Neurobiomechanical Influences
on Nerve Conduction
Daniel Robbins
Overview
• This presentation is intended to bring together a fuller understanding
on nerve conduction and the mechanics that support and influence
conduction.
• It is not intended to be an introduction to nerve conduction, or to
replace text book based learning. I aim to summarise the concepts
and illustrate areas that I found confusing whilst learning. Hopefully
this will aid the learning of others and create some areas for further
thought.
• Initially I will provide a review of the principles of nerve conduction,
how the structure of nerves help control conduction and the areas
that I found difficult. Next I will cover how mechanical stresses affect
nerve conduction, how we measure stresses in nerves and how the
structure of nerves deals with these stresses.
Principles of Conduction
• Principle of dynamic polarisation.
– States that electrical signals within a nerve cell flow only in one
direction: from the receiving sites of the neuron (usually dendrites and
cell body) to the trigger region at the axon. From there the signal
(action positional) is propagated unidirectionally along the length of the
axon.
• Principle of connectional specificity
– States the nerve cells do not connect indiscriminately with one another
to form random networks; rather each cell makes specific connections
– at particular contact points – with certain postsynaptic target cells but
not others.
Nerve Structure and Conduction
Dendrites
Image taken from http://www.britannica.com/EBchecked/topic-art/409665/66781/Conduction-of-the-action-potential-Ina-myelinated-axon-the
Quantifying Conduction
•
Image taken from http://media.wiley.com/assets/7/95/0-7645-5422-0_0704.jpg
Conduction/membrane physiology
Action Potential:
a) Resting membrane potential (RMP) at
-70mV. Na+ on outside and K+ on inside
of cell
b) As depolarisation reaches threshold of
-55mV, the action potential is triggered.
Na+ rushes into cell. Membrane potential
reaches +30mV on action potential
c) Propagation of the action potential
d) Repolarisation occurs with K+ exiting
the cell to return to -70mV RMP
e) Return of ions (Na+ and K+) to their
extracellular and intracellular sites by the
sodium/potassium pump
Image taken from https://eapbiofield.wikispaces.com/nervous+system+emily?f=print
Molecular Channels
The transfer of sodium and potassium molecules during nerve conduction
occurs via ion channels. The image below explains how these function.
Image taken from http://ibs.derby.ac.uk/~steve/neuroscience/action_potential.gif
•
'Nerve impulse' Produced
when 'threshold potential' (55mV) reached
•
Sodium channels open
– Sodium ions enter
– Potential rises to
+30mV
•
Potassium channels open
– Potassium ions exit
– Potential sinks to
-70mV
Sodium/Potassium Pump
In addition to ion channels, molecule transfer occurs via sodium/potassium pumps
which work in the following manner.
Three sodium ions from inside the
cell first bind to the transport protein.
Then a phosphate group is
transferred from ATP to the transport
protein causing it to change shape
and release the sodium ions outside
the cell.
Two potassium ions from outside the
cell then bind to the transport protein
and as the phosphate is removed,
the protein assumes its original
shape and releases the potassium
ions inside the cell.
Animation taken from http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/sppump.html
Confusion???
• At this point I always struggled to picture everything in its
entirety.
– If Molecule transfers provided electrochemical gradients which
created the action potentials what constrained them?
Intracellular fluids/contents where controlled by membrane
physiology but what happens outside of the cell? What controls
extracellular fluids?
– After long, and fairly painful, research I found one article (Nakao
et al.1997) which seems to answer the question (at least in
rabbit facial nerves anyway). It seems that in addition to
intracellular pathways (axoplasmic transport systems), there are
also extracellular pathways. To illustrate this we have to go back
to nerve structure….
Nakao Y. Tabuchi T.Sakihama N.; Nakajima S. (1997) Extracellular fluid pathway inside the facial nerve fascicles The Annals
of otology, rhinology & laryngology 1997, vol. 106, no6, pp. 503-505
Nerve structure cont
Axons
Endoneurium (Endo = inner)
Intracellular fluid (Intra = inside)
Perineurium (Peri = around)
Extracellular fluid 1 (Extra = outside)
Intrafascicular epineurium (Epi = upon)
Extrafascicular epineurium
Image taken from Topp (2006)
Extracellular fluid 2
Conduction Summary
Image taken from http://biologyclass.neurobio.arizona.edu/images/actionpotential1.jpg
Image taken from
http://openwetware.org/images/a/a6/Actionpotential.jpg
Neurobiomechanics
• At the most basic level tissue stresses can be
divided into two areas : type and intensity.
• Type is simply: tensile (pulling) or compressive
(pressing)
• Intensity is simply: low, medium, high or
excessive.
– Mueller and Maluf provided a good overview of the
effects of these stresses in their ‘Tissue stress theory’.
Tissue Stress Theory
Organ System
Stress/Activity Level
Low
Normal
High
No change
Max discharge
Excessive
Neuromuscular:
Max discharge
Axonal
rate
rate
Demyelination
Recruitment threshold
Recruitment
Activation during
MVC
threshold
Degeneration
activation during
MVC
motor unit synchronization
dendritic arborization
serotonergic neural activity
synaptic transmission
&
Mueller M, Maluf K. Tissue adaptation to physical stress: a proposed “physical stress theory” to guide physical
therapist practice, education, and research. Phys Ther. 2002;82:383– 403.
