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Transcript
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
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 and 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
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
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
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
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
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
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
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
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
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