Lecture 3: Somatosensory System Flashcards
Somatosensory system:
Transmission of somatic (i.e.
body) sensations from body
receptors to CNS.
in the somatosensory system, the information is transmitted to (5) :
In order:
1. Spinal cord
2. Brainstem
3. Cerebellum
4. Thalamus
5. Cerebral cortex
three important elements of the somatosensory system:
- Sensory receptors (reception)
- Sensory pathways (transmission)
- Sensory centers (processing)
Mechanoreceptive sensations stimulated by:
mechanical
displacement (i.e. tissue deformation)
What are the two subtypes of mechanoreceptive sensations?
(1) Tactile sensations (skin)
(2) Proprioceptive (position) sensations (muscle and joints)
What are the different tactile sensations (skin) (4):
- touch
- pressure
- vibration
- tickle and itch
Proprioceptive (position) sensations (muscles & joints) (2):
- Muscle condition sense (e.g. muscle contracted, stretched, relaxed)
- Joint position sense
Nociceptive sensations (2):
- Detect pain
- Stimulated by any factor that damages tissue.
Thermoreceptive sensations:
- Detect temperature (heat and cold).
- Stimulated by change in temperature.
What are the three subdivisions of somatic sensations?
(1) mechanoreceptive sensations (tactile + proprioception)
(2) nociceptive sensations
(3) thermoreceptive sensations
Somatosensory receptors are classified based
on:
the type of sensation they detect
Mechanoreceptors detect:
Tissue deformation
Skin tactile receptors (4) (1/2 mechanoreceptors):
- Free nerve endings
- Expanded tip receptor (e.g. Merkel’s discs)
- Encapsulated endings (e.g. Meissner’s
corpuscle, Pacinian corpuscle, Krause’s
corpuscle Ruffini’s end-organ…) - Hair end-organ (e.g. tactile hair)
Muscle receptors (1/2 mechanoreceptors):
- Muscle spindles
- Golgi tendon receptor
Nociceptors detect:
pain (i.e. tissue damage)
What are the somatosensory receptors of nociceptors:
Pain receptors: specialized free nerve
endings
Thermoreceptors detect :
change in temperature
What are the somatosensory receptors of thermoreceptors:
Specialized free nerve endings:
-Cold receptors
-Warmth receptors
Resting membrane potential:
At resting state (in absence of stimulus).
Action potential:
Results from transient changes in the resting
membrane potential of a stimulated neuron.
Electrical signal that travels along axons.
Long range transmission of information within the nervous system
Receptor potential:
Result from transient changes in the resting
membrane potential in the receptor of sensory
neurons by external stimuli
Synaptic potential:
➢ Results from the communication between neurons at synaptic contacts
➢ Recorded at the post-synaptic neuron by the stimulation of the pre-synaptic neuron
Receptor potential:
When a stimulus (e.g. touch, pain…) excites the receptor, the electrical
potential of the receptor membrane changes and creates a receptor
potential (like EPSP). Stimulation causes opening of ion channels (Na+,
Ca2+), depolarization of the receptor membrane.
Mechanisms of stimulation of the receptor (4):
- Mechanical deformation, which stretches the membrane (e.g.
mechanoreceptors) - Application of chemicals (e.g acid, alcohol, drugs…)
- Change in temperature (e.g. thermoreceptors)
- Tissue damage (e.g. pain receptors)
How does transduction of Sensory Stimuli occcur?
The receptor potential rises the membrane potential of the nerve
fiber attached to the receptor.
If the threshold is reached, an
action potentials appear in the nerve fiber.
Action potential travel through the nerve fiber to reach the brain
(i.e. AP transmits encoded sensory information to the brain).
Transduction of Sensory Stimuli: The greater the intensity of the stimulus, the
greater the receptor potential amplitude, and the
greater the RATE of action potential generation.
Adaptation of receptors:
When a continuous sensory stimulus is
applied, the receptors respond at a high impulse rate at first stimulus
and then progressively slow down their rate of response until many of
them no longer respond
Common examples of adaptation of the receptor:
-Adaptation to cold water
-Feeling of clothes on skin
Speed of adaptation varies with:
Type of receptors
Slowly adapting receptors:
decrease their rate of response ___
slowly
Slowly adapting receptors (3):
(1) Pain receptors
(2) Merkel’s discs
(3) Ruffini’s organ
Rapidly adapting receptors:
decrease their rate of response __
rapidly
Rapidly adapting receptors (4):
- Thermoreceptors
- Pacinian corpuscle
- Hair receptor
- Meissner’s corpuscle
When a continuous sensory stimulus is applied, the receptors
respond :
At a high impulse rate at first stimulus and then
progressively slow down their rate of response until many of them no longer respond.