Tissue stress theory
- Overview of consequences
Neural tension in the upper limb
(A) With elbow extension from 90° of flexion to 0° of
flexion, the median nerve bed lengthens and the
median nerve glides toward the elbow (converges).
With the same joint motion, the ulnar nerve bed
shortens and the ulnar nerve glides away from the
elbow (diverges).
(B) With wrist extension from 0° of extension to 60° of
extension, both nerve beds lengthen; thus, both nerves
converge toward the wrist. The magnitude of excursion
is greatest closest to the moving joint.
Measurements are presented in proximal (P) or distal
(D) millimetres
Topp, K.S, Boyed, B.S. Structure and Biomechanics of Peripheral Nerves: Nerve Responses to Physical Stresses
and Implications for Physical Therapist Practice. Physical Therapy . Volume 86 . Number 1 . January 2006
Methods of strain measurement
Linear displacement transducer
Linear transducers work
via mechanical
displacement of a
sensor which emits an
increasing voltage with
increased movement.
This voltage is
externally monitored
and used to gauge the
distance moved which
in turn is related to
strain.
Coppietersa,M.W, Butler, D.S. Do ‘sliders’ slide and ‘tensioners’ tension? An analysis of neurodynamic techniques
and considerations regarding their application. Manual Therapy 13 (2008) 213–221
Buckle force transducer
Buckle force transducers are
more often used on tendons,
however sometimes are used
on nerves in cadaver studies.
They work in a similar
principle to that of Golgi
tendon organs.
A bent ‘E’ shaped clip is
placed around the nerve with
the limb positioned so the
nerve is not in maximum
tension. When tension occurs
the clip is forced straight. The
strain gauge on the clip
measures the strain.
Kleinrensink G, Stoeckart R, Vleeming A, et al. Mechanical tension in the median nerve: the effects of joint
position. Clin Biomech.1995;10:240 –244.
Load Cells
Load cells measure force via a direct attachment, much in the same way the a
fisherman will use a strain gauge to measure the weight of a caught fish.
The results of the above imposed stretch (displayed as relative strain) are
displayed on the next slide, the stretch was imposed for 60 minutes then
released. Continuous monitoring of nerve conduction was undertaken
simultaneously on both limbs to provide a baseline in addition to effects of
stretch on nerve conduction.
Wall E, Massie J, Kwan M, et al. Experimental stretch neuropathy: changes in nerve conduction under tension. J
Bone Joint Surg Br. 1992;74:126 –129.
Stretch neurobiomechanics
Wall E, Massie J, Kwan M, et al. Experimental stretch neuropathy: changes in nerve conduction under tension. J
Bone Joint Surg Br. 1992;74:126 –129.
Neurobiomechanics cont.
A similar study by Jou et al. displays the difference in somatosensory
evoked potentials (SSEP’s ) when stretch is applied to the left limb only.
Jou I, Lai K, Shen C, Yamano Y. Changes in conduction, blood flow, histology, and neurological status following
acute nerve-stretch injury induced by femoral lengthening. J Orthop Res. 2000;18:149 –155.
Effects of stretch on nerve structure
Effects on structure can be split into
vascular and functional effects. The
major vascular consequence is initially
reduced vascular function (especially
via oblique blood vessels).
Long term vascular issues can also
lead to increased pressure via
reduced vascular return.
The structure of the nerve itself also
changes during stretch. The nodes of
Ranvier open further as do the
Schmidt-Lanterman clefts. Both of
these changes affect the levels of local
cytoplasm.
Butler, D. (1991) Mobilisation of the Nervous System, Churchill Livingstone
Structural defences
Nerve fibres are crimped which
helps to provide some defence
against stretch induced damage
Images from Butler (1991) and Topp (2006)
Nerve Damage
Crimping of fibres is not
equal across all fibres.
Therefore, when high
levels of stretch (or
duration) occurs different
sections of the nerves
structures will be
affected before others.
Jou I, Lai K, Shen C, Yamano Y. Changes in conduction, blood flow, histology, and neurological status following
acute nerve-stretch injury induced by femoral lengthening. J Orthop Res. 2000;18:149 –155.
Tension at the Brachial Plexus
•
An additional benefit of nerve
structure can be observed at a
much larger scale (than
crimping) in that of the
plexus’s. Observe the diagram
to the right (the brachial
plexus). If force is applied to
one of the lower branches in
the form of tension, the tension
is divided fairly equally
amongst the nerve roots.
•
N.B This force is not actually
across all nerve roots, for a
fuller understanding refer to
the paper by Kleinrensink
referenced on the buckle
transducers slide.
Butler, D. (1991) Mobilisation of the Nervous System, Churchill Livingstone
Questions or feedback???
Contact Dan Robbins on d.w.e.robbins@googlemail.com