10. Lower and Upper Motor Neurons and the Internal Capsule
Revised January 25, 2021
The objectives of this chapter are to:
- Describe the anatomy of lower and upper motor neurons.
- Describe the organization of the internal capsule.
I. Lower and Upper Motor Neurons
Lower motor neurons are neurons that directly innervate skeletal muscle. The cell bodies of these neurons are located within the ventral horns of the spinal cord and within brainstem motor nuclei.
Upper motor neurons, as defined clinically, are cortical neurons that innervate lower motor neurons (either directly or via local interneurons). The axons of upper motor neurons are contained within the pyramidal system, which is composed of the corticospinal (pyramidal) and corticobulbar tracts.
A. The corticospinal (pyramidal) tract
Primary motor cortex: The corticospinal tract (pyramidal tract) originates from several frontal and parietal cortical areas. The focus here is on its origin from the primary motor cortex (Brodmann’s area 4) (#74246) of the precentral gyrus (#4321) and the anterior part of the adjacent paracentral lobule (#74248). The precentral gyrus, similar to the postcentral gyrus, is somatotopically organized. The foot and lower limb are represented on the medial surface of the hemisphere and the trunk, upper limb, hand and face, on the lateral surface. What cerebral arteries supply the primary motor cortex? What part of the somatotopic representation does each cerebral artery supply?
Pyramidal cells: Two types of neurons occur in the cerebral cortex, pyramidal (#6131) and stellate cells (#8319). Stellate ("star-shaped") neurons are local interneurons. Their axons remain within the cerebral cortex. Pyramidal cells are the output neurons of the cortex. Their axons end in other areas of the cortex on the same or opposite side, or they project to the basal ganglia, thalamus, brain stem, and spinal cord. The name pyramidal is derived from the shape of their cell bodies, "pyramid-like" (#5799). The fact that the term "pyramidal" is applied both to this category of cortical cell and to the corticospinal tract is coincidental. The tract is named after the pyramids in the medulla (#6411).
Corona radiata and internal capsule: Pyramidal tract axons leave the cerebral cortex and go through the subcortical white matter. Within the subcortical white matter, the pyramidal system fibers converging from all parts of the cerebral cortex into the internal capsule (fig 10a, #6390) form part of the corona radiata.
Remember, though, that the subcortical white matter contains three kinds of fibers:
- Commissural fibers that travel to the opposite hemisphere through the corpus callosum and anterior commissure.
- Association fibers that travel to other parts of the same cerebral hemisphere.
- Projection fibers, which include
- All fibers projecting (in the corona radiata) from the cerebral cortex to noncortical areas.
- All fibers from the thalamus projecting to the cerebral cortex.
Thus, the subcortical white matter is composed of these three kinds of fibers (commissural, association and projection fibers) traveling at roughly right angles to each other. The focus here is on the corona radiata fibers.
Brainstem: The pyramidal fibers descend through the internal capsule into the middle three-fifths of each cerebral peduncle (#6461, #6397). The cerebral peduncle fibers become incorporated into basilar pons (pons proper) (#4020). In transverse sections through the pons, the corticospinal fibers are the large fascicles in the center of basilar pons (#10144, #6403). As these axons continue caudally, they come to the surface of the medulla as the pyramids (#6411).
Pyramidal decussation and spinal cord: The corticospinal axons decussate (#5306) at the junction of the medulla spinal cord (#6414), which is at the level of the foramen magnum. The axons move laterally (#6213) into the lateral funiculus (#6416). Consequently, the axons that originate in the left cerebral cortex become located on the right side of the spinal cord. In the spinal cord, these axons form the lateral corticospinal tract. Some of the axons in the pyramids do not decussate but continue directly into the spinal cord as the ventral (anterior) corticospinal tract (#6417), but most of these fibers eventually cross and the ventral corticospinal tract is of little clinical significance. There are considerably fewer corticospinal fibers in lumbar than in cervical segments (#6420). Why?
In summary, the upper motor neurons of the left motor cortex innervate the lower motor neurons of the right body. A stroke or other lesion involving all of the corticospinal fibers from the left cerebral cortex produces weakness of the right upper and lower limbs (right hemiparesis).
Lower motor neurons: The corticospinal fibers terminate in the ventral horn on lower motor neurons, either directly or via local interneurons. Of course, these motor neurons receive input from several other sources. In fact, their dendrites and cell bodies are literally outlined by synaptic buttons. The axons of these motor neurons leave the cord via the ventral roots (#4497). The axons of the large ventral horn neurons (alpha motor neurons) terminate as motor endplates on skeletal muscle (#52178). Alpha motor neurons are also designated as "final common path neurons." What deficits result from the interruption of upper motor neurons? Compare these deficits with those that occur after lower motor neuron disease or damage.
B. The Corticobulbar Tract
Axons that are homologous to corticospinal fibers, but terminate in the motor nuclei of cranial nerves in the brain stem [e.g., nuclei V (#6198), VII (#6695), IX, X, XI and XII (#4311)], form the corticobulbar tract. Thus, they are the axons of the upper motor neurons that synapse on the lower motor neurons of the cranial nerves. The corticobulbar fibers accompany the corticospinal axons through the internal capsule (#6430) and cerebral peduncle (#6463) and then gradually leave the corticospinal tract to enter the tegmentum of the pons and medulla to terminate in the different nuclei.
