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Sensory perception
MD1009pgSensoryPercep1onlynchma2013_14 PG2000pgSensoryPercep1onlynchma2013_14 Peripheral Nervous System
Nerve fibers that carry information between CNS & other parts of the body
Afferent division –
Sends information from
internal and external
environment to CNS
Visceral afferent
Incoming pathway
for information
from internal viscera
(organs in body cavities)
Efferent division –
Sends information from
the CNS
Sensory afferent
Somatic
Sensation from
body surface
& proprioception Special senses
Vision, hearing
Smell, taste
Perception
Conscious interpretation of external world derived from sensory input
Why sensory input does not give true reality perception
–  Humans have receptors that detect only a limited number of
existing energy forms
–  Information channels in our brains are not high-fidelity
recorders
–  Cerebral cortex further manipulates the data
Stimulus
A change detectable by the body
Exists in different modalities (eg heat, light, sound ……)
Perceived by receptors (afferent neurons have sensory receptors)
Receptors convert heat/light/sound etc into electrical signals
Sequence
Stimulus
Receptor
Receptor
potential
Sensory transduction
Receptors
Structures at peripheral endings of afferent neurons
Detect stimuli (change detectable by the body)
Convert forms of energy into electrical signals (action potentials)
–  Process is called transduction
Action
potential
(in afferent fibre)
Sensory receptors
Sense
Stimulus
Chemoreceptors
Taste/Smell
O2/CO2/pH
Photoreceptors
Rods/Cones
Mechanoreceptors
Touch/Pressure
Balance, Motion
Hearing
Muscle fibre length
Joint position
Hair cell movement
Blood pressure
Thermoreceptors
Skin
Deep (in CNS)
Nociceptors
Nerve endings
Pain
Osmoreceptors
OTHERS
Electroreceptors
Blood osmolarity
Magnetoreceptors –in animals (not humans)
Sense organs have the property of transduction
The energy of stimulus is altered to electrical energy
The energy of stimulus is usually increased (amplified) or reduced during transduction
Receptors
•  May be
–  Specialized ending of an afferent neuron
–  Separate cell closely associated with peripheral ending of a neuron
•  Stimulus alters receptor’s permeability which leads to graded receptor
potential
•  Usually causes nonselective opening of all small ion channels
•  This change in membrane permeability can lead to the influx of sodium
ions. This produces receptor (generator) potentials.
•  The magnitude of the receptor potential represents the intensity of
the stimulus.
•  A receptor potential of sufficient magnitude can produce an action
potential. This action potential is propagated along an afferent fiber to
the CNS.
Intensity
Amplitude of stimulus is coded by the frequency of action potentials (APs)
(Weber –Fechner law)
500 AP
frequency
(Hz)
0 stimulus intensity (log scale)
Generator Potential
Local depolarization = Na+ influx
Generator potential = local depolarization of
the membrane of a sensory neuron.
Graded response correlates with the
strength of a stimulus applied to the
associated receptor organ
If the generator potential is large enough (ie
threshold is reached), an action potential is
triggered.
Generator Potential in a Pacinian corpuscle
Conversion of Receptor Potentials into an AP
Receptor Potential
Ini1a1on site of AP is different in efferent neurons or interneuron compared with afferent neurons duration of
stimulus
-40 mV
Sensory receptor
potential
sensory
potential
(mV) -70 mV
Time Generation of sensory potential
• the sensory stimulus generates a sensory receptor potential in the sensory cell
• usually a depolarization
• amplitude is proportional to stimulus
• generated by Na+ influx via channels of specific sensory receptor protein channels
• sensory potential may decline if stimulus is prolonged (called sensory adaptation)
Sensory adaptation
• sensory receptors vary in degree of adaptation
• tonic receptors are non or slowly adapting
• phasic receptors are rapidly adapting
stimulus
on
off
tonic receptors
| | | | | | | phasic receptors
| | | action potentials
generated in sensory
axon
Tonic receptor
Phasic receptor
AP frequency (Hz)
200
s1mulus Increase in AP
frequency
is maintained
0
Time
s1mulus Increase in AP
frequency is
not maintained
-only lasts a brief
period
How is sensory information coded (perceived)?
IMPORTANT FACTORS
Type of stimulus
Location of stimulus
Intensity of stimulus
Type of stimulus (Modality)
Is coded by receptor and pathway eg
light, photoreceptors, visual cortex
Specific receptor type
Activates specific pathway
Transmits to specific area of cerebral cortex
How is sensory information coded (percieved)?
IMPORTANT FACTORS: Type, Location & Intensity of stimulus
Location of stimulus
Is coded by receptive field
eg skin
Location of receptor determines…
Specific pathway and……
Transmits information to somatosensory cortex
How is sensory information coded (perceived)?
