Chapter 13: Spinal Cord

Question 1

Three types of muscles referring to locations

Answer 1

Axial muscles: trunk movement, maintains posture (most medial)

Proximal muscles: shoulder, elbow, pelvis, knee movement, important for locomotion

Distal muscles: hands, feet, digits (fingers and toes) movement (most lateral)

Question 2

What are the three main roles of the spinal cord?

Answer 2

1) Carrie’s motor information from the brain to the periphery
2) Carries sensory input from periphery to the brain
3) mediates reflexes for nody

Question 3

What is the anatomy of the spinal cord?

Answer 3

• dorsal root-sensory (afferents)
• dorsal root ganglion- contains somas of sensory afferents entering the cord
• ventral root - motor (efférents)
• spinal nerves - mixed sensory and motor roots

Question 4

What’s e the three divisions of the gray matter?

Answer 4

Dorsal horn
Intermediate gray
Ventral horn

Question 5

What’s are the types of neurons you can find in gray matter?

Answer 5

Ventral horn: mainly motor neurons whose axons exit the spinal cord
Dorsal horn: mainly neurons responding to sensory inputs
Intermediate gray: sensory neurons, motor neurons and interneurons

Question 6

Define a motor unit

Answer 6

One alpha motor neuron and all the muscle fibers it innervates

Question 7

What is a motor neuron pool?

Answer 7

The collection of alpha motor neurons that innervates a single muscle

Question 8

What are the two types of lower motor neurons in the spinal cord? What do they innervate and trigger?

Answer 8

1) alpha motor neurons: - innervate extrafusal muscle
- trigger the generation of force in the muscle

2) gamma motor neurons: - innervate intrafusal muscle fibers of the muscle spindles

Question 9

LMN vs UMN

Answer 9

• lower motor neuron (LMN) - provide direct input into muscle contraction, split into Alpha and Gamma motor neurons, cell body in ventral horn, death leads to ALS (paralysis)
• upper motor neuron (UMN) - provide input into LMN, cell body in cortex, death leads to spasticity

Question 10

What does the descending spinal pathway command?

Answer 10

Modulates spinal reflexes or directly drive motor output

Question 11

What can you say about the lateral pathways?

Answer 11

• voluntary movement of distal musculature
• under direct cortical control
• innervate distal musculature

Question 12

What are the lateral pathways?

Answer 12

Corticospinal tract

Rubrospinal tract

Question 13

What can you say about ventromedial pathways?

Answer 13

• control of posture, locomotion, orienting and balance
• under brainstem control
• innervate axial and proximal musculature

Question 14

Why are the ventromedial pathways?

Answer 14

• tectospinal tract
• vestibulospinal tract
• pontine reticulospinal tract
• medullary reticulospinal tract

Question 15

What’s the the two mechanisms for the force of muscle contraction by lower motor neurons? Explain each one

Answer 15

1. By varying firing rate of motor units
   ——-> 1 AP in the alpha motor neuron causes an end plate potential (epp) strong enough to generate one postsynaptic AP in the muscle. 1 AP in the muscle causes a twitch. Increasing the AP frequency causes twitch summation and eventually sustained contraction
2. Recruitment of synergistic motor units: large motor units (fast twitch) generate more force but tire easily
   Small motor units(slow twitch) generate less force for endurance

Question 16

What are the three sources of input for alpha motor neurons?

Answer 16

• sensory input from muscle spindles
• input from upper motor neurons in the brain
• input from spinal inter neurons

Question 17

What do muscle spindles detect?

Answer 17

Changes in muscle length

Question 18

Where is the muscle spindle located?

Answer 18

In parallel with extrafusal muscle fibers

Question 19

What wraps around intrafusal muscle fibers? What do they do?

Answer 19

1a afferents inform CNS about muscle length

Question 20

What composes a muscle spindle?

Answer 20

1a afferents

Intrafusal muscle fibers

Question 21

Which motor neurons innervate intrafusal muscle fibers?

Answer 21

Gamma motor neurons

Question 22

What do gamma motor neurons do?

Answer 22

They adjust the sensitivity of the muscle spindle, reflex length when muscle contracts so it is not limp.

Question 23

What does stretching the muscle entail?

Answer 23

It stretches the 1a nerve, activating alpha motor neurons

Question 24

What happens when extrafusal muscle fibers contract?

Answer 24

Activity in 1a axon stops- spindle becomes limp and no longer sensitive to stretch, until gamma motor neuron resets length of spindle

Question 25

What resets spindle length?

Answer 25

Gamma motor neurons activity contracts intrafusal fibers

Question 26

Which afferents are linked with muscle tension?

Answer 26

1b afferents in Golgi tendons

Question 27

Explain reciprocal inhibition

Answer 27

The contraction of one set of muscles accompanied by the relaxation of their antagonist muscles

Question 28

Explain the process of t reciprocal inhibition

Answer 28

1a axon projects from muscle spindle to either an inhibitory inter neuron then the alpha motor neuron or directly the alpha motor neuron. In the case Of only the alpha motor neuron, there is contraction of bicep extrafusal fibers. In the case of the triceps, which is the antagonist muscle, there will be a “reciprocal inhibition” thanks to the effect of the inhibitory interneuron

Question 29

Explain ipsilateral and contralateral flexor crossed-extensor reflex

Answer 29

Ipsilateral: excite flexors and inhibit extensors (bending leg that is hurt)

Contralateral: inhibit flexors, excite extensors (extending leg so you don’t fall over)

Question 30

Differences between Muscle Spindles and Golgi Tendons

Answer 30

Muscle Spindles: send info about stretch (change in length), in intrafusal fibers, 1a afferent axons, in parallel, fires more in passive stretch, no activity when actively contracted

Golgi tendons: Muscle Spindles: send info about tension (change in force), 1b afferent axons, in series, fires less in passive stretch, massive firing activity when actively contracted

Question 31

2 Descending Spinal Tracts and their functions

Answer 31

1. Lateral Pathways: voluntary movement of distal muscles, under cortical control
2. Ventromedial Pathways: posture, locomotion, under brain stem control

Question 32

Explain Corticospinal Tract

Answer 32

Carrying motor information
Decussate in caudal medulla
Longest axon in CNS
Damage leads to impaired Babinski reflex

Question 33

Rubrospinal tract

Answer 33

Originates in red nucleus of mid brain
Decussate in pons
Function is reduced in humans, coricospical tract takes over