1. Sensory Systems Picture: Rene Descartes (1596-1650)
Dr W Kolbinger, Sensory Systems (2009) 1

2. Lecture Outline Common Plan of Sensory Systems
Four Sensory Receptor Classes
Three Basic Processes in a Sensory Receptor
Encoding of Four Stimulus Attributes
Convergence, Divergence and Lateral Inhibition 2

3. Common Plan of Sensory Systems
Pathway
Perception
Behavior
Receptor
Sensory systems have a common plan. They are designed to detect a physical
stimulus (physical energy), such as light, sound, etc. This stimulus is
“translated” into cell activity in sensory receptor cells or sensory receptor
neurons. The scientific term for this “translation” process is signal transduction,
another term for sensory receptors is therefore transducers.
These receptors (when we talk about sensory systems, the term “receptors”
usually refers to receptor cells or receptor neurons, rather than to receptor
molecules) transmit the signal on to second order neurons, which are part of a
sensory pathway. Most sensory pathways include the thalamus as a relay
station.
Finally, the encoded sensory information reaches specialized regions of the
cerebral cortex. In general, most sensory cortex areas are located in the parietal,
occipital, temporal, or insular lobes.
The figure on the right,
which has been originally
drawn by Rene Descartes, a
17th century French
philosopher, includes
already all the essential
elements of the common
plan of sensory systems.
In the cortex, sensory
information reaches
consciousness. Sensory
information is interpreted
there and may trigger behavior. The process of interpretation of sensory input is
called perception.
Some behavioral responses, such as reflexes, can be triggered sub-cortically, at
the level of the spinal cord, for example.
Stimulus
Picture: Rene Descartes (1596–1650) 3

4. Fourth-order sensory
afferent neurons
Third-order sensory
afferent neurons
Second-order sensory
afferent neurons
First-order sensory
afferent neurons
Sensory receptors

5. Sensory Systems and their Receptors
Sensory Receptor Class
Somatosensory System
(touch, vibration, proprioception, pain and temperature)
Nociceptors
Visual System
Vestibular System
Auditory System
Olfactory System
Gustatory System
Mechanoreceptors
Thermoreceptors
Chemoreceptors
Photoreceptors
Mechanoreceptors
During the lecture series we will cover the six sensory systems, the
somatosensory system, the visual system, the vestibular system, the auditory
system, the olfactory system (smell) and the gustatory system (taste). The
olfactory system and the
gustatory system together
are also called the chemical
senses.
The six sensory systems
and their Receptors are
shown in the table . You realize that all
sensory systems together
function with a total of four
basic sensory receptor
classes, mechanoreceptors,
which are sensitive to
mechanical stimuli,
thermoreceptors, which are sensitive to temperature, chemoreceptors, which are
sensitive to chemical ions or molecules and photoreceptors, which are sensitive
to light.
Mechanoreceptors
Chemoreceptors
Chemoreceptors 5

6. Examples of Sensory Receptors
Sensory receptor neuron(somatosensory and olfactory systems
Sensory receptor cell(visual, taste, and auditory systems 6

7. 1. Sensory receptors A: Free nerve endings (pain, temperature)B C D
A: Free nerve endings (pain, temperature)
B: Pacinian corpuscle (pressure)
C: Meissner’s corpuscle (touch)
D: Muscle spindle (stretch)
Mechanoreceptors are activated by pressure or changes in pressure. Mechanoreceptors include, but are not limited to, the pacinian corpuscles in subcutaneous tissue, Meissner's corpuscles in nonhairy skin (touch), baroreceptors in the carotid sinus (blood pressure), and hair cells on the organ of Corti (audition) and in the semicircular canals (vestibular system). Photoreceptors are activated by light and are involved in vision. Chemoreceptors are activated by chemicals and are involved in olfaction, taste, and detection of oxygen and carbon dioxide in the control of breathing. Thermoreceptors are activated by temperature or changes in temperature. Nociceptors are activated by extremes of pressure, temperature, or noxious chemicals.

