nerve physiology

Lining

Epineurium

· loose connective tissue

· internal epineurium surrounds individual fascicles

· external epineurium is condensation of tissue that ensheaths fascicles as a group

· composes 25-75% of cross-section of nerve (greater near joints for padding)

· cushions fascicles and allows movement of nerve in its bed

Perineurium

· surrounds individual fascicles

· in form of concentric lamellae

· acts as extension of blood-brain barrier

· resists longitudinal traction

Endoneurium

· acts as packing between axons of individual fascicles

· participates in formation of Schwann cell tube

Vascular supply

· segmental nutrient vessels join plexus of longitudinal vessels in epineurium

· plexus supplies second plexus in perineurium

· vessels enter endoneurium from perineurial plexus

· nerve may survive extensive mobilisation from its bed

Fascicles

· smallest unit of nerve structure that can be manipulated surgically

· consists of axons packed within endoneurium and enclosed by perineurium

· number of fascicles within nerve varies greatly

· fascicles undergo numerous interconnections that form intraneural plexus

· despite this, there is partial localisation of distal function at proximal levels

Fascicular groups

· fascicles clustered into groups of three to six

· interfascicular connections uncommon between fascicular groups

· matching of groups may be important in repair

Neurone

· consists of three parts

Cell body

· metabolically active

· has ribosomes and ER (Nissl substance)

· activity related to

· maintenance of ionic gradient

· synthesis of neurotransmitters

· transport of neurotransmitters and nutrients

Axon

· efferent cytoplasmic extension

· ends in synaptic knobs

· contain synaptic transmitters

Dendrites

· afferent cytoplasmic extensions

· arborize extensively

· provide a receptor membrane

Myelin

Peripheral nerves

· some neurones myelinated

· axon surrounded by myelin

· myelin consists of many layers of membrane of Schwann cells

· Schwann cells are glia-like cells along peripheral nerves

· myelin surrounds axon except at nodes of Ranvier

· periodic 1 um contractions about 1 mm apart

CNS

· most cells myelinated

· myelin produced by oligodendrocytes

· form myelin on many neighbouring axons

Nerve fibre types

Universal

Type A

· large diameter

· rapid conduction velocity

· concerned with

· proprioception and motor (alpha)

· light touch (beta)

· motor to muscle spindles (gamma)

· pain and temperature (delta)

· most sensitive to pressure

Type B

· smaller and slower

· preganglionic autonomic fibres

· most sensitive to hypoxia

Type C

· smallest and slowest

· consist of

· dorsal root fibres for reflex responses and deep pain

· postganglionic sympathetics

· most sensitive to local anaesthetic

Sensory

Ia

· muscle spindle annulospiral

· A alpha

Ib

· Golgi tendon organ

· A alpha

II

· muscle spindle flower spray

· A beta

III

· pain and temperature

· A delta

IV

· deep pain

· dorsal C