3.1 Neuroanatomy, Motor and Sensory Pathways, and Cranial Nerves

Neuroanatomy, Motor and Sensory Pathways, and Cranial Nerves

Trace a sensory or motor finding through the brain, spinal cord, root and peripheral nerve. The point at which a pathway crosses helps you identify which side of the nervous system to examine.

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1. Central Nervous System Organization

Cerebral Cortex and Cortical Homunculi

The cerebral cortex orchestrates sensorimotor integration, cognitive processing, and voluntary motor planning. The primary motor cortex (M1) resides in the precentral gyrus of the frontal lobe (Brodmann area 4), working in concert with the premotor cortex (area 6, planning motor actions guided by sensory cues) and the supplementary motor area (SMA, initiating internally generated movement sequences). Immediately posterior across the central sulcus lies the primary somatosensory cortex (S1) within the postcentral gyrus of the parietal lobe (Brodmann areas 3, 1, and 2), which processes tactile discrimination, kinesthesia, and conscious proprioception.

Electrical stimulation studies mapped the body across both sides of the central sulcus. The maps overlap and vary between individuals.

• Medial sensorimotor cortex: Lower-limb representations extend onto the medial surface near the paracentral lobule, typically in anterior cerebral artery (ACA) territory. A cortical ACA stroke may produce contralateral leg-predominant findings.
• Lateral sensorimotor cortex: Hand, arm and face representations lie more laterally, typically in middle cerebral artery (MCA) territory. Contralateral face and arm involvement greater than leg involvement can suggest an MCA distribution. Confirm the site and cause through the acute stroke pathway.

Basal Ganglia Loops

The basal ganglia consist of interconnected subcortical nuclei—the caudate nucleus, putamen (collectively termed the striatum), globus pallidus (internal [GPi] and external [GPe] segments), subthalamic nucleus (STN), and substantia nigra (pars compacta [SNc] and pars reticulata [SNr]). Together, they modulate motor program initiation, scaling, and inhibition of unwanted movements mainly through thalamocortical and brainstem circuits.

1. Direct pathway in the classic model: Cortical inputs stimulate striatal GABAergic neurons expressing D1 dopamine receptors. These neurons directly inhibit the GPi/SNr complex. Because the GPi/SNr normally exerts continuous tonic GABAergic inhibition on the ventrolateral (VL) and ventral anterior (VA) motor thalamus, inhibiting the GPi disinhibits the thalamus, allowing thalamocortical projections to fire and facilitate voluntary movement.
2. Indirect pathway in the classic model: Cortical inputs stimulate striatal GABAergic neurons expressing D2 dopamine receptors, which inhibit the GPe. Reduced GPe output disinhibits the STN, which fires glutamatergic excitatory signals to the GPi/SNr. Excited GPi/SNr neurons drive intense tonic inhibition onto the thalamus, effectively suppressing extraneous or competing motor programs.

In the classic model, loss of nigrostriatal dopamine reduces facilitation of the direct pathway and removes inhibition of the indirect pathway. The resulting increase in inhibitory output helps explain bradykinesia in Parkinson disease. Early Huntington disease can preferentially affect indirect-pathway neurons. A subthalamic lesion can cause contralateral hemiballismus. Similar flinging movements can also arise from lesions at other points in the network.

Cerebellar Circuitry and Clinical Ataxia

The cerebellum integrates sensory feedback and motor signals to help coordinate timing, balance and motor learning. Three broad functional divisions explain common examination patterns:

• Vestibulocerebellum (Flocculonodular Lobe): Connected with vestibular nuclei; regulates equilibrium, static and dynamic postural balance, and the vestibulo-ocular reflex (VOR). Lesions produce trunk titubation, broad-based balance ataxia, and pathological nystagmus.
• Spinocerebellum (Vermis and Intermediate Hemispheres): Receives spinocerebellar tract inputs; coordinates ongoing axial and appendicular limb movements during locomotion. Vermis lesions cause prominent trunk and gait ataxia; intermediate hemisphere lesions cause ipsilateral limb coordination deficits.
• Cerebrocerebellum (Lateral Hemispheres): Receives cortical input via pontine nuclei and influences cortex via cerebellar output and thalamus; coordinates multi-joint motor planning, trajectory timing, and motor learning. Lesions impair fine motor control, finger dexterity, and rapid alternating movements.

Brainstem Integration

The midbrain, pons and medulla contain ascending and descending tracts, reticular networks, autonomic centers and most cranial nerve nuclei. The spinal component of the accessory nerve arises in the upper cervical cord. A lateral medullary lesion can cause ipsilateral facial and contralateral body loss of pain and temperature, sometimes with ipsilateral Horner syndrome. Both vertebral and posterior inferior cerebellar artery disease can cause a lateral medullary infarct.

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2. Major Ascending (Sensory) Pathways

Two major body pathways differ in the location of their first relay and crossing. The face mainly uses trigeminal pathways.

