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I. Ocular Physiology > Corneal Innervation (M2) > Flashcards

Corneal Innervation (M2) Flashcards

1
Q

Where are the cell bodies of the corneal sensory neurons?

A

trigeminal ganglion

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2
Q

What are the subsets of corneal innervation?

A

1) Corneal stromal nerves
2) Subepithelial nerve plexus
3) Sub-basal nerve plexus
4) Intraepithelial nerve terminals

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3
Q

Where are stromal nerves found?

A

anterior stroma

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4
Q

What innervates the perilimbal and peripheral cornea?

A

small nerve fascicles originating at limbal plexus and conjunctiva

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5
Q

Are stromal nerves myelinated?

A

yes, Schwann

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6
Q

Where is anastimosis more frequent in the central cornea?

A

anterior 1/3

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7
Q

Where is the subepithelial nerve plexus located in the cornea? 1. What part of the cornea has a more dense plexus? 2

A
  1. anterior stroma immediately beneath Bowman’s membrane

2. peripheral more dense than center

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8
Q

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

A
  1. peripherally

2. Schwann cells

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9
Q

What forms the sub-basal nerve plexus?

A

penetrating nerve fibers from the subepithelial nerve plexus that divides into multiple, thinner nerve fascicles (2-20) and then branch into epithelial leash fascicles

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10
Q

What nerves make a vortex? 1. What is the location of the center of this vortex? 2.

A
  1. sub-basal nerve plexus

2. inferior and nasal

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11
Q

How far may individual axons in the sub-basal plexus travel?

A

6 mm

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12
Q

How many intraepithelial nerve terminals does each sub-basal nerve give rise to?

A

10-20

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13
Q

How do intraepithelial nerve terminals end? 1. Where are these most common? 2

A
  1. free nerve endings with a bulbous morphology

2. basal and wing cells

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14
Q

How much area can a individual axon branch cover?

A

1-50 mm2

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15
Q

Which nuclei of the spinal nucleus of the medulla of the Trigeminal Brainstem Nuclear Complex (TBNC) mediate corneal sensation?

A
  1. Interpolaris nucleus

2. Caudalis nucleus

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16
Q

Where do third order neurons of the central sensory pathway synapse? 1. And where do they project to? 2

A
  1. Edinger Westphal nucleus

2. somatosensory cortex (pain)

17
Q

Describe the Aβ nerve fiber:

  1. relative thickness
  2. innervated structures
  3. myelinated or not?
  4. relative conduction velocity
  5. type of sensation
A
  1. thick
  2. Limbus, palpebrae, conjunctiva
  3. myelinated
  4. medium
  5. touch. pressure
18
Q

Describe the Aδ nerve fiber:

  1. relative thickness
  2. innervated structures
  3. myelinated or not?
  4. relative conduction velocity
  5. type of sensation
A
  1. thin
  2. Cornea (~30%), Uvea
  3. myelinated (except cornea)
  4. medium
  5. Touch, pressure, temperature, pain
19
Q

Describe the C nerve fiber:

  1. relative thickness
  2. innervated structures
  3. myelinated or not?
  4. relative conduction velocity
  5. type of sensation
A
  1. thinnest
  2. Cornea (~70%), Uvea
  3. non-myelinated
  4. slowest
  5. pain
20
Q

What are the three types of cornea somatosensory receptors?

A
  1. Polymodal nociceptors
  2. Mechano-nociceptors
  3. Cold thermal receptors
21
Q

Describe polymodal nociceptors in cornea:

  1. percent of total
  2. nerve fiber type
  3. relatie receptive field
  4. firing style
A
  1. 70%
  2. C-type&raquo_space; Aδ-type
  3. large
  4. continuous irregular discharge of nerve impulses that persist as long as the stimulus is maintained. Frequency roughly proportional to intensity
22
Q

What do polymodal nociceptors respond to (along with thresholds of each)?

A
  1. heat (>39-40oC)
  2. cold (<6.5)
  3. mechanical forces (very low threshold)
23
Q

Describe Mechano-nociceptors in cornea:

  1. percent of total
  2. nerve fiber type
  3. relatie receptive field
  4. firing style
  5. Threshold
A
  1. 15-20%
  2. all Aδ fibers
  3. large
  4. Fire a few nerve impulses in response to a brief or sustained stimuli
  5. very low 0.6mN
24
Q

Describe cold thermal receptors in cornea:

  1. percent of total
  2. nerve fiber type
  3. relatie receptive field
  4. firing style
  5. Threshold
A
  1. 10-15%
  2. Aδ- and C-fibers
  3. small
  4. Spontaneously discharges at a low rate without stimulus. Rate of discharge increases when <33oC and decreases when high temp
  5. small (0.1oC) decrease
25
Q

What convert suprathreshold noxious stimuli into coded electrical impulses by opening cation-permeable channels along small caliber Aδ and C fibers?

A

Transient receptor potential channels (TRP channels)

26
Q

What do TRP channels usually form as?

A

tetramers

27
Q

What fibers innervate the perilimbal episclera? 1. What stimuli do they respond to? 2

A
  1. thick, fast conducting (AB) fibers

2. gentle mechanical stimulation (Non-noxious touch)

28
Q

During injury, what is it called when non-stimulated branches receive impulses (in reverse direction)?

A

antidromic stimulation

29
Q

What are the neuropeptides released by nerves during corneal injury? 1. What do these do? 2

A
  1. CGRP (Calcitonin gene-related peptide) and (SP) Substance P
  2. Contribute to local inflammatory response through stimulation of cytokine production and stimulate epithelial mitosis and migration
30
Q

What causes neurotrophic keratopathy? 1. What does it result in? 2

A
  1. Impairment of trigeminal nerve

2. Loss of corneal sensation and poor healing/homeostasis so form ulcers

31
Q

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?

A
  1. sensitization
  2. hyperalgesia
  3. large number of cytokines released by locally damaged cells
32
Q

Does refractive surgery increase or decrease sensitivity? 1. Contact lens wear? 2

A
  1. decrease

2. decrease

33
Q

What is the mechanism of topical anesthetics? 1. What are examples of these? 2

A
  1. Block propagated action potentials by inhibiting Na+ channels
  2. Proparacaine, lidocaine

I. Ocular Physiology (20 decks)

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