Osteoarthritis is a disease that affects your joints. The surfaces within your joints become damaged so the joint doesn’t move as smoothly as it should. The condition is sometimes called arthrosis, osteoarthrosis, degenerative joint disease or wear and tear.
When a joint develops osteoarthritis, some of the cartilage covering the ends of the bones gradually roughens and becomes thin. This can happen over the main surface of your knee joint and in the cartilage underneath your kneecap. The bone underneath the cartilage reacts by growing thicker and becoming broader. All the tissues within the joint become more active than normal – as if your body is trying to repair the damage.
These changes in and around the joint are partly the result of the inflammatory process and partly your body’s attempt to repair the damage. In many cases, the repairs are quite successful and the changes inside the joint don’t cause much pain or, if there is pain, it’s mild and may come and go. However, in other cases, the repair doesn’t work as well and your knee becomes damaged. This leads to instability and more weight being put onto other parts of the joint, which can cause symptoms to become gradually worse and more persistent over time.
Almost anyone can get osteoarthritis, but it’s most likely if:
There are several ways you can help yourself, including:
If you still have pain after trying self-help measures, your doctor may recommend the following treatments:
A joint is where two or more bones meet. The joint allows the bones to move freely but within limits. The knee is the largest joint in the body and also one of the most complicated. It needs to be strong enough to take our weight and must lock into position so we can stand upright. But it also has to act as a hinge so we can walk and must withstand extreme stresses, twists and turns, such as when we run or play sports.
The knee joint is where your thigh bone (femur) and shin bone (tibia) meet. The end of each bone is covered with cartilage which has a smooth, slippery surface that allows the ends of the bones to move against each other almost without friction. Your knees have two additional rings of cartilage between the bones. These are called menisci, which act a bit like shock absorbers to spread the load more evenly across the joint.
Your knee joint is held in place by four large ligaments. These are thick, strong bands which run within or just outside the joint capsule. Together with the capsule, the ligaments prevent the bones moving in the wrong directions or dislocating. The thigh muscles also help to hold the knee joint in place.
Your muscles are attached to your bones by strong connecting tissues called tendons. These tendons run on either side of the joint, which they also help to keep in place. When your muscles contract they shorten, and this pulls on the tendon attached to the bone and makes the joint move.
Your kneecap (patella) is fixed firmly in the middle of the large tendon that attaches your thigh muscles (quadriceps) to the bone just below your knee joint at the front of your shin bones. The underside of your kneecap is also covered with cartilage.
The joint is surrounded by a membrane (the synovium) that produces a small amount of synovial fluid, which helps to nourish the cartilage and lubricate the joint. The synovium has a tough outer layer called the capsule, which helps hold your knee in place.
The main symptoms of osteoarthritis are pain and sometimes stiffness, which can affect one or both knees. The pain tends to be worse when you move the joint or at the end of the day. You may have pain all around your knee or just in a particular place, most likely at the front and sides, and it may be worse after a particular movement, such as going up or down stairs. The pain is usually better when you rest.
It’s unusual, but some people have pain that wakes them up at night. This generally only happens with severe osteoarthritis. You’ll probably find that your pain will vary and that you have good days and bad days, sometimes depending on how active you’ve been but sometimes for no obvious reason.
Your knee may feel stiff at certain times, often in the mornings or after a period of rest. Walking for a few minutes will usually ease it. However, many people don’t have any stiffness at all, even with quite severe osteoarthritis.You may not be able to move your knee as freely or as far as normal, and it may creak or crunch as you move. If your osteoarthritis is severe, your knee may become bent and bowed. Sometimes the joint gives way, either because the muscles have become weak or because the joint structure has become less stable.
You may notice that your knee looks swollen. The swelling may be hard (caused by osteophytes around the sides of the joint) or soft (caused by extra fluid in the joint). The muscles at the front of your thigh that help straighten your knee may look thin and wasted.
There are many factors that can increase the risk of osteoarthritis, and it’s often a combination of these that leads to the condition.
Osteoarthritis usually starts from the late 40s onwards. We don’t fully understand why it’s more common in older people, but it might be due to factors like weakening of the muscles, the body being less able to heal itself or gradual wearing out of the joint with time.
