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 main control center for processing and responding to sensory information.

• CNS: Brain and spinal cord; responsible for integration and command.

• PNS: All neural tissue outside CNS; connects CNS to limbs and organs.

Protection of the Brain

Meninges

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

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

• Arachnoid mater: Middle layer with web-like extensions; contains CSF and blood vessels.

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

Table: Layers of the Meninges

Layer	Location	Main Features
Dura mater	Outermost	Strong, fibrous, forms partitions
Arachnoid mater	Middle	Web-like, contains CSF
Pia mater	Innermost	Delicate, vascular

Inflammation in the Nervous System

Meningitis and Encephalitis

Meningitis is inflammation of the meninges, which can spread to the CNS and cause encephalitis (inflammation of the brain). Diagnosis is often made by analyzing CSF obtained via lumbar puncture.

• Symptoms: Fever, headache, neck stiffness, neurological deficits.

• Risks: Swelling can compress brain tissue, leading to severe complications.

Cerebrospinal Fluid (CSF)

Functions and Formation

CSF forms a liquid cushion around the brain, providing buoyancy, protection, and chemical stability.

• Buoyancy: Reduces effective brain weight by 97%.

• Protection: Shields CNS from trauma.

• Nourishment: Delivers nutrients and removes waste.

CSF is produced mainly by the choroid plexus, a network of capillaries in the ventricles.

Hydrocephalus

Pathophysiology and Treatment

Hydrocephalus occurs when CSF circulation or drainage is blocked, increasing intracranial pressure. In infants, this causes skull enlargement; in adults, it can compress brain tissue.

• Treatment: Drainage of CSF via ventricular shunt to abdominal cavity.

Blood Brain Barrier (BBB)

Structure and Function

The BBB maintains a stable environment for the brain by regulating the passage of substances from blood to neural tissue.

• Tight junctions: Prevent passage of harmful substances.

• Astrocytes: Support barrier function.

• Transport mechanisms:

  • Simple diffusion for lipid-soluble substances.

  • Facilitated diffusion for glucose, amino acids.

  • Transcytosis for larger molecules.

Some areas lack BBB (e.g., vomiting center, hypothalamus) to monitor blood composition.

Traumatic Brain Injuries

Types and Effects

• Concussion: Temporary alteration in function.

• Contusion: Permanent damage.

• Subdural/Epidural hemorrhage: Bleeding increases pressure, can be fatal.

• Cerebral edema: Swelling due to injury.

Cerebrovascular Accidents (CVAs)

Stroke and Related Conditions

CVAs, or strokes, result from interrupted blood supply to the brain, causing tissue death.

• Ischemia: Lack of blood flow, often due to clot.

• Hemiplegia: Paralysis on one side.

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

• Tissue plasminogen activator (TPA): Used to treat ischemic stroke.

Degenerative Brain Disorders

Alzheimer's Disease

• Progressive dementia, memory loss, disorientation.

• Beta-amyloid plaques and neurofibrillary tangles disrupt neuron function.

Parkinson's Disease

• Degeneration of dopamine-producing neurons.

• Symptoms: tremor, rigidity, slow movement, depression.

• Treatment: L-dopa, deep brain stimulation.

Huntington's Disease

• Hereditary, caused by mutant huntingtin protein.

• Symptoms: wild movements, mental deterioration.

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

Diagnostic Procedures for CNS Dysfunction

Clinical and Imaging Techniques

• Reflex tests: Assess basic CNS function.

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

• Cerebral angiography: X-rays with dye for blood flow.

• Ultrasound: Evaluates blood flow in arteries.

Spinal Anatomy and Protection

Structure and Function

• Spinal cord extends from foramen magnum to L1/L2 vertebra.

• Major reflex center; two-way communication with brain and body.

• Conus medullaris: Cone-shaped end of spinal cord.

• Cauda equina: Bundle of nerve roots at lower end.

Protection

• Bone, meninges, and CSF protect spinal cord.

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

• Subarachnoid space: Contains CSF.

Spinal Cord Cross-sectional Anatomy

Gray and White Matter

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

• White matter: Surrounds gray matter; contains ascending (sensory), descending (motor), and transverse tracts.

Table: Spinal Cord Horns and Functions

Horn	Function
Dorsal	Sensory input
Ventral	Motor output
Lateral	Autonomic (visceral) motor

Spinal Cord Trauma and Disorders

Types of Injury

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

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

• Flaccid paralysis: No voluntary/involuntary muscle control; muscles atrophy.

• Spastic paralysis: Reflex activity remains; no voluntary control.

Transection Syndromes

• Paraplegia: Loss of function below T1-L1.

• Quadriplegia: Loss of function in all limbs (cervical region).

• Spinal shock: Temporary loss of function below injury.

Poliomyelitis (Paralytic)

Pathology and Effects

• Destruction of ventral horn motor neurons by poliovirus.

• Muscle atrophy, paralysis, possible respiratory failure.

• Postpolio syndrome: Late-onset symptoms in survivors.

Amyotrophic Lateral Sclerosis (ALS)

Features and Progression

• Destruction of upper and lower motor neurons.

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

• Death within 5 years typical; no cure.

• Drug riluzole slows progression by interfering with glutamate signaling.

Neuronal Pathways

Key Features

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

• Relay: Chains of two or three neurons.

• Somatotopy: Spatial relationship in CNS reflects body arrangement.

• Symmetry: Pathways are paired (right and left).

Ascending Pathways

Organization

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

• Second-order neuron: Spinal cord/medulla to thalamus/cerebellum.

• Third-order neuron: Thalamus to somatosensory cortex.

Main Tracts

• 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 touch and vibration.

• Composed of fasciculus cuneatus and gracilis.

Spinocerebellar Tracts

• Convey muscle/tendon stretch information for coordination.

Spinothalamic Pathways

• Transmit pain, temperature, and pressure.

Discussion Questions

• Why is meningitis dangerous for neurological function?

• Compare Alzheimer's and Parkinson's diseases.

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

Additional info: These notes cover topics from Ch. 11 (Fundamentals of the Nervous System and Nervous Tissue) and Ch. 12 (The Central Nervous System) of a standard Anatomy & Physiology curriculum.