IMPORTANT FACTORS: Type, Location & Intensity of stimulus
Intensity of stimulus
2 factors involved in coding
Frequency of action
potentials (frequency coding)
No of receptors
activated (population coding)
Frequency of action potential in neuron
Number of receptor activated
Increased
frequency of
action potential
Leads to an
increased
receptor
potential
And higher
stimulus
strength
The somatic sensory/somatosensory
system
The system enables reaction to stimuli that are relayed from
-Thermoreceptors
-Nociceptors
-Mechanoreceptors
-Chemoreceptors
Information passes through sensory nerves to tracts in the spinal cord to
the brain
The primary brain area involved in processing is
the primary somatosensory area in the parietal lobe of the cerebral cortex
MD1009pgSoma1csensorySystemlynchma2013_14 PG2000pgSoma1csensorySystemlynchma2013_14 Somatic sensory organs (somatosensory)
Sensory organs in skin, muscles and tendons are exteroceptors
(detect external stimulus)
Sensory organs in internal organs are interoceptors
(detect internal stimulus)
Sensory endings have proteins embedded in membrane which detect a specific type
of sensory stimulus
A) Touch & pressure receptors
• class Aβ axons (A are myelinated and biggest, and α>β>γ>δ)
• myelinated.
• large diameter (10 mm).
• conduction velocity - 50 m/s
• include sensory axons with free nerve endings and those with a terminal accessory
organ or corpuscle
i) rapidly adapting
• detect light touch on smooth skin and low frequency vibrations
ii) slowly adapting
-detect prolonged touch and pressure
Meisner's corpuscles
- rapid adaptation
- not as but not as fast as Pacinian
- most common receptors of fingers, palms
and soles
- smaller receptive field than Pacinian
Pacinian corpuscle
-rapid adaptation
-very sensitive
- large receptive field
Ruffini
-slow
-large receptive field
-sensitive to stretching
(deep skin, ligaments
and tendon)
Merkel's disks
-slow
-small receptive field
-for light touch (eg finger tips
lips and genitals
Acuity
•  Refers to discriminative ability
•  Influenced by receptive field size and lateral inhibition
The receptive field determines whether 2 distinct points
can be perceived
Figs 9.15 and 10.11
Lateral Inhibition: Enhances contrast between areas
of strong and weak stimulation
Fig. 10.10
Pain
Is primarily protective
Triggers awareness that tissue damage is occurring
Memory of pain triggers learning and future avoidance
Pain is accompanied by motivated behavioral responses and emotional reactions
Perception -influenced by other experiences
Nociceptors
1. Mechanical nociceptors - Respond to mechanical damage eg cut, crush or pinch
2. Thermal nociceptors - Respond to temperature extremes
3. Polymodal nociceptors - Respond equally to all kinds of damaging stimuli
Properties
Non adapting
Prostaglandins greatly enhances receptor response to noxious stimuli
Nociceptors ………..
release transmitters in CNS & also neuropeptides peripherally
These can induce vasodilation and have other actions
Peripheral and central terminals respond to chemicals & pH which regulate
their sensitivity
Hyperalgesia (sensitization) to pain
-caused by prolonged severe tissue damage (burns, arthritis)
-an increased sensitivity (lowering of the threshold) of C fibre nociceptors,
either at site of injury (primary) or at adjacent sites (secondary).
Itch receptors
-specialised free nerve ending
Temperature receptors
-further specialised free nerve endings for hot and cold
B) Nociception (pain perception)
-nociceptors are specialized receptors sensitive to noxious stimuli (unpleasant, aversive,
potentially tissue damaging)
-pain is the conscious experience of noxious stimuli
-the receptors are free nerve endings devoid of myelin sheath
-the ending have specific proteins sensitive to noxious stimuli
Two major types of fibres that respond to nociception
1. Aδ (Adelta) fibres (fast pain system)
-axons are medium diameter (2 mm), thinly myelinated, 15 m/s conduction velocity
-sensitive to abnormally high mechanical stimuli
-rapidly adapting
-well localised pain
-sensation of sharp pain, eg, pinprick, and initial response to noxious heat
2. C fibres (slow pain system)
-axons are non-myelinated, <1mm diameter, 0.5m/s velocity
-sensitive to high mechanical, heat & cold & chemical stimuli
-non-adapting
-continuous throbbing pain
-not well localised pain
-very heterogeneous
SUMMARY TABLE How pain is
perceived
Somatosensory pathway
1st order sensory neuron
Afferent neuron with its
peripheral receptor that
first detects stimulus
2nd order sensory neuron
Either in spinal cord or
medulla. Synapses with 3rd
-order neuron
3rd order sensory neuron
Located in thalamus
Pain
2 best known pain neurotransmitters
–  Substance P
•  Activates ascending pathways that transmit nociceptive signals
to higher levels for further processing
