Anatomy and Physiology of the Brain: Structure, Meninges, and White/Grey Matter

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Brain Structure and Organization

Cerebral Hemispheres

The cerebral hemispheres are the largest parts of the human brain, divided into right and left halves. They are responsible for higher-order functions such as reasoning, memory, voluntary movement, and sensory perception.

Definition: The two halves of the cerebrum, separated by the longitudinal fissure.
Functions: Each hemisphere controls the opposite side of the body (contralateral control).
Higher Functions: Involved in language, abstract thought, and complex motor skills.
Corpus Callosum: A thick band of nerve fibers connecting the two hemispheres, allowing communication between them.
Example: The left hemisphere is typically dominant for language in right-handed individuals.

Gyri and Sulci

The surface of the brain is highly folded, forming ridges called gyri and grooves called sulci. This folding increases the surface area, allowing for more neurons and greater processing power.

Gyri (singular: gyrus): Elevated ridges on the brain's surface.
Sulci (singular: sulcus): Shallow grooves between gyri.
Purpose: Increases cortical surface area within the limited volume of the skull.
Example: The precentral gyrus is involved in voluntary motor control.

Gray Matter and White Matter

The brain is composed of two main types of tissue: gray matter and white matter.

Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons. Found mainly in the cerebral cortex and nuclei deep within the brain.
White Matter: Consists of myelinated axons that connect different brain regions. Located beneath the cortex.
Example: The corpus callosum is a large white matter tract connecting the two hemispheres.

Brain Meninges and Protective Structures

Meningeal Layers

The brain is protected by three connective tissue membranes called meninges. These layers provide physical protection and support for the brain.

Dura Mater: The tough, outermost layer. It has two sublayers: the periosteal layer (attached to the skull) and the meningeal layer (closer to the brain).
Arachnoid Mater: The middle, web-like layer. Contains the subarachnoid space filled with cerebrospinal fluid (CSF).
Pia Mater: The delicate, innermost layer that closely follows the contours of the brain.
Function: Protects the brain, contains blood vessels, and helps circulate CSF.
Example: The subdural space is a potential space between the dura and arachnoid mater, where subdural hematomas can occur.

Cerebrospinal Fluid (CSF)

Cerebrospinal fluid is a clear, colorless liquid that surrounds the brain and spinal cord, providing cushioning and maintaining a stable environment.

Production: Produced by the choroid plexus in the brain's ventricles.
Functions: Cushions the brain, removes waste, and transports nutrients.
Circulation: Flows through the ventricles, subarachnoid space, and is reabsorbed into the bloodstream via arachnoid granulations.
Example: Blockage of CSF flow can lead to hydrocephalus (accumulation of fluid in the brain).

White Matter Tracts

Types of White Matter Tracts

White matter tracts are bundles of myelinated axons that connect different parts of the brain and spinal cord. They are classified based on their function and direction.

Type of Tract	Direction	Function	Example
Association Tracts	Within the same hemisphere	Connect different cortical areas	Arcuate fasciculus
Commissural Tracts	Between hemispheres	Connect corresponding areas of both hemispheres	Corpus callosum
Projection Tracts	Between cortex and lower CNS	Connect cortex with brainstem and spinal cord	Internal capsule

Additional info: These tracts are essential for integrating sensory input, coordinating motor output, and enabling complex cognitive functions.

Summary of Key Terms

Cerebrum: Largest part of the brain, responsible for higher functions.
Gyri/Sulci: Folds and grooves that increase cortical surface area.
Gray Matter: Neuron cell bodies; White Matter: Myelinated axons.
Meninges: Dura mater, arachnoid mater, pia mater.
Cerebrospinal Fluid (CSF): Cushions and protects the brain.
White Matter Tracts: Association, commissural, and projection tracts.