Reflex Reactions

If our sense of the outside world comes from our senses, our reactions to the outside world are controlled by our reflexes.

When we think of reflexes we typically think of an unexpected reaction to an outside stimulus. We might duck in response to a loud sound, dodge if we see something coming towards us, retract if we touch something hot or pull away if we stand on something sharp.

All of these “reflex” reactions occur when muscles contract in response to an external stimulus.

The reality is that outside forces are stimulating our muscles every time we move, run, jump, walk, throw, catch, get out of bed or hold our head up and our reflexes are governing our reaction to those forces. Reflexes are ultimately responsible for the action of our muscles and so govern the way we move.

Reflexes are actions that occur before the brain is aware of what is happening. The reaction to the stimulus is processed locally, in the spinal cord, bypassing brain control. Naturally, if movement results, the brain will become aware of such movement and can regulate it somewhat, but cannot control it.

Reflexes have at least four (usually five) separate parts.

  • A sensor
  • A sensory nerve that carries the message from the sensor to the spinal cord
  • A relay nerve called an interneuron – some reflexes bypass the interneuron, others go via 3 or 4 interneurons.
  • A motor nerve that carries the message from the spinal cord to the muscle
  • A muscle fibre that contracts

The Stretch (Myotatic) Reflex

One of the best-known reflexes is the knee-jerk reflex. You sit with your leg dangling towards the floor and your leg is tapped just below the kneecap. Hopefully, your foot will shoot out and then relax back to its starting point.

The sensors are the muscle spindles located in the quadriceps or thigh muscle. You will remember that these sensors constantly generate nerve signals.

As the tendon is tapped below the kneecap, a small but sudden stretch is felt by the spindles which respond by “revving” the engine, suddenly increasing the frequency of messages back to the spinal cord.

The increased rate of firing from the muscle spindles is sent back to the spinal cord, where it relays directly and indirectly to the anterior motor neurons.

The Anterior Motor Neurons now have more sensory input so their output suddenly increases, the muscle contracts and your foot shoots out.

This reflex does not exist simply for the amusement of doctors and young children. In life, this reflex works to protect our joints from injury.

Imagine a gymnast landing on a mat after a somersault. With her legs bent she is ready for impact. As her feet touch the mat, her knees start to bend. The quadriceps muscles on the front of her thigh start to stretch. Sensors within the muscle detect the speed and force of the stretch and fire rapidly. The nerve from the muscle carries the massively increased rate of firing back to the spinal cord where a direct connection is made to the nerves that control the tone (strength) of the quadriceps. The massive increase in nerve activity is transmitted straight back to the quadriceps, causing it to contract instantly. The faster the muscle stretches, the faster the response will be. Failure to contract hard enough would cause the gymnast to end her leap sprawled on the ground; too much contraction and she would be flung straight up in the air again.

The example of the gymnast teaches us that feedback from the stretch of a muscle dynamically alters the muscle strength output in a way that protects our joints and our bodies from injury.

Most of us will never be doing somersaults, but we use those same reflexes every time we walk down stairs, jump off a chair, throw a ball or bend over to pick up a pen.

Think back to the last time you jumped off a chair, or if you haven’t done it for a while, try it now. You may have had time to plan the landing but did your planning really help? Most of the time the landing is done on autopilot. Feedback from muscle spindles in your thigh tells the motor neurons in your spine exactly how much force to generate so that you land without injury.

Constant Adjustment

Muscles constantly adjust their tone to control our movements.

If you can – squat down on the floor right now. Don’t hold onto anything, just balance. Notice what is happening to your feet and ankles. You feel tiny adjusting movements which keep you on balance. These movements are the result of reflexes coming from the muscles themselves. Those same reflexes are operating in every muscle, throughout your body, all day, every day.

Imagine you were walking across rocks and your foot suddenly slipped. The muscles holding your foot would be stretched very rapidly, causing the spindle cells to increase their rate of firing back to the spinal cord. Direct connections to the nerves that contract the muscle are increased and the muscle suddenly fires, holding the foot and preventing a sprain of your ankle ligaments. The same reflex that caused your foot to shoot out when the kneecap was tapped and stopped our gymnast from collapsing on the floor is actually the same reflex that keeps your ankle safe from sprain or strain while you walk.

It is unfortunate that the myotatic mechanism was ever called a reflex because most people link reflexes with one-off events, like the knee-jerk reflex. The myotatic mechanism is unlike all other reflexes because it functions 24 hours per day. It constantly monitors and maintains appropriate muscle tone.

When we bend forward to pick up a pen off the ground, we often forget that muscles all the way up our spine are working to hold our weight; that our calf muscles are contracting to push our toes into the ground and our neck muscles are active to hold our head up. When we pick up a chair or a bag of cement, the sensors in the muscles that move the vertebrae in our spine would have more or less tension placed upon them and they will vary their output accordingly. As we lift the weight, the amount of tension in the muscles increases and the muscles increase their power to hold our spine firmly.

These stabilising muscles are vital for our joint function and safety. If the stabilizers don’t work with enough speed or strength then the joint is likely to be sprained, strained or damaged. This can happen in one major accident, leading to strain or sprain – or it can happen over years – causing the wear and tear we know as osteoarthritis.

Our reflexes then, control our muscle tone and protect us from injury. Learning how to test for muscle tone should be of the utmost importance for any clinician concerned with injury prevention.