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 develops from a structure called the neural tube.

• Rostral: Toward the forehead (upper)

• Caudal: Toward the spinal cord (lower)

Embryonic Development of the Brain

Neural Tube and Brain Vesicles

The brain and spinal cord originate from the neural tube. As development proceeds, the neural tube forms three primary brain vesicles:

• Prosencephalon (forebrain)

• Mesencephalon (midbrain)

• Rhombencephalon (hindbrain)

These vesicles further differentiate into secondary brain vesicles and eventually into the major adult brain structures:

• Telencephalon → Cerebrum (cerebral hemispheres)

• Diencephalon → Thalamus, hypothalamus, epithalamus, retina

• Mesencephalon → Midbrain

• Metencephalon → Pons and cerebellum

• Myelencephalon → Medulla oblongata

Brain Regions and Organization

Major Brain Regions

The adult brain is organized into four main regions:

• Cerebral hemispheres

• Diencephalon

• Brain stem (midbrain, pons, medulla oblongata)

• Cerebellum

Brainstem Structure

The brainstem consists of three main parts, often referred to as "bumps":

• Midbrain

• Pons (largest bump)

• Medulla oblongata

The brainstem transitions into the spinal cord as it passes through the foramen magnum.

Gray Matter and White Matter in the CNS

Distribution Patterns

The CNS exhibits a characteristic pattern of gray matter (neuron cell bodies, nonmyelinated neurons) and white matter (myelinated and nonmyelinated axons):

• In the spinal cord: Central cavity surrounded by gray matter, with white matter external to gray matter.

• In the brainstem: Additional gray matter nuclei are scattered within white matter.

• In the cerebrum and cerebellum: An outer cortex of gray matter is present, with islands of gray matter (nuclei) within white matter.

Cerebral Hemispheres

Surface Features and Lobes

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

• Gyri: Ridges or folds

• Sulci: Shallow grooves

• Fissures: Deep grooves

The hemispheres are divided into five lobes by various sulci:

• Frontal lobe

• Parietal lobe

• Temporal lobe

• Occipital lobe

• Insula (not visible on the surface)

Major Fissures

• Longitudinal fissure: Separates the left and right hemispheres

• Transverse cerebral fissure: Separates the cerebrum from the cerebellum

Ventricles of the Brain

Structure and Function

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

• Paired lateral ventricles (deep in each hemisphere)

• Third ventricle (in the diencephalon)

• Fourth ventricle (in the hindbrain, continuous with the central canal)

Ventricles are connected by the interventricular foramen and cerebral aqueduct.

Cerebral Cortex

Functional Areas

The cerebral cortex is the site of conscious mind functions, including awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding. It contains three main types of functional areas:

• Motor areas: Control voluntary movements (located in the frontal lobe)

• Sensory areas: Conscious awareness of sensation (parietal, temporal, occipital lobes)

• Association areas: Integrate diverse information

Motor Areas

• Primary (somatic) motor cortex: Located in the precentral gyrus; initiates voluntary movement of skeletal muscle

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

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

• Frontal eye field: Controls voluntary eye movements

Sensory Areas

• Primary somatosensory cortex: Located in the postcentral gyrus; receives sensory information from skin, muscles, and joints

• Somatosensory association cortex: Integrates sensory input for understanding

• Primary visual cortex: Receives visual information from the retinas (occipital lobe)

• Visual association area: Interprets visual stimuli

• Primary auditory cortex: Interprets information from the inner ear (temporal lobe)

• Auditory association area: Stores memories of sounds and permits perception of sound stimulus

Lateralization of Cortical Functioning

Lateralization refers to the division of labor between the two hemispheres. The left hemisphere typically controls language, math, and logic, while the right hemisphere is associated with visual-spatial skills, intuition, emotion, and artistic abilities. Most people have left hemisphere dominance, which usually results in right-handedness.

Cerebral White Matter

Fiber Tracts

Cerebral white matter is responsible for communication within the brain and between the brain and spinal cord. It consists of myelinated fibers organized into three types:

• 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 the cerebral cortex with lower brain regions or the spinal cord

Somatotopy and Body Maps

Homunculus Representation

The primary motor cortex and primary somatosensory cortex contain spatial maps of the body, known as homunculi. These maps reflect the amount of cortex devoted to motor or sensory functions of different body regions.

• Areas with fine motor control (e.g., hands, face) have larger cortical representation.

• Areas with greater sensory input (e.g., lips, fingertips) are similarly enlarged in the sensory cortex.

Ascending and Descending Pathways

Major Spinal Tracts

Spinal tracts are organized into multi-neuron pathways with four key features:

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

• Relay: Consist of chains of two or three neurons

• Somatotopy: Spatial correspondence between CNS regions and body regions

• Symmetry: Pathways are paired on both sides of the CNS

Ascending Pathways

• Dorsal column–medial lemniscal pathways: Discriminative touch and vibration

• Spinothalamic pathways: Pain, temperature, coarse touch, and pressure

• Spinocerebellar tracts: Muscle or tendon stretch to the cerebellum for coordination

Meninges of the CNS

Protective Membranes

The meninges are three connective tissue membranes that surround the brain and spinal cord:

• Dura mater: Strongest, outermost layer; forms dural septa to limit brain movement

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

• Pia mater: Delicate, innermost layer; clings tightly to the brain surface

Spinal Cord Structure

Organization and Function

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

Key anatomical features:

• Cervical and lumbar enlargements: Regions where nerves serving the limbs arise

• Conus medullaris: Tapered end of the spinal cord

• Cauda equina: Collection of nerve roots at the inferior end

• Filum terminale: Fibrous extension anchoring the spinal cord