11 - Motor II Flashcards
For this lecture only, you do not have to memorize contralateral/ipsilateral
FYI
Origin of corticotectal tract
Origin: visual association cortex, of occipital lobe.
Termination of corticotectal tract
Termination: in the following nuclei of the midbrain:
- Oculomotor accessory nuclei
- Superior colliculus
Describe the oculomotor accessory nuclei
These nuclei project via the medial longitudinal fasciculus (MLF) to the nuclei that innervate the extraocular muscles – to control reflex movements of the eyes
Describe the superior colliculus
- Gives rise to the tectospinal tract, it crosses, and terminates in the cervical and thoracic spinal cord levels
- Is associated with reflex movements of the head and upper thorax, in response to visual, auditory, vestibular or cutaneous stimuli
Example of corticotectal tract: Auditory
Auditory : (pathway)
- Loud noise (auditory stimulus) in back of classroom
- Ears
- Auditory nuclei in brainstem
- Inferior colliculi
- Superior colliculi
- Tectospinal tract
- Neck and upper thoracic spinal cord levels
- Reflex turning of head, and upper thorax toward back of room to “see” what the source of startling noise may be
Example of corticotectal tract: Cutaneous
Cutaneous : While working in the attic and looking to the left, you feel
something crawling (cutaneous stimulus) on the back of your right hand.
You quickly turn your head to look and see what is on the back of your
hand…and you see a spider…!
Sensory information from the hand is transmitted to the…
1 - Reticular formation (r.f.)
It projects to the → superior colliculus which gives rise to the → tectospinal fibers that project to the upper cervical and thoracic
spinal cord to cause reflex turning of the head, and upper thorax in the direction of the surprising cutaneous stimulus to “see” the source of the sensation.
2 - Sensory cortex
Projects to the → visual cortex → which gives rise to corticotectal fibers that descend to the oculomotor accessory
nuclei (that will cause reflex turning of the eyes)
Relationship between corticorubral and rubrospinal tract
- The sensorimotor cortex gives rise to fibers that form the corticorubral tract.
- The corticorubral tract terminates in the red nucleus of the midbrain.
- The red nucleus gives rise to the rubrospinal tract.
- It crosses in the midbrain, and descends to the upper spinal cord.
Hint: cortico=cortex, rubral=red nucleus, spinal=spinal cord… the names tell you where they run
Function of rubrospinal tract
Although present, is relatively insignificant in the human and only functions to supplement the more important corticospinal tract (more important in primates for swinging on trees - flexing hand at wrist and fingers)
Functions: flexing hand and digits
- facilitating motor neurons that innervate the distal flexor muscles of the UL.
- inhibiting motor neurons that innervate the distal extensor muscles of the UL.
Does the rubrospinal tract contain UMNs or LMNs?
The rubrospinal tract consists of UMN’s. A lesion will result in UMN signs.
Reticulospinal system stimulation
The reticulospinal system is stimulated by ipsilateral…
- descending cortical projections (corticoreticular fibers)
- input from the cerebellum and vestibular nuclei (to influence muscle tone)
- ascending somatosensory projections (spinoreticular fibers) relaying nociception
Describe how the reticulospinal tract forms
- The sensorimotor cortex gives rise to the corticoreticular fibers. These fibers descend with the corticonuclear and corticospinal tract fibers.
- Corticoreticular tract descends ipsilaterally, to terminate in the brainstem reticular formation (in the pons and medulla).
- Pontine nuclei give rise to the medial (pontine) reticulospinal tract which
descends with ipsilateral predominance to terminate at all spinal cord levels. - In the spinal cord, it descends in the anterior funiculus and then its axons
synapse in the gray matter with interneurons and gamma motor neurons.
Function of medial (pontine) reticulospinal tract
The medial (pontine) reticulospinal tract has an excitatory influence on motoneurons that innervate paravertebral and limb extensors.
Is simultaneously inhibitory to the flexors.
Formation and pathway of lateral (medullary) reticulospinal tract
- A medullary nucleus gives rise to the lateral (medullary) reticulospinal tract
which descends mostly ipsilaterally to terminate at all spinal cord levels. - In the spinal cord, it descends in the anterior funiculus and then its axons synapse in the gray matter with interneurons.
