Descending Motor Pathways

Question 1

What are motor pathways divided into?

Answer 1

Upper and lower motor neuron regions (UMN and LMN)

Question 2

Where do UMNs originate? Where do they end?

Answer 2

• Originate in the cerebrum and subcortical structures
• End/synapse in spinal cord or brainstem
  
  • Synapse with cranial nerve nuclei in brainstem which then go on to supply head
  • Synapse in spinal cord with nuclei that go on to supply the body

Question 3

Function of UMNs?

Answer 3

• Influence LMN activity
• Modify local reflex activity
• Superimpose more complex patterns of movement

Question 4

Where do LMNs originate?

Answer 4

Brainstem or spinal cord (ventral grey horn)

Question 5

What are motor end plates?

Answer 5

Also called neuromuscular junctions - specialised chemical synapses formed at the sites where the terminal branches of the axon of a motor neuron contact a target muscle cell.

Question 6

Where does the cell body of a LMN lie?

Answer 6

Within the ventral horn of the spinal cord OR the brainstem motor nuclei of the cranial nerves (within the CNS)

Question 7

After the axon of a LMN exits the CNS, it forms the somatic motor part of the PNS. Where does it terminate?

Answer 7

On the muscle fibres which it innervates.

N.B. It is important to note that although one LMN will innervate several muscle fibres, a single muscle fibre is innervated by only one LMN.

Question 8

What is released by LMNs at the neuromuscular junction (NMJ)? What does this cause?

Answer 8

The neurotransmitter acetylcholine, which causes firing of an action potential in the receiving muscle fibre.

Question 9

What is acetylcholine? Function?

Answer 9

The chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate.

Question 10

Cross section through spinal cord. Sensory info enters via dorsal root. How does motor info leave the spinal cord?

Answer 10

Cell bodies of LMNs found in ventral grey horn. Axons (efferent fibres) then exit via the ventral root to go towards muscle

Question 11

Different types of descending motor pathways:

Answer 11

N.B. the name of the pathway tells you the origin and destination of the pathway

Question 12

The motor tracts can be functionally divided into two major groups: Pyramidal tracts and Extrapyramidal tracts.

Describe the origin and function of both of these tracts.

Answer 12

• Pyramidal:
  
  • These tracts originate in the cerebral cortex
  • Carry motor fibres to the spinal cord and brain stem
  • They are responsible for the voluntary control of the musculature of the body and face.
• Extrapyramidal:
  
  • These tracts originate in the brain stem
  • Carry motor fibres to the spinal cord
  • They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion

Question 13

Where do the pyramidal tracts derive their name from?

Answer 13

the medullary pyramids of the medulla oblongata, which they pass through

Question 14

Functionally, the pyramidal tracts can be subdivided into two. What are these?

Answer 14

1. Corticospinal tracts
2. Corticobulbar/corticonuclear tracts

Question 15

What do the corticospinal tracts supply?

Answer 15

Supplies the musculature of the body.

Question 16

Where do the corticospinal tracts originate/terminate?

Answer 16

• Originate in cerebral cortex
• After originating from the cortex, the neurones converge, and descend through the internal capsule
• Then the neurones pass through the crus cerebri of the midbrain, the pons and into the medulla.
• Terminate in ventral horn of spinal cord

Question 17

Where do the corticonuclear tracts orignate/terminate?

Answer 17

• Arise from the lateral aspect of the primary motor cortex
• The fibres converge and pass through the internal capsule to the brainstem.
• The neurones terminate on the motor nuclei of the cranial nerves
  
  • ​Here, they synapse with lower motor neurones, which carry the motor signals to the muscles of the face and neck.

Question 18

What is the internal capsule?

Answer 18

A white matter pathway, located between the thalamus and the basal ganglia

Question 19

Why is the internal capsule clinically relevant?

Answer 19

The internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as a ‘capsular stroke‘. Such an event could cause a lesion of the descending tracts.

Question 20

What are the 4 extrapyramidal tracts?