A clinically important point is that the lower motor neurons of the brain stem receive bilateral corticobulbar input. Therefore, unilateral corticobulbar tract lesions usually produce no clinical effect on head and neck muscles with two exceptions:
- Facial nucleus (VII): The neurons that innervate the muscles of the lower face (below the forehead) receive mainly crossed input from the opposite motor cortex. Therefore, a stroke or other lesion involving the left motor cortex or internal capsule causes weakness of the right facial muscles below the forehead, but the patient can still wrinkle her right forehead.
- Hypoglossal nucleus (XII): The neurons that innervate the genioglossus muscle receive mainly crossed input from the opposite motor cortex. Therefore, a stroke or other lesion involving the left motor cortex or internal capsule causes weakness of the right genioglossus muscle. When the patient protrudes his tongue, the normal left genioglossus muscle pushes the tongue to the right. However, this deficit is generally transient, lasting a few days.
In summary, a lesion involving all of the corticospinal and corticobulbar fibers from the left cerebral cortex produces
- Right hemiparesis (weakness of the right upper and lower limbs).
- Weakness of the right face below the forehead.
- Deviation of the tongue to the right upon protrusion (transient).
C. Control of voluntary eye movements
Voluntary control of eye movements is not executed through the corticobulbar tract. There are discrete cortical areas subserving the control of eye movements. One of these is called the frontal eye field (FEF). The FEF has recently been localized in humans to the junction of the superior frontal sulcus with the precentral sulcus (#74245) in area 6, not in area 8 as in the monkey (#4351). Electrical stimulation of the FEF produces conjugate horizontal gaze to the opposite side. Thus, the left hemisphere controls horizontal gaze to the right. A left FEF lesion causes inability to voluntarily look to the right (and the patient looks toward the left for the first few days).
FEF cortical cells send axons through the internal capsule and cerebral peduncle into the pons. In the case of horizontal gaze, these axons decussate and terminate in an area of the opposite reticular formation near the abducens nucleus called the paramedian pontine reticular formation (PPRF). The PPRF, in turn, connects with the abducens nucleus of the same side, thus exciting the motor neurons of cranial nerve VI (#6174). The abducens nucleus also has interneurons whose axons ascend in the opposite medial longitudinal fasciculus (MLF) (#6333) to the oculomotor nucleus to cause concurrent activation of the medial rectus muscle of the opposite side. A right PPRF lesion impairs ability to voluntarily look to the right.
The PPRF is the pontine center for horizontal gaze. Similarly, there is a center for vertical gaze located in the midbrain near the posterior commissure and pretectal area (fig 7k, #6302). This center connects directly with the oculomotor (#6217) and trochlear nuclei. Upward gaze depends on the posterior commissure and a lesion affecting the posterior commissure causing loss of upward gaze (dorsal midbrain or Paranaud's syndrome) does not affect horizontal gaze. Likewise, patients with impaired horizontal gaze may have no difficulty with upward gaze. Why? Predict what gaze problem would occur with a tumor of the pineal gland (#2397).
II. The Internal Capsule
The internal capsule is the fibrous expressway that connects the cerebral cortex to the basal ganglia, thalamus, brain stem (motor nuclei, pontine nuclei and reticular formation) and spinal cord. In addition, it carries the fibers connecting the thalamus to the cerebral cortex.
The internal capsule is divided into three regions: the anterior limb, genu, and posterior limb. These three regions are best seen in a axial (horizontal) section (fig 10b ).
- Anterior limb: The anterior limb (#6446 horizontal, #6426 coronal) is located between the putamen and and the caudate nucleus. It does not contain any pyramidal system axons and it does not contain any thalamic axons to primary sensory cortex. It is of relatively little clinical significance.
- Genu: The genu (#6447 horizontal, #15106 coronal) is located where the anterior and posterior limbs meet at the rostral end of the thalamus, at the level of the foramen of Monro and the anterior commissure on coronal section. The genu contains corticobulbar fibers.
- Posterior limb: The posterior limb (#6448 horizontal, #6390 coronal) is located between the thalamus medially and the basal ganglia laterally. It contains corticobulbar fibers, corticospinal fibers, and the thalamocortical somatosensory radiations. The corticobulbar fibers are closest to the genu. Caudal to them are the corticospinal fibers, which are grouped in a somatotopic manner. The thalamic somatosensory radiation partly interdigitates with the corticospinal axons but is also caudal to the upper motor neuron axons. The optic (visual) radiations and auditory radiations are in the most caudal part of the posterior limb. Where are the cell bodies for the somatosensory, auditory, and optic radiations located?
It has frequently been observed that a small lesion in the internal capsule causes more damage than many large lesions of the cortex. Why? Contrast the clinical results of hemorrhage into the anterior limb with hemorrhage into the posterior limb.
The course of the corticospinal tract can be followed from the internal capsule as far caudally as the pyramid in fig 10c, a sagittal section from a monkey brain and fig 10a, a coronal section from a human.
Click for the Syllabus Quiz for Chapter 10
|Chapter 9||Chapter 11|