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Somatosensory system
Peripheral
Somatosensory
neurons
Sensory receptors
Peripheral nerve axon
Peripheral nerve axon
Dorsal root ganglion
Spinal cord
Brain
Introduction
• Somatosensation:
– the sensory information from the skin and
musculoskeletal systems
• Skin :
– Superficial sensory or cutaneous
– Superficial sensory information :
- touch, pain, temperature
- Touch
- Superficial pressure and vibration
• Musculoskeletal :
– proprioception and pain
• Proprioception
– stretch of muscles, tension on tendons, position of joints,
and deep vibration
– both static joint position sense and kinesthetic sense,
sensory information about movement
• information in the somatosensory system proceeds
from the receptor through a series of neuron to the
brain
• Sensory information:
– nerve impulses generated from the original stimuli
• Sensation:
– awarness of stimuli from the senses
• Perception:
– the integration of sensation into meaningful forms
– (an active process of interaction between the brain and the
environment)
• Pathway convey Somatosensory information
share similar anatomic arrangements
• Receptors in the periphery encode the
mechanical, chemical or thermal stimulation
received into receptor potentials
• Action potential  peripheral axon soma in
dorsal root ganglion  proximal axon in
spinal cord via axons white matter brain
• The diameter of the axons, the degree of
axonal myelination, and the number of
synapse in the pathway determine how
quickly the information is processed
Peripheral somatosensory
neurons
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Sensory receptors
Somatosensory peripheral neurons
Cutaneous innervation
Musculoskeletal innervation
• Sensory receptors : located at the distal ends
of peripheral nerves
– mechanoreceptors, responding to mechanical
deformation of the receptor by touch, pressure,
stretch, or vibration
– chemoreceptors, responding to substances
released by cells, including damaged cells
following injury or infection
– thermoreceptors, responding to heating or cooling
– Nociceptors
– Tonic receptors
– Phasic receptors
• Somatosensory peripheral neurons
– cell body of most peripheral sensory
neurons : outside the spinal cord in the
dorsal root ganglion or outside the brain in
cranial nerve ganglia
• Two axons :
– distal axons conduct message from
receptors to the cell body
– proximal axons project from the cell body
into the spinal cord or brain stem
Classification of afferent axons
Factors that influence nerve conduction velocity
-Diameter of the axons(the larger is the faster)
-The degree of axonal myelination
Cutaneous innervation
- receptive field : the area of skin innervated by each neuron
• Receptive field: tend to be smaller
distally and larger proximally
• Skin sensations:
– Touch
– Pain
– Temperature
• Fine touch:
– superficial fine touch receptors : Meissner’s
corpuscles, Merkel’s disks, hair follicle
receptors
– subcutaneous fine touch : pacinian
corpuscles, Ruffini’s corpuscles
• Coarse touch:
-free endings throughout the skin (localized touch or
pressure and sensations of tickle and itch)
• Nociceptors
-Free nerve endings, responsive to stimuli that
damage or threaten tissue.
• Thermal receptors
-Also free nerve endings, respond to either warmth
or cold within the temperature
• Cutaneous receptors
-respond to touch, pressure, vibration, stretch,
noxious stimuli, and temperature
• Dermatome
-the area of skin innervated by axons from cell
bodies in a single dorsal root
Musculoskeletal innervation
• Muscle spindle
– Are embedded in skeletal muscle.
– the sensory organ in muscle – muscle fibers,
sensory endings, and motor endings
• Sensory endings
– stretch that is changes in muscle length and the
velocity of length change
• Quick and tonic stretch of the spindle
– type Ia afferents
• Tonic muscle stretch
– type II afferents
• Intrafusal and extrafusal fibers
– Intrafusal fibers contractile only at their
ends; the central region cannot contract
– nuclear bag fibers
• Have a clump of nuclei in the central region
– nuclear chain fibers
• Have nuclei arranged single fiber
• Two different sensory endings
– primary endings : phasic and tonic
(annulospiral ending)
– secondary endings : tonic(flower-spry
ending)
• Muscle length is signaled by type Ia and
II afferents reflecting stretch of the
central region of both types of intrafusal
fibers.
– Spindle sensitivity to changes in length is
adjusted by gamma static efferents.
• Velocity of change in muscle length is
signalized only by type Ia afferents,
– with information mainly from nuclear bag
fibers whose sensitivity is adjusted by
gamma dynamic efferents.
