Study Guide: The Central Nervous System – Brain Anatomy

The Central Nervous System: Brain Anatomy

Brain Regions and Organization

The adult human brain is organized into four primary regions, each with distinct functions and anatomical features. Understanding these regions is fundamental to grasping the structure and function of the central nervous system.

• Cerebral Hemispheres: The largest region, responsible for higher cognitive functions.

• Diencephalon: Contains structures such as the thalamus and hypothalamus, crucial for sensory relay and homeostasis.

• Brain Stem: Composed of the midbrain, pons, and medulla oblongata; regulates vital functions.

• Cerebellum: Coordinates movement and balance.

Brain Development

Brain development follows a predictable pattern, with the adult brain exhibiting distinct regions and convolutions. The organization of gray and white matter is established early and persists throughout life.

Gray Matter and White Matter in the CNS

The central nervous system (CNS) is characterized by a specific distribution of gray and white matter. This pattern is essential for understanding neural processing and signal transmission.

• Gray Matter: Composed of short, nonmyelinated neurons and cell bodies; responsible for processing information.

• White Matter: Consists of myelinated and nonmyelinated axons; facilitates communication between different CNS regions.

• Basic Pattern: Central cavity surrounded by gray matter, with white matter external to gray matter.

Ventricles of the Brain

The brain contains fluid-filled chambers called ventricles, which are continuous with each other and the central canal of the spinal cord. These ventricles are filled with cerebrospinal fluid (CSF) and lined by ependymal cells.

• Lateral Ventricles: Paired, C-shaped chambers deep in each hemisphere, separated by the septum pellucidum.

• Third Ventricle: Located in the diencephalon, connected to lateral ventricles via the interventricular foramen.

• Fourth Ventricle: Located in the hindbrain, connected to the third ventricle via the cerebral aqueduct; continuous with the central canal of the spinal cord.

• Openings: Three openings connect the fourth ventricle to the subarachnoid space: paired lateral apertures and a median aperture.

Cerebral Hemispheres

Surface Markings and Lobes

The cerebral hemispheres form the superior part of the brain and account for the majority of its mass. Surface markings include gyri (ridges), sulci (shallow grooves), and fissures (deep grooves).

• Longitudinal Fissure: Separates the two hemispheres.

• Transverse Cerebral Fissure: Separates the cerebrum from the cerebellum.

Lobes of the Cerebral Hemispheres

Each hemisphere is divided into five lobes by prominent sulci:

• Frontal Lobe

• Parietal Lobe

• Temporal Lobe

• Occipital Lobe

• Insula: Buried under portions of the temporal, parietal, and frontal lobes.

Major Sulci and Regions

Major sulci divide the lobes and define the boundaries of functional regions:

• Central Sulcus: Separates the precentral gyrus (frontal lobe) and postcentral gyrus (parietal lobe).

• Parieto-Occipital Sulcus: Separates occipital and parietal lobes.

• Lateral Sulcus: Outlines temporal lobes.

Each hemisphere has three basic regions:

• Cerebral Cortex: Superficial layer of gray matter.

• White Matter: Internal region.

• Basal Nuclei: Deep within white matter.

Cerebral Cortex

Structure and Function

The cerebral cortex is the "executive suite" of the brain, responsible for conscious mind functions such as awareness, sensory perception, voluntary motor initiation, communication, memory storage, and understanding.

• Thickness: 2–4 mm, composed of neuron cell bodies, dendrites, glial cells, and blood vessels.

• Mass: Accounts for 40% of the brain's mass.

Functional Areas of the Cerebral Cortex

The cortex contains three types of functional areas:

• Motor Areas: Control voluntary movement.

• Sensory Areas: Provide conscious awareness of sensation.

• Association Areas: Integrate diverse information.

Each hemisphere is concerned with the contralateral (opposite) side of the body. Lateralization of cortical function can occur in only one hemisphere, and conscious behavior involves the entire cortex.

Motor Areas

Motor areas are located in the frontal lobe and control voluntary movement:

• Primary Motor Cortex: Located in the precentral gyrus; contains pyramidal cells for conscious control of precise, skilled movements.

• Premotor Cortex: Anterior to the precentral gyrus; plans movements and coordinates learned motor skills.

• Broca’s Area: Motor speech area, usually in the left hemisphere; directs muscles of speech production.

• Frontal Eye Field: Controls voluntary eye movements.

Somatotopy and Motor Homunculi

Somatotopy refers to the mapping of all muscles of the body to specific areas on the primary motor cortex. Motor homunculi are visual representations of this mapping.

Clinical Note: Homeostatic Imbalance

Damage to the primary motor cortex (e.g., stroke) results in paralysis of muscles controlled by the affected area, typically on the opposite side of the body. Muscle strength is not impaired, but voluntary control is lost. Other neurons can be reprogrammed to take over lost skills with practice.

Sensory Areas

Sensory areas are located in the parietal, insular, temporal, and occipital lobes. They provide conscious awareness of sensation and include:

• Primary Somatosensory Cortex: Located in the postcentral gyri; receives sensory information from skin and proprioceptors.

• Somatosensory Association Cortex: Integrates sensory input for understanding objects.

• Visual Areas: Primary visual cortex and visual association area in the occipital lobe.

• Auditory Areas: Primary auditory cortex and auditory association area in the temporal lobe.

• Vestibular Cortex: Responsible for balance awareness.

• Olfactory Cortex: Involved in conscious awareness of odors.

• Gustatory Cortex: Perception of taste.

• Visceral Sensory Area: Perception of visceral sensations.

Multimodal Association Areas

Multimodal association areas receive inputs from multiple sensory areas and send outputs to various regions. They allow for integration of information, memory storage, and decision-making. These areas are divided into:

• Anterior Association Area (Prefrontal Cortex): Involved in intellect, cognition, recall, and personality.

• Posterior Association Area: Recognizes patterns and faces, localizes us in space, and is involved in language comprehension.

• Limbic Association Area: Provides emotional impact and helps establish memories.

Clinical Note: Homeostatic Imbalance

Tumors or lesions in the anterior association area can cause mental and personality disorders, including loss of judgment and inhibitions. Lesions in the posterior association area may result in loss of awareness of self in space.

Lateralization of Cortical Functioning

Lateralization refers to the division of labor between the hemispheres. The left hemisphere is dominant for language, math, and logic in 90% of humans, while the right hemisphere specializes in visual-spatial skills, intuition, emotion, and artistic abilities. Hemispheres communicate via fiber tracts for functional integration.

Hemisphere	Dominant Functions
Left	Language, math, logic
Right	Visual-spatial skills, intuition, emotion, artistic and musical skills

Example: Most people are right-handed due to left hemisphere dominance for motor control.

Additional info: The cerebral cortex's functional areas are interconnected, and damage to one area can often be compensated by others through neuroplasticity.