Organization of the Motor System

Question 1

Which structures are involved in the following?

a) Reflexes
b) Posture
c) Voluntary movement

Answer 1

a) LMN and spinal cord circuits
b) Brainstem and spinal cord
c) cortex, brainstem and spinal cord

Question 2

What is the difference between a closed and open loop?

Answer 2

Closed loops are reflex control done by our axial muscles (muscles of trunk and head) for balance, posture
and locomotion

Open loops are for sensory cue or desire to move done by distal muscles for fine motor skills

Question 3

Match the following structures or control centres with its corresponding structure or control centre

a) postcentral gyrus
b) precentral gyrus
c) body sensations
d) temporal
e) Vision
f) Frontal lobe

Answer 3

a) Primary somatosensory cortex
b) Primary motor cortex
c) Parietal lobe
d) Hearing, advanced visual processing
e) Occipital
f) Planning of movements, recent memory and some aspects of emotion

Question 4

What are the following structures responsible for?

• Insert photo after
  a) Premotor cortex
  b) Primary motor cortex
  c) Primary somatosensory cortex

Answer 4

a) motor planning and sequencing for contralateral body
b) controls contralateral body motor functions
c) receives contralateral sensory input from the body

Question 5

What are the following structures responsible for?

a) Posterior parietal cortex (superior lobule)
b) primary visual cortex
c) Primary auditory cortex

Answer 5

a) Integration of sensory input - stereognosis
b) receives contralateral visual field info from both eyes
c) receives bilateral auditory sensory input

Question 6

a) What is the special term for the terminal branches of the axon?
b) What is the special term for the sheath of schwann?

Answer 6

a) telodendria

b) Neurilemma

Question 7

Which structures have direct control over skeletal muscle fibres? What effect will it have on skeletal muscle fibres when stimulated?

Answer 7

The axons of lower motor neurons from the brainstem or spinal cord has direct control over skeletal muscle fibres, and stimulation of the lower-motor neuron always has an excitatory effect on skeletal muscle fibres

Question 8

Describe the basic overview of normal motor function/the motor system

Answer 8

Transmission of signal from cortex - muscle goes via an UMN; which are neurons of the cerebral motor cortex and brainstem nuclei to a LMN; defined as cells of the ventral horn of the spinal cord OR cranial nerve motor nuclei which give rise to axons supplying skeletal muscle

Signals may travel to LMNs via the conscious pathway (pyramidal) or unconscious pathways (extrapyramidal)

Once reached the spinal cord -> anterior horn cell -> peripheral nerve -> NMJ -> Muscle

*insert photo from workbook

Question 9

Where does the corticospinal tract begin?

Answer 9

The motor cortex on the pre-central gyrus

Question 10

Which tracts are conscious and which are unconscious? Where do they originate and end?

Answer 10

Conscious: pyramidal or corticospinal tract:
The pyramidal system consists of UMNs originating in the cerebral cortex, descending through the internal capsule and medullary pyramids to synapse with LMNs in the spinal cord

Unconscious: Extrapyramidal tracts originating in the brainstem, basal ganglia and cerebellum which target the lower motor neurons in the spinal cord

Question 11

What are the four subconscious extrapyramidal tracts?

Answer 11

1. Rubrospinal
2. Anteromedial tracts: these subconsciously control the axial and proximal muscles
   a) . Tectospinal
   b) . Vestibulospinal
   c) . Reticulospinal

*insert photo after

Question 12

What are Betz cells and where are they? What do they synapse with?

Answer 12

Upper motor/pyramidal neurons with long axons in the primary motor cortex which travel along the corticospinal/pyramidal tract to synapse with interneurons or alpha motor neurons

Question 13

What are alpha motor neurons?

Answer 13

A LMN is the cell body and axon of an alpha motor neuron.

Question 14

Name the three tracts of the corticospinal tract, what do they each control?

Answer 14

1. Corticobulbar - conscious control over eye and face muscles
2. Lateral corticospinal - conscious control over limb skeletal muscles
3. Anterior corticospinal - conscious control over axial skeletal muscle

Question 15

Describe the pathway of the corticospinal tract(s), including its origin and decussations

Answer 15

Originates in the primary motor cortex on the precentral gyrus.

Axons descend through the internal capsule to the medulla via the pyramids of the medulla

• 75-90% decussate in the pyramids of the medulla oblongata and descend in the lateral corticospinal tract to distal musculature/limbs
• 10-25% remain ipsilateral and don’t decussate until they are nearer to where they terminate in the spinal cord. These descend in the ventral corticospinal tract and go mainly to axial skeletal muscle

They all synapse onto the ventral horn of the spinal cord onto their LMN targets

Question 16

Name five general functions that the extrapyramidal motor pathways are responsible for

Answer 16

Involuntary actions, reflexes, locomotion, complex movements and posture

Question 17

Where does the rubrospinal tract originate from and descend through/to?

