Classify nerve fibres. Describe the mechanism of impulse conduction and compare saltatory with continuous conduction. Add a note on nerve degeneration and regeneration.

Structure of a Neurone

• Cell body (soma) — contains Nissl granules (rough ER), the site of protein synthesis
• Dendrites — receive impulses; conduct toward the soma
• Axon — conducts away; arises from the axon hillock, which has the lowest threshold and is the site of impulse initiation
• Myelin sheath — by Schwann cells in the periphery and oligodendrocytes in the CNS
• Nodes of Ranvier — gaps in myelin, 1 mm apart; rich in Na+ channels

Classification BY Erlanger and Gasser

• Sensory classification — Ia (annulospiral), Ib (Golgi tendon), II (flower spray, touch), III (pain, cold), IV (unmyelinated, pain)

Properties of Nerve Fibres

• Excitability — the ability to respond to a stimulus
• Conductivity — transmission of the impulse
• All-or-none law — applies to a single fibre, not to a whole nerve trunk
• Refractory period
• Accommodation — a slowly rising stimulus fails to excite
• Summation — of subthreshold stimuli
• Indefatigability — nerve does not fatigue easily (unlike the synapse)

Conduction of the Impulse

A. In unmyelinated fibres — continuous conduction

Active region depolarised → Local circuit current flows to adjacent region → Adjacent region reaches threshold → Action potential regenerated → Process repeats point to point

B. In myelinated fibres — saltatory conduction

• Myelin is a good insulator → current cannot flow across the internode
• The impulse therefore jumps from node to node (Latin saltare, to leap)

Factors Affecting Conduction Velocity

• Diameter — velocity ∝ diameter (∝ √diameter in unmyelinated fibres)
• Myelination — the single greatest factor
• Temperature — cooling slows conduction; a nerve block occurs at about 8 °C
• Internodal distance — longer internodes conduct faster

Degeneration and Regeneration

• Wallerian degeneration — the segment distal to injury degenerates within 3–4 days; axon and myelin break up and are removed by macrophages
• Retrograde degeneration — proximal stump degenerates back one or two nodes
• Chromatolysis — in the cell body; Nissl granules disperse and the nucleus moves peripherally
• Regeneration — occurs in peripheral nerves only, at about 1 mm/day, guided by Schwann cell tubes (bands of Büngner)
• CNS axons do not regenerate — oligodendrocytes release inhibitory factors and glial scarring occurs

Axoplasmic Transport

• Retrograde transport carries tetanus toxin, rabies and herpes virus from the periphery to the CNS

Applied Aspects

• Demyelinating disease — multiple sclerosis (CNS), Guillain–Barré syndrome (peripheral) → slowed or blocked conduction
• Nerve conduction velocity studies — demyelination slows velocity; axonal loss reduces amplitude
• Pressure block — large fibres are affected first (order: motor, touch, pain); local anaesthetics block small fibres first (order: pain, temperature, touch, motor)
• Hypoxia affects fibres in the order B > A > C; this is why autonomic function fails early in ischaemia