How do muscle spindles provide information about muscle length?

Muscle spindles provide information about muscle length by detecting changes in muscle stretch and sending signals to the brain.

Muscle spindles, also known as stretch receptors, are specialised sensory receptors located within the muscle fibres. They are composed of intrafusal fibres, which are encapsulated in a sheath of connective tissue. These intrafusal fibres are innervated by sensory neurons, which are responsible for transmitting information about muscle length to the central nervous system.

When a muscle is at rest, the muscle spindles maintain a baseline level of activity, known as the resting discharge rate. This provides the brain with constant information about the muscle's length. When the muscle is stretched, the muscle spindles are also stretched. This causes the intrafusal fibres to deform, which in turn stimulates the sensory neurons. The sensory neurons then increase their firing rate, sending a barrage of signals to the brain. This increase in firing rate is proportional to the degree of stretch, allowing the brain to determine how much the muscle has lengthened.

Conversely, when the muscle contracts and shortens, the muscle spindles also shorten. This reduces the deformation of the intrafusal fibres, causing the sensory neurons to decrease their firing rate. This decrease in firing rate signals to the brain that the muscle has shortened.

In addition to providing information about muscle length, muscle spindles also play a crucial role in the stretch reflex. When a muscle is stretched rapidly, the muscle spindles detect this sudden change and send signals to the spinal cord. This triggers a reflex contraction of the muscle, helping to prevent overstretching and potential injury.

In summary, muscle spindles provide information about muscle length by detecting changes in muscle stretch and transmitting this information to the brain. This allows the brain to monitor and control muscle length, ensuring that movements are smooth and coordinated.