White Matter - Ascending and Descending Tracts

Question 1

White Matter - Definition and Division

Answer 1

Collection of myelinated nerve fibers that carry signals up and down, to and from the brain.
3 pairs of Columns or Funiculi:
- Dorsal (posterior)
- Lateral
- Ventral (anterior)
Each column (funiculi) is filled with tracts (fasciculi)

Question 2

Tracts (Fasciculi)

Answer 2

Described according to the funiculus in which they are located.
CNS uses ascending and descending tracts to communicate with external environment.

Question 3

First Order Neuron

Answer 3

Carry signals from the periphery(environment) to the spinal cord.

Question 4

Second Order Neuron

Answer 4

Carry Signals from the spinal cord to the thalamus.

Question 5

Third Order Neuron

Answer 5

Carry signals from the thalamus to the primary sensory cortex.

Question 6

Ascending Tracts - Function and Characteristics

Answer 6

Transport sensory information in afferent pathways from the body to the brain.
Bilateral paired structures.
Touch (pain, thermal changes, pressure, light/crude touch, vibration, two-point discrimination, proprioception) transmitted through ascending tracts.

Question 7

Known Ascending Tracts

Answer 7

8
Dorsal column and medial lemniscus
Spinothalamic tracts and spinoreticular
Spinocerebellar tracts

Question 8

Dorsal Column Medial Lemniscus (DCML) - Function and Divisions

Answer 8

Transports information about vibration, proprioception, fine touch and two-point discrimination.
Divided in two regions:
Fasciculus gracilis (medial): info from below T6-T8, lower limbs.
Fasciculus cuneatus (lateral): info from aboveT6-T8 but below the head, upper limbs.

Question 9

Dorsal Column Medial Lemniscus - Pathway (first, second, third order neuron)

Answer 9

First-order neuron: Ipsilateral: peripheral sensation → dorsal root → dorsal horn of the spinal cord → fasciculus gracilis or cuneatus (dorsal column)
Second-order neuron (decussation): Ipsilateral: Nucleus gracilis or cuneatus → Contralateral: medial lemniscus after NG/NC.
Third-order neuron: Contralateral: Ventralposterolateral thalamus → thalamocortical tract → somatosensory cortex

Question 10

Spinothalamic Tract - Function and Division

Answer 10

Anterior (ventral): crude (non-discriminative) touch and pressure changes .
Lateral: painful and thermal stimuli.

Accompanied with spinoreticular pathway.

Question 11

Spinothalamic Tract - Pathway (first, second, third order neuron)

Answer 11

First-order neuron: Ipsilateral: Peripheral sensation → Dorsal root → Dorsal horn of spinal cord.
Second-order neuron (decussation): Ipsilateral: Substantia gelatinosa → Contralateral: Anterior grey commissure (anterior funiculus) → Spinothalamic tracts.
Third-order neuron: Contralateral: Ventralposterolateral thalamus → Thalamocortical tract → Somatosensory cortex (postcentral gyrus).

Question 12

Spinoreticular Pathway - Function

Answer 12

Pain signals from tissue injury.
Pathway is the same as the spinothalamic. Decussation in the spinal cord and ascend with the spinothalamic tract, end in the reticular formation (medulla and pons).
3rd and 4th order neurons continue to thalamus and somatosensory cortex (postcentral gyrus).

Question 13

Spinocerebellar Tract - Function and Division (4)

Answer 13

Transmit unconscious proprioceptive signals from limbs and trunk to the cerebellum. Info about muscle stretch and rate of muscle stretch from GTO and muscle spindle.
4 Tracts:
Dorsal spinocerebellar: MS and some GTO from lower limb.
Cuneocerebellar: MS and some GTO from upper limb.
Ventral spinocerebellar: GTO from lower limb.
Rostral spinocerebellar: GTO from upper limb.

Question 14

Spinocerebellar Tract - Pathway (DCVR)

Answer 14

First-order neuron: same for all DCVR: Ipsilateral: Peripheral sensation → Dorsal root → Dorsal horn of spinal cord.
Second-order neuron: DCR: Ipsilateral: Spinocerebellar tract → inferior cerebellar peduncle. V: (decussation x2) Contralateral: Spinocerebellar tract → Midbrain (inferior colliculus) → Ipsilateral: Superior cerebellar peduncle.
Third-order neuron: DCVR: Ipsilateral: Cerebellum.