Dorsal columns: dorsal root ganglion → ipsilateral spinal ascent → medullary relay and crossing → thalamus → cortex
Anterolateral: dorsal root ganglion → dorsal horn relay and spinal crossing → contralateral ascent → thalamic and other relays

Dorsal Column-Medial Lemniscal (DCML) System

• Modalities Carried: Discriminative touch, vibration and conscious proprioception. Two-point discrimination and recognizing an object by touch also require cortical processing.
• Anatomical Course:
  1. First-order neuron: The ascending branches of dorsal root ganglion neurons enter the ipsilateral posterior funiculus. Other branches can participate in spinal circuits. Afferents from the lower extremity and trunk (below T6) travel medially in the Fasciculus Gracilis. Afferents from the upper trunk, upper extremity, and neck (above T6) enter laterally to form the Fasciculus Cuneatus.
  2. Second-order neuron: First-order axons relay in the ipsilateral gracile and cuneate nuclei of the caudal medulla. Second-order axons cross as internal arcuate fibers to form the medial lemniscus.
  3. Third-order neuron: Medial lemniscal fibers terminate in the Ventral Posterolateral (VPL) nucleus of the thalamus. Third-order thalamocortical axons project through the posterior limb of the internal capsule to the postcentral gyrus (S1).

Spinothalamic Tract (Anterolateral System)

• Modalities Carried: The anterolateral system carries pain and temperature information and contributes to crude touch.
• Anatomical Course:
  1. First-order neuron: Incoming nociceptive afferents can travel a short distance in Lissauer’s tract before synapsing in the dorsal horn. Lamina II is rich in local interneurons. Projection neurons occur in lamina I and deeper laminae, including lamina V.
  2. Second-order neuron: Projection axons cross the anterior white commissure near their segment of entry and ascend in the contralateral anterolateral cord. The clinical sensory boundary may be a few segments below a hemicord lesion.
  3. Third-order neuron: Thalamic targets include the VPL and other nuclei. Thalamocortical pathways distribute information to somatosensory cortex and other regions involved in pain perception.

Comparative Table: Primary Ascending Pathways

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3. Major Descending (Motor) Pathways

Lateral Corticospinal Tract (LCST)

The lateral corticospinal tract is the primary motor pathway mediating voluntary, fractionated, skilled distal extremity movements, especially individual finger movements required for manual dexterity.

Cortical layer V → internal capsule → cerebral peduncle → pons → medullary pyramids
→ pyramidal crossing → contralateral lateral spinal cord → spinal interneurons or motor neurons

1. Origin: Corticospinal axons arise from layer V neurons in primary motor, premotor and other cortical areas, including somatosensory cortex. Betz cells form one subset of these neurons.
2. Descent: Fibers descend through the corona radiata and posterior limb of the internal capsule, then through the cerebral peduncle and basilar pons to the medullary pyramids.
3. Crossing: Most fibers cross at the caudal medulla and descend as the lateral corticospinal tract. A lesion above this crossing usually affects the opposite side of the body. A lesion of the tract in the cord affects the same side below it.
4. Anterior pathway: Some fibers descend in the anterior corticospinal tract without crossing at the pyramids. Segmental crossing and bilateral connections contribute to axial and proximal control.
5. Spinal targets: Many corticospinal axons act through interneurons. Some contact motor neurons directly, including those involved in skilled hand movements.

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4. Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN) Lesions

Upper motor neurons and their axons lie within the central nervous system. Lower motor neuron cell bodies are in the spinal anterior horn or cranial motor nuclei. Their axons carry the final signal to skeletal muscle.

Spinal roots and the stretch reflex

Sensory axons enter via dorsal roots, with their cell bodies in dorsal root ganglia. Motor axons exit via ventral roots. The roots join into mixed spinal nerves. A tendon tap briefly stretches a muscle and activates spindle Ia afferents. Their spinal connections excite alpha motor neurons to the same muscle, while interneurons can inhibit the antagonist. A knee jerk therefore depends on the femoral nerve and its lumbar root contributions as well as central modulation. Compare both sides and interpret the reflex with strength and sensation. Neuromuscular junction disease can cause weakness but is a separate localization from a lower motor neuron lesion.

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5. Cranial Nerves: Functions, Screening, and Clinical Testing

Cranial nerve findings can help localize a problem when interpreted alongside the history and limb examination. In an acute presentation, activate the appropriate emergency pathway before extending the screening examination.

Summary Table: Cranial Nerves I through XII

An absent gag can occur in healthy people. Use an approved swallowing screen when indicated. Gag alone cannot establish swallowing safety.

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6. Clinical Application Scenario

Clinical Vignette: Acute Hemiparesis and Cranial Nerve Localization

A 68-year-old with atrial fibrillation suddenly develops right lower facial droop and right-sided weakness and numbness, with the arm more affected than the leg. He can still raise both eyebrows. Activate the local stroke response or emergency medical services immediately and establish last known well. Record findings available without delaying transfer.

The pattern supports a left central lesion. Face and arm predominance makes a left MCA distribution plausible. Clinical findings alone cannot determine the vessel, exact division or whether the stroke is ischemic or hemorrhagic. Brain imaging and the acute stroke team guide those decisions.