Osteoarthritis of the knee is twice as common in women as in men. It’s most common in women over the age of 50, although there’s no strong evidence that it’s directly linked to the menopause. It’s often associated with mild arthritis of the joints at the ends of the fingers (nodal osteoarthritis), which is also more common in women.
Being overweight is an important factor in causing osteoarthritis, especially in the knee. It also increases the chances of osteoarthritis becoming progressively worse.
Normal activity and exercise don’t cause osteoarthritis, but very hard, repetitive activity or physically demanding jobs can increase the risk. Injuries to the knee often lead to osteoarthritis in later life. A common cause is a torn meniscus or ligament, which can result from a twisting injury.
A torn meniscus is a common injury in footballers, and an operation to remove the damaged cartilage (meniscectomy) or repair cruciate ligaments also increases the risk of osteoarthritis in later life.
Genetic factors play a major part in osteoarthritis of the knee. If you have a parent, brother or sister with knee osteoarthritis then you’ll have a greater chance of developing it yourself. We don’t know a lot about the genes that cause the increased risk, but we do know that a number of genes will have a small effect rather than one particular gene being responsible.
Sometimes osteoarthritis is a result of damage from different kinds of rarer joint disease, such as gout, that occurred in earlier years.
Although there’s no evidence that different conditions such as cold or wet weather actually cause or worsen osteoarthritis, many people find that their pain and stiffness may vary with the weather. This may be because nerve fibers in the capsule of affected joints are sensitive to changes in atmospheric pressure.
It’s impossible to predict how osteoarthritis will develop for any one person. It can sometimes develop over just a year or two and cause a lot of damage to a joint, which may cause some deformity or disability.But more often osteoarthritis is a slow process that develops over many years and results in fairly small changes in just part of the joint. This doesn’t mean it won’t be painful, but it’s less likely to cause severe deformity or disability.
In severe osteoarthritis the cartilage can become so thin that it no longer covers the ends of the bones. The bones start to rub against each other and eventually wear away. The loss of cartilage, the wearing of bone and the bony spurs can alter the shape of the joint, forcing the bones out of their normal alignment.
In addition, the muscles that move the joint gradually weaken and become thin or wasted. This can make the joint unstable so that the knee gives way when weight is put on it.
Changes in lifestyle can greatly reduce the risk of osteoarthritis of the knee progressing. Regular exercise, protecting the joint from further injury and keeping to a healthy weight will all help.
Osteoarthritis doesn’t lead to rheumatoid arthritis or other types of joint disease and won’t spread through the body like an infection might. However, deformity caused by osteoarthritis in one joint may lead to uneven loading of other joints. This could result in osteoarthritis in those joints. Because there’s little, if any inflammation in osteoarthritic joints, osteoarthritis doesn’t make you feverish or unwell. However, some people with osteoarthritis will develop other illnesses purely by chance.
There can sometimes be rarer complications with osteoarthritis of the knee:
Osteoarthritis with crystals occurs when chalky deposits of calcium crystals form in the cartilage. This is called calcification or chondrocalcinosis. It can happen in any joint, with or without osteoarthritis, but it’s most likely to occur in a knee that’s already affected by osteoarthritis, especially in older people. It can cause sudden pain and noticeable swelling of the joint. The crystals may show up on x-rays and they can also be seen under a microscope in samples of fluid taken from the joint.
Osteoarthritis tends to become more severe more quickly when there are crystals present. Sometimes the crystals can shake loose from the cartilage, causing a sudden attack of very painful swelling called acute calcium pyrophosphate crystal arthritis (acute CPP crystal arthritis), which was sometimes previously called ‘pseudogout’.
lining. They’re often painless, but you may be able to feel a soft-to-firm lump at the back of your knee. Sometimes a cyst can cause aching or tenderness when you exercise.
Occasionally a cyst can press on a blood vessel, which can lead to swelling in your leg, or the cyst may burst (rupture) and release joint fluid into your calf muscle, which can be very painful.
A cyst may not need treatment, but if it does it can generally be treated by drawing off the extra fluid from your knee using a syringe (this is called aspiration) and injecting a steroid solution.
It’s very important to get an accurate diagnosis if you think you might have arthritis. There are many different types of arthritis and some, such as rheumatoid arthritis, need very different treatments.
Osteoarthritis is usually diagnosed based on your symptoms and the physical signs that your doctor finds when examining your joint, for example:
There’s no blood test for osteoarthritis, although your doctor may suggest them to help rule out other types of arthritis.
