Multiple Sclerosis

Neurons send electrical signals known as action potentials. Action potentials are when there is a change in the voltage across the cell membrane; this occurs when different ions (with different charges) move in and out of the cell through different channels. These channels are concentrated at the unmyelinated nodes of Ranvier; consequently, action potentials can only be produced at the nodes of Ranvier.

The key ions involved in creating an action potential (nervous impulse) are potassium (K⁺), sodium (Na⁺), and chloride (Cl^–). These ions move in and out of the neuron cell, changing the membrane potential (charge). The ions move through either passive diffusion i.e., down their concentration gradient through sodium and potassium channels, or are actively pumped i.e., against their concentration gradient via the Na⁺– K⁺ pump.

Neurons have a high number of potassium channels and consequently have a large volume of positive potassium ions on the inner cell membrane. Outside the cell membrane there is a large volume of negatively charged chloride ions. This difference in charge creates a membrane potential of approximately -60mV and a concentration gradient.

When the neuron is stimulated:

• Sodium channels open and there is a surge of positively charged sodium ions into the cell.
• The membrane potential becomes more positive.
• A threshold potential is reached, creating an action potential.
• An electrical impulse jumps to the next node of Ranvier.

The myelin sheath acts to speed up the conduction of the electrical impulse by:

• Increasing membrane resistance i.e., large sections of the neuron have ion movement inhibited, only allowing ion movement at the nodes of Ranvier. This encourages the electrical impulse to jump between the nodes of Ranvier.
• Reducing the membrane’s capacitance i.e., fewer ions are required to move in order to create an electrical impulse.

In MS, the myelin sheath is repeatedly attacked by the body’s immune system. Eventually this leads to demyelination (loss of the myelin sheath) and the above process of nervous impulse propagation is slowed i.e., the areas of demyelination lead to:

• Reduced membrane resistance i.e., there electrical impulse does not jump from the nodes of Ranvier and the impulse is not conducted as quickly.
• Increase membrane capacitance i.e., more ions are required to move to create an electrical impulse.

MS presents with a focal sensory disturbance e.g., burning sensation in a part of the body. Depending on which part of central nervous system is affected, MS will manifest in various ways.

• Visual impairment, double vision, hemianopia, and optic neuritis.
• Facial weakness
• Deafness
• Positional vertigo
• Ataxia
• Loss of temperature/pain sensation
• Tingling, burning, and tearing sensations
• Numbness and paraesthesia
• Transverse myelitis (medical emergency)
• Incontinence and sexual dysfunction
• Depression

MS can follow a variety of patterns, including:

• Relapsing-remitting: individuals will have periods of good health followed by acute episodes of symptom.
• Secondary progressive: as the condition progresses from relapsing-remitting, symptoms become more severe with fewer periods of good health.
• Primary progressive: individuals experience symptoms that gradually worsen with no episodes of remission.