Topic 6-Organization of Motor System

Question 1

What are three types of movements

Answer 1

Reflexes
Rhythmic Movements
Voluntary movements

Question 2

What are the two modes of motor control discussed?

Answer 2

The two modes of motor control are phasic control (transient muscle activation) and tonic control (steady muscle activation).

Question 3

Describe the functioning of the spinal cord in coordinating sensory and motor pathways (steps)

Answer 3

1) Sensory fibers enter the spinal cord from the back; motor fibers exit from the front.
2) The spinal cord contains circuits for automatic and stereotyped responses.
3) Alpha motor neurons are referred to as the “final common pathway” because they consolidate signals from various sources.
4) Once an alpha motor neuron sends a signal to the muscles, the brain temporarily loses control.
5) Feedback from muscle spindles and vision is required to correct any errors in the signal.
6) The initial action is based on the brain’s interpretation of the correct response, in coordination with the brainstem and spinal circuits.

Question 4

What is the function of the descending spinal tracts from the brainstem?

Answer 4

The descending spinal tracts from the brainstem project to the spinal cord and modulate the actions of spinal circuits.

Question 5

Which muscles do the medial descending tracts from the brainstem influence?

Answer 5

The medial descending tracts from the brainstem influence postural muscles.

Question 6

What is the role of the lateral descending tracts from the brainstem?

Answer 6

The lateral descending tracts control the distal muscles, which are located farther from the center of the body.

Question 7

In which part of the spinal cord do the vestibulospinal and reticulospinal tracts descend?

Answer 7

The medial vestibulospinal and reticulospinal tracts descend ipsilaterally in the ventral columns of the spinal cord.

Question 8

Where do the vestibulospinal and reticulospinal tracts terminate in the spinal cord?

Answer 8

These tracts terminate on interneurons in the intermediate zone and medial motor neurons.

Question 9

Where does the medial vestibulospinal tract originate?

Answer 9

The medial vestibulospinal tract originates from the vestibular nuclei in the brainstem.

Question 10

What is the primary target of the medial vestibulospinal tract?

Answer 10

The medial vestibulospinal tract primarily targets “antigravity” muscles, playing a role in reflex control of balance and posture.

Question 11

How are motor nuclei are grouped in relation to muscles they innervate​? According two what two rules?

Answer 11

1) Proximal-distal rule​

2) Flexor-extensor rule​

Question 12

What does the proximal-distal rule in motor nuclei organization state?

Answer 12

The proximal-distal rule states that motor neurons located medially innervate proximal and axial muscles, while lateral motor neurons innervate distal muscle

Question 13

What does the flexor extensor rule in motor nuclei organization state?

Answer 13

Flexor muscles are typically controlled by motor neurons located on the ventral side of the spinal cord.
Extensor muscles are generally controlled by motor neurons located on the dorsal side of the spinal cord.

Question 14

What is a motor unit?

Answer 14

Composed of a single alpha motor neuron and all the muscle fibers it innervates, allowing for coordinated contraction of those fibers.

Question 15

How are motor units classified?

Answer 15

Motor units are classified based on the speed of muscle contraction in response to action potentials

Question 16

How does the number of fibers per motor unit affect muscle control?

Answer 16

The number of fibers per motor unit affects the precision of muscle control; fewer fibers allow for finer control, while more fibers are associated with gross, powerful movements.

Question 17

What is a motor neuron pool?

Answer 17

All the motor neurons innervating a single muscle​

Question 18

What is the size principle in the context of muscle contraction?

Answer 18

States that during muscle contraction, the recruitment of motor neurons from the motor neuron pool follows a specific order, with small motor units being recruited before larger ones.

Question 19

How is electrical resistance related to the surface area of motor neurons according to the size principle?

Answer 19

Electrical resistance is inversely related to the surface area of motor neurons, meaning that neurons with larger surface areas have lower resistance and are recruited later during muscle contraction as more force is required.

Question 20

Describe 3 defining features of spinal reflexes

Answer 20

-stereotyped motor response to a given sensory stimulus
- circuitry responsible is entirely in spinal cord (fast!)
- modified to suit task

Question 21

What is the Stretch Reflex?

Answer 21

The Stretch Reflex is a type of reflex that causes muscle contraction in response to lengthening.

Question 22

What is the receptor involved in the Stretch Reflex?

Answer 22

The receptor in the Stretch Reflex is the muscle spindle, which is monitored by Ia afferent fibers.

Question 23

What type of neural pathway does the Stretch Reflex follow?

Answer 23

Follows a monosynaptic pathway, which means it involves a single synapse and is a very rapid reflex with a delay of approximately 1 millisecond.

Question 24

Why is the monosynaptic stretch reflex important and how is it tested?

Answer 24

Crucial for quickly assessing spinal cord health non-invasively. It’s tested by tapping a tendon with a reflex hammer, which should trigger a muscle contraction if the spinal cord is intact.

Question 25

What is Reciprocal Inhibition?

Answer 25

Reciprocal Inhibition is a neural mechanism that involves the Ia inhibitory interneuron and reciprocal innervation. It serves to excite one group of muscles while simultaneously inhibiting their antagonistic muscles.