8. Three Basic Processes in Different Components of a Sensory Receptor
Cell body
transduction site
synaptic
Terminal
“graded potentials”
2) Action potentials
The components of a sensory receptor in general are the cell body (soma,
perikaryon), the transduction site, where the signal transduction takes place, an
axon, and a synaptic terminal where the signal is transmitted, usually to the
second order neuron of the sensory pathway.
As an example, the diagram below shows a typical sensory receptor of the
somatosensory system, a Pacinian corpuscle, which is, like all somatosensory
receptors, a pseudounipolar neuron.
Three basic processes occur in different parts of the sensory receptor:
Receptor Potential
Receptor potentials are
generated in the
transduction site of a
sensory receptor. These
receptor potentials are
“graded potentials”, which
means they do not follow
the all-or-nothing rule. They
are more like “analog
signals”, with variable
durations and intensities.
Action Potentials
In case the receptor potential is above threshold, action potentials are generated
in a defined area of the axon, which is called the trigger zone. Action potentials
are then conducted, first along the distal part of the axon, followed by the
proximal part of the axon until they reach the synaptic terminal. Please note that
in this example of a sensory receptor neuron, the action potentials are not
generated at the axon hillock, as it occurs for example in multipolar motor
neurons.
Transmitter Release
Transmitter release takes place at the synaptic terminals of the sensory receptor.
The transmitter released into the synaptic cleft will then bind to receptors (in this
case receptor molecules) in the postsynaptic membrane of the postsynaptic cell,
the second order neuron of the sensory pathway.
Not all sensory receptors are receptor neurons, like the one shown above. Some
sensory receptors, such as haircells, which are the mechanoreceptors of the
vestibular and the auditory systems, are receptor cells. These receptor cells,
instead of being of neuronal origin, are specialized epithelial cells. Their apical
pole is still the transduction site, but they lack an axon, and they don’t produce
action potentials. Instead, the graded receptor potential directly induces the
release of a signaling substance (transmitter) at the base of this cell.
1) Receptor potential
3) Transmitter release 8

9. Encoding of Four Stimulus Attributes
Modality
labeled lines
Intensity
Amplitude of graded receptor potentials
Frequency code of action potentials
Duration
Mechanisms depending on receptor adaptation
Location
Concept of receptive fields and other mechanisms
Four attributes characterize a sensory stimulus:
Modality refers to the physical type (or quality) of stimulus energy. This might
be light (for the visual system), sound (for the auditory system, touch (for the
somatosensory system), etc.
Intensity refers to the amplitude (or quantity) of a stimulus.
Duration is defined by the time between the start and the end point of the
stimulus.
Location refers to the place where the stimulus is located or originated. This
might be some place in our three dimensional environment around us (as for
light, or sound), or a place on the surface of our body (as for touch). 9

10. Encoding of Stimulus Intensity
Sensory Transduction and Receptor Potentials
1. The environmental stimulus interacts with the sensory receptor and causes a change in its properties
2. receptor potential or generator potential.
3.receptor potentials are graded in amplitude

11. Encoding of Stimulus Intensity
Encoding of Stimulus Intensity starts at the transduction site of a sensory
receptor. Low stimulus intensity causes only minor changes in the polarity of the
receptor, i.e. causes small graded receptor potentials.
Sub-threshold stimuli may
well induce small receptor
potentials, without inducing
any action potentials.
Frequency Code Once
receptor potentials are
above threshold, action
potentials are induced.
Higher stimulus intensities
induce higher receptor
potentials, which in turn
lead to a generation of
higher numbers of action
potentials per time unit.
This primary encoding mechanism for stimulus intensity, which is demonstrated
in the diagram above, is called the frequency code of individual axons.
Population Code refers to the fact that high intensity stimuli can activate
more individual axons than low intensity stimuli. Think about increasing the
pressure on your skin. The more you press, the larger the area and the number
of axons that are stimulated.

