceptors Found in Skin
• Pacinian corpuscle
– Found in dermis
• Meissner corpuscle
– Found in ridges of
glabrous skin
• Merkel’s disc in
epidermis
• Rufini endings
– Found in hairy &
glabrous skin
Primary Afferent Axon
• Axons of varying diameters with soma in
DRG and enter Spinal cord through dorsal
roots into dorsal horn
• Different diameter axons carry different
types of somatosensory information
• Project locally in spinal cord and have long
ascending branches to contact secondary
somatosensory axons
Receptive Field
• Area of skin that is monitored by a single
mechanoreceptor
– Meissner & merkel have small RF (2-3mm)
– Pacinian & ruffini have large RF (entire finger/
½ palm)
F 12.2
Dermatomes
• The area of skin innervated by the right and
left dorsal roots (1st order neuron) of a
single spinal segment
• When mapped, dermatomes form sets of
bands representing surface of body
innervated by axons in one level of sc.
Add F 12.10-12
2-point discrimination
• Ability to discern 2 closely position points
as 2 rather than 1.
• Varies 20 fold throughout body
• Fingertips have highest resolution
– Due to high density of mechanoreceptors
– Receptor subtypes with small receptive fields
– More cortical neurons dedicated to deciphering
sensory information
F 12.6
First and Second Order Neurons
• Primary and secondary somatosensory or
afferent neurons
– The primary neuron has the sensory receptor
– The secondary neurons gets information from
the primary and can project information or
modify locally activity of primary neurons.
Second order Neurons aka
Interneurons
• Neurons that receive synaptic input form
DRG neurons
• Reside in dorsal horn and trigger reflex
responses
• Also ascend to brainstem and thalamus
• Also reside in brain stem and are involved
in perception
Add 12.13
Dorsal Column-Medial
Leminiscus Pathway
• Touch/vibration & position (proprioception)
info travels to brain separate from
pain/temperature
• Afferent/central axon of large sensory (AB)
fibers ascend ipsilaterally in dorsal columns
with tactile info and limb position info
• DC also have 2nd order ascending axons
from dorsal horn neurons
Dorsal column nuclei
• Axons terminate in DCN in medulla
• Then decussate (cross) and ascend as
medial lemniscus tract through pons &
midbrain to synapse in ventral posterior
nucleus (VPN) of thalamus
• VPN axons then project to primary
somatosensory neurons in parietal cortex
(S1)
Somotopic Organization
Somotopy
• Mapping of body areas sensation onto the
cortex
• Somotopic map called homunculus that
shows that the largest number of neurons in
S1 receive sensory information from hand
and mouth
Somatosensory Cortex
• In Parietal lobe, posterior to central sulcus
• Carries on higher order processing of sensory
information. Called S1
• Receives synaptic input from VP nucleus of
thalamus
• Respond to somatosensory info
• Lesions in S1 impair somatic sensation
• Electrically stimulate S1 and you “feel” a
sensation on the appropriate body part
S1 Cortex
• Reciprocal (bidirectional) connections
between cortical areas
• Association pathways
• Restriction of information: some cortex
areas specialize in decoding texture, size &
shape
• Thalamic input is to cortical layer IV which
send axons to other layers in same area
S2 and Parietal Posterior Cortex
• S2 is lateral to S1 and is association area
• PPC is posterior to S1 and is involved in
perception/recognition of sensation
• Neurons in S2 and PPC have complex
receptive fields which can include sensory
information as well as attention and visual
and movement planning.
Posterior Parietal Cortex
• Injury causes neglect syndrome: Do not
recognize body part as your own so you do
not dress it, wash it.
• Agnosia: Inability to recognize objects
including your own body parts.
• Astereoagnosia: inability to recognize
something by touch, but recognized by sight
Trigeminal Touch Pathway
• Two trigeminal nerves CN5
• Each divided into 3 PNs that innervate face,
mouth and anterior 2/3 tongue & dura mater
•
Sensory CNs
• CN 7 facial, 9 glossopharyngeal, 10 vagus
• Large diameter axons carry tactile info from
skin mechanoreceptors
• Synapse on ipsilateral trigeminal nucleus in
pons
• Decussate and project to medial VP nucleus
of thalamus
• Project to somatosensory cortex
Add 12.15-19