Central Nervous System: Structure, Development, and Functional Organization

Central Nervous System (CNS)

Overview

The Central Nervous System (CNS) is composed of the brain and spinal cord. It is responsible for integrating incoming sensory signals and coordinating outgoing motor signals, serving as the main control center for the body.

• CNS: Includes the brain and spinal cord.

• Integration: Processes sensory input and initiates motor output.

• Cephalization: Evolutionary development of the anterior CNS, resulting in a concentration of neurons in the head; highest level in humans.

Embryonic Development of the CNS

Neural Tube and Primary Brain Vesicles

The brain and spinal cord originate from the neural tube during embryonic development. At the anterior end, three primary brain vesicles form:

• Prosencephalon (forebrain)

• Mesencephalon (midbrain)

• Rhombencephalon (hindbrain)

• The posterior end becomes the spinal cord.

Secondary Brain Vesicles and Adult Structures

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

• Forebrain → Telencephalon (cerebral hemispheres) and Diencephalon (epithalamus, thalamus, hypothalamus, retina)

• Midbrain remains as Mesencephalon

• Hindbrain → Metencephalon (pons, cerebellum) and Myelencephalon (medulla oblongata)

Brain Growth and Folding

As the brain grows faster than the membranous skull, it folds to fit the available space. The forebrain moves toward the brain stem, and the cerebral hemispheres envelop the diencephalon and midbrain, increasing surface area through creasing and folding.

Developmental Timeline

• Week 5: Major flexures form, angling telencephalon and diencephalon toward the brain stem.

• Week 13: Cerebral hemispheres grow to enclose the diencephalon and rostral brain stem.

• Birth: Adult pattern of brain structures and convolutions is established.

Regions and Organization of the CNS

Adult Brain Regions

• Cerebral hemispheres

• Diencephalon

• Brain stem (midbrain, pons, medulla oblongata)

• Cerebellum

Spinal Cord Structure

• Central cavity surrounded by gray matter (non-myelinated neurons and neuron cell bodies)

• External white matter composed of myelinated fiber tracts

Brain Structure

• Similar pattern to spinal cord

• Cerebral hemispheres and cerebellum have an outer layer of gray matter called the cortex

• In the brain stem, the cortex disappears and gray matter nuclei are scattered amid white matter

Ventricles of the Brain

Structure and Function

The brain contains hollow chambers called ventricles that are interconnected and continuous with the central canal of the spinal cord.

• Filled with cerebrospinal fluid (CSF)

• Lined by ependymal cells

Types of Ventricles

• Lateral ventricles: Paired, C-shaped, located in cerebral hemispheres

• Third ventricle: Located in the diencephalon

• Fourth ventricle: Located in the hindbrain

Cerebral Hemispheres

Surface Markings

• Gyri: Ridges

• Sulci: Shallow grooves

• Fissures: Deep grooves

• Longitudinal fissure: Separates the two hemispheres

• Transverse cerebral fissure: Separates cerebrum and cerebellum

Lobes of the Cerebrum

• Frontal

• Parietal

• Temporal

• Occipital

• Insula

Cerebral Cortex

Structure and Function

The cerebral cortex is a thin, superficial layer of gray matter, accounting for 40% of the brain's mass. It is the site of conscious mind, responsible for awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding.

Functional Areas

• Motor areas: Control voluntary movement

• Sensory areas: Conscious awareness of sensation

• Association areas: Integrate diverse information

General Considerations

• Each hemisphere controls the contralateral (opposite) side of the body

• Lateralization: Specialization of cortical function; hemispheres are not functionally identical

• Conscious behavior involves the entire cortex

Motor Areas of the Cerebral Cortex

Location and Components

• Located in the frontal lobe

• Primary (somatic) motor cortex: Precentral gyrus

• Premotor cortex: Anterior to precentral gyrus

• Broca's area: Anterior to inferior premotor area (usually left hemisphere)

• Frontal eye field: Controls voluntary eye movements

Primary Motor Cortex

• Allows conscious control of precise, skilled, skeletal muscle movements

• Motor homunculus: Map representing contralateral motor innervation; more area means more control

Premotor Cortex

• Plans movements; staging area for skilled motor activities

• Controls learned, repetitive, or patterned motor skills

• Coordinates simultaneous or sequential actions

• Controls voluntary actions dependent on sensory feedback

Broca's Area

• Motor speech area; directs muscles of speech production

• Active in planning speech and voluntary motor activities

Frontal Eye Field

• Controls voluntary eye movements

Sensory Areas of the Cerebral Cortex

Overview

Sensory areas are responsible for conscious awareness of sensation and are located in the parietal, insular, temporal, and occipital lobes.

• Primary somatosensory cortex

• Somatosensory association cortex

• Visual areas

• Auditory areas

• Vestibular cortex

• Olfactory cortex

• Gustatory cortex

• Visceral sensory area

Primary Somatosensory Cortex

• Located in postcentral gyri of parietal lobe

• Receives sensory information from skin and proprioceptors (sense position) of skeletal muscle, joints, and tendons

• Capable of spatial discrimination: Identifies body region being stimulated

• Somatosensory homunculus: Map representing contralateral sensory input

Somatosensory Association Cortex

• Posterior to primary somatosensory cortex

• Integrates sensory input for object understanding

• Determines size, texture, and relationship of parts of objects being felt

Visual Areas

• Primary visual cortex: Receives visual information from retinas

• Visual association area: Surrounds primary visual cortex; interprets visual stimuli using past experiences (e.g., color, form, movement, face recognition)

Auditory Areas

• Primary auditory cortex: Superior margin of temporal lobes; interprets pitch, loudness, and location

• Auditory association area: Posterior to primary auditory cortex; stores memories of sounds and permits perception of sound stimulus

Table: Embryonic Brain Development and Adult Structures

Stage	Vesicle/Region	Adult Structure	Associated Ventricles
Neural Tube	Entire tube	Brain, Spinal Cord	Central canal
Primary Vesicles	Prosencephalon	Forebrain
	Mesencephalon	Midbrain
	Rhombencephalon	Hindbrain
Secondary Vesicles	Telencephalon	Cerebral hemispheres	Lateral ventricles
	Diencephalon	Epithalamus, thalamus, hypothalamus, retina	Third ventricle
	Mesencephalon	Midbrain	Cerebral aqueduct
	Metencephalon	Pons, cerebellum	Fourth ventricle
	Myelencephalon	Medulla oblongata	Fourth ventricle
Spinal Cord	Posterior neural tube	Spinal cord	Central canal

Example: Motor and Sensory Homunculus

The motor homunculus and sensory homunculus are visual representations of the body mapped onto the precentral and postcentral gyri, respectively. Regions requiring finer control or more sensitive perception occupy larger areas on these maps (e.g., hands, face).

Additional info:

• Further details on association areas, multimodal integration, and clinical relevance (e.g., lesions, localization) are covered in advanced chapters.