Lecture 10 - Muscle Spindles

Question 1

Proprioception and Kinaesthesia

Answer 1

Proprioception is your brains understanding of where your body is in space. What position you are in

Kinaesthesia is your brains understanding of movement. Proprioception but while moving

Question 2

When to use surface vs indwelling EMG

Answer 2

Surface EMG for largest surface level muscles. Surface EMG doesn’t allow for us to measure MU because of too much noise.

Indwelling EMG is for smaller and deeper muscles get more accurate motor unit readings. Less input because of smaller mu and axon

How to read MU: when the graph picks up in comparison, later it starts the bigger it is. Surface EMG and torque also help tell. Later the torque lines up the later

Question 3

Muscle spindle anatomy

Answer 3

Muscle spindles are specialized sensory receptors that provide feedback on muscle stretching

In skeletal muscle anf extra fusal

In between skeletal muscle fibers in a interfusal muscle fibre with a cross section. Lots of space because muscle contracts it becomes thicker

Intrafusal muscle fibers run in parallel with skeletal muscle fibers

Question 4

Intrafusal fibers

Answer 4

NON force producing muscle fibers then lengthen and stretch muscle length (inside)

Question 5

Afferent axons

Answer 5

Sensory ending of muscle spinal. Sends sensory info to CNS through spinal cord from muscle

Question 6

Efferent axons

Answer 6

Effect movement. Coming from brain telling muscle to contract. Gamma innervate polar ends and sends info from spinal cord

Question 7

Gamma motor neuron endings

Answer 7

Stimulate intrafusal muscle fibers. Contraction only here

Question 8

Extrafusal muscle fibers

Answer 8

Fibers that cause muscle contraction, parallel with intrafusal fibers.

Question 9

Capsule

Answer 9

connective tissue
surrounding intrafusal
fibers and sensory
receptors that compose
the spindle

Question 10

Sensory endings

Answer 10

Sense length of intrafusal muscle fibers

Central/Non-contractile Region: of muscle spindle = middle

Question 11

MOST IMPORTANT

Primary vs secondary afferents

Answer 11

Primary afferents (Group Ia): wrap around all types of intrafusal fibres, bag and chain type. A primary afferent and the intrafusal fibres it wraps around is called a MUSCLE SPINDLE PRIMARY
ENDING

Secondary afferents (Group II) make ‘flower spray endings’ onto ONLY chain type intrafusal fibres. A secondary
afferent and the intrafusal fibres
it contacts is called a MUSCLE
SPINDLE SECONDARY ENDING

Question 12

MOST IMPORTANT

Bag type vs Chain type

Answer 12

Nuclear Bag Intrafusal Fibres – nuclei arranged in a central ‘bag’ region. Floating and wider

Nuclear Chain Intrafusal Fibres – nuclei arranged along a straight ‘chain. In a row/line

Question 13

Mechanically gated ion channels

Answer 13

Mechanically gated ion channels: Mechainical energy into AP. Axon type that wraps around muscle fiber.

Cytoskeletal Strands:
Binds MGIC together. opens channels when stretched

Patch Clamp:
By suctioning onto the sensory neuron,
we can even record the Na+ currents when stretch is applied to the membrane

Question 14

Receptor potential

Answer 14

Voltage in the sensory neuron to different amounts of muscle stretch. Larger stretches lead to larger receptor
potentials.

Larger stretch= larger receptor= larger depolarization

Question 15

Sensory coding of muscle stretch

Answer 15

More stretched = more AP. Rastor clot measuring system.

When muscle is stretched it is in dynamic phase, this is velocity and bag type. Chain type is overall length

The firing rate (AP) of muscle spindles corresponds to changes in length of the muscle

Difference in the firing rate between the initial and final length of the muscle

During the dynamic phase of muscle stretch – this provides the CNS with a sensory input proportional to the velocity of muscle stretch

Question 16

Muscle Stretch Velocity Coding by Primary Muscle Spindles

Answer 16

Primary endings have higher dynamic index values than secondary endings

Stretch responses of primary and secondary endings have different qualities. Primary endings are more sensitive to the dynamic phase of movement – their firing rates during
this dynamic phase closely correspond to the velocity of muscle stretching.

Higher velocities result in LARGER DYNAMIC RESPONSES

Bag type or Kinaesthesia

Question 17

Muscle Length Coding by Secondary Muscle Spindles

Answer 17

Secondary endings DO NOT respond as much as primary endings during the dynamic phase of muscle stretching

Stretch responses of primary and secondary endings have different qualities. Secondary endings are more sensitive to muscle length – they are not sensitive to muscle stretch velocity, however, their firing rates more closely correspond to muscle length than primary endings

Chain type

Question 18

Coding of Muscle Length AND Velocity

Answer 18

Comparing the responses of primary (Ia) and secondary (II) muscle spindles to a series of muscle stretches, we can see primary muscle spindle firing rate is more related to the velocity of
stretch (BLUE is where velocity is HIGH), whereas secondary muscle spindle firing is related to length of the muscle (RED is where the offset from initial muscle length – “static offset” – is HIGH)

Question 19

Role of Gamma

Answer 19

Without: if the muscle were to contract to a shorter length, the intrafusal
fibres would become slack (floppy)

With: when Gamma are stimulated at the same time as the alpha. neurons, the intrafusal fibres contract along with
the extrafusal (skeletal) muscle fibres