Motor Neuron Pathways and Tracts

Motor Neuron Pathways

• Motor neurons in the ventral horn of the spinal cord and brainstem motor nuclei are the final pathway for muscle control, synapsing directly on striated muscles.
• Alpha (α) motor neurons, the large motor neurons, innervate extrafusal fibers in skeletal muscles.
• Gamma (γ) motor neurons, smaller motor neurons scattered among α motor neurons, supply intrafusal fibers within muscle spindles.
• Muscle spindles serve as receptors for muscle stretch reflexes.
• The motor unit consists of a motor neuron, its axon, and all the muscle fibers it innervates.
• The neuromuscular junction is the connection between the axon's terminal branches and the muscle fiber.
• Motor neurons controlling axial muscles are located in ventromedial columns in the spinal cord's ventral gray horn.
• Neurons for proximal muscles are in the mid-region, while those for distal limb muscles are in lateral columns.

Corticospinal and Corticobulbar Tracts

• Motor neuron cell groups receive input from the contralateral motor cortex (MC) via the corticospinal and corticobulbar tracts.
• The corticospinal tract consists of approximately one million fibers on each side; only 3% originate from Betz cells in layer V of the primary motor cortex.
• Corticospinal fibers are excitatory, using glutamate as a neurotransmitter.
• Upper motor neurons give rise to the corticospinal and corticobulbar tracts.
• The corticospinal pathway controls skilled, voluntary movements of the distal limbs.
• It arises from somatotopically organized areas of the primary MC, lateral premotor cortex (PMC), and supplementary motor area (SMA).
• Fibers originate from both precentral (60%) and postcentral (40%) cortical areas.
• Corticospinal neurons are primarily in Brodmann area 4 (40%) in the precentral gyrus (primary MC).
• The lateral PMC and SMA are located in Brodmann area 6 (20%).
• Axons also arise from neurons in the primary sensory cortex (Brodmann areas 3, 1, and 2), anterior paracentral gyri, superior parietal lobule (Brodmann areas 5 and 7), and cingulate gyrus.
• Corticospinal fibers descend through the corona radiata, internal capsule, cerebral peduncle, basis pontis, and medullary pyramids.
• About 90% of corticospinal fibers decussate at the caudal end of the medulla oblongata (pyramidal decussation) forming the lateral corticospinal tract.
• In the internal capsule, hand fibers are lateral and slightly anterior to foot fibers in the corticospinal tract.
• Fibers controlling proximal muscles are dorsal to those controlling distal muscles in the pons.
• Pontine lesions typically cause pure motor hemiparesis due to the ventral location of the pyramidal tract.
• Lower extremity fibers are lateral and decussate more rostrally than upper extremity fibers within the medullary pyramids.
• Fibers that do not decussate descend in the ipsilateral ventral funiculus as the ventral corticospinal tract.
• Most ventral corticospinal fibers decussate at lower spinal cord levels, with only about 2% remaining ipsilateral (bundle of Barnes) to control axial musculature.
• Corticobulbar fibers originate in the lower third of the cortical motor fields, especially the MC and SMA.
• They descend through the genu of the internal capsule, cerebral peduncle, and basis pontis, intermixed with corticospinal fibers.
• The corticobulbar pathway has bilateral input to the trigeminal, hypoglossal, and facial nerve nuclei (upper facial muscles).
• Ventral brainstem lesions rostral to the lower pons can result in contralateral central facial paresis, while lower dorsolateral pontine lesions can result in ipsilateral peripheral facial paresis.
• An aberrant fiber bundle branches off the corticospinal tract in the midbrain and upper pons.
• Muscles of the lower face receive predominantly crossed corticobulbar input, while muscles of the upper face have both ipsilateral and contralateral representation.

