Proprioception is defined as body position sense – the ability to know where our body is at all times.
It refers to our uncanny ability to know almost exactly where our arm, leg or finger is, without looking at it. With proprioception, you can estimate the angle of your elbow, the position of your hand and the spread of your fingers. If you had to touch your nose, you could do so, even with your eyes closed. As impressive as this ability is, it is only part of the proprioception story.
The ultimate purpose of proprioception is to control the way we move. It gathers continuous input from the millions of sensors in the skin, muscles, joints and ligaments, combines it with the input from our five main senses, and uses it to control our balance, co-ordination, posture and movement.
Whether we’re picking up a glass, throwing a ball, watching TV or walking down a step, proprioception is constantly monitoring the input from the nerve sensors in your body, to make sure that the output to your muscles is perfect. It is a system so complex, diverse and adaptable that no amount of robotic or computing power can come close to duplicating the smooth and coordinated movements of the human body.
Your brain is just a processing centre. It doesn’t actually generate anything, it just connects the mass of incoming signals in a way that predictably controls our outgoing signals. Our every thought and action starts life as an incoming nerve signal.
The barrage of incoming nerve signals is the raw material that creates all brain function and therefore all human function. Most of those incoming nerve signals originate in the muscles themselves.
The brain’s function is to turn those incoming nerve signal into action. Action in a human sense will mean making muscles work, and to a lesser extent, controlling our glands and organs.
Nerve messages are classified as either incoming or outgoing. Incoming nerve messages are heading towards the spinal cord or brain and are known as sensory or afferent. Outgoing nerve messages travel from the brain or spinal cord and are intended to make something happen, they are therefore known as motor or efferent.
The ability for us to move at all, let alone perform somersaults, catch a ball, go to the toilet or climb stairs depends on the seamless integration of these two types of messages.
Our awareness of the outside world is maintained through the five senses of sight, smell, taste, hearing and touch. Our awareness of our inside world is maintained through proprioception. It really is our “sixth sense.”
Proprioceptors are a group of sensors found throughout the body. They provide information on the movement, position, tension and force in every area of the body.
Most proprioceptors, especially those in the skin, send messages only when they are stimulated, but a special class of proprioceptors known as muscle spindle cells, send a constant stream of messages to the spinal cord and brain, even when the muscle is resting.
Spindle cells are are smaller muscle fibres that sit alongside ordinary muscle fibres. Their job is to detect the slightest changes in the tension of the muscle.
Every muscle spindle cell constantly generates signals which travel from the muscle back to the spinal cord. Like the rev counter of an engine, they are constantly active, responding to the activity of the muscle. Whether the muscle is fully stretched or semi-contracted, the muscle spindles will adjust their output to reflect the amount of tension in the muscle. This way, the muscle is ready to react at any time, no matter what it’s position.
All proprioceptive information is sent to the spinal cord where it has two functions. First, it controls muscle tone and second, it supplies the brain and higher processing centres with vital feedback.
Although most sensory information is ultimately sent to the brain, much of it is filtered before it gets to the consciousness. Many nerve messages never get as far as the brain, their reflex effects are managed within the spinal cord, totally independent of brain control.
In animal experiments (of which I do not approve), researchers have found that that when they tickle a decerebrate dog (a dog that has had it’s spinal cord cut in the neck) on the shoulder blade, the dog will lift it’s hind leg on the same side to “scratch” the “flea” that is annoying it. When the tickling crosses the spine, the other leg starts scratching. This movement is occurring without any input from the brain, it is purely under reflex control. Similarly, a walking movement will be provoked simply by pushing on the pads of the dog’s paws.
Motor output simply refers to messages sent from the brain or the spinal cord to the body. These messages are designed to produce an action.
Although most of our conscious movement occurs via muscles that are traditionally classified as voluntary, up to 90% of our voluntary muscle activity is totally subconscious or involuntary, controlled by reflexes that originate in our proprioceptors. It is this involuntary activity that stabilises, holds, controls and limits the movements of our bones and joints.
All muscles are made of muscle fibres which contract when they receive an impulse from special nerves cells in the spinal cord called Anterior Motor Neurons, which is a silly name because there are no posterior motor neurons.
Motor neurons are fascinating cells. They only have a single output. All they do is send impulses or messages to muscle fibres. A single motor neuron will supply one or more muscle fibres so thousands of motor neurons have to act together to bring a muscle to full contraction. Every time they send a message, a muscle fibre, or fibres, will contract. The faster the messages are sent, the faster the fibre will contract. The more motor neurons that are sending messages, the stronger the contraction will be.
What makes motor neurons fascinating is not their output, but their input. Each motor neuron receives up to 10,000 different inputs. An input is a message from another nerve which is bringing information from other muscles, skin, tendons, ligaments, bones and the brain.
A motor neuron will not send a message to a muscle fibre before it has accumulated enough inputs. For your average motor neuron, this would be about 70mV (nerves work in millivolts). However, each input supplies only half to one mV, so it is impossible for any one input to cause an output. To make life even more complicated, not all inputs are positive. Some are negative. Negative inputs are called inhibitory, positive inputs are called facilitatory.
Each neuron waits until it receives enough facilitatory inputs to outweigh the inhibitory inputs. When the total reaches 70mV, the neuron fires, sending a signal to make a muscle fibre (or several fibres) contract. To put this in mind-boggling terms, we also need to remember that even at rest, each motor neuron is firing 50 times per second. At maximum effort, each one is firing 500 times per second.
Motor neurons (and therefore muscles) receive their instructions from three main sources.
Muscle spindle inputs we have covered. Instructions from the brain are fairly self-explanatory and far too complicated for this discussion. Interneurons are special nerve cells that interconnect messages from other nerve cells within the spinal cord. It is interneurons that supply the bulk of the 10,000 inputs to each motor neuron. These inputs come from the mass of proprioceptors throughout the body, mainly from skin and muscles.
Interneurons allow muscles to work together, sometimes moving, sometimes stabilising, sometimes balancing. Even simple operations like walking involve the whole body; trunk, spine, neck and arms, as well as the legs. As the tension changes in one area, proprioception will increase or decrease the tone of many others.
Most people, whether medically trained or not, believe that muscle tone is under conscious control. We are told to “sit up straight” as if our posture is a conscious decision. Obviously, it is partly under conscious control, so we can sit up straight, for a while, if we have to, but ultimately our posture will be controlled by the output from our motor neurons, much of which is beyond the influence of our consciousness.
Proprioceptive Medicine is primarily concerned with the influence our SENSORY input is having on our muscle tone, and our overall health.