Answer 20

1. Vestibulospinal Tracts
2. Reticulospinal Tracts
3. Rubrospinal Tracts
4. Tectospinal Tracts

The extrapyramidal tracts originate in the brainstem, carrying motor fibres to the spinal cord.

Question 21

What are the extrapyramidal tracts responsible for?

Answer 21

They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion.

Question 22

Where do the reticulospinal tracts originate/terminate?

Answer 22

Originate in the reticular formation.

Terminates in spinal cord.

Question 23

Where do the vestibulospinal tracts originate/terminate?

Answer 23

Originate in vestibular nuclei in brainstem.

Terminate in spinal cord.

Question 24

Where do the tectospinal tracts originate/terminate?

Answer 24

Originate in tectum of brainstem.

Terminate in spinal cord.

Question 25

Where do the rubrospinal tracts originate/terminate?

Answer 25

Originate in red nucleus (a midbrain structure). Terminate in spinal cord.

Question 26

Corticospinal and corticonuclear pathways summary:

Answer 26

Cerebral cortex --\> Precentral gyrus --\> internal capsule --\> braimstem/spinal cord * Corticospinal: * cortex to spinal cord * spinal nerves * Corticonuclear: * cortex to brainstem nuclei * cranial nerves

Question 27

Somatotopic organisation of descending fibres: motor homunculus

Answer 27

Fibres originate at **specific location** in precentral gyrus; leg medially, face laterally

Question 28

The internal capsule consists of **three parts** and is V-shaped when cut horizontally, in a transverse plane. What are these 3 parts?

Answer 28

* The **genu** is the bend in the V * **Anterior limb**: the part in front of the genu, between the head of the caudate nucleus and the lentiform nucleus * **Posterior limb**: between the thalamus medially and lentiform nucleus laterally

Question 29

The descending fibres fibres retain somatotopic representation as they pass through internal capsule. Where do the fibres going to the a) face b) arm c) trunk d) leg

Answer 29

a) travel via the genu Fibres of the corticospinal tract use the **posterior limb** --\> somatotopically organised: b) fibres going to arm travel more anteriorly in the posterior limb c) fibres going to trunk travel in the middle in the posterior limb d) fibres going to leg travel more posteriorly in the posterior limb

Question 30

What are the descending fibres of the internal capsule continuous with inferiorly?

Answer 30

Crus cerebri (cerebral peduncles)

Question 31

After the internal capsule, where do the descending neurones pass?

Answer 31

through the crus cerebri of the midbrain, the pons and into the medulla.

Question 32

Somatic representation is still present as the fibres pass to the crus cerebri. What is the location of a) face b) arm c) trunk d) leg?

Answer 32

* Face fibres: medially * Leg: laterally

Question 33

What is the blood supply of the internal capsule?

Answer 33

The internal capsule and basal nuclei are supplied by perforating branches of the anterior cerebral artery (ACA), Heubner's artery, middle cerebral artery (MCA), internal carotid artery (ICA) and anterior choroidal artery (AChA). Also known as **lenticulostriate arteries**

Question 34

What happens if there is a haemorrhage of the lenticulostriate arteries? Why is the internal capsule particularly vulnerable?

Answer 34

* Vulnerable to vascular damage. * Many important sensory and motor fibres in small area * Serious sensory and motor deficits

Question 35

Lenticulostriate arteries are **small end arteries**. In which condition are they at risk?

Answer 35

Hypertension

Question 36

Brainstem path of descending pathways

Answer 36

1. Start in cortex 2. Pass through genu/posterior limb of internal capsule 3. Pass through crus cerebri 4. Fibres pass through **ventral pons** 5. In the medulla, they pass through medullary pyramids and pass through midline at decussation

Question 37

Cross sections through brainstem

Question 38

Where do corticospinal fibres cross the midline?

