Central Nervous System: Structure, Protection, Disorders, and Pathways

Divisions of the Nervous System

Overview

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, serving as the integration and command center. The PNS includes all neural elements outside the CNS, responsible for communication between the CNS and the rest of the body.

• CNS: Brain and spinal cord; processes and integrates information.

• PNS: Nerves and ganglia outside the CNS; transmits sensory and motor signals.

Protection of the Brain

Meninges

The brain is protected by three connective tissue membranes called meninges, which cover the CNS, protect blood vessels, enclose venous sinuses, contain cerebrospinal fluid (CSF), and form partitions in the skull.

• Dura mater: Outermost, toughest meninx; limits excessive movement of the brain.

• Arachnoid mater: Middle layer with web-like extensions; separated from dura by subdural space; subarachnoid space contains CSF and blood vessels.

• Pia mater: Innermost, delicate layer; clings tightly to brain and contains many tiny blood vessels.

Main Dural Septa

• Falx cerebri: In longitudinal fissure, attached to crista galli.

• Falx cerebelli: Along vermis of cerebellum.

• Tentorium cerebelli: Horizontal fold over cerebellum and in transverse fissure.

Inflammation in the Nervous System

Meningitis and Encephalitis

• Meningitis: Inflammation of the meninges, often diagnosed by lumbar puncture and CSF analysis.

• Encephalitis: Inflammation may spread to CNS, affecting the brain.

Cerebrospinal Fluid (CSF)

Functions and Formation

• Buoyancy: Reduces effective brain weight by 97%.

• Protection: Cushions CNS from trauma.

• Nourishment: Carries chemical signals.

CSF is formed from blood plasma by the choroid plexus, a cluster of capillaries in each ventricle, and is filtered at a constant rate.

CSF Location

• CSF circulates in ventricles, central canal of spinal cord, and subarachnoid space.

Hydrocephalus

Pathophysiology and Treatment

• Hydrocephalus: Obstruction of CSF flow increases intracranial pressure.

• In newborns, unfused skull bones allow head enlargement.

• In adults, increased pressure can damage brain tissue.

• Treatment: Drain CSF via ventricular shunt.

Blood Brain Barrier (BBB)

Structure and Function

• Maintains stable environment for brain; prevents uncontrolled neuron firing.

• Substances from blood must pass through capillary endothelium and tight junctions.

• Astrocyte feet and pericytes support barrier integrity.

Transport Mechanisms

• Simple diffusion: Lipid-soluble substances and gases.

• Facilitated diffusion: Glucose, amino acids, specific ions.

• Transcytosis: Larger substances.

Exceptions

• BBB is absent in areas like the vomiting center and hypothalamus for monitoring blood composition.

Traumatic Brain Injuries

Types and Effects

• Concussion: Temporary alteration in function.

• Contusion: Permanent damage.

• Subdural/Epidural hemorrhage: Blood pressure may force brain stem through foramen magnum.

• Cerebral edema: Swelling associated with injury.

Cerebrovascular Accidents (CVAs)

Stroke and Related Conditions

• Ischemia: Loss of blood supply leads to tissue death.

• Hemiplegia: Paralysis on one side; sensory/speech deficits.

• Transient ischemic attacks (TIAs): Temporary, reversible episodes.

• Tissue plasminogen activator (TPA): Treatment for ischemic stroke.

Degenerative Brain Disorders

Alzheimer's Disease

• Progressive dementia; misfolded proteins, beta-amyloid plaques, neurofibrillary tangles.

• Brain shrinks as cells die.

Parkinson's Disease

• Degeneration of dopamine neurons in substantia nigra.

• Symptoms: tremor, shuffling gait, depression, cognitive decline.

• Treatment: L-dopa, deep brain stimulation, gene therapy.

Huntington's Disease

• Hereditary; accumulation of huntingtin protein.

• Degeneration of basal nuclei and cortex; wild movements, mental deterioration.

• Treatment: drugs blocking dopamine effects; stem cell research ongoing.

Diagnostic Procedures for CNS Dysfunction

Clinical and Imaging Techniques

• Reflex tests: Knee-jerk reflex for basic CNS function.

• Imaging: CT, MRI, PET for tumors, lesions, infarcts.

• Cerebral angiography: X-rays with dye for stroke diagnosis.

• Ultrasound: Evaluates blood flow in brain arteries.

Spinal Anatomy and Protection

Structure and Terminology

• Spinal cord begins at foramen magnum, ends at L1 or L2 vertebra.

• Provides two-way communication; major reflex center.

• Conus medullaris: Cone-shaped terminal structure.