X-rays are taken to assess the severity of the changes caused by osteoarthritis, although often they won’t be needed. They may show changes such as bony spurs or narrowing of the space between the bones where the cartilage has worn thin. They may also show whether there are any calcium deposits within the joint. However, x-rays aren’t a good indicator of how much pain or disability you’re likely to have. Some people have a lot of pain from fairly minor joint damage, while others have little pain from more severe damage.
Rarely, a magnetic resonance imaging (MRI) scan of your knee can be helpful. This will show the soft tissues (for example cartilage, tendons, muscles) and changes in the bone that can’t be seen on a standard x-ray.
There’s no cure for osteoarthritis as yet, but there’s a lot that you can do to improve your symptoms. Self-help measures play a very important part in relieving the pain and stiffness, and reducing the chances of your arthritis becoming worse.
There’s a great deal of evidence that being overweight increases the strain on your joints, especially your knees. Research shows that being overweight or obese not only increases your risk of developing osteoarthritis but also makes it more likely that your arthritis will get worse over time.
Because of the way the joints work, the force put through your knees when you walk, run or go up and down stairs can be up to five to six times your body weight. Losing even a small amount of weight can make a big difference to the strain on weight-bearing joints such as the knees.
No special diet has shown to help specifically with osteoarthritis, but if you need to lose some weight you should follow a balanced, reduced-calorie diet combined with regular exercise.
Even if you don’t need to lose weight it’s very important to keep moving if you have osteoarthritis of the knee. You’ll need to find the right balance between rest and exercise – most people with osteoarthritis find that too much activity increases their pain while too little makes their joints stiffen up. Little and often is usually the best approach to exercise if you have osteoarthritis.
There are two types of exercise that you’ll need to do:
Strengthening exercises will improve the strength and tone of the muscles that control the affected joint. Osteoarthritis of the knee can weaken your thigh muscles (quadriceps), so regular exercising of the muscles, such as straight-leg raises, helps to stabilise and protect the joint. It’s also been shown to reduce pain and is particularly helpful in preventing your knee giving way, reducing the tendency to stumble or fall.
Aerobic exercise is any exercise that increases your pulse rate and makes you a bit short of breath. Regular aerobic exercise should help you sleep better, is good for your general health and well-being and can reduce pain by stimulating the release of pain-relieving hormones called endorphins.
A physiotherapist can advise you on the best exercises to do, but you’ll need to build them into your daily routine to get the most benefit from them. The pull-out section at the back of this booklet will give you some simple exercises to try at home. You can also talk to your GP about the Exercise on Prescription scheme that’s available in some areas.
Swimming can be very good for osteoarthritis. Because the water supports the weight of your body, you won’t be putting a lot of strain on your joints as you exercise. Your physiotherapist may also recommend special exercises in a hydrotherapy pool. This can help get muscles and joints working better and, because the water is warmer than in a typical swimming pool, it can be very soothing and relaxing. If you know you’re going to be more active than usual, try taking a painkiller before you start to avoid increased pain later.
There are a number of tablets and creams that can help the symptoms of osteoarthritis, and because they work in different ways you can combine different treatments if you need to. Your chemist can advise you and supply paracetamol and some low-dose tablets and creams without a prescription.
Painkillers often help with the pain and stiffness, although they don’t affect the arthritis itself and won’t repair the damage to the joint. They’re best used occasionally when the pain is very bad or when you’re likely to be exercising.
Paracetamol is usually the best and most well tolerated painkiller to try first, but make sure you take the right dose as most people take too little. You should try taking 1 g (usually two tablets) three or four times per day. It’s best to take them before the pain becomes very bad but you shouldn’t take them more often than every four hours.
Combined painkillers (for example co-codamol) contain paracetamol and codeine and may be helpful for more severe pain. They’re stronger than paracetamol on its own, but codeine can cause side-effects such as constipation or dizziness.Over-the counter non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, can also help. You can use these for a short course of treatment, but if they’ve not helped within this time then they’re unlikely to. If the pain returns when you stop taking the tablets, try another short course.
You shouldn’t take ibuprofen or aspirin if you’re pregnant, or if you have asthma, indigestion or a stomach (gastrointestinal) ulcer, until you’ve spoken with your doctor or pharmacist.
