CHAPTER 12 — CENTRAL NERVOUS SYSTEM

Brain Development & Tissues

The central nervous system (CNS) develops from the neural tube, which forms the brain and spinal cord. The organization of gray and white matter is fundamental to CNS function.

• Neural Tube Formation: By week 4 of embryonic development, the neural tube forms the brain and spinal cord. Its cavity becomes the ventricles and central canal.

• Brain Vesicles: By week 5, the neural tube differentiates into regions:

  • Telencephalon: Forms the cerebral hemispheres and lateral ventricles.

  • Diencephalon: Forms the thalamus, hypothalamus, epithalamus, and the third ventricle.

  • Mesencephalon: Forms the midbrain and cerebral aqueduct.

  • Metencephalon/Myelencephalon: Form the pons, cerebellum, medulla, and the fourth ventricle.

• Gray Matter: Composed of neuron cell bodies, dendrites, and synapses; responsible for processing information.

• White Matter: Consists mainly of myelinated axons; transmits signals between regions.

• Cortex: The outer layer of gray matter in the brain.

• Nucleus: Cluster of neuron cell bodies within the CNS.

• Tract: Bundle of axons in the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

Cerebrum & Cortex

The cerebrum is the largest part of the brain, responsible for higher functions such as awareness, sensation, movement, language, and memory.

• Structure: Each hemisphere contains cortex (gray matter), inner white matter, and basal nuclei (deep gray matter). The insula is a buried cortical region.

• Functions of Cortex: Supports conscious awareness, sensation, voluntary movement, language, memory, and understanding.

• Motor Areas:

  • Precentral Gyrus: Controls voluntary movement of the opposite side of the body; mapped by the motor homunculus (hands and face have large representation).

  • Premotor Cortex: Plans learned, patterned movements.

  • Broca’s Area: Produces speech; damage causes nonfluent aphasia.

  • Frontal Eye Field: Controls voluntary eye movements.

• Sensory Areas:

  • Postcentral Gyrus: Receives touch and proprioceptive input from the opposite side.

  • Association Cortex: Identifies texture and shape.

  • Occipital Lobe: Processes vision.

  • Temporal Lobe: Processes hearing.

  • Insula: Processes visceral sensation, balance awareness, and taste.

  • Uncus: Processes smell.

• Association Areas:

  • Prefrontal Cortex: Responsible for planning, judgment, working memory, and personality.

  • Posterior Association Area: Recognizes patterns, faces, and spatial relationships.

  • Wernicke’s Area: Comprehends language; damage causes fluent but nonsensical speech.

  • Limbic Association Area: Links emotion and memory.

• Hemispheric Specialization: Left hemisphere often handles language, math, and logic; right hemisphere often handles spatial, artistic, and emotional functions. Both hemispheres work together.

• Projection Fibers: Connect cortex to lower CNS regions; internal capsule fibers spread as corona radiata.

• Basal Nuclei: Regulate movement and suppress unwanted actions.

Diencephalon, Brainstem & Cerebellum

The diencephalon, brainstem, and cerebellum are essential for sensory relay, autonomic control, movement coordination, and consciousness.

• Thalamus: Major relay for sensory and motor signals to the cortex; supports arousal, learning, and memory.

• Hypothalamus: Maintains homeostasis, controls autonomic functions, links to the pituitary gland, and contains the suprachiasmatic nucleus (SCN) for circadian rhythm.

• Epithalamus/Pineal Gland: Releases melatonin to regulate sleep-wake cycles.

• Brainstem: Composed of midbrain, pons, and medulla; contains pathways and vital centers.

  • Midbrain: Contains cerebral peduncles, colliculi, and cranial nerves III-IV.

  • Pons: Relays signals between cortex and cerebellum; contains cranial nerves V-VII.

  • Medulla: Contains pyramids, sensory nuclei, cranial nerves VIII-X, XII; controls heart rate, blood vessel diameter, breathing, and reflexes (cough, swallow, vomit).

• Cerebellum: Coordinates movement, balance, and posture; compares intended movement with sensory feedback. Arbor vitae is its inner white matter.

