Neurobiomechanical Influences on Nerve Conduction - PowerPoint PPT Presentation

Title: Neurobiomechanical Influences on Nerve Conduction


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Neurobiomechanical Influences on Nerve Conduction

• Daniel Robbins

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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.

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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.

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Nerve Structure and Conduction
Dendrites
Image taken from http//www.britannica.com/EBcheck
ed/topic-art/409665/66781/Conduction-of-the-action
-potential-In-a-myelinated-axon-the
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Quantifying Conduction

• Image taken from http//media.wiley.com/assets/7/9
  5/0-7645-5422-0_0704.jpg

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Conduction/membrane physiology
Action Potentiala) 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.co
m/nervoussystememily?fprint
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Molecular Channels
The transfer of sodium and potassium molecules
during nerve conduction occurs via ion channels.
The image below explains how these function.

• '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

Image taken from http//ibs.derby.ac.uk/steve/neu
roscience/action_potential.gif
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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/gk
aiser/biotutorials/eustruct/sppump.html
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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. 5
03-505 
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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 Extracellular
fluid 2
Image taken from Topp (2006)
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Conduction Summary
Image taken from http//openwetware.org/images/a/a
6/Action-potential.jpg
Image taken from http//biologyclass.neurobio.ariz
ona.edu/images/action-potential1.jpg
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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.

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Tissue Stress Theory
Organ System Stress/Activity Level Low
Normal
High Excessive Neuromuscular
Max discharge No change
Max discharge Axonal rate
rate Demyelination Recruitment
threshold Recruitment
Activation during threshold
Degeneration MVC
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. 200282383 403.
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Tissue stress theory - Overview of consequences
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Neural tension in the upper limb

• 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
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Linear displacement transducer
Methods of strain measurement
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) 213221
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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.199510240 244.
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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. 199274126
129.
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Stretch neurobiomechanics
Wall E, Massie J, Kwan M, et al. Experimental
stretch neuropathy changes in nerve conduction
under tension. J Bone Joint Surg Br. 199274126
129.
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Neurobiomechanics cont.
A similar study by Jou et al. displays the
difference in somatosensory evoked potentials
(SSEPs ) 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. 200018149 155.
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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
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Structural defences
Nerve fibres are crimped which helps to provide
some defence against stretch induced damage
Images from Butler (1991) and Topp (2006)
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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. 200018149 155.
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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 plexuss. 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
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Questions or feedback???
Contact Dan Robbins on d.w.e.robbins_at_googlemail.co
m