3. Functional neuroanatomy and vasculature, part 2

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Steve Lampa
PhD · Clinical Associate Professor
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Table of Contents

Long ascending and descending pathways

The three major longitudinal pathways the corticospinal tract, posterior column-medial lemniscus, and anterolateral system can be followed systematically to and from the cerebral cortex, through brain stem and to and from spinal cord. A key feature of all these pathways is to note the location where their tracts cross to the contralateral side. Remember that the information that the cerebral cortex is sending or receiving crosses (decussates) to the contralateral side and knowing the level of the decussation can aid in the localization of a lesion.

Check yourself 3.6.

Which tracts are labeled? (Tap the right arrow for answers)

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Cervical spinal cord with locations of the corticospinal, posterior column-medial lemniscus, and anterolateral (spinothalamic) system. Image designed using biorender.com.

Sensory:

A and B Posterior Column (fasciculus gracilis (A) and cuneatus (B) combined)

C Anterolateral System (aka Anterior and lateral spinothalamic tracts)

Motor:

D Lateral corticospinal tract

The corticospinal (pyramidal) tracts have their origin from an upper motor neuron, in the primary motor area of the precentral gyrus and premotor area of the frontal lobe. Axons of these neurons descend through the posterior limb of the internal capsule to eventually reach the rostral midbrain, descending as distinct bundles (tracts) in the cerebral peduncles.

Fibers in the corticospinal tracts continue their descent in the ventral-most portion of the brain stem, traversing the cerebral peduncle, basal pons, and medullary pyramid (Interactive 3.7). There are subsets of these axons that are carrying information to motor nuclei of cranial nerves in the brain stem, these are called corticobulbar or corticonuclear fibers.

Axons of the corticospinal/corticobulbar tracts descend through the rostral pons as a series of longitudinally oriented bundles of fibers, accompanied by numerous corticopontine bundles.

At the junction of the spinal cord and caudal medulla, most of the fibers in found in the pyramids decussate, the pyramidal decussation, forming the lateral corticospinal tracts.

The lateral corticospinal tracts descend through the lateral funiculus of spinal cord and upon reaching their target level synapse on lower motor neurons in the anterior horn of the spinal cord.

The anterolateral system forms after the 1st order afferent neurons carrying information regarding pain, temperature and crude touch from the periphery synapse on 2nd order neurons in the dorsal horn of the spinal cord. The information coming in usually ascends a few spinal cord segments superiorly before synapsing. After synapsing the 2nd order neurons cross (decussate) in the anterior white commissure of the spinal cord to form the anterolateral (spinothalamic) tracts. The ascending anterolateral system (spinothalamic tract) at all levels of the brain stem is in anterolateral position in the tegmentum.

The second order axons in the anterolateral system will continue to ascend to synapse in a sensory relay nucleus in the thalamus, where the 3rd order neurons will continue through the posterior limb of the internal capsule to end mostly in the primary somatosensory cortex found in the postcentral gyrus of parietal lobe.

The posterior (dorsal) column tracts, the gracile and cuneate fascicles, are carrying fine touch and conscious proprioception as they ascend as axons of 1st order neurons in the medial (gracile) and lateral (cuneate) portions of the posterior column of the spinal cord. These 1st order neurons synapse in the caudal medulla and in the gracile and cuneate nuclei.

The 2nd order axons exit the gracile and cuneate nuclei ventrally after synapsing and the axons cross in the midline forming the contralateral medial lemniscus; a vertically oriented band of ascending fibers. The decussating fibers are called the internal arcuate fibers and the region is sometimes referred to as the sensory decussation.

Throughout the medulla, the medial lemniscus is organized so that fibers representing cervical region of the body are dorsal, as if the sensory homunculus were standing on the pyramid.

The medial lemniscus in the caudal medulla starts near the midline and moves progressively laterally, rotating nearly 180° as it ascends through the brain stem.

In the caudal pons, the sensory homunculus is arranged such that the head and arms are medially located and with the feet lateral. As the medial lemniscus fibers ascend in the pons they gradually flatten in profile and come to lie in a medial-lateral orientation. At the junction between the basal pons and the pontine tegmentum it has a transverse orientation.

Finally, at level of rostral pons and midbrain the medial lemniscus has moved into the tegmentum and is closely associated with the anterolateral system.

