Foundations and Professional Practice · 6 min read
Neurological Foundations
Organization of the Nervous System
The central nervous system is the brain and spinal cord; the peripheral nervous system (12 pairs of cranial nerves, 31 pairs of spinal nerves) carries afferent (sensory) and efferent (motor) signals. A neuron receives input on its dendrites and sends output down its axon to a synapse, where a neurotransmitter crosses to the next cell (acetylcholine at the neuromuscular junction). Myelin (oligodendrocytes in the CNS, Schwann cells in the PNS) speeds conduction.
White-matter tracts are projection fibers (cortex to lower centers), association fibers within a hemisphere (arcuate, superior longitudinal, and uncinate fasciculi), and commissural fibers between hemispheres (corpus callosum).
Cerebral Cortex and Language Areas
| Area and location | Function and effect of damage |
|---|---|
| Primary motor cortex (BA 4), precentral gyrus | Voluntary movement of the opposite side; premotor and supplementary motor areas (BA 6) plan and initiate movement |
| Broca's area (BA 44/45), posterior inferior frontal gyrus | Language formulation, grammar, speech motor planning; damage is associated with nonfluent aphasia |
| Primary somatosensory cortex (BA 3, 1, 2), postcentral gyrus | Touch, pressure, and proprioception from the opposite side |
| Primary auditory cortex (BA 41/42), Heschl's gyrus | First cortical reception of sound, with input from both ears |
| Wernicke's area (BA 22), posterior superior temporal gyrus | Language comprehension; damage is associated with fluent aphasia |
| Supramarginal gyrus (BA 40) and angular gyrus (BA 39), inferior parietal lobule | Phonological processing (supramarginal); reading, writing, word retrieval, and calculation (angular). Angular gyrus damage produces alexia with agraphia and anomia |
| Arcuate fasciculus | Links temporal and frontal language areas; damage is associated with conduction aphasia (repetition disproportionately impaired) |
The prefrontal cortex supports executive functions. The left hemisphere is dominant for language in approximately 95% of right-handed and about 70% of left-handed people.
| Left hemisphere | Right hemisphere |
|---|---|
| Phonology, morphology, syntax, literal meaning | Prosody, pragmatics, inference, humor, nonliteral language |
| Sequential, analytic processing; calculation | Visuospatial processing; attention; facial recognition |
| Damage: aphasia, apraxia of speech, right-sided weakness | Damage: left neglect, anosognosia, aprosodia |
Subcortical Structures, Cerebellum, and Brainstem
| Structure | Role | Clinical link |
|---|---|---|
| Basal ganglia (caudate, putamen, globus pallidus) | Regulate tone and movement scale; suppress unwanted movement | Parkinson's disease: hypokinetic dysarthria. Huntington disease: hyperkinetic dysarthria |
| Thalamus | Relay for sensory pathways (all but smell) and motor loops | Sensory loss; thalamic aphasia |
| Cerebellum | Coordinates timing, force, and range of movement; ipsilateral control | Ataxic dysarthria |
| Brainstem (midbrain, pons, medulla) | Cranial nerve nuclei; arousal, respiration, and swallowing centers | Dysarthria and dysphagia |
Motor Pathways and Upper vs. Lower Motor Neurons
The pyramidal (direct activation) system carries skilled voluntary movement: the corticobulbar tract ends on cranial nerve nuclei in the brainstem, and the corticospinal tract ends in the spinal cord after about 85–90% of its fibers cross at the medullary pyramids. The extrapyramidal (indirect activation) system is multisynaptic and regulates tone, posture, and reflexes. Both are upper motor neurons (UMNs); the cranial and spinal nerves are lower motor neurons (LMNs), the final common pathway.
| Feature | UMN lesion | LMN lesion |
|---|---|---|
| Tone, reflexes, bulk | Spasticity; hyperreflexia; positive Babinski sign; little atrophy | Flaccidity; hyporeflexia; atrophy and fasciculations |
| Distribution | Movement patterns; contralateral when above the brainstem | Muscles of the damaged nerve; ipsilateral |
| Dysarthria type | Spastic (bilateral damage) or unilateral UMN | Flaccid |
Cranial Nerves for Speech, Swallowing, and Hearing
| Nerve | Motor | Sensory and autonomic | Sign of unilateral damage |
|---|---|---|---|
| V Trigeminal | Muscles of mastication; tensor veli palatini; tensor tympani | Face and mouth; touch from the anterior two-thirds of the tongue | Jaw deviates toward the weak side |
| VII Facial | Facial expression; stapedius | Taste, anterior two-thirds of the tongue; salivary glands | LMN: whole ipsilateral face. UMN: contralateral lower face |
| VIII Vestibulocochlear | None | Hearing and balance | Hearing loss, tinnitus, vertigo |
| IX Glossopharyngeal | Stylopharyngeus | Taste and sensation, posterior third of the tongue; pharynx | Reduced pharyngeal sensation and gag |
| X Vagus | Velum (except tensor), pharynx, larynx | Larynx, pharynx; viscera | Uvula points away from the lesion; hypernasality; breathy voice |
| XI Accessory | Sternocleidomastoid; trapezius | None | Weak head turn away from the lesion |
| XII Hypoglossal | All tongue muscles except palatoglossus | None | LMN: tongue deviates toward the lesion; atrophy, fasciculations |
Cerebral Blood Supply
The internal carotid arteries give rise to the anterior and middle cerebral arteries. The vertebral arteries join to form the basilar artery, which ends as the posterior cerebral arteries. The circle of Willis links the two systems through the communicating arteries, allowing collateral flow.
| Artery | Territory | Typical deficits |
|---|---|---|
| Anterior cerebral (ACA) | Medial frontal and parietal lobes | Contralateral leg weakness; reduced initiation |
| Middle cerebral (MCA) | Lateral hemisphere, including Broca's and Wernicke's areas | Left: aphasia, right face and arm weakness. Right: left neglect. The most common stroke site |
| Posterior cerebral (PCA) | Occipital and inferior temporal lobes, thalamus | Contralateral visual field loss; alexia without agraphia (left) |
| Vertebrobasilar | Brainstem and cerebellum | Dysarthria, dysphagia, vertigo, ataxia |
Neuroplasticity
Neuroplasticity is the brain's capacity to reorganize in response to experience. Kleim and Jones described ten principles that guide treatment dosage and task selection: use it or lose it; use it and improve it; specificity (the practice dictates the change); repetition matters; intensity matters; time matters; salience matters (practice must be meaningful); age matters; transference (training can enhance similar behaviors); and interference (training can impede others).
- Broca's area: posterior inferior frontal gyrus. Wernicke's area: posterior superior temporal gyrus.
- Left hemisphere: language form and content. Right hemisphere: prosody, pragmatics, attention, visuospatial skills.
- UMN signs: spasticity, hyperreflexia. LMN signs: flaccidity, atrophy, fasciculations.
- Basal ganglia: hypokinetic or hyperkinetic dysarthria. Cerebellum: ataxic dysarthria.
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