Lecture 2 - Introduction to Motor Systems

Question 1

What is a descending motor system and what two main structures do they include?

Answer 1

-upper motor neurons
-found in the motor cortex and the brainstem centers

Question 2

In the descending system what do upper motor neurons in the motor cortex do?

Answer 2

-plan, initiate, and direct voluntary movements

Question 3

In the descending system what doe upper motor neurons in the brainstem centers do?

Answer 3

-rhythmic stereotyped movements and postural control

Question 4

What structure provides input to the the upper motor neurons in the motor cortex?

Answer 4

-basal ganglia by initiating intended movements and suppressing unwanted movements

Question 5

What structure provides input to the upper motor neurons in the brainstem centers?

Answer 5

-the cerebellum by coordinating via a negative feedback system ongoing movements

Question 6

The upper motor neurons in the descending pathway send their projections onto what two possible structures?

Answer 6

-local circuit neurons in the spinal cord which have sensorimotor integration and CPGs
-motor neuron pools which are lower motor neurons
(note - either the upper motor neurons will synapse directly onto the motor neuron pool or will synapse onto local circuit neurons in the spinal cord which then synapse onto the motor neuron pools themselves?

Question 7

Where can local circuit neurons also receive input from in addition to from upper motor neurons in the brainstem and motor cortex?

Answer 7

-sensory inputs from environment

Question 8

Where doe the projections of motor neuron pools aka lower motor neurons project to?

Answer 8

skeletal muscles

Question 9

What is a motor unit?

Answer 9

-an individual motor neuron and all the muscle fibers it innervates

Question 10

What is a motor pool?

Answer 10

-all of the neurons innervating a single muscle; aka the neurons in the ventral horn of the spinal cord

Question 11

What muscles do lateral motor neurons innervate and medial motor neurons in the ventral horn innervate?

Answer 11

lateral muscles
medial muscles

Question 12

Whats is an alpha motor neuron and what is their size?

Answer 12

-large, multipolar motor neurons found in the brainstem and spinal cord and are responsible for controlling muscle contraction for voluntary motor movements
-large axon and diameter and they are powerhouse of muscle contraction

Question 13

What is a gamma motor neuron and what is its size?

Answer 13

-smaller in comparison to alpha motor neurons
-found in brainstem and spinal cord
-responsible for proprioceptive feedback and changes in muscle tension

Question 14

What is a fiber and is part of the nervous system?

Answer 14

-not part of nervous system
-a muscle fiber is how we classify skeletal muscles and their respective fibers and they have different behaviors, contraction times

Question 15

What is a slow-twitch (ST) muscle fiber, what neuron is it recruited by, what is contraction time, size, and resistance to fatigue?

Answer 15

-generally for long periods of time
-recruited by gamma motor neurons
-contraction time - slow
-size of motor neuron - small
-resistance to fatigue - high
-tend to have a lot of mitochondria and blood vessels

Question 16

What is a fast-twitch A (FT-A) motor neuron, what is its contraction time, size of the motor neuron, resistance to fatigue?

Answer 16

contraction time - fast
size - large
-resistance to fatigue - intermediate

Question 17

What is a fast-twitch B (FT-B) motor neuron, what is it innervated by what is its contraction time, size of the motor neuron, resistance to fatigue?

Answer 17

-innervated by alpha motor neurons
-quick bursts of movement
-contraction time - very fast
-size - very large
-resistance to fatigue - low
-do not have many vessels or mitochondria and tend to be pale

Question 18

What is a muscle spindle and what neuron does it signal by?

Answer 18

-in the intra and extra fusal fibers of muscle tissue
-detect when a muscle stretch as well as the change in stretch over time in response to muscle contraction
-have an afferent projection which send a signal to alpha motor neurons
-are sensory organs that wrap around the skeletal muscle and their job is to detect when the muscle stretches which can occur when you carry a load

Question 19

What is a golgi tendon organ (GTO) and what does it signal by?

Answer 19

-embedded in muscle tissue and they detect the degree of tension in muscle tissue
-signal via gamma motor neurons
-integrate the external side of muscle fiber exterior and have to do with tension
-cells will release glycine so the contramuscle will relax while one muscle is tense

Question 20

Where are motor neurons located in the spinal cord?