Supranuclear Facial Paresis

• There are two types of supranuclear facial paresis: one for voluntary and one for involuntary emotional facial movements.
• The ventral part of the facial nerve nucleus (lower face) has predominantly crossed supranuclear control for voluntary movements.
• Emotional involuntary movements and voluntary facial movements may be clinically dissociated, suggesting a separate supranuclear pathway for involuntary movements.
• The SMA and/or cingulate motor areas control emotional facial innervation.
• Fibers mediating emotional facial movements do not descend in the internal capsule.
• The right cerebral hemisphere is "dominant" for emotional facial expression.
• Some facial corticobulbar fibers descend ipsilaterally to the medulla, then decussate and ascend to the contralateral facial nucleus (perioral muscles).
• Involvement of the dorsolateral pons can result in emotional facial paresis.

Motor Cortex Organization

• Within the MC, corticospinal neurons are somatotopically organized based on functional importance (motor homunculus).
• The size of cortical representation varies; the lips, jaw, thumb, and index finger have large representations.
• Forehead, trunk, and proximal portions of the limbs have small representations.
• Cortical hand area is located in the middle to lower portion of the anterior aspect of the central sulcus (Brodmann area 4).
• Neurons in the medial MC and anterior paracentral gyrus innervate muscles of the foot, leg, and thigh.
• Neurons in the medial two-thirds of the precentral gyrus innervate the upper extremity and trunk.
• Neurons in the ventrolateral precentral gyrus contribute to the corticobulbar tract.
• They project to motor nuclei of CN V, VII, IX, X, XI, and XII to influence cranio-facial-oral musculature.
• Each hypoglossal nucleus usually receives impulses from both sides of the cerebral cortex, except for the genioglossus muscle.
• Lingual paresis may occur with lesions at different anatomic levels.

Role of Sensory Pathways, Cerebellum, and Basal Ganglia

• Sensory cortical pathways, corticofugal projections, direct corticospinal projections, and projections to the basal ganglia and cerebellum are active in planning/executing movements.
• The cerebellum coordinates movements and controls equilibrium and muscle tone.
• It controls ipsilateral limbs through connections with the spinal cord, brainstem, and contralateral MC.
• The corticopontine pathway, a corticofugal pathway, arises from the precentral and postcentral gyri, PMC, SMA, and posterior parietal cortices.
• Fibers descend through the internal capsule and cerebral peduncle to the pontine nuclei.
• Second-order neurons from the pontine nuclei cross to the contralateral basis pontis, forming the pontocerebellar pathway.
• The basal ganglia control posture and movement and participate in motor planning through connections with the ipsilateral MC.
• The corticostriate pathway includes direct and indirect projections from the cerebral cortex to the striatum.
• Corticostriate projections arise mainly from motor-sensory cortex (Brodmann areas 4, 3, 1, and 2), PMC (Brodmann area 6), and the frontal eye fields (Brodmann area 8).
• Direct corticostriate projections reach the striatum through the internal and external capsules and the subcallosal fasciculus.
• The indirect pathways include the cortico-thalamo-striate pathway, and collaterals from the corticoolivary and corticopontine pathways.
• All parts of the cerebral cortex project to the caudate and putamen.
• Cortical association areas project mainly to the caudate nucleus, while sensorimotor areas project preferentially to the putamen.
• Corticostriate projections terminate ipsilaterally in a topographic pattern.
• The cortex also sends fibers to the substantia nigra, subthalamic nucleus, and claustrum.

Corticothalamic Pathway

• The corticothalamic pathway arises from cortical areas receiving thalamic projections, serving as a feedback mechanism.
• Examples of reciprocal connections include the anterior nucleus and the posterior cingulate cortex, the ventral lateral nucleus and the MC, etc.
• Corticothalamic fibers descend in various parts of the internal capsule and enter the thalamus in a bundle known as the thalamic radiation.

Additional Corticofugal Tracts

• The corticoreticular pathway arises from one cerebral hemisphere and projects to both sides of the brainstem reticular formation.
• The corticohypothalamic tract arises from the prefrontal cortex, cingulate gyrus, amygdala, olfactory cortex, hippocampus, and septal area.
• Corticofugal areas project to the superior colliculi and centers in the brainstem reticular formation to influence the motor nuclei of CN III, IV, and VI.

