Spinal Cord: Ascending and Descending Pathways

What is a funiculus, and what does it contain?

A region of spinal cord white matter containing tracts or fascicles. Each tract transmits specific types of sensory or motor information

What is the difference between ascending and descending spinal pathways?

Ascending pathways carry sensory (afferent) information toward the brain. Descending pathways carry motor (efferent) information toward the spinal cord

Which pathway carries discriminative touch, deep pressure, vibration, and conscious proprioception?

The dorsal column mediated lemniscus (DCML) pathway

Which pathway carries nociception and temperature? Which carries non-discriminative touch?

The lateral spinothalamic tract carries nociception and temperature. The anterior spinothalamic tract carries non-discriminative touch

Which descending pathways control skilled voluntary movement versus gross movement of proximal and axial muscles?

Lateral corticospinal is precise, skilled voluntary movement

Anterior corticospinal is gross motor function of proximal and axial muscles

Which pathways are associated with unconscious proprioception and involuntary postural control?

Spinocerebellar tracts are unconscious proprioception for balance and motor control

Extrapyrimidal tracts are involuntary motor control, posture, and balance

On an unlabeled spinal cord cross section, identify the posterior, lateral, and anterior funiculi. Which one contains the dorsal columns?

The posterior funiculus is dorsal and contains the dorsal columns. The lateral funiculus lies along the sides. The anterior funiculus lies ventrally

Where is the DCML pathway located within the spinal cord?

The posterior funiculus, also called the dorsal columns

Does the DCML ascend ipsilaterally or contralaterally in the spinal cord? Where does it decussate?

It ascends ipsilaterally in the spinal cord and decussates in the caudal medulla

A DCML signal from the right side of the body ultimately reaches which primary somatosensory cortex?

The left primary somatosensory cortex, because the pathway crosses in the medulla

What are the first, second, and third order neuron locations in the DCML pathway?

First order: dorsal root ganglion

Second order: nucleus gracilis or nucleus cuneatus in the medulla

Third order: ventral posterolateral (VPL) nucleus of the thalamus

The third order neuron projects to the postcentral gyrus

Which fibers enter the medial root entry zone for the DCML pathway, and what do they carry?

Ia fibers: proprioception
Aβ: touch

What happens to DCML first order fibers after entering the medial root entry zone?

They enter the dorsal columns, ascend ipsilaterally, and give off branches that synapse with spinal reflex pathways in laminae IV and V

Fasciculus cuneatus

Lateral; cervical/upper extremity information

Fasciculus gracilis

Medial; thoracic/lower extremity information

Which medullary nucleus receives information from each dorsal column fasciculus?

Fasciculus cuneatus—>nucleus cuneatus

Fasciculus gracilis—>nucleus gracilis

What happens to DCML second order neurons after the first synapse?

They cross the midline in the caudal medulla at the medial lemniscus decussation, then ascend through the pons and midbrain in the thalamus

How are upper and lower limb fibers positioned in the medial lemniscus at the pons and midbrain?

Upper limb: Medial
Lower limb: lateral

Fibers overlap as the decussate in the caudal medulla

Where do DCML second order neurons synapse with third order neurons?

The ventral posterolateral (VPL) nucleus of the thalamus

How are upper and lower limb representations arranged in the VPL

VPL Thalamus: Upper limb is medial, lower limb is lateral

Postcentral Gyrus: Upper limb is lateral, lower limb is medial

Where do DCML third order neurons travel after leaving the VPL?

Through the internal capsule to the corresponding body region representation in the postcentral gyrus

Put the complete DCML pathway in order

Medial root entry zone → ipsilateral fasciculus gracilis/cuneatus → nucleus gracilis/cuneatus in the caudal medulla → medial lemniscus decussation → ascend through pons and midbrain → VPL thalamus → internal capsule → primary somatosensory cortex

How does dorsal column organization change between sacral, thoracic and cervical levels?

Lower body fibers occupy the medial gracile region. At higher spinal levels, additional fibers enter and the lateral cuneate region is present. Identify the sections labeled L2-S3, T2-L2/3, and C4-T1

Where are the spinothalamic tracts located?

In the anterolateral system, within the anterior and lateral funiculi

What modalities are transmitted by the lateral spinothalamic tract?

Nociception and temperature

What modalities are transmitted by the anterior spinothalamic tract?

Non-discriminative touch

Where does the spinothalamic pathway decussate, and which side of the body does a spinal cord tract represent?

It decussates in the spinal cord. After crossing, a spinothalamic tract carries information from the contralateral body

Which peripheral fibers enter the lateral root entry zone, and what modalities does the lecture associate with each?

