2003A03 Explain how lignocaine prevents the conduction of a nerve action potential.

 

List:

·      Arrival

·      Action

·      Differential effects

 

Arrival:

Subcut

Peripheral nerve

Plexus

·  Diffusion into nerve

Intrathecal

Diffusion into: (check)

·  Cauda equina

·  Spinal nerve roots

·  Dorsal root ganglia

·  Spinal cord
(also: into epidural space)

Epidural

Diffusion into:

·  Intrathecal space (as above)

·  Lateral spread (including to paravertebral SNS chain?)

·  Longitudinal spread
(Also: binding to epidural fat, absorption into venous blood)

 

Action:

Target

·  Voltage-dependent Na+ channel (VDNaC)

·  (less active at VDKC, VDCaC)

Entry

·  From ICF: unionised form diffuses into axoplasm, ionised form enters active receptor

·  From ECF: ionised form enters active receptor (less common)

Binding

·  Rate time spent in active state rate of cycling nerve activity

·  Affinity Inactive > resting > activated

Effect

·  ↓Rate of cycling -> no action potential propagation

·  Initial: incomplete blockade, tonic

·  Repeated stimulation: complete blockade, phasic

·  No change to resting potential

 

Differential effects:

Type

Function

Size (μm)

Conduction velocity (m/s)

Myelination

LA sensitivity

Aα

Motor

Proprioception

12-20

70-120

Heavy

+

Aβ

Light touch

Vibration

5-12

30-70

Heavy

++

Aγ

Tone

Spindles

3-6

15-30

Heavy

+++

Aδ

Sharp pain

Cold temperature

2-5

12-30

Heavy

++++

B

Pre-ganglionic:

SNS and PSNS

<3

3-15

Light

++++++

C-polymodal

Dull pain

Warm temperature

0.4-1.2

0.5-2.3

None

+++++

C-SNS

Post-ganglionic:

SNS vasomotor

0.3-1.3

0.7-2.3

None

+++++

 

 

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