5.1.1 Nerve Trunks Flashcards
What are the characteristics of M Waves?
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
What source will block small diameter axons first vs big diameter?
Blocking small diameter axons first:
- Local anesthetics
Blocking large diameter axons first:
- Temperature
- Pressure
- Positive charge applied extracellularly (anodal block)
- Asphyxia/anoxia
What is a compound action potential?
extracellularly recorded nerve or muscle potential resulting from the summation of many underlying individual action potentials evoked by a stimulus
What are the 4 types of Dorsal Root Axons? How are they distributed?
- 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
How does intracellular APs change with Axon diameter?
-Size of intracellular action potential does NOT vary with cell size
How does threshold for electrical stimulation from an outside source vary with small and large axons? Why?
-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
What are the characteristics of a compound action potential?
- 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
How does axon conduction velocity relate to diameter? How can myelination affect this?
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
What are the 4 different types of Ventral Root Axons? What is their distribution?
- 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
What are the characteristics of the H Reflex?
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
What is the Bell-Majendie Law?
-All sensory axons enter dorsal root and all motor neurons exit through the ventral root
The amplitude of extracellular APs is directly related to?
-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
What is the order in which axons will be activated intrinsically from small to large?
-Smallest cells will be recruited first, followed by largest
What is the exception to the Bell-Majendie Law?
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
What are the characteristics of F Waves?
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