• Golgi tendon organs
- sensitive to very slight changes in the tension on a tendon
and respond to tension exerted both by active contraction
and by passive stretch of muscle
- Ib afferents
• Joint receptors
- respond to mechanical deformation of capsule and
ligaments
-ruffini’s endings(II) : extremes of joint range and respond
more to passive than to active movement
- paciniform corpuscles(II) : respond to movement
- free nerve endings(Aδ and C) : stimulated by inflammation
- ligament receptors(Ib) : similar to Golgi tendon organs and
signal tension
• Fully normal proprioception
– Requires muscle spindles, joint receptors,
and cutaneous mechanoreceptors.
Golgi tendon organs and muscle
spindles: how do they compare?
l
GTO
Muscle spindle
Location
Within tendon near the
Interspersed among
muscle-tendon junction in muscle fibers in parallel
series with muscle fibers
with the fibers
Stimulus
Increase in muscle tension Increase in muscle length
responses
1)Inhibits tension
development in streteched
muscle
2)Initiate tension
development in antagonist
muscles
1)Initiate rapid
contraction of stretched
muscle
2)Inhibits tension
development in
antagonist muscles
• Summary of the function of differentdiameter axons
– large-diameter afferents:
• muscle, tendons, and joints
– medium-sized afferents:
• joint capsule, muscle spindles, and cutaneous
touch, stretch, and pressure receptors
– smallest-diameter afferents:
• crude touch, nociceptive, and temperature
information
Pathway to the brain
• Conscious relay pathways
• Divergent pathways
• Unconscious relay pathways
Conscious relay pathways to
cerebral cortex
- touch
- proprioception
- pain
- temperature
Three projection neurons
Two routes
- discriminative touch and conscious proprioception
(dorsal columns, medial leminiscus system)
- discriminative pain and temperature (anterolateral
tracts=neospinothalamic and trigeminospinothalamic) :
fast pain
- To be aware of sensory information
- The information must reach the thalamus
Discriminative touch and conscious
proprioception pathway
• Discriminative touch:
– localization of touch and vibration and ability to
discriminate between two closely spaced points
touching the skin
• Conscious proprioception:
– awareness of the movements and relative position
of body parts
• Stereognosis:
– the ability to use touch and proprioceptive
information to identify an object
• Three-neuron relay
– the primary, or first-order, neuron conveys
information from the receptors to the
medulla
– the secondary, or second-order, neuron
conveys information from the medulla to
the thalamus
– the third-order, neuron conveys
information from the thalamus to the
cerebral cortex
Dorsal Column/Medial Lemniscus
System
• Fasciculus gracilis
– Axons from the lower limb occupy the more
medial section of the dorsal column
– Nucleus gracilis
• Fasciculus cuneatus
– Axons from the upper limb occupy the
more the lateral section of the dorsal
column
– Nucleus cuneatus
• Sensory information essential for
identifying objects by palpation,
distinguishing between closely spaced
stimuli, and controlling fine movement
and smoothness of movements travels
in the dorsal columns  medial
leminiscus VPLS1 cortex
• Tactile information from the face
– travels in the trigeminal nerve
thalamus(VPM)  sensory cortex
Discriminative pain and
temperature, coarse touch
• Anterolateral columns
– Pain
– Temperature
– Coarse touch
• Conveys less information than the dorsal column/MLS,
and thus coarse touch cannot be tested independently
when discriminative touch is intact.
• Is involved with pleasant touch and skin-to-skin contacts.
• Spinothalamic tract
• Temperature sensation
– warmth and cold
– free nerve ending of small myelinated and
unmyelinated neurons
– Aδ : cooling, C : heat
– proximal axon (segment of the spinal
cord) the second axon (cross the midline
and then ascend contralaterally to VPL
nucleus of the thalamus) sensory cortex
• Pain
– complex phenomenon; persistent pain affects emotional,
autonomic, and social functioning
– Pain is composed
• Protective and the emotional response
• Nociceptive
– Describes receptors or neurons that receive or transmit
information about stimuli that damage or threaten to damage
tissue.
– Several different pathways
• Neospinothalamic pain:
– fast pain (conscious relay pathway)
• Spinolimbic pain:
– slow pain (divergent pathway)
• Both pain:
– anterolateral section of the spinal cord
• Fast, localized pain
– the first-order neuron brings information
into the dorsal horn of the spinal cord
– the axon of the second-order neuron
crosses the midline and projects from the
spinal cord to the thalamus
– the third-order neuron projects from the
thalamus to the cerebral cortex
Divergent pathways
• Medial pain system
– Projection neurons synapse in medial
locations in the central nervous system
– Several pathways with variable numbers of
projection neurons, not a three neuron
pathway like fast pain.
– The information from the medial pain
systems is not somatotopically organized,
so slow pain cannot be precisely localized.