12. Encoding of Duration: Different Strategies
Slowly adapting receptors
remain active for the duration of a stimulus
Rapidly adapting receptors
are active only during times of changes (on/off)
Encoding of Stimulus Duration
Encoding of stimulus duration is another feature of sensory systems. It has its
origin again at the receptor level.
We all know from every day experience that a while after the onset of a sensory
stimulus, the sensation induced by the stimulus may disappear, until it is no
longer recognized. This process takes already place at the receptor level and is
called receptor adaptation.
For example while you dress in the morning, sensory information caused by your
clothes touching the skin help you in the process. During the day, you no longer
feel the touch of your clothiers, although you are still dressed and the fabric
touches your skin.
Sensory receptors differ widely in their adaptation rate. We differentiate between
slowly adapting receptors and rapidly adapting receptors.
Ideally, slowly adapting
receptors remain
depolarized and produce
increased rates of action
potentials for the whole
duration of the stimulus.
The duration of the stimulus
is indicated by the time the
frequency of action
potentials is elevated.
Slowly adapting receptors
are important for regulatory
functions, when a
physiological parameter has
to be kept at a certain
value.
Rapidly adapting receptors, on the other hand, may only signal the onset of the
stimulus, as shown in the lower portion of the diagram. The receptor potential
quickly returns to baseline values and no action potentials are generated after
this initial phase, although the stimulus is still on with the same intensity.
In reality, it is rare that sensory receptors act as simple. Rapidly adapting
receptors often have a basal discharge in the absence of a stimulus. Their firing
rate may increase during the onset of the stimulus, it may return to the basal
rate during the duration of the stimulus, and the offset of the stimulus might
cause a short drop in firing rate.
As a consequence, slowly adapting receptors are better in constantly monitoring
levels of stimulation, whereas rapidly adapting receptors are most sensitive to
changes, not to constant stimulation.
slowly adapting receptors are better in constantly monitoring
levels of stimulation, whereas rapidly adapting receptors are most sensitive to changes, not to constant stimulation. 12

13. Encoding of Location: Receptive Field of a Sensory Receptor
Stimulation
Recordings 1 1 2 2
Encoding of Stimulus Location
The way a stimulus is localized may vary in different sensory systems and we
will focus on mechanisms for the somatosensory system, for the visual system
and for the auditory system later in the lecture series.
One basic principle I like to
introduce at this point is the
concept of a receptive field
of a sensory receptor.
The diagram on the right
shows a schematic drawing
of a somatosensory touch
receptor neuron
(pseudounipolar neuron)
innervating a certain area of
the skin. The square on the
left of the diagram depicts a
part of the body’s surface. A
recording electrode
measures the action
potentials at one point along the axon and the recordings are shown on the right
hand side.
When the skin is touched at the tip of arrow # 1, the recording shows baseline
activities. When the skin is touched at # 2, the number of action potentials
increases. You would repeat the experiment until you could outline the whole
area where a stimulus would induce a response in this neuron. The area is then
called the receptive field of this neuron.
A receptive field defines an area of the body that when stimulated results in a change in firing rate of a sensory neuron 13

14. Convergence and Divergence
convergence answers the
question “Where does the information come from?”
divergence answers the question
“Where does the information go to?” 14

15. lateral inhibition Receptive fields can be excitatory or inhibitory.
The areas of inhibition contribute to a phenomenon called lateral inhibition, and aid in the precise localization of the stimulus by defining its boundaries and providing a contrasting border

16. Somatosensory System Lecture Outline
Five Modalities and their Receptors
Different Fibers for Different Receptors
Segmental Spinal Nerves and Dermatomes
Pathways for Different Modalities
Clinical Correlations 16

17. Modalities of the Somatosensory System
Touch (discriminative touch)
Vibration
Proprioception
Pain (nociception)
Temperature
most of the somatic sensory modalities refer to sensations of the skin, proprioception refers to sensory receptors originating in the skeleto-muscular system.
The somatic sensory system comprises of five different modalities: touch,
vibration, proprioception, pain and temperature. Whereas most of the somatic
sensory modalities refer to sensations of the skin, proprioception refers to
sensory receptors originating in the skeleto-muscular system.
All receptors of the somatic sensory system are pseudo-unipolar neurons. Their
sensory endings can be found in skin, or in (or close to) the muscle. Their fibers
run in peripheral nerves and their cell bodies are located in ganglia (dorsal root
ganglia in case the fibers run in spinal nerves or in cranial nerve ganglia).
All receptors of the somatic sensory system are pseudo-unipolar neurons. Their sensory endings can be found in skin, or in (or close to) the muscle. Their fibers run in peripheral nerves and their cell bodies are located in ganglia (dorsal root ganglia in case the fibers run in spinal nerves or in cranial nerve ganglia). 17

18. Touch is transduced by
Merkel’s disks (discriminative touch) and Ruffini’s endings (skin stretch).
Vibration is transduced
by Meissner’s corpuscles (for lower frequencies of about 50 Hz) and Pacinian corpuscles (for higher frequencies of about 300 Hz).
Pain (pricking pain by rapidly adapting mechano-sensitive or thermo-sensitive
receptors, burning pain by slowly adapting polymodal receptors) and
Temperature (cold receptors and warm receptors) are transduced by free nerve endings.