Internal Capsule

• Compact lamina of white matter contains fibers passing between brainstem and cerebral hemispheres.
• Continues rostrally with the corona radiata and caudally with the cerebral peduncles.
• Located laterally to the caudate nucleus and thalamus, and medially to the lenticular nucleus.
• Curved shape with five segments: anterior limb, genu, posterior limb, retrolenticular segment, and sublenticular segment.
• Anterior Limb contains frontopontine, thalamocortical, corticothalamic, and caudate-putaminal fibers.
• Genu contains corticobulbar fibers and potentially motor corticopontine fibers.
• Caudal half of the posterior limb contains the corticospinal bundle, somatotopically organized (upper extremity anteriorly, followed by trunk, lower extremity).
• Corticorubral, corticoreticular, and corticopontine fibers intermix with corticospinal fibers.
• Corticorubral, corticothalamic, and thalamocortical fibers are dorsal to the corticospinal fibers in the posterior limb.
• Sublenticular segment contains auditory and visual radiations.
• Retrolenticular segment contains the visual radiations of Gratiolet and corticotectal, corticonigral, and corticotegmental fibers.
• Vascular supply: Anterior limb - recurrent artery of Heubner. Genu/Posterior limb - anterior choroidal artery. Superior aspect - lenticulostriates.

Motor Deficits and Localization

• Motor deficits include plegia (complete paralysis) and paresis (weakness). Paralysis is commonly used for both complete and partial loss of motor function.
• Muscle strength is graded per the Medical Research Council’s scale.
• When examining patients with weakness, determine: lesion location, whether the lesion is focal/multifocal/diffuse, and the underlying cause.
• Lesions in the descending motor system: cerebral cortex, internal capsule, brainstem, or spinal cord.
• Cortical lesions leading to spasticity involve the primary motor and premotor cortical areas.
• Upper motor neuron refers to lesions involving more than the pyramidal tract.
• Lower motor neuron lesions located in the cells or axons in the ventral gray column of the spinal cord or brainstem.
• Upper motor neuron syndrome may follow head injuries, perinatal brain injuries, stroke, demyelinating diseases, or motor neuron diseases.

Upper Motor Neuron Syndrome Characteristics

• Muscles initially weak and flaccid then become spastic, exhibiting hypertonia and hyperactive muscle stretch reflexes.
• Muscle stretch reflexes consist of a monosynaptic arc.
• Clonus, rhythmic contraction and relaxation of muscles, is best seen at the ankle.
• Spasticity is a velocity-dependent increase in tonic stretch reflexes.
• Predominates in antigravity muscles (flexors of the upper extremities and extensors of the lower extremities).
• Evaluation of muscle tone shows variable resistance to passive movements with changes in speed and direction. Clasp-knife character.
• Weakness of the upper extremity is most marked in the deltoid, triceps, wrist extensors, and finger extensors.
• Affected arm is adducted at the shoulder, flexed at the wrist and fingers (spastic hemiparesis).
• Weakness of lower extremity muscles is most marked in hip flexors, knee flexors, foot dorsiflexors, and foot evertors.
• Different anatomic substrates may underlie hyperreflexia and spasticity. As an example, corticospinal lesions in the cerebral peduncle do not result in spasticity
• Associated with pathologic reflexes, such as the extensor plantar reflex or Babinski sign.
• Severe flexor or extensor muscle spasms may also occur.
• Spinal cord lesions are often associated with marked flexor spasms, except for incomplete or high spinal cord lesions that usually have a dominant extensor tone.
• Severe flexor spasms may also be accompanied by bladder and fecal incontinence.
• Observed negative features: muscle weakness, muscle slowness, impaired dexterity, and fatigability.
• Severe spasticity may exhibit muscle deformities, contractures, and associated reactions including synkinesias.
• Superficial reflexes are absent on the affected side.
• With lesions above the pyramidal decussation, signs are detected contralaterally. Below, ipsilaterally.

