Spinal muscular atrophy

Spinal muscular atrophy, or SMA, is a genetic disorder where nerve cells in the spinal cord die prematurely, and this causes the muscles that would normally be controlled by those nerves to atrophy, or wither away, which causes weakness.

When the brain wants a muscle to contract, it sends a signal through an upper motor neuron, which takes the impulse from the brain to the spinal cord, and then through a lower motor neuron, which goes from the spinal cord to the neuromuscular junction, which is where the lower motor neuron touches the muscle cell.

The lower motor neurons which cause voluntary contraction of skeletal muscle are called alpha motor neurons, and these alpha motor neurons are the ones that die in SMA. Their cell bodies are located in the anterior horn, or front part, of the spinal cord, and their axons project from the spinal cord all the way to the muscles they innervate. A group of these neurons is called a motor nerve.

If a lower motor neuron dies or if the entire nerve is injured, the motor unit, which includes the neuron and the muscle fibers it innervates, stops working.

Depending on how many muscle fibers stop contracting, there can be overall muscle weakness or in an extreme situation, a flaccid, or low-tone paralysis.

This denervated muscle also atrophies over time, a classic example of “use it or lose it”. This contrasts with the increased muscle tone and spasticity that develops after an upper motor neuron is damaged.

When a lot of these muscle fibers are affected, fasciculations can happen which are, spontaneous, involuntary muscle contractions.

Alpha motor neurons also carry the signal for muscle contraction in deep tendon reflexes, like the knee-jerk reflex, and they diminish or disappear when alpha motor neurons are damaged.

Now, it turns out that there are a few types and subtypes of SMA.

Type 1a, congenital SMA, is the most severe of all and it starts even before birth, when mothers may notice decreased fetal movements.

SMA type Ib, also called infantile SMA or Werdnig-Hoffman disease, is the classic form where babies often appear normal at birth and then in the first few weeks of life develop hypotonia or low muscle tone.

These infants have progressive weakness, which is worse proximally than distally, and is initially more obvious in the legs, making it hard for them to do things like sit up.