Week 10 - The Ascending & Descending Tracts Flashcards
What is the difference between upper and lower motor neurons?
Upper motor neurons (UMNs) arise above the level of the pyramidal decussation and innervate lower motor neurons (LMNs) in the spinal cord or brainstem. LMNs are located in the motor nuclei of the brainstem and anterior horn of the spinal cord.
Where do corticospinal tracts originate and what is their function?
Corticospinal tracts originate in the cerebral cortex and control fine movements and voluntary motor functions.
Where do corticobulbar tracts originate and what is their function?
Corticobulbar tracts originate in the cerebral cortex and project to lower motor neurons in the brainstem, influencing facial and neck muscle movements.
What are the paths and functions of the reticulospinal tracts?
Reticulospinal tracts originate in the brainstem and influence posture and limb movements by modulating spinal reflexes.
What are the paths and functions of the vestibulospinal tracts?
Vestibulospinal tracts arise from the vestibular nuclei in the brainstem and regulate posture and balance by controlling antigravity muscles.
What is the function of the rubrospinal tract?
The rubrospinal tract originates in the red nucleus and facilitates flexor muscle activity while inhibiting extensor muscles.
What is the function of the tectospinal tract?
The tectospinal tract originates in the superior colliculus and coordinates head and eye movements in response to visual stimuli.
What layers of neurons are primarily involved in the motor cortex?
The motor cortex contains granular cells, fusiform cells, and pyramidal cells, with pyramidal cells being the primary output neurons.
What is the role of columns in the motor cortex?
Columns in the motor cortex function as integrative processing systems, stimulating groups of synergistic muscles.
What are the two main types of descending motor pathways?
The two main types are corticospinal tracts and corticobulbar tracts.
Fill in the blank: The corticospinal tract is involved in the control of _______.
[fine movements]
True or False: The anterior corticospinal tract primarily innervates distal muscle groups.
False
What happens to muscle tone after a stroke affecting the corticospinal tract?
Initially, affected muscles lose tone, and after several days, they may become spastic and exhibit hyperreflexia.
What is the Babinski sign indicative of?
The Babinski sign indicates damage to the corticospinal tract.
What are the symptoms of lower motor neuron paralysis?
Symptoms include loss of muscle tone, muscle atrophy, and loss of all reflex and voluntary movement.
What is the significance of giant Betz cells in the corticospinal tract?
Giant Betz cells send branches back to the cerebral cortex to inhibit activity in adjacent regions, sharpening the excitatory signal.
What is the function of the anterior corticospinal tract?
The anterior corticospinal tract is important for bilateral postural adjustments during voluntary movements.
Where do the fibers of the lateral corticospinal tract primarily terminate?
They primarily terminate on interneurons in the intermediate zone of the spinal cord gray matter.
What is the relationship between the red nucleus and the corticospinal tract?
The red nucleus receives fibers from the corticospinal tract and serves as an accessory route for motor control.
True or False: The corticospinal tract is the only pathway for voluntary movements.
False
What occurs when corticospinal fibers are destroyed but the corticorubrospinal pathway remains intact?
Discrete movements can still occur, but fine control of the hands and fingers is impaired.
What is the main role of the descending pathways originating from the brainstem?
They exert different influences on spinal motor neurons, affecting flexor and extensor reflexes.
What are the indicators of upper motor neuron paralysis?
Movement, spasticity, increased tendon reflexes, loss of superficial reflexes, and Babinski sign.
These symptoms indicate damage to the upper motor neurons.
What are the symptoms of lower motor neuron paralysis?
Loss of muscle tone, muscle atrophy, loss of all reflex and voluntary movement.
Lower motor neuron paralysis indicates damage to the lower motor neurons.
What is the corticonuclear tract?
A system of cortical neurons that influence movements of striated muscles innervated by cranial nerve motor nuclei.
Formerly known as the corticobulbar tract.
Where do corticonuclear tract fibers originate?
Lateral aspect of the primary motor, premotor, supplementary motor, and somatosensory cortex.
These fibers convey commands for voluntary movements to the brainstem.
What is the primary function of the rubrospinal tract?
Facilitates neurons that innervate flexor muscles of the upper limb and inhibits extensor antigravity muscles.
It primarily influences upper limb movement.
What is the origin of the rubrospinal tract?
The red nucleus in the tegmentum of the midbrain.
