MTC
4.03 General Senses
COLLEGE OF MEDICINE CLASS 2022
PHYSIOLOGY
LE 4
Reino Palacpac, MD | January 14, 2019
TRANS 3
OUTLINE
I. GENERAL SENSES VS.
SPECIAL SENSES
A. CLASSIFICATION BY
STRUCTURAL COMPLEXITY
B. COMPLEXITY RANGE OF
RECEPTORS
II. GENERAL SENSES
A. UNENCAPSULATED NERVE
ENDINGS
B. ENCAPSULATED NERVE
ENDINGS
III. SENSATION
A. SENSATION VS.
PERCEPTION
IV. SENSORY RECEPTORS
A. CLASSIFICATION OF
SENSORY RECEPTORS BY
LOCATION
B. TYPES OF SENSORY
RECEPTORS
C. NERVE FIBER TYPES
CLASSIFICATION
STIMULUS OF SENSORY
RECEPTORS
Remember
Lecturer
D. STIMULUS OF SENSORY
RECEPTORS
V. SENSORY UNIT
A. TWO-POINT
DISCRIMINATION
B. STEREOGNOSIS
C. ADAPTATION/
DESENSITIZATION
VI. SENSORY RECEPTORS
A. FOUR GENERAL SENSES
VII. NOCICEPTIVE SOMATIC
SENSE
A. NOCICEPTIVE SENSATION
B. TYPES OF PAIN
C. DUAL PAIN PATHWAYS IN
THE CNS
D. PAIN SENSATIONS
E. CNS HAS ANALGESIC
MECHANISMS
VIII. DUAL SENSORY ASCENDING
PATHWAY
IX. MECHANORECEPTORS
X. CHEMORECEPTORS
LEGEND
Book
Previous
Trans
• Free nerve ending
• Encapsulated nerve ending
• Specialized receptor cells
II. GENERAL SENSES
A. UNENCAPSULATED NERVE ENDINGS
Presentation
I. GENERAL SENSES VS. SPECIAL SENSES
• General Senses
→ Receptors throughout the body
▪ Pain
▪ Temperature
▪ Light touch
▪ Pressure
▪ Sense of body and limb position
• Special Senses
→ Receptors located in sense organs (e.g., ear, eye)
▪ Taste
▪ Smell
▪ Vision
▪ Hearing
▪ Balance
A. CLASSIFICATION BY STRUCTURAL COMPLEXITY
• General Senses
→ Nociceptors
→ Thermoceptors
→ Mechanoceptors
→ Chemoreceptors
• Special Senses
→ Olfaction
→ Gustation
→ Hearing
→ Equilibrium
→ Vision
B. COMPLEXITY RANGE OF RECEPTORS
Figure 2. Free nerve endings
• Skin, Bones, Internal organs, Joints
• Free Nerve Endings
→ Pain
→ Temperature
• Merkel’s Discs
→ Light Touch
→ Pressure
• Root Hair Plexuses
→ Light Touch
B. ENCAPSULATED NERVE ENDINGS
Figure 3. Pacinian corpuscle
• Naked nerve endings surrounded by one or more layers
• Deeper tissue, Muscles
• Pacinian Corpuscles
→ Deep Pressure
• Meissner’s Corpuscles
→ Discriminative Touch in Hairless Skin Areas
• Krause’s End-Bulbs
→ Discriminative Touch in Mucous Membranes
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PHYSIOLOGY 1 of 10
4.03 General Senses
• Ruffini’s Corpuscles
→ Deep Pressure & Stretch (Proprioception)
III. SENSATION
• Feeling produced following effective stimulation of the sensory
division of the nervous system
• Types according to stimulus:
→ Exteroceptive
→ Interoceptive
• Categorized as:
→ Epicritic
▪ Easily localized
→ Protopathic
• Information Flow
→ Sensory receptors → Impulse in sensory fibers
→ Impulse reaches CNS → Sensation (new experience,
recalled memory) → perception
A. SENSATION VS. PERCEPTION
• Sensation
→ The conscious or subconscious awareness of external or
internal stimuli
• Perception
→ The conscious awareness and the interpretation of meaning
of sensations
LE 4
TRANS 3
• Proprioceptors
→ Monitor degree of stretch in skeletal muscles, tendons, joints
and ligaments
→ Which provide information about the position of the body in
space at any given instant
• Teleceptors ("distance receivers")
→ concerned with events at a distance
B. TYPES OF SENSORY RECEPTORS
• Mechanoreceptors
→ Mechanical energy
▪ touch
▪ pressure
▪ vibration
• Thermoreceptors
→ Temperature changes
• Chemoreceptors
→ Changes in chemical concentration
• Photoreceptors
→ Electromagnetic receptors
• Nociceptors
→ Tissue damage
▪ Pain
C. NERVE FIBER TYPES CLASSIFICATION
IV. SENSORY RECEPTORS
• Specialized cells or cell processes that monitor the stimuli or
conditions in/outside body (→ extero- and interoceptors)
• Also called sensory endings
