#16

Question 1

largest descending motor control system in mammals

Answer 1

Corticopontine system

Question 2

the corticopontine system is comprised of fibers originating from all parts of the cerebral cortex, passing inferiorly through the X to terminate on Y in the basis pontis.

Answer 2

X internal capsule and basis pedunculi
Y deep pontine nuclei

Question 3

The corticopontine system conveys the “motor plan” and integrated sensory data to the

Answer 3

cerebellum

Question 4

Pontine nuclei send
projections contralaterally
into the X to participate in
cerebellar processes.

Answer 4

middle cerebellar
peduncle

Question 5

There are five main descending motor systems that reach the spinal cord

Answer 5

• Corticospinal system
• Reticulospinal system
• Vestibulospinal system
• Tectospinal system
• Rubrospinal system

Question 6

A few comments applicable to all of the decending motor systems:

Answer 6

• The corticospinal (and corticobulbar) tract is referred to as the pyramidal tract. All the rest are extrapyramidal tracts.
• Damage to any of these will result in upper motor neuron signs, but there are some differences in pyramidal vs. extrapyramidal signs, and pyramidal signs are by far the most common
  and easiest to interpret in patients.

Question 7

Corticospinal and corticobulbar tracts
origin:
UMN:
Course:

Answer 7

• Origin from cerebral cortex: mostly motor cortex (M1) but also from the premotor cortex (PMA and SMA) and parietal cortex (PC)
• Upper motor neurons: M1 (primary motor cortex) contains a representation of the opposite side of the body (motor homunculus) with roughly 40% dedicated just to the hand (particularly the thumb) and the jaw and lower face muscles
• Course: Fibers (upper motor neurons) descend through the posterior limb of the internal capsule, a major white matter bundle in the inferomedial telencephalon
  containing all descending fibers from the cortex, as well as ascending fibers from the thalamus. The somatotopic organization of corticospinal fibers is maintained through the internal capsule, as reiterated below, with the arm represented anteriorly (closest to the genu) and the leg represented posteriorly (farthest from the genu). The genu of the internal capsule contains descending corticobulbar fibers.

Question 8

Somatotopy is maintained as the

Answer 8

corticobulbospinal fibers descend into the basis pedunculi (cerebral peduncles) of the midbrain, and the system stays ventral as it descends through the brainstem

Question 8

Somatotopy is maintained as the

Answer 8

corticobulbospinal fibers descend into the basis pedunculi (cerebral peduncles) of the midbrain, and the system stays ventral as it descends through the brainstem

Question 9

Most (90%) of corticospinal fibers cross in the

Answer 9

pyramids, forming the lateral corticospinal tract

Question 10

Somatotopy of the lateral corticospinal tract within the spinal cord is similar to the
spinothalamic tract:

Answer 10

leg on outside, arm on inside:

Question 11

Targets of lateral corticospinal fibers

Answer 11

: Majority of lateral corticospinal
fibers end in the intermediate zone; about 20% end on alpha motorneurons, particularly in the dorsolateral part of the ventral horn in the cervical enlargement

Question 12

Intrinsic hand muscle motorneurons are located in the dorsal part of the ventral horn at

Answer 12

C7-T1 and their direct innervation by corticospinal fibers is thought to allow individual and fractionated movements of the hand and
forearm.

Question 13

The (uncrossed) anterior corticospinal pathway is thought to mediate

Answer 13

voluntary control over posture by acting on alpha motor neurons in the medial part of the ventral horn. The exact function is controversial.

Question 14

Function of lateral corticospinal tract

Answer 14

The lateral corticospinal tract exerts
the strongest influence on flexor muscle groups and is thought to be make adaptive modifications to spinal CPGs, such as when
attempting to navigate complex physical environments

Question 15

: Supraspinal activation of gamma motor neurons by extrapyramidal tracts (brainstem UMNs)
leads to

Answer 15

stretching of muscle spindles, which activates alpha motor neurons via the stretch reflex, partially
contracting muscles and increasing tone

Question 16

These extrapyramidal pathways (reticulospinal, vestibulospinal, and tectospinal) all originate in the brainstem and work together to

Answer 16

adjust tone and posture, each
contributing in different ways.

Question 17

Pontine (medial) reticulospinal tract (MRST)
origin:
spinal course:
targets:
functions:

Answer 17

• Origin: pontine tegmentum
• Spinal course: anterior funiculus
• Targets: mostly interneurons
• Functions: Facilitates anti-gravity, extensor muscles
  and increases muscle tone (gamma motor system)

Question 18

Medullary (lateral) reticulospinal tract (LRST)
origin:
spinal course:
targets:
functions:

Answer 18

Origin: medullary reticular formation
* Spinal course: lateral funiculus
* Targets: mostly interneurons
* Functions: Suppresses extensor muscle activity and
reduces muscle tone (gamma motor system)

Question 19

retricospinal tracts control the….

Reticulospinal tracts also coordinate….

Answer 19

gamma motor
system (thus regulating muscle tone), and activity in these pathways is modulated by the
cerebellum, as we’ll see, making the
reticulospinal system a critical player in motor control

Reticulospinal neurons are also involved in “switching on” and integrating central pattern
generators (CPGs) by facilitating (MRST) or inhibiting (LRST) stereotypic movements (e.g.,
reaching and grasping) and compound limb movements (both upper and lower limbs). This
has an important facilitatory effect on generating locomotor patterns.

