Chapter 20 – Nervous System Structures & Disorders

Brain & Spinal Functional Areas (Matching Style Reference)

Nervous System Overview

1. Motor Area (Primary Motor Cortex – Pre-Central Gyrus)

• Function: Generates voluntary movements by sending descending signals through corticospinal and corticobulbar tracts.

• Structure: Topographic map known as the motor homunculus, where larger cortical areas are allocated for fine motor control (e.g., hands, face).

• Damage Effects: Leads to contralateral flaccid paralysis, potentially progressing to spasticity following an upper motor neuron lesion.

• Clinical Correlation: Upper motor neuron (UMN) lesions result in hyperreflexia, which can be tested with the Babinski sign.

2. Sensory Areas (Primary Somatosensory Cortex – Post-Central Gyrus)

• Function: Receives sensory information (proprioception, touch, pain, temperature) from the contralateral body via thalamic relay.

• Structure: Sensory homunculus parallels the motor map.

• Damage Effects: Can cause loss of discriminative touch and proprioceptive deficits. Integration with the association cortex allows for stereognosis (object recognition by touch).

3. Association Areas

• Function: Integrate incoming data with memory, emotion, and reasoning to produce context, comprehension, and planning.

• Examples:

• Prefrontal cortex (executive functions)

• Wernicke’s area (language comprehension)

• Parietal-temporal-occipital areas (multimodal integration).

• Lesion Effects: Can lead to neglect syndromes, agnosias, and aphasias, emphasizing the importance of integration beyond primary cortices.

4. Cerebrum (Cerebral Hemispheres)

• Composition: Contains cortical gray matter, basal nuclei, and limbic structures; responsible for conscious thought, perception, and intellect.

• Features: Fissures and sulci increase surface area (~2,500 cm²).

• Lateralization: Left hemisphere is associated with language and analytic tasks; right hemisphere is linked to visuospatial skills and emotions.

5. Cerebellum

• Function: Produces skilled muscle movements, helps with spatial orientation, and controls posture.

• Mechanism: Coordinates timing, force, and precision of skeletal muscle contractions; adjusts postural tone through an error-correction loop comparing intended and actual movements using proprioceptive feedback.

• Damage Effects: Can lead to ataxia (wide-based gait), intention tremors, dysmetria, and nystagmus. Alcohol consumption acutely depresses cerebellar function.

6. Thalamus

• Function: Acts as a bilateral relay station atop the brainstem, filtering and amplifying sensory (except olfaction) and motor signals.

• Role in Sleep: Participates in sleep-wake regulation via the reticular activating system (RAS).

• Pathology: Thalamic pain syndrome and coma can occur if RAS is disrupted.

7. Hypothalamus

• Function: Major center for visceral control and homeostasis, regulating temperature, hunger, thirst, circadian rhythms, and autonomic output through its connection to the pituitary gland.

• Nuclei: Contains supraoptic and paraventricular nuclei producing antidiuretic hormone (ADH) and oxytocin.

• Lesion Effects: Can lead to diabetes insipidus, obesity, and thermoregulatory failure.

8. Pons

• Function: Acts as a bridge of fibers between the cerebrum and cerebellum; contains nuclei for cranial nerves V–VIII.

• Respiratory Role: Modulates respiratory rhythm through pneumotaxic and apneustic centers.

• Reflexes: Involves corneal blink and mastication reflexes.

• Damage Effects: Damage may result in locked-in syndrome if corticospinal tracts are bilaterally destroyed.

9. Medulla Oblongata

• Function: Connects the brain to the spinal cord and houses vital autonomic centers for cardiovascular and respiratory functions.

• Decussation: Approximately 90% of corticospinal fibers cross here, leading to contralateral control.

• Lesion Effects: High lesions can result in respiratory arrest.

10. Limbic System

• Components: Includes the hippocampus, amygdala, cingulate gyrus, and fornix.

• Function: Governs emotion, motivation, and memory consolidation.

• Clinical Relevance: Hyperactivity in the amygdala is linked to PTSD, while damage to the hippocampus can cause anterograde amnesia (e.g., in Alzheimer’s disease).

11. Corpus Callosum

• Function: The largest commissural fiber bundle linking the left and right cerebral hemispheres (approximately 200 million axons).

• Role: Enables bilateral integration of functions such as language (left) and prosody (right), as well as bimanual coordination.

• Surgical Consideration: Corpus callosotomy may be performed to treat intractable epilepsy, leading to “split-brain” phenomena.

12. Cerebrospinal Fluid (CSF)

• Production: Clear, colorless fluid produced primarily in the lateral ventricles by the choroid plexus (ependymal cells + fenestrated capillaries).

• Circulation Pathway: Lateral ventricles → interventricular foramina → 3rd ventricle → cerebral aqueduct → 4th ventricle → median/lateral apertures → subarachnoid space → arachnoid villi → dural venous sinuses.

• Volume & Composition: Approximately 150 mL volume; turnover rate is about 3–4 times per day (~500 mL/day). Low protein content (<45 mg/dL), glucose around 60% of serum levels, and optimized ion milieu for neuronal excitability.

• Functions: Provides buoyancy, acts as a mechanical cushion, aids in waste removal, and maintains chemical stability.

• Clinical Application: Lumbar puncture at L3–L4 can be used to analyze CSF for conditions like meningitis or subarachnoid hemorrhage.

13. Spinal Nerve Roots

• Dorsal (Posterior) Root: Sensory/Afferent fibers carry sensory information from the periphery to the spinal cord; lesions can cause anesthesia or paresthesia in a dermatomal pattern.

• Ventral (Anterior) Root: Motor/Efferent fibers conduct axons of lower motor neurons to skeletal muscles, with lesions leading to flaccid paralysis and loss of reflexes.

14. Neurological Diseases & Disorders

• Parkinson’s Disease: Progressive movement disorder due to degeneration of dopaminergic neurons; characterized by resting tremor, bradykinesia, and rigidity. Treatment includes L-DOPA and deep-brain stimulation.

• Dementia: Global cognitive decline with various causes, most commonly Alzheimer’s. Treatment focuses on symptomatic relief.

• Cerebral Palsy (CP): Non-progressive motor disorders resulting from early brain insults; manifests as abnormal muscle tone and delayed milestones.

• Huntington’s Disease: Genetic disorder causing progressive chorea and dementia; treatment is symptomatic.

• CVA (Stroke): Sudden neurologic deficit due to brain ischemia or hemorrhage; acute management includes tPA and thrombectomy.

• Poliomyelitis: Viral infection leading to paralysis; vaccination has significantly reduced incidence.

15. Integrative & Ethical Considerations

• Understanding neuroanatomy is crucial for clinical localization and management.

• Neurodegenerative diseases raise ethical issues regarding autonomy, resource allocation, and genetic counseling.

• Public health significance: Education on stroke prevention is vital as strokes are a leading cause of death in the US.

• Global equity in healthcare is important, as evidenced by polio eradication campaigns highlighting disparities in vaccination coverage.

16. Numerical & Statistical Highlights

• CSF Volume: 150 mL; production rate of ~500 mL/day.

• Corticospinal Decussation: About 90% of fibers cross.

• Alzheimer’s Prevalence: Over 5 million Americans affected; incidence doubles every five years after age 65.

• Stroke Statistics: Approximately 795,000 cases in the US annually; about 140,000 deaths.