Notes on Multiple Sclerosis, Guillain-Barré Syndrome, and Axon Regeneration

Action Potential Propagation: In healthy individuals, electrical impulses (action potentials) propagate along nerves coated by a myelin sheath.
Myelin Sheath Formation: The myelin sheath is formed by an oligodendrocyte cell, which tightly wraps around the axon in multiple layers.
Function of Myelin:
- Its high resistance prevents the electrical impulse from leaking out of the axon.
- Its weak ability to store an electrical charge allows for rapid conduction.
- Requires little energy for conduction.
Importance: The myelin sheath is a critical component for efficient axonal conductance of electrical impulses.

Hallmark of MS: The defining characteristic of MS is the loss of this protective myelin sheath, a process known as demyelination.
Physiological Impact: Demyelination leads to an uncontrolled loss or leakage of intracellular potassium ( $K^+$ ) from the axon.
Consequences on Action Potential: This potassium leakage alters the action potential. When an electrical impulse reaches a demyelinated segment, it becomes diminished, resulting in:
- Slowing of conduction velocity.
- Inability to sustain repetitive impulse discharges.
- Ultimately, conduction failure.

Indication: Ampyra (dalfampridine) is the first oral therapy specifically indicated to improve walking in patients with MS, as demonstrated by an increase in walking speed in clinical trials.
Mechanism of Action (Partial Elucidation):
- Dalfampridine is a small, lipid-soluble molecule that can cross the axon cell membrane.
- Once inside the axon, it blocks open potassium channels.
- This action significantly reduces the uncontrolled loss of potassium ( $K^+$ ).
- As a result, there is an increase in the action potential and improved neuronal conduction.
- This improved neuronal conduction may lead to the observed improvement in walking ability.

Walking Impairment: Patients often experience difficulties with walking, which Ampyra aims to address.
Vision Loss: Partial loss of vision can occur. For example, a person might see students but not a chair directly in front of them, or experience images that appear and disappear intermittently, despite the physical presence of objects.
Intermittent Muscle Control: The disease can lead to periods of intermittent muscle function and weakness.

The alpha four beta one integrin is a molecule found on the outside of the cell that is involved in attacking myelin. (This was mentioned in the context of the immune system's attack on myelin, likely relevant to the pathogenesis of MS).

Immune System Attack: In Guillain-Barré syndrome, the body's immune system erroneously attacks its own nervous system.

Sensory and Motor Deficits: Patients typically experience weakness and tingling sensations, often starting in the legs and then spreading to the arms and upper body.
Loss of Sensation: Sufferers can lose the ability to feel heat, pain, or other sensations.
Paralysis: The condition can progress to paralysis.

Rarity: Guillain-Barré syndrome is rare, affecting approximately $1$ in $100,000$ people.
Demographics: It can strike individuals of any age and either gender.

No Specific Cure: There is no specific cure for Guillain-Barré syndrome.
Treatments Available: However, various treatments are available to manage the condition.
Recovery: Most people do recover from GBS.
Long-term Effects: Despite recovery, some individuals may experience paralysis for weeks, months, or even years.

Capacity for Regrowth: In the peripheral nervous system, if nerves have been grafted together, axons have the capacity to regrow and reinnervate.
Functional Restoration: This regeneration can restore function within months, possibly up to a year. For example, a finger that initially has no movement can regain function over time.
Well-Known Phenomenon: This regenerative capability in the PNS is a well-established fact.

Lack of Regeneration: In contrast, there is generally no axon regeneration in the central nervous system (CNS) after injuries such as trauma, stroke, multiple sclerosis damage, or spinal cord injury.
Unclear Reasons: The definitive reasons for this differential regenerative capacity between the PNS and CNS are not fully known, though many clues suggest biological differences in CNS axons.

Impaired Reflexes: If there is demyelination affecting spinal nerves more proximally, leading to an absence of sensory nerves entering the spinal cord or motor nerves exiting it, reflexes will be absent.
Example (Patellar Tendon Reflex): If there is no innervation to the patellar tendon, striking it will not elicit a reflex.