Central Nervous System: Structure, Protection, and Disorders

Central Nervous System (CNS)

Overview

The Central Nervous System (CNS) consists of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly, serving as the main control center for the body.

Protecting the CNS

Physical and Chemical Barriers

• Bones: The skull protects the brain, and the vertebral column protects the spinal cord from mechanical injury.

• Meninges: Three connective tissue membranes that envelop the CNS:

  • Dura mater: The tough, outermost layer.

  • Arachnoid mater: The middle, web-like layer.

  • Pia mater: The delicate, innermost layer adhering to the CNS surface.

• Blood-Brain Barrier (BBB):

  • Formed by tight junctions between endothelial cells, astrocytes, and the presence of Cerebrospinal Fluid (CSF).

  • Limits the passage of harmful substances from the blood into the CNS, while allowing essential nutrients.

  • CSF also cushions and protects the CNS from trauma.

The Central Nervous System: Nervous Tissue

Gray Matter vs. White Matter

• Gray Matter: Contains neuron cell bodies and unmyelinated fibers. Responsible for processing and integrating information.

• White Matter: Composed of myelinated axons. Functions in the rapid transmission of nerve signals.

Disorder Example: Multiple Sclerosis (MS)

• Definition: A degenerative disease where the myelin sheath of CNS neurons is destroyed, disrupting signal transmission.

• Symptoms: Muscle weakness, impaired coordination, and other neurological deficits.

• Treatment: No cure; managed with medication and physical therapy.

The CNS: Brain

Major Brain Regions

• Cerebrum: The largest part, responsible for conscious thought, memory, speech, intelligence, sensory processing, and voluntary motor movement.

  • Divided into lobes: Frontal, Parietal, Temporal, Occipital.

  • Highly folded (gyri and sulci) to increase surface area.

  • Hemispheres: Left and right, each with specialized functions (lateralization).

• Cerebellum: Coordinates skeletal muscle movements and maintains posture and balance. Contains about 50% of the brain's neurons.

• Diencephalon: Relay center connecting cerebral hemispheres and the rest of the CNS.

  • Thalamus: Major sensory relay and regulator of endocrine functions.

  • Hypothalamus: Maintains homeostasis by controlling autonomic functions (e.g., temperature, hunger, thirst).

• Brain Stem: Connects the brain to the spinal cord and controls vital functions.

  • Midbrain: Relays sensory information for vision and hearing; contains substantia nigra (dopamine production).

  • Pons: Relay station for motor control and regulation of breathing rhythm.

  • Medulla Oblongata: Regulates heart rate, blood pressure, respiratory rate, and reflexes like vomiting and swallowing.

Ventricles and Cerebrospinal Fluid (CSF)

• Ventricles: Cavities within the brain filled with CSF.

• Functions of CSF:

  • Cushions and protects the brain.

  • Provides nutrients and removes waste.

  • Keeps the brain buoyant.

• Ependymal cells: Line the ventricles and produce CSF.

CNS: Spinal Cord

Anatomy and Protection

• Location: Begins at the foramen magnum of the skull and ends at approximately the L2 vertebra (cauda equina).

• Protection: Like the brain, protected by bone, meninges, and CSF.

• Growth: The spinal cord stops growing around age 4; spinal nerves continue to extend between vertebrae.

Internal Structure

• White Matter: Located on the outside; organized into columns.

• Gray Matter: Located on the inside; forms an H-shaped region (horns).

• Spinal Nerves: Extend from the spinal cord via roots:

  • Dorsal (posterior) root: Sensory input; contains dorsal root ganglion.

  • Ventral (anterior) root: Motor output.

Disorders and Diseases of the CNS

Major Disorders

• Alzheimer's Disease:

  • Progressive neurodegenerative disease affecting synapses in the cerebrum.

  • Associated with increased plaque and acetylcholine (ACh) deficiency.

  • Leads to memory loss, language and reasoning impairment, and is the leading cause of dementia.

• Parkinson's Disease:

  • Degenerative disorder affecting dopamine-producing neurons (especially in the substantia nigra).

  • Symptoms include movement difficulties, tremors, and changes in mood and behavior.

  • May be genetic or caused by trauma.

• Multiple Sclerosis (MS):

  • Progressive disease destroying myelin in the CNS.

  • Disrupts and slows nerve signal transmission, affecting multiple organs and functions.

• Meningitis:

  • Infection and inflammation of the meninges and CSF.

  • Usually caused by bacterial or viral infection.

  • Diagnosis confirmed by spinal tap (lumbar puncture).

Summary Table: Major Brain Regions and Functions

Region	Main Functions
Cerebrum	Conscious thought, memory, speech, intelligence, sensory processing, voluntary movement
Cerebellum	Coordination of movement, balance, posture
Diencephalon (Thalamus & Hypothalamus)	Sensory relay, endocrine regulation, homeostasis
Brain Stem (Midbrain, Pons, Medulla)	Relay of information, autonomic regulation (heart rate, breathing), reflexes

Key Terms and Definitions

• Meninges: Three protective membranes surrounding the CNS.

• Blood-Brain Barrier: Selective barrier that protects the brain from harmful substances.

• Cerebrospinal Fluid (CSF): Fluid that cushions the CNS and provides nutrients.

• Gray Matter: CNS tissue containing neuron cell bodies.

• White Matter: CNS tissue containing myelinated axons.

• Multiple Sclerosis: Disease characterized by demyelination in the CNS.

• Alzheimer's Disease: Neurodegenerative disorder causing dementia.

• Parkinson's Disease: Disorder affecting movement due to dopamine deficiency.

• Meningitis: Inflammation of the meninges, often due to infection.