KIN 313 Lecture 7 - Muscle Spindle 1: Afferent/Efferent System

25 September 2025

Relationship between spindle density and muscle location

distal muscles = higher density

proximal muscles = lower density

what is muscle spindle density

how many spindles per muscle mass

spindles are split into what regions

polar, equatorial, polar

equatorial is the middle where the nuclei are

polar includes the contractile tissue

how long are muscle spindles

about 1cm or less

muscle spindle capsule is made of

connective tissue

spindles are made up of

capsule (connective tissue)

intrafusal fibres

• bags

• chains

bag vs chain fibres

bags are longer and nuclei are all clumped together

chains are shorter and the nuclei are lined up neatly in a row

all have contractile tissue at the end

Primary afferent a (Ia) goes to…

the muscle spindle

pierces the capsule then wraps around the center of every intrafusal fibre in the spindle (where the nuclei are)

secondary (II) afferent goes to…

muscle spindle

connects to all intrafusal fibres except Bag1

connects off center (not over the nuclei)

can wrap around like Ia does or connect in a flower spray

motor axons go to ___ part of spindle and sensory axons go to ___ part of spindle

sensory axons go to center of spindles

motor axons go to ends of spindles on the contractile tissue

primary afferent b (Ib) goes…

from GTO to CNS

bag 1 = ___ bag

bag 2 = ____ bag

bag 1 = dynamic bag

bag 2 = static bag

dynamic bag vs static bag

dynamic bag is shorter than static bag

dynamic bag recieves input from gamma dynamic motor neuron, and codes really well for velocity. sends info via Ia afferent

static bag receives input grom gamma static motor neuron and codes really well for length. sends info via Ia afferent and II afferent

how many intrafusal fibres per spindle

how many of these are bags

how many of these are chains

2-20 intrafusal fibres per spindle, usually a minimum of 4 or 5

2-3 are usually bag fibres

the rest (about 3 or more) are chain fibres

bag fibres or chain fibres are longer

bag fibres are longer than chain fibres

what happens to the spindles when you stretch  a muscle

the spindles also strech and respond in some way

what is the efferent motor system specific to muscle spindles

fusimotor or gamma (y) system

skeletal muscle motor system is also called

alpha motor system

motor neurons that make up the gamma system

dynamic and static gamma motor neurons

y Dynamic motor neuron vs y Static motor neuron

y dynamic only goes to bag 1 and mostly codes for velocity. tuning y dynamic results in a change in info being received by the Ia afferent

y static goes to bag 2 and all the chain fibres, mostly coding for length. tuning y static results in a change of info being received by Ia AND II afferents

how to gamma motor neurons get to the intrafusal fibres?

they pierce the capsule and biforcate to go to either end of the spindle/intrafusal fibres and connect to the contractile tissue

what kind of receptor is the msucle spindle

mechanoreceptor

how to spindles cause the msucle to contract?

the spindles are stretched,

the stretch-sensitive channels in the sensory nerves connecting to the spindle (primary a and secondary) are pulled open

channels leak ions proportional to the degree of stretch

these bring membrane potential up to threshold

once it reaches threshold, the channels open even more and flood, causing an AP

the axon becomes myelinated as it exits the capsule, sending the AP to the CNS

monosynaptic connection to Motor Neuron Pool, giving them EPSPs and having them fire APs back to the muscle to have it contract as a result of the spindle being stretched

Ia afferents are sensitive to 

velocity of stretch and some sensitivity to length of stretch

II afferents are sensitive to

length change with some sensitivity to velocity

the fact that secondary afferent is not very good at velocity coding implies that…

implies that Bag 1 plays a large role in velocity coding because that is the only intrafusal fiber that the secondary afferent does not go to

what happens to 1a discharge rate during unloading

stops leaking/firing during unloading

what happens to secondary afferent discharge rate during unloading

depends on the speed of the unloading

• quick stretch quick release = stops firing during shortening/unloading

• small and fast oscillations = doesnt pick up on the change, codes for average or initial length

• extended release = slowed firing rate