Question 26

What is the role of Renshaw Cells in Recurrent Inhibition?

Answer 26

Renshaw Cells are inhibitory neurons that have a side branch extending back toward their own cell body, allowing them to control the “gain” of the neural circuit, similar to adjusting a volume knob.

Question 27

How does Recurrent Inhibition work?

Answer 27

Recurrent Inhibition works by Renshaw Cells inhibiting their own alpha motoneuron, which adjusts the neural circuit’s sensitivity, similar to tuning a volume knob.

Question 28

Explain when recurrent vs reciprocal inhibition is used:

Answer 28

Recurrent inhibition is used for fine-tuning neural circuits within the spinal cord, while reciprocal inhibition is employed to coordinate muscle activity during voluntary movements.

Question 29

Descending Control 3 Features

Answer 29

-descending pathways control Ia inhibitory interneuron output
-can have antagonist relaxation or co-contraction
-Can affect excitability of the Renshaw cell as well (not shown)

Question 30

How does the Golgi Tendon Organ (GTO) reflex work? What is the role of the 1B inhibitory interneuron?

Answer 30

The GTO reflex activates when a muscle contracts too forcefully, at which point the 1B afferent neurons from the GTO sense the tension and stimulate 1B inhibitory interneurons. These interneurons then inhibit the alpha motor neurons, reducing muscle contraction to prevent potential damage.

Question 31

How do the Ia and Ib Effects work together?

Answer 31

The Ia and Ib Effects, involving the stretch reflex and Golgi Tendon Organ reflex, collaborate to stabilize posture and prevent changes in position.

Question 32

What two reflexes work together to stabilize a posture?

Answer 32

The stretch reflex and the Golgi tendon organ (GTO) reflex work together to stabilize a posture and prevent changes in position

Question 33

What is the primary function of the flexion reflex?

Answer 33

The primary function of the flexion reflex is to move a limb away from a painful stimulus.

Question 34

What type of muscle action is involved in the flexion and crossed-extension reflex?

Answer 34

The flexion and crossed-extension reflex involves a coordinated pattern of muscle action.

Question 35

How does the neural input behave in the flexion and crossed-extension reflex?

Answer 35

In the flexion and crossed-extension reflex, a single input diverges into polysynaptic pathways.

Question 36

What allows rhythmic patterns of activity to occur without the need for brain input in spinal circuitry?

Answer 36

Rhythmic patterns of activity can occur without brain input due to the presence of central pattern generators (CPGs).

Question 37

Where is all the essential circuitry for basic locomotion primarily located in the body?

Answer 37

All necessary circuitry for basic locomotion is primarily located in the spinal cord.

Question 38

What initiates the generation of rhythmic patterns for basic locomotion in the spinal cord?

Answer 38

The generation of rhythmic patterns for basic locomotion in the spinal cord is initiated by a sensory input.

Question 39

When are spinal rhythm-generating networks present?

Answer 39

Spinal rhythm-generating networks are present almost from the time of birth.

Question 40

Technological Approach for Spinal Cord Injury:

Answer 40

1) Stimulating electrodes in spinal cord activate neural pathways, bypassing damaged areas and facilitating coordinated muscle movements,
2) Brain-Machine Interphase: Decode neural signals from the brain and translate them into commands for robotic exoskeletons, enabling individuals with spinal cord injuries to regain the ability to walk

Question 41

Where does the tectospinal tract originate from? What is its function?

Answer 41

Originates from the superior colliculus, coordinating head and eye movements.

Question 42

What are three medial brainstem pathways and the 1 lateral one?

Answer 42

Reticulospinal tract​, tectospinal tract, vesitbulospinal tract
Lateral: Rubrospinal tract ​

Question 43

Where does the reticulospinal tract originate and what is its function?

Answer 43

Originates from pontine reticular formation​, maintain posture

Question 44

Which two pathways are descending contralaterally from where they start?

Answer 44

1) Tectospinal
2) Reticulospinal

Question 45

What is the predominant role of the lateral rubrospinal tract?

Answer 45

The lateral rubrospinal tract provides excitatory input to motor neurons to upper limb flexors

Question 46

What are 3 other tasks for the motor system?

Answer 46

1) Convey timed commands to muscles
2) Account for body posture
3) Account for “motor plant”: mechanical properties of the body part like inertia

Question 47

What are cranial nerves II, IV, VI? Where are they located and what are their function?

Answer 47

Optic nerve, Trochlear nerve, Abducent Nerve
Functions: Control eye lid and eyelid movement (blinking)
Locations: midbrain and pons

Question 48

What are cranial nerves V (5) and VII (7)? Their location and functions?

Answer 48

Trigeminal nerve; Facial nerve
Location: pons
Function: Innervate the face muscles for chewing

Question 49

Name Cranial Nerves IX (9) to XII (12)

Answer 49

glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI), and hypoglossal nerve (CN XII).
Function: Innervate speech articulators
Location: Have nuclei in the medulla and spinal cord

Question 50

Where is the vestibular nerve located and function?

Answer 50

pons and medulla of the brainstem
function: transmits sound and equilibrium (balance) information from the inner ear to the brain.