Lecture 9 - Spinal Cord, Brainstem, and Cortex Control of Motor Function

Question 1

Differentiate between the different types of motor neurons in the CNS.

L9 S7

Answer 1

Upper motor neuron:

• descends spinal cord to synapse with lower motor neuron or interneuron
• uses glutamate

Lower motor neuron:

• typically originates in anterior horn and terminates at effector (muscle)
• uses acetylcholine

Question 2

What is a motor unit and what are the different classifications of neurons or groups of neurons related to motor units?

L9 S8

Answer 2

Motor unit:
-a single motor neuron and the muscle fibers it innervates

Motor neuron pool:
-group of motor neurons that innervate fibers within the same muscle

Small motor neurons:

• innervate few muscle fibers
• low threshold; fires first
• smaller force

Large motor neuron:

• innervate many muscle fibers
• high threshold; fires last
• larger force

Question 3

What is recruitment with regards to motor units?

L9 S8

Answer 3

Control of the tension of a muscle fiber by controlling the number of associated motor units that are activated.

Question 4

What are the characteristics of motor interneurons?

L9 S12

Answer 4

• 30x more abundant than anterior motor neurons
• bipolar cell shape
• capable of spontaneous activity
• responsible for integrative function
• primarily inhibitory (GABA) but can be excitatory (glutamate)

Question 5

What are the characteristics of Renshaw cells?

L9 S13

Answer 5

• inhibitory (glycine) resulting in lateral inhibition

- receive collateral branches of at least one motor neuron and can transmit to several motor neurons

Question 6

Differentiate between he types of inhibitory and excitatory motor interneurons.

L9 S14-16

Answer 6

Ia inhibitory:

• inhibits antagonistic muscle
• signal from muscle spindle

Ib inhibitory:

• autogenic inhibition reflex stimulated by Golgi tendon organ; prevents muscles from tearing
• connects with Ib afferent neuron to inhibit same muscle

Excitatory:

• involved in spinal reflex to remove body from undesirable stimulus
• sends ipsilateral and contralateral signals in response to undesirable stimulus
• receives signals from sensory group III afferent neurons

Question 7

What are the different types of sensory neurons?

L9 S17

Answer 7

Muscle spindle (group Ia and II afferents):

• parallel with extrafusal fibers
• detect dynamic and static changes in muscle length

Golgi tendon organs (group Ib afferents):

• in series with extrafusal fibers
• detect muscle tension (from tendons)

Pacinian corpuscles (group II afferents):
-detect vibration

Free nerve endings (group III and IV afferents):
-detect noxious stimuli (e.g. pain)

Question 8

What types of motor and sensory fibers are myelinated and which aren’t?

L9 S19

Answer 8

Myelinated:

• motor (A; B lightly myelinated)
• sensory (I, II, and III)

Unmyelinated:

• motor (C)
• sensory (IV)

Question 9

Describe the muscle spindle.

L9 S22-23

Answer 9

• encapsulated group of 3-12 intrafusal fibers innervated by gamma motor neurons (group II afferent)
• has no contractile fibers in central region
• detects changes in muscle length
• associated with fine muscle movements (increased spindles for finer movement)

Question 10

Differentiate between he different types of intrafusal fibers.

L9 S23

Answer 10

Nuclear bag fibers:

• detect rate of change in muscle length
• innervated by group Ia afferents and dynamic γ efferents
• multiple nuclei in “bag-like” configuration

Nuclear chain fibers:

• detect static charge in muscle length
• innervated by group II afferents and static γ efferents
• multiple nuclei in a row

Question 11

Describe the stretch reflex.

L9 S33

Answer 11

• muscle spindle is stimulated by stretch and excites group Ia afferents
• group Ia afferents excite α motor neuron associated with stretched muscle
• stretched muscle contracts, reducing tension
• synergistic muscles are also excited while antagonistic muscles are inhibited

Question 12

Describe the Golgi tendon organ reflex.

L9 S35

Answer 12

• Golgi tendon organ is stimulated by tension and excites type Ib afferents
• type Ib afferents excite inhibitory interneurons
• inhibitory interneurons inhibit anterior motor neurons associated with tense muscle
• tensed muscle relaxes

Question 13

What are the different classifications of upper motor neurons?

L9 S48

Answer 13

Medial activation system:
-innervate postural and girdle muscles

Lateral activation system:
-innervate muscles used for fine movements in the limbs

Nonspecific activating system:
-create local reflex arcs