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Sensory Physiology
Keri Muma
Bio 6
Sensory
Function – specialized cells that monitor internal
and external conditions
Two types of senses
General – receptors are distributed throughout the
body
Afferent impulses are sent to the somatosensory cortex
Special – receptors are concentrated in sense
organs
Afferent impulses are sent to special sense cortexes
PNS Organization
Afferent (sensory) division –
carries sensory info from
receptors to the CNS
Somatic afferent fibers –
carries impulses from skin,
skeletal muscles and joints
Visceral afferent fibers –
carries impulses from organs
within ventral body cavities
Special sense afferent fibers –
eyes, ears, taste, smell
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Anatomy of Sensory Neurons
Pseudounipolar neuron
Cell body lies in the dorsal
root ganglia
Action potential is initiated at
the peripheral end of the
neuron
Receptors
Sensory Receptors – found on the peripheral end of sensory
neurons
Act as transducers – change an incoming stimulus of one
type into an electrical impulse
Adequate stimulus - receptors are specific for the type of
stimulus it is sensitive to an can transduce
Receptors can respond to other types of stimuli if strong
enough
“Law of Specific Nerve Energies” – regardless of the type
of stimulus the sensation is perceived as what the receptor
is specific for
Types of Receptors
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Sensory Transduction
A stimulus alters the membrane permeability of the
receptor. This leads to the production of a graded receptor
potential
Receptors that are modified endings of the afferent neuron
produce generator potentials
Receptors that are separate cells from the afferent neuron
produce receptor potentials
Release chemicals that open ligand gated channels on the
afferent neuron
Types of Receptors
Nociceptors – activation causes the sensation of pain
Free nerve endings found in the skin, joints, bones, and blood
vessels
Sensitive to chemicals, tissue damage, and extreme
temperatures
Themoreceptors – react to changes in
temperature
Free nerve endings in the dermis,
hypothalamus, and liver
You have 3X more cold receptors than
warm receptors
Types of Receptors
Mechanoreceptors – stimulated by physical
change such as pressure or movement
Merkel cells – cells in the stratum basale associated
with free nerve endings, detect fine touch
Hair root plexus – free nerve endings associated with
hair follicles, detect movement of hair
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Types of Receptors
Mechanoreceptors (encapsulated)
Meissner’s corpuscles –detect touch, found in dermal
papillae
Ruffini corpuscles – sensitive to stretch and distortion,
found in the dermis
Pacinian corpuscles – deep pressure receptors found in
the dermis, vibration
Types of Receptors
Baroreceptors – sensitive to internal pressures
Monitors blood pressure in vessels (carotid sinus and
aorta), lungs, bladder, intestines
Proprioceptors – monitors position and stretch of
muscles, tendons, ligaments, and joint capsules
Helps maintain posture and
sense of body position
Types of Receptors
Chemoreceptors – detect chemicals
dissolved in solution
Olfactory, taste, osmolarity, pH, CO2, O2
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Organization of the Somatosensory System
Ascending tracts – a
bundle of axons
transmitting impulses
towards the brain
Usually involves three
successive neurons
Levels of Integration – three levels of processing
Receptor level - Primary neuron running from the
receptor to the posterior horn of the spinal cord or
medullary nuclei
Levels of Integration – three levels of processing
Circuit level – Secondary neuron synapses with the
first and transmit impulses to the thalamus
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Levels of Integration – three levels of processing
Perception level – Tertiary neuron running from
the thalamus to the primary somatosensory
cortex or a special senses cortex
Processing at the Perception Level
The cerebral cortex is responsible for:
Sensation - the awareness of the stimuli
Perception – the interpretation of the stimuli
Processing at the Perception Level
Each sensory fiber is a “labeled line”
The type, location, strength, and the intensity
of the stimuli is encoded in the area of the
cortex it travels to and the frequency of the
impulse
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Coding of a Stimulus
Determining type of stimulus
or receptor activated
Depends on the pathway it
takes and the area of the
cerebral cortex it travels to
The area of the cortex
devoted to each region is
related to the regions
sensitivity (# of receptors)
Referred Pain & Phantom Pain
Referred pain
Some visceral pathways are shared with somatosensory
so the brain perceived it as the more frequently
stimulated pathway
Phantom pain
Nerve endings are irritated
Remodeling of the cortex
Coding of a Stimulus
Localization of the stimulus
Also based on area of cortex
Accuracy of localization depends on the size of the
receptor field
The smaller the receptor field the more accurate
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Localization of the Stimulus
Accuracy also depends on lateral inhibition
Only the most intensely stimulated pathway is excited
Less excited pathways surrounding the stimulated area
are inhibited
Coding of a Stimulus
Intensity of the stimulus
Number of receptors stimulated – stronger
stimuli usually affect larger areas
Frequency of action potentials – stronger
stimuli generate larger receptor potential,
therefore a greater frequency of action
potentials
Coding of a Stimulus
Duration of the stimulus
Adaptation – receptors change their sensitivity in
the presence of constant stimulus
Phasic receptors – fast adapting
Report changes in the environment
Burst of firing at the beginning
and end of stimulus
Examples: temperature, smell,
touch
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Coding of a Stimulus
Duration of the stimulus
Tonic receptors – slow adapting or not at all
Constant firing rate
For situations were continuous information about a
stimulus is valuable
Examples: proprioceptors
pain, muscle stretch
Pain
The perception of pain is subjective
Influenced by past and present experiences
Unlike other senses pain is coupled with
behavioral and emotional responses
Pain Pathways
Fast pain pathways – myelinated
Immediate sharp pain
Easy to localize
Slow pain pathways – unmyelinated
Dull aching pain
Persists longer and hard to localize
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What stimulates the pain pathways?
Nociceptors are sensitized to stimuli by
chemicals released in damaged tissues
Prostaglandins – which are inhibited by aspirin
Bradykinins –chemicals activated by enzymes
released by damaged tissue
Histamine – triggers itching
Capsaicin – chemical in chili peppers, stimulates
pain and thermoreceptors
Pain Pathway
Primary neurons releases neurotransmitter
substance P or glutamate to stimulate
ascending pathway
Carried to the thalamus then to the somatosensory
cortex for perception
Also sent to the reticular formation which increases
level of alertness
And the hypothalamus (limbic system) which triggers
behavioral and emotional responses to pain
Pain Pathways
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Analgesic Pathway
The CNS has a
descending analgesic
pathway that can inhibit
the pain pathway
By presynaptic
inhibition, the release of
endorphins or
enkephalins blocks the
release of substance P
Analgesic Pathway
Endorphins bind to opiate receptors on the axon
terminal and block the release of substance P
Morphine binds to these opiate receptors to inhibit pain
Gate-Control Model
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