Rate of adaptation varies with
type of
receptor
Pacinian corpuscle adapts
very rapidly (< second)
Hair receptor adapts:
rapidly (within
a second)
Joint capsule and muscle spindle receptors adapt
slowly (> second)
Adaptation of Receptors: Mechanisms of adaptation: 2 ways:
(1) Accommodation of the receptor
(2) Accommodation of the nerve fiber
Accommodation of the receptor:
The receptor potential
appears at the onset of the stimulus (e.g. compression) but disappears rapidly even though the stimulus continues (i.e. receptor still under compression) (e.g. squeezed ball)
Accommodation of the nerve fiber:
Decrease in the firing
rate of APs that is caused by an inactivation of the Na+
channels (i.e. saturation of ions channels).
There are __ different types of tactile receptors:
SIX
There are six different types of tactile receptors:
(1) Free nerve endings
(2) Meissner’s corpuscles
(3) Merkel’s discs
(4) Hair end-organ
(5) Ruffini’s end-organ
(6) Pacinian corpuscles
Free nerve endings (1/6 types of tactile receptors):
-Found everywhere in the skin (mostly at
SUPERFICIAL layers)
- Connected with a SMALL UNMYELINATED sensory nerve fiber
-Detect CRUDE TOUCH and PRESSURE sensations
-SLOWLY ADAPTING RECEPTORS
Meissner’s corpuscles:
- Elongated ENCAPSULATED nerve endings
- Connected with a LARGE MYELINATED sensory nerve fiber
- Located in the SUPERFICIAL LAYERS of the skin (non-hairy part)
- Detect FINE TOUCH (discriminative, movement of the objects on the surface of the skin) and LOW FREQUENCY VIBRATION
-RAPIDLY ADAPTING RECEPTORS
Merkel’s discs:
(1) Located in the SUPERFICIAL layers of the skin (epidermis)
(2) Detect TOUCH and LIGHT PRESSURE
(3) SLOWLY ADAPTING RECEPTORS
Hair end-organ (free nerve endings of hair root):
- In contact with the ROOT of the SKIN HAIR
- Detects HAIR MOVEMENT
- RAPIDLY ADAPTING RECEPTORS
Ruffini’s end-organ:
-ENCAPSULATED endings located in the DEEPER LAYERS of the skin (dermis)
- Detects HEAVY and PROLONGED TOUCH and DEEP PRESSURE
signals
- SLOWLY ADAPTING RECEPTORS
Pacinian corpuscles:
-ENCAPSULATED endings located in the DEEPER LAYERS of the skin (dermis)
- Detect TISSUE VIBRATION or other RAPID CHANGES IN THE MECHANICAL STATE OF THE TISSUES (deep pressure,
stretch) - RAPIDLY ADAPTING RECEPTORS
Pain sensation occurs when:
Tissue is being damaged (e.g. injury, inflammation)
Pain sensations are a __ for the body
protective mechanism –> causes individual to remove painful stimulus
What are the two types of pain:
(1) Fast pain (pricking pain)
(2) slow pain (aching pain)
Fast pain (pricking pain):
Felt within less a second of the stimulus and is sharp in character (e.g. hit a finger or toes by a hard object)
slow pain:
begins after a second or more and is throbbing or aching in nature (e.g. headache, tooth, pain)
Pain receptors are:
specialized free nerve endings
pain receptors are __ in many locations of the body
widespread
Pain receptors are widespread in many locations of the body:
(1) Superficial layers of the skin
(2) Internal tissues (e.g. tooth, stomach, …)
(3) Bones, joints and muscle surfaces
(4) Arterial walls (blood)
There are no pain receptors in:
The brain
Pain receptors can be stimulated by:
(1) Mechanical (tissue damage, strong stretch)
(2) Chemical ( alcohol, bradykinin, potassium ions, lactic acid…)
(3) Thermal (> 45 celscius)
(4) Inflammation
Pain receptors are __ adapting receptors
slow
for pain receptors, more stimulus =
more response
pain receptors are a protective mechanism to :
remove painful stimulus
Thermal sensations can be:
graduated
Thermal gradations are discriminated by:
3 types of sensory receptors (specialized free nerve endings)
What are the 3 types of sensory receptors (specialized free nerve endings) that discriminate thermal gradations:
(1) Cold receptors: sensible to cold temperatures
(2) Warm receptors: sensible to hot temperatures
(3) Pain receptors: sensible to extreme temperatures
Regarding thermal sensations, there are more _ than _ receptors
There are more cold receptors than warm receptors
Sensations of Freezing cold and burning hot are:
the same sensation because of the stimulation of pain receptors
Thermal receptors are:
Rapidly adapting receptors
what is an example of thermal receptors being rapidly adapting receptors?