You can apply anti-inflammatory creams and gels directly onto painful joints three times a day. There’s no need to rub them in – they absorb through the skin on their own. They’re especially helpful for osteoarthritis of the knee, and they’re extremely well tolerated as very little is absorbed into the bloodstream. If you have trouble taking tablets then anti-inflammatory creams are a particularly good option to try. You can decide if they help your pain within the first few days of trying them.
If you’re already taking NSAID tablets, speak to your doctor about non-NSAID creams (for example capsaicin cream) to avoid taking too much of one type of drug.
Apart from keeping an eye on your weight, there are a number of other ways you can reduce the strain on your knees.
Applying warmth to a painful knee often relieves the pain and stiffness of osteoarthritis. Heat lamps are popular, but a hot-water bottle or reheatable pad is just as effective. This can be helpful if you have a flare-up of pain when you’ve done a bit too much. An ice pack can also help. Don’t apply ice/heat packs or hot-water bottles directly to your skin.
More evidence to support the use of knee braces for osteoarthritis is becoming available. There are several types that can help to stabilize the kneecap and make it move correctly. You can buy knee braces from sports shops and chemists, but you should speak to your doctor or physiotherapist first. They may also be able to provide braces or recommend the best ones for you.
There are many different complementary and herbal remedies that claim to help with arthritis, and some people do feel better when they use them. However, on the whole these treatments aren’t recommended for use on the NHS because there’s no conclusive evidence that they’re effective.
Many people try glucosamine and chondroitin tablets. These are compounds that are normally present in joint cartilage, and some studies suggest that taking supplements may improve the health of damaged cartilage. Glucosamine and chondroitin, which are similar to each other, are available from your chemist or health food store. You’ll need to take a dose of 1.5 g of glucosamine sulphate a day, possibly for several weeks before you can tell whether they’re making a difference. Glucosamine hydrochloride doesn’t appear to be effective, so always check that you’re taking the sulphate.
Most brands of glucosamine are made from shellfish. If you’re allergic to shellfish, make sure you take a vegetarian or shellfish-free variety. Glucosamine can affect the level of sugar in your blood, so if you have diabetes you should keep an eye on your blood sugar levels and see your doctor if they increase. You should also see your doctor for regular blood checks if you’re taking the blood-thinning drug warfarin.
Many people are interested in homeopathic remedies, and a number are used for osteoarthritis. However, there’s no conclusive scientific evidence that they’re effective.
There’s some research showing that acupuncture can sometimes provide relief from arthritis pain, although the effect may be short-lived. For longer-lasting benefits, you may need to have regular sessions of acupuncture. There’s also some evidence that electro-acupuncture may be effective for pain associated with osteoarthritis of the knee. This technique is similar to conventional acupuncture except that an electrical impulse is applied via the needles.
Although manipulation by a chiropractor or osteopath may be helpful for back or neck pain, the use of manipulation for osteoarthritis in other joints is limited.
If you do want to try it, make sure you choose a practitioner who is registered with the appropriate regulatory body.Generally speaking complementary and alternative therapies are relatively well always before risks with therapies than to or one fully critical base you
Complementary and alternative medicine for arthritis; Complementary and alternative medicines for the treatment of rheumatoid arthritis, osteoarthritis and fibromyalgia; Practitioner-based complementary and alternative therapies for the treatment of rheumatoid arthritis, osteoarthritis, fibromyalgia and low back pain.
Many people find that self-help measures, such as those listed above, are enough to help them manage their symptoms, but your healthcare team will be able to suggest other treatments if you need them.
Capsaicin cream is made from the pepper plant (capsicum) and is an effective and very well-tolerated painkiller. It’s only available on prescription. It needs to be applied three times a day to be effective and, like NSAID creams and gels, it’s particularly useful for osteoarthritis of the knee.
Most people feel a warming or burning sensation when they first use capsaicin, but this generally wears off after several days. The pain-relieving effect starts after several days of regular use and you should try it for at least two weeks before deciding if it has helped.
If you have severe pain, for example while you’re waiting for a knee replacement operation, and other medications aren’t giving enough relief, your doctor may recommend stronger painkillers (or opioids) such as tramadol, nefopam or meptazinol. Stronger painkillers are more likely to have side-effects – especially nausea, dizziness and confusion – so you’ll need to see your doctor regularly and report any problems you have with these drugs.
