Chapter 12A- CNS

Human Anatomy and Physiology Eleventh Edition - Chapter 12 Part A: The Central Nervous System

Introduction

  • Importance of Understanding CNS: Understanding the central nervous system (CNS) is essential for knowledge application regarding brain and spinal cord injuries, such as strokes.

Overview of the Central Nervous System (CNS)

  • Cephalization: The evolutionary process that led to the development of an extensive anterior (rostral) portion of the CNS.

    • Embryological Development:

    • The brain and spinal cord originate from a structure known as the neural tube.

    • The surrounding tissue of the neural tube develops into three primary vesicles, which will evolve into the brain.

    • The central cavity formed by these vesicles will become the brain's ventricles and the spinal cord's central canal.

Development of Primary Vesicles

  • The anterior end of the neural tube expands and creates three primary vesicles:

    • Prosencephalon (Forebrain): Develops into the cerebrum and diencephalon (DE).

    • Mesencephalon (Midbrain): Represents the midbrain of the brainstem.

    • Rhombencephalon (Hindbrain): Transforms into the pons, medulla oblongata (MO), and cerebellum (CB).

    • The posterior end morphs into the spinal cord.

Secondary Brain Vesicles

  • The primary vesicles give rise to five secondary brain vesicles:

    • Forebrain (Prosencephalon):

    1. Telencephalon (Endbrain): Develops into the left and right hemispheres of the cerebral cortex, each surrounding a lateral ventricle.

    2. Diencephalon (Interbrain): Develops into the thalamus, hypothalamus, and epithalamus, surrounding the third ventricle.

    • Midbrain (Mesencephalon): Remains undivided and develops around the cerebral aqueduct.

    • Hindbrain (Rhombencephalon): Develops around the fourth ventricle and divides into:

    1. Metencephalon (Afterbrain): Forms the pons and cerebellum.

    2. Myelencephalon (Spinal brain): Develops into the medulla oblongata.

Embryonic Development of the Brain

Neural Tube and Brain Vesicles

  • Neural Tube: Contains the central canal.

  • Primary Brain Vesicles: Expanded regions that provide foundational areas of the adult brain.

  • Secondary Brain Vesicles: Derived from the primary vesicles that indicate specific adult structures (e.g., lateral ventricles, cerebrum).

  • Adult Neural Canal Regions: Transformation occurs during development to form adult brain structures such as cerebral hemispheres, thalamus, and the spinal cord's central canal.

Brain Growth and Organization

  • The brain grows faster than the surrounding skull, leading to folding:

    • The forebrain moves toward the brainstem including midbrain, pons, and medulla oblongata.

    • Cerebral hemispheres envelop the diencephalon and midbrain while increasing the surface area through creasing and folding.

Key Structural Features of the Brain

  • Adult brains comprise four main regions:

    • Cerebral Hemispheres: Major part of the brain, accounting for approximately 83% of its mass.

    • Diencephalon: Central region of the brain.

    • Brain Stem: Includes midbrain, pons, and medulla oblongata.

    • Cerebellum: Responsible for coordination and balance.

Gray Matter vs. White Matter

  • Gray Matter: Comprises short, unmyelinated neurons and neuron cell bodies.

  • White Matter: Composed mainly of myelinated axons and some unmyelinated axons.

    • CNS's Basic Organizational Pattern:

    1. Central cavity is surrounded by gray matter.

    2. Additional outer layer of white matter surrounding the gray matter.

    • Organizational patterns vary in the spinal cord, brain stem, and cerebellum.

Ventricles of the Brain

Structure and Function

  • Ventricles: Fluid-filled chambers that help cushion the brain and circulate cerebrospinal fluid (CSF).

    • Arrangement: Lateral ventricles > 3rd ventricle > cerebral aqueduct > 4th ventricle > central canal.

    • Cerebrospinal Fluid (CSF): Provides mechanical protection, chemical stability, and removes waste.

    • Lining: Ventricles are lined by ependymal cells (neuroglial cells).

Lateral Ventricles

  • Two large, C-shaped chambers deep within each hemisphere, separated anteriorly by the septum pellucidum.

  • Interventricular Foramen: Connects each lateral ventricle to the third ventricle.

Third and Fourth Ventricles

  • Third Ventricle: Located in the diencephalon and connects to the fourth ventricle via the cerebral aqueduct.

  • Fourth Ventricle: Positioned in the hindbrain, continuous with the central canal and connects to the subarachnoid space.

  • Apertures: Three openings (paired lateral apertures and median aperture) connect the fourth ventricle to the surrounding spaces.

Cerebral Hemispheres

Basic Structure and Function

  • Form the superior portion of the brain, accounting for 83% of the total brain mass.

  • Surface Markings:

    • Gyri: Ridges.

    • Sulci: Shallow grooves.

    • Fissures: Deep grooves, such as:

    • Longitudinal Fissure: Separates the two hemispheres.

    • Transverse Cerebral Fissure: Separates cerebrum from cerebellum.

Division into Lobes

  • Each hemisphere consists of five distinct lobes, delineated by several sulci:

    • Frontal Lobe

    • Parietal Lobe

    • Temporal Lobe

    • Occipital Lobe

    • Insula: Situated beneath parts of the temporal, parietal, and frontal lobes.

Major Sulci of the Cerebral Hemispheres

  • Central Sulcus: Divides the precentral gyrus of the frontal lobe from the postcentral gyrus of the parietal lobe.

  • Parieto-Occipital Sulcus: Separates the occipital lobe from the parietal lobe.

  • Lateral Sulcus: Divides the temporal lobe from the frontal and parietal lobes.

Functional Regions of the Hemispheres

  • Each hemisphere has three basic regions:

    • Cerebral Cortex: Superficial layer of gray matter.

    • White Matter: Located internally.

    • Basal Nuclei: Deep within white matter.