1. Classification of nerve fibres; properties of mixed nerves
Dr Rekha Prabhu

2. Nerve Conduction Velocity
Cell diameter: The greater the cell diameter, the greater the conduction velocity.
Myelin: The greater the myelination, the greater the conduction velocity.
• Large myelinated fibers = fast conduction
• Small unmyelinated fibers = slow conduction

3. Three types of nerves
Afferent nerves are composed of sensory nerve fibers (axons) grouped together to carry impulses from receptors to the central nervous system.
Efferent nerves are composed of motor nerve fibers carrying impulses from the central nervous system to effector organs, such as muscles or glands.
Mixed nerves are composed of both afferent and efferent nerve fibers.
(SAME: Sensory Afferent , Motor Efferent)

4. Classification of nerve fibers
The first system, described by Erlanger and Gasser, applies to both sensory (afferent) and motor (efferent) nerve fibers and uses a lettered nomenclature of A, B, and C.
The second system, described by Lloyd and Hunt, applies only to sensory nerve fibers and uses a Roman numeral nomenclature of I, II, III, and IV.

5. ERLANGER - GASSER CLASSIFICATION
Fiber type
Diameter(m)
Conduction velocity(m/s)
Function A α
12-20
70-120
Proprioception: somatic motor β
5-12
30-70
Touch, pressure γ
3-6
15-30
Motor to muscle spindles δ
2-5
12-30
Pain, cold, touch B
<3
3-15
Preganglionic autonomic C
Dorsal Root
0.5-2
Pain, temperature
Sympathetic
Postganglionic sympathetic

6. REMEMBER
A and B fibers are myelinated
C fibers are unmyelinated.

7. Numerical classification for sensory neurons
Number
Origin
Fiber type Ia
Muscle spindle, annulospiral ending Aα Ib
Golgi tendon organ II
Muscle spindle, flower spray endings; touch, pressure Aβ
III
Pain and cold receptors Aδ IV
Pain , temperature
Dorsal root C

8. Remember Number IV---- not myelinated
Demyelinating Diseases: Affect somatic, visceral afferents, motor
Pain and temperature not affected

9. Relative susceptibility of A, B, C nerve fibers to conduction block produced by various Agents
Susceptibility to:
Most susceptible
intermediate
Least susceptible
Hypoxia B A C
Pressure
Local anaesthetics

10. Compound Action Potential(CAP)
Property of mixed nerves
Appearance of multiple peaks in the action potential. The multipeaked action potential is called a compound action potential
Unique shape because a mixed nerve is made up of families of fibers with various speeds of conduction.

11. Compound Action potential

12. Learning Outcomes What is a mixed nerve?
What is the Erlanger-Gasser classification of nerve fibers in a mammalian peripheral nerve?
What is meant by a compound nerve action potential (CNAP)?
Draw a schematic diagram of a CNAP.
Describe the classification used for sensory neurons.