Central Nervous System: Structure, Development, and Functional 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 development of the CNS, known as cephalization, has resulted in an increased number of neurons, with the highest level of complexity found in the human brain.

• Cephalization: Evolutionary trend toward concentration of nervous tissue at the anterior end of an organism, leading to the formation of a head and brain.

• Brain and Spinal Cord: The two main components of the CNS, responsible for processing and transmitting information throughout the body.

Brain Development

Embryological Origins

During embryonic development, the brain and spinal cord begin as a 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)

• Secondary Brain Vesicles:

  • Forebrain: Telencephalon and Diencephalon

  • Midbrain: remains as Mesencephalon

  • Hindbrain: Metencephalon and Myelencephalon

• Adult Brain Structures:

  • Telencephalon: Cerebral hemispheres (cerebrum)

  • Diencephalon: Epithalamus, thalamus, hypothalamus, retina

  • Mesencephalon: Midbrain

  • Metencephalon: Pons and cerebellum

  • Myelencephalon: Medulla oblongata

• Central cavity of neural tube: Becomes the ventricles of the brain.

Example: The telencephalon develops into the two cerebral hemispheres, which together form the cerebrum, the largest part of the adult human brain.

Growth and Flexures

• As the brain grows, it folds to fit within the skull, forming flexures that angle the telencephalon and diencephalon toward the brain stem.

• Cerebral hemispheres grow posteriorly and laterally, enveloping the diencephalon and midbrain, and 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 (composed of midbrain, pons, and medulla oblongata)

4. Cerebellum

Gray and White Matter

• Gray matter: Consists of short, nonmyelinated neurons and cell bodies.

• 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: In the brain stem, additional gray matter nuclei are scattered within white matter. The cerebrum and cerebellum have an outer layer of gray matter called the cortex, with scattered gray matter nuclei amid white matter.

Ventricles of the Brain

Structure and Function

The 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 (a type of neuroglia).

• Lateral ventricles: Large, C-shaped chambers deep within each cerebral hemisphere, separated by the septum pellucidum.

• Third ventricle: Located in the diencephalon, connected to each lateral ventricle via the interventricular foramen.

• Fourth ventricle: Located in the hindbrain, connected to the third ventricle via the cerebral aqueduct. It is continuous with the central canal of the spinal cord.

• Three openings connect the fourth ventricle to the subarachnoid space:

  • Lateral apertures (in side walls)

  • Median aperture (in the roof)

Cerebral Hemispheres

Surface Features

• Cerebral hemispheres form the superior part of the brain and account for 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

  • Parietal

  • Temporal

  • Occipital

  • Insula (buried under portions of the other lobes)

• Major sulci:

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

  • Parieto-occipital sulcus: Separates occipital and parietal lobes

  • Lateral sulcus: Outlines temporal lobes

• Each hemisphere has three basic regions:

  • Cerebral cortex (gray matter, superficial)

  • White matter (internal)

  • Basal nuclei (deep 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, but no axons. The cortex accounts for about 40% of the brain's mass.

Functional Areas of the Cortex

• Three main types of functional areas:

  • Motor areas: Control voluntary movement

  • Sensory areas: Conscious awareness of sensation

  • Association areas: Integrate diverse information

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

• Lateralization (specialization) of cortical function can occur in only one hemisphere.

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

Motor Areas

• Located in the frontal lobe.

• Primary motor cortex: Located in the precentral gyrus; contains pyramidal cells (large neurons for conscious control of precise, skilled skeletal muscle movements).

• Pyramidal (corticospinal) tracts: Formed from long axons that project down the spinal cord.

• Somatotopy: All muscles of the body can be mapped to an area on the primary motor cortex.

• Motor homunculus: Upside-down caricatures represent contralateral motor innervation of body regions.

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

• Broca's area: Motor speech area that directs muscles of speech production; active in planning speech and voluntary motor activities.

• Frontal eye field: Controls voluntary eye movements.

Sensory Areas

• Concerned with conscious awareness of sensation.

• Occur in parietal, insular, temporal, and occipital lobes.

• Main areas include:

  • Primary somatosensory cortex

  • Somatosensory association cortex

  • Visual areas

  • Auditory areas

  • Vestibular cortex

  • Olfactory cortex

  • Gustatory cortex

  • Visceral sensory area

Primary Somatosensory Cortex

• Located in the postcentral gyri of the parietal lobe.

• Receives general sensory information from skin and proprioceptors of skeletal muscle, joints, and tendons.

• Capable of spatial discrimination: identification of body region being stimulated.

• Somatosensory homunculus: Upside-down caricatures represent contralateral sensory input from body regions.

Clinical Relevance: Homeostatic Imbalance

• Damage to areas of the primary motor cortex (e.g., from a stroke) paralyzes muscles controlled by those areas; paralysis occurs on the opposite side of the body from the damage.

• Muscle strength or ability to perform discrete individual movements is not impaired, only control over movements is lost.

• Other premotor neurons can be reprogrammed to take over the skill of damaged neurons, but this requires practice, similar to the initial learning process.

Table: Summary of Brain Vesicle Development

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