swimming in cold water
The somatosensory receptor is:
The peripheral end-
process of the sensory neuron (pseudo-unipolar)
that transmits the sensory information to the CNS
The cell body of the sensory neuron is located in:
sensory ganglion (spinal cord).
Sensory ganglion (spinal cord)
Its specific sensory nerve (labeled line
principle).
Transmission of Somatosensory information
from the Receptor to the Brain: Velocity (speed) of transmission depends on:
The fiber diameter and the
myelinization of the fiber
The larger the nerve fiber diameter:
The faster the rate of transmission of
the signal and the highest the quality of information (high discrimination).
Velocity of transmission varies from
slow (0.5 m/s) to fast (120 m/s)
Nerve fiber classification (2):
(1) Type A - myelinated fibers of varying size
(2) Type C - unmyelinated fibers: small size - slow transmission speed
What are the four subtypes of Type A nerves
Muscle spindle and Golgi tendon organ
transmit signals through:
Type A-a myelinated nerve fibers (e.g. proprioceptive sensation), fast transmission, high discrimination.
Meissner’s corpuscles, hair receptors,
Pacinian corpuscles and Ruffini’s organs
transmit signals through:
Type A-β & A y -
myelinated nerve fibers (e.g. tactile sensations), fast transmission, high
discrimination
Free nerve endings receptors transmit signals through (2):
(1) Type A-delta, myelinated nerve fibers (e.g.
fast (pricking) pain, cold sensations)
(2)Type C, unmyelinated fibers (e.g. slow
(aching) pain, cold, warmth, crude touch &
tickle sensations), slow transmission, low
discrimination
The more critical the information the _ the rate of transmission
The more critical the information the faster
the rate of transmission.
Sensory fibers from the body (except the face) transit to the brain through the
spinal cord
Segmental distribution
each part of the body is connected to a specific
portion of the spinal cord
Sensory fibers enter the spinal cord through
Dorsal root ganglion
Where are the cell body of sensory fibers located?
In the dorsal root ganglion
Two pathways for sensory information:
(1) Dorsal Column-Medial Lemniscal System
(2) Antero-Lateral System
Sensory fibers in both pathways for sensory information (Dorsal column-medial lemniscal system and anterao-lateral system) :
Decussate
Decussation:
(i.e. cross the midline to the contralateral
side): sensory information in one side of the body will be transmitted to the opposite
side of the brain
In both somatosensory pathways (dorsal column-medial lemniscal system and antero-lateral system):
Three order neurons make the relay to transmit sensory stimulation from the receptor
to the final point in the cerebral cortex
The Dorsal Column -
Medial Lemniscal Pathway transmits:
tactile sensations (touch, vibration,
fine pressure) and Proprioceptive sensations
(e.g. muscle stretch, joint position).
The Dorsal Column -
Medial Lemniscal Pathway, signal originates from:
Tactile receptors (e.g.
meisner’s, Pacinian, Ruffini’s, Merkel’s) or
Proprioceptive receptors (e.g. muscle spindle,
Golgi tendon).
The Dorsal Column -
Medial Lemniscal Pathway uses:
Large myelinated nerve fibers, such as A-a
A-β fibers for fast signal transmission, and
with high degree of discrimination (e.g. fine
touch)
The Dorsal Column -
Medial Lemniscal Pathway : 1st order neuron:
1st order neuron has cell body in the DORSAL ROOT GANGLION of the spinal cord. It makes
synapse with 2nd order neuron at the DORSAL COLUMN NUCLEI (Cuneate, Gracial) in the
MEDULLA.
The Dorsal Column -
Medial Lemniscal Pathway : 2nd order neuron:
-DECUSSSATES at the level of
MEDIAL LEMNISCUS (medulla), and then forms the
MEDIAL LEMNISCUS TRACT (up to the thalamus)
-makes synapse with the 3rd
order neuron at the THALAMUS (ventral posterior
lateral nucleus).