Question 15

Descending Tracts - Function, Division (2) and Neurons (2)

Answer 15

Transport motor information in efferent pathways from brain to body.
Pyramidal tracts, Extrapyramidal tracts.
Upper motoneuron (UMN), Lower motoneuron (LMN).
Conscious and unconscious regulation of LMN of the anterior gray horn is achieved by numerous UMN pathways.

Question 16

Pyramidal Tracts (2) - Function

Answer 16

Conscious control of muscles from the cerebral cortex to the muscles of body and face.
Corticospinal Tracts
Corticobulbar Tract

Question 17

Extrapyramidal Tracts (4) - Functions

Answer 17

Unconscious, reflexive, responsive control of muscles from various brainstem structures to postural and anti-gravity muscles to control balance, locomotion, posture and tone.
Do not pass through the pyramids.
- Reticulospinal
- Vestibulospinal
- Rubrospinal
- Tectospinal

Question 18

Upper Motoneurons (UMN) - Function and Location

Answer 18

Brain and brainstem to the ventral horn of spinal cord.
Actions of UMNs provide stimulatory and inhibitory modulation of activity of the anterior horn cells, and by extension, the activity of the motor system.

Question 19

Lower Motoneuron (LMN) - Location

Answer 19

From the ventral horn of the spinal cord to the peripheral muscles.

Question 20

Corticospinal Tracts - Divisions (4) and their Functions

Answer 20

Communicate with 3 major cortical areas:
- Primary motor cortex (precentral gyrus): execution of movements (walking, running, writing…).
- Premotor cortex: control of behavior, particularly of the trunk muscles (while sitting).
- Supplementary motor cortex: stabilization and coordination of the body during bimanual movements.

• Posterior parietal cortex: for fine tuning.

Question 21

Corticospinal Tracts - Pathway

Answer 21

Divided into anterior and lateral corticospinal tracts.
UMN: Ipsilateral: a&l: cerebral cortex → internal capsule → crus cerebri → pons → rostral medulla →
a: caudal medulla (pyramids) → spinal cord
l: (decussation) Contralateral: caudal medulla → spinal cord.
LMN: (decussation) Contralateral: a&l: level of nerve root → peripheral nerve.
Anterior decussates at the LMN level of nerve root while lateral decussates at the UMN caudal medulla (pyramids).

Question 22

Corticobulbar Tract - Function and Location

Answer 22

Arise from lateral primary motor cortex, terminate in the brainstem (pons and medulla) to activate cranial nerves. Don’t go to the spinal cord. For facial and tung muscles.
Left controls right side and right controls left.
Cranial nerve VII: contralateral UMN for lower half of face.
Cranial nerve XII: contralateral UMN for tongue.

Question 23

Vestibulospinal Tract - Function and Divisions (2)

Answer 23

Do not decussate.
2 tracts that control anti-gravity muscles, for processing special afferent signals from semicircular canal system.
Inhibit flexor and promote extensor muscle activity to maintain balance.
Medial: originates in medial vestibular nucleus. Controls ipsilateral postural and tone adjustments.
Lateral: originates in lateral, superior and inferior nuclei. Controls contralateral postural and tone adjustments.

Question 24

Rubrospinal Tract - Function and Location

Answer 24

Begins in red nucleus, decussates and descends through pons and medulla into spinal cord. Supply upper limb and trunk flexors. Flexion reflex.
Disinhibition leads to upper limb flexion.
Inhibition leads to upper limb extension.

Question 25

Reticulospinal Tract - Function and Division (2)

Answer 25

Do not decussate.
Medial: originates in pons and contributes to voluntary movements and increases muscle tone.
Lateral: originates in medulla and contributes to inhibition of voluntary movements and reduces muscle tone.

Question 26

Tectospinal Tract - Function and Location

Answer 26

Decussates.
Begins in tectum, where superior and inferior colliculi are.
Superior colliculi is involved in reflexive responses to visual stimuli. Inferior is involved in reflexive responses to auditory stimuli. Supply muscles of the head and neck.