Muscle Spindles

What are muscle spindles?

Specialized sensory receptors that provide feedback on muscle stretching to the CNS
- stretch detectors: sense how much & how fast a muscle is lengthened or shortened

Intrafusal Fibres vs. Extrafusal Fibres?

Intrafusal Fibres:
- non-force producing muscle fibres that lengthen & stretch with changes in muscle length
- inside the membrane, deep inside muscle fibres
- only contract at the ends & not in middle because the sensory axons are there
- if contracted in middle the axons are compressed, no AP, & they go limp

Extrafusal Fibres:
- fibres that cause contractions
- are parallel with intrafusal fibres
- outside the membrane

Afferent vs. Efferent Axons?

Afferent Axons:
- innervate sensory endings of a muscle spindle & send feedback to the spinal cord
- carry info from sensory receptors to CNS

Efferent Axons:
- of gamma motor neurons that innervate polar ends (contractile) of intrafusal fibres & send excitatory inputs from spinal cords
- carry motor info away from CNS to muscles

Muscle Spindle Anatomy

Gamma Motor Neurons:
- stimulate intrafusal muscle fibres

Sensory Endings:
- sense length of intrafusal muscle fibres

Central/Non-contractile Region:
- of the muscle spindle

Capsule:
- connective tissue surrounding intrafusal fibres & sensory receptors that compose the spindle

Primary vs Secondary Afferents?

Primary (group Ia):
- wrap around all types of intrafusal fibres ("annulospiral endings") but most commonly the nuclear bag type
- these are the curly ones that wrap around intrafusal fibres

Secondary (group II):
- make 'flower spray endings' onto ONLY nuclear chain type intrafusal fibres

Nuclear Bag vs Nuclear Chain?

Nuclear Bag Intrafusal Fibres:
- nuclei arranged in a central 'bag' region

Nuclear Chain Intrafusal Fibres:
- nuclei arranged along a straight 'chain'

What is primary & secondary muscle spindle ending?

Muscle Spindle Primary Ending:
- a primary afferent & the intrafusal fibres it wraps around

Muscle Spindle Secondary Ending:
- a secondary afferent & the intrafusal fibres it contacts with

Mechanically Gated Ion Channels & muscle stretch?

These channels are connected by cytoskeletal strands that pull on them to open channels up so sodium can flow in
- the patch clamp recrod Na+ cuurent when stretch is applied to membrane

- more stretch = more ion flow = larger receptor potential
- traces that are low are from Na+ entering sensory endings (-ve current) with differnt amount of stretch applied to cell membrane (Hg = pressure)

What is the relation of firing rate of muscle spindles with changes in length of muscle?

1) Difference in firing rate of initial & final length of muscle
- provides CNS with a sensory input proportional to length of muscle

2) difference in firing rate of muscle spindles during dynamic phase of muscle stretch
- provides CNS with a sensory input proportional to velocity of muscle

- chain type fibre experiences slower stretch whereas beg fibre experiences it quickly (steep slope)

What is the Dynamic Phase?

The phase where the lines are closer together
- the dynamic phase shortens when the velocity increases
- ^ this happens in bag type fibre

Dynamic Phase & the different endings?

1) Primary endings are more sensitive to the dynamic phase of movement than secondary endings
- primary firing rates during dynamic phase closely correspond to velocity of muscle stretching

2) Secondary endings are more sensitive to muscle length not muscle stretch velocity
- their firing rates more closely correspond to muscle length than primary endings

The x-axis is the time & higher density of line means more action potential

In a slow stretch the dynamic index value is lower but it is lower for secondary compared to primary

What is human microneurography?

Technique of inserting an electrode directly into a peripheral nerve to record AP from sensory neurons & motor neurons
- a primary ending with high velocity results in a larger dynamic reponse

Muscle Length & Velocity?

- if high concentration in early stretch period it is primary (1a)
- more crowded before release of stretch
- relates to velocity

- if high concentration when already stretched it is chain or secondary (II)
- more crowded during release of stretch
- relates to length

- releasing stretch faster = less AP

The Fusimotor system - what is the role of gamma motor neurons?

With GMN's:
- when they are stimulated at same time as alpha MN, the intrafusal fibres contract along with extradusal (skeletal) muscle fibres
- this allows the muscle spindles to keep responding during contractions

Without GMN's:
- if muscle were to contract to a shorter length, the intrafusal fibres would become floppy & the spindle would stop providing feedback about where your limbs are

What is Alpha-Gamma Co-Activation?

Co-activation occurs because the same motor commands coming the cerebral cortex connects both to AMNs & GMNs in spinal cord
- thus they contract together
- this is to increase sensory information

What is proprioception & kinesthesia?

Muscle spindles are the most important source of sensory information used for proprioception & kinesthesia

Proprioception:
- body's ability to perceive the limb position (static)

Kinesthesia:
- body's ability to sense limb movement (dynamic)

Vibration & proproception & kinesthesia?

Vibration-evoked illusions of limb movement demonstrate that muscle spindles play a critical role in limb position & perception
- the vibrator probe pushes into tendon which pulls on muscle & activates its muscle spindles
- this tricks our brain into thinking our muscle is being stretched