Fibers + Axons

What 2 factors primarily determine axon conduction velocity?

Axon diameter + myelination. Larger diameter and more myelin = faster conduction.

Why do larger-diameter axons conduct faster?

Larger diameter ↓ internal resistance to current flow → faster propagation.

Why does myelination increase conduction velocity?

Myelin prevents current leakage and allows the AP to travel by saltatory conduction between Nodes of Ranvier.

What is saltatory conduction?

AP appears to “jump” from one Node of Ranvier to the next → faster conduction.

What happens at the Nodes of Ranvier?

High concentration of voltage-gated Na⁺ channels regenerates the action potential.

What is the general speed order of peripheral nerve fibers?

Aα → Aβ → Aδ → B → C (fastest → slowest).

What are Aα fibers?

Largest, heavily myelinated, fastest fibers; primarily proprioception and somatic motor.

What sensory information is carried by Aα fibers?

Proprioceptive information from muscle spindles and Golgi tendon organs.

What are Aβ fibers?

Large, myelinated, fast sensory fibers that primarily carry touch, pressure, and vibration.

What cutaneous sensations are primarily transmitted by Aβ fibers?

Fine/discriminative touch, pressure, vibration, and skin deformation.

What are Aδ fibers?

Small, thinly myelinated sensory fibers that transmit fast/sharp pain and cold temperature.

What type of pain is carried by Aδ fibers?

Fast, sharp, well-localized “first pain.”

What are C fibers?

Smallest, unmyelinated, slowest fibers; carry slow pain, warmth, itch, and some autonomic information.

What type of pain is carried by C fibers?

Slow, dull, aching/burning, poorly localized “second pain.”

Compare Aδ vs C pain.

Aδ = thinly myelinated, faster, sharp/well-localized first pain; C = unmyelinated, slower, dull/burning/poorly localized second pain.

Which fibers primarily detect cold?

Aδ fibers.

Which fibers primarily detect warmth?

C fibers.

Which fibers primarily transmit itch?

C fibers.

Which fibers primarily carry discriminative touch and vibration?

Aβ fibers.

Which sensory fibers conduct fastest?

Aα fibers because they have the largest diameter and greatest myelination.

Which sensory fibers conduct slowest?

C fibers because they are small and unmyelinated.

Why do you feel a sharp pain before an aching pain after an injury?

Fast Aδ fibers deliver sharp first pain before slower C fibers deliver dull/aching second pain.

What are Group Ia afferents?

Large, heavily myelinated Aα fibers from primary muscle spindle endings; detect muscle length and especially rate of stretch.

What are Group Ib afferents?

Large, heavily myelinated Aα fibers from Golgi tendon organs; detect muscle/tendon tension.

What are Group II afferents?

Mainly Aβ fibers; include secondary muscle spindle afferents and many cutaneous mechanoreceptor afferents.

What are Group III afferents?

Aδ fibers; small, thinly myelinated fibers associated with fast pain, cold, and some mechanical stimuli.

What are Group IV afferents?

C fibers; small, unmyelinated fibers associated with slow pain, warmth, and chemical/metabolic stimuli.

Match sensory fiber groups to letter classifications.

Group I = Aα; Group II = Aβ; Group III = Aδ; Group IV = C.

Which fiber comes from the primary muscle spindle ending?

Group Ia / Aα.

Which fiber comes from the Golgi tendon organ?

Group Ib / Aα.

Which fibers are commonly associated with cutaneous mechanoreceptors?

Group II / Aβ.

Which fibers are commonly associated with nociceptors?

Group III/Aδ and Group IV/C.

What are cutaneous mechanoreceptors?

Sensory receptors in the skin that detect mechanical stimuli such as touch, pressure, vibration, and skin stretch.

What type of axons are associated with most cutaneous mechanoreceptors?

Large, myelinated Aβ afferents → rapid transmission of mechanical sensory information.

What are rapidly adapting receptors?

Respond strongly when a stimulus begins/changes but decrease firing during a sustained stimulus → detect movement/change.

What are slowly adapting receptors?

Continue firing while a stimulus is present → provide information about sustained pressure, shape, or stretch.

