Chapter 12: The Central Nervous System – Brain Structure and Function

Brain Structure and Function

Cranial Meninges

The cranial meninges are three protective connective tissue layers that surround the brain, providing physical stability and shock absorption.

• Dura mater: The tough, outermost layer, also called the "tough mother." It forms a durable protective covering.

• Arachnoid mater: The middle, web-like layer that cushions the brain.

• Pia mater: The delicate, innermost layer that closely adheres to the surface of the brain.

Divisions of the Cranial Vault

Several dural folds partition the cranial cavity and support the brain's structure:

• Falx cerebri: Located in the longitudinal fissure, it separates the two cerebral hemispheres.

• Falx cerebelli: Separates the two cerebellar hemispheres in the transverse fissure.

• Tentorium cerebelli: Separates the cerebellum from the cerebrum.

Blood Supply to the Brain

The brain receives a rich blood supply to meet its high metabolic demands.

• Internal carotid arteries: Supply the anterior portion of the brain.

• Vertebral arteries: Supply the posterior portion of the brain.

• Internal jugular veins: Drain venous blood from the brain.

Blood-Brain Barrier (BBB)

The blood-brain barrier is a selective permeability barrier formed by endothelial cells and astrocytes. It protects the brain from harmful substances while allowing essential nutrients to pass through.

Cerebrospinal Fluid (CSF)

CSF is a clear, colorless fluid that circulates in the subarachnoid space, providing cushioning, nutrient delivery, and waste removal for the brain and spinal cord.

• Produced by ependymal cells of the choroid plexus in the ventricles.

• Acts as a shock absorber and brings nutrition to nervous tissue.

• Reabsorbed into the venous system via arachnoid villi.

Ventricles and CSF Flow

• Lateral ventricles: Two large cavities separated by the septum pellucidum.

• Third ventricle: A narrow slit between the two halves of the thalamus.

• Fourth ventricle: Located between the brainstem and cerebellum.

CSF Flow Pathway:

• Lateral ventricles → Interventricular foramina → Third ventricle → Cerebral aqueduct → Fourth ventricle → Subarachnoid space → Arachnoid villi (reabsorption)

Brain Stem

The brain stem connects the spinal cord to the brain and controls vital functions.

• Medulla oblongata (inferior): Contains the pyramids where the corticospinal tract crosses. Functions include cardiovascular and respiratory rhythm centers. Contains nuclei for cranial nerves VIII–XII.

• Pons (middle): Acts as a bridge. Contains nuclei for cranial nerves V–VIII.

• Midbrain (superior): Contains the cerebral aqueduct (connecting ventricles 3 & 4), cerebral peduncles (motor tracts), and tectum (with superior and inferior colliculi for visual and auditory reflexes).

Reticular Activating System (RAS)

The RAS is a network of neurons in the brainstem that maintains consciousness and filters sensory input to help focus attention.

Cerebellum

The cerebellum is responsible for coordinating posture, equilibrium, and balance.

Diencephalon

• Thalamus: Acts as a relay station for sensory information.

• Hypothalamus: Regulates the autonomic nervous system and homeostasis.

Cerebrum

The cerebrum is the largest part of the brain, responsible for higher brain functions.

• Cerebral cortex: Outer gray matter; inner white matter.

• Gyri (singular: gyrus): Folds formed as gray matter develops faster than white matter.

• Longitudinal fissure: Separates the two hemispheres.

• Central sulcus: Separates the frontal and parietal lobes.

• Precentral gyrus: Primary motor area (Brodmann areas 1, 2, 3).

• Postcentral gyrus: Primary somatosensory area (Brodmann area 4).

• Parieto-occipital sulcus: Separates parietal and occipital lobes.

• Lateral (cerebral) sulcus: Separates frontal and temporal lobes.

White Matter Tracts

• Association tracts: Connect gyri within the same hemisphere.

• Commissural tracts: Connect gyri in opposite hemispheres (e.g., corpus callosum).

• Projection tracts: Convey impulses to and from lower parts of the CNS.

Basal Nuclei

Deep gray matter regions that regulate muscle tone and coordinate movement.

Limbic System

Known as the "emotional brain," it is involved in emotion, motivation, and memory.

Sensory Homunculus

A graphical representation of the distribution of sensory receptors in the cortex, illustrating the relative sensory space each body part occupies.

Primary Sensory Areas

• Primary visual area: Occipital lobe

• Primary taste area: Inferior to the somatosensory area

• Primary auditory area: Temporal lobe

• Primary olfactory area: Temporal lobe

Brain Waves

Brain activity can be measured as electrical patterns called brain waves:

• Alpha waves: Present when awake and relaxed.

• Beta waves: Occur during mental activity.

• Theta waves: Seen during emotional stress.

• Delta waves: Present during sleep in adults; if seen while awake, may indicate brain damage.

Cranial Nerves

There are 12 pairs of cranial nerves, classified as sensory, motor, or mixed. Each has specific functions related to sensation, movement, or both.

Cranial Nerve	Type	Main Function
I. Olfactory	Sensory	Smell
II. Optic	Sensory	Vision
III. Oculomotor	Motor	Eye movement, pupil constriction
IV. Trochlear	Motor	Eye movement
V. Trigeminal	Mixed	Facial sensation, chewing
VI. Abducens	Motor	Eye movement
VII. Facial	Mixed	Facial expression, taste
VIII. Vestibulocochlear	Sensory	Hearing, balance
IX. Glossopharyngeal	Mixed	Taste, swallowing
X. Vagus	Mixed	Viscera control, taste
XI. Accessory	Motor	Head movement
XII. Hypoglossal	Motor	Tongue movement

Example: The Role of the Medulla Oblongata

The medulla oblongata contains centers that regulate heart rate and breathing, and is the site where motor tracts cross to the opposite side of the body (decussation of pyramids).

Additional info: The Brodmann areas referenced for the precentral and postcentral gyri are typically 4 (primary motor cortex) and 1, 2, 3 (primary somatosensory cortex), respectively. The notes may have these reversed.