Chapter 12: The Central Nervous System – Structure, Function, and Protection

Chapter Overview: The Central Nervous System

The central nervous system (CNS) is composed of the brain and spinal cord, serving as the primary control center for the body. This chapter explores the major anatomical structures, functional regions, protective coverings, and developmental aspects of the CNS.

Latin and Greek Terminology in Neuroanatomy

Key Terms and Etymology

Understanding the roots of neuroanatomical terms aids in mastering the vocabulary of the CNS.

• Cephalo-: Head

• Cerebral: Pertaining to the brain

• Encephalo-: In the head

• Myelo-: Marrow, spinal cord (related to myelin)

• Rostral: Toward the beak (front)

• Caudal: Toward the tail (back)

• Ventricle: Little belly (cavity in the brain)

Directional Terms in Neuroanatomy

Understanding Anatomical Directions

Directional terms are essential for describing locations and orientations within the CNS.

• Anterior–Posterior: Front to back

• Rostral–Caudal: Toward the nose/beak to toward the tail

• Dorsal–Ventral: Back to belly

• Superior–Inferior: Above to below

• Medial–Lateral: Toward the midline to away from the midline

CNS Major Structures

Overview from Rostral to Caudal

The CNS is organized from the most rostral (anterior) to the most caudal (posterior) structures:

• Cerebrum (2 cerebral hemispheres)

• Diencephalon (thalamus, hypothalamus, etc.)

• Cerebellum

• Brain Stem (midbrain, pons, medulla oblongata)

• Spinal Cord

Cerebral Hemispheres

Structure and Communication

The cerebrum consists of two hemispheres separated by the longitudinal fissure and connected by the corpus callosum. The wrinkled surface increases cortical area for higher processing.

• Gyri: Elevated ridges ("hills")

• Sulci: Shallow grooves ("valleys")

• Fissures: Deep grooves

Corpus Callosum

The corpus callosum is a thick band of nerve fibers that enables communication between the right and left hemispheres.

Hemispheric Lateralization

Each hemisphere has specialized functions, though they work together:

• Left Hemisphere: Language, logic, math

• Right Hemisphere: Art, emotion, intuition, spatial skills

Some individuals may have reversed or shared dominance.

Lobes of the Cerebrum

Each hemisphere is divided into five lobes, separated by sulci:

• Frontal Lobe: Voluntary movement, executive functions

• Parietal Lobe: Somatosensation (body sensation)

• Temporal Lobe: Hearing, smell, memory

• Occipital Lobe: Vision

• Insula: Taste, visceral sensation (internal)

Functional Areas of the Cerebral Cortex

Specific regions of the cortex are responsible for distinct functions:

• Broca’s Area: Motor speech (usually left hemisphere)

• Wernicke’s Area: Language comprehension

• Precentral Gyrus: Primary motor cortex

• Postcentral Gyrus: Primary somatosensory cortex

Gray and White Matter

The cerebrum is organized into layers:

• Cerebral Cortex: Superficial gray matter (neuron cell bodies, dendrites)

• White Matter: Deeper myelinated axon tracts

• Basal Nuclei: Deep gray matter involved in movement control

Basal Nuclei (Basal Ganglia)

Basal nuclei include the caudate nucleus, putamen, and globus pallidus. They regulate movement initiation, intensity, and termination. Disorders include Parkinson’s and Huntington’s diseases.

Diencephalon

Major Components and Functions

The diencephalon is located deep within the brain and includes:

• Thalamus: Relay station for sensory information to the cortex

• Hypothalamus: Homeostasis, autonomic and endocrine control, regulates temperature, hunger, thirst, sleep-wake cycles, and links to the pituitary gland

• Epithalamus: Contains the pineal gland, which produces melatonin for sleep regulation

Brain Stem

Structure and Basic Life Functions

The brain stem consists of the midbrain, pons, and medulla oblongata. It controls vital functions such as breathing and heart rate, houses cranial nerve nuclei, and contains the reticular activating system (RAS) for consciousness and alertness.

Cerebellum

Coordination and Balance

The cerebellum, separated from the cerebrum by the transverse cerebral fissure, has two hemispheres and a highly folded surface. It fine-tunes motor activity, balance, and coordination. The arbor vitae is the tree-like arrangement of white matter within the cerebellum.

