CH 12 I

Central Nervous System (CNS)

General Characteristics

The central nervous system (CNS) is composed exclusively of brain and spinal cord tissues. It is responsible for integrating sensory information and responding accordingly. The CNS is highly organized and demonstrates a strong relationship between structure and function.

• Cephalic: Refers to the cranium or head.

• Cephalization: The evolutionary trend of concentrating neural and sensory organs toward the anterior (head) end of the body. Humans exhibit the highest level of cephalization, resulting in advanced intelligence and complex neural systems.

• Structure/Function Relationship: Any structural change (e.g., injury or disease) in a CNS area can result in a corresponding functional loss (e.g., damage to the brain's motor area leads to movement issues; damage to sensory areas leads to loss of stimulus response).

Brain: Characteristics and Surface Anatomy

Physical Characteristics

• Appearance: The brain is a mass of wrinkled, pinkish-gray tissue.

• Weight: Approximately 1,450–1,600 g, about 2% of total body weight.

• Size: Determined by body mass; generally, male brains are larger than female brains. The adult brain is about four times larger than a newborn's brain.

Surface Anatomy

• Cerebral hemispheres (cerebrum)

• Cerebellum

• Brain stem

CNS Development

Neural Tube & Primary Brain Vesicles

The neural tube is the embryonic precursor to CNS structures, originating from the ectoderm layer. Around the fourth week of development, the anterior (rostral) end of the neural tube expands and constricts to form three primary brain vesicles:

• Prosencephalon (forebrain)

• Mesencephalon (midbrain)

• Rhombencephalon (hindbrain)

The posterior (caudal) end becomes the spinal cord, which is continuous with the hindbrain and exits through the skull's foramen magnum.

Secondary Brain Vesicles

By week 5, the primary vesicles give rise to secondary vesicles:

Primary Vesicle	Secondary Vesicle
Prosencephalon (forebrain)	Telencephalon, Diencephalon
Mesencephalon (midbrain)	Mesencephalon (unchanged)
Rhombencephalon (hindbrain)	Metencephalon, Myelencephalon

Additional info: The telencephalon develops into the cerebral hemispheres, the diencephalon into thalamic structures, the metencephalon into the pons and cerebellum, and the myelencephalon into the medulla oblongata.

Adult Structures and Ventricles

• Most adult brain structures develop from a single secondary vesicle (exception: metencephalon forms both cerebellum and pons).

• Ventricles develop from the neural canal, becoming the fluid-filled central portion of the adult brain and remaining continuous with the spinal cord's central cavity.

Brain Development Pattern

Brain tissue grows faster than the fetal skull, causing it to fold and create midbrain/cervical flexures. This allows the cerebral hemispheres to grow posteriorly and laterally, resulting in a horseshoe or C-shaped growth pattern. By week 26, the outer hemispheres develop surface convolutions, greatly increasing the brain's surface area and allowing more neurons to occupy limited space (tripling the surface area for approximately 100 billion neurons).

Brain Structural Organization

Distinctive Structures

• Cerebral hemispheres (cerebrum)

• Diencephalon: thalamus, hypothalamus, epithalamus

• Cerebellum

• Brain stem: midbrain, pons, medulla

Neural Patterns

• White matter: Long, myelinated fibers arranged in tracts (bundles).

• Gray matter: Short, nonmyelinated fibers with associated neuron cell bodies (soma).

CNS Gray/White Matter Patterns

The basic pattern is a fluid-filled central cavity surrounded by gray matter, which is itself enclosed by white matter. This pattern varies by region:

• Spinal cord: gray matter inside, white matter outside

• Brain stem: gray matter inside, white matter outside, with scattered gray matter nuclei embedded in white matter

• Cerebrum/Cerebellum: gray matter cortex on the outside, white matter beneath, and additional gray matter (basal nuclei) deep within the white matter

Brain Ventricles

Brain ventricles are central, fluid-filled cavities lined by glial ependymal cells and are continuous with each other and the spinal cord. The main ventricles are:

Ventricle	Location	Connection
Lateral ventricles (2)	Cerebral hemispheres	Separated by septum pellucidum; connected to third ventricle via interventricular foramen (IVF)
Third ventricle	Diencephalon	Connected to fourth ventricle via cerebral aqueduct
Fourth ventricle	Hindbrain, dorsal to pons	CSF moves into external spaces via lateral/median apertures

Cerebral Hemispheres (Cerebrum)

General Features

• Paired superior brain structures, making up 83% of total brain mass.

Cortical Convolutions

• Gyrus (gyri): Elevated ridge-like areas (tops of convolutions).

• Sulcus (sulci): Shallow grooves between gyri.

• Fissure: Deep grooves separating large brain areas (e.g., longitudinal fissure separates left/right hemispheres; transverse cerebral fissure separates cerebrum from cerebellum).

Major Regions (Superficial to Deep)

• Cortex: Superficial gray matter providing cognitive functions (contains neuron cell bodies, unmyelinated fibers, and glial cells).

• White matter: Subcortical area allowing communication between cerebrum/cerebellum and lower CNS structures (contains myelinated fiber tracts).

• Basal nuclei: Organized subcortical gray matter areas involved in motor control, attention, and cognition (centers of activity deep within cerebral white matter).

Additional info: The organization of the cerebrum allows for complex processing, integration of sensory information, and execution of voluntary movements.