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

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

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)

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.

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 0.4-1.2 0.5-2 Pain, temperature Sympathetic 0.3-1.3 0.7-2.3 Postganglionic sympathetic

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

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

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

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

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.

Compound Action potential

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.