Central Nervous System: Structure, Development, and Functional Organization

Central Nervous System (CNS)

Definition and Overview

• CNS refers exclusively to the brain and spinal cord.

• It is the main control center for processing and integrating information in the body.

Cephalization

• Cephalization is the evolutionary trend toward concentration of nervous tissue at the anterior (rostral) end of the organism.

• This process results in an increased number of neurons and the highest level of nervous system organization in the human brain.

• Cephalization allows for advanced sensory input, integration, and complex behaviors.

Embryonic Development of the Human Brain

Neural Tube and Brain Vesicles

• The neural tube forms from ectoderm and gives rise to the entire CNS.

• It develops into three primary brain vesicles:

  • Prosencephalon (forebrain)

  • Mesencephalon (midbrain)

  • Rhombencephalon (hindbrain)

• These further divide into secondary brain vesicles, which form the major adult brain structures.

Primary Vesicle	Secondary Vesicle	Adult Brain Structure	Associated Ventricles
Prosencephalon	Telencephalon	Cerebral hemispheres	Lateral ventricles
Prosencephalon	Diencephalon	Thalamus, hypothalamus, epithalamus	Third ventricle
Mesencephalon	Mesencephalon	Midbrain	Cerebral aqueduct
Rhombencephalon	Metencephalon	Pons, cerebellum	Fourth ventricle
Rhombencephalon	Myelencephalon	Medulla oblongata	Fourth ventricle

Additional info: The central canal of the spinal cord is continuous with the brain's ventricular system and is filled with cerebrospinal fluid (CSF).

Brain Regions and Organization

Major Brain Regions

• The adult brain is divided into four main regions:

  1. Cerebral hemispheres

  2. Diencephalon

  3. Brain stem (composed of midbrain, pons, and medulla oblongata)

  4. Cerebellum

Distribution of Gray and White Matter in the CNS

Brain

• Gray matter consists of neuron cell bodies and short, nonmyelinated neurons.

• White matter is composed mainly of myelinated axons (with some nonmyelinated axons).

• In the brain, gray matter forms the outer cortex and deeper nuclei, while white matter lies beneath the cortex.

Spinal Cord

• The spinal cord has a central cavity surrounded by gray matter, with outer white matter.

• This arrangement allows for efficient transmission of signals to and from the brain.

Ventricles of the Brain

Structure and Function

• The brain contains four interconnected ventricles filled with cerebrospinal fluid (CSF):

  • Lateral ventricles (one in each cerebral hemisphere)

  • Third ventricle (in the diencephalon)

  • Fourth ventricle (between the brain stem and cerebellum)

• Ventricles are important for cushioning the brain, removing waste, and distributing nutrients.

Lobes, Sulci, and Fissures of the Cerebral Hemispheres

Major Lobes and Landmarks

• The cerebral hemispheres are divided into four main lobes:

  • Frontal lobe

  • Parietal lobe

  • Temporal lobe

  • Occipital lobe

• Key surface features include:

  • Gyri: elevated ridges

  • Sulci: shallow grooves

  • Fissures: deep grooves (e.g., longitudinal fissure separates left and right hemispheres)

• The insula (insular lobe) is located deep within the lateral sulcus.

Cerebral Cortex

Structure and Function

• The cerebral cortex is the site of the conscious mind, responsible for perception, voluntary movement, and higher cognitive functions.

• It is a thin layer of gray matter, making up about 40% of the brain's mass.

• The surface area is greatly increased by the presence of gyri and sulci.

Functional Areas

• The cortex contains three types of functional areas:

  • Motor areas: control voluntary movement

  • Sensory areas: conscious awareness of sensation

  • Association areas: integrate diverse information for purposeful action

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

• Some functions are lateralized, meaning they are more dominant in one hemisphere (e.g., language in the left hemisphere for most people).

• Conscious behavior involves the entire cortex; no area acts alone.

Motor Areas

• Primary motor cortex (precentral gyrus): controls voluntary movements via pyramidal cells (upper motor neurons).

• Somatotopic arrangement: different body parts are mapped onto specific regions of the cortex, forming the motor homunculus.

• Premotor cortex: plans complex movements involving multiple muscle groups.

• Broca's area: involved in speech production.

• Frontal eye field: controls voluntary eye movements.

Sensory Areas

• Primary somatosensory cortex (postcentral gyrus): receives sensory input from the body.

• Somatosensory association cortex: interprets sensory information.

• Somatotopic arrangement forms the sensory homunculus, reflecting the amount of cortex devoted to each body part.

• Other sensory areas include:

  • Visual areas (occipital lobe): primary and association visual cortices

  • Auditory areas (temporal lobe)

  • Vestibular cortex (balance)

  • Olfactory cortex (smell)

  • Gustatory cortex (taste)

  • Visceral sensory area (internal organ sensations)

Association Areas

• Multimodal association areas integrate information from multiple senses and are involved in higher cognitive functions.

• Three main parts:

  • Prefrontal cortex (anterior): executive functions, decision-making

  • Posterior association area: links information from different senses

  • Limbic association area: involved in emotions

Lateralization of Cortical Function

• About 90% of people are left hemisphere dominant (especially for language, math, and logic).

• The right hemisphere is more involved in visual-spatial skills, intuition, emotion, and artistic abilities.

• In left-handed individuals, dominance may be reversed.

• Neither hemisphere is superior overall; each specializes in certain functions.

Cerebral White Matter

Types of Fibers

• Association fibers: connect different parts of the same hemisphere.

• Commissural fibers: connect the two hemispheres (e.g., corpus callosum).

• Projection fibers: connect the cortex with lower brain regions and the spinal cord.

Basal Nuclei

• Basal nuclei are clusters of gray matter deep within the cerebral hemispheres.

• Main components: caudate nucleus, putamen, and globus pallidus.

• They are involved in the indirect coordination of movement, especially in starting and stopping motions.

The Diencephalon

Major Structures

• Thalamus: relay station for sensory and motor signals to the cerebral cortex.

• Epithalamus: includes the pineal gland, which regulates the sleep/wake cycle via melatonin secretion.

• Hypothalamus: major homeostatic control center, regulating the autonomic nervous system, body temperature, hunger, thirst, sex drive, sleep/wake cycles, and hormone release.

Brain Stem

• Composed of the midbrain, pons, and medulla oblongata.

• Controls automatic, hard-wired behaviors necessary for survival (e.g., heart rate, breathing).

• Contains nuclei for 10 of the 12 cranial nerves.

Midbrain

• Involved in the fight-or-flight response (amygdaloid body), pain suppression, and dopamine production (substantia nigra).

• Contains the corpora quadrigemina (superior and inferior colliculi) for visual and auditory reflexes.