Answer 17

From the red nuclei of the midbrain (which receives inputs from the cerebral cortex and cerebellum);

Since it doesn’t pass through the pyramids of the medulla its an extrapyramidal pathway: It descends in the lateral part of the brainstem and then in the lateral funiculus adjacent to the lateral corticospinal tract

Question 18

What happens if there is a lesion to the rubrospinal tract?

Answer 18

Minimal effects (demonstrating the presence of considerable overlap!)

Question 19

What is the rubrospinal tract especially in control of?

Answer 19

Voluntary control of upper limb movements; specifically large muscle flexor movement and inhibiting extensor tone of the upper limbs

*workbook also said for fine motor activity of upper limbs

Question 20

What is the reticulospinal tract responsible for?

Answer 20

Automatic movement: Body posture, controlling reflexes and muscle tone

Question 21

What is the reticulospinal tract comprised of?

Answer 21

Pontine and medullary reticulospinal tracts

Question 22

Where does the pontine reticulospinal tract originate from and what is it responsible for?

Answer 22

It’s the medial pathway originating in the pontine reticular nucleus responsible for exciting extensor muscles

Question 23

Where does the medial reticulospinal tract originate from and what is it responsible for?

Answer 23

It’s the lateral pathway originating in the medullary reticular nuclei responsible for inhibiting excitatory axial extensor muscles and automatic breathing

Question 24

What is the vestibulospinal tract responsible for?

Answer 24

Involuntary control over head balance and body posture

Question 25

Where does the vestibulospinal tract receive information from and what does it control?

Answer 25

Recieves info from the vestibulocochlear nerve about changes in head orientation. It then descends via the vestibulospinal tract and alters muscle tone, extension and maintains posture

Question 26

What is the tectospinal tract responsible for?

Answer 26

Orientation and positioning of the head for balance in response to stimuli - “eye-head coordination”

Question 27

Where does the tectospinal tract originate from?

Answer 27

The tectum of the midbrain

Question 28

Where is a LMN’s cell body? How does it interact with muscle?

Answer 28

Either in the ventral horn of the spinal cord or cranial nerve nuclei of the brainstem and directly innervates a muscle with its axon

*another question like this in other quiz

Question 29

What is a motor unit and how do they influence control over musculature?

Answer 29

A motor unit is the number of muscle fibres innervated by a single axon, a smaller number means finer control

Question 30

What are the two types of LMNs? What do they do and what is their primary difference?

Answer 30

They feedback to the brain and the reflex arc

alpha-MN: innervates the muscle itself
gamma-MN: innervates the muscle spindle and keeps it taught

Question 31

What is a muscle spindle and what does it detect and allow for/provide?

Answer 31

An encapsulated sense organ found within the muscle

Since it detects muscle contraction it’s co-activated with alpha-motor neurons and tightens as the muscle contracts - and is therefore responsible for tendon reflex

Allows for finer motor control and can detect the extent of muscle tension (contraction/relaxation) and inform the cerebellum (unconscious), therefore it can provide limb positional information to the CNS (proprioception)

Question 32

What does a muscle spindle activate (and what stimulates this activation)?

Answer 32

Activates LMNs in response to muscle stretch

Question 33

Describe what takes place in the neuromuscular junction and how it leads to a muscular contraction

Answer 33

1. A motor nerve impulse causes the release of Ach into the synaptic cleft, which activates nicotinic Ach receptors on muscle fibre membranes
2. This opens ligand-gated sodium channels depolarising the sarcolemma
3. AP is propagated deep into muscle fibre through T tubules (invaginations in the sarcolemma) as voltage gated sodium channels open
4. T tubule depolarisation activates ryanodine receptors in the membrane of the SR - releasing stored Ca2+ into the cytoplasm
5. Ca2+ stimulates muscle contraction by binding a troponin subunit (TnC) allowing myosin binding sites to be revealed
   * Ca2+ is returned to the SR via ATPase pumps

Question 34

How does the activation of a single motor neuron compare to the activation of multiple motor neurons?

Answer 34

Increased FOC the more motor neurons are activated

Activation of a single motor neuron will lead to a weak but distributed contraction

Question 35

Where are golgi tendon organs found?

Answer 35

At the myotendinous junction between muscle and tendon

Question 36

What do golgi tendon organs do?

Answer 36

They monitor the degree of muscle contraction by sensing changes in the collagen within the tendon (as the collagen stretches and straightens with muscle contraction)

They prevent OVER contraction of muscles as when they’re activated they inhibit the muscle contraction and activate its antagonist

Question 37

What is muscle tone and what does it rely on?

Answer 37

The minimal background muscle power that allows us to maintain posture and minimal stiffness in skeletal muscles, it relies entirely on the muscle stretch reflex and rises/falls depending on the number and size of motor units involved.

