1 Dr W Kolbinger, Sensory Systems (2009)
Sensory Systems
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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
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Common Plan of Sensory Systems Perception Behavior
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Pathway
Stimulus
Receptor
Sensory receptors
First-order sensory afferent neurons
Second-order sensory afferent neurons
Third-order sensory afferent neurons
Fourth-order sensory afferent neurons
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Sensory Systems and their Receptors
Sensory System Sensory Receptor Class
Somatosensory System (touch, vibration, proprioception, pain and temperature)
• NociceptorsVisual System
Vestibular System
Auditory System
Olfactory System
Gustatory System
• Mechanoreceptors• Thermoreceptors• Chemoreceptors
• Photoreceptors
• Mechanoreceptors
•Mechanoreceptors•Chemoreceptors •Chemoreceptors
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Examples of Sensory Receptors• Sensory receptor neuron(somatosensory and
olfactory systems• • • • Sensory receptor cell(visual, taste, and auditory
systems
1. Sensory receptors
A: Free nerve endings (pain, temperature)
B: Pacinian corpuscle (pressure)
C: Meissner’s corpuscle (touch)
D: Muscle spindle (stretch)
A
B C
D
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Three Basic Processes
• Cell body• transduction site• synaptic Terminal
• “graded potentials”
1) Receptor potential
2) Action potentials
3) Transmitter release
in Different Componentsof a Sensory Receptor
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Encoding of Four Stimulus Attributes
Attributes Encoding
• 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
Sensory Transduction and Receptor Potentials1. 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
Encoding of Stimulus Intensity
Encoding of Stimulus Intensity
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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)
slowly adapting receptors are better in constantly monitoringlevels of stimulation, whereas rapidly adapting receptors are most sensitive to changes, not to constant stimulation.
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A receptive field defines an area of the body that when stimulated results in a change in firing rate of a sensory neuron
Encoding of Location:Receptive Field of a Sensory Receptor
1
2
1
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Stimulation Recordings
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• Convergence • Divergence
Convergence and Divergence
convergence answers thequestion “Where does the information come from?”divergence answers the question “Where does the information go to?”
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
lateral inhibition
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Lecture Outline
• Five Modalities and their Receptors • Different Fibers for Different Receptors • Segmental Spinal Nerves and Dermatomes • Pathways for Different Modalities • Clinical Correlations •
Somatosensory System
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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.
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).
Touch is transduced byMerkel’s disks (discriminative touch) and Ruffini’s endings (skin stretch).
Vibration is transducedby 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-sensitivereceptors, burning pain by slowly adapting polymodal receptors) andTemperature (cold receptors and warm receptors) are transduced by free nerve endings.
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. Thisreceptor 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 arepositioned close to the border between muscle and tendon.Their Ib afferent fibers form the afferent limb of the reverse myotatic reflex(inverse myotatic reflex).
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Afferent Fiber Classification
A alpha A beta
A delta
C
I
II
III
IV
72-120 m/sec
36-72 m/sec
4-36 m/sec
0.4-2 m/sec
12-20 μm
6-12 μm
1-6 μm
0.2-1 μm
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Afferent Fiber Classificationand Somatosensory Modalities
12-20 μm
6-12 μm
1-6 μm
0.2-1 μm
Touch, Vibration
Pain, Temperature
Proprioception
• From skin: • From muscle:
Segmental Organization of Spinal Nerves and Dermatomes- segmental organization of the sensory innervation of the skin
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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
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Topographical Organization of the Spinal Cord
• Dorsal column
MidlineFrom leg
From trunk
From arm
Spatial Orientation of Signals from Different Parts of the Bodyin Somatosensory Area
• Somatosensory area has a high degree of localizationof the different parts of the body
Sensory Neurons: Two-Point Discrimination
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Pathway for Pain and Temperature:Anterolateral System
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Topographical Organization of the Spinal Cord
• ALS
MidlineFrom leg
From trunk
From arm
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Topographical Organization of the Spinal Cord
L T A
Right Left
LT
A
Touch, vibration, proprioception from left side of body
Pain, temperature from right side of body
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Lissauer’s Tract and the Anterolateral System
Right Left
Chief Complaint: Left Leg Paralysis, Right Leg NumbnessHistory:A 75 year old retired pastry maker was in good health until about one year ago, whenhe started to develop gait difficulties and numbness of his right leg. Recently, he alsoexperienced urinary urgency with occasional incontinence. Though he started takinglaxatives, he experienced problems with bowel movements. His wife reported that healso had stiffness in the legs bilaterally. At times, the left leg has unexpectedly not beenable to support his body weight for a brief period, causing him to stumble to maintainbalance. He also reported that in addition to the numbness in the right leg, he also hasa 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 softand non-tender. No palpable abdominal masses. Digital rectal examination showedsignificantly reduced muscle tone in the external sphincter and weakness of voluntarycontraction. 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. Uponmotor examination, the upper extremities had normal strength, bulk, and tone, and thereflexes 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 wasweaker than the right (4/5). The muscle bulk in both legs was normal. Reflexes in theright leg were 2+, knee jerk on the left leg was 3+, and ankle jerk was 4+. Plantarresponse was extensor the left and flexor on the right. Finger to nose and heel to shintesting was normal. Pinprick testing and temperature sensation showed decreasedsensitivity on the right side of the trunk below the umbilicus. Vibration and joint positionsense was significantly reduced in the left leg and foot.
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Brown Sequard Syndrome
Sensory loss at level of lesion
Ipsi- lateral
Contra- lateral
Touch, Vibration, Proprioception
Pain, Temperature
Sensory loss below level of lesion
Ipsi- lateral
Contra- lateral
Touch, Vibration, Proprioception
Pain, Temperature
X
X
X
X
33 Dr W Kolbinger, Visual System (2009)
Visual System
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Lecture Outline
• Structures of the Eye
• Refraction and Image Formation
• Visual Acuity
• Autonomic Control of Pupil Diameter
• Clinical Correlations •
Anatomic Considerations
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The Ocular Fundus
Fovea
Macula
Optic disc
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 receivevisual information.
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Optics of the Eye
Cornea refractive power: 42 D
Flat lens refractive power: 13 D
Rounded lens refractive power: 26 D
Plasticity: 13 D
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Accommodation
• Far Vision
Focus on the Retina
Flat lens refractive power: 13 D
Ciliary muscle relaxed
Suspensory ligaments tightened
Accommodation Adjusts the Refractive Power of the Eye
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Accommodation
• Near Vision
Rounded lens refractive power: 26 D
Focus on the Retina
Ciliary muscle constricted
Suspensory ligaments floppy
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• Near Vision
Blurred picture on the Retina
Flat lens
Presbyopia
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 elasticityduring aging, therebyreducing the ability to focuson near objects, a conditioncalled presbyopia.