Functional Brain Systems

Limbic System and Reticular Activating System (RAS)

Functional systems span multiple brain regions:

• Limbic System: Emotional and visceral responses, includes structures in the cerebrum and diencephalon

• RAS: Maintains alertness and consciousness, spans the brain stem and influences the cerebrum

Protection of the CNS

Meninges

The CNS is protected by three connective tissue membranes called meninges:

• Dura Mater: Tough, outermost layer with periosteal and meningeal layers

• Arachnoid Mater: Middle, web-like layer filled with cerebrospinal fluid (CSF)

• Pia Mater: Delicate, vascular inner layer adhering to the brain and spinal cord

Spaces:

• Subdural Space: Between dura and arachnoid mater

• Subarachnoid Space: Between arachnoid and pia mater, contains CSF

Blood Supply and Blood Brain Barrier (BBB)

The brain receives blood from the vertebral and internal carotid arteries, which form the Circle of Willis. The BBB is a selective barrier formed by endothelial tight junctions, astrocytes, and pericytes, protecting the CNS from harmful substances but also limiting drug delivery.

Cerebrospinal Fluid (CSF)

CSF is produced by the choroid plexus in the ventricles, circulates through the ventricular system and subarachnoid space, and is reabsorbed into the blood at arachnoid granulations. CSF cushions the CNS and removes waste.

Developmental Aspects of the CNS

Neural Tube Formation and Defects

The CNS develops from the neural tube, which forms early in embryonic development. Failure of the neural tube to close can result in defects such as anencephaly (rostral end) or spina bifida (caudal end). Folic acid supplementation before and during early pregnancy greatly reduces the risk of neural tube defects.

The Spinal Cord

Structure and Function

The spinal cord extends from the foramen magnum to the level of L1/L2. It transmits sensory information to the brain and motor commands from the brain, and mediates reflexes.

• Cervical and Lumbar Enlargements: For limb innervation

• Conus Medullaris: Tapered end of the spinal cord

• Cauda Equina: Bundle of nerve roots below the spinal cord

• Filum Terminale: Fibrous extension anchoring the cord to the coccyx

Cross-Sectional Anatomy

• Gray Matter: H-shaped, contains neuron cell bodies; divided into dorsal (sensory), ventral (motor), and lateral (autonomic) horns

• White Matter: Myelinated axon tracts; divided into dorsal (sensory), lateral (mixed), and ventral (motor) columns

Spinal Meninges

The spinal cord is covered by the same three meningeal layers as the brain. The pia mater forms denticulate ligaments and the filum terminale for anchoring.

Neuronal Pathways

Descending (Motor) Pathways

Motor commands travel from the brain to the body via two main pathways:

• Direct (Pyramidal) Pathways: Two-neuron chain (upper and lower motor neurons) for voluntary movement

• Indirect (Extrapyramidal) Pathways: Multineuronal, control balance, posture, and coarse movements

Ascending (Sensory) Pathways

Sensory information ascends to the brain via three-neuron chains:

• Dorsal Columns: Touch and proprioception (cross in medulla)

• Spinothalamic Tracts: Pain and temperature (cross in spinal cord)

• Spinocerebellar Tracts: Subconscious proprioception

Decussation

Most pathways cross (decussate) to the opposite side of the CNS, explaining why brain injuries affect the contralateral side of the body.

Clinical Note: Brown-Sequard Syndrome

Hemisection of the spinal cord causes loss of touch/proprioception on the same side and pain/temperature on the opposite side below the lesion.

Summary Table: Major CNS Structures and Functions

Structure	Main Function(s)
Cerebrum	Higher cognitive functions, voluntary movement, sensory perception
Diencephalon	Sensory relay, homeostasis, endocrine regulation
Brain Stem	Basic life functions, cranial nerves, consciousness
Cerebellum	Coordination, balance, fine motor control
Spinal Cord	Sensory/motor relay, reflexes

Additional info: This guide integrates foundational concepts from OpenStax, Marieb & Hoehn, and F.A. Davis, with expanded academic context for clarity and exam preparation.