What are Muscle Spindles and How do They Work?

Improve your knowledge of the human body by learning more about Muscle Spindles and what they do.

What are Muscle Spindles?

Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle.

Source: Brain Kart

What do Muscle Spindles do?

They convey length information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception.

“Almost every muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching.

With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, for maintaining posture and for a stable gait. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch.”

Where do Muscle Spindles Originate?

Muscle spindles are located within extrafusal (skeletal) muscle fibre sand consist of connective tissue surrounding intrafusal fibres in a capsular structure. Muscle spindles are innervated by myelinated afferent nerve fibres, which enter the capsule of and spiral around the intrafusal fibres.

How are Muscle Spindles Activated?

When muscles lengthen, the spindles are stretched. This stretch activates the muscle spindle which in turn sends an impulse to the spinal cord. This impulse results in the activation of more motor neurons at spinal level that send an impulse back to the muscle.

What is the Function of Muscle Spindles and Golgi Tendon Organs?

Two important proprioceptors that play a role in flexibility are the muscle spindle and the golgi tendon organ (GTO), together reflexively work to regulate muscle stiffness.

The function of the GTO can be considered opposite of the muscle spindle, which serves to produce muscle contraction.

How do Golgi Tendons Work?

The Golgi Tendon Organ is a proprioceptive receptor that is located within the tendons found on each end of a muscle.

It responds to increased muscle tension or contraction as exerted on the tendon, by inhibiting further muscle contraction. Golgi tendon organs are arranged in series with the extrafusal muscle fibres.

What is the Function of Muscle Proprioceptors?

Sensory nerve endings wrap around the proprioceptors to send information to the nervous system. The proprioceptors can sense when tissues are stretched or experience tension and pressure. For example, the proprioceptors in muscles are called muscle spindles.

What is Muscle Proprioception?

General proprioception describes the position of muscles, joints, and tendons because proprioceptors are located in neuromuscular spindles and Golgi tendon organs. Axons project within peripheral nerves and enter the spinal cord via dorsal roots. Neurons are located in the spinal ganglia.

What do Muscle Spindles Respond to?

Muscle spindles are sensitive to changes in velocity and are innervated by type 1a nerve fibres. These afferent nerve fibres conducting the impulse directly to the spinal cord, where they are immediately conducted via interneurons to alpha motor neurons, which stimulate muscle contraction.

How Long is a Muscle Spindle?

Muscle spindles are composed of very fine muscle fibres of lengths varying between 4 and 10 millimetres. The intrafusal fibres composing the muscle spindle are so small that they contribute little to the force of contraction.

How do Muscle Spindles Help Maintain Posture?

It is a source of muscle tone as it provides a resistance to stretch and is involved in the stretch reflex.

The stretch reflex allows for protection from painful or damaging stimuli. The static component of the spindles is involved in maintaining upright posture.

What are the Three Types of Proprioceptors

Most vertebrates possess three basic types of proprioceptors: muscle spindles, which are embedded in skeletal muscles, Golgi tendon organs, which lie at the interface of muscles and tendons, and joint receptors, which are low-threshold mechanoreceptors embedded in joint capsules.

Understanding Muscle Spindles

Now you have learnt more about muscle spindles, educate yourself about the TFL, digastric and oblique muscles.