Lower Motor Neuron Lesions

• Damaged lower motor neurons or axons will cause weakness or paralysis of involved muscles, flaccidity, hypotonia, decreased or absent reflexes (hyporeflexia or areflexia), and eventually muscle atrophy.
• In the spinal muscular atrophies, weakness and amyotrophy predominate in proximal segments of the limbs, distal, facioscapulohumeral, scapulohumeral, and segmental forms are well known.
• Some patterns of discrete muscle atrophy have localizing value.
• Fasciculations may be present.
• No pathologic reflexes are elicited.

Hemiplegia Topographic Diagnosis

• Facial weakness may result from an upper or lower motor neuron lesion.
• Upper half of the face, which has bilateral cortical innervation, is not affected in supranuclear lesions.
• If there is an upper motor neuron type of facial paresis on the same side of the hemiplegia, the lesion is localized above the upper pons (MC, corona radiata, or internal capsule).
• A lesion on the cerebral peduncles and upper pons can also cause a hemiplegia with an associated upper motor neuron facial paresis.
• Disproportionate hemiparesis (e.g., faciobrachial predominance) suggests a corticosubcortical lesion.
• Crural predominance suggests the lesion most likely involves the contralateral paracentral region.
• Internal capsular lesions often result in pure motor hemiplegia, as do lesions of the basis pontis, cerebral peduncle, and medullary pyramid.
• Anterior choroidal artery territory infarctions cause hemiparesis, hemisensory loss, and hemianopia due to involvement in the posterior limb of the internal capsule.
• Alternating hemiplegia results in a "crossed" syndrome, with CN involvement ipsilateral to the lesion, and contralateral hemiparesis or hemiplegia.

Paraparesis or Quadriparesis

• Paraparesis/paraplegia can be located in the cerebrum or cervical or thoracic spinal cord, or may be peripheral.
• Quadriparesis/quadriplegia may be in the high cervical cord (can also result from diffuse peripheral nervous system involvement).
• Muscle stretch reflex assessment is useful in determining the lesion location.
• In spinal cord lesions, muscle stretch reflexes are lost at the level of the lesion and increased below.
• Low cervical spinal cord compression causes lower motor neuron signs at the corresponding segmental level and upper motor neuron signs below.
• May be inverted or paradoxical reflexes resulting from combined spinal cord and nerve root pathology.

Single Limb Weakness (Monoplegia)

• May result from an upper motor neuron lesion or an extramedullary spinal cord lesion.
• In patients with isolated monoplegia and no involvement of the face, consider a lower motor neuron type of syndrome.

Motor Unit Conditions

• Lesions of the lower motor neuron may involve the motor neurons, roots, plexus, peripheral nerves , neuromuscular junction, and muscle.
• Muscle weakness, atrophy, fasciculations, and exaggerated muscle stretch reflexes suggest motor neuron disease.
• Peripheral nervous system diseases may affect motor, sensory, or autonomic neurons.
• Absent muscle stretch reflexes indicate dysfunction of large-diameter sensory fibers.
• Muscle stretch reflexes diminish with age.
• Generalized distal weakness likely results from a peripheral neuropathy, although proximal weakness may be present.
• Severe unilateral pain made worse with movements of the arm, minor sensory loss, and weakness greater proximal than distal suggest Parsonage–Turner syndrome or neuralgic amyotrophy.
• Generalized proximal weakness is likely due to a myopathy or neuromuscular junction disorder.
• Fluctuating weakness with predilection for involvement of extraocular muscles and proximal limb muscles, exacerbated by activity, is the sign for myasthenia gravis.
• Symmetric upper and lower girdle muscle involvement associated with muscle pain and dysphagia is often with inflammatory myopathies.
• Delayed relaxation of skeletal muscles following voluntary contraction is characteristic of myotonic disorders.
• Episodic attacks of flaccid limb muscle weakness, with sparing of ocular and respiratory muscles classify as periodic paralysis.
• Pseudohypertrophy of the calves is seen with Duchenne muscular dystrophy
• Patients may use their hands to rise from the ground (Gowers maneuver) due to weakness of proximal hip muscles.
• Other early findings include lumbar hyperlordosis, wide-based gait, and toe walking.