Peripheral nerves

Location + function of cell bodies

Locations/ details	Function
Sensory neuron: dorsal root ganglion  Motor neuron: ventral horn  Autonomic system neurons: lateral horn	Controls protein synthesis

Location + function of schwann cells

Location: Around axon

Function: Produces myelin sheath( myelin sheath as the extension of schwann cell

Location of Node of ranvier

Between schwann cells → distance bwt nodes of Ranvier: internodal distance

What is endoneurium

Closes to neuron → loose areolar connective tissue

Location + function of perineurium

Location: Around each fascicle ( a bundle of neurons ) → dense + irregular connective tissue proper

Function: Circular/ oblique/ longitudinal biochemical diffusion barrier → controls intra-fascicular environment  → has type I + II collagen + elastic fibre in multidirectional arrangement  → has role in biomechanical behaviour

Location + function of Epineurium

Location: 2 layers:  Interfascicular epineurium → loose  CTP → spider web arrangement areolar connective tissue + adipose tissue  Epifascicular epineurium → dense + irregular CTP → outer layer: border bwt nerve + nerve bed

Function: Epifascicular epineurium: offers greater mechanical resistance Interfasciular epineurium: cushioning w/ adipose tissue / protection against compression

Location + function of mesoneurium

Loose CTP around nerve

Allows movement/ excursion of nerve in nerve bed

How are materials transported in the neuron

→ cell body: contains all organelles for production of components needed for function 

→ cell body can be quite remote → bi-directional axonal transport used to transport components produced by soma( cell body ) to axon or axon terminal

Anterograde transport: materials transported from cell body to the axonal terminal 

Retrograde transport: materials transported from axonal terminal to cell body → needed for communication of events in synaptic terminal + reuse of materials 

→ constant + adequate amount of energy + oxygen needed 

How do nerves get oxygen and energy supply + features of arteries

Anastomotic network + relative tortuosity of blood vessels accommodates strain + gliding of nerves during motion 

Features of arteries to accommodate for nerve movement: Arteries have tortuosity: slack to accommodate movement 

How are arteries arranged in nervous system

Type of  structures	Arrangement
Extrinsic arteries	longitudinal
Segmental arteries	from extrinsic arteries → epineurial arterioles ( perpendicular to extrinsic arteries )
Anastomotic network of blood vessels formed by epineural arterioles bwt interfascicular + epifascicular epineurium	Parallel to fascicle

Lymph vessels

In epineurial but X in perineurial regions

Function of epineurial + perineurial arterioles and endoneurial capillaries

Epineurial arterioles		constrict to control blood flow to nerves
Perineurial arterioles	pierce perineurium obliquely	have tortuosity to accommodate for movement + little smooth muscles → X control blood flow → quickly change to capillary proportions
Endoneurial capillaries		provide blood nerve barrier → controls movements of substances into nerve environment

How do structures in nervous system vary

How does it vary: 

1. No. of fascicles : larger size of fascicle → smaller number of fascicle given the nerve overall diameter is the same 

2. Contribution of fascicle + interfascicular epineurium to total cross-sectional area→ areas of greater compression: fascicles make up less of the total cross sectional area + interfascicular  epineurium makes up more of total cross sectional area 

3. Endoneurial capillary density 

What is excursion

Excursion( displacement/ gliding of nerve relative to surrounding nerve bed )  + strain

Principle of nerve response to tensile loading in regards to direction + magnitude of nerve excursion

1. Relationship depends on the anatomical relationship bwt nerve + axis of rotation in moving joint 

2. Movement of adj segments of nerve: in elongation of nerve bed( extension): adj segments of nerve glide towards moving joint; in shortening of nerve bed ( flexion ): adj segments of nerve glide away from moving joint 

3. Sequence of events in nerve excursion: excursion happens first in nerve segments immediately adj to moving joint → happens in segments progressively distant from moving joint 

Magnitude of excursion decreases from nerve segments adj to moving joint to the segment most distant to moving joint

Where does greatest strain occur in tensile loading

Strain: elongation of nerve bed → increases strain + magnitude of strain greatest in segment closest to moving joint

Response of median and ulnar nerve in response to elbow extension

Median nerve: middle of M in brachial plexus → crosses anterior to the axis of elbow → when elbow moves from 90’ F to 0’F →increase in tension by lengthening median nerve bed →  median nerve segment in arm moves towards the elbow + median nerve segment in forearm moves towards the wrist

Ulnar nerve: inferior trunk of medial cord → crosses posterior to axis of elbow → when elbow moves from 90’F to 0’F → relief of tension by shortening ulnar nerve bed  → ulnar nerve segments in arm + forearm moves away from elbow 

What is happening at the nerve level when nerves glide

1. Epineurium + fascicles + axons in fascicles undulate 

2. Fascicles in the nerve straighten at the start of nerve excursion 

3. Axons in the fascicles straighten + become tensioned 

What happens to neuron level when excessive tension is applied

1. Nerve straightens 

2. Fascicles straighten → perineurium tensioned 

3. Axons straighten + tensioned → less strong than perineurium 

4. Some axons rupture → 4% strain

5. Some fascicles rupture 

6. When critical number of fascicles rupture → whole nerve fails → rapid plastic deformation 

Elastic properties of nerves retained until perineurium fail but rupture of axons happen before the rupture of perineurium 

Endoneurial pressure contributes to nerve stiffness: 

1. Fascicles elongate → cross sectional area reduced → intrafascicular pressure ↑ → intrafascicular microcirculation compromised 