• Sensory receptors are the peripheral endings of afferent nerve
fibers
• Part of a neuron or specialized cells that generate receptor
potentials
• Biological transducers (converts any form of stimulus into an
action potential)
• All sensory receptors are transducers, changing incoming
stimulus of pressure, vibration, light, etc., into electro-chemical
neuron impulses
• Receptors are specific for a certain type of stimulus
→ “Receptor specificity”
• Simplest receptor type: free nerve endings
D. STIMULUS OF SENSORY RECEPTORS
Figure 4
A. CLASSIFICATION OF SENSORY RECEPTORS BY
LOCATION
• Exteroceptors
→ Sensitive to stimuli arising from outside body
• Interoceptors
→ Or visceroreceptors, from internal viscera
→ Concerned with the internal environment
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• Adequate Stimulus
→ stimulus that excites the receptor with the LOWEST
threshold
• Specific sensory receptor
→ sensitive to one type of stimulus
• Stimulus transduction
→ Differential sensitivity of receptors
▪ Each receptor is highly receptive to one type of stimulus
and appraises the NS of one kind of modality of sensation
− Adequate stimulus
• Transduction of Sensory Signal to Nerve Impulses
→ Regardless of the type of stimulus, the effect on all receptors
is the same: a change in the electrical potential of the
receptor
▪ Receptor potential
• Action potential initiation
→ Receptor potential (depolarizing, hyperpolarizing)
→ Generator potential
→ Threshold
→ Action potential
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TRANS 3
Location – Body Part Stimulated
● Sensory receptors transmit impulses to specific sensory nuclei of
the thalamus → Somatic sensory cortex (Brodmann’s area 1,2,3)
● Sensory Homunculus
Figure 5
● Adequate stimulus → action potential → sensation
● **High intensity → increased rate of firing
● Inadequate stimulus → generator potential → no sensation
Parameters determined with adequate stimulus
●
●
●
●
Quality or modality of sensation
Stimulus intensity
Stimulus location
Stimulus duration
Quality or Modality of Sensation
● Muller’s Doctrine of Specific energies
→ Implies that the modality or submodality of a sensation is
determined not by the stimulus, but by what specific receptor
or nerve fiber is stimulated
● Labeled Line Principle
→ Specificity of nerve fibers for transmitting ONLY ONE modality
of sensation
Figure 7
● Topognosis
→ Ability to localize precisely a body part stimulated without the
aid of vision
● Law of Projection
→ Conscious sensation is referred to the location of the receptor
no matter where a particular sensory pathway is stimulated
along its course to the cortex
→ Explains “phantom limb” ----- (plasticity in sensory system)
→ Location-Body part stimulated
V. SENSORY UNIT
Intensity and Duration of Sensation
● Intensity
→ Coded by # of receptors activated and frequency of AP coming
from receptor
● Duration
→ Coded by duration of APs in sensory neurons
● Recruitment of Sensory Units
→ The number of sensory receptors activated in a sensory unit
is proportionate to the intensity of stimulus
▪ Weak stimuli → lower threshold receptors
▪ Strong stimuli → higher threshold receptors
Judgment of stimulus intensity
● Weber-Fechner Principle
→ The magnitude of sensation is proportionate to the log of
stimulus intensity
▪ E.g., Hold 30 g weight, detect 1 g increase. Hold 300 g
weight, detect 30 g increase
→ Gradations of stimulus strength are discriminated in proportion
to the logarithm of stimulus strength
→ As the sensory intensity increases, there is decrease in ability
to discriminate small changes.
