Central Nervous System: Structure, Organization, and Functional Areas

Divisions of the Nervous System

Central Nervous System (CNS)

The Central Nervous System (CNS) is one of the two major divisions of the nervous system, consisting of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly. Understanding the CNS is crucial for interpreting neurological injuries and diseases.

• Cephalization: Evolutionary development of the anterior portion of the CNS, resulting in increased neuron numbers and complexity, reaching its highest level in humans.

• Components: Brain and spinal cord.

• Clinical relevance: CNS injuries (e.g., stroke, spinal cord injury) can have profound effects on function.

Brain Regions and Organization

Major Brain Regions

The adult brain is organized into four main regions, each with distinct functions and structures.

• Cerebral hemispheres

• Diencephalon

• Brain stem (composed of midbrain, pons, and medulla oblongata)

• Cerebellum

Example: The brain stem controls basic life functions such as breathing and heart rate.

Spinal Cord

The spinal cord is a major CNS structure that transmits signals between the brain and the rest of the body.

• Gray matter: Contains short, nonmyelinated neurons and cell bodies.

• White matter: Composed of myelinated and some nonmyelinated axons.

Brain Stem

The brain stem contains additional gray matter nuclei scattered within white matter, contributing to its complex functions.

• Gray matter nuclei: Involved in autonomic functions and reflexes.

• White matter: Contains ascending and descending tracts.

Pattern of Distribution of Gray and White Matter in the CNS

Gray and white matter are distributed in specific patterns throughout the CNS.

• Cerebral hemispheres and cerebellum: Outer layer of gray matter called the cortex.

• Scattered gray matter nuclei are also found within white matter.

Ventricles of the Brain

Structure and Function

Ventricles are fluid-filled chambers within the brain that are continuous with the central canal of the spinal cord.

• Filled with cerebrospinal fluid (CSF).

• Lined by ependymal cells (a type of neuroglia).

• Paired lateral ventricles are large, C-shaped chambers located deep in each hemisphere.

Ventricular Connections

• Each lateral ventricle connects to the third ventricle via the interventricular foramen.

• The third ventricle lies in the diencephalon.

• The third ventricle connects to the fourth ventricle via the cerebral aqueduct.

• The fourth ventricle is continuous with the central canal of the spinal cord and connects to the subarachnoid space via three openings.

Cerebral Hemisphere Features

Surface Markings

The cerebral hemispheres form the superior part of the brain and account for 83% of its mass. Their surface is marked by ridges and grooves.

• Gyri: Ridges on the brain surface.

• Sulci: Shallow grooves.

• Fissures: Deep grooves that separate major regions.

• 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 sulci:

• Frontal

• Parietal

• Temporal

• Occipital

• Insula (buried under portions of the other lobes)

Major Sulci

• Central sulcus: Separates precentral gyrus (frontal lobe) from postcentral gyrus (parietal lobe).

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

• Lateral sulcus: Outlines temporal lobes.

Basic Regions of the Cerebral Hemispheres

• Cerebral cortex: Superficial layer of gray matter.

• White matter: Internal myelinated fibers.

• 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.

• Thin (2–4 mm) superficial layer of gray matter.

• Composed of neuron cell bodies, dendrites, glial cells, and blood vessels.

• Accounts for 40% of brain mass.

• Functional imaging (PET, MRI) shows specific motor and sensory functions are located in discrete cortical areas called domains.

General Considerations

• Contains three types of functional areas:

  1. Motor areas: Control voluntary movement.

  2. Sensory areas: Conscious awareness of sensation.

  3. Association areas: Integrate diverse information.

• Each hemisphere is concerned with the contralateral (opposite) side of the body.

• Lateralization: Specialization of cortical function can occur in only one hemisphere.

• Conscious behavior involves the entire cortex in one way or another.

Functional and Structural Areas of the Cerebral Cortex

The cortex is divided into motor, sensory, and association areas, each with specific functions and locations.

Motor Areas

• Primary motor cortex: Located in the precentral gyrus; controls precise, skilled skeletal muscle movements via pyramidal cells.

• Premotor cortex: Anterior to the precentral gyrus; helps plan movements and coordinates learned motor skills.

