10/7/2010
Sensory Receptors
Sensory Receptors
(some basic properties)
Organs which
receive information
from outside or
within the body and
send it to the
central nervous
system for
processing
Sensory Receptors
(some basic properties)
Sensory Receptors Are Transducers
Definition of a Transducer or Transduction
Conversion of one form of energy (e.g. heat,
pressure, chemical) into another form: e.g. electrical
energy or depolarisation
Specialised nerve ending
Activated by
various forms
of energy:
light heat
light,
heat,
mechanical
chemical etc.
The depolarisation occurring at the sensory terminal is
a). Graded – Non-propagating, confined to terminal
b). Called the Receptor Potential (R.P.)
Specialised
cells
Sensory receptors are either specialized endings of afferent
neurones or separate cells that signal to the afferent neurone
The Receptor Potential
The Receptor Potential:
If large enough will evoke all or nothing action
potentials that propagate along the nerve fibre
Intensity of Stimulus is Linearly Encoded
Variable stimulus
intensity
10mV
Produces graded
receptor potentials
Adequate stimulus
Produces variable
patterns of
action potentials
in the CNS
More Stimulus = More Transmitter Released
The number of action potentials generated by a pressuresensitive sensory afferent neuron is directly proportional
to the stimulus intensity.
1
10/7/2010
Receptor Potential:
more or less proportional to stimulus
stimulus
receptor potential
Who can guess how a membrane
of a neurone could react to
pressure or temperature by a
change in membrane potential?
adaptation
mV
Adaptation to stimulus depends on receptor type
Adaptation: A Mechanism Preventing
Sensory Information Overload
The off
response
Specific types of sensory receptors for
different mechanosensory stimulation
Skin:
Paccinian
Corpuscles
pressure/touch
Muscle:
stretch
receptors
Different receptors vary in their speed of adaptation to stimuli
Receptor Specificity
Sensory Processing:
spatial discrimination
Receptors have a differential sensitivity to
different energies or stimuli
Mechanoreceptors
Photoreceptors
Chemoreceptors
nerve ending
Sensory afferent (A)
has a more spatially
limited receptive field
than sensory afferent
neuron (B).
A receptor is specific to one energy (or stimulus), because
it is most sensitive to that energy. Other energies may
stimulate but only at high intensities.
2
10/7/2010
Sensory Processing:
Sensory Processing:
spatial discrimination – the paired point test
spatial discrimination
Lips
Back
Overlapping stimulation between
neighboring receptive fields
provides
id generall information
i f
ti
about the location of a stimulus.
Sensory Processing:
Sensory Processing:
density of sensory afferent terminals
descending modulation from higher centres
Painful Condition
Stimulus A
occurs in an area
of greater nerve
ending density
Stimulus A will
generate
a greater number
of action
potentials than
stimulus B.
Context and Sensory Perception
Monty Python & the Holy Grail
Painful Condition
& Fight or Flight
Descending input
Sensory Processing: sensory discrimination
Deliver information about locationspecific changes in touch and
temperature in the periphery.
Provide background
information about
touch and temperature
3
10/7/2010
Convergence of sensory pathways is one of the reasons
for “referred” pain from visceral organs
Sensory Modality Representation is Location
Specific in the Cerebral Cortex
Sensory Representation is Cortex
Summary:
Fine discrimination
is highly represented
cortical homunculus
1. Sensory receptors allow our body to interact with the external environment
2. Sensory receptors in internal organs are essential for homeostasis and
also alert the body in case of some anomaly
3. Some receptors adapt fast, others slow and some – almost not at all.
4. The intensity of the stimulus is encoded by the frequency of the action
potentials.
5. Most receptors detect the stimulus via a subset of highly specialised ion
channels which open (Na+) or close (K+) to evoke depolarisation of the
membrane of the sensory ending in response to the stimulus.
6. Information sent from the receptors is called AFFERENT information. In the
CNS it is usual that several afferent neurones contact the same
postsynaptic cell. This is the convergence, which is also a reason for
referred pain.
7. The organs with the highest tactile sensitivity have the largest
representations in the “sensory homunculus”
4