Figure 8
● All the receptors or the area innervated by one sensory afferent
neuron
→ Receptor field
▪ Well defined area where the sensory unit reacts to a
stimulus
▪ Is the area monitored by one receptor
▪ The smaller the receptive field, the more precise is the
encoding of stimulus localization
▪ Explained by lateral inhibition mechanism
Lateral Inhibition Mechanism
▪ Every sensory pathway when stimulated gives rise
simultaneously to lateral inhibitory signals and inhibit
adjacent neurons
▪ Essential to increase the degree of contrast in the
sensory pattern received by the cerebral cortex
▪ Also known as surround inhibition
▪ Enhances the topognostic ability of individuals
Figure 6
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TRANS 3
Figure 9
Figure 12
B. STEREOGNOSIS
Figure 10
● The larger the receptive field, the poorer ability to localize
stimulus (2 pt. discrimination test)
A. TWO-POINT DISCRIMINATION
● Involves placing the tips of two objects (toothpicks) a certain
distance apart on the skin and ask the patient if they are able to
sense the presence of one or two stimuli.
● Moving the points closer or further apart can identify the range
of sensitivity in different regions of the body
● Ability to perceive two touch stimuli as two separate sensation
at a minimum distance
● Minimal distance by which two touch stimuli must be separated
to be perceived as separate is called “two-point threshold”
● Smallest area - greater number of receptors
→ Two Point Threshold
▪ Smallest distance wherein the stimuli are perceived as
two points by the subject
▪ Lesser on the fingertips and greater o the person’s back
▪ Reasons:
− Different numbers of sensory receptors
− Size of the sensory unit
● Ability to identify objects by handling them without looking at
them.
● Depends on intact touch and pressure sensation
C. ADAPTATION/DESENSITIZATION
• When maintained stimulus of constant strength is applied to a
receptor, the frequency of action potentials in its sensory
nerves declines over time
• Two types of adapting receptors
Rapidly Adapting Receptors
• “Rate receptors”, “Phasic receptors” or “Movement
receptors”
• Produces action potentials only at the beginning or end of the
stimulus → encode changes in stimulus strength but not
stimulus duration
• Cannot be used to transmit a continuous signal to the brain
• Has predictive function
• Examples:
→ Photoreceptors
→ Pacinian corpuscles
→ Semicircular canal
→ Hair follicle
→ Meissner’s corpuscles
• Better at coding changes in stimulus intensity but not the
duration
Slowly Adapting Receptors
Figure 11
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• “Tonic Receptors”
• Detect continuous stimulus strength as long as the stimulus is
present
• Keeps the higher center constantly informed of the status of the
body and its relation to the environment
• Examples:
→ Muscle spindle
→ Golgi tendon
→ Macula
→ Nociceptors
→ Baroreceptors
→ Chemoreceptors
→ Merkel’s disc
→ Ruffini’s ending
• Better at coding the intensity of a stimulus for its entire duration
PHYSIOLOGY 4 of 10
4.03 General Senses
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Table 2. Somatic Sensory Receptors
RECEPTORS
LOCATION
STIMULUS
Figure 13. Action potentials generated in slow and rapid adapting
receptors.
Table 1. SUMMARY
Rapidly Adapting
Rate/Phasic/Movement
Receptors
Detects changes in stimulus
strength
Cannot transmit continuous
signal to brain
Pacinian’s and Meissner’s
Corpuscles, semicircular
canals, hair follicles
Free nerve
endings
Everywhere in
the skin,
tissues
Pacinian
Corpuscles
Underneath
the skin and
fibrous tissue
Ruffini’s
ending
Underneath
the skin,
deeper tissues
Hair end
organs
Merkel’s Discs
Meissner’s
Corpuscles
Beneath hair
follicles
Hairy parts
Non-hairy
parts (Low
threshold)
Touch,
pressure,
tickle, itch,
pain
Touch,
pressure,
vibratory
signals
Touch,
pressure,
changes in
position
Hair
movement
Touch
Touch,
vibration
(low
frequency)
TRANS 3
TYPE
Tonic
Phasic
Phasic
Phasic
Tonic
Phasic
Slow Adapting
Tonic Receptors
A. FOUR GENERAL SENSES
Nociceptors
Detects continuous stimulus
strength as long as it’s
present
Can transmit continuous
signal to the brain
Muscle spindle, Golgi tendon,
Macula, Nociceptors, Ruffini’s
ending, Merkel’s discs
VI. SENSORY RECEPTORS
• Mechanoceptive Sensation
→ Tactile sensation
▪ Touch
▪ Pressure
▪ Vibration
▪ Tickle and itch
→ Position Sensation
▪ Static position
▪ Dynamic Position (Kinesthesia)
• Thermoceptive Sensation
→ Hot
→ Cold
• Nociceptive Sensation
→ Pain
• Skin receptors
→ Cutaneous receptors (somatic sensory)
Figure 14. Skin receptors
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Figure 15
• Respond to heat, mechanical stress and chemicals
→ Associated with tissue damage
• Mostly concentrated in skin
• Fast pain
→ Towards cortex
→ Usually triggers reflex
▪ i.e. pulling the affected limb towards the body when burnt
• Slow pain
→ Gradual, persistent
→ Indistinct source
• Referred pain
→ Visceral
→ “incorrect” source perceived
Figure 16. Referred/Visceral pain.