The Dorsal Column -
Medial Lemniscal Pathway : 3rd order neuron:
Projects into the primary
somatosensory cortex.
The Antero-Lateral Pathway
26 (Spinothalamic Tract) transmits:
a broad spectrum of sensory
modalities (e.g. pain, thermal sensations,
crude touch & pressure, tickle and itch,
sexual sensations)
The Antero-Lateral Pathway
(Spinothalamic Tract) transmits:
A broad spectrum of sensory
modalities (e.g. pain, thermal sensations, crude touch & pressure, tickle and itch,
sexual sensations)
The Antero-Lateral Pathway
(Spinothalamic Tract) signal originates from:
FREE NERVE ENDING RECEPTORS (e.g. pain receptors, thermal
receptors, skin free nerve endings)
The Antero-Lateral Pathway
(Spinothalamic Tract) uses:
smaller myelinated (A-δ) and
unmyelinated (C) fibers
The Antero-Lateral Pathway
(Spinothalamic Tract) uses smaller myelinated (A-δ) and
unmyelinated (C) fibers for:
slow transmission, and low degree of discrimination (e.g. vague types of sensory
information, aching pain)
The Antero-Lateral Pathway
(Spinothalamic Tract): 1st order neuron:
Has cell body in the DORSAL ROOT of the spinal cord.
It makes synapse with 2nd order neuron at the SUBSTANTIA GELATINOSA in the dorsal horn of SPINAL CORD
The Antero-Lateral Pathway
(Spinothalamic Tract): 2nd order neuron:
-Decussates at the level of the
ANTEROLATERAL QUADRANT in the SPINAL CORD, and then forms the ANTEROLATERAL (SPINOTHALAMIC) tract
(up to the thalamus).
- makes synapse with the 3rd
order neuron at the thalamus (ventral posterior lateral nucleus)
The Antero-Lateral Pathway
(Spinothalamic Tract): 3rd order neuron:
Projects into the primary
somatosensory cortex.
Somatosensory information from the
face & head is conveyed through :
the
trigeminal nerve (a cranial nerve).
The trigeminal nerve enters into the
CNS trough the
brainstem (not spinal
cord).
Somatosensory Pathway
of the Face: 1st order neuron:
has cell body in the
trigeminal ganglion. It makes synapse
with 2nd order neuron at the Principal
nucleus of trigeminal complex
(brainstem).
Somatosensory Pathway
of the Face: 2nd order neuron:
-decussates at the level
of Medial lemniscus and form
Trigeminal lemniscus tract (up to the thalamus)
-makes synapse with the
3rd order neuron at the Ventral posterior
medial nucleus of thalamus
Somatosensory Pathway
of the Face: 3rd order neuron:
projects into the
primary somatosensory cortex.
The primary somatosensory cortex is the __ of the somatosensory pathways
TERMINAL STATION
The primary somatosensory cortex is located in the :
Post-central gyrus (the
parietal lobe)
Primary Somatosensory Cortex: Highly organized with distinct
Spatial orientation (i.e. each area of the cortex is devoted to a given body region, map of the sensor representation)
Primary Somatosensory Cortex: Each side of the cortex
receives information from the opposite side of
the body
Penfield’s Homunculus:
Unequal representation
of the body
Penfield’s Homunculus:
unequal representation
of the body: which have greatest vs least area of representation?
Lips have greatest
area of representation
followed by the face,
thumb, fingers, hand…
Trunk and lower body
have the least area of
representation.
More a region of the
body has receptors, the
greatest is
its area of
representation in the
somatosensory cortex
Somatosensory Cortex is
composed of
six cellular layers
(cytoarchitectonic organization).
Layer IV in the somatosensory cortex is:
The input layer (i.e.
3rd order neurons terminate in layer IV)
Within the layers in the somatosensory cortex, the neurons are also arranged in:
vertical columns
In the primary somatosensory cortex, each vertical column is:
Reserved for a specific sensory modality (i.e, pain, touch, pressure, stretch)
In the primary sensory cortex, the different columns interact between each other which:
allows
the beginning of processing
sensory signals (i.e. giving
meaning to sensory information)
The somatosensory association
cortex is located
behind the
primary somato-sensory cortex in
the parietal lobe.