• Cauda equina: Nerve roots at inferior end.

• Filum terminale: Anchors spinal cord to coccyx.

Protection

• Protected by bone, meninges, and CSF.

• Epidural space: Fat and veins between vertebrae and dura mater.

• Subarachnoid space: Contains CSF.

• Site of lumbar puncture: Below L2.

Spinal Cord Cross-sectional Anatomy

Gray and White Matter

• Gray matter: Central, butterfly-shaped; includes dorsal, ventral, and lateral horns.

• White matter: Surrounds gray; contains ascending, descending, and transverse tracts.

• Central canal: Runs length of cord, filled with CSF.

Organization of Gray Matter

• Dorsal root: Sensory input.

• Ventral root: Motor output.

• Gray commissure: Connects sides, encloses central canal.

White Matter Columns

• Dorsal (posterior), lateral, and ventral (anterior) funiculi.

• Tracts are composed of axons with similar functions.

Major Ascending (Sensory) and Descending (Motor) Tracts

Tract Organization

• Ascending tracts: Dorsal columns, spinocerebellar, spinothalamic.

• Descending tracts: Corticospinal (pyramidal), rubrospinal, vestibulospinal, reticulospinal, tectospinal.

Spinal Cord Trauma and Disorders

Types of Injury

• Paresthesia: Sensory loss from dorsal root/tract damage.

• Paralysis: Motor loss from ventral root/horn damage.

• Paralysis can be flaccid (no voluntary/involuntary control, muscle atrophy) or spastic (reflex activity remains, no voluntary control).

Transection Effects

• Paraplegia: Transection between T1 and L1.

• Quadriplegia: Transection in cervical region.

• Spinal shock: Temporary loss of function below lesion.

Poliomyelitis (Paralytic)

Pathogenesis and Effects

• Destruction of ventral horn motor neurons by poliovirus.

• Muscle atrophy, possible death from respiratory/cardiac failure.

• Postpolio syndrome: Late-onset symptoms from neuron loss.

Amyotrophic Lateral Sclerosis (ALS)

Features and Progression

• Destruction of upper and lower motor neurons in brain, brainstem, and spinal cord.

• Symptoms: Loss of speech, swallowing, breathing ability.

• Death within 5 years typical.

• Likely gene/environment interaction; glutamate excitotoxicity involved.

• Drug riluzole interferes with glutamate signaling (only treatment, not cure).

Neuronal Pathways

Key Features

• Decussation: Most pathways cross from one side of CNS to the other.

• Relay: Pathways consist of chains of two or three neurons.

• Somatotopy: Spatial relationship in CNS reflects body arrangement.

• Symmetry: Pathways are paired right and left.

Ascending Pathways

Neuron Chains

• First-order neuron: Sensory receptor to spinal cord/brainstem.

• Second-order neuron: Interneuron in dorsal horn/medullary nuclei; axons to thalamus/cerebellum.

• Third-order neuron: Interneuron in thalamus; axon to somatosensory cortex.

Main Pathways

• Dorsal column–medial lemniscal: Discriminative touch, vibration.

• Spinothalamic: Pain, temperature, coarse touch.

• Spinocerebellar: Muscle/tendon stretch to cerebellum.

Ascending Spinal Cord Tracts

Dorsal Column–Medial Lemniscal Pathways

• Transmit input for discriminative touch and vibration.

• Composed of fasciculus cuneatus and fasciculus gracilis.

Spinocerebellar Tracts

• Ventral and dorsal tracts; coordinate muscle activity.

Spinothalamic Pathways

• Lateral and ventral tracts; transmit pain, temperature, pressure.

Discussion Questions

• Why is meningitis so dangerous for neurological function?

• Compare Alzheimer's and Parkinson's diseases.

• What spinal cord injury could cause loss of voluntary movement but preserved reflexes?

Table: Comparison of Major Degenerative Brain Disorders

Disorder	Main Features	Symptoms	Treatment
Alzheimer's Disease	Progressive dementia, misfolded proteins, plaques, tangles	Memory loss, disorientation, mood changes	Supportive, cholinesterase inhibitors
Parkinson's Disease	Dopamine neuron degeneration	Tremor, shuffling gait, depression	L-dopa, deep brain stimulation
Huntington's Disease	Hereditary, huntingtin protein accumulation	Wild movements, mental decline	Dopamine blockers, stem cell research

Additional info:

• Equations are not directly relevant for this section, but the concept of CSF pressure can be described by hydrostatic pressure equations in physiology.

• Neuronal pathway organization is essential for understanding sensory and motor deficits in spinal cord injuries.