19. Golgi tendon organs are
Muscle spindles are embedded in extrafusal fibers of the working musculature of the muscle.
The primary receptor of a muscle spindle is a rapidly adapting receptor with a Ia
("one a") afferent fiber. It carries sensory information of muscle stretch. This
receptor forms the afferent limb of the myotatic reflex (deep tendon reflex),
The secondary receptor of a muscle spindle is a slowly adapting receptor carrying sensory information of muscle length. It uses a class II ("two") afferent fiber.
Golgi tendon organs are
positioned close to the border between muscle and tendon.
Their Ib afferent fibers form the afferent limb of the reverse myotatic reflex
(inverse myotatic reflex).

20. Afferent Fiber Classification
12-20 μm
6-12 μm
1-6 μm
0.2-1 μm
A alpha
A beta
A delta C I II
III IV
m/sec
36-72 m/sec
4-36 m/sec
0.4-2 m/sec
Three types of primary afferent fibers (with cell bodies in the dorsal root ganglia) mediate cutaneous sensation: (1) large myelinated A and A fibers that transmit impulses generated by mechanical stimuli; (2) small myelinated A fibers, some of which transmit impulses from cold receptors and nociceptors that mediate pain and some of which transmit impulses from mechanoreceptors; and (3) small unmyelinated C fibers that are concerned primarily with pain and temperature. However, a few C fibers also transmit impulses from mechanoreceptors. 20

21. Afferent Fiber Classification and Somatosensory Modalities
From skin:
From muscle:
12-20 μm
6-12 μm
1-6 μm
0.2-1 μm
Touch,
Vibration
Proprioception
Pain,
Temperature 21

22. Segmental Organization of Spinal Nerves and Dermatomes- segmental organization of the sensory innervation of the skin

23. Pathway for Touch, Vibration, Proprioception: Dorsal Column / Medial Lemniscus System
Touch, vibration and proprioception are carried in a pathway called the dorsalcolumn
/ medial lemniscus system 23

24. Topographical Organization of the Spinal Cord
Dorsal column
From arm
From trunk
From leg
Midline 24

25. Spatial Orientation of Signals from Different Parts of the Body
in Somatosensory Area
Somatosensory area has a high degree of localization
of the different parts of the body

26. Sensory Neurons: Two-Point Discrimination

27. Pathway for Pain and Temperature: Anterolateral System27

28. Topographical Organization of the Spinal Cord
ALS
From arm
From trunk
From leg
Midline 28

29. Topographical Organization of the Spinal Cord
Touch, vibration, proprioception
from left side of body L T A
Right
Left L T A
Pain, temperature
from right side of body 29

30. Lissauer’s Tract and the Anterolateral System
Right
Left 30

31. Chief Complaint: Left Leg Paralysis, Right Leg Numbness
History:
A 75 year old retired pastry maker was in good health until about one year ago, when
he started to develop gait difficulties and numbness of his right leg. Recently, he also
experienced urinary urgency with occasional incontinence. Though he started taking
laxatives, he experienced problems with bowel movements. His wife reported that he
also had stiffness in the legs bilaterally. At times, the left leg has unexpectedly not been
able to support his body weight for a brief period, causing him to stumble to maintain
balance. He also reported that in addition to the numbness in the right leg, he also has
a constant tingling feeling in the same limb, which he describes as “intolerable.”
General Examination:
Normal vital signs. Patient has no significant cardiovascular history. Abdomen was soft
and non-tender. No palpable abdominal masses. Digital rectal examination showed
significantly reduced muscle tone in the external sphincter and weakness of voluntary
contraction. Prostate was felt to be enlarged with a highly nodular, irregular surface.
Neurological examination:
Patient was fully alert and oriented x 3. Cranial nerve exam was unremarkable. Upon
motor examination, the upper extremities had normal strength, bulk, and tone, and the
reflexes were 2+ throughout the C5 to C8 spinal level. In the lower extremities,
however, the muscle tone was increased in left leg and the left iliopsoas muscle was
weaker than the right (4/5). The muscle bulk in both legs was normal. Reflexes in the
right leg were 2+, knee jerk on the left leg was 3+, and ankle jerk was 4+. Plantar
response was extensor the left and flexor on the right. Finger to nose and heel to shin
testing was normal. Pinprick testing and temperature sensation showed decreased
sensitivity on the right side of the trunk below the umbilicus. Vibration and joint position
sense was significantly reduced in the left leg and foot.