Some opioids can be given as a plaster patch that you wear on the skin. These can give pain relief for a number of days.
If inflammation in the joint is contributing to your pain and stiffness, a short course of NSAID tablets (for example ibuprofen, naproxen) may be useful.
Like all drugs, NSAIDs can sometimes have side-effects, but your doctor will take precautions to reduce the risk of these – for example, by prescribing the lowest effective dose for the shortest possible period of time.
NSAIDs can cause digestive problems (stomach upsets, indigestion or damage to the lining of the stomach) so in most cases they’ll be prescribed along with a drug called a proton pump inhibitor (PPI), which will help to protect your stomach.
Because a lot of drug treatments for osteoarthritis work in different ways, they can be combined to help ease your symptoms.Self-help methods like looking after your joints will also help to prevent further damage.
NSAIDs also carry an increased risk of heart attack or stroke. Although the increased risk is small, your doctor will be cautious about prescribing them if there are other factors that may increase your overall risk – for example, smoking, circulation problems, high blood pressure, high cholesterol or diabetes.
If you have trouble opening childproof containers, your pharmacist will put them in a more suitable container for you. Contact us for our special request card which you can hand to your pharmacist with your prescription.
Steroid injections are sometimes given directly into a particularly painful knee joint. The injection can start to work within a day or so, and it may improve pain for several weeks or even months. This is mainly used for very painful osteoarthritis where the knee is swollen, for sudden painful attacks caused by the shedding of calcium pyrophosphate crystals or to help people through an important event (such as a holiday or family wedding). However, it’s important to remember that steroid injections can’t be given frequently or indefinitely. If you need repeated steroid injections into an osteoarthritic knee then you may need to consider surgery.
When steroid injections don’t work, some doctors give injections of this lubricating substance into the knee joint, either as a single injection or as a course of several injections. However, this form of treatment isn’t approved by the National Institute for Health and Clinical Excellence (NICE) and isn’t widely used because the evidence that it works isn’t convincing.
Some people find that transcutaneous electrical nerve stimulation (TENS) can help to relieve pain, although research evidence on its effectiveness is mixed. A TENS machine is a small electronic device that sends pulses to the nerve endings via pads placed on your skin. It produces a tingling sensation and is thought to modify pain messages transmitted to your brain. TENS machines are available from pharmacies and other major stores, but a physiotherapist may be able to loan you one to try before you decide whether to buy one.
Surgery may be recommended if pain is very severe or you have mobility problems. Many thousands of knee replacements are performed each year for osteoarthritis, and the operation can give substantial pain relief in cases where other treatments haven’t helped enough. Surgical techniques are improving all the time and replacements now last on average over 15 years.
Sometimes keyhole surgery techniques may be used to wash out loose fragments of bone and other tissue from your knee. This is called arthroscopic lavage, and it’s not recommended unless your knee locks.
If pain is a problem at night, heat may help. Try a hot bath before going to bed, or use a hot-water bottle, wheat bag (which you can heat in a microwave) or electric blanket. Taking a painkiller before going to bed can ease night-time pain so you can get to sleep more easily. Placing a pillow between your knees can also help to ease pain.
Most people with osteoarthritis are able to continue in their jobs, although you may need to make some alterations to your working environments, especially if you have a physically demanding job. Speak to your employer’s occupational health service if they have one, or your local Jobcentre Plus can put you in touch with Disability Employment Advisors who can arrange work assessments. They can advise you on changing the way you work and on equipment that may help you to do your job more easily. If necessary, they can also help with retraining for more suitable work.
Living with a long-term condition like osteoarthritis can lower your morale and may affect your sleep. It’s important to tackle problems like these as they could lead to depression and will certainly make the osteoarthritis itself more difficult to cope with. It often helps to talk about negative feelings, so it could be useful to speak to your healthcare team, or your family and friends.
Research has already shown the importance of exercise and weight management in reducing the pain of osteoarthritis, particularly of the knee. There are many studies going on around the world to find and test new treatments for osteoarthritis.
Researchers are looking into ways to help GPs make a quicker diagnosis of osteoarthritis. A new technique, dGEMRIC (delayed gadolinium-enhanced MRI of cartilage), which aims to diagnose osteoarthritis at an earlier stage, is currently being investigated by Swedish scientists.