2. Transverse contraction greatest in the middle of the section 

3. Compliance increases when blood vessels severed 

Behaviour of of nerves under different types of tension loading

Variables of loading	Behaviour
Tensile loading parallel to long axis	Rupture at Lower strain values  Zones of low compliance
Tensile loading at an angle to long axis	Rupture at  Higher strain values  Branch point where nerve is restrained
Total funicular cross sectional area vs total nerve cross sectional area	Strength of nerve increases w/ number of funiculi  Greater total funicular cross sectional area → greater strength → ruptures at higher stress points  Spinal roots( X perineurium + funicular plexus ) → elastic failure at lower stress + strain  *less relation to total nerve cross sectional area

Behaviour under compression

Modes of compression: 

1. Transverse contracture from lengthening 

2. External compression: 

*Uniform circumferential: like a cuff

→ transverse displacement

→ damage greatest at edge of compression where shear forces are greatest

*Lateral compression: compression by 2 parallel structures 

→ shape of nerve changes but volume X → less injurious 

Cellular healing process from acute crush and transection injuries

1. Cell body reaction: 

Cell body swells + nucleus moves peripherally 

→ metabolic priority changes from production of ntm to production of structural materials for axon repair + growth 

*primary sensory neurons more vulnerable to apoptosis than motor neurons

2. Wallerian degeneration: distal axonal disintegration + myelin fragmentation 

3. Macrophage infiltration: 1-4 days 

4. De-differentiation of Schwann cells → detach from axons + proliferate to help macrophages clear cellular debris 

5. Schwann cells align longitudinally to express stimulating factors → stimulate nerve regrowth to target organ

6. Regeneration of axons from proximal stump: very slow growth 1-3mm/ day 

→ muscle atrophy + undergo interstitial fibrosis 

→ sensory nerves seek target sensory organs → sensory organs degenerate 

→ early re-innervation produces superior functional return

Changes to nerve from chronic compression

1. Preservation of axonal integrity → neurapraxia 

2. Gradual increase in macrophage infiltration 

3. Schwann cells proliferates + undergoes apoptosis → turnover at 2 weeks + peak 4 weeks 

→ caused by mechanosensitivity to compression + X macrophage induced 

4. Focal locations of demyelination → neuropraxia 7-10 days → remyelination 

5. New myelin sheath thinner 

6. Nerve conduction velocity decreased 

7. Normal neuromuscular junction → X denervation atrophy 

8. Late stage: axonal damage → obvious motor weakness 

Changes to intraneurial blood flow from chronic nerve compression

1. Intraneural oedema 

Effects: 

1. Ischaemic damage to endoneurial capillary endothelial cells 

2. Damage to blood nerve barrier 

3. Increase in intraneurial pressure 

4. Impairment of intraneural blood flow + axoplasmic flow 

5. Activation of fibroblasts + fibrosis

Primary repair of acute nerve injuries

severe transection of nerves where there is a larger gap bwt two parts → primary repair in 24 hrs 

Epineurial repair: suturing the epineurium tgt 

Fascicular repair: suturing the fascicles tgt + identify the neurons by electrical stimulation/ staining

Nerve grafting

When primary repair X be performed w/o undue tension  Autografting: finding cutaneous nerves e.g. sural + medial antebrachial cutaneous lateral femoral cutaneous + superficial radial nerve → immobilise limb for protection of graft → graft regain same tensile strength

Pre-op goals in denervated extremity

Protection + maintenance of ROM → prevent contractures + deformity + support joints + limit oedema  Direct muscle stimulation to reduce muscle atrophy

Post-op for management of acute nerve injuries

1. Protection + monitoring 

2. Early phase sensory re-education →  decreases mislocalization + hypersensitivity + reorganizes tactile submodalities 

3. Hydrotherapy to improve joint contractures + enhances muscular performance 

4. Electrical stimulation → promote regeneration 

Management of chronic nerve injuries

Neurodynamic: nerve elongation + excursion techniques  → vary intraneural pressure dynamically when applied  → facilitate reduction of intraneural oedema + reduction of symptoms  → limit fibroblast activity + minimise scar formation through early use of mesoneurial gliding tissue —> reduce pressure in carpal tunnel in wrist E

Electrical stimulation →  increases sensory + motor neuron regeneration

Factors affecting predisposition to carpal tunnel syndrome

High repetition wrist/ finger activities  → shear injury to subsynovial connective tissue → fibrosis

Occupational exposure to vibration

Diabetes → increase in amount of collagen in perineurium + endoneurium → larger cross sectional area of the nerve [ systemic ]

Presence of nerve disorder → increased risk of development of secondary nerve disorder in same quadrant → double crush phenomena  → accumulative compression in multiple sites of where the nerve passes through → serial constraints to axoplasmic flow  → possible reason for predisposition:  Axonal transport  Altered ion channels  Neuroinflammation  Central sensition  Altered neural biomechanics

Principle of injury of carpal tunnel syndrome

Reduction in dimensions of carpal tunnel by wrist F + E  Increase in volume of contents in carpal tunnel  reason:  *oedema  *incursion of muscles e.g. lumbricles in MCP F/ wrist E → MCP flexion causes the proximal part of the lumbricals to move into the distal part of the carpal tunnel as it is inserted onto flexor digitorum profundus tendons that glide proximally in finger flexion ; extrinsic flexor muscle bellies enter the carpal tunnel in wrist E