Fundamentals of neuroscience: cells, axons, and synapses

The Erlanger / Gasser Classification of Nerve FibresFibreTypeFunction (examples)Avg. Fibre Dia.(m)AAAABCThe Lloyd Hunt Classification of Nerve FibresGroup Function (examples)Avg. Fibre Dia.(m)IIIIIVPrimary Muscle Spindle AfferentsMotor to Skeletal MusclesCutaneous Touch and Pressure AfferentsMotor to Muscle SpindlesCutaneous Temperature and Pain AfferentsSympathetic PreganglionicCutaneous Pain AfferentsSympathetic PostganglionicPrimary Muscle Spindle AfferentsAfferents from Tendon OrgansCutaneous Mechano-receptorsDeep Pressure Sensors in MuscleUnmyelinated Pain Afferents1585< 331(unmyelinated)13931Avg. Cond. Vel.(m/s)100 (70-120)50 (30-70)20 (15-30)15 (12-30)7 (3-15)1 (0.5-2)Avg. Cond. Vel.(m/s)75 (70-120)55 (25-70)11 (10-25)1 (0.5-2)Myelinated Nerve Fibres• Conduction velocity increases with fibre diameter, but the greatest influence iswhether or not an axon is myelinated.• Myelin sheath consists of membranes of Schwann Cells wrapped around theaxons. This wrapping is periodically interrupted at what are termed the Nodes ofRanvier.• The effect of this sheath is to increase the resistance across the cell membranefrom the inside of the axon to the extracellular space, and to concentrate chargedistribution at the Nodes of Ranvier.Action Potential Propagation in Myelinated AxonsAction Potential Propagation in Myelinated Axons+ ++ +Myelinated nervemembrane+ +ExtracellularNa ++ +Myelinated nervemembrane+ +Extracellular- -- -- -- -- -IntracellularIntracellularIncreased chargedistribution at theNodes of RanvierAction Potential Propagation in Myelinated AxonsAction Potential Propagation in Myelinated AxonsNa ++ +Myelinated nervemembrane+ +ExtracellularMyelinated nervemembraneNa ++ +Extracellular- -Local Circuit Currents- -IntracellularK +- -Intracellularlocal circuit current flow is from one Node of Ranvier to the next6