Function of lateral (medullary) reticulospinal tract
The lateral (medullary) reticulospinal tract has an inhibitory influence on motoneurons that innervate the paravertebral and limb extensors.
Is simultaneously excitatory to the flexors.
Does the reticulospinal tract contain UMNs or LMNs?
Reticulospinal tracts consist of UMN’s. A lesion will result in UMN signs.
What do BOTH reticulospinal tracts have in common?
medial pontine and lateral medullary
Both reticulospinal tracts:
• Are controlled by corticoreticular fibers.
• Arise from the reticular formation.
• Descend to terminate at all spinal cord levels.
• Terminate on interneurons which synapse with alpha and gamma motor neurons that innervate the antigravity muscles (paravertebral and proximal limb (girdle) musculature – not fingers).
• They play a role in modulation of muscle tone and posture maintenance
via commands that arise from the basal ganglia and cerebellum (the basal ganglia and cerebellum project to the thalamus which in turn influences the activity of the UMN’s)
Vestibulospinal tract origin
Medial and inferior vestibular nuclei
Vestibulospinal tract termination
Bilaterally at cervical and upper thoracic spinal cord levels alpha and gamma motoneurons that innervate the neck muscles
Function of vestibulospinal tract
Maintain equilibrium (balance) in response to vestibular input
Control head movement to keep an image on the retina
There is another section of the vestibulospinal tract. Where does it originate?
Lateral vestibular nucleus
receives input from the vestibular apparatus. Information is used to coordinate orientation of the head and body via the LVST
Termination of lateral portion of vestibulospinal tract
Ipsilaterally at all spinal cord levels interneurons (mostly), and alpha motoneurons
Function of lateral portion of vestibulospinal tract
Excitatory to antigravity muscles (trunk paravertebral and proximal limb
extensor muscles)
Simultaneously inhibit LMN’s that innervate limb flexor muscles.
Function in the maintenance of posture and balance.
What are the three functional classifications of the descending motor pathways?
- Ventromedial (Anteromedial) Group
- Lateral Group
- Cortical Group
Ventromedial (Anteromedial) Group
Tracts consist of:
• anterior corticospinal tract
• medial and lateral vestibulospinal tracts
• medial and lateral reticulospinal tracts
• tectospinal tract
Bilateral termination in spinal cord
Influence gross movements of the axial (trunk) and proximal limb (girdle) musculature to maintain balance as postural adjustments are made
Lateral Group
Tracts consist of:
• Lateral corticospinal tract
• Rubrospinal tract
The rubrospinal tract is rudimentary and clinically not very important in humans
Tracts influence mostly the distal muscles of the limbs (muscles that control digits)
Cortical Group
Consists ONLY of axons of the lateral corticospinal tract which terminate on lower motoneurons (but not interneurons) that innervate the distal muscles of the limbs, especially the muscles of the hand
These axons mediate distinct, fractionated movements of the digits (as in playing the piano)
Sequence of motor neuron bodies in ventral horn from medial to lateral
Ventral horn contains nerve cell bodies of motor neurons that innervate
muscles in this sequence (from medial to lateral : T S A F H)
T = trunk S = shoulder A = arm F = forearm H = hand
Brown-Séquard Syndrome
Results following hemisection of the spinal cord and is characterized by the following:
- Loss of discriminative sense AT and BELOW the level of the lesion, ipsilaterally.
- Loss of pain and temperature sensation, about 2 segments below the level of the
lesion, contralaterally. - Loss of reflex activity involving the level of the lesion, ipsilaterally.
- UMN signs below the level of the lesion, ipsilaterally.
- LMN lesion signs at the level of the lesion, ipsilaterally.