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4.03 General Senses
• Stimuli
→ Thermal
→ Mechanical
→ Chemical
▪ Bradykinin, serotonin, histamine, potassium ions, acids
▪ Acetylcholine, proteolytic enzymes, lactic acids,
prostaglandins, substance P
LE 4
TRANS 3
• Types of thermoreceptors:
→ Cold Sensitive (10°C - 38°C)
▪ Greater in number
▪ Transmitted to type Aδ and type C
▪ Activity is greatest at 25°C
→ Warmth Sensitive (30°C - 45°C)
▪ Transmitted mostly to type C
▪ Activity is greatest at 40°C
→ Pain Sensitive (⬇10°C and ⬆45°C)
▪ Transmitted to type Aδ and type C fibers
▪ Freezing cold and burning hot
• Cold and warmth sensitive receptors are capable of adaption
which is observed at temperature range between 20°C and
40°C
Figure 17. Pain sensation.
• Classification of Pain
→ Acute or Physiologic Pain
▪ Sudden onset
▪ Recedes during the healing process
▪ Good pain (indicative of injury/protective mechanism)
→ Chronic or Pathologic Pain
▪ Gradual onset
▪ Includes inflammatory and neuropathic pain
▪ Bad pain
• Pain
→ Almost all specialized somatic sensory receptors transmit
their signals in type Aβ fibers
→ The free nerve endings transmit impulses to type A and type
C fibers
• Pain receptors: all free nerve endings (tonic receptors)
→ Primarily located in:
▪ Superficial layers of skin
▪ Periosteum
▪ Arterial walls
▪ Joint surfaces
▪ Falx and tentorium of the cranial vault
→ Devoid of pain receptors:
▪ Brain tissue, nails, hair
Thermoreceptors
•
•
•
•
Respond to changes in temperature
In dermis, skeletal muscles, liver, and hypothalamus
Free nerve endings
Cold receptors > warm receptors
Figure 19. Types of pain fibers.
VII. NOCICEPTIVE SOMATIC SENSE
● HYPERPATHIA – chronic exposure to noxious stimuli increases
the threshold for excitation of the pain receptors
● HYPERALGESIA – low threshold for painful stimuli
A. NOCICEPTIVE SENSATION
● Occurs whenever a tissue is damaged (protective mechanism)
● Damaged skin releases a variety of chemical substances from
itself, blood cells, and nerve endings. These substances include
bradykinin, prostaglandins, serotonin, substance P, K+, H+, and
others; they trigger the set of local responses that we know as
inflammation. As a result, blood vessels become leakier and
cause tissue swelling (or edema) and redness. Nearby mast
cells release the chemical histamine, which directly excites
nociceptors.
● Bradykinin, acetylcholine, serotonin, proteolytic enzymes,
histamine, H+ & K+ ions, acids – lactic acid
● Prostaglandins and substance P - enhance the sensitivity of
pain endings but do not directly excite them
● Three types of nociceptors exist:
→ Mechanical nociceptors- detects sharp, pricking pain
→ Thermal and mechano-thermal nociceptors - detects
sensations which elicit pain which is slow and burning, or
cold and sharp in nature
→ Polymodal nociceptors - detects mechanical, thermal and
chemical stimuli
Figure 18. Distribution of cold and warm receptors.