Answer 38

At the decussation of pyramids

Question 39

Overview of pathway of corticospinal tracts:

Answer 39

* UMN in pre-central gyrus sends axon via the posterior limb of the internal capsule * Axon descends and travels through cerebral peduncles * Axon descends in ventral pons and then medullary pyramids * At level of caudal medulla, the **majority of fibres** cross the midline * Then join the **lateral corticospinal tract** * Here fibres are contralateral descending fibres * At a specific spinal cord level, then synapse with a LMN at the ventral grey horn * LMN then exits spinal cord

Question 40

In the most inferior (caudal) part of the medulla, the corticospinal tract divides into two. What are these 2 tracts it divides into?

Answer 40

* **_Lateral corticospinal tract (85%):_** * These fibres decussate (cross over to the other side of the CNS). * They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels). * From the ventral horn, the lower motor neurones go on to supply the muscles of the body. * **_Anterior/ventral corticospinal tract (15%):_** * Remains ipsilateral, descending into the spinal cord. * They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels OR remain ipsilateral

Question 41

Which tract do the UMNs that descend the cord ipsilaterally enter?

Answer 41

The anterior corticospinal tract --\> then at each relevant spinal level, fibres decussate (cross the anterior spinal commissure of the spinal cord) to synapse with ventral horn motor neurones. Provides **bilateral innervation**.

Question 42

Which tract do the UMNs that cross at the decussation of pyramids enter?

Answer 42

Lateral corticospinal tract --\> contralateral innervation

Question 43

Where does the anterior corticospinal tract terminate?

Answer 43

at the level of the thoracic vertebrae.

Question 44

What 2 ways is the anterior corticospinal tract different from the lateral corticospinal tract?

Answer 44

1. Ipsilateral 2. Terminates at the level of the thoracic vertebrae

Question 45

The skeletal muscles are divided into axial and appendicular muscles. What is each category?

Answer 45

* Axial: muscles of the trunk and head * Appendicular: muscles of the arms and legs

Question 46

Which category of muscles does the lateral corticospinal tract innervate?

Answer 46

Appendicular limb musculature (contralateral innervation)

Question 47

Which category of muscles does the anterior corticospinal tract innervate?

Answer 47

Axial musculature (bilateral innervation)

Question 48

How does the anterior corticospinal tract provide bilateral innervation?

Answer 48

At appropriate cord level some fibres cross, others remain ipsilateral. E.g. postural muscles tend to work in pairs not individually

Question 49

Somatotopic representation is retained through the cord and through the lateral corticospinal tract. * Fibres to arm travel more medially in tract * Fibres to trunk in middle * Fibres to leg more laterally

Answer 49

Question 50

Corticospinal Tract Summary:

Answer 50

* Leaves precentral gyrus and descends through the posterior limb of the internal capsule * Fibres descend through cerebral peduncle of midbrain, ventral pons and pyramids of medulla * 85% of CST decussate at pyramids to descend spinal cord in Lateral CST * Contact LMN in contralateral ventral grey horn * 15% remain ipsilateral in anterior/ventral CST * Contact LMN that project to both sides of the respective spinal cord level (ipsilateral and contralateral ventral grey horns) * Exit the spinal cord via the ventral rootlets and roots * Extend to skeletal/striated muscle via the segmental spinal nerves

Question 51

LMN lesions can occur at the cell bodies or at the axons as they leave the spinal cord (peripheral spinal nerves or cranial nerves). What are some causes of LMN lesions?

Answer 51

* Trauma * Infection e.g. poliomyelitis * IV disc herniation

Question 52

How can LMN lesions manifest?

Answer 52

* **Flaccid paralysis** of muscles involved * a neurological condition characterised by weakness or paralysis and reduced muscle tone without other obvious cause * Diminished (hyporeflexia) or absent (areflexia) tendon reflexes at level of lesion * Muscle wasting * Muscle weakness (paresis)/reduced power * Hypotonia (decreased muscle tone) * Fasciculation (muscle twitches)/fibrillation

Question 53

Where do UMN lesions occur?

Answer 53

In cerebral hemispheres or as they descend to lateral white column of the spinal cord

Question 54

How do UMN lesions present initially and then long-term?

