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

Brain Structure and Function

Cranial Meninges

The brain is protected by three connective tissue layers known as the cranial meninges. These layers provide structural support and help maintain the environment necessary for proper neural function.

• Dura mater: The tough, outermost layer, often referred to as the "tough mother." It provides durable protection for the brain.

• Arachnoid mater: The middle layer, characterized by a web-like appearance. It contains the subarachnoid space where cerebrospinal fluid (CSF) circulates.

• Pia mater: The innermost layer, which closely adheres to the surface of the brain, following its contours.

Divisions of the Cranial Vault

Several folds of dura mater help compartmentalize the cranial cavity:

• Falx cerebri: Located in the longitudinal fissure, 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 blood through two main arterial systems and returns blood via veins:

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

• Vertebral arteries: Supply the posterior regions.

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

Blood Brain Barrier (BBB)

The BBB is a selective barrier formed by endothelial cells and astrocytes, restricting passage of substances from the blood into the brain, thus protecting neural tissue from toxins and pathogens.

Cerebrospinal Fluid (CSF)

CSF is a clear fluid produced by ependymal cells in the choroid plexus of the ventricles. It acts as a shock absorber, provides nutrients, and removes waste.

• Flow of CSF: CSF circulates through the ventricles and the subarachnoid space, eventually being reabsorbed by arachnoid villi.

• Ventricles:

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

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

  • Fourth ventricle: Located between the brain stem and cerebellum.

• Arachnoid villi: Structures that reabsorb CSF into the venous system.

Brain Stem

The brain stem is composed of three main regions and is responsible for vital functions and cranial nerve nuclei.

• Medulla oblongata: The most inferior part. Contains pyramids where the corticospinal tract crosses. Functions include cardiovascular and respiratory rhythm centers. Houses nuclei for cranial nerves VIII-XII.

• Pons: The middle region, acting as a bridge. Contains nuclei for cranial nerves V-VIII.

• Midbrain: The superior region. The cerebral aqueduct passes through, connecting ventricles 3 and 4. Contains cerebral peduncles (axons from various tracts) and the tectum (superior and inferior colliculi for visual and auditory reflexes).

Reticular Activating System (RAS)

The RAS is a network within the brain stem that maintains consciousness and filters sensory input to help focus attention.

Cerebellum

The cerebellum is responsible for posture, equilibrium, and balance. It coordinates voluntary movements and maintains muscle tone.

Diencephalon

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

• Hypothalamus: Controls the autonomic nervous system and regulates homeostasis.

Cerebrum

The cerebrum is the largest part of the brain, responsible for higher functions such as thought, memory, and voluntary movement.

• Cerebral cortex: The outer layer (gray matter); inner layer is white matter.

• Gyri: Folds of the cortex, formed because 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 (areas 1, 2, 3).

  • Postcentral gyrus: Primary somatosensory area (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 between opposite hemispheres.

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

• Corpus callosum: Major commissural tract connecting the right and left hemispheres.

Basal Nuclei

Deep areas of gray matter within the white matter. They regulate muscle tone and coordinate movement.

Limbic System

The limbic system is involved in emotion, motivation, and memory. Sometimes called the "emotional brain."

Sensory Homunculus

A graphical representation of the distribution of sensory receptors in the cortex, illustrating which body parts have the most sensory input.

Primary Sensory Areas

• Primary visual area: Located in the occipital lobe.

• Primary taste area: Inferior to the somatosensory area.

• Primary auditory area: Located in the temporal lobe.

• Primary olfactory area: Located in the temporal lobe.

Brain Waves

Brain activity can be measured by EEG, which records different types of waves:

• Alpha: Present when awake and relaxed.

• Beta: Occur during mental activity.

• Theta: Associated with emotional stress.

• Delta: Occur during sleep in adults; presence while awake may indicate brain damage.

Cranial Nerves

There are 12 pairs of cranial nerves, classified as sensory, motor, or mixed. They emerge from the brain and brain stem and control various functions.

Cranial Nerve	Type	Main Function
I (Olfactory)	Sensory	Smell
II (Optic)	Sensory	Vision
III (Oculomotor)	Motor	Eye movement
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	Autonomic control of heart, lungs, digestive tract
XI (Accessory)	Motor	Head and shoulder movement
XII (Hypoglossal)	Motor	Tongue movement

Example: Flow of CSF

CSF is produced in the choroid plexus of the lateral ventricles, flows through the interventricular foramen to the third ventricle, then through the cerebral aqueduct to the fourth ventricle, and finally into the subarachnoid space. It is reabsorbed by arachnoid villi into the venous system.

Additional info:

• The brain's structural organization is essential for its function, with each region specialized for particular tasks.

• Damage to specific areas can result in distinct neurological deficits, highlighting the importance of understanding brain anatomy.