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PERIPHERAL AXONS: CHARACTERISTICS

Axons
  Caliber
  Conduction velocity
  Motor unit action potentials
  Transport
  Unmyelinated
Cutaneous afferents
Dorsal root ganglia
Mechanotransduction
Motor
Muscle spindle
Muscle & tendon afferents
Nociceptors
Thermoreceptors
  Temperature-sensitive channels

External links
  Images
  Nerve impulse

Mills

Sensory Axons

CUTANEOUS SENSORY AFFERENT AXONS
Mechanoreceptors: Low Threshold
Name Stimulus Type End Organ Class Sensation Trophic Factor
Survival Functional
Type I Displacement Touch spot Aαβ ? ? ?
Slow
  adapting I
Displacement Merkel cell Aαβ Sustained
  pressure
Shape
Texture
Light touch
NT3 BDNF

Marker: Atoh1
Type II
SA II
Displacement Ruffini Aαβ ? ? ?
G2 hair Displacement
  velocity
Hair follicle
Pallisade ending
BNC1 Na+ channel
Aαβ ? Hair
  movement
? ?
Rapidly
  adapting
Displacement
  velocity
Meissner
  corpuscle
Aαβ Tapping ? ?
Field Displacement
  velocity
? Aαβ ? ? ?
D hair Displacement
  velocity
Lowest
  mechanothreshold
T-Ca++ channel
  (Cav3.2) 2
? NT4; NT3
via TrkB
± p75; ?NT4
Pacinian
  corpuscle
Displacement
  transient
  (e.g. vibration)
Pacinian
  corpuscle
Aαβ Buzzing;
  Flutter
? ?
G1 hair Displacement
  transient
  (e.g. vibration)
Hair follicle Aαβ ? ? ?
Thermoreceptors
Name Stimulus Type End Organ Class Sensation Trophic Factor
Survival Functional
Cold Cooling
Menthol
Unmyelinated
  neurite complex

C
Cool
  Innocuous
? ?
Warm Warming ? C Warm
  Innocuous
? ?
Also see: Temperature-sensitive channels
Nociceptors
Name Stimulus Type End Organ Class Sensation Trophic Factor
Survival Functional
Myelinated
  nociceptor
Pain Unmyelinated
  Neurite-Schwann
  cell complex
Basal epidermis
Myelinated axons
Aαβ
Pain
  Sharp
  Stinging
NGF p75; NGF
C nociceptor Inflammation
  Hyperalgesia
Unmyelinated
  axons
Project to spinal
  Lamina I &
  Lamina IIouter
C Pain
  Dull or
  Sharp Burn
NGF
trkA
p75; NGF;
trkA; CGRP;
Substance P &
  NK1 receptor
Nerve injury
  Hyperalgesia
Unmyelinated
  axons
Project to spinal
  Lamina IIinner
Noxious heat: TRP-V1
C Pain
  Dull or
  Sharp Burn
NGF
trkA
GDNF; Ret
GFRα;
FRAP & TMP;
P2X3 ATP
  Receptor
Itch
Histamine
Unmyelinated
  axons
Project to spinal
  Lamina I
C Itch ? ?

Muscle Axons

MUSCLE & TENDON AFFERENT AXONS
Name Stimulus Type End Organ Effect Class Trophic
Factor
1° sensory Muscle stretch
  Phasic
Spindle
Intrafusil fibers
  Nuclear region
  DB1; 1 branch
  SB2 & chain;
    2nd branch
Excite
  homonymous muscle
Monosynaptic reflex
Ia NT3
2° sensory Muscle stretch
  Tonic
Spindle
Nuclear chain fibers
  ± SB2
Excite
  homonymous muscle
Mono & Polysynaptic
II .
Golgi tendon Tendon stretch
  Phasic &
  Sustained
Myotendinous junction
Golgi tendon organ
Inhibit
  homonymous muscle
Ib .
Proprioception 8
Current stimulus: Mechanical activation (MA); Muscle stretch
Current type: Rapidly adapting
Neuron currents
  Ion: Na+ > > Ca++
  Channel: Piezo2
    Location: Muscle spindle & Golgi tendon organ sensory endings
    Function: Mechanotransduction
Afferent neuron molecular marker: Parvalbumin



MUSCLE SPINDLES

Disorders
General
Histology
Innervation
  Afferent
  Efferent

Spindles: General 7
MUSCLE SPINDLE INNERVATION (simplified)

From: University of Glasgow
Muscle Spindle Afferents
  • Axon size: IA
  • Express α3 isoform of Na+/K+-ATPase


MOTOR EFFERENT AXONS
Name Low frequency
stimulus effect
Muscle Fibers Innervated Ending type Class Trophic
Factor
Dynamic γ Minor contraction
Increased stretch response
Spindle (Intrafusil)
  Dynamic Bag1 (DB1)
Indented plate
at poles of fibers
II NT3
Static γ Smooth contraction (SB2) Spindle (Intrafusil)
  Static Bag2 (SB2)
Indented plate II NT3
Oscillation Spindle (Intrafusil)
  Chain
Trail
Dynamic β ? Extrafusil
Intrafusil: DB1
Compact (p1) endplate . .
Static β ? Extrafusil
Intrafusil: Chain
Compact (p1) endplate . .
α Muscle fiber contraction Extrafusil Compact (p1) endplate I .

Muscle Spindles: Normal


Spindle with Intrafusil muscle fibers & associated axons

Gomori trichrome stain

Perimysial Structures
Spindle
Intramuscular nerve
Vessels
    Vein
    Artery

VvG stain

Muscle spindle with esterase-stained neuromuscular junctions

Esterase stain

Muscle Spindle
Coarse internal architecture similar to regenerating or immature extrafusil muscle fibers

NADH stain

Muscle Spindle Disorders


Axon: Caliber


Axon: Conduction velocity


Axon: Transport 4

Disorders
General substrates
Types
  Anterograde
  Retrograde
  Synaptic regions

Motor Unit Action Potentials (MUAP)


Dorsal Root Ganglion

Dorsal Root Ganglion (DRG) Development: Cell origins DRG anatomy DRG Neurochemistry DRG Sensory modalities: Neurophysiology & Anatomy Also see: Sensory neuronopathies

Temperature Sensing Channels 3

Ion channel Activating temperature Activators
VRL-1 (TRPV2) 53 °C Noxious heat
VR1 (TRPV1) 45 °C Noxious heat
Capsaicin
pH < 5.9
Vanilloids
Endocannabinoids
TRPV3/TRPV4 (VRL-2) 30 to 40 °C Warm
Painless > 41 °C Noxious heat
CMR1/TRPM8 < 25 °C Cold
Menthol
TRPA1 (ANKTM1) < 17 °C Noxious cold
Icilin
See



Mechanotransduction Channels 5


Remak: 1st description of non-myelinated fibers (axons) 9


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Return to Molecular Classifications

References
1. Nature Neurosci 2002;5:254-260
2. Nature Neurosci 2003;July
3. Gut 2003;52:1233–1235
4. Neuron 2010;68:610-638
5. TINS 2012; Online May 21
6. Neuron 2012;74:961-963, Neuron 2012;74:1015-1022
7. Pflugers Arch 2014 Jun 3
8. Nat Neurosci 2015;18:1756-1762
9. Remak 1838

7/25/2017