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Figure 21: referred pain
Figure 20: chemical stimuli
B. TYPES OF PAIN
● Fast Pain (Acute Pain)
→ felt within 0.1 sec after the application of stimulus
→ sharp pain, pricking pain, acute pain, bright pain or electric
pain
→ elicited by mechanical and thermal stimuli
→ easily localized (epicritic)
→ Transmitted by type A delta fibers
→ Evokes withdrawal reflex
● Slow Pain (Chronic Pain)
→
→
→
→
→
→
→
Felt after 1 sec
slow burning pain, aching pain, throbbing pain
usually associated with greater tissue destruction
felt both in the skin and deep tissues
transmitted by Type C (Slow-chronic pain pathway)
poorly localized
Elicited by all types of pain stimuli
C. DUAL PAIN PATHWAYS IN THE CNS
● Acute Fast Pain Pathway (Neospinothalamic tract)
→
→
→
→
fast pain: epicritic sense
termination in the brainstem and thalamus
neurotransmitter: glutamate
pain localization precise
● Chronic Slow Pain Pathway (Paleospinothalamic tract)
→
→
→
→
slow chronic pain: protopathic sense
terminates widely in the brainstem
do not reach the cortex
neurotransmitter: substance P
D. PAIN SENSATIONS
● Referred pain- pain felt considerably away from the structure
causing the pain. Pain sensations originating in visceral organs
are often perceived as involving specific regions of the body
surface innervated by the same spinal nerves.
● EXAMPLES OF REFERRED PAIN:
→ Cardiac pain is felt at the inner part of left arm and left
shoulder.
→ Pain in ovary is referred to umbilicus.
→ Pain from testis is felt in abdomen.
→ Pain in diaphragm is referred in right shoulder.
→ Pain in gallbladder is referred to epigastric region.
→ Renal pain is referred to loin
TRANS 3
● DERMATOMAL RULE
→ Pain is referred to a structure, which is developed from the
same dermatome from which the pain producing structure is
developed.
→ A dermatome includes all the structures or parts of the body
which are innervated by afferent nerve fibers of one dorsal
root. For example, the heart and inner aspect of left arm
originate from the same dermatome.
● NEUROPATHIC PAIN
→
→
→
→
→
occurs in the absence of nociceptor stimulation
most likely occurs when nerve endings are injured
excruciating and difficult to treat
Examples: Phantom limb pain (projected pain)
Causalgia – develop after traumatic damage to a peripheral
nerve
● VISCERAL PAIN
→ Causes of true visceral pain:
→
1. ischemia of visceral tissue
→
2. chemical damage
→
3. spasm of the smooth muscle of the hollow viscera
→
4. overdistention
→
5. stretching of the connective tissue surrounding or
within the viscus
→ difficult to localize
● PAIN PERCEPTION
→ function of brainstem, reticular formation, thalamus and other
lower brain centers
● PAIN INTERPRETATION
→ function of cerebral cortex
● What causes differences in pain intensity? Frequency of firing of
the nociceptor
E. CNS HAS ANALGESIC MECHANISMS
● Endogenous opioids
→ Enkephalins (200 times as potent as opium, morphine,
heroin)
→ Endorphins
● Secreted by CNS, pituitary
→ Blocks pain transmission in brain by stopping transmission at
dorsal horn
→ prevents first-order neurons from producing substance P
(thus, no pain information transferred)
● GATE CONTROL THEORY
→ Cells present in substantia gelatinosa could inhibit the entry
of pain signals into the dorsal horn of spinal cord
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→ Mechanism: presynaptic inhibition using GABA
Figure 22 analgesic fibers
● ANALGESIA SYSTEM OF BRAIN & SPINAL CORD consists
Figure 23
of 3 major components:
→ 1. Periaqueductal gray and periventricular areas of
mesencephalon and upper pons
→ 2. Raphe magnus nucleus and nucleus reticularis
paragigantocellularis
→ 3. Pain inhibitory complex located located in the dorsal
horns of the spinal cord
● Transmitter substances in the analgesia system:
→ ENKEPHALIN- morphine-like neurotransmitter that
can block pain signals at the dorsal horn
→ SEROTONIN- enhances activity of enkephalin secreting
neurons in the pons and spinal cord
● TRANSMISSION OF SOMATIC SIGNALS INTO CNS:
→ BELL-MAGENDIE LAW
▪ dorsal roots → sensory function
▪ ventral roots → motor function
VIII. DUAL SENSORY ASCENDING PATHWAY
Dorsal column / Medial lemniscal Pathway
Figure 24
Brodmann areas of the brain. Areas of note include areas 3–1–2 = primary
sensory cortex; area 4 = primary motor cortex; area 8 = frontal eye felds; area
44 = Broca area;
area 22 = Wernicke area; and area 17 = primary visual cortex.