What is the difference between a small and large receptive field?

Small receptive field = precise localization/high spatial resolution; large receptive field = poorer localization but detects stimuli over a larger area.

What is a Merkel receptor?

Slowly adapting, small receptive field Aβ mechanoreceptor; detects sustained pressure, edges, shape, and texture.

What is a Meissner corpuscle?

Rapidly adapting, small receptive field Aβ mechanoreceptor; detects light touch, motion across skin, and low-frequency vibration.

What is a Pacinian corpuscle?

Rapidly adapting, large receptive field Aβ mechanoreceptor; detects high-frequency vibration and rapid pressure changes.

What is a Ruffini ending?

Slowly adapting, large receptive field Aβ mechanoreceptor; detects skin stretch and sustained deformation.

Which receptors have small receptive fields and therefore better localization?

Merkel + Meissner.

Which receptors have large receptive fields?

Pacinian + Ruffini.

Which mechanoreceptors are rapidly adapting?

Meissner + Pacinian.

Which mechanoreceptors are slowly adapting?

Merkel + Ruffini.

Which receptor is best for detecting edges and texture?

Merkel receptor.

Which receptor is best for light touch/movement across the skin?

Meissner corpuscle.

Which receptor is especially sensitive to high-frequency vibration?

Pacinian corpuscle.

Which receptor is especially sensitive to skin stretch?

Ruffini ending.

What is a free nerve ending?

Unencapsulated sensory ending commonly associated with pain, temperature, itch, and crude mechanical sensation.

What fiber types commonly terminate as free nerve endings?

Aδ and C fibers.

Why are Aβ fibers appropriate for fine touch?

Large diameter + myelination → rapid conduction needed for precise tactile information.

Why are C fibers much slower than Aβ fibers?

C fibers have a small diameter and no myelin; Aβ fibers are larger and myelinated.

APPLICATION: A patient steps on a sharp object and immediately feels a sharp localized pain. Which fiber?

Aδ → thinly myelinated → fast first pain.

APPLICATION: Seconds after an injury, pain becomes diffuse, burning, and aching. Which fiber?

C fiber → unmyelinated → slow second pain.

APPLICATION: A patient can feel pain but cannot accurately detect vibration. Which fiber system is more likely impaired?

Aβ fibers are more affected because they carry vibration/touch.

APPLICATION: A patient loses sharp pain and cold sensation but maintains vibration. Which fibers are more affected?

Aδ fibers.

APPLICATION: A patient loses slow burning pain and warmth sensation. Which fibers are more affected?

C fibers.

APPLICATION: Which receptor would be important when identifying the edge/shape of an object through touch?

Merkel receptor.

APPLICATION: Which receptor helps detect an object beginning to slip from the hand?

Meissner corpuscle because it rapidly detects movement across the skin.

APPLICATION: Which receptor is strongly activated by vibration from a power tool?

Pacinian corpuscle.

APPLICATION: Which receptor provides information about sustained skin stretch while gripping an object?

Ruffini ending.

APPLICATION: Why can demyelination slow or block neural transmission?

Loss of myelin causes current leakage and disrupts efficient saltatory conduction.

APPLICATION: How would MS affect conduction?

CNS demyelination → slower or failed AP propagation.

APPLICATION: How would Guillain-Barré syndrome affect conduction?

PNS demyelination → slowed/blocked peripheral nerve conduction.

APPLICATION: How can Charcot-Marie-Tooth disease affect nerve conduction?

Peripheral nerve/myelin abnormalities can reduce conduction velocity and impair sensory/motor transmission.

APPLICATION: Why does temperature matter clinically for nerve conduction?

Temperature alters ion-channel activity and conduction velocity; cooling generally slows nerve conduction.

APPLICATION: Why are fiber properties relevant to electrical stimulation/TENS?

Different axon diameters and thresholds influence which sensory/motor fibers are recruited by electrical stimulation.

APPLICATION: What happens if voltage-gated Na⁺ channels are blocked by tetrodotoxin?

Na⁺ influx cannot produce/regenerate the AP → action potential propagation stops.