Central Nervous System: Structure, Development, and Functional Organization

Central Nervous System Overview

Definition and Components

The Central Nervous System (CNS) is composed of the brain and spinal cord. It is responsible for integrating sensory information and coordinating bodily functions. Embryologically, the CNS originates from the neural tube, which develops into the brain and spinal cord.

• Rostral: Refers to structures toward the forehead (upper).

• Caudal: Refers to structures toward the spinal cord (lower).

Embryonic Development of the Brain

Neural Tube and Brain Vesicles

The neural tube forms three primary brain vesicles:

• Prosencephalon: Forebrain

• Mesencephalon: Midbrain

• Rhombencephalon: Hindbrain

These primary vesicles further differentiate into secondary brain vesicles, which give rise to adult brain structures:

• Telencephalon: Cerebral hemispheres

• Diencephalon: Thalamus, hypothalamus, epithalamus, retina

• Mesencephalon: Midbrain

• Metencephalon: Pons and cerebellum

• Myelencephalon: Medulla oblongata

Brain Regions and Organization

Major Regions of the Adult Brain

The adult brain is organized into four main regions:

• Cerebral Hemispheres

• Diencephalon

• Brain Stem: Midbrain, pons, medulla oblongata

• Cerebellum

Structural Features

The brainstem consists of three prominent structures: midbrain, pons (largest bump), and medulla. The spinal cord continues from the brainstem through the foramen magnum.

Gray Matter and White Matter in the CNS

Distribution Patterns

The CNS exhibits a basic pattern of central cavity surrounded by gray matter, with white matter external to gray matter. This pattern changes as one ascends from the spinal cord to the brainstem and cerebrum.

• Gray Matter: Contains neuron cell bodies and short, nonmyelinated neurons.

• White Matter: Composed mostly of myelinated axons, with some nonmyelinated axons.

In the cerebrum and cerebellum, an outer layer of gray matter called the cortex is present, along with scattered gray matter nuclei within white matter.

Cerebral Hemispheres

Surface Markings and Lobes

The cerebral hemispheres form the superior part of the brain and account for 83% of its mass. Surface features include:

• Gyri: Ridges (folds)

• Sulci: Shallow grooves

• Fissures: Deep grooves

Each hemisphere is divided into five lobes: frontal, parietal, temporal, occipital, and insula (not shown in all diagrams). The hemispheres are separated by the longitudinal fissure and connected by the corpus callosum.

Functional Regions

• Cerebral Cortex: Thin outer region of gray matter

• Cerebral White Matter: Deeper region, appears white

Ventricles of the Brain

Structure and Function

Brain ventricles are fluid-filled chambers lined by ependymal cells and filled with cerebrospinal fluid (CSF). They are continuous with each other and the central canal of the spinal cord.

• Lateral Ventricles: Paired, C-shaped chambers in each hemisphere

• Third Ventricle: Located in the diencephalon

• Fourth Ventricle: Located in the hindbrain, continuous with the central canal

Cerebral Cortex Functional Areas

Motor Areas

Motor areas are located in the frontal lobe and control voluntary movement:

• Primary (Somatic) Motor Cortex: Located in the precentral gyrus; initiates voluntary movement of skeletal muscle.

• Premotor Cortex: Plans movements and controls learned, patterned motor skills.

• Broca’s Area: Motor speech area, usually in the left hemisphere.

• Frontal Eye Field: Controls voluntary eye movements.

Sensory Areas

Sensory areas are located in the parietal, temporal, and occipital lobes. They include:

• Primary Somatosensory Cortex: Located in the postcentral gyrus; receives sensory information from skin, muscles, and joints.

• Visual Cortex: Located in the occipital lobe; receives and interprets visual information.

• Auditory Cortex: Located in the temporal lobe; interprets sound information.

Lateralization of Cortical Functioning

Lateralization refers to the division of labor between the hemispheres:

• Left Hemisphere: Language, math, logic

• Right Hemisphere: Visual-spatial skills, intuition, emotion, artistic and musical skills

Hemispheres communicate via fiber tracts and functional integration. Cerebral dominance refers to the hemisphere dominant for language (usually left).

Cerebral White Matter

Fiber Tracts

Cerebral white matter is responsible for communication within the brain and between the brain and spinal cord. Fiber tracts are classified by direction:

• Association Fibers: Connect different parts of the same hemisphere

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

• Projection Fibers: Connect hemispheres with lower brain or spinal cord

Somatotopy and Body Maps

Homunculus Representation

The body is spatially represented in the primary motor cortex (precentral gyrus) and primary somatosensory cortex (postcentral gyrus). This mapping is depicted by the homunculus, which shows the relative size of cortical areas dedicated to different body regions.

Ascending and Descending Pathways

Spinal Tracts and Pathways

Major spinal tracts are part of multi-neuron pathways:

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

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

• Somatotopy: Spatial relationships in the CNS correspond to those in the body.

• Symmetry: Pathways are paired symmetrically.

Ascending Pathways

• Dorsal Column–Medial Lemniscal Pathways: Discriminative touch and vibration

• Spinothalamic Pathways: Pain, temperature, coarse touch, pressure

• Spinocerebellar Tracts: Muscle or tendon stretch, coordination

Meninges of the CNS

Dura Mater

The dura mater is the strongest meninx, consisting of two layers in the brain (periosteal and meningeal). It forms partitions called dural septa, which limit excessive movement of the brain.

• Falx Cerebri: In longitudinal fissure

• Falx Cerebelli: Along vermis of cerebellum

• Tentorium Cerebelli: Over cerebellum and in transverse fissure

Arachnoid Mater

The arachnoid mater is the middle meningeal layer, separated from the dura mater by the subdural space. The subarachnoid space contains CSF and blood vessels. Arachnoid granulations allow CSF reabsorption into venous blood.

Pia Mater

The pia mater is a delicate layer that clings tightly to the brain, following every convolution and containing many blood vessels.

Spinal Cord

Structure and Function

The spinal cord is enclosed in the vertebral column and begins at the foramen magnum. It provides two-way communication between the brain and body and carries out spinal reflexes.

Summary Table: Major Brain Regions and Their Functions

Region	Main Function
Cerebral Hemispheres	Conscious mind, voluntary movement, sensory perception
Diencephalon	Relay and processing of sensory information, homeostasis
Brain Stem	Basic life functions, pathway for information between brain and body
Cerebellum	Coordination of movement, balance

Additional info: The notes include expanded academic context and definitions to ensure completeness and clarity for exam preparation.