Sometimes Ruth Griffiths finds it hard to smile. It's not because she isn't happy, but because she suffers from a disorder which prevents her muscles from working.

Myasthenia gravis, commonly known as rag doll disease, often leaves Ruth floppy and debilitated.

But thanks to medical research, the 37-year-old is able to enjoy life as an actress, as well as a busy mum.

She was diagnosed in October 1998, a few months after her 18-month-old son Tomos was born. "It is frustrating that it has in some ways affected what I feel is my ability to be a good mother, although I am quite capable of looking after him," she says.

"But perhaps I'm not as able to do everything that a normal mother could do. I'd love to be able to kick a ball around with him or go down a slide with him. I'd like to go for a long walk, but I just have to accept that I can't."

Myasthenia gravis is caused by mutant antibodies which affect Ruth's voluntary muscles the ones she controls herself for movement.

Her natural defence system is now attacking her rather than the illness and disease it should be fighting against. As a result, she finds it difficult to raise her arms above her head and sometimes get neck ache leaving her unable to hold up her head up.

She says: "It started when my son was born, but I can keep it under control now.

"I can do things, but I get warnings that I'm going to have an attack and I have to rest for a while. But if I get an infection it seems to get worse."

Ruth successfully fends off her disease with treatment and still leads an ordinary life, looking after her son with husband Marc at their home in Bell Street, Hornton.

Although to some, her life as an actress is far from ordinary. She recently appeared in the television series Casualty, playing the part of a social worker. She says: "I am doing bits at the moment, but gradually I am doing more and more and coming back into it.

"My husband also works as an actor and is in fact one of the regular paramedics in the series and you can see him in the background quite a lot."

Ruth is only able to carry on working because of the steroids she takes.

And she has had her thymus gland removed. The gland is an integral part of the human defence system, but is abnormal in MG sufferers.

Thanks to research, she has also received a plasma transplant. This straw-coloured fluid, present in blood, is made up of antibody-producing cells. And Prof John Newson-Davies, a retired neurologist, based at the Radcliffe Infirmary, was the first person to discover that plasma played an important part in the battle against the disease.

And Ruth, the founder of the Myasthenia Gravis Association Oxfordshire and Buckinghamshire branch, is keen for the research to continue.

She said: "There's a possibility that there could be a cure and that's why the research is so important, but there's still a long way to go.

"If we think we can help someone then it's worth doing it.

"One day they might be able to find a way of helping people more than they can now."