Motor functions of the spinal cord

Question 1

Definition of a motor unit

Answer 1

A single motor neuron and the muscle fiber that it innervates.
Can innervate a few fibers or thousands, depending on how precise the movement has to be

Question 2

Motor neuron types

Answer 2

α motor neuron: innervates extrafusal fibers. Generate force

ɣ motor neuron: Innervates intrafusal fibers = muscle spindle.

Question 3

Function of a muscle spindle

Answer 3

Correct for any change in muscle fiber length after a contraction or a stretch via stimulation of α motor neuron to oppose change

Question 4

Muscle spindle shape/type

Answer 4

Nuclear bag: nucleus in a central region

Nuclear chain: nucleus in a row

Question 5

Sensory innervation of nuclear bag

Answer 5

Ia sensory afferent (fast)

Question 6

Sensory innervation of nuclear chain

Answer 6

Ia and II (slower)

Question 7

The function of Ia sensory fiber?

Answer 7

Detect velocity of the fiber’s length change

Question 8

The function of II sensory fiber?

Answer 8

Detect the length of the fiber

Question 9

ɣ motor neurons innervation on muscle spindle/intrafusal fibers?

Answer 9

Nuclear chain: static ɣ motor neurons

Nuclear bag: dynamic ɣ motor neurons

Question 10

Different types of interneurons

Answer 10

1. Ia - inhibitors of antagonist muscles depending on the situation
2. Renshaw - modulate the strength and firing rate of the α motor neuron
3. Ib - the interneuron of the Golgi tendon organ reflex
4. Excitatory - used in the flexor-withdrawal reflex

Question 11

Stretch reflex = patella reflex arch = myotatic reflex

Answer 11

Monosynaptic reflex
Triggered by stretching a muscle and finishes by contraction the muscle
Stretch - intrafusal fiber activated - synapses directly on the α motor neuron in ventral horn - cause contraction of the same muscle - antagonist muscles are inhibited -

Question 12

Golgi tendon reflex = inverse myotatic reflex

Answer 12

1 inhibiting neuron
Is triggered by muscle contraction and finishes by muscle relaxation
Golgi tendon organ - located in the muscle tendon
Arranged perpendicularly to the intrafusal fibers
Sense contraction of the muscle - Ib afferent - synapse on an inhibitory interneuron Ib - inhibit the α motor neuron on the same muscle - activate antagonist muscles

Question 13

Flexor - withdrawal reflex

Answer 13

Polysynaptic reflex
Stimulated by pain or temperature
Triggers ipsilateral muscle flexion and contralateral muscle extension
With a pain sensation, afferent fibers are activated and synapse on multiple interneurons that cause the muscles near the pain source to flex thus pulling
away while the opposite side of the body is stabilized by activation of the extensors (and inhibition of flexors).