Central Nervous System: Brain Structure and Function

Central Nervous System Overview

The Brain

The brain is the central organ of the nervous system, responsible for processing sensory information, controlling motor functions, and enabling higher cognitive abilities.

• Composition: Made of wrinkled, gray tissue; weighs about 1.4 kg (3 lbs).

• Brain Volume: Approximately 1200 ml.

• Sex Differences: Males have larger brains than females (about 10%). No direct correlation between size and intelligence.

• Myelination: Brain has both myelinated (white matter) and non-myelinated (gray matter) areas.

Brain Development

Embryonic Development

The embryonic ectoderm thickens to form a neural plate, which folds to create the neural tube. The neural tube forms the CNS structures, while neural crest cells form parts of the PNS.

• Neural tube forms three primary vesicles: forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).

• These develop into five secondary vesicles: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

Major Brain Regions

• Telencephalon: Becomes the cerebrum (largest part of the brain).

• Diencephalon: Includes thalamus, hypothalamus, and epithalamus.

• Mesencephalon: Forms the midbrain.

• Metencephalon: Forms the pons and cerebellum.

• Myelencephalon: Forms the medulla oblongata.

Brainstem

Components and Functions

The brainstem consists of the medulla oblongata, pons, and midbrain. It controls vital functions such as breathing, heart rate, and reflexes.

• Midbrain: Located between diencephalon and pons; contains nuclei involved in pain suppression and motor control.

• Pons: Bulging region; contains conduction tracts and cranial nerve nuclei.

• Medulla Oblongata: Most inferior part; controls cardiovascular and respiratory centers, vomiting, hiccuping, and other autonomic functions.

Ventricles of the Brain

Structure and Function

Ventricles are fluid-filled cavities derived from the embryonic neural tube, providing cushioning and nutrient transport.

• Lateral Ventricles: Paired C-shaped cavities in each cerebral hemisphere.

• Third Ventricle: Located in the diencephalon; connected to lateral ventricles via interventricular foramen.

• Fourth Ventricle: Located in the hindbrain; connected to third ventricle via cerebral aqueduct.

• Corpus Callosum: Connects the two cerebral hemispheres.

Cerebral Hemispheres

Structure and Lobes

The cerebrum is divided into two hemispheres, each containing five lobes separated by sulci and fissures.

• Frontal Lobe: Personality, voluntary movement, and higher cognitive functions.

• Parietal Lobe: Sensory processing.

• Temporal Lobe: Auditory and olfactory processing.

• Occipital Lobe: Visual processing.

• Insula: Deep within lateral sulcus; involved in taste and visceral sensation.

Functional Areas of the Cortex

The cerebral cortex is responsible for conscious thought, voluntary movement, and sensory perception. It is divided into three main functional areas:

• Motor Areas: Control voluntary movement.

• Sensory Areas: Receive and process sensory information.

• Association Areas: Integrate information for complex functions.

Motor Areas

• Primary Motor Cortex: Located in the frontal lobe; controls precise voluntary movements.

• Premotor Cortex: Plans and coordinates learned motor skills.

• Frontal Eye Field: Controls voluntary eye movements.

Clinical Note

• Damage to Primary Motor Cortex: Paralysis of muscles controlled by the affected area.

• Damage to Premotor Cortex: Loss of skill memory, but not muscle strength.

Sensory Areas

• Primary Somatosensory Cortex: Receives sensory input from skin, muscles, and joints.

• Somatosensory Association Cortex: Integrates sensory information for understanding objects.

• Visual, Auditory, Olfactory, Gustatory, and Visceral Areas: Process respective sensory modalities.

Cerebral White Matter

Types of Fibers

White matter consists of myelinated axons that connect different brain regions.

• Commissural Fibers: Connect corresponding areas of the two hemispheres (e.g., corpus callosum).

• Association Fibers: Connect different parts of the same hemisphere.

• Projection Fibers: Connect the cortex with lower brain regions and spinal cord.

Basal Nuclei

Structure and Function

Basal nuclei are clusters of gray matter deep within each hemisphere, involved in motor control and coordination.

• Functions: Regulate voluntary movement, inhibit unnecessary movements, and are associated with disorders such as Parkinson's and Huntington's disease.

Diencephalon

Major Structures

• Thalamus: Relay station for sensory information; forms walls of the third ventricle.

• Hypothalamus: Regulates homeostasis, body temperature, hunger, thirst, and endocrine functions.

• Epithalamus: Contains the pineal gland; involved in sleep-wake cycles.

Cerebellum

Structure and Function

The cerebellum is located dorsal to the pons and medulla, accounting for 11% of total brain mass. It coordinates voluntary movements, balance, and posture.

• Vermis: Connects the two cerebellar hemispheres.

• Cerebellar Peduncles: Fiber tracts connecting cerebellum to brain stem.

Protective Coverings of the Brain

Meninges

The brain is protected by three connective tissue membranes called meninges:

• Dura Mater: Tough, outermost layer.

• Arachnoid Mater: Middle layer; contains cerebrospinal fluid (CSF) in the subarachnoid space.

• Pia Mater: Delicate, innermost layer adhering to the brain surface.

Blood-Brain Barrier

Function and Structure

The blood-brain barrier (BBB) protects the brain from harmful substances in the blood while allowing essential nutrients to pass.

• Structure: Formed by tight junctions between endothelial cells of capillaries, astrocyte end-feet, and a thick basement membrane.

• Function: Selectively permeable to nutrients, some metabolic wastes, and ions; restricts passage of toxins and pathogens.

Summary Table: Major Brain Regions and Functions

Region	Main Structures	Primary Functions
Cerebrum	Frontal, Parietal, Temporal, Occipital, Insula	Conscious thought, voluntary movement, sensory perception
Diencephalon	Thalamus, Hypothalamus, Epithalamus	Sensory relay, homeostasis, endocrine regulation
Brainstem	Midbrain, Pons, Medulla Oblongata	Autonomic functions, reflexes, cranial nerve nuclei
Cerebellum	Vermis, Hemispheres, Peduncles	Coordination, balance, posture

Key Equations and Terms

• Equation for Brain Volume: $\text{Brain Volume} \approx 1200\ \text{ml}$

• Corpus Callosum: Major commissural fiber connecting the two hemispheres.

• Basal Nuclei: Involved in motor control and coordination.

• Blood-Brain Barrier: Selective permeability for brain protection.

Additional info: Some academic context and definitions have been expanded for clarity and completeness.