The somatic sensory system
 Sensory stimuli that reach the conscious level of
perception
 Somatic senses of touch, temperature, pain, itch
and proprioception.
 Special senses
Receptor react to specific forms of energy
 Classification of receptors based on:
 Stimulus type
 Location
 Structural complexity
Classification by Stimulus Type
 Mechanoreceptors — respond to touch, pressure,
vibration, stretch, and itch
 Thermoreceptors — sensitive to changes in
temperature
 Photoreceptors — respond to light energy (e.g.,
retina)
 Chemoreceptors — respond to chemicals (e.g.,
smell, taste, changes in blood chemistry)
 Nociceptors — (noci = harm) sensitive to paincausing stimuli (e.g. extreme heat or cold, excessive
pressure, inflammatory chemicals)
Classification by Location
1. Exteroceptors
 Respond to stimuli arising outside the body
 Receptors in the skin for touch, pressure, pain, and
temperature
 Most special sense organs
2. Interoceptors (visceroceptors)
 Respond to stimuli arising in internal viscera and blood
vessels
 Sensitive to chemical changes, tissue stretch, and
temperature changes
Classification by Location
3. Proprioceptors
 Respond to stretch in skeletal muscles, tendons,
joints, ligaments, and connective tissue coverings
of bones and muscles
 Inform the brain of one’s movements
Classification by Structural Complexity
1. Complex receptors (special sense organs – will be
discussed separately)
 Vision, hearing, equilibrium, smell, and taste
2. Simple receptors for general senses:
 Tactile sensations (touch, pressure, stretch,
vibration), temperature, pain, and muscle sense
 Simple receptors can be
 Unencapsulated (free)
 Encapsulated dendritic endings
Unencapsulated dendritic endings
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Found all over the body
Abundant in the ET and CT
Unmyelinated
Respond to temperature changes and pain and some to pressure
changes
 Cold response are more superficial and receptors that
respond to heat – deeper
 Temperature out of the range of the thermoreceptors will
activate nociceptors
Other receptors respond to itch (respond among other to
histamine) and light touch (detect changes in shape like
bending)
Encapsulated dendritic endings
 One or more fiber terminals of sensory neurons
enclosed in connective tissue capsule
 All are mechanoreceptors:
 Touch
 proprioceptors
Touch receptors
Meissner’s corpuscle
Ruffini's endings
Pacinian corpuscle
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Receptor type Structure
Location
Function
Meissner’s
Few spiral
corpuscle/tacti terminals
le corpuscle
surrounded by
Schwann cell
and CT capsule
Pacinian
Single dendrite
corpuscle/lame surrounded by
llated
capsule with up
corpuscle
to 60 layers of
collagen fibers
Ruffini’s
Receptor endings
corpuscle
enclosed by
flatten capsule
Between dermal
papillae in
hairless skin
Touch, pressure
Skin,
interosseous
membrane,
viscera
Deep pressure. Respond
only when the pressure
is first applied (on/off
pressure stimulation)
All skin, joint
capsule
Stretching of skin –
continuous pressure
Proprioceptors
Muscle receptors
Joint receptors
Tendon receptors
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Receptor type Structure
Location
Function
Muscle
spindles
Perimysium of
skeletal muscles
Detect muscle stretch and
initiate reflex that resist
stretch
In tendons close to
skeletal muscle
insertion
When tendon fibers are
stretched by muscle
contraction the nerve
endings are activated by
compression. When
activated, the contraction of
the muscle is inhibited
which causes relaxation
Monitor stretch in in the
articular capsule and
provide information on the
position and motion of the
joint (conscious)
Golgi tendon
organs
Joint
receptors
Spindle-shape
proprioceptors.
Modified skeletal
muscle fibers enclosed
in CT capsule
Proprioceptors.
Consist of bundle of
collagen fibers
enclosed in CT capsule
with sensory endings
coiling between and
around the fibers
Proprioceptors
Joints’ CT capsule
(combination of
several receptors types
– Pacinian, Raffini,
free ending and Golgi
tendon)
From Sensation to Perception
 Sensation: the awareness of changes in the internal
and external environment
 Perception: the conscious interpretation of those
stimuli
Processing of the sensory information
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Levels of neural integration in sensory systems:
1. Receptor level — the sensor receptors
2. Circuit level — ascending pathways in the CNS
3. Perceptual level — neuronal circuits in the cerebral
cortex
Processing at the Receptor Level
 The receptor must have specificity for the stimulus energy (as
previously discussed)
 The receptor’s receptive field must be stimulated
 The stimulus need to be converted to a nerve impulse
 Receptors have different levels of adaptation
 Information is encoded in the frequency of the stimuli – the
greater the frequency, the stronger is the stimulus.
The stimulation of the receptive field affects the
discharge of the sensory neurons
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The receptive field is the a specific physical area that, when
stimulated, affect the discharge of the stimulus.
Most receptive fields activation will result in message sending
– excitatory receptive field
Sensory receptors in the CNS can have inhibitory receptive
field (we will mention some examples later when talking about
vision).
