Spinal Reflexes

Question 1

Where are the cell bodies of the motoneurons innervating limb muscles located?

Answer 1

In the ventral horns of the spinal cord.

Question 2

What is a motor pool? Where are they found?

Answer 2

The collection of the cell bodies of motoneurons innervating a single muscle - they are clustered together in a cross-sectional region of the ventral horn.

Question 3

Can a muscle fiber be innervated by many motoneurons?

Answer 3

Not skeletal. Multiunit smooth muscle can be.

Question 4

Can a single motoneuron innervate many muscle fibers?

Answer 4

Yeah

Question 5

What is a motor unit?

Answer 5

One motoneuron + all the muscle fibers it innervates.

Question 6

What is an isometric contraction?

Answer 6

When no joint movement occurs (like when holding a book up to read it).

Question 7

What is an isotonic contraction?

Answer 7

When the muscle shortens but does so with a uniform tension.

Question 8

In a dynamic contraction, ________ _______ is performed.

Answer 8

external work

Question 9

A single AP in a motoneuron gives rise to a single ______ of the muscle fiber.

Answer 9

twitch

Question 10

During a contractile event in which increasing force is needed, what is the order of muscle fiber types that are recruited? This leads to a ______ increase in muscle tension as the number of active motor units increases.

Answer 10

Type 1 (red, slow) first, then Type II (white, fast). Leads to a nonlinear increase in tension.

Question 11

What two receptors are classified as Group I receptors?

Answer 11

Primary spindle and Golgi tendon organ

Question 12

What receptor is a Group II receptor?

Answer 12

Secondary spindle

Question 13

What types of receptors are Groups III and IV receptors?

Answer 13

Chemoreceptors and nociceptors

Question 14

Which group of receptors has the largest diameter and fastest conduction velocity?

Answer 14

Group I

Question 15

What do muscle spindles monitor?

Answer 15

Muscle length

Question 16

Intrafusal muscle fibers are arranged in _______ with the extrafusal muscle fibers.

Answer 16

parallel

Question 17

Intrafusal muscle fibers are innervated by ______ motoneurons.

Answer 17

gamma

Question 18

In which Group of afferent receptors does the muscle spindle receptor belong?

Answer 18

Ia

Question 19

Ia afferents (muscle spindle) are more responsive to ______ in muscle length rather than overall muscle length.

Answer 19

changes

Question 20

Why are gamma motoneurons activated concomitantly with alpha motoneurons that innervate extrafusal muscle fibers?

Answer 20

When alpha motoneurons activate, the muscle contracts and shortens. To keep the muscle spindle stretched (and maintain its sensitivity), the gamma motoneurons activate the intrafusal muscles flanking the muscle spindle to maintain a degree of muscle spindle stretch and sensitivity.

Question 21

What three connections do Ia afferents make within the spinal column?

Answer 21

1. Homonymous
2. Synergistic
3. Antagonistic

Question 22

In a homonymous connection, the afferent fiber synapses directly with an ________ _________ that projects back to the SAME MUSCLE.

Answer 22

excitatory motoneuron

Question 23

In a synergistic connection, the afferent fiber synapses directly with an excitatory motoneuron that innervates a ________ _______.

Answer 23

synergistic muscle

Question 24

In a antagonistic connection, the afferent fiber synapses directly with an _________ that then releases the neurotransmitter _________ in a synapse with a motoneuron of an ___________ muscle.

Answer 24

afferent synapses with an interneuron that releases GABA in a synapse with a motoneuron of an antagonistic muscle.

Question 25

What does the H wave represent in EMG testing?

Answer 25

The muscle twitch corresponding to the signal that traveled up the afferent into the spine, then back to the muscle via the motoneuron that the afferent synapsed with.

Question 26

What does the M wave represent in EMG testing?

Answer 26

If the external stimulus is high enough, the motoneuron is directly activated –> muscle twitch, and this represents the M wave.

Question 27

When both a M wave and H wave is present, which one is seen first?

Answer 27

The M wave

Question 28

What accounts for a smaller H wave upon increasing the external stimulus strength during an EMG test?

Answer 28

A strong stimulus can send a retrograde action potential in the motoneuron which interferes with the efferent signal, creating a smaller H wave.