It crosses to the contralateral side at the ventral tegmental decussation.
What is the primary role of the medial reticulospinal tract?
To facilitate voluntary movements and increase motor tone by acting on gamma motor neurons.
It excites axial muscles to support the body against gravity.
What is the function of the lateral reticulospinal tract?
Inhibits voluntary movements and reduces muscle tone by acting on gamma motor neurons.
It counterbalances the excitatory effects of the medial reticulospinal tract.
What is decerebrate posturing?
A posture characterized by extended arms and legs due to unopposed hyperactivity in extensor musculature.
It occurs in patients with lesions at the level of the midbrain or rostral pons.
What is the medial vestibulospinal tract responsible for?
Causes rotation and lifting of the head as well as rotation of the shoulder blade around its axis.
It helps maintain appropriate orientation in response to posture and balance changes.
What characterizes the lateral vestibulospinal tract?
It arises from the lateral vestibular nucleus and projects to all levels of the ipsilateral spinal cord.
It plays a role in postural stabilization.
What is the tectospinal tract associated with?
Orientating the head and neck during eye movements in response to visual stimuli.
It arises from the superior colliculus of the midbrain.
What is the olivospinal tract thought to influence?
The activity of motor neurons in the anterior grey column of the spinal cord.
Its existence is now considered doubtful.
What are the primary cranial nerves influenced by the corticonuclear tract?
Cranial nerves V, VIII, XII, IX, X, and XI.
These nerves are responsible for various motor functions in the face and head.
True or False: The corticonuclear tract fibers cross in the pyramidal decussation.
False.
Most corticonuclear fibers innervate cranial nerve motor nuclei bilaterally.
Fill in the blank: The reticulospinal tract is described as the most _______ descending motor system.
[primitive].
It includes components from the reticular formation throughout the brainstem.
What happens to the tongue when there is a lesion in the corticonuclear fibers to the hypoglossal nucleus?
The tongue deviates towards the weak side (contralateral to the lesion) when protruded.
This is due to the unopposed pull of the intact muscle.
What is the function of the anterior white column in the spinal cord?
Innervates motor neurons responsible for neck movements
It orients the head and neck during eye movements in response to visual stimuli.
What is the current status of the olivospinal tract?
Considerable doubt exists about its existence
It was thought to arise from the inferior olivary nucleus and influence motor neurons.
Where do descending autonomic fibers arise from?
Cerebral cortex, hypothalamus, amygdaloid complex, reticular formation
They descend in the lateral white column of the spinal cord.
What are intersegmental tracts?
Short ascending and descending tracts within the spinal cord
They interconnect neurons of different segmental levels and are important in intersegmental spinal reflexes.
What occurs during spinal cord transection?
Loss of muscle tone, motor function, reflex activity, visceral sensation, and somatic sensation below the transection
Spinal shock occurs immediately after damage.
What is Brown-Séquard Syndrome?
Hemisection of the spinal cord leading to dissociation of function between proprioception and pain/temperature sensations
Loss of proprioception occurs ipsilaterally and loss of pain/temp sensations occurs contralaterally.
What characterizes Amyotrophic Lateral Sclerosis (ALS)?
Degeneration of corticospinal tracts and ventral horn cells
Results in muscle weakness, fasciculations, and impaired bladder/bowel function.
What is syringomyelia?
Expansion of the central canal of the spinal cord
Causes segmental loss of pain and thermal sensation while tactile sensation is preserved.
What is tabes dorsalis?
Late consequence of syphilitic infection affecting the nervous system
Characterized by degeneration of dorsal root ganglion neurons and loss of vibration and proprioception.
What is combined systems disease?
Result of vitamin B12 deficiency affecting the spinal cord
Symptoms include tingling, loss of vibration/position senses, and upper motor neuron dysfunction.
What happens when three consecutive dorsal roots are sectioned?
Abolition of all sensory function in a particular dermatome
One dorsal root section does not cause much sensory loss due to overlap between dermatomes.
What is the effect of lesions on the ventral root?
Abolish or impair motor functions of affected segment muscles
Sectioning the ventral root of C8 results in weakness of hand muscles.
What are spinal tracts or fasciculi?
Bundles of fibres that have the same origin, course, and termination
What are the three main regions of spinal cord tracts?
