Central Nervous System: Structure, Development, and Organization

Central Nervous System (CNS)

Overview of the CNS

The central nervous system (CNS) is composed of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly. The evolutionary process known as cephalization has led to the concentration of nervous tissue at the anterior end, resulting in the complex human brain.

• Cephalization: Evolutionary development of the anterior (rostral) portion of the CNS, resulting in increased neuron numbers and complexity.

• The human brain represents the highest level of cephalization among animals.

Brain Development

Embryological Origins

Embryologically, the brain and spinal cord originate from a structure called the neural tube. The anterior end of the neural tube expands and forms three primary brain vesicles, which further differentiate into secondary vesicles and adult brain structures.

• Primary brain vesicles:

  • Prosencephalon (forebrain)

  • Mesencephalon (midbrain)

  • Rhombencephalon (hindbrain)

• The posterior end of the neural tube becomes the spinal cord.

Secondary Brain Vesicles and Adult Structures

• Forebrain divides into:

  • Telencephalon → cerebral hemispheres (cerebrum)

  • Diencephalon → epithalamus, thalamus, hypothalamus, retina

• Midbrain (mesencephalon) remains undivided.

• Hindbrain divides into:

  • Metencephalon → pons and cerebellum

  • Myelencephalon → medulla oblongata

• The central cavity of the neural tube becomes the ventricles of the brain.

Neural Tube Region	Primary Vesicle	Secondary Vesicle	Adult Brain Structure	Adult Neural Canal Region
Anterior	Prosencephalon	Telencephalon	Cerebral cortex, basal nuclei	Lateral ventricles
		Diencephalon	Thalamus, hypothalamus, epithalamus, retina	Third ventricle
	Mesencephalon	Mesencephalon	Midbrain	Cerebral aqueduct
Posterior	Rhombencephalon	Metencephalon	Pons, cerebellum	Fourth ventricle
		Myelencephalon	Medulla oblongata	Fourth ventricle, central canal

Additional info: Table inferred and expanded from the embryonic development diagram and slide content.

Brain Growth and Folding

• The brain grows faster than the membranous skull, causing it to fold to fit the available space.

• The forebrain moves toward the brain stem (midbrain, pons, medulla oblongata).

• Cerebral hemispheres double back and envelop the diencephalon and midbrain, increasing surface area through creasing and folding.

Brain Regions and Organization

Major Brain Regions

The adult brain is organized into four main regions:

1. Cerebral hemispheres

2. Diencephalon

3. Brain stem (midbrain, pons, medulla oblongata)

4. Cerebellum

Gray and White Matter

• Gray matter: Contains neuron cell bodies and nonmyelinated neurons.

• White matter: Composed of myelinated and nonmyelinated axons.

• Basic CNS pattern: Central cavity surrounded by gray matter, with white matter external to gray matter.

• Pattern changes as you ascend from the spinal cord to the brain stem and cerebrum.

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

Pattern of Distribution of Gray and White Matter in the CNS

Structural Organization

• In the spinal cord: Central gray matter (butterfly-shaped) surrounded by white matter.

• In the brain stem: Additional gray matter nuclei scattered within white matter.

• In the cerebrum and cerebellum: Outer cortex of gray matter with scattered gray matter nuclei within white matter.

Example: The cerebral cortex is a thin layer of gray matter covering the cerebral hemispheres, responsible for higher brain functions.

Ventricles of the Brain

Structure and Function

• Ventricles are fluid-filled chambers within the brain, continuous with each other and the central canal of the spinal cord.

• Filled with cerebrospinal fluid (CSF) and lined by ependymal cells (a type of neuroglia).

• Paired lateral ventricles are large, C-shaped chambers deep within each hemisphere, separated by the septum pellucidum.

• Each lateral ventricle connects to the third ventricle (in the diencephalon) via the interventricular foramen.

• The third ventricle connects to the fourth ventricle (in the hindbrain) via the cerebral aqueduct.

• The fourth ventricle is continuous with the central canal of the spinal cord and connects to the subarachnoid space via three openings: paired lateral apertures and a median aperture.

Cerebral Hemispheres

Surface Features

• The cerebral hemispheres form the superior part of the brain and account for about 83% of its mass.

• Surface markings include:

  • Gyri: Ridges

  • Sulci: Shallow grooves

  • Fissures: Deep grooves

• Longitudinal fissure: Separates the two hemispheres.

• Transverse cerebral fissure: Separates the cerebrum from the cerebellum.

Lobes of the Cerebral Hemispheres

• Each hemisphere is divided into five lobes by sulci:

  • Frontal lobe

  • Parietal lobe

  • Temporal lobe

  • Occipital lobe

  • Insula (buried under portions of the temporal, parietal, and frontal lobes)

• Major sulci:

  • Central sulcus: Separates precentral gyrus (frontal lobe) from postcentral gyrus (parietal lobe)

  • Parieto-occipital sulcus: Separates occipital and parietal lobes

  • Lateral sulcus: Outlines temporal lobes

Basic Regions of Each Hemisphere

• Cerebral cortex: Superficial layer of gray matter

• White matter: Internal communication tracts

• Basal nuclei: Deep clusters of gray matter within white matter

Cerebral Cortex

Structure and Function

• The cerebral cortex is the "executive suite" of the brain, responsible for conscious mind functions such as awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding.

• It is a thin (2–4 mm) superficial layer of gray matter, composed of neuron cell bodies, dendrites, glial cells, and blood vessels (no axons).

• The cortex makes up about 40% of the total brain mass.

Functional Organization

• Functional imaging (PET, MRI) shows that specific motor and sensory functions are localized in discrete cortical areas called domains, while higher functions are distributed over many areas.

• Four general considerations:

  1. Contains three types of functional areas:

     • Motor areas: Control voluntary movement

     • Sensory areas: Conscious awareness of sensation

     • Association areas: Integrate diverse information

  2. Each hemisphere is concerned with the contralateral (opposite) side of the body.

  3. Lateralization: Specialization of cortical function can occur in only one hemisphere.

  4. Conscious behavior involves the entire cortex in one way or another.

Motor Areas of the Cortex

• Located in the frontal lobe, these areas control voluntary movement:

  • Primary (somatic) motor cortex: Located in the precentral gyrus; contains large pyramidal cells responsible for conscious control of precise, skilled skeletal muscle movements. Their axons form the pyramidal (corticospinal) tracts.

  • Somatotopy: All muscles of the body can be mapped to specific areas on the primary motor cortex, forming a motor homunculus (a visual representation of body regions controlled by the cortex).

  • Premotor cortex: Plans movements, controls learned, repetitious, or patterned motor skills, and coordinates simultaneous or sequential actions.

  • Broca's area: Usually present in the left hemisphere; directs muscles involved in speech production.

  • Frontal eye field: Controls voluntary eye movements.

Example: Damage to the primary motor cortex (e.g., by stroke) can cause paralysis of muscles controlled by that area, typically on the opposite side of the body.