Last reviewed: September 2019

To understand why some people have SMA, you have to know more about:

Our bones and muscles

Inside us we all have a skeleton and its job is to support and protect our body. It’s made of bones that grow as we grow.

The bones that go from our bottom to our head are small and moveable and are called vertebra. They make up our backbone or spine.

As we grow, some of our bones join up together. Where these bones meet up, it’s called a joint – like at our knees or elbows.

Our skeleton can bend at these joints because the bones have muscles attached to them. When our muscles move, so do our bones so that we can do things like wave a hand or nod our head. Our muscles need to stay strong and be able to move when we want them to.

A baby’s body has about 300 bones. As they grow, some bones join up. By the time they’re an adult, they only have 206 bones.

Everyone has more than 640 muscles attached to their bones.

One of the jobs our brain has is to tell our body what to do. It has to be able to send instructions to our muscles so that they can move and move our bones. Our brain does this by sending electrical messages through our nerve cells.

Nerve cells are long and thin and different shapes so that they can carry messages from one part of our body to another.

• The nerve cells you need to know about to understand SMA are our lower motor neurons.

These are inside our spinal cord which is protected by our backbone. Their job is to pick up the messages that come from our brain and down our spinal cord and take them to our muscles and tell them to move.

Everyone’s body is made up of cells – trillions of them. Most cells are so small you’d need a microscope to see them. There are all sorts of cells with different jobs. Some are building blocks that make up our bones, our muscles, our hair and skin. Nerve cells are one sort of cell. Some nerve cells stretch from inside our spinal cord right down to our big toe!

Inside every cell there are thousands of genes – these are very important. They control lots of the instructions about us and how our bodies are put together – a bit like a cake recipe. They control things like the colour of our eyes and our hair.

When one of our genes doesn’t work properly, this is sometimes called a ‘faulty’ gene. Having a ‘faulty’ gene is a bit like having a cake recipe with part of the instructions missing.

Lots of people have ‘faulty’ genes of one sort or another – for example, people who are colour blind – but this doesn’t mean this is anybody’s fault.

The important genes you need to know about to understand SMA are:

• The Survival Motor Neuron 1 or the SMN1 gene

Everyone has two of these SMN1 genes in all the cells in their body; they were passed to them by their parents.

Half our genes come from our mum, half from our dad. The exact set of genes that mums and dads pass on to their children is different each time. This is why brothers and sisters are different from each other.

• The other gene is called Survival Motor Neuron 2 or the SMN2 gene.

As well as controlling lots of instructions about us, genes also carry information that’s needed to make proteins.

Proteins are even smaller than cells and have different jobs to do, like fighting off infections and building muscles.

Our SMN1 genes make an important protein called: the Survival Motor Neuron Protein or SMN Protein for short.

We need SMN protein to keep our lower motor neurons healthy and working so that they can tell our muscles to move. Lower motor neurons pick up the messages that have come from our brain, all the way down our spinal cord.

Our SMN2 genes are like the SMN1 gene. They can only make a small amount of protein that works but they’re still important. These genes are a bit different – we can have between 0 and 8 copies of them. The more copies we have, the more SMN protein we’re likely to be able to make. These genes are sometimes called ‘back-up’ genes.

Our SMN1 and SMN2 genes work together to make SMN protein.