Human Brain Anatomy: Structure and Function

Brain Protection and General Composition

The brain is enclosed and protected by the skull. Beneath the skull, the meninges provide further protection, and Cerebrospinal Fluid (CSF) flows within this space to reduce friction and cushion the brain.

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

• Gray Matter: Primarily involved in processing information and generating responses. It has a cortical arrangement (forming the outer layer or cortex) and is also found in deeper clusters called nuclei.
• White Matter: Forms a large mass connecting different brain areas. The corpus callosum, a large white matter structure, connects the two cerebral hemispheres across the interhemispheric fissure.

Important functions are associated with the gray matter, including the origin of the 12 pairs of cranial nerves, which can be sensory, motor, or mixed.

White Matter Fiber Types

White matter contains nerve fibers that transmit signals:

• Association Fibers: Connect different cortical areas within the same cerebral hemisphere.
• Commissural Fibers: Connect corresponding areas in both hemispheres. This connection is primarily established through the corpus callosum, a white matter mass located deep within the brain.
• Projection Fibers: Transmit sensory impulses from the body towards the gray matter nuclei and cerebral cortex, and carry motor commands from the gray matter towards the effector muscles and glands.

Subcortical Gray Matter Nuclei

These clusters of gray matter are located deep within the white matter, beneath the cerebral cortex. Alterations to their physiology can disrupt various functions. Key subcortical nuclei include:

• Hypothalamus
• Putamen
• Globus Pallidus
• Substantia Nigra
• Optic Thalamus (Thalamus)
• Red Nucleus
• Subthalamic Nucleus (Corpus Luysi)
• Striatum (typically includes Putamen and Caudate Nucleus)

Primary Brain Functions

The brain is responsible for a vast array of functions, including:

• Producing sensations
• Controlling willpower and voluntary movements
• Center for learning and memory
• Basis of consciousness
• Controlling conditioned reflexes
• Center for emotions
• Enabling reasoning
• Controlling appetite and satiety
• Regulating sexual behavior
• Governing body temperature
• Center for oral and written language
• Controlling bodily secretions
• Regulating social behavior
• Regulating the sleep-wake rhythm

Key Brain Structures Summary

General Organization

• Cerebral Hemispheres: Left and Right
• Corpus Callosum: Connects hemispheres
• Gray Matter: Includes the cerebral cortex and subcortical nuclei
• White Matter: Composed of association, projection, and commissural fibers
• Gyri (Convolutions): The folds on the brain's surface

Lobes

• Frontal Lobe
• Parietal Lobe
• Temporal Lobe
• Occipital Lobe
• Insula Lobe (deep)

Major Fissures (Sulci)

• Interhemispheric Fissure: Separates the hemispheres
• Calcarine Fissure: Located in the occipital lobe
• Parieto-occipital Fissure: Separates parietal and occipital lobes
• Sylvian Fissure (Lateral Sulcus): Separates temporal lobe from frontal and parietal lobes
• Rolandic Fissure (Central Sulcus): Separates frontal and parietal lobes

Functions of Specific Basal Nuclei

Subcortical nuclei, often referred to as basal nuclei or ganglia when discussing motor control, have distinct roles:

Hypothalamus Functions

Present in birds and mammals, the hypothalamus:

• Regulates body temperature, reducing reliance on ambient temperature.
• Responsible for manifestations of hunger and thirst.
• Regulates the sleep rhythm.
• Controls the pituitary gland through neurosecretions (releasing factors), initiating processes like sexual maturation.

Optic Thalamus (Thalamus) Functions

The thalamus acts as a major relay station:

• Most sensory impulses pass through the thalamus, where they synapse before reaching the cerebral cortex.
• Its connections with other basal nuclei contribute to non-conscious reactions and attitudes.
• It directs sensory information to specific areas of the cerebral cortex for precise localization and interpretation of sensations.
• Responsible for imprecise sensitivity (e.g., perceiving light, touch, temperature broadly) but does not determine the full meaning or context of sensations, which occurs in the cortex.