Sensory Physiology
Receptor transduction
Receptive fields and perception
Phasic and tonic receptors
Different somatosensory
modalities
Five special senses
Somatic (= general)
senses
1. Touch
2. Temperature
3. Nociception
4. Itch
5. Proprioception
Special senses
1. Vision
2. Hearing
3. Taste
4. Smell
5. Equilibrium
Touch Receptors
Free or encapsulated dendritic endings
In skin and deep organs, e.g.: Pacinian
corpuscles
•
concentric layers of c.t. ⇒ large receptive field
detect vibration
opens
mechanically
gated ion channel
adaptation ⇒
receptor type?
rapid
Temperature Receptors
• AKA thermoceptors or thermorecetors
• Free dendritic endings in hypodermis
• Function in thermoregulation
• Cold receptors (< body temp.)
• Warm receptors (> body temp.)
• Test if more cold or warm receptors in
lab
• Adaptation only between 20 and 40°C
• Nociceptors activated if T > 45°C
General Principles
The process of stimulus transduction involves the opening or closing of
ion channels on a specialised plasma membrane.
The size of the receptor potential, just like the synaptic potential is
graded and is determined by:
•
•
•
•
The magnitude of the stimulus
The rate of change of the stimulus strength
The temporal summation of other receptor potentials
The degree of ‘adaptation’
Sensory Physiology: General
Principles
Sensory information exists in a variety of forms:
Light
Sound
Temperature
Vibration
Pressure
Chemical
Osmolarity
Tension
Sensory Physiology: General Principles
Sensory systems convert one form of energy into an electrical
signal
This conversion of one energy form (eg. light) into an electrical
signal (receptor potential) is known as stimulus transduction
Sensory Physiology: General Principles
Different sensory receptors for different sensory modalities
Photoreceptors are responsive to light. (eyes)
Mechanoreceptors are responsive to mechanical energy. (skin, ears)
Thermoreceptors are response to either heat or cold. (skin)
Osmoreceptors are responsive to the concentration of solutes in the
extracellular
fluid. (tongue, hypothalamus)
Chemoreceptors are responsive to specific chemicals (tongue,
hypothalamus)
Nocireceptors are sensitive to tissue damage (pH, histamine, extreme
heat/cold, extreme mechanical pressure). (skin)
Types of Sensory Receptors
1. Chemo- (specific ligands) and Osmo(conc. of solutes)
2. Mechano- (touch, pressure, vibration,
stretch)
3. Thermo- (temp. change)


Cold receptors lower than body temp.
Warm receptors (37 - 45oC)
> 45oC ?
1. Photo- (light)
Somatosensory System
MECHANORECEPTORS (touch, pressure vibration)
Skin: Meissners corpuscle, Merkles corpuscle / discs, Pancinian
corpuscle
THERMORECEPTORS (free nerve endings activated by a rising
temperature OR a falling temperature)
NOCIRECEPTORS (free nerve endings activated by mechanical
deformation chemicals and temperature)
In the dermis of the skin and are responsible for superficial somatic pain
In muscles, tendons and joints and are responsible for deep somatic
pain
In the visceral organs and are responsible for visceral pain
PROPRIOCEPTORS (muscle spindles in muscle; tendon organs in
tendon) are responsible for the awareness of muscle length and body
position in space
Somatosensory System
Mechanoreceptors
Thermoreceptors
Nocireceptors
Proprioceptors
General Principles
Receptor potentials have the same properties as synaptic
potentials
A receptor may be either a specialised nerve ending of an afferent
neuron or a separate cell that is intimately associated with the
peripheral endings of the neuron.
Figure 46-3 Excitation of a sensory nerve fiber by a receptor potential
produced in a pacinian corpuscle. (Modified from Lo&#235;wenstein WR:
Excitation and inactivation in a receptor membrane. Ann N Y Acad Sci
94:510, 1961.)
Receptor Potentials
Typical relation between receptor potential and action potentials when the receptor
potential rises above threshold level.
Relation of amplitude of receptor potential to strength of a
mechanical stimulus applied to a pacinian corpuscle. (Data from
Lo&#235;wenstein WR: Excitation and inactivation in a receptor
membrane. Ann N Y Acad Sci 94:510, 1961.)
Adaptation of different types of receptors, showing rapid adaptation of some
receptors and slow adaptation of others.
Figure 46-8 Translation of signal strength into a frequency-modulated series
of nerve impulses, showing the strength of signal (above) and the separate
nerve impulses (below). This is an example of temporal summation.
Figure 46-17 The rhythmical output of summated nerve impulses from the respiratory center,
showing that progressively increasing stimulation of the carotid body increases both the intensity
and the frequency of the phrenic nerve signal to the diaphragm to increase respiration.
Figure 46-18 Successive flexor reflexes showing fatigue of conduction through the reflex
pathway.
Sensory
Pathway
Stimulus
Sensory receptor (= transducer)
Afferent sensory neurons
CNS
Integration, perception
Somatosensory Pathways
Pathways are composed of a first, second and third order neuron
First Order neuron is the 10 afferent. It conveys information to the spinal
cord (via the dorsal roots) or the brainstem.
Second Order neuron conduct impulses from either the spinal cord or
brainstem to the thalamus.
Third Order neuron conducts impulses from the thalamus to the
somatosensory cortex.
