Spinal Muscular Atrophy (SMA) primarily affects the nervous system by causing the degeneration and loss of motor neurons, leading to muscle weakness and atrophy.
The Role of SMN Protein
- SMN1 Gene Deficiency: SMA is typically caused by a mutation or deletion in the SMN1 gene. This gene is responsible for producing the Survival Motor Neuron (SMN) protein.
- Essential for Motor Neuron Survival: The SMN protein is crucial for the health and proper functioning of motor neurons, which are nerve cells in the spinal cord and brainstem that control voluntary muscle movements.
- Insufficient SMN Protein: In individuals with SMA, the body does not produce enough SMN protein.
Impact on Motor Neurons
- Motor Neuron Degeneration: The lack of sufficient SMN protein causes motor neurons to shrink and eventually die.
- Impaired Signal Transmission: As motor neurons degenerate, the brain's ability to send signals to control muscles is severely compromised.
- Muscle Weakness and Atrophy: The loss of motor neuron function leads to progressive muscle weakness (paresis) and muscle wasting (atrophy). This primarily affects muscles closest to the center of the body (proximal muscles), impacting movements like sitting, crawling, and walking.
Effects on Voluntary Movements
- Impaired Voluntary Control: SMA particularly affects voluntary movements, especially those involving the head, neck, chest, and legs.
- Respiratory Problems: Weakness of the respiratory muscles can lead to breathing difficulties and increased susceptibility to respiratory infections.
- Swallowing Difficulties: In some cases, SMA can also affect the muscles involved in swallowing, causing feeding difficulties.
Severity and Types of SMA
The severity of SMA varies depending on the amount of functional SMN protein produced and the specific type of SMA:
| SMA Type | Onset | Severity |
|---|---|---|
| Type 0 | Prenatal | Most severe; profound weakness at birth; typically fatal within months. |
| Type 1 | 0-6 months | Severe weakness; difficulty breathing and swallowing; most do not survive beyond 2 years without intervention. |
| Type 2 | 6-18 months | Moderate weakness; may be able to sit but not stand or walk independently. Life expectancy varies. |
| Type 3 | After 18 months | Milder weakness; may walk independently but have difficulty with activities like running or climbing stairs. Normal life expectancy possible. |
| Type 4 | Adulthood | Adult onset, muscle weakness and atrophy. |
Summary
In summary, spinal muscular atrophy affects the nervous system by disrupting the production of the SMN protein, leading to the degeneration of motor neurons. This results in muscle weakness, atrophy, and impaired voluntary movement, primarily affecting the head, neck, chest, and legs, and the severity of the condition varies depending on the specific type of SMA.
