According to "Textbook of Medical Physiology," input into the nervous system is provided by sensory receptors that tell the brain what is happening in the body and allows the central nervous system to respond and adapt to the changing environment. Many types of receptors exist in the human body. These receptors monitor pain, temperature, sensation, blood pressure and everything else. Two types of specialized receptors in the muscles are called muscle spindles and golgi-tendon organs, which are essential to posture and movement, and can be utilized to increase muscular strength, power and performance during exercise.
Purpose
Muscle spindles respond to stretch on the muscle and golgi-tendon organs respond to tension on the tendons. When a muscle is stretched it causes a rapid lengthening and a contraction of the agonist muscle, which is known as the stretch reflex, and it is seen in dynamic or ballistic flexibility exercise.
Anatomy and Neurology
The muscle spindle lies between skeletal muscle fibers, called extrafusal, and consists of small specialized fibers, called intrafusal, and nerve endings surrounded by a connective tissue sheath. When the spindle is stretched it activates the sensory nerve fibers inside the spindle alerting the central nervous system to the changes to the muscle and load, and allows the central nervous system to adapt to its changing environment. This triggers a reflexive muscle contraction that resists further stretching.
Function
Muscle spindles send information to higher brain centers, which is essential for maintaining muscle tone, posture and for executing movement. Muscle spindles are found in virtually all muscles and with golgi-tendon organs contribute to proprioception. Proprioception is the ability of the body to tell where it is in space. When you close your eyes, you can still touch your nose with your hand because proprioceptors alert the brain where everything is in the body. Proprioception is also important in balance and in injury prevention. For example, poor ankle proprioception can lead to sprained ankles.
Muscle Spindles and Performance
The muscle spindle is activated each time a muscle is stretched and allows complex movements to occur. Muscle spindle involvement in proprioception contributes to the specific movements and exercises that are performed in a training program.
Expert Insight
To avoid injury in the neuro-musculoskeletal system it is important to understand how the body works. Muscle spindles are a key player in the function of the nervous system with the proprioceptive nature of their function. Without them our body would be clumsy and we could not perform delicate and fine movement.
During rehabilitation of injured joints, a major portion of the rehab process is re-establishing proprioceptive control to avoid future injury. This utilizes various nerve receptors including muscle spindles and the stretch reflex.
References
- "Textbook of Medical Physiology Tenth Edition"; A.C. Guyton and J.E. Hall; 2000
- "Physiology of Sport & Exercise 3rd ed."; J.H. Willmore and D.L. Costill; 2004
- "Resource Manual for Guidelines for Exercise Testing and Prescription 6th Ed."; American College of Sports Medicine; 2009
