NEUR 2026: Motor Neuron Circuits

What skeletal muscle does the alpha-motor neuron connect to?

extrafusal muscle, to cause contraction

What skeletal muscle does the gamma-motor neuron connect to?

intrafusal muscle (nuclear bag and chain fiber)

What are in the extrafusal muscles?

myofilaments (actin + myosin), that shorten the muscle

What fiber comes off the nuclear bag fiber?

Ia fiber

What endings are on the nuclear bag fibers?

annulospiral endings

What fibers come off the nuclear chain fibers?

Ia and II fibers

What endings come from the Ib fibers in the nuclear chain?

flower-spray ending

When the extrafusal muscle that contains muscle spindles (intramural muscles) contracts, what happens to the annulospinal endings?

they have less contraction and more Piezo2 channels are pulled open

When the extrafusal muscle that contains muscle spindles (intramural muscles) relaxes, what happens to the annulospinal endings?

they have more contraction and less Piezo2 channels open

What is the alpha-gamma coavtivation for the biceps brachii when the arm angle is at 180º?

low alpha, low gamma

What is the alpha-gamma coactivation for the triceps brachii when the arm angle is at 180º?

high alpha, high gamma

What is the frequency level of the Ia fiber coming from the biceps brachii at 180º?

high frequency

What are the spikes per second of the Ia fiber coming from the biceps brachii at 180º?

275 s/s

What is the frequency level of the Ia fiber coming from the triceps brachii at 180º?

low frequency

What are the spikes per second of the Ia fiber coming from the triceps brachii at 180º?

2 s/s

What is the alpha-gamma coavtivation for the biceps brachii when the arm angle is at 100º?

normal alpha, normal gamma

What is the alpha-gamma coavtivation for the triceps brachii when the arm angle is at 100º?

normal alpha, normal gamma

What is the frequency level of the Ia fiber coming from the biceps brachii at 100º?

normal frequency

What are the spikes per second of the Ia fiber coming from the biceps brachii at 100º?

85 s/s

What is the frequency level of the Ia fiber coming from the triceps brachii at 100º?

normal frequency

What are the spikes per second of the Ia fiber coming from the triceps brachii at 100º?

85 s/s

What is the alpha-gamma coavtivation for the biceps brachii when the arm angle is at 30º?

high alpha, high gamma

What is the alpha-gamma coavtivation for the triceps brachii when the arm angle is at 30º?

low alpha, low gamma

What is the frequency level of the Ia fiber coming from the biceps brachii at 30º?

low frequency

What are the spikes per second of the Ia fiber coming from the biceps brachii at 30º?

2 s/s

What is the frequency level of the Ia fiber coming from the triceps brachii at 100º?

normal frequency

What are the spikes per second of the Ia fiber coming from the triceps brachii at 30º?

275 s/s

collagen + sensory endings = ?

golgi tendon organ

What does the golgi tendon organ reflex do?

prevents tendon / musculotendinous rupture

What is the knee-jerk reflex triggerd by?

activation of the muscle spindle (including the nuclear bag and nuclear chain fibers)

Where do the muscle spindles send signals?

the somatosensory cortex and the cerebellum

Where do alpha-motor neurons innervate on?

the extrafusal muscle fibers (responsible for muscle contraction)

What is the crossed extension reflex?

the limbs perform opposite motions, allowing one to withdraw while the other extends to main balance

(T/F) The muscle spindle triggers the crossed extension reflex

false

What triggers the crossed extension reflex?

touch

What is a motor unit?

one motor neuron and all the muscle fibers it innervates

The interneuron frequency controls…

muscle tension