Download Somatosensory system

Somatic Sensation
(MCB160
Lecture by Mu-ming Poo, Friday March 9, 2007)
• Introduction
– Adrian’s work on sensory coding
– Spinal cord and dorsal root ganglia
– Four somatic sense modalities
• Touch
– Mechanoreceptors and signal transduction
– Receptive fields of DRG neurons
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Proprioception
Nociception
Thermal Sensation
Central Projection and Somatotopic Maps
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Origin of Sensory Physiology and Neuronal Information Coding
Weak pressure
Higher pressure
very high pressure
Some of Adrian’s first recordings from a very small number of
nerve fibers in the sensory nerves of cat’s toe.
Adrian’s Findings:
1. The nerve impulse (action potential) is “all-or-none”
2. The strength of stimulus is coded by the firing frequency
3. There is adaptation of neuronal firing after stimulus onset
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Weber-Fechner’s Law
I = k Log (S/So) where I= intensity of sensation (freq of firing)
k, So (threshold intensity) are constants
Power law: I = k (S-So)n , where n is a constant
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Organization of the Spinal Cord
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Spinal Cord and Dorsal Root Ganglia (DRG)
(ventral horn)
Ventral root
Spinal nerve
Dorsal root
Dorsal root ganglia - spinal nerve - limb and trunk
Trigeminal ganglia - cranial nerve - head and face
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Stimuli: mechanical force, temperature change, tissue damage,
or chemical action; mediated specifically by different receptors
Nociceptors
Thermoceptors
Mechanoceptors
Mechanoceptors
Nociceptors, thermoceptors – bare endings, small diameter unmyelinated axon
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Mechanoreceptors: encapsulated (wrapped) endings, large diameter myelinated axon
Peripheral endings of DRG neurons
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Transduction by mechanoreceptors
• Stress-gated ion
channels open when
membrane is deformed
• Na+ influx
• Depolarization
(generator potential)
• Initiation of action
potential
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Four types of mechanoreceptors
glaborous (hairless) skin
1.
2.
3.
4.
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Subcutaneous
Pacinian
Superficial
Meissner
Raffini
Merkel
disk
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Receptive Field
• The area in the periphery within which
sensory stimulus can modulate the firing of
the sensory neuron.
• Spatial resolution of the RF:
Size: smaller RF - higher resolution
Density: higher density – higher resolution
“Two-point discrimination test”
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RFs of mechanoreceptor neurons
RF size: 2-10 mm
Each DRG axon receives convergent
inputs from 10-25 corpuscles
RF size: several cm
Each DRG axon receives input from 12
single corpuscle or ending
Proprioception
(Sense of position and movement of limb and body)
Mechanoreceptors in muscle and joints
1. Muscle spindle receptors – detect the extent and
rate of muscle contraction, endings in parallel
with muscle fibers
2. Golgi tendon organs – detect tension exerted by
the muscle, ending in series with muscle fibers
3. Joint capsule receptors – detect flexion or
extension of joints
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Nociception (Pain Sensation)
Nociceptors
-- Respond to noxious (mechano-, thermo) stimuli
directly or chemical released by damaged cells
-- Chemicals: histamine, bradykinin, substance P,
ATP, serotonin, acetylcholine, acids, high K+
Three types:
1. Mechano-sensitive nociceptors (high threshold)
2. Thermo-sensitive nociceptors (high threshold)
3. Polymodal (chemo-sensitive) nociceptors
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Thermal Sensation
Thermoreceptors
-- Ending of unmyelinated C-fibers
-- Spontaneous firing at low-freq at skin T of 34oC, respond only
within innocuous range.
Two Types:
1. Cold receptors – fire when T decreases from 34oC, maximal
firing at 25oC
2 . Warmth receptors – fire when T increases from 34oC,
maximal firing at 45oC
Hot sensation – noxious stimulus detected by nociceptors, not
thermal receptors
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Projection to CNS
(you should know these pathways,
although not covered in the lecture, see textbook for detail)
Dorsal Column-Medial Lemniscal System
(Touch and proprioception)
-- afferent to spinal cord laminae III-VI
-- ascend ipsilaterally (dorsal column)
-- cross midline in medulla
-- ascend to midbrain via medial lemnicus
-- to thalamus and somatosensory cortex
Anterolateral System
(Pain and Thermal Sensation)
-- afferent to spinal cord laminae I & II
-- cross midline to contralateral anterolateral
column
-- ascend via direct (spinothalamic) and
indirect pathways to thalamus
-- to somatosensory cortex
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Somatotopic map in the cortex
Sensory homunculus: This model
shows what a man's body would look
like if each part grew in proportion to the
area of the cortex of the brain
concerned with its sensory perception. 18