5.1.1 Nerve Trunks

Question 1

What are the characteristics of M Waves?

Answer 1

M Waves: results of stimulation of alpha motoneuron axons

• AP evoked by this stimulation will travel both proximally and distally
• Distal = Orthodromic – travels in normal direction of normal conduction
• Proximal= Antidromic– travels in opposite direction of normal conduction

Question 2

What source will block small diameter axons first vs big diameter?

Answer 2

Blocking small diameter axons first:

• Local anesthetics

Blocking large diameter axons first:

• Temperature
• Pressure
• Positive charge applied extracellularly (anodal block)
• Asphyxia/anoxia

Question 3

What is a compound action potential?

Answer 3

extracellularly recorded nerve or muscle potential resulting from the summation of many underlying individual action potentials evoked by a stimulus

Question 4

What are the 4 types of Dorsal Root Axons? How are they distributed?

Answer 4

• I, A-alpha: muscle spindle primary afferents, GTO
• II, A-beta & A-gamma: muscle spindle secondary afferents, skin touch and pressure
• III, A-delta: skin touch, temperature, fast pain, autonomic
• IV, C: autonomic, temperature, slow pain

Question 5

How does intracellular APs change with Axon diameter?

Answer 5

-Size of intracellular action potential does NOT vary with cell size

Question 6

How does threshold for electrical stimulation from an outside source vary with small and large axons? Why?

Answer 6

-Is indirectly proportional to axon diameter

• -Small axons require a larger extracellular current to depolarize the membrane to threshold than large axons
• -Large axon has a lower internal resistance to longitudinal current flow than a small axon, allowing more current to flow through the large axon from an extracellularly applied stimulus

Question 7

What are the characteristics of a compound action potential?

Answer 7

• Multiple peaks in the action potential is a property of mixed nerves, not single axons
• Potential changes recorded extracellularly from peripheral nerves represent an algebraic summation of the all-or-none action potentials of many axons
• Unique shape due to the variously sized diameter axons present in the peripheral nerve
• Fast-conducting fibers arrive at recording electrodes before slower fibers

Question 8

How does axon conduction velocity relate to diameter? How can myelination affect this?

Answer 8

Is directly proportional to axon diameter

While both axon diameters vary directly with conduction velocity, myelinated axons have a higher slope

• Myelinated Axons: CV (m/s) = 6 x Axon Diam
• Unmyelinated Axons: CV = 1.7 x Axon Diam

Question 9

What are the 4 different types of Ventral Root Axons? What is their distribution?

Answer 9

• A-alpha: Alpha motorneurons innervating extrafusal muscles (aka LMNs)
• A-gamma: Gamma motorneurons innervating intrafusal muscle
• B: Autonomic preganglionic axons
• C: Ventral root sensory axons

Question 10

What are the characteristics of the H Reflex?

Answer 10

H Reflex: result of orthodromic activation of Ia afferents in peripharal nerve, causing motoneuron activation of muscle fibers

• Occurs only at subthreshold intensities, due to size of Ia afferents

Question 11

What is the Bell-Majendie Law?

Answer 11

-All sensory axons enter dorsal root and all motor neurons exit through the ventral root

Question 12

The amplitude of extracellular APs is directly related to?

Answer 12

-Is directly proportional to axon diameter OR neuronal cell size

• Large axons generate large extracellular currents, producing large action potentials
• Small axons generate small extracellular currents, producing small action potentials

Question 13

What is the order in which axons will be activated intrinsically from small to large?

Answer 13

-Smallest cells will be recruited first, followed by largest

Question 14

What is the exception to the Bell-Majendie Law?

Answer 14

Exception: sensory axons from pelvic viscera (L5-S3) that are involved with pain and temperature enter the spinal cord through the ventral roots, but have their cell bodies in DRG

• Sectioning the dorsal roots in this region will not interrupt all sensory afferents

Question 15

What are the characteristics of F Waves?

Answer 15

F Waves: results of antidromic volley re-exciting axon hilloc of alpha motorneuron

• NOT a reflex
• At subthreshold intensities, F wave is not observed because antidromic activity is not induced