• Limbic System: Links emotion, memory, and behavior.

• Reticular Formation/RAS: Supports alertness, consciousness, filtering of sensory input, and autonomic/motor control; severe damage may cause coma.

CSF, Meninges & Higher Functions

Cerebrospinal fluid (CSF) and meninges protect the CNS, while higher functions include memory, consciousness, and sleep.

• Ventricles: CSF-filled cavities connected to the spinal central canal; lined by ependymal cells.

• CSF Production & Flow: CSF is produced by the choroid plexus, flows through ventricles, exits via fourth ventricle apertures, enters the subarachnoid space, and is absorbed by arachnoid granulations into venous sinuses.

  • Functions: Provides buoyancy, cushioning, chemical stability, and nourishment.

  • Hydrocephalus: Blockage of CSF flow causes accumulation and increased pressure.

• Meninges:

  • Dura Mater: Outer layer; contains venous sinuses.

  • Arachnoid Mater: Middle layer; contains CSF space and granulations.

  • Pia Mater: Inner layer; closely adheres to brain surface.

  • Falx Cerebri: Separates cerebral hemispheres.

• Memory: Progresses from sensory to short-term to long-term; consolidation aided by rehearsal, emotion, and association. The hippocampus is crucial for memory formation.

  • Anterograde Amnesia: Inability to form new memories.

  • Retrograde Amnesia: Loss of past memories.

• Consciousness: Includes awareness, movement control, and higher thought. Fainting is brief loss, coma is prolonged, and brain death is irreversible.

• EEG: Records electrical activity of the brain; used to assess seizures, sleep, and brain disorders.

• Sleep: Includes NREM and REM stages; regulated by circadian rhythm (~24 hours) via SCN. Sleep supports memory and restoration; CSF may clear waste during sleep.

Spinal Cord & Pathways

The spinal cord conducts signals between the brain and body and mediates reflexes. Its structure and pathways are essential for sensory and motor function.

• Structure: Begins at the foramen magnum, ends near L1–L2. Enlargements serve limbs; conus medullaris is the tapered end; cauda equina is the bundle of lower nerve roots; filum terminale anchors the cord. There are 31 pairs of spinal nerves.

• Clinical Procedures: Lumbar puncture (LP) is commonly performed at L4–L5. Epidural space contains fat and veins; CSF is in the subarachnoid space.

• Cross-Section: Central gray matter (horns), outer white matter (columns); central canal contains CSF.

• Roots: Dorsal roots are sensory (dorsal root ganglia contain sensory cell bodies); ventral roots are motor; they join to form mixed spinal nerves.

• White Columns: Dorsal/posterior, lateral, and ventral/anterior columns contain ascending and descending tracts.

• Pathway Rules:

  • Decussation: Pathways cross from one side to the other.

  • Relay: Pathways involve chains of neurons.

  • Somatotopy: Pathways are organized according to body regions.

  • Bilateral: Pathways are present on both sides of the CNS.

• Ascending Sensory Pathways: Usually involve three neurons. The dorsal column pathway carries fine touch and proprioception:

  • Gracilis: Lower body.

  • Cuneatus: Upper body.

  • Pathway: Receptors → medulla → thalamus → sensory cortex.

• Descending Motor Pathways: Upper motor neuron (cortex) → lower motor neuron (ventral horn) → muscle.

  • Corticospinal (Direct) Tracts: Control skilled voluntary movement.

  • Indirect Tracts: Support posture, balance, coarse movement, and head/eye orientation.

Table: CNS Gray and White Matter Structures

Table: Major Brain Regions and Functions

Example:

Damage to Broca’s area results in nonfluent aphasia, where speech production is impaired but comprehension may remain intact. Conversely, damage to Wernicke’s area causes fluent but nonsensical speech, indicating impaired language comprehension.

Additional info: The organization of ascending and descending pathways is crucial for understanding neurological deficits following injury or disease. The spinal cord's segmental arrangement allows for targeted clinical interventions, such as lumbar puncture or epidural anesthesia.