Answer 20

-the ventral horn of the spinal cord

Question 21

Is there a ventral root ganglion like there is a dorsal root ganglion?

Answer 21

no

Question 22

What muscles do lower motor neurons in the lateral ventral horn innervate?

Answer 22

distal muscles, like muscles of extremities

Question 23

What muscles do lower motor neurons in the medial ventral horn innervate?

Answer 23

proximal muscles such as the torso

Question 24

What is the dorsal root ganglion?

Answer 24

an additional set of sensory neurons which exist outside of the doral horn of the spinal cord

Question 25

How does cresyl violet stain the gray matter versus the white matter?

Answer 25

-gray matter is lighter while the white matter is purple since it has great amounts of fat

Question 26

What are the two types of movement that can be executed and by what muscles are they mediated?

Answer 26

–voluntary movements - mediated by skeletal muscles
-involuntary movements - mediated by smooth muscle

Question 27

What are reflex arcs and why are they special?

Answer 27

involuntary and unconscious movements that are not executed by a central motor command but rather by the exclusive usage of local circuitry in the spinal cord

Question 28

What are two examples of a reflex arc?

Answer 28

-knee jerk reflex and a withdrawal reflex

Question 29

In the knee jerk reflex which muscle needs to relax and which one needs to contract?

Answer 29

flexor contracts whule extensor relaxes

Question 30

What is a central pattern generator or CPG?

Answer 30

-spinal cord mediated movements that is a local circuit of motor and inhiitory neurons within the spinal cord that mediate simple rhythmic, motor tasks or patterns such as walking, breathing

Question 31

What is thought to cause locomotion or walking?

Answer 31

a CPG with local motor and inhibotry neurons
-spinal cord injuries complicate this hypothesis though

Question 32

On what test subject has the majority of research on CPGs been conducted on?

Answer 32

cats

Question 33

How is research conducted on cats to study CPGs and locomotion?

Answer 33

1. induce an incomplete spnal injury via a hemilateral lesion on a cat
   -recover locomotion thorugh treadmill ytranining since proprioceptive feedbac k from paws moving along with the treadmill stimulates CPGs to trigger movement

Question 34

How have neural implants beend found to restore some locomotion in animals?

Answer 34

through a biofeedback to stimulate CPGs

Question 35

What are the two major descending motor pathways?

Answer 35

corticospinal and corticobulbar tracts

Question 36

What does the corticospinal tract convey voluntary motor signals from and to what?

Answer 36

from the motor cortex to the body

Question 37

What does the corticobulbar tract convey voluntary motor signals from and to what?

Answer 37

from the cranial nerve motor nuclei to the muscles in the face head and neck

Question 38

How are visceral or involuntary motor commands conveyed?

Answer 38

through the vagus nerve and involve the parasympathetic and sympathetic autonomic nervous system

Question 39

Can you live without a cerebellum?

Answer 39

the inner ear helped maintain movement, posture, and balance

Question 40

What is the ventral root?

Answer 40

it serves as an off ramp for the axonal projections of the ventral horn motor neurons to their respective skeletal muscle targets

Question 41

What is involved in the withdrawal reflex?

Answer 41

ascending pathways which detect stimuli and descending pathways which cause motor neurons in skeletal muscle to move

Question 42

What are renshaw cells?

Answer 42

-inhibitory interneurons which use glycine and play an important role in relaying info from sensory neurons and inhibit downstream efferent motor neurons to prevent muscle injury

Question 43

Where does the corticobulbar tract branch to and what nerves receive contralateral inputs from it?

Answer 43

-branches to the caudal medulla and receives inputs from the hypoglossal nerve, trigeminal nerve, and facial nerve which innervate the lower portion of the face

Question 44

What is the corticobulbar tract?

Answer 44

brainstem —>midbrain, pons (skeletal muscles of the face), ends in caudal medulla (skeletal muscles of tongue)

Question 45

What do upper motor neurons of the corticospinal tract that are in the cerebral cortex innervate?

Answer 45

lateral limb muscles via innervation of local motor neurons in the lateral ventral horn

Question 46

What do upper motor neurons of the corticobulbar tract that are in the brainstem innervate?

Answer 46

medial muscles such as posture balance and locomotion via innervation of lower motor neurons in the medial ventral horn