The second order axons in the medial lemniscus pathway will continue to ascend and synapse in a sensory relay nucleus in the ventroposterior lateral (VPL) nucleus of the thalamus, where the 3rd order neurons will continue through the posterior limb of the internal capsule to end mostly in the primary somatosensory cortex found in the postcentral gyrus of parietal lobe.

Interactive 3.7.

(Tap to open; use your Apple Pencil to draw.)

Interactive 3.8.

(Tap to open; use your Apple Pencil to draw.)

Interactive 3.9.

(Tap to open; use your Apple Pencil to draw.)

Key concept

See Figure 3.12, showing the locations for the medial lemniscus and spinothalamic tract, anterolateral, through the different regions of the brain stem.

Figure 3.12. Brain stem with the medial lemniscus and spinothalamic tract. Access Medicine.
Figure 3.12. Brain stem with the medial lemniscus and spinothalamic tract. Access Medicine.

Thalamus and internal capsule

Figure 3.13. Thalamus and internal capsule T2 axial MRI.
T: Thalamus

C: Caudate Nucleus

L: Lentiform Nucleus

*: Anterior limb of Internal Capsule

**: Posterior limb of internal capsule

Topographically the thalamus (diencephalon) is located lateral to the 3rd ventricle and medial to the posterior limb of the internal capsule (Figure 3.13).

The thalamus relays information to and from the cortex, thalamocortical or corticothalamic fibers. The thalamus consists of a multitude of named nuclei that that have association (intra-laminar projections), sensory, motor, and limbic functions (Figure 3.14). These nuclei are involved in relaying information to and from specific areas of the cerebral cortex.

An example of these relay nuclei was mentioned previously with discussion of the spinothalamic tract and medial lemniscus which both synapsed in the ventroposterior lateral (VPL) nucleus of the thalamus.

The thalamus is primarily supplied by branches of the posterior cerebral artery or the posterior communicating artery.

Bundles of axons that are destined for either the thalamus or cortex travel through the internal capsule.

Figure 3.14. Thalamic nuclei and their targets.

Thought question

Compare and contrast the information that is carried in the spinothalamic and medial lemniscus pathways.

These bundles of axons in the are located between the lenticular (lentiform) nucleus (laterally) and the thalamus and head of the caudate nucleus (medially). Almost all the neural traffic to and from the cerebral cortex passes through the internal capsule.

Figure 3.15. Internal capsule relationships. Relationships between internal capsule, basal ganglia, and thalamus in horizontal section. Notice that descending motor fibers for the face, arm, and leg (F, A, L) run in front of ascending sensory fibers (f, a, l) in the posterior limb of the internal capsule. (Modified from Simon RP, Aminoff MJ, Greenberg DA: Clinical Neurology. 4th ed. Appleton & Lange, 1999.); Accessed 10/6/19.

The internal capsule has parts that can be divided by its relationship to the lenticular nucleus, the putamen and globus pallidus. An anterior limb between the head of the caudate nucleus and the lenticular nucleus, a posterior limb between the thalamus and lenticular nucleus (Interactive 3.8 and Figure 3.12). Two other portions a retrolenticular part posterior to the lenticular nucleus and sublenticular part inferior to the lenticular nucleus.

Also traveling in the internal capsule are corticopontine, corticobulbar, and corticospinal fibers.

Like other regions of the CNS, the internal capsule has a specific somatotopic arrangement of functional information that the axons of passage are carrying.

In the posterior limb of the internal capsule, for example, there is both descending information to the brain stem and spinal cord, as well as ascending sensory information that is destined for the parietal lobe.

The principal artery supply to the internal capsule is via lenticulostriate and anterior choroidal arteries (Figure 3.16). Small localized, lacunar, strokes of the internal capsule can present clinically on the specific functional arrangement of the axons found in the internal capsule.

Figure 3.16. Deep arterial supply to internal capsule and basal ganglia. Ischemic Stroke, Jameson J, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J. Harrison’s Principles of Internal Medicine, 20e; 2018. Accessed: November 13, 2018; Modified Lampa, November 2018.

Medial Medullary Syndrome

Figure 3.17. Medial Medullary Syndrome. ClinicalKey.com. Accessed 10/02/19; edited by Lampa.