Sensory neurons of neighboring receptive field may exhibit
 convergence (many sub-threshold stimuli to sum in the
postsynaptic neuron)
 Overlapping with another receptor’s receptive field –
sending 2 sensations from the same area (pressure and
pain)
The smaller the receptive field the greater the ability of the
brain to localize the site
Transduction allows sensory receptors to respond to
stimuli – converting sensation into a nerve impulse
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Sensory transduction – the process that enables a sensory receptor to
respond to a stimulus.
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The sensory transduction induces a receptor potential in the peripheral
terminal of the sensory neuron
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A receptor potential is a depolarization event that if brings the membrane
to a threshold, will become a nerve impulse (AP)
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The conversion from receptor potential to AP happens in the trigger zone
that can be in the first node of Ranvier.
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In some cases, the peripheral terminal is a separate sensory cell (ex. Photo
receptors). In this case there is an involvement of a synapse and NT
Receptors adaptation
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The duration of a stimulus is coded by duration of action
potentials.
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A longer stimulus generates longer series of APs.
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If a stimulus persists, some receptors adapt or stop responding
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There are 2 classes of receptors according to how they adapt:
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Tonic receptors – slowly adapting – they fire rapidly when first
activated, than they slow and maintain firing as long as the
stimulus is present (baroreceptors, proprioceptors)
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Phasic receptors – rapidly adapting receptors – rapidly firing
when first activated but stop firing if the strength of stimulus
remains constant
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This type of reaction allows the body to ignore information that
was evaluated and found not to be a threat to homeostasis (smell)
Receptors adaptation
 The mechanisms for receptors’ adaptation depends on
the receptors:
 Potassium channels in the receptor’s membrane
open causing the membrane repolarization
 Sodium
channels
inactivated
stopping
depolarization
 Accessory structure may contribute to decrease
sensitivity (muscle in the ear contract and limit the
movement of the auditory oscicles)
Processing at the circuit Level
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A sensory pathway is a set of neurons arranged in series.
The circuit level role is to deliver the impulses to the appropriate
region in the cerebral cortex.
The ascending tract typically consists of 3 neurons
First order neurons
 cell bodies in a ganglion (dorsal or cranial)
 Impulses from skin and proprioceptors to spinal cord or brain stem
to a 2nd order neuron
Second order neuron
 In the dorsal horn of the spinal cord or in the medulary nuclei
 Transmit impulses to thalamus or cerebellum
Third order neurons
 Cell bodies in the thalamus (no 3rd-order neurons in the
cerebellum)
 Transmit signals to the somatosensory cortex of the cerebrum
The neural pathway of nociception from primary afferent neurons (PANs) to the superficial
lamina in the dorsal horn of the spinal cord.
3rd order neuron
2nd order neuron
1st order neuron
White F A et al. PNAS 2007;104:20151-20158
©2007 by National Academy of Sciences
Processing at the circuit Level
 Impulses ascend in :
 Non specific pathway that in general transmit pain,
temperature and touch
 Give branches to reticular formation and thalamus on
the way up
 Sends general information that is also involved in
emotional aspects of perception
 Specific ascending pathways involve in more precise
aspect of sensation
Processing at the Perceptual Level
 Interpretation of sensory input occurs in the cerebral
cortex
 The ability to identify the sensation depends on the
specific location of the target neurons in the sensory
cortex not on the nature of the message (all messages
are action potentials)
The CNS integrate sensory information
 Most of the somatic sensory information enters the
spinal cord and travels via ascending pathways to the
brain
 Some information goes directly to the brain through
the cranial nerves
 Autonomic sensory information does not arrive
conscious perception
 Each area of the brain is processing different
information
Main Aspects of Sensory Perception
 Perceptual detection – detecting that a stimulus has
occurred and requires summation
 Magnitude estimation – the ability to detect how intense
the stimulus is
 Spatial discrimination – identifying the site or pattern of
the stimulus
 Feature abstraction – used to identify a substance that has
specific texture or shape
 Quality discrimination – the ability to identify
submodalities of a sensation (e.g., sweet or sour tastes)
 Pattern recognition – ability to recognize patterns in stimuli
(e.g., melody, familiar face)
Properties of the sensory system - summary
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Stimulus – works on a receptor
 The receptor is a transducer that converts the stimulus into a
change of membrane potential
The message from the receptor will be sent in the form of action
potential to the CNS
Stimuli that will reach the cerebral cortex will be come
conscious
Somatosensory information ascends the spinal column along
several pathways, which synapse at the midbrain &/or thalamus
before reaching the cortex
Sensory processes have different sub-modalities of
somatosensory information
Later stages of processing combine information across the submodalities, & with information from other senses
What sensory pathways we will discuss?
 Pathways for somatic perception that project to the
somatosensory cortex and cerebellum
 Somatovisceral sensations
 Touch receptors
 Temperature receptors
 Pain and itching receptors
 Hearing
 Equilibrium
 vision