Acupuncture – A method of obtaining pain relief which originated in China. Very fine needles are inserted, virtually painlessly, at a number of sites (called meridians) but not necessarily at the painful area. Pain relief is obtained by interfering with pain signals to the brain and by causing the release of natural painkillers (called endorphins).
Aerobic exercise – Any exercise that increases your pulse rate and makes you a bit short of breath.
Analgesics – Painkillers. As well as dulling pain they lower raised body temperature, and most of them reduce inflammation.
Cartilage – A layer of tough, slippery tissue that covers the ends of the bones in a joint. It acts as a shock absorber and allows smooth movement between bones.
Chiropractor – A specialist who treats mechanical disorders of the musculoskeletal system, often through spine manipulation or adjustment. The General Chiropractic Council regulates the practice of chiropractic.
Gout – An inflammatory arthritis caused by a reaction to the formation of urate crystals in the joint. Gout comes and goes in several flare-ups at first, but if not treated it can eventually lead to joint damage. It often affects the big toe.
Hydrotherapy - Exercises that take place in water (usually a warm, shallow swimming pool or a special hydrotherapy bath) which can improve mobility, help relieve discomfort and promote recovery from injury.
Inflammation – A normal reaction to injury or infection of living tissues. The flow of blood increases, resulting in heat and redness in the affected tissues, and fluid and cells leak into the tissue, causing swelling.
Ligaments – Tough, fibrous bands anchoring the bones on either side of a joint and holding the joint together. In the spine they’re attached to the vertebrae and restrict spinal movements, therefore giving stability to the back.
Magnetic resonance imaging (MRI) scan –A type of scan that uses high-frequency radio waves in a strong magnetic field to build up pictures of the inside of the body. It works by detecting water molecules in the body’s tissue that give out a characteristic signal in the magnetic field. An MRI scan can show up soft-tissue structures as well as bones.
Manipulation –A type of manual therapy used to adjust parts of the body, joints and muscles to treat stiffness and deformity. It’s commonly used in physiotherapy, chiropractic, osteopathy and orthopaedics.
Menisci (singular meniscus) – Rings of cartilage, like washers, lying between the cartilage-covered bones in the knee. They act as shock absorbers and help the movement of the joint. Each knee has an inside (medial) and an outside (lateral) meniscus.
Menopause – The time when menstruation ends, usually when a woman is in her 50s. This means the ovaries stop releasing eggs every four weeks, and it’s no longer possible to have children. If this happens before the age of 45, it’s known as premature menopause.
Non-steroidal anti-inflammatory drugs (NSAIDs) - A large family of drugs prescribed for different kinds of arthritis that reduce inflammation and control pain, swelling and stiffness. Common examples include ibuprofen, naproxen and diclofenac.
Occupational therapist – A trained specialist who uses a range of strategies and specialist equipment to help people to reach their goals and maintain their independence by giving practical advice on equipment, adaptations or by changing the way you do things (such as learning to dress using one handed methods following hand surgery).
Osteopath – A trained specialist who treats spinal and other joint problems by manipulating the muscles and joints in order to reduce tension and stiffness, and so helps the spine to move more freely.
Osteophytes –An overgrowth of new bone around the edges of osteoarthritic joints. Spurs of new bone can alter the shape of the joint and may press on nearby nerves.
Physiotherapist – A trained specialist who helps to keep your joints and muscles moving, helps ease pain and keeps you mobile.
Proton pump inhibitor (PPI) –A drug that acts on an enzyme in the cells of the stomach to reduce the secretion of gastric acid. They’re often prescribed along with non-steroidal anti-inflammatory drugs (NSAIDs) to reduce the side-effects of those drugs.
Rheumatoid arthritis –A common inflammatory disease affecting the joints, particularly the lining of the joint. It most commonly starts in the smaller joints in a symmetrical pattern – that is, for example, in both hands or both wrists at once.
Synovium – The inner membrane of the joint capsule that produces synovial fluid.
Transcutaneous electrical nerve stimulation (TENS) – A small battery-driven machine which can help to relieve pain. Small pads are applied over the painful area and low-voltage electrical stimulation produces a pleasant tingling sensation, which relieves pain by interfering with pain signals to the brain.