• Broca's area: Anterior to the premotor area; involved in motor speech production.

• Frontal eye field: Controls voluntary eye movements.

• Somatotopy: All muscles of the body can be mapped to areas on the primary motor cortex.

• Motor homunculus: Diagram representing the relative size of cortical area devoted to motor control of different body regions.

Damage to Motor Cortex

• Damage to primary motor cortex (e.g., stroke) paralyzes muscles controlled by those areas, typically on the opposite side of the body.

• Damage to premotor cortex can result in apraxia (inability to carry out skilled actions) and deficits in fine motor control.

Somatosensory Areas

• Primary somatosensory cortex: Receives general sensory information from skin and proprioceptors.

• Somatosensory homunculus: Diagram representing the relative size of cortical area devoted to sensory input from different body regions.

• Somatosensory association cortex: Integrates sensory input for object recognition.

Visual Areas

• Primary visual (striate) cortex: Located in the occipital lobe; receives visual information from the retinas.

• Visual association area: Interprets visual stimuli using past experiences (e.g., recognizing faces).

Auditory and Vestibular Areas

• Primary auditory cortex: Superior margin of temporal lobes; interprets pitch, loudness, and location.

• Auditory association area: Stores memories of sounds and permits perception of sound stimuli.

• Vestibular cortex: Responsible for conscious awareness of balance.

Olfactory, Gustatory, and Visceral Areas

• Olfactory cortex: Medial aspect of temporal lobes; conscious awareness of odors.

• Gustatory cortex: In insula; perception of taste.

• Visceral sensory area: In insula; perception of visceral sensations (e.g., upset stomach).

Association Areas

• Multimodal association areas: Receive inputs from multiple sensory areas and send outputs to multiple areas, allowing integration and interpretation of information.

• Divided into:

  • Anterior association area (prefrontal cortex): Involved with intellect, cognition, recall, personality, and working memory.

  • Posterior association area: In temporal, parietal, and occipital lobes; involved in pattern recognition, spatial awareness, and language understanding.

  • Limbic association area: Part of limbic system; provides emotional impact and helps establish memories.

Lateralization of Cortical Functioning

• Lateralization: Division of labor between hemispheres; hemispheres are not identical.

• Cerebral dominance: Refers to the hemisphere dominant for language (usually left hemisphere).

• Left hemisphere: Controls language, math, logic.

• Right hemisphere: Controls visual-spatial skills, intuition, emotion, artistic and musical skills.

• Hemispheres communicate via fiber tracts and functional integration.

Cerebral White Matter

Structure and Function

Cerebral white matter is responsible for communication between cerebral areas and between the cortex and lower CNS. It consists of myelinated fibers bundled into large tracts.

• Association fibers: Connect different parts of the same hemisphere.

• Commissural fibers: Connect gray matter of two hemispheres (e.g., corpus callosum).

• Projection fibers: Connect hemispheres with lower brain or spinal cord.

Basal Nuclei (Ganglia)

Structure and Function

The basal nuclei are deep gray matter structures within each hemisphere, including the caudate nucleus, putamen, and globus pallidus. Together, the caudate nucleus and putamen form the striatum.

• Influence muscle movements.

• Play a role in cognition and emotion.

• Regulate intensity of slow or stereotyped movements.

• Filter out inappropriate responses.

• Inhibit antagonistic/unnecessary movements.

• Clinical relevance: Disorders such as Parkinson's disease and Huntington's disease are associated with basal nuclei dysfunction.

Summary Table: Major Regions and Features of the CNS

Region	Main Structures	Key Functions
Cerebral Cortex	Gray matter (outer layer)	Conscious thought, sensory perception, voluntary movement
White Matter	Myelinated fibers (internal)	Communication between regions
Basal Nuclei	Caudate nucleus, putamen, globus pallidus	Motor control, cognition, emotion
Ventricles	Lateral, third, fourth ventricles	CSF production and circulation

Additional info:

• Functional imaging (PET, MRI) is used to study the localization of brain functions.

• Damage to specific cortical areas can result in loss of function, such as paralysis or sensory deficits.

• Motor and sensory homunculi illustrate the disproportionate representation of body regions in the cortex (e.g., face and hands have larger areas due to fine motor and sensory control).