The Motor System

Question 1

What constitutes a motor unit?

What does the needed degree of control determine for a motor unit? How much of something is that…more or less?

Answer 1

One motor unit.
All the muscle fibers it innervates.

The number of muscle fibers in a motor unit varies based on the needed degree of control.
The smaller the number of fibers, the greater the control.

Question 2

Describe the systematic mapping arrangement of the anterior horn of the spinal cord for motor cell bodies of lower motor neurons.

Answer 2

The axial spine is medial to distal structures.

Neuronal soma of flexor muscles are dorsal to the extensors. This occurs throughout the entire spinal cord.

Question 3

Name and describe the 3 muscle fiber types.

Answer 3

Red fibers–thin, contain ABUNDANT MITOCHONDRIA, for WEAK, SLOW but SUSTAINABLE CONTRACTIONS for LONG PERIODS OF TIME.
White fibers–larger, contain relatively few mitochondria and contract in BRIEF POWERFUL TWITCHES.
Intermediate fibers–a blend of characteristic of White and Red fibers.

Question 4

Name and describe the 3 kinds of motor units and the type of muscle fiber they innervate

Answer 4

1. Smallest alpha motor neurons innervate red muscle fiber. slow twitch, “S”
2. The largest alpha motor neurons innervate white muscle fiber. Fast twitch, Fatigues; “FF”
3. Intermediate alpha motor neurons innervate intermediate muscle fiber. Fast twitch, fatigue-Resistant; “FR”
   (most muscles have all 3 fiber types randomly intermingled within the muscle…Proportions vary)

Question 5

What does the size of the motor neuron have to do with type of fiber it innervates?

Answer 5

There is a correlation between the size of motor neuron and the type of fiber it innervates.
Small…slow
Large…Fast

Question 6

How are motor units recruited?
Which are recruited first and why?
What is the order of motor units recruited?
What is the mechanical result of this system?

Answer 6

In order of size.
Smaller are recruited first because the size allows smaller ones to hit action potentials first…Not transmission of nerve impulse but how easily they are recruited.
Smaller neurons have greater membrane potential changes for the given amount of synaptic current.
As the synaptic drive reaches the anterior horn, S units are 1st, FR are 2nd, and FF are 3rd.
This order of recruitment gives a progressively smooth increase during an increase in muscle contractile force. This is an automatic phenomenon.

Question 7

What do Hierarchical and Parallel Connections within motor control systems do?
What are their 3 classes?

Answer 7

These inputs determine the level of activity of lower motor neurons. These neurons may be divided into 3 broad overlapping classes.
1. Built-in patterns of neural connections.
2. Descending pathways that modulate the activity of the motor neurons
3. Higher centers that influence the activity of descending pathways.
(internal wiring at the spinal level. Neuronal demand effects plasticity to meet demand. i.e. children walking with cervical demand.)

Question 8

Motor Control System 1. Give 2 examples of Built-in Patterns of Neural Connections.
What are their purpose?

Answer 8

“stretch-reflex”–aka myotatic reflex, aka deep tendon reflex (2 neuron pathway) stretched muscle activates muscle spindle, whose afferents end on motor neurons that in turn result in muscle contraction.
“Flexor withdrawal reflex” step on a tack.
Networks of interneurons in the brainstem and spinal cord that can act for pattern generators for rhythmic movements such as breathing and walking.

Question 9

Motor Control System 2. What is an upper motor neuron?

Where do they originate?

Answer 9

Neurons whose axons descend to the spinal cord OR cranial nerve motor nuclei to affect the activity of lower motor neurons.
They originate from the Cerebral Cortex (corticospinal tracts) or the brainstem.

Question 10

Motor Control System 2. Generally, distinguish between injuries in upper and lower motor neurons and higher centers.

Answer 10

upper/lower motor neuron damage causes weakness and distinctive signs and symptoms.
Higher centers have distinctive movement abnormalities also (involuntary movements, incoordination, difficulty initiating movement), but is not accompanied by substantial weakness.

Question 11

Motor Control System 2. Review: What is the path of the Corticospinal Tract?

Answer 11

Fibers descend from Precentral Gyrus

1. The posterior Internal Capsule.
2. Cerebral Peduncle of the rostral midbrain. (crus cerebri)
3. Basal Pons
4. Pyramids of the Medulla.
5. At the Caudal Medulla level, 85% of fibers decussate at Pyramidal Decussation.
6. 85% descends in the Lateral Corticospinal Tract.
7. 15% descend in the uncrossed Anterior Corticospinal Tract. Most are headed to axial skeletal muscle.