Answer 54

* Initially: * Flaccid paralysis of opposite limbs * Loss of tendon reflexes * After several days to a week motor function recovers but hypertonia * Long-term: * Increased, brisk (hyperreflexia) spinal reflex below lesion * Spastic paralysis of the involved muscles * Loss of fine motor control: permanent inability to carry out fine movements of hands and feet

Question 55

After several days to a week motor function recovers in UMN lesions. Why?

Answer 55

Other pathways appear to take over most 'corticospinal' functions but movements are not as fine-tuned.

Question 56

In UMN lesions, why are axial muscle groups spared?

Answer 56

Axial muscles receive **bilateral innervation** --\> if there is a lesion on one side, the other side takes over to innervate

Question 57

Why are the pyramidal tracts susceptible to damage?

Answer 57

Because they extend almost the whole length of the central nervous system

Question 58

If there is only a unilateral lesion of the left or right corticospinal tract, how do the symptoms appear?

Answer 58

symptoms will appear on the contralateral side of the body (except axial muscles)

Question 59

What is the Babinski reflex? Who is this seen in?

Answer 59

* Sole of foot is firmly stroked * **A positive Babinski sign**: happens when the _big toe bends up_ and back to the top of the foot and the other toes fan out * ONLY normal in children under 2 years * In people above 2 years, this is a sign of neurological condition (UMN lesion) * **Normal plantar reflex**: _downward flexion_ of toes towards source of stimulus

Question 60

Where do the corticonuclear neurons originate?

Answer 60

Arise in the head region (lateral aspect) of primary motor cortex/precentral gyrus

Question 61

The UMNs of the corticonuclear pathway influence LMNs where?

Answer 61

in **cranial nerve motor nuclei**.

Question 62

Innervation of LMNs of corticoncuclear pathway is largely **bilateral** (with some exceptions). What is benefit of this?

Answer 62

* E.g. fibres from the right side of the precentral gyrus innervate muscles in right AND left side of face * UMN lesions aren't as severe

Question 63

Overview of corticonuclear pathway

Answer 63

* Cell body in precentral gyrus (lateral aspect) * Send axons via the **genu** of the internal capsule * Axons then contact cell bodies of LMNs on BOTH SIDES (bilateral) * This occurs for most LMNs * LMN then sends fibres via cranial nerves to innervate head and neck

Question 64

The innervation of the corticonuclear tract is largely bilateral. What are the exceptions to this?

Answer 64

* Lower facial nucleus * E.g. right side of precentral gyrus innervates the left lower facial nucleus * Hypoglossal nucleus * Extrinsic tongue muscles innervates **contralaterally** These are under **contralateral** innervation.

Question 65

Ventral view of emergence of facial nerve from brainstem

Answer 65

Cerebellopontine angle

Question 66

View of motor terminal branches of facial nerve

Answer 66

Question 67

What is the facial colliculus?

Answer 67

An elevated area located on the pontine tegmentum (**dorsal pons**) in the **floor of the fourth ventricle**. It is formed by **fibres from the facial motor nucleus of the facial nerve** as they **loop over the abducens nucleus** (NOT formed by the facial nucleus).

Question 68

Cross section through caudal pons: view of facial colliculus

Answer 68

Question 69

What is also at risk in lesions of the abducens nucleus?

Answer 69

Facial colliculus

Question 70

Corticonuclear input to facial motor nuclei:

Answer 70

* At the level of the _upper facial region_ of the precentral gyrus: * UMN sends axon via genu of internal capsule * Sends **bilateral** projections to both sides of upper facial nucleus * Synapses with LMNs which go on to innervates the **upper face** regions * At the level of the _lower facial region_ of the precentral gyrus: * UMN sends axon via genu of internal capsulse * Axon **crosses midline** to supply the **contralateral lower facial nucleus** * Synapses with LMNs which go on to innervates the **lower face contralateral** regions All of these LMNs travel together via the **facial nerve**.

Question 71

Describe the innervation of upper and lower face

Answer 71

Upper face --\> bilaterally Lower face --\> contralaterally

Question 72

* Which case presents a **transient ischaemic attack** at the level of the internal capsule? * Which case presents an **infection within the facial canal** (Bell's palsy)? Which side is the lesion? At what level (UMN or LMN)?