*The smaller the size of the motor units, the less energy is expended (i.e maintaining normal posture)

Question 38

What generates resting motor tone?

Answer 38

When we’re awake LMNs continually activate muscle to generate the resting motor tone

Question 39

Describe the golgi tendon reflex, which other reflex does it act opposingly to?

Answer 39

It’s polysynpatic:
1. 1b afferent sensory fibres detect overstretch of a tendon (straightening/stretching of its collagen)

2. Connects to inhibitory and excitatory interneurons to cause relaxation of the muscle and contraction of the antagonist muscle - opposite to stretch/myotatic reflex

Question 40

Where are motor programs generated?

Answer 40

Basal ganglia

Question 41

Describe the hierarchy of the motor system

Answer 41

Highest: cerebral cortex
Intermediate: brainstem nuclei and cerebellum
Lowest: motor neurons of cranial and spinal nerves

Question 42

What are LMNs defined as?

Answer 42

Cells of the ventral horn of the spinal cord OR cranial nerve motor nuclei which give rise to axons supplying skeletal muscle

Question 43

How should alpha motor neurons be influenced in a healthy nervous system?

Answer 43

They SHOULD be under constant inhibition from UMNs, particularly the extrapyramidal system, this inhibition is only temporarily lifted to allow us to carry out voluntary movements.

The intensity of this constant inhibition varies continually; i.e during sleep; descending inhibition paralyses virtually all skeletal muscles apart from those responsible for breathing or those supplying extraocular muscles

Question 44

What is the role of the cerebellum in the motor cortex?

Answer 44

Involved in the timing and coordination of the motor cortex

Question 45

What happens to stretch reflexes in an UMN lesion?

Answer 45

Since UMNs (especially extrapyramidal ones) are inhibiting most stretch reflexes, a lesion will lead to less descending inhibition and a rise in motor tone

*muscle tone to the antagonistic muscle will be equally affected

Question 46

What may happen to the upper and lower limbs when there is a severe UMN lesion?

Answer 46

Since there is now LESS descending inhibition; spastic paralysis may occur in both upper and motor limbs in severe cases

Since flexors are usually stronger than extensors in the upper body, there will be overall flexion of the limb. However, a lower limb with spastic paralysis will have overall extension, since extensors are dominant.

Question 47

In addition to alterations of muscle tone, name four other signs which may occur as a result of an extrapyramidal disorder?

Answer 47

Excessive movements such as

1. Pill rolling tremor
2. Choreiform movements (jerking) and hemiballismus (unilateral, usually violent chorea)
3. Tics
4. Bradykinesia which can progress to akinesia (absence of movement) and tardive dyskinesia - an iatrogenic cause of parkinsonism like effects (oral grimaces, sucking, chewing)

Question 48

Describe what is meant by the term “orderly recruitment” of motor units

Answer 48

The recruitment of motor units is always orderly and is based on the cell body size, i.e; recruitment begins with motor units capable of minimal forces and ends with those capable of heaviest forces

Question 49

Where would you expect to find an alpha motoneuron in the spinal cord and what would be the effect of directly activating it on a) muscle length and b) motor tone?

Answer 49

Cell bodies of all alpha motor neurons are within the CNS in either the cranial nerve motor nuclei of the brainstem or the ventral horn of the spinal cord

Activation of an alpha motoneuron will result in

a) shortening of muscle it innervates and development of force
b) increase in the muscle’s motor tone

Question 50

Describe the pathway of the axons of alpha motoneurons, where do they terminate?

Answer 50

Axons of alpha motor neurons leave the CNS to course the PNS as either motor divisions of cranial nerves or spinal segmental nerves. They terminate on skeletal muscles at the NMJ

Question 51

Where would you expect to find the cell body of a gamma-motor neuron? What would be the effect of direclty activating it on a) muscle length and b) muscle tone?

Answer 51

Cell bodies of gamma motor neurons are found in the ventral horn of the grey matter

Activation will result in
a) shortening of the intrafusal muscle fibres of the muscle spindle sense organ it innervates -> this activates the muscle spindle afferent -> recruiting the alpha motoneuron via the stretch reflex which shortens the muscle length

b) Contraction results in increased development of force and thus increase in motor tone

Question 52

Identify whether the following are pyramidal (UMN), extrapyramidal and LMN

a) Festinating gait (PD)
b) Clasp knife rigidity
c) dysdiadochokinesia
e) marked muscle wasting
f) intention and pill rolling tremor
g) Babinski sign
h) Paucity of facial expression
i) complete absence of unilateral facial expression
j) Cogwheel rigidity
k) intention tremor

Answer 52

a) extrapyramidal
b) UMN
c) extrapyramidal
e) LMN
f) extrapyramidal
g) UMN
h) extrapyramidal
i) LMN
j) extrapyramidal
k) extrapyramidal