1. The Nerve Flashcards
The nervous system (central and peripheral) is derived from
ectoderm
Vertebral column is derived
mesodermal
During their development, the limbs rotate in what direction
opposite directions;
upper limbs rotate laterally.
- extensors posterolaterally
lower limbs rotate medially
- extensors anteromedially)
What is preaxial
anterior to the bone or fascial plane or axis of the limb
postaxial
structures posterior to bone or fascial plane or axis of the limb
postaxial
How many sheaths for tibial and peroneal nerve
In the lower limb,
tibial nerve (preaxial) and common peroneal nerve
(postaxial) lie enclosed within a single sheath.
a typical neuron possesses a
how many axons / dendrites
cell body (often called the soma),
dendrites
and an axon
Each neuron has only one axon,
although it may have many dendrites
Neurons may be classified according to
polarity functionality direction myelination characteristics of peripheral nerve fibres
neurotransmitters produced
Nerve polarity and examples of each
unipolar (primary sensory neurons),
bipolar (bipolar cell of the retina)
or multipolar (cortical neurons, motor neurons,
interneurons)
neurotransmitters produced by nerves
cholinergic, adrenergic, glutamatergic,
GABAergic, dopaminergic, serotonergic and so forth.
Myelin
Myelin is a dielectric (electrically insulating) material that forms a layer,
the myelin sheath
usually around only the axon of a neuron.
It is essential for the proper functioning of the nervous system
dielectric
electrically insulating
Schwann cells
(a type of glial cell)
supply the myelin
for peripheral neurons,
oligodendrocytes
myelinate the axons
of the
central nervous system.
A and B fibres possess
layers of
Also have
several layers of myelin.
They have nodes of Ranvier
resulting in a continuous
but non-homogenous
saltatory conduction.
C fibres
Myelin?
C fibres are the
only unmyelinated fibres
(Schwann cells do not form myelin)
Their impulse conduction is uniform and homogenous
endoneurium
Within a nerve,
each axon is surrounded by a
layer of connective tissue
fascicles
axons are bundled together
perineurium
fascicle wrapped in a layer of connective tissue
blood–nerve barrier and acts as a
diffusion barrier to local anaesthetics.
epineurium
entire nerve is wrapped in a layer of connective tissue
Aa
fxn
diameter um
conduction vel
myelin
sens to LA
Motor
12-20
70-120 ms
+++
++
Ab
fxn
diameter um
conduction vel
myelin
sens to LA
Touch / pressure
5-12
30-70
+++
++
Ay
fxn
diameter um
conduction vel
myelin
sens to LA
Proprioception / muscle tone
1-4
15-30
++
+++
Ad
fxn
diameter um
conduction vel
myelin
sens to LA
pain / temp
1-4
12-30
++
+++
B
fxn
diameter um
conduction vel
myelin
sens to LA
Preganglion autonomic
1-3
3-15
+
++
C
fxn
diameter um
conduction vel
myelin
sens to LA
post ganglionic autonom/
pain / temp / mechanorec
.5-1
0.5-2
–
-
+ least
K ion concentration intrcell vs excell
x10 intracell
Na ion excell vs intracell
x 10 excell
How is diff maintained
RMP is
Na/K atpase
resting nerve cell
selective permeabilty to K+
->
net efflux of small K+
=
RMP -60 to -70mV
Action potential
Propagated impulse
Rise RMP to threshold potential -55mV
Depolarising phase
how occur
Rise in NA perm
= influx of Na
RMP less negative
Then
Repolarisation phase
Repolarisation phase
Slowing of Na influx
opening voltage depend K channel
Large outward K
rapidly restoring to RMP (or beyond - Hyperpolarisation
Na/K atpase
restores gradient
refractory period
absolute refractory period - firing till 1/3
not stimulated
relative refractory period - 1/3 on to afterdepol
supranormal stimulus - cause excitation
Subthreshold stimulus
No AP
follows all or none
Margin of safety block
The current flowing through the nerve
is 5–10 times that needed to depolarise it;
this is called ‘the margin of safety’
for impulse conduction.
Because of this, 80% of Na+ channels must be blocked
before conduction failure occurs
Decremental impulse conduction
Slower rate depol / slower conduction velocity
When exceeds margin of safety = conduction failure
Underlies local anaes in RA
How many nodes should be blocked for failure of conduction
3 successive nodes ranvier min
Sensitivity of nerve fibres to LA
depends on fibre type and size
smaller beore larger
exception is B
Sequence of blockade
B Ad = C Ay Ab Aa
sympatheitc before sensory before motor
Sequence of block reversal
Aa Ab Ay Ad=C B
Na channel
how many subunits
How many domains in the subunit
The Na+ channel is made up of two types of subunit:
α (one)
and
β (one or two).
The α subunit has four domains,
each having six helical
membrane-spanning segments.
Na channel alpha subunit contains
The α subunit contains a
voltage sensor,
an ion selectivity filter,
gating structures
and
P segment,
which forms the pore.
How many isoform of Na channel
9
7 in nerves
Nav 1.4 skeletal
Na 1.5 cardiac
Where are the Na clustered
Around nodes of ranvier
essential high speed conduction
What happens in MS
involves only central nervous system
(brain and spinal cord)
and not the PNS.
It is associated with demyelination in brain and spinal cord.
the loss of clustering
underlies the
electrophysiological consequences
(decreased conduction velocity)
and
resultant manifestations.
The Na+ channel exists in three states
resting
Open
Inactivated
Na channel
3 states transition
Resting state
->
open when stimulated
= Na influx
Triggers AP
Closed by going inactivated
What does local have higher affinity for
Inactivated over resting
= More Na channel in inacivated state
Increase in refractory period
limits frequency firing
What happens @ Higher frequency of impulse
Greater channels open as well as inactivated
Chance LA binding higher
= phase or use dependent block
Phase (use) dependent block
@ higher frequencies of impulse firing, a greater number of channels
are in open as well as inactivated state; hence, the chance for LA
binding is greater.
Why are hydrophobic LA > toxic than hyphilic
rate dependent block of Cardiac Na channel
What crosses Axon
Unionised base of LA
Ionises intracellularly
Cationic form acts within cell inactivate Na channel
Cations
are positively-charged ions
Anions are
negatively-charged
Is blockade proportional to power functional loss
Impulse conduction is vital for
functioning of a peripheral nerve,
its blockade is not proportional
to functional loss produced
define minimum blocking concentration of LA
The lowest concentration of LA
that blocks all impulse conduction
within a nerve within a
reasonable period of time
Is 50% enough of a blockade of fibres
50% fibres may result in limb
movement and, more important,
pain, which would be unacceptable
clinically
Differential block i
observation of motor preservation,
despite complete sensory loss.
This may be clinically employed in
‘walking epidurals’.
pharmacokinetics of LA in nerve block includes the following
phases
1 delivery,
2 permeation of nerve sheath,
3 induction
4 recovery
How does anaesthesia progress in a block
What direction
why
proximal to distal direction
outer fibres (mantle bundle) innervate the proximal structures
while the inner fibres (core bundle)
innervate the distal structures.
LA diffuses down the
mantle to the core gradient,
How does recovery of a block occur
why
distal to proximal direction
vascularity of core fibres is higher, the
recovery occurs in
