The document discusses the propagation of action potentials along myelinated and unmyelinated nerve fibers. It describes how a stretch receptor in a muscle generates a receptor potential that triggers an action potential to propagate along the axon to the motor neuron terminal. This causes neurotransmitter release and initiation of a synaptic potential in the motor neuron, propagating another action potential to the muscle fiber, causing contraction. Saltatory conduction in myelinated fibers increases velocity by allowing action potentials to hop between nodes of Ranvier without needing to regenerate at each segment, in contrast to the continuous regeneration in unmyelinated fibers. Factors like axon diameter, myelination, membrane resistance and capacitance affect conduction velocity.

1. Propagation of the Action Potential
Myelinated and unmyelinated nerve fibers

2. NEURON STRUCTURE

3. The sequence of signals that produces reflex action.
A. The stretching of a muscle produces a receptor potential in the specialized receptor (the muscle spindle). The amplitude of the
receptor potential is proportional to the intensity of the stretch. This potential spreads passively to the integrative or trigger zone at
the node of Ranvier. If the receptor potential is suficiently large, it triggers an action potential that then propagates actively and
without change along the axon to the axon terminal. At specialized sites in the terminal the action potential leads to an output signal,
the release of a chemical neurotransmitter. The transmitter diffuses across the synaptic cleft between the axon terminal and a target
motor neuron that innervates the stretched muscle; it then binds to receptor molecules on the external membrane of the motor
neuron.
B. This interaction initiates a synaptic potential that spreads passively to the trigger zone of the motor neuron’s axon, where it initiates
an action potential that propagates actively to the terminal of the motor neuron’s axon. The action potential releases a neuro
transmitter where the axon terminal meets a muscle fiber.
C. The neurotransmitter binds receptors on the muscle fiber, triggering a synaptic potential in the muscle. If suficiently large, or if
combined with signals from other motor neurons, the synaptic potential will generate an action potential in the muscle, causing
contraction of the muscle fiber.

5. Theory of propogation: Local circuit theory
The inward current in an active region must be opposed by an equal
outward flow of current elsewhere in the membrane. This concept is known
as local circuit theory. In a region just in front of the wave of inward current
(propagated action potential), the outward current has an excitatory effect on
the membrane

6. Passive electrical properties and propagation of
action potential: Cable Model of Axon
Simplified model
Rm: parallel resistance of
membrane due to all ionic
conductances
Cm: capacitance of membrane
Er:resting potential
Ri: internal resistance of
axoplasm Da:axon diameter
Length constant
Electrotonic conduction A stimulus
ʎ =
𝐷𝑎 𝑅𝑚
4𝑅𝑖

10. Conduction velocity in unmyelinated and
unmyelinated fibers
1) Increase axon diameter
2) saltatory conduction produced due to insulation
3) increased membrane resistance Rm
4) decreases membrane capacitance (Cm)
Factors affecting electrotonic conduction velocity
conduction increases velocity by
All these factors affected by Myelination
V𝑒𝑙𝑜𝑐𝑖𝑡𝑦 ≈
1
𝐶𝑚
𝐷𝑎
𝑅𝑚𝑅𝑖

11. https://biology.stackexchange.com/questions/8282/why-is-saltatory-conduction-in-myelinated-
axons-faster-than-continuous-conductio
Decreased Membrane Capacitance:

12. Increased Membrane Resistance:

13. gm=1/R
Smaller fibers without myelin, like the ones carrying pain information, carry signals
at about 0.5-2.0 m/s
Larger, myelinated axons found in neurons that transmit the sense of touch or
proprioception – 80-120 m/s

14. instead of having to constantly
generate new action potentials
along each segment of the axon,
the ionic current from an action
potential at one node of Ranvier
provokes another action potential
at the next node. This apparent
"hopping" of the action potential
from node to node is known as
saltatory conduction.
https://www.khanacademy.org/sci
ence/biology/human-
biology/neuron-nervous-
system/v/electrotonic-action-
potential

15. Conduction velocity in myelinated and
unmyelinated fibers
Action potential conduction affected by
1. Axon structure (large or small, myelinated or unmyelinated)
2. Path of positive charge
It is fast inside of axon
Slower across membrane
3. Axonal excitability
Number of voltage gated channels
Axon diameter
Level of resting membrane potential
Level of threshold potential
4. Temperature
Slower at low temperature