32. Brown Sequard SyndromeX X X X 32

33. Visual System Lecture Outline
Structures of the Eye
Refraction and Image Formation
Visual Acuity
Autonomic Control of Pupil Diameter
Clinical Correlations
Dr W Kolbinger, Visual System (2009) 33 33

34. Anatomic Considerations

35. The Ocular Fundus Optic Fovea disc Macula
The diagram on the right
superimposes the passage
of light originating from the
point of fixation and from
the blind spot region of the
visual field on the
underlying anatomical
structures.
Light originating from the
point of fixation is projected
onto the fovea. Light
originating from the blind
spot region of the visual
field ends up in the optic
disk region of the retina. Since the optic disk region does not contain
photoreceptor, the visual information of this region is lost. Most of the time we
are not aware of the loss of visual information, since our brain “completes” the
picture with the “most likely” visual information.
The optic disc region itself only contains axons of retinal ganglion cells, the output elements of the retina, but it lacks photoreceptors. As a consequence, the optic disc is responsible for the blind spot, a region inside the boundaries of the visual field, where we don’t receive
visual information. 35

36. Optics of the Eye Cornea refractive power: 42 D
Rounded lens refractive power: 26 D
Flat lens refractive power: 13 D
Refractive Power of an Optical System
When light rays pass from air into another medium, such as water (or human
tissue, which also has a high water component), or a glass lens for example,
they change their direction. This interaction between light and its environment is
called refraction.
When incoming parallel light rays pass through a convex lens, all light rays after
the passage will focus at a defined distance from the midline of the lens. This
distance is called focal distance. The focal distance depends on the material and
on the curvature of the lens.
Refractive power is the reciprocal value of the focal distance in meters. Its unit
of measurement is called diopter, abbreviated as “D” (plural: diopters).
In the human eye, the cornea is responsible for the main refractive power of the
eye of about 42 D, whereas the lens is responsible for the modulation of
refractive power (refractive plasticity) during accommodation.
Plasticity: 13 D 36

37. Accommodation Far Vision
Ciliary muscle relaxed
Flat lens refractive power: 13 D
Suspensory ligaments tightened
During far vision, light rays
originating from a distant
object can be considered
(almost) parallel. The focus
of light rays during far
vision largely depends on
the high refractive power of
the cornea (42 D) and only
a small refractive power of
the flat lens (13 D).
During far vision, the ciliary
muscle, which is a circular
muscle around the lens, is
relaxed. The diameter of
this circular structure is large, causing tightening of the suspensory ligaments
(zonule fibers). As a consequence, these ligaments pull on the lenses’ equator,
thereby flattening it and minimizing its refractive power.
Focus on the Retina
Accommodation Adjusts the Refractive Power of the Eye 37

38. Accommodation Near Vision Ciliary muscle constricted
Rounded lens refractive power: 26 D
Suspensory ligaments floppy
During near vision
incoming light rays can no
longer be considered
parallel. Therefore, a
stronger refractive power is
needed to focus the light
rays on the retina.
This is achieved by
constriction of the circular
ciliary muscle, which
reduces its’ diameter and
relaxes the suspensory
ligaments. The lens follows
its own elasticity and gets a
more rounded (more convex) shape, which increases its refractive power to
about 26 D in a young individual.
The ciliary muscle is activated by parasympathetic fibers of the autonomic
nervous system (ANS). Cell bodies of the first order efferent parasympathetic
neurons are located in the upper midbrain, in the Edinger-Westphal nucleus of
cranial nerve III. Together with the somatic efferent fibers innervating the
extraocular muscles, the preganglionic fibers originating in the midbrain form
the oculomotor nerve (CN III). The preganglionic parasympathetic fibers synapse
on second order neurons in the ciliary ganglion. Postganglionic fibers originating
in this ganglion form the short ciliary nerves which innervate the ciliary muscle.
Focus on the Retina 38

39. Presbyopia Near Vision Flat lens Blurred picture on the Retina
The variability of the refractive power of the lens between far vision (13 D)
and near vision (26 D) is called refractive plasticity.
Unfortunately,
the lens looses its elasticity
during aging, thereby
reducing the ability to focus
on near objects, a condition
called presbyopia.
Flat lens
Blurred picture on the Retina 39