Divergent pathways
• Solw, aching pain:
– depends on a divergent network, not a
three-neuron pathway like fast pain
• First neuron:
– small, unmyelinated C fibers, sensitive to
noxious heat, chemical, or mechanical
stimulation
• Ascending projection neurons:
– wide-dynamic-range neuron
• The ascending axons reach the
midbrain, reticular formation, and
thalamus via three tracts
– Spinomesencephalic
– Spinoreticular
– Spinolimbic
• Spinomesencephalic tract
– nociceptive information to the superior
colliculus and to an area surrounding the
cerebral aqueduct, the periaqueductal gray
– involved in turning the eyes and head
toward the source of noxious input and in
activating descending tracts that control
pain
– periaqueductal gray is part of the
descending pain control system
• Spinoreticular ascending neurons:
– Reticular formation is a neural network in the
brainstem that includes the reticular nuclei and
their connections.
– Arousal, attention, and sleep/waking cycles are
modulated by the reticular formation.
– reticular formation  midline and intralaminar
nuclei of the thalamus
• Spinolimbic track
– midline and intralaminar nuclei of the
thalamuscerebral cotrex involved with emotions,
sensory integration, personality, and movement
• The slow pain pathways:
– produces automatic movements and
autonomic and emotional responses to
noxious stimuli
• Slow pain information from the face :
– trigeminoreticulothalamic pathway: C fibers
in the trigeminal N. reticular formation
intralaminar nuclei
Tempeature
• Reticular formation, nonspecific nuclei
of the thalamus, to subcortical nuclei,
and to the hypothalamus
• This temperature information that does
not reach conscious awareness
contributes to arousal, provides gross
localization, and contributes autonomic
regulation
Unconscious relay to the
cerebellum
• Information from proprioceptors and
information about activity in spinal
interneurons are transmitted to the
cerebellum via Spinocerebellar tracts
• Adjusting movements
High-fidelity pathways
 Two pathways
-arranged information to the cerebellar cortex
 posterior spinocerebellar pathway
-Transmits information from the legs and the lower half of the
body
-1st order; nucleus dorsalis
-2nd order; posterior spinocerebellar tract.
-the tract remains ipsilateral and projects to the cerebellar
cortex via the inferior cerebellar peduncle.
 cuneocerebellar pathway
-begin with primary afferents from the arm and upper half of
the body
High fidelity pathway
 Posterior spinocerebellar pathway :
• Transmit information from the leg and
the lower half of the body
• Dorsal column to the thoracic or upper
lumbar spinal cord(T1-L2)  nucleus
dorsalis  posterior spinocerebellar
tract
• Ipsilateral and projects to the cerebellar
cortex
 Cuneocerebellar pathway :
• Afferents from the arm and upper half
of the body
• Lateral cuneate nucleus (first – and
second-order neurons) in the medulla
 cuneocerebellar tract  ipsilateral
inferior cerebellar penduncle
cerebellar cortex
Internal feedback tracts
• the two internal feedback tracts monitor
the activity of spinal interneurons and of
descending motor signals from the
cerebral cortex and brain stem
• anterior spinocerebellar tract
– Transmits information from the
thoracolumbar spinal cord
• rostrospinocerebellar tract
Internal feedback tracts
 Anterior spinocerebellar tract :
• Transmit information from the thoracolumbar spinal
cord
• Lateral and ventral horns -> cross the opposite side
and ascend in the contralateral anterior
spinocerebellar tract to the midbrain -> cerebellum
via superior cerebellar penduncle
• Gets information from both sides of the lower body
• Bilateral projection : automatic coordination of lower
limb activities
 Rostrospinocerebellar tract :
• Transmits information from the cervical spinal
cord to the ipsilateral cerebellum and enters
the cerebellum via both the inferior and
superior cerebellar peduncles
• The anterior and rostrospinocerebellar tracts :
- apprise the cerebellum of the descending
commands delivered to the neurons that
control muscle activity via interneurons
located between descending motor tracts and
motor neurons that innervate muscles
- the activity of spinal reflex circuits
• Functions of spinocerebellar tracts
- not consciously perceived
- unconscious adjustments to movements and
posture
- internal feedback tract :
*convey descending motor information to the
cerebellum prior to the information reaching the
motor neurons
- high-fidelity pathway
*convey information spindles, tendon organs, and
cutaneous mechanoreceptors; cerebellum obtains
information about movements commands and
about the movements or postural adjustments
-cerebellum can compare the intended motor
output with the actual movement output
• Information in the spinocerebellar tracts
is from proprioceptors, spinal
interneurons, and descending motor
pathways.
• This information, which does not reach
conscious awarness, contributes to
automatic movements and postural
adjustments