Answer 72

* Weak lower face on right side * LEFT corticonuclear damage * Upper face spared as fibres coming from left precentral gyrus innervate right upper facial nucleus (bilateral) * Transient ischaemic attack at the level of the internal capsule * Weak upper and lower face on right side * RIGHT facial nerve or nucleus damage * Fibres from upper and lower right facial nuclei lost

Question 73

What does a **supranuclear** lesion to the corticonuclear/corticobulbar tracts result in?

Answer 73

* Deprives the **lower half** of the **opposite facial motor nucleus** of corticobulbar input * Results in paralysis of the **lower half of the face on the opposite side** to the lesion

Question 74

What does damage to the facial nerve itself result in?

Answer 74

Paralysis of the whole of one side of the face (on the side of the lesion)

Question 75

Is corticonuclear/corticobulbar damage UMN lesion or LMN lesion? What about facial nerve damage?

Answer 75

* Corticobulbar damage --\> UMN * Facial nerve damage --\> LMN

Question 76

Diagram of **right** corticobulbar damage (UMN) and **left** facial nerve damage (LMN)

Answer 76

Question 77

Facial innervation: UMN and LMN Summary

Answer 77

* Upper motor neuron lesion * Contralateral lower quadrant weakness * Angle of the mouth * Opposite side * Lower motor neuron lesion * Ipsilateral half of face * orbicularis oculi muscle and facial muscles involved * unable to close eyes * weakness of angle of the mouth * cannot elevate eyebrows * Same side

Question 78

Which cranial nerve supplies the extrinsic muscles of the tongue?

Answer 78

CN XII hypoglossal (purely motor function)

Question 79

Is the innervation of the extrinsic muscles of the tongue contralateral/bilateral/ipsilateral?

Answer 79

Contralateral

Question 80

describe the corticonuclear route regarding CN XII

Answer 80

* UMN fibres originate from (e.g. left) precentral gyrus * Pass through **genu** of internal capsule * Reach contralateral hypoglossal nucleus where they synapse with LMN * LMNs then travel via hypoglossal nerve to extrinsic muscles of tongue

Question 81

Where do you think the tongue will deviate if we lesion the left hypoglossal nerve?

Answer 81

The tongue will deviate to the left (ipsilateral to the lesion) - fibres have already decussated

Question 82

Where do you think the tongue will deviate if we lesion the fibres coming from the right side of the cortex ?

Answer 82

The tongue will deviate to the left (contralateral to the lesion)

Question 83

Hypoglossal innervation to extrinsic tongue: UMN and LMN Summary

Answer 83

* Upper motor neuron lesion * Deviation is contralateral to lesion * Lower motor neuron lesion * Peripheral hypoglossal nerve itself is paralysed * Deviation is ipsilateral to the lesion

Question 84

Muscles can be controlled by structures other than the cerebral cortex. What do these structures include?

Answer 84

* Basal ganglia * Tectum, Red nucleus * Reticular formation (in brainstem) * Vestibular system

Question 85

What is the reticulospinal pathway responsible for?

Answer 85

* Reticular formation (pons & medulla) to spinal cord * Voluntary movement/ breathing/consciousness

Question 86

What is the vestibulospinal pathway responsible for?

Answer 86

* Vestibular nuclei (pons and rostral medulla) to spinal cord * Controls posture

Question 87

What is the rubrospinal pathway responsible for?

Answer 87

* Red nucleus (midbrain) to spinal cord * Controls muscle tone

Question 88

Summary of descending tract location:

Answer 88

Question 89

Major ascending and descending fibre tracts: Heavily shaded tracts are sensory or motor to the right side of the body

Answer 89

Question 90

What is the neurotransmitter of the somatic motor system?

Answer 90

Acetylcholine

Question 91

Via which structure do fibres of the anterior corticospinal tract decussate?

Answer 91

Anterior spinal commissure