The somatosensory association cortex receives input from:
primary somatosensory cortex,
ventro-basal nuclei of the thalamus,
visual and auditory cortex.
The somatosensory association function:
is to decode
somatosensory meaning
(information processing) and
integrate it with other sensory
information (e.g. visual, auditory)
loss of somatosensory area results in:
various symptoms:
- Inability to recognize complex objects.
- Loss of high degree of perception on the opposite side of the body
(e.g. neglect syndrome
Sensation modality :
Each type of sensation: touch (fine,
crude) pressure (deep, light), stretch, vibration, hair
movement, pain, temperature is called a sensation
modality
The labeled line principle:
Each sensation modality is
transmitted by specific nerve fibres that terminate at a
specific point in the CNS.
The labeled line principle: When the nerve fiber is stimulated (at any level of the
pathway and by any type of stimulus):
the sensation is felt
at the original point (e.g. pain receptor in the finger) and
this is determined (processed) by the point in the CNS to
which the fiber leads (i.e. the brain who feels the
sensation, not the peripheral receptor).
free nerve ending (Receptor) shape;
free nerve endings
free nerve ending (receptor) location:
Superficial layer of skin
Free nerve endings (Receptor) ; sensory modality
crude touch
pressure
Free nerve endings (receptor): adaptation:
Slow
free nerve endings; Nerve fiber:
C- unmyelinated
Free nerve endings: Transmission speed:
Slow transmission
Low discrimination
Meisner’s corpuscule: Shape:
encapsulated nerve ending
Meisner’s corpuscule: location:
superficial layer of skin
Meisner’s corpuscule sensory modality
(1) fine touch
(2) low frequency vibration
meisner’s corpuscule adaptation:
rapid
Meissner’s corpuscule: Nerve fiber:
A-beta
Myelinated
Meissner’s corpuscule: transmission speed
Fast transmission
High discrimination
Merkel’s discs: shape:
expanded tip
Merkel’s discs: location:
Superficial layer of skin
Merkel’s discs: sensory modality;
-touch
-light pressure
Merkel’s discs: adaptation;
slow
Merkel’s discs: nerve fiber:
A-beta
Myelinated
Merkel’s discs: transmission speed:
Fast transmission
High discrimination
Ruffini’s organ: shape
encapsulated nerve endings
Ruffini’s organ: location:
Deep layer of skin
Ruffini’s organ: sensory modality:
prolonged touch
Deep pressure
Ruffini’s organ: adaptation:
slow
Ruffini’s organ: nerve fiber:
A-gamma
Myelinated
Ruffini’s organ: transmission speed:
Fast transmission/high discrimination
Pacinian corpuscule : Shape:
Encapsulated nerve endings
Pacinian corpuscule: Location:
Deep layer of skin
Pacinian corpuscule: sensory modality
vibration
stretch
Pacinian corpuscule: adaptation:
rapid
Pacinian corpuscule: Nerve fiber:
A-beta
Myelinated
Pacinian corpuscule: transmission speed
fast transmission
high discrimination
Hair end-organ: shape
free nerve endings
Hair end-organ location:
root of skin hair
hair end organ: sensory modality:
hair movement
hair end-organ : adaptation:
rapid
hair end-organ nerve fiber:
A-beta
A-gamma
Myelinated
Hair end-organ transmission speed:
fast transmission
high discrimination
Specialized free nerve endings- pain: shape:
free nerve endings
specialized free nerve endings - PAIN :location
-Superficial layer of skin
-internal tissues
specialized free nerve endings - PAIN : sensory modality
pain
specialized free nerve endings - PAIN : nerve fiber:
A-delta myelinated
C - unmyelinated
specialized free nerve endings - PAIN : adaptation
slow
specialized free nerve endings - PAIN : transmission speed
slow transmission
low discrimination
Specialized free
nerve endings-
Thermal: shape
free nerve endings
Specialized free
nerve endings-
Thermal: location
-Superficial
layer of
skin
Specialized free
nerve endings-
Thermal: sensory modality:
- Temperature
Specialized free
nerve endings-
Thermal: adaptation:
rapid
Specialized free
nerve endings-
Thermal: Nerve fiber:
A-delta
Myelinated
C-Unmyelinated
Specialized free
nerve endings-
Thermal: transmission speed
Slow transmission/ Low
discrimination
loss of the somatosensory association cortex results in:
-inability to recognize complex obects
-loss of high degree of perception on the opposite side of the body (e.g. neglect syndrome)