Question 12

Motor Control System 2. What is the path of the Pontine Reticulospinal Tract?

Answer 12

Begins in the Reticular formation of the Pons.

It descends in the Medial Pontine Reticulospinal tract.

Question 13

Motor Control System 2. What is the path of the Medullary Reticulospinal Tract?

Answer 13

Begins in the reticular formation of the Medulla.

It descends in the lateral medullary reticulospinal tract.

Question 14

Motor Control System 2. Describe pathway and purpose of Tectospinal tract

Answer 14

Originates in the Superior Colliculi and decussates in the midbrain.
Descends in the contralateral anterior funiculus.

Reflexively oriented with vision. Goes to upper extremities so arms, head and neck can protect vision.

Question 15

Motor Control System 2. Describe pathway and purpose of Rubrospinal tract.

Answer 15

Originates in the red nucleus of the rostral midbrain.
Descussates while in the midbrain,
First descends in the lateral tegmentum of the brainstem.
Continues in the spinal cord in the contralateral lateral funiculus near the lateral corticospinal tract.
(Ruby = red Rubro = red)

Question 16

Describe the Hierarchy theory of motor system operation.

Answer 16

Association areas of the cortex decide that a movement is needed.
Premotor cortex areas devise a plan for movement and pass this information on to…
Primary Motor Cortex then issues commands for the motor neurons, either directly or indirectly with interneurons in the brainstem and spinal cord.

Question 17

Describe the Parallel Circuit theory of motor system operation.

Answer 17

pre-motor areas send messages to the spinal cord, but not to the extent of the primary motor cortex.
Basal ganglion and cerebellum are involved in aspects of planning and monitoring movements but do not have direct outputs. (These never have direct connection. Therefore their input has to go somewhere else before a movement is initiated.) Basal ganglia and cerebellum influence the output from cerebral cortex to brainstem and spinal cord while also having additional outputs to the brainstem nuclei. In addition, they receive inputs from cerebral cortex as well as other parts of basal ganglia system. Cerebellum receives a lot of sensory information than basal ganglia in terms of motor control.

Question 18

Where do collaterals of Corticospinal fibers go?4

Where do they synapse?

Answer 18

Basal ganglia
thalamus
Reticular formation
Various sensory nuclei, (posterior column nuclei)

Posterior horn,
Intermediate gray matter
anterior horn with motor neurons or interneurons.
(communicating with sensory neurons and interneurons in addition to lower motor neurons allows system to rewire itself to allow walking even after spinal cord lesions. The system is more complicated than we thought.)

Question 19

Because Corticospinal collateral fibers end in various places, what does that indicate for motor neurons and their pathways?

Answer 19

The collateral fibers have a variety of effects from regulating information in the ascending pathways to affecting the activity of motor neurons via interneurons.
(Damage allows for another pathway to be created…Monkey’s spine was injured and it rewired itself. Create the demand to get the potential.)

Question 20

Define Decorticate Posturing.

Answer 20

Indicates there has been severe damage ABOVE the level of the Red Nucleus in the midbrain. (Middle or anterior cerebral artery.)

The INHIBITION of the Red Nucleus is damaged from the telencephalon and diencephalon. Since the Red Nucleus facilitates Cervical level flexor tone to the upper extremities, this becomes the dominant upper extremity resting muscle tone. (arms drawn in and hands near heart.)

The lateral corticospinal tract, which facilitates flexion in the lower extremity motor neurons, is damaged. Extension tone from 1. pontine reticulospinal and 2. medial and lateral vestibulospinal tracts become dominant in the legs. (extended legs and feet)

Question 21

Define Decerebrate Posturing

Answer 21

A result of additional damage to the red nucleus or the rubrospinal tract.

The flexor tone to the upper extremities as sen in decorticate postrueing is lost with additional damage to the Rubrospinal tract. The result is extensor tone to the neck and upper extremities. (extensory tone is pretty rare)

Common in:
1. Lesion or compression (from free edge of tentorum cerebelli) in the midbrain.
2. Lesions in the cerebellum
3. Pontine strokes. (Basilar system feeding into the pons.)
(Extensor tone in legs with both postures. Synaptic push because the lateral corticospinal tract is not working like it should. Other tracts take over and extensor tone happens.)