There are three main spinal pathways that carry somatosensory
information
The Anterolateral Pathway (also called Spinothalamic)
The Dorsal Column Pathway
The Spinocerebellar Pathway (important for proprioception)
Sensory Nerves, Spinal Cord & Spinal Nerves
Central ‘butterfly’ shaped
grey matter contains the
cell bodies of motor
neurons and
interneurons
Surrounding white matter
contains the axons of
ascending and
descending neurons
(more on this later)
Dorsal roots (sensory) join with
ventral roots (motor) to form a
spinal nerve.
Spinal nerve contains sensory
and motor information.
Spinal Cord & Spinal Nerves
Central ‘butterfly’ shaped
grey matter contains the
cell bodies of motor
neurons and
interneurons
Surrounding white matter
contains the axons of
ascending and
descending neurons
(more on this later)
Dorsal roots (sensory) join with
ventral roots (motor) to form a
spinal nerve.
Spinal nerve contains sensory
and motor information.
Dermatomes
The area of skin that that is innervated by the sensory nerves of a
single spinal nerves is called a dermatome
Somatosensory Pathways
3O neuron
2O neuron
1O neuron
Somatic Senses
• Primary sensory neurons from receptor to
spinal cord or medulla
• Secondary sensory neurons always cross
over (in spinal cord or medulla) →
thalamus
• Tertiary sensory neurons →
somatosensory cortex (post central gyrus)
Somatosensory Pathways
There are specific pathways which covey sensory (ascending)
information up the spinal cord to the brain
Somatosensory Pathways
Crossover at spinal cord
Pain and Temperature
Tickle and Itch
Poorly localised touch
Crossover in medulla
Discriminative touch
Shape, size texture, weight
Vibration
Proprioception
DORSAL COLUMN PATHWAY
• CARRIES FINE TOUCH,
POSITION, PRESSURE,
VIBRATION, TWO POINT
DESRIMINATION stereognosis
• AFFERENT SENSORY FIBERS Aβ
TYPE.
• VERY FAST VELOCITY 30 – 70
m/s
• 3 NEURON SYSTEM
ANTEROLATERAL PATHWAY
• CARRIES PAIN & TEMPRATURE
(lat. Sp.Th)
• CRUDE TOUCH & PRESSURE
( VENT, Sp. Th)
• AFFERENT SENSORY FIBERS Aδ
(MYELINATED) FAST PAIN
• C FIBERS( UNMYELINATED)
SLOW PAIN
• RELATIVELY SLOW VELOCITY Aδ
– 6 – 30 m/s. C – 0.5 – 2 m/s.
• 3 NEURON SYSTEM
CNS Distinguishes 4
Stimulus Properties
• Modality (nature) of stimulus
– Type of receptor
• Location
– lateral inhibition
– population coding
• Intensity
• Duration
Somatosensory Cortex
Area on somatosensory cortex related to degree of innervation
Receptive Fields
• Each 1° sensory neuron picks up information from
a receptive field
• Often convergence onto 2° sensory neuron ⇒
summation of multiple stimuli
• Size of receptive field determines sensitivity to
stimulus → Two point discrimination test
Pattern of stimulation of pain fibers in a nerve leading from an area of
skin pricked by a pin. This is an example of spatial summation.
Two-points discrimination varies
Lateral inhibition
Lateral pathways are strongly inhibited causing most accurate localization.
e.g. movement of skin hairs can be well located, temperature and pain are
poorly located.
Intensity & Duration of Stimulus
• Intensity is coded by # of receptors activated
and frequency of AP coming from receptor
• Duration is coded by duration of APs in
sensory neurons
• Sustained stimulation leads to adaptation
– Tonic receptors
– Phasic receptors
Tonic Receptors
Phasic Receptors
• Slow or no adaptation
• Rapid adaptation
• Continuous signal
transmission for duration of
stimulus
• Cease firing if strength of a
continuous stimulus
remains constant
• Monitoring of parameters
that must be continually
evaluated, e.g.:
baroreceptors
• Allow body to ignore
constant unimportant
information, e.g.:
– Smell
Tonic and Phasic Receptors
•
Two classes of receptors that encode stimulus duration
– Phasic – produce APs only at the beginning or end of the
stimulus  encode changes in stimulus, but not stimulus
duration
– Tonic – produce APs as long as the stimulus continues
• Receptor adaptation – AP frequency decreases if
stimulus intensity is maintained at the same level
Tonic and Phasic Receptors
Nociceptors
• Free dendritic endings
• Activation by strong, noxious stimuli - Function?
• 3 categories:
– Mechanical
– Thermal (menthol and cold / capsaicin and hot)
– Chemical (includes chemicals from injured tissues)
• Inflammatory Pain
• May activate 2 different pathways:
– Reflexive protective – integrated in spinal cord
– Ascending to cortex (pain or pruritis)
Epicritic-electric, lancinating, shock-like
Protopathic-constant, burning
Pain
•
Aβ, and AΔ fibers mediate pain
•
C fibers mediate pruritis
•
Fast (Aδ fibers) pain is sharp
•
Slow pain (C) is throbbing
•
Ascend to limbic system and hypothalamusEmotional Distress
– Modulation
•
Gate Control Theory: We can inhibit the pain response (fig 1012c)
•
Pain control
– NSAIDs (inhibit COX)
– Opiates (inhibit NT release)
Referred Pain
Pain in organs is
poorly localized
May be displaced if
Multiple 1° sensory
neurons converge
on single ascending
tract
Referred Pain
Referred Pain: Heart