SBMS1431: Conduction of Action Potential along Myelinated and Unmyelinated Nerves

Factors affecting propagation

Presence or absence of myelin
• Myelination conduction (saltatory)
• Unmyelinated conduction (continuous)

Axon diameter
• Larger: less resistance to local current flow
• Smaller: more resistance

Refractory period
• Unidirectional

Myelin

Consists of concentric multiple layers of cell membrane which are mainly phospholipids from certain glial cells (neuroglia)
• Wraps around the axon of nerve fiber to provide electrical insulation and increases conduction speed of AP

Conductions in unmyelinated axons- Big Idea

• The action potential spreads by depolarizing the adjacent region of the axon membrane.

• This happens as a chain reaction along the axon. It is a self-regenerative process: one action potential gives rise to a new action potential right next to it, which gives rise to another, and so on down the line.

Conductions in unmyelinated axons- 1. Initiation (Segment 1)

• An action potential develops at the initial segment (the axon hillock).

• At this point, voltage-gated Na⁺ channels open, and Na⁺ rushes into the cell, making the inside positive (+30 mV).

Conductions in unmyelinated axons- 2. Spread of Current (From Segment 1 to Segment 2)

• The sodium ions (Na⁺) that entered at Segment 1 spread away from the open channels.

• They travel sideways (laterally) inside the axon. This is shown as the "Local current."

• This local current causes a graded depolarization in the adjacent, resting membrane at Segment 2.

Conductions in unmyelinated axons- 3. Reaching Threshold (Segment 2)

• The local current quickly brings the membrane potential at Segment 2 from its resting state (-70 mV) up to the threshold (-60 mV).

• Once threshold is reached, voltage-gated Na⁺ channels in Segment 2 open.

Conductions in unmyelinated axons- 4. The Chain Reaction (Segment 2 to Segment 3)

• Now Segment 2 fires its own action potential (Na⁺ rushes in).

• Meanwhile, Segment 1 begins repolarization (K⁺ leaves the cell) and enters its refractory period. This ensures the signal cannot go backward.

• The Na⁺ entering at Segment 2 now spreads laterally to create a local current that depolarizes Segment 3.

• Segment 3 reaches threshold, fires, and the cycle repeats all the way down the axon.

Conduction in Myelinated Axons (Saltatory Conduction)- The Big Idea

• Myelin is a fatty sheath that wraps around the axon like electrical tape.

• It limits the movement of ions across the axon membrane.

• Because ions can't cross the myelin, the action potential cannot happen under the myelin. Instead, it must "jump" from node to node (the gaps in the myelin). This jumping is why it's called saltatory conduction (saltare means "to jump" in Latin).

Conduction in Myelinated Axons (Saltatory Conduction)- Why the Signal Doesn't Leak

• The depolarizing current does not leak through the internodal region (the parts wrapped in myelin) because of two things:

  1. The myelin sheath provides high resistance (it acts as an insulator).

  2. There is a very low density of Na⁺ and K⁺ channels in that region.

• As a result, the electrical current is forced to travel quickly down the inside of the axon until it reaches the next bare spot.

Conduction in Myelinated Axons (Saltatory Conduction)- 3. Where the Action Happens (The Nodes)

• Voltage-gated Na⁺ and K⁺ channels, and the Na⁺/K⁺-ATPase pump, are highly concentrated at the nodes of Ranvier.

• This is the only place where the membrane can depolarize and fire an action potential.

Conduction in Myelinated Axons (Saltatory Conduction)- 1. Initiation (Segment 1)

• An action potential occurs at the initial segment (axon hillock).

• At Node 1, voltage-gated Na⁺ channels open, and Na⁺ rushes into the cell (making the inside positive, +30 mV).

Conduction in Myelinated Axons (Saltatory Conduction)-

2. Production of a Local Current (Jumping to the Next Node)

• Because there are no channels under the myelin, the positive charge (cations) travels inside the axon (shown as the "Local current" arrow).

• The myelin forces this current to move quickly and efficiently down the inside of the axon.

Conduction in Myelinated Axons (Saltatory Conduction)- 3. Depolarizing the Next Node (Segment 2)

• The local current reaches the next Node of Ranvier (Node 2).

• Because all the voltage-gated Na⁺ channels are clustered here, the current easily brings this node to threshold (-60 mV).

• Once threshold is reached, the node fires a full-sized action potential.