→ Limited to mechanorecceptive sensation
▪ Fine touch and pressure
▪ Vibration
▪ Proprioception
→ Large, myelinated neurons
→ higher degree of spatial orientation
→ With fine gradations of stimulus intensity
→ Epicritic sensation
Destruction of SSA I
→ Broad spectrum
▪ Crude touch and presssure
▪ Tickle and itch
▪ Thermal
▪ Nociception
→ Small unmyelinated neurons
→ Lesser degree of spatial orientation
→ Lacks the fine gradations
→ Protopathic sensations
Destruction of SSA II
Anterolateral / Spinothalamic Pathway
→ Atopognosis
▪ inability to localize discreetly body part stimulated
→ Astereognosis
▪ inability to identify shapes and textures by palpation
→ Inability to approximate weight of an object
→ Inability to judge critical degrees of pressure sensation
→ Poor localization of pain and thermal sensations
→ Amorphosynthesis
▪ loss of ability to recognize complex objects and complex
forms by feeling them on the opposite side of the body
Spinal Cord Transection
→ All sensations and motor functions distal to the segment of
transection are blocked
Brown Sequard Syndrome
→ If only half of the spinal cord is injured
→ Symptoms:
1. Ipsilateral paralysis
2. Ipsilateral loss of proprioceptive sensations, vibration
sensations
3. Ipsilateral atopognosis and 2 point discrimination
4. Contralateral loss of pain and thermal sensations below
the level of spinal injury
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IX. MECHANORECEPTORS
→ Respond to physical distortion of cell membrane (e.g.:
stretching, twisting, compression)
● Subdivided into
→ Baroreceptors
▪ Sensitive to internal pressures: blood pressure, lung
stretch, digestive tract tension
→ Proprioceptors
▪ Monitors of muscle stretch
→ Tactile receptors
▪ Touch, pressure, vibration
− Unencapsulated:
o Free nerve endings
o Merkels dics - fine touch
− Encapsulated:
o Meissners corpuscles - fine touch,
o Pacinian corpuscles - deep pressure
Encapsulated Nerve Endings
→ Proprioceptors
▪ Muscle Spindles
− Skeletal Muscle Stretching
▪ Golgi Tendon Organs
− Tendon Stretching
▪ Joint kinesthetic receptors
− monitors stretch in synovial joints; sends info to
cerebellum and spinal reflex arcs
o to cerebrum,
o cerebellum and
o spinal reflex arcs
▪ Tickle and Itch
− Generally, results from stimulation of free nerve
endings which is exclusively found in the superficial
layer of the skin which is the only tissue from which this
sensation is elicited.
− Transmitted by small type C fibers
Mechanoceptive Sensation
→ Tactile Sensation
▪ Touch
− generally results from stimulation of tactile receptors in
the skin or in tissues immediately beneath the skin
▪ Pressure
− generally results from stimulation of tactile receptors
deeper tissue
− maintained touch
→ Position sensation
− detected by proprioceptors ( muscle spindle and golgi
tendon organ ) detecting the position of the body in
space at a given time.
o Static
o Dynamic (Kinesthesia)
▪ Vibration
− generally results from stimulation of tactile receptors of
rapidly repetitive sensory signal
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REFERENCES
Doc Palacpac PPT.
Teachmephysiology.com
● Tickling Sensation
→ pleasurable
● Itching Sensation
→ annoying
● Pain Sensation
→ unpleasant
X. CHEMORECEPTORS
→ Respond to small concentration changes of specific
molecules (chemicals)
→ Internal chemoreceptors monitor blood composition (e.g.
Na+, pH, pCO2 )
→ Found within aortic and carotid bodies
→ Very important for homeostasis
Figure 25: Referred Pain- Felt on body Surface
Transmission of Signals of Different Intensities in Nerve
Tracts
● Spatial Summation
→ increasing signal strength is transmitted by using
progressively greater numbers parallel fibers
Figure 26
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