–  Glutamate
•  Major excitatory neurotransmitter
AN IMPORTANT NOTE: The brain has built in analgesic system
(a) Suppresses transmission in pain pathways as they enter spinal cord
(b) Depends on presence of opiate receptors
•  Endogenous opiates – endorphins, enkephalins, dynorphin
Proprioceptors
• 
• 
detect position of body in space, particularly position of body limbs
gives rise to the kinaesthetic or "6th sense"
(an automatic non-conscious awareness of body and limb position)
• 
two main proprioceptive sense organs:
i) Golgi organs
-located on tendons
-free nerve endings
-detect length and movement of joints
ii) Muscle spindle organs
• 
• 
• 
• 
• 
• 
detect length and movement of muscles
composed of 3-12 modified muscle fibres called the intrafusal muscle fibres
the 2 end regions of the intrafusal muscle fibres have contractile filaments
innervated by gamma motor (efferent) axons
the central region of each intrafusal muscle fibre does not contain contractile
filaments, but has a swollen sensory region innervated by sensory (afferent) axons
sensory endings on the muscle spindles are activated by stretch
2 types of muscle spindles
nuclear bag
nuclear chain
NOTE: Normal contractile
fibres are called
extrafusal fibres, and
innervated by alpha motor
axons)
Role of intrafusal muscle fibres
Contraction maintains sensitivity
of the muscle spindles when a
muscle contracts
Properties of sensory axons to muscle spindles
Group 1 axons
Group 2 axons
Type
Aα
Aβ
Diameter
20µm
10µm
Conduction
velocity
100m/sec
50m/s
Endings
Primary
Secondary
Adaptation
Phasic
Tonic
Primary endings (group 1 axons)
• 
• 
• 
• 
• 
resting discharge rate of firing of AP in the sensory axons is ~10 / sec
increase in AP discharge when muscle spindle is stretched, ie, when
muscle is stretched
increase in AP frequency is proportional to rate of stretch - detect
velocity of muscle stretch
AP frequency rises as high as 500/sec during very rapid stretch
are phasic receptors, ie, rapidly adapt in response to stretch of muscle
and muscle spindle
500
Rapid adaptation
AP frequency
10
stretch
time
Secondary endings (group II axons)
• 
• 
• 
• 
increase in AP discharge when muscle is stretched - from resting frequency of
10 / sec
increase in AP frequency is proportional to increased length of muscle spindle
AP frequency rises only to a maximum of ~ 50/sec during stretch
are tonic receptors, ie, only slow adaption in response to stretch of muscle and
muscle spindle
detect absolute of muscle length
AP frequency
• 
50
10
stretch
time
Chemical Senses
Taste and smell
Receptors are chemoreceptors
Influence digestion & appetite
(digestive juices)
Stimulation of receptors induces
pleasurable or objectionable
sensations and signals presence
of something to seek or to avoid
Taste (Gustation)
• 
Chemoreceptors in oral cavity and throat
• 
Taste receptors life span =10 days
• 
• 
• 
• 
Taste bud consists of
(a) Taste pore
Fluids in mouth contact
receptor cells
• 
• 
• 
• 
• 
• 
(b) Taste receptor cells
Modified epithelial cells
with microvilli
Microvilli contain
receptor sites that bind
chemical molecules
Taste
Tastant = taste-provoking chemical
Tastant binds receptor
Ion channels open
Depolarizing receptor potential
Action potentials set up in terminals
of afferent nerve fibers with which
receptor cell synapses
Signals conveyed to brain stem,
thalamus to cortical gustatory area
5 Primary Tastes
Salty - Stimulated by chemical salts, especially
NaCl
Sour - Caused by acids which contain a free
hydrogen ion, H+
Sweet - Evoked by configuration of glucose
Bitter - Chemically diverse tastants eg
alkaloids, toxic plant derivatives, poisonous
substances
Umani - Meaty or savory taste
Influenced by
•  Information from other receptors, especially odor
•  Temperature and texture of food
•  Psychological experiences associated with past experiences
Smell (Olfaction)
•  Olfactory
receptors in nose
are specialized
endings of
renewable afferent
neurons
•  Olfactory mucosa -
3cm2 of mucosa in
ceiling of nasal cavity
Olfactory mucosa contains 3 cell types
Olfactory receptor cell is the afferent
neuron. Its receptor is in the olfactory
mucosa & signals to the brain.
Axons of olfactory receptor cells form the
olfactory nerve
Supporting cells secrete mucus
Basal cells are precursor cells. Olfactory
receptor cells are replaced about every 2
months.
•  Odorants
–  Molecules that can be smelled
•  To be smelled, substance must be
–  Sufficiently volatile that some of its molecules
can enter nose in inspired air
–  Sufficiently water soluble that it can dissolve
in mucus coating the olfactory mucosa
1000 different types of olfactory receptors
Odorants act through 2nd messenger systems to trigger action potentials
Afferent signals - sorted according to scent component by
glomeruli within olfactory bulb