Conduction in Myelinated Axons (Saltatory Conduction)-

4. The Chain Reaction Continues

• As Node 2 fires, Node 1 begins repolarization (K⁺ leaves) and enters its refractory period (preventing the signal from going backward).

• The new action potential at Node 2 creates a new local current that jumps to Node 3.

• The cycle repeats. The signal effectively "hops" from node to node all the way down the axon.

Why This Is Faster?

• In unmyelinated axons: The action potential has to be regenerated at every single patch of membrane along the axon. This takes time.

• In myelinated axons: The action potential only has to be regenerated at the nodes. The signal travels passively and very quickly through the internodes. It's like taking an express train that skips most of the stops.

• Result: Much faster propagation along the axon.

Synaptic Transmission — How does a nerve signal flow through a neuron?

1. Input (Dendrites & Cell Body)

• Receives signals from multiple sources.

• Convergence: Many inputs integrated into one neuron.

2. Integration (Axon Hillock)

• Decision point: sums inputs to decide if threshold is reached.

• Abundant in voltage-gated Na⁺ channels.

3. Propagation (Axon)

• Conducts signal away from cell body.

• Myelin sheath speeds up conduction.

4. Output (Axonal Terminal)

• Releases neurotransmitters to target cells.

• Divergence: One neuron sends signals to many cells.

Synaptic Transmission- Examples

• Neuron-neuron: Communicates with another neuron.

• Neuron-muscle: Communicates with a muscle (motor end plate).

Synaptic Cleft

• 30–50 nm wide gap between presynaptic (PRE) and postsynaptic (POST) neurons.

• Prevents nerve impulses from directly passing from one neuron to the next.

Transmission Across the Cleft

• It is a chemical event.

• Depends on the release, diffusion, and receptor binding of neurotransmitters.

• Ensures unidirectional communication between neurons.

• The depolarizing phase of the nerve impulse opens voltage gated Ca2+ channels, which are present in the membrane of the axonal terminal, Ca2+ flows inward through the opened channels

• An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles.

• The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron’s plasma membrane
  • Binding of neurotransmitter molecules to their receptors on ligand- gated channels opens the channels and allows particular ions to flow across the membrane
  • As ions flow through the opened channels, the voltage across the membrane changes
  • Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition)

Neuronal pattern of communication

• Depolarisation at the dendrites often results
  in a graded potential

• Graded potentials degrade over distance and
  so a small depolarization at a single dendrite
  is unlikely to cause an action potential

Ways to increase successful generation of action potential

Each neuron possess many dendrites and each is a potential site for neuronal synapses

• Two types of signals:
  - Excitatory postsynaptic potential signal (ESPSs)
  - Inhibitory postsynaptic potential signal (IPSPs)

• Summation
  - The process of adding up postsynaptic potentials and responding to their net effect
  - It occurs in the trigger zone (axon hillock)
  - Can be Spatial or Temporal

Spatial

Occurs when the graded potentials from multiple locations arrive simultaneously and combine

Spatial summation = adding signals from different places at the same time.

Temporal

Occurs when two (or more) graded potentials from one presynaptic neuron arrive close together in time

Temporal summation = adding signals from the same place at different times (close together).

Excitatory Postsynaptic Potential (EPSP)

• Effect: Depolarizes the postsynaptic neuron.

• Mechanism: Opens Na⁺ channels (and closes K⁺ channels) → Na⁺ moves in → net depolarization.

• Result: Makes the cell more likely to fire an action potential.

• Is a graded potential

Inhibitory postsynaptic potential (IPSP)

• Effect: Hyperpolarizes the postsynaptic neuron.

• Mechanism: Opens K⁺ channels and Cl⁻ channels → K⁺ moves out and Cl⁻ moves in → hyperpolarization.

• Result: Moves the membrane potential farther from threshold, making the cell less likely to fire an action potential.

• Is a graded potential

Disease of Myelin- Multiple Sclerosis (MS)

What it is: A chronic autoimmune disorder of the CNS.

• Type: A demyelinating disease of the CNS.

• Location: Affects the white matter (myelinated axons) of the brain and spinal cord.

• Pathology: Multiple plaques form during demyelination (loss of myelin sheath) throughout the CNS.

• Consequence:

  • Leakage of K⁺ ions through exposed channels.

  • Causes hyperpolarization and failure of action potential conduction.