Aδ: non-discriminative touch, nociception, and temperature
C: nocicpetion, temperature, and itch

What is the first order neuron location for the spinothalamic pathway?

The dorsal root ganglion (DRG)

What happens after spinothalamic first order fibers enter the lateral entry zone?

They enter Lissauer’s tract, ascend 1-2 spinal segments, and synapse with second order neurons in the dorsal horn

Where does the first spinothalamic tract occur?

In the substantia gelatinosa or nucleus proprius of the spinal cord. These are associated with laminaew II and III

Where do spinothalamic second order neurons cross the midline?

Through the anterior white commisure of the spinal cord

What happens after spinothalamic second order neurons decussate?

They ascend in the contralateral spinothalamic tract toward the VPL thalamus

Where do spinothalamic second order neurons synapse with third order neurons?

In the ventral posterolateral (VPL) nucleus of the thalamus

Where do spinothalamic third order neurons terminate?

In the postcentral gyrus, the primary somatosensory cortex, after traveling through the internal capsule

How are upper and lower body fibers organized in the spinothalamic VPL and postcentral gyrus?

VPL thalamus upper limb/cervical is medial, lower limb/sacral is lateral

Postcentral gyrus upper limb/cervical is lateral, lower limb/sacral is medial

Put the complete spinothalamic pathway in order

Lateral root entry zone → Lissauer’s tract (ascend 1–2 segments) → substantia gelatinosa/nucleus proprius → anterior white commissure decussation → contralateral spinothalamic tract → VPL thalamus → internal capsule → primary somatosensory cortex

What are pyramidal tracts, and what type of information do they transmit?

They contain the corticospinal system and transmit motor information

Primary functions of the lateral corticospinal tract

Precise, skilled voluntary motor function

Primary function of the anterior corticospinal tract

Gross motor function of proximal and axial muscles

Where do corticospinal upper motor neurons originate?

The precentral gyrus, which contains the primary motor cortex

Which major structures do corticospinal upper motor neurons pass through while descending from the cortex?

Posterior limb of the internal capsule → cerebral peduncles of the midbrain → pons → pyramids of the medulla

Where does the lateral corticospinal tract decussate?

At the pyramidal decussation in the medulla

In the spinal cord, a lateral corticospinal tract carries commands from which cerebral hemisphere to which side of the body?

From the contralateral primary motor cortex to the ipsilateral body, because its fibers already crossed in the medulla

Describe the anterior corticospinal tract’s projections and decussation

It has bilateral (ipsilateral and contralateral) projections. Its contralateral projections cross in the anterior white commissure of the spinal cord

Where do corticospinal upper motor neurons synapse with lower motor neurons?

In the ventral horn of the spinal cord gray matter

How do lower motor neurons leave the spinal cord and reach their target muscles?

They exit through the ventral root of a spinal nerve and project to their target muscles

Put the complete lateral corticospinal pathway in order

Primary motor cortex → posterior limb of internal capsule → midbrain/cerebral peduncles → pons → medullary pyramids → pyramidal decussation → lateral corticospinal tract → ventral horn upper-to-lower motor neuron synapse → ventral root → target limb muscle

Put the anterior corticospinal pathway in order, emphasizing how it differs from the lateral pathway

Primary motor cortex → internal capsule → brainstem → descend in anterior corticospinal tract → bilateral spinal projections, with contralateral fibers crossing in the anterior white commissure → ventral horn lower motor neurons → ventral root → proximal/axial muscles

What information do the spinocerebral tracts carry, and why is it needed?

Unconscious proprioception, required for balance and motor control

Which four extrapyramidal tracts are listed in the lecture?

Rubrospinal, reticulospinal, olivospinal, and vestibulospinal tracts

What is the rubrospinal funiculus?

Lateral

What is the reticulospinal funiculus

Anterior and lateral

What is the olivospinal funiculus?

Anterior

What is the vestibulospinal funiculus?

Anterior

What are the primary functions of the extrapyramidal tracts?

Involuntary motor control, postural control, and balance

What determines which impairment occurs following a spinal cord injury?

The location of the damage and which sensory or motor tracts are affected

Which major pathways are affected vs relatively spared in a spinal artery stroke?

Pathways such as the spinothalamic and corticospinal tracts are affected, vs the posterior dorsal columns/DCML are relatively spared

Which major pathways are affected by Vitamin B12 deficiency?

The posterior columns/DCML

Brown sequard syndrome

Damage to one side of the spinal cord. The damaged side is the DCML and corticospinal pathways

What effect does a large central cord lesion have?

involves the central gray matter and nearby white matter, including the region of the anterior white commissure where spinothalamic fibers cross.