Corneal Innervation (M2) Flashcards
Where are the cell bodies of the corneal sensory neurons?
trigeminal ganglion
What are the subsets of corneal innervation?
1) Corneal stromal nerves
2) Subepithelial nerve plexus
3) Sub-basal nerve plexus
4) Intraepithelial nerve terminals
Where are stromal nerves found?
anterior stroma
What innervates the perilimbal and peripheral cornea?
small nerve fascicles originating at limbal plexus and conjunctiva
Are stromal nerves myelinated?
yes, Schwann
Where is anastimosis more frequent in the central cornea?
anterior 1/3
Where is the subepithelial nerve plexus located in the cornea? 1. What part of the cornea has a more dense plexus? 2
- anterior stroma immediately beneath Bowman’s membrane
2. peripheral more dense than center
Where does penetration of Bowman’s membrane by nerve bundles mainly occur? 1. What must this type of subepithelial plexus shed to do this? 2
- peripherally
2. Schwann cells
What forms the sub-basal nerve plexus?
penetrating nerve fibers from the subepithelial nerve plexus that divides into multiple, thinner nerve fascicles (2-20) and then branch into epithelial leash fascicles
What nerves make a vortex? 1. What is the location of the center of this vortex? 2.
- sub-basal nerve plexus
2. inferior and nasal
How far may individual axons in the sub-basal plexus travel?
6 mm
How many intraepithelial nerve terminals does each sub-basal nerve give rise to?
10-20
How do intraepithelial nerve terminals end? 1. Where are these most common? 2
- free nerve endings with a bulbous morphology
2. basal and wing cells
How much area can a individual axon branch cover?
1-50 mm2
Which nuclei of the spinal nucleus of the medulla of the Trigeminal Brainstem Nuclear Complex (TBNC) mediate corneal sensation?
- Interpolaris nucleus
2. Caudalis nucleus
Where do third order neurons of the central sensory pathway synapse? 1. And where do they project to? 2
- Edinger Westphal nucleus
2. somatosensory cortex (pain)
Describe the Aβ nerve fiber:
- relative thickness
- innervated structures
- myelinated or not?
- relative conduction velocity
- type of sensation
- thick
- Limbus, palpebrae, conjunctiva
- myelinated
- medium
- touch. pressure
Describe the Aδ nerve fiber:
- relative thickness
- innervated structures
- myelinated or not?
- relative conduction velocity
- type of sensation
- thin
- Cornea (~30%), Uvea
- myelinated (except cornea)
- medium
- Touch, pressure, temperature, pain
Describe the C nerve fiber:
- relative thickness
- innervated structures
- myelinated or not?
- relative conduction velocity
- type of sensation
- thinnest
- Cornea (~70%), Uvea
- non-myelinated
- slowest
- pain
What are the three types of cornea somatosensory receptors?
- Polymodal nociceptors
- Mechano-nociceptors
- Cold thermal receptors
Describe polymodal nociceptors in cornea:
- percent of total
- nerve fiber type
- relatie receptive field
- firing style
- 70%
- C-type»_space; Aδ-type
- large
- continuous irregular discharge of nerve impulses that persist as long as the stimulus is maintained. Frequency roughly proportional to intensity
What do polymodal nociceptors respond to (along with thresholds of each)?
- heat (>39-40oC)
- cold (<6.5)
- mechanical forces (very low threshold)
Describe Mechano-nociceptors in cornea:
- percent of total
- nerve fiber type
- relatie receptive field
- firing style
- Threshold
- 15-20%
- all Aδ fibers
- large
- Fire a few nerve impulses in response to a brief or sustained stimuli
- very low 0.6mN
Describe cold thermal receptors in cornea:
- percent of total
- nerve fiber type
- relatie receptive field
- firing style
- Threshold
- 10-15%
- Aδ- and C-fibers
- small
- Spontaneously discharges at a low rate without stimulus. Rate of discharge increases when <33oC and decreases when high temp
- small (0.1oC) decrease
What convert suprathreshold noxious stimuli into coded electrical impulses by opening cation-permeable channels along small caliber Aδ and C fibers?
Transient receptor potential channels (TRP channels)
What do TRP channels usually form as?
tetramers
What fibers innervate the perilimbal episclera? 1. What stimuli do they respond to? 2
- thick, fast conducting (AB) fibers
2. gentle mechanical stimulation (Non-noxious touch)
During injury, what is it called when non-stimulated branches receive impulses (in reverse direction)?
antidromic stimulation
What are the neuropeptides released by nerves during corneal injury? 1. What do these do? 2
- CGRP (Calcitonin gene-related peptide) and (SP) Substance P
- Contribute to local inflammatory response through stimulation of cytokine production and stimulate epithelial mitosis and migration
What causes neurotrophic keratopathy? 1. What does it result in? 2
- Impairment of trigeminal nerve
2. Loss of corneal sensation and poor healing/homeostasis so form ulcers
What is a long lasting, irregular, low frequency nociceptor firing that reappears after the impulse caused by the direct stimulus? 1. What is it accompanied by? 2. What is the cause of it?
- sensitization
- hyperalgesia
- large number of cytokines released by locally damaged cells
Does refractive surgery increase or decrease sensitivity? 1. Contact lens wear? 2
- decrease
2. decrease
What is the mechanism of topical anesthetics? 1. What are examples of these? 2
- Block propagated action potentials by inhibiting Na+ channels
- Proparacaine, lidocaine
