The Muscle Spindle

Why do we need to know about the muscle spindle?

It is so important for us because it provides the CNS a tremendous amount of information. It helps to regulate muscle contraction and ensure efficient movements. It helps to stabilise joints during contraction. It maintains the stretch reflex, which is so integral to any clinician’s complete physical examination. It’s like the millions of operators inside the mission control room at NASA who are constantly updated on the condition of the rocket and in turn modify and regulate its functions- all in real time.

A muscle spindle is a proprioceptor- a sensory receptor involved in sensing body movement and position. It is located within the intrafusal fibres of the skeletal muscle. Before we go into detail regarding the structure and function of the muscle spindle, it’s important to know that the intrafusal muscle fibres do not really contain as much actin and myosin as the surrounding extrafusal fibres. In effect, their main function is sensory, rather than in contraction and relaxation as part of motor function. 

Intrafusal fibres lie within the extrafusal fibres and are connected to the latter's glycocalyx

Each muscle spindle is 3-10 mm in length. The muscle spindle itself consists of two type of fibres- bag fibres and chain fibres. Bag fibres are present in the centre of the muscle fibre and chain fibres are present in the either ends of the muscle fibre. Something like this:

When a muscle is stretched, the stretch is applied to the extrafusal as well as the intrafusal fibres. The stretch activates the proprioreceptors within the intrafusal fibres (i.e. the muscle spindle) and afferent signals are sent to the spinal cord via the appropriate nerves.

Type of Nerve Ending	Type of Nerve	Diameter (in μm)	Velocity of Impulse	Bag	Chain
Primary	Ia	17	70 -120 m/s	Yes	Yes
Secondary	II	8			Yes

- (annulospiral nerve endings)

There are two types of responses from stretching the muscle- dynamic and static. As we know from the definitions of the words themselves, dynamic would have to indicate that impulses are being transmitted while there is movement. In other words, during the fraction of a secondthat the length of the intrafusal fibres change during a muscle stretch, an impulse is sent to the spinal cord signalling, “Look, there’s a change in the length of the muscle!” The muscle spindle bag fibres are responsible for this dynamic response. The static response detects a continuous change in the length. Due to the stretching force, the muscle’s length is now altered, and remains altered for as long as the stretch remains. This is detected by the chain fibres and sent to the spinal cord through the secondary afferent nerves. In effect, bag fibres are activated by a rapid rate of the change in length but only while the length is changing and chain fibres fire as there is a continuous change in the length. 

I’m sure you all would have studied the stretch reflex and its pathway. Receptor, afferent, spinal cord, efferent, muscle. Essentially, for practical purposes, let’s retain this pathway. 

In addition to this, you would have read about this being a monosynaptic pathway. That means one nerve sends the impulses and another one receives them. This kind of arrangement is the reason why reflexes are so fast- there is only one path (here, it is through the Ia fibres) to follow. Polysynaptic pathways cause delays in the impulse reaching its destination, which is why it is convenient that reflexes are monosynaptic.

Alright, it would appear that we know the events leading up to the spinal cord receiving information from the muscle spindle. What happens after that?

The second part of the reflex arc is maintained by the gamma efferent system of nerves. (Please keep in mind that the extrafusal fibres are innervated by large A Alpha nerve fibres and the intrafusal fibres are innervated by gamma nerve fibres.) The end result is simple- muscle contraction. We see that muscle stretch results in muscle contraction.It is as if the muscle spindle “opposes” the sudden change in length of the muscle. 

If you’ll recall, I had mentioned two responses- a dynamic and static response. Where do you think these come into play? As soon as the muscle gets stretched, the dynamic response causes muscle contraction. What about the static response? It allows the muscle to remain in a state of contraction for a reasonable amount of time. 

This really helps the spinal cord and the motor areas of the cerebrum to ensure there are no jerky, irregular movements. As the muscle spindles are there to prevent excessive stretch and to maintain proper contraction, there is smooth motor function. The elimination of extraneous and irregular motor responses is known as “damping” or “signal averaging”.

Let’s just say there’s a plate of gulab jamuns in front of you that your mother is saving for a guest. You reach your hand out to take one but as soon as your mother sees that slight movement, she charges out of nowhere and slaps your hand back into place. Your mother is the muscle spindle.

There is one more thing I would like to mention here- a small phenomenon known as “coactivation”. All through this article we have been speaking of intrafusal muscle fibres, as though they are completely separate from extrafusal fibres. They are simply two sides of the same coin, and one cannot speak of them as different entities. Whenever signals from the motor cortex descend to the muscles, it is the A alpha nerves that cause actual visible muscle contraction. Along with this, the gamma nerve fibres are also activated. In essence, whenever there is motor activation, both the extrafusal and intrafusal fibres are activated, despite them having different innervation. What’s the point of this, you may ask.

1. Stretch of the extrafusal fibres allows the  muscle spindle to also be stretched- i.e. the muscle spindle is constantly being updated on the status of the extrafusal fibres
2. The damping function is maintained

Let’s apply these concepts to the classic stretch reflex example- the knee jerk. 

Here’s one more important function of muscle spindles - ensuring stabilisation of joints during precise and delicate movements. Certain actions require us to maintain a specific posture or to keep certain groups of muscle contracted. For example, when painting or drawing, the artist needs to hold his fingers in such a way as to not mess up the picture. A cameraman holding his camera must maintain that posture so that the scene shot is perfect. Obviously, as well know, this requires the interplay of many, many factors- the motor cortex, the bulboreticular system, etc. How the muscle spindle helps is such: 

Definitions:
Muscle stretch reflex: when a muscle is stretched suddenly, excitation of the spindles causes reflex contraction of the large skeletal muscle fibres of the stretched muscle and also closely allied synergistic muscles. 

Terms:

• Proprioceptor
• Extrafusal 
• Intrafusal
• Gamma nerves
• Alpha nerves
• Monosynpatic pathway
• Coactivation
• Damping
• Stretch reflex

Author: Shruthi Sivakumar

Sources and citations

Guyton and Hall Textbook of Medical Physiology- 13^th edition
Human Physiology - From Cells to System, 8^th edition, Lauralee Sherwood

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