Dorsal, ventral, and lateral funiculi
What is the primary function of the dorsal column/medial lemniscal system?
Pathway for discriminative touch, vibratory sense, and proprioception
Where do the axons of neurons mediating tactile sensation and proprioception ascend in the spinal cord?
In the ipsilateral dorsal columns to the medulla
What are the two fascicles of the dorsal columns at upper spinal levels?
- Gracile fascicle
- Cuneate fascicle
What does the fasciculus gracilis contain?
Ascending fibres from sacral, lumbar, and lower thoracic segments T6-T12
What is the role of neurons in the dorsal root ganglia?
Represent the 1st order neurons and innervate sensory receptors
What happens to axons of the 2nd order neurons in the dorsal column pathway?
They cross the midline to form the medial lemniscus
What are the symptoms of damage to the dorsal columns?
- Loss of tactile sense
- Loss of kinaesthetic sense
What is the primary pathway believed to transmit pain and temperature sensations?
The spinothalamic tract
What are the two components of the spinothalamic tract?
- Direct pathway
- Indirect pathways
What is the role of the neospinothalamic tract?
Encodes location, intensity, and quality of pain
What is the consequence of sectioning the neospinothalamic pathway at the cervical level?
Complete loss of pain, temperature, and simple tactile sensation on the contralateral side
What does the paleospinothalamic tract transmit?
Sensory information to the reticular formation and limbic system
What is the function of the spinoreticular tract?
Transmits sensory information to the reticular formation influencing levels of consciousness
What is the primary function of the dorsal spinocerebellar tract?
Transmits muscle joint sense pathways to the cerebellum
Where does the dorsal spinocerebellar tract synapse?
In the nucleus dorsalis of Clarke
What type of proprioceptive information does the dorsal spinocerebellar tract provide?
Nonconscious proprioception regarding muscle status
What is unique about the anterior spinocerebellar tract?
It crosses twice, once in the spinal cord and again in the superior cerebellar peduncle
What types of receptors do the first order neurons of the anterior spinocerebellar tract innervate?
Golgi tendon organs
Fill in the blank: Damage to the dorsal spinocerebellar tract results in loss of _______.
Nonconscious proprioception and coordination
What is the path of the anterior spinocerebellar tract?
It crosses first in the spinal cord and again when it joins the superior cerebellar peduncle, terminating in the vermis of the anterior lobe of the cerebellum
What types of inputs does the anterior spinocerebellar tract receive?
It receives input from modulating descending tracts to lower motor neurons and from reflex flexor arcs
What is the role of descending pathways in the anterior spinocerebellar tract?
They adjust the output from lower motor neurons independently from the corticospinal tract
What information does the anterior spinocerebellar tract convey to the cerebellum?
Information about whole limb movement and postural stability
What is the consequence of damage to the anterior spinocerebellar tract?
Loss of nonconscious proprioception and coordination in the lower limb
Where do afferent fibers of the cuneocerebellar tract originate?
They enter the spinal cord above C8 and ascend ipsilaterally in the fasciculus cuneatus
What is the function of the cuneocerebellar tract?
It conveys nonconscious proprioception related to the upper limb
How do fibers in the cuneocerebellar tract reach the cerebellum?
They enter through the inferior cerebellar peduncle after synapsing in the accessory cuneate nucleus
What happens to the processing in the accessory cuneate nucleus?
It is analogous to the processing in Clarke’s column
What is the similarity between the rostral spinocerebellar tract and the ventral spinocerebellar tract?
The course of the tract is similar; both convey proprioceptive information
What type of proprioceptive input does the rostral spinocerebellar tract receive?
Input from Golgi tendon organs located in the upper limb
What is the consequence of damage to the rostral spinocerebellar tract?
Loss of nonconscious proprioception and coordination in the ipsilateral upper limb
What does the spinotectal tract provide information for?
Spinovisual reflexes and movements of the eyes and head toward the source of stimulation
Where do axons of the spinotectal tract terminate?
They synapse with neurons in the superior colliculus of the midbrain
What is the origin of the spino-olivary tract?
Axons enter the spinal cord from the posterior root ganglion
Where do the axons of the spino-olivary tract synapse?
They synapse on 3rd order neurons in the inferior olivary nucleus of the medulla oblongata
What type of information does the spino-olivary tract convey to the cerebellum?
Information from cutaneous and proprioceptive organs