What accounts for the loss of discriminative sense AT and BELOW the level of the lesion, ipsilaterally?
because the FG and FC ascend on ipsilateral side of spinal cord, so loss is on the same side as the lesion
What accounts for the loss of pain and temperature sensation, about 2 segments below the level of the lesion, contralaterally?
because the central processes of first order neurons enter the spinal cord and
bifurcate into short ascending and descending branches which ascend or descend 1 – 3 spinal cord levels. Fibers coming from below or bringing input from lower levels are severed. Always go backward to determine the side / location of the deficits
Polyomyelitis
A. Viral infection of the spinal cord
B. Affects LMN’s
C. Results in LMN signs
Combined Systems Disease
(Subacute Combined Degeneration, Posterolateral Sclerosis) cause
Results from deficiency in vitamin B12 intake or metabolism
Combined Systems Disease
(Subacute Combined Degeneration, Posterolateral Sclerosis) characteristics
Characterized by degeneration of the…
- Dorsal funiculus (containing sensory fibers of the FG and FC)
- Lateral funiculus (containing motor fibers of the corticospinal tract)
What happens when there is degeneration of the dorsal funiculus in combined systems disease?
Damage to these fibers results in loss of discriminative touch, two-point discrimination, vibratory sense, and proprioception, and sensory ataxia
What happens when there is degeneration of the lateral funiculus in combined systems disease?
Damage to these fibers results in UMN signs (Babinski sign, hyperreflexia, muscle weakness, and spasticity)
Three arteries of the spinal cord
- Anterior spinal artery
- Posterior spinal artery
- Spinal branches of segmental arteries
What does the anterior spinal artery supply?
Supplies the anterior 2/3 of the spinal cord (the anterior and lateral funiculi and most of the gray matter).
What do the posterior spinal arteries supply?
Note: there are 2 of them
The 2 posterior spinal arteries supply the posterior 1/3 of the spinal cord (the dorsal funiculi and the posterior part of the dorsal horns)
How do the spinal branches of segmental arteries divide and branch off?
- divide into dorsal and ventral radicular and dorsal and ventral spinal medullary arteries
What do the radicular arteries supply?
The radicular arteries supply the spinal nerve roots
What do the medullary arteries supply?
The medullary arteries supply the spinal cord
What anastomoses occur BELOW the cervical line?
This means they occur in the thoracic, lumbar and sacral regions…
The radicular arteries anastomose with the anterior and posterior spinal arteries
What anastomoses occur ABOVE the cervical line? What is the clinical implication of this?
In contrast, the cervical region of the spinal cord is supplied ONLY by the anterior and posterior spinal arteries. Thus, without the contribution of the radicular arteries’ anastomoses, the spinal cord is most susceptible (to damage from interruption of blood flow in the anterior or posterior spinal arteries), at cervical levels. So if the anterior or posterior spinal arteries become occluded or compressed in the cervical region, there is no backup from the radicular anastomoses and the spinal cord neural tissue will be damaged
Anterior spinal artery syndrome
Occlusion by a thrombus, or compression of the anterior spinal artery may…
a. damage the anterolateral system /pathways causing loss of pain and temperature sensation several levels below the level of the lesion
b. damage the anterior horn cells producing lower motor neuron weakness
c. produce a large lesion affecting the lateral corticospinal tracts, producing upper motor neuron signs
d. damage the descending pathways controlling sphincter function and cause incontinence
Posterior spinal artery syndrome
Results from occlusion or compression of one or both of the posterior spinal arteries.
a. lesions of the posterior white columns will result in loss of proprioception (position sense from muscles and joints), loss of vibratory sense and loss of tactile (fine touch) discrimination
Case 11
A 45-year-old man complains of left foot drop which started 5 months ago. This was very subtle at first, but has been getting progressively worse. The patient can now barely lift his left foot up. He walks with difficulty. More recently he has noticed some clumsiness of his left hand. Upon further questioning he admits to frequent “twitching” of the muscles in his distal lower limbs (particularly in the calf), forearms, and shoulders. His family thinks he has lost some weight. There is no numbness, tingling, pain, or any other neurologic symptom noted. Family history is negative for any neurologic disorder. Examination shows severe weakness and flaccidity of left ankle dorsiflexion and mild weakness of the intrinsic left hand muscles and left finger extension. Otherwise, strength seems adequate. There is atrophy noted in the muscles of the left hand and the left pretibial muscles. Fasciculations are noted in several muscle groups of the legs, arms, and back. Deep tendon reflexes are pathologically brisk in the arms and legs bilaterally. A Babinski response is present bilaterally. The rest of the neurologic exam was unremarkable.