Question 20

Lateral vestibulospinal tract:
origin:
target:
function:

Answer 20

• Origin: Lateral vestibular nucleus (ipsilateral)
• Target: Ipsilateral intermediate zone and medial motor neuron groups
• Function: Excites gamma motor neurons, which increase tone in trunk and proximal limb extensors,
  maintaining upright posture (vestibulo-spinal reflex)

Question 21

Medial vestibulospinal tract:
origin:
target:
function:

Answer 21

• Origin: Medial vestibular nucleus (bilateral)
• Target: Bilateral intermediate zone and medial motor neuron groups for neck and shoulder muscles, ends at T1
• Function: Maintenance of head in upright position (vestibulo-collic reflex)

Question 22

TECTOSPINAL TRACT
origin:
target:
function:

Answer 22

Origin: superior colliculus (midbrain).
Crosses immediately in midbrain.
Targets: axial alpha motors neurons controlling neck muscles..
Function: reflexive head turn toward/away from visual or auditory stimulus. This actions is velocity dependent – at most velocities,
tectospinal reflexes will turn the head toward a novel stimulus. Above threshold velocities, tectospinal activation is protective, turning the
head away from fast-approaching stimuli.

Question 23

The corpora quadrigemina – the 4 colliculi

Answer 23

The tectum consists of the superior and inferior colliculi. The superior colliculus has a retinotopic map
giving it the ability to localize objects in visual space and make eye and head movements towards/away from the object. The inferior colliculus maps sound in space and is
an obligatory relay for auditory information going to the
thalamus, but it also sends signals to the superior colliculus. Auditory and visual stimuli are integrated in the superior colliculus, which drives the tectospinal tract to turn the head toward/away from stimuli of either type.

Question 24

RUBROSPINAL TRACT Origin Target Function

Answer 24

Origin: red nucleus (midbrain), which itself gets inputs from numerous cortical areas including premotor and primary motor cortex. Crosses immediately in midbrain. Targets: intermediate zone interneurons that synapse on distal limb alpha MNs in neck and upper limb only Function: facilitates flexor activity (like corticospinal tract) in the cervical spinal cord, but only in a coarse manner.

Question 25

the red nucleus is a site of significant convergence of inputs from

Answer 25

both cortex and cerebellum, much like primary motor cortex M1, suggesting that M1, red nucleus, and the cerebellum form a recurrent network involved in the feedback control of voluntary motor actions

Question 26

The red nucleus plays a significant role in the generation of

Answer 26

primitive grasp reflexes of infants, and is heavily engaged in crawling, presumably through activity in the rubrospinal tract (which influences lower motor neurons in the cervical spinal cord, so upper limb, but not lower limb

Question 27

decorticate posture

Answer 27

the lesion is in the rostral midbrain or higher, above the red nucleus. Rubrospinal biased flexion is thought to override vestibulospinal and reticulospinal biased extension, but only for the upper limb. In decorticate rigidity, the red nucleus is spared, so damage is rostral to the midbrain.

Question 28

decerebrate posture

Answer 28

an upper pontine lesion, activity in the intact vestibulospinal system (arising lower down in the pontomedullary region) is biased for extension, causing rigidity of all four limbs. In decerebrate rigidity, the red nucleus is damaged bilaterally, so damage involves the midbrain or down to the mid-pons

Question 29

Spasticity

Answer 29

paresis (weakness) Hypertonia Hyperreflexia Spasticity indicates pyramidal damage More resistence in one direction Velocity dependent

Question 30

Rigidity if the basal ganglia is involved?

Answer 30

Extrapyramidal damage or basal ganglia damage same resistence in both sides not velocity dependent When the basal ganglia is involved (as in Parkinson’s disease), a “cogwheel” phenomenon can often be appreciated. This is a circular, jerking rigidity in flexion and extension in the background of tremor, which continues throughout an entire range of movement.

Question 31

Hyper-reflexia in upper motor neuron disease is due to

Answer 31

loss of supraspinal inputs to neurons that inhibit α / γ motor neurons Loss of excitatory corticospinal fiber innervation to inhibitory interneurons, which inhibit α and γ motor neurons via spindle type II pathways, is one explanation for exaggerated stretch reflex associated with UMN signs. Under these circumstances, fast-adapting Type Ia fibers encode changes in muscle length and velocity without temporally precise reference to immediate length. Result: hyperreflexia.

Question 32

Clonus

Answer 32

Clonus is a pattern of involuntary and rhythmic muscle contraction resulting from lesion in descending motor pathways. The exact mechanism remains unclear. The two most widely accepted hypotheses illustrate concepts introduced in this unit: 1) Clonus is caused by hyperactive stretch reflexes via self-excitation, producing rhythmic stimulation of lower motor neurons. 2) Clonus is caused by central pattern generator activity arising as a consequence of peripheral sensory events, producing the rhythmic effect. Clonus may be found at the ankle, knee, wrist, jaw, and elbow, but can occur in any muscle with a frequency of 5-8 Hz.