Medial medullary syndrome is a rare but illustrative condition to introduce lesion localization and relating this to function of structures in the brain stem. This syndrome affects the antero-medial part of the medulla (see Figure 3.17) and most often is caused by an infarction of branches of the anterior spinal or vertebral artery. In Interactive 3.5, fill in either the deficit or structures damaged in this condition.

Check yourself 3.10.

In Medial Medullary Syndrome, fill in the appropriate deficit and/or structure involved. (tap the right arrow for answers)

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Peer teaching instructions

Practice predicting the signs of focal cerebral artery–related ischemia.

Figure 3.18.

For each of the arteries listed in Figure 3.18, in a group describe an area of the cortex or demonstrate on the interactive diagrams on the cortex, cerebellum, and brainstem the regions that are supplied by each assigned artery. Then relate this area to the predictive signs resulting from focal ischemia to this area. Use Table 3.1 and Table 3.2 to help predict the loss of function.

Interactive 3.11.

Lateral (top) and medial (bottom) view. (Tap to open; use your Apple Pencil to draw.)

Table 3.1. Anterior arterial circulation territories

Anterior circulation

Territory

Ischemic loss of function

Internal carotid/central artery of retina

Retina

Monocular blindness

Middle cerebral artery

Lateral motor area (precentral gyrus)—upper division

Contralateral face and arm weakness

 

Lateral sensory area (postcentral gyrus)—upper division

Contralateral face and arm anesthesia

 

Internal capsule (lateral striate artery)

Contralateral arm, face, leg weakness and anesthesia

 

Inferior frontal gyrus—lower division

Motor (Broca’s) aphasia

 

Frontal eye fields

Gaze to affected side

 

Superior temporal gyrus—lower division

Receptive (Wernicke’s) aphasia

 

Angular gyrus

Hemianopia (either), alexia (left)

 

Inferior parietal lobule (non-dominant side)

Contralateral sensory neglect, dressing apraxia, constructional apraxia

 

Entire territory of middle cerebral artery (proximal to bifurcation into superior and inferior divisions)

Contralateral hemiparesis and hemianesthesia, plus global aphasia if dominant hemisphere affected

Anterior cerebral artery

Medial motor and sensory area (paracentral lobule)

Contralateral leg weakness and anesthesia

 

Prefrontal cortex (orbital gyri and anterior pole of frontal lobe)

Apathy w/ potential memory loss

Table 3.2. Posterior arterial circulation territories

Posterior  circulation

Territory

Ischemic loss of function

Posterior cerebral artery

Occipital lobe

Homonymous hemianopia (with macular sparing), alexia without agraphia

 

Thalamus (diencephalon)

Hemi-sensory loss

 

Midbrain

Oculomotor nerve palsy (mydriasis, diplopia), contralateral hemiparesis, hemi-sensory loss, possible loss of consciousness

Basilar artery*

Basal pons

Contralateral hemiparesis, CN VI signs

 

Anterior midbrain

Limb ataxia, medial rectus palsy/fixed and dilated pupil

Superior cerebellar artery**

Lateral rostral pons

Optokinetic nystagmus, contralateral sensory loss (vibration, touch, position and pain and temperature)

Anterior inferior cerebellar artery**

Lateral caudal pons

Ipsilateral facial paralysis, gaze palsy, deafness and tinnitus

 

Vermis, anterior lobe of cerebellum

Ipsilateral limb ataxia, nystagmus, vertigo, nausea

Posterior inferior cerebellar artery**

Lateral medulla

Vertigo, loss of pain and temperature from the limbs and trunk on contralateral side, loss of pain and temperature over the face on the ipsilateral side, truncal ataxia, dysphagia/dysphonia/palatal paralysis, Horner Syndrome

 

Inferior cerebellar peduncle

Ataxia (limb and truncal)

Vertebral artery*

Medial medulla, anterior spinal cord

Contralateral hemiparesis; touch, vibration and positional sense loss; CN XII sign

Note: Many cerebellar signs are very similar if superior cerebellar, anterior inferior cerebellar, and posterior inferior cerebellar arteries are occluded.

*Paramedian branches of the vertebral-basilar (both short and long.

**Circumferential branches of the vertebral-basilar (long).