Standing in a queue at the checkout of his local supermarket, Gerry McCann, of Easterhouse, Glasgow, suddenly slumped to the ground, unable to move any part of his body. As a crowd of anxious shoppers gathered around him, the 42-year-old concentrated hard on his speech, managing to convey in a whispery voice, through fixed jaws, his urgent requirement for medication.

Acting on his directions, a shop assistant found McCann a glass of water and helped him ingest a small pill she had retrieved from his pocket. A few minutes later, he was on his feet and moving shakily but freely.

For McCann, this frustrating - and clearly embarrassing - incident had become what he now describes self-effacingly as "one of numerous inconveniences of living with Parkinson's disease". The former charity co-ordinator, who was diagnosed with the progressive neurological condition aged 35, explains: "Going suddenly rigid, as I did in the supermarket, is called switching off', which can be a common occurrence for many people with Parkinson's. I would switch off at least three times a day, without notice, despite being on a programme of drugs.

"This had been going on for many years, and taking the drugs on time and in larger measures would help, but I still found it hard to get the dose right or predict the switching-off episodes.

"Too many times I'd find myself lying on the floor at home, in a shop or in the street unable to communicate my problem at all. Obviously, it was difficult to live with such unpredictable symptoms."

Three years on, however, McCann, who is married and has two children, says his quality of life "could not be more improved". While he still takes at least 40 pills a day to control Parkinson's - he had an "early-onset" diagnosis similar to that of 46-year-old Hollywood actor Michael J Fox, who first learned he had the disease aged 30 - the switching-off episodes have been vastly reduced to "only a couple of minor episodes each week".

This breakthrough, McCann believes, is thanks to the involvement of a specialist Parkinson's nurse. Since late 2004, when McCann requested a GP referral to the Lightburn Hospital, one of five health centres in Glasgow with Parkinson's disease clinics, he has been cared for by Parkinson's specialist nurse Jacqui Kerr.

"Before I found the Lightburn, my illness had been monitored by a neurology consultant who I'd maybe see every six months. In between these visits I'd pretty much manage my drug intake myself. I could refer to my doctor as well, but I felt I didn't have a great amount of support," says McCann. "I thought I was doing fine - the best I could expect given the illness. I'd tried lots of different drugs but in the end the only one that controlled many of the symptoms was Levodopa.

"This can work brilliantly, but you have to be very vigilant about doses. If I didn't get this right then I'd have some very bad days with tremors and spasms, as well as the horrible switching-off episodes.

"But as soon as Jacqui became involved in monitoring my drugs, making sure I took the right ones exactly on time and adjusting the pills on a frequent basis to better control my symptoms, my life improved immeasurably."

For McCann's family, too, Kerr offered support. Before Parkinson's, he had an active life, playing the church organ and co-ordinating charity furniture removals, but the change had a profound effect on his wife, Jo, and children Darren, now 18, and Natalie, 21. "I'd gone from being an independent person, an able husband and father, to relying on my family for care and assistance," he says. "While I was still generally optimistic about things, I had very black periods.

"All this obviously affected my family. So to have someone like Jacqui there for Jo to consult was a big bonus. It was the first time she felt there was anyone to talk to. While this might sound like a cliche, having Jacqui come into our lives has changed our world from night to day. It felt that much better."

McCann considers himself to be lucky. Living in Glasgow, he is one of 1400 people with Parkinson's disease who have access to seven specialist nurses. In fact, the city's framework of specialist Parkinson's care is cited as a model case. While the health advisory body Sign (Scottish Intercollegiate Guidelines Network) is still in the process of setting recommendations of Parkinson's care in Scotland, the equivalent organisation south of the border, Nice (the National Institute for Clinical Excellence), recommends a caseload of 300 patients to one specialist nurse.

According to the Parkinson's Disease Society (PDS), nowhere in Scotland outside Greater Glasgow does the care meet the same standard. In some health board areas, such as Lothian, where there are approximately 2000 people diagnosed with Parkinson's, the charity reports there is only one specialist nurse. There are no specialist nurses for a total of 1250 patients in the Borders, Ayrshire and Arran and Dumfries and Galloway.

The PDS, which raises funds and campaigns to support all people with Parkinson's, and their families, believes a lot more could be done. Andrew Sim, PDS Scotland manager, says: "For us, the evidence is clear that where there is at least one specialist nurse to 300 Parkinson's cases then care is extremely effective and could undoubtedly improve the lives of the estimated 12,000 people diagnosed with the condition in Scotland.

"We are keen to campaign the government and to work with the 14 individual health boards in Scotland to roll out the same framework of support and care as there is currently in Glasgow."

To this end, a parliamentary reception on November 27, hosted by Christine Grahame, convener of the parliament's health committee, will highlight the vital role of Parkinson's disease nurse specialists such as Jacqui Kerr. "We want to get across the message that if Greater Glasgow Health Board can provide, and find the resources, for the ideal level of care for people with Parkinson's, there should be scope for other health boards to do the same," says Sim. "We're simply talking about a basic level of care that each person with Parkinson's should have the right to receive."

McCann, who is chair of the East Glasgow Parkinson's Disease Support Group, could not agree more. "The stark reality of the situation is brought home to me every time I talk to friends with the condition living in Edinburgh. I feel very fortunate indeed to be living where I do - but it doesn't seem fair that others do not have the same quality of life as me."

A condition that affects 120,000 people in the UK
What is Parkinson's?
It is a progressive and debilitating neurological disorder that makes daily tasks such as walking, talking and writing difficult.

Why does it occur?
Medics still do not know why some people are affected by Parkinson's but they claim to have identified the cause. When the brain transmits messages to the body to make movements it uses chemical messengers (neurotransmitters), including one called dopamine. People with Parkinson's display a short supply of dopamine.

How does it affect people?
Symptoms vary between people but they can include tremors, stiffness and "switching off", slowness of movement, poor balance and problems with speech, facial expressions and handwriting. The condition starts slowly and gets worse over time.

Who has it?
There are estimated to be 120,000 people with the condition in the UK, and 10,000 are diagnosed every year. Approximately 12,000 Scots are thought to have it. The disease is more common in the elderly, although one in 20 cases is in those aged under 40.

How is it treated?
There is no cure for Parkinson's, although drugs can treat the symptoms. The main aim of the drugs is to increase the dopamine that gets to the brain. Treatment varies from one person to the next, and levels of drugs need to be adjusted carefully. Levodopa is perhaps the most well-known drug and has been used to treat Parkinson's since the 1960s. Other treatments include exercise, good diet, physiotherapy and speech therapy.

Surgery is occasionally recommended as a way of aiding the stimulation of parts of the brain affected by Parkinson's.

Where can I find out more?
The Parkinson's Disease Society provides field staff and local information and maintains 330 branches and support groups in the UK. For information about specialist clinics and nurses in Scotland, see www.parkinsons.org.uk In the Glasgow area there are five PDS groups including Mansion House Day Hospital, Lightburn Hospital, Stobhill Hospital, Southern General Hospital and the Murray Owen Centre in East Kilbride.