Your body is able to sense its position in the environment. This is critical for movement, playing basketball, or even sitting still. Find out what this means as we explore proprioceptors, muscle spindles, and tendon organs.
Muscle Position and Movement
Long ago, there was a great mythological fighter known as Achilles. He fought for the Greek army in the Trojan War. His skill in battle was exceptional, and this warrior's talent did not come from nothing.
Besides talent and practice, Achilles unconsciously relied on certain receptors in his muscles in order to allow him to move his arms and legs in battle to avoid being killed. These receptors sensed position and movement and allowed him to balance his body, run, hit, and block.
Let's find out exactly what these structures are and why Achilles eventually succumbed to his weakness, the Achilles heel.
Proprioceptors sense changes in muscles and help you keep your balance.
When Achilles went into battle, he had to use not only his weapons, but also eyesight, hearing, and other senses in order to gather information about his surroundings. In addition, Achilles' body had to know where his limbs were in relation to his torso. If his body was unable to tell where his limbs were, it would be kind of difficult to fight. Imagine Achilles flailing his arms and legs around all over the place without purpose; he would have been killed very quickly.
This latter scenario is exactly what would have happened if he didn't have sensory receptors located in the inner ear, muscles, tendons, and joints that use internal stimuli to detect changes in position or movement of the body or its limbs. We call these receptors proprioceptors. Basically, proprioceptors help to sense the body's position and movement in space without the need for external stimuli like eyesight and hearing.
You can prove that this is the case yourself. If you were to stand up right now, close your eyes, and cover your ears and try to remain standing without falling down, you'd be using proprioceptors to accomplish this task. These receptors are able to sense changes in your muscles, joints, tendons, and inner ear in order to keep you standing. If you begin to lean just a little to the left, the proprioceptors send signals to the central nervous system to tell it to contract or extend certain muscles in order to keep you balanced, upright, and standing in one place.
However, proprioception, which is the act of sensing your body's motion, posture, and balance, is also critical in hand-eye coordination, a type of conscious proprioception. This is exactly why Achilles would have used a combination of both conscious and unconscious proprioception to move his limbs during a fight in the correct motion in order to hit the correct point, all without losing his balance.
Information is passed along to the central nervous system by muscle spindles.
When Achilles took his sword in order to go fight, his biceps muscle had to first stretch as his arm extended to pick up the sword, and then the biceps muscle had to contract in order to provide the strength to lift the sword. His central nervous system was able to sense all of this movement thanks to special receptors called muscle spindles. A muscle spindle is a special type of proprioceptive mechanoreceptor, called a stretch receptor, located in skeletal muscle that helps to detect changes in the length of a muscle.
As Achilles extended or contracted his muscles, the muscle spindles sent information to the central nervous system, which helped it determine how far away or how close his limbs were to his body, based on the stretch of the muscles that moved the limbs in the first place.
Despite his war hero status, Achilles eventually died when an arrow from another mythological figure, called Paris, pierced his Achilles heel, which led to his downfall and death. The Achilles heel has a structure running through it commonly called the Achilles tendon. This is a tendon that, if severed, would prevent you from walking or running very well, to say the least. If this were to happen during a battle, you would pretty much be toast.
Tendons like the Achilles tendon have what is known as a tendon organ. This is a proprioceptor located in the area where a tendon and muscle connect that helps to detect the tension being applied to a muscle. We sometimes call tendon organs Golgi tendon organs or Golgi organs for short. Do not, however, confuse the Golgi tendon organs with the Golgi apparatus found inside of your cells.
The Golgi organ relays tendon changes to the brain.
Once Achilles' heel was pierced by the arrow, the Golgi organ in the Achilles tendon and the muscles connecting to it would have sent signals up to the central nervous system to relay the position of Achilles' damaged muscles and leg.
Oftentimes, due to the pain and poor positioning of the leg after the Achilles tendon ruptures, people lose their balance and fall. If this were to happen in a battle, it would be difficult for Achilles to get up and protect himself. Therefore, once the arrow pierced his heel, poor Achilles eventually succumbed to his one deadly weakness, something we colloquially now term the Achilles heel.
With that sad note in mind, let's review the most important concepts of our lesson.
Sensory receptors located in the inner ear, muscles, tendons, and joints that use internal stimuli to detect changes in position or movement of the body or its limbs are called proprioceptors.
One of these proprioceptors is called a muscle spindle, which is a special type of proprioceptive mechanoreceptor, called a stretch receptor, located in skeletal muscle that helps to detect changes in the length of a muscle.
Another type of proprioceptor is called a tendon organ, which is a proprioceptor located in the area where a tendon and muscle connect that helps to detect the tension being applied to a muscle. Don't forget that we sometimes call tendon organs Golgi tendon organs or Golgi organs for short.
Following this lesson, you'll be able to:
- Define and describe the importance of proprioceptors
- Explain the function of two proprioceptors: muscle spindles and tendon organs