Divisions and Organization of the Central Nervous System: Structure and Function of the Brain and Spinal Cord

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 clinical work involving brain and spinal cord injuries, such as stroke.

• CNS Components: Brain and spinal cord

• Cephalization: Evolutionary development of the anterior (rostral) portion of the CNS, resulting in increased neuron numbers and higher functional levels, especially in humans.

Example: Stroke affecting the CNS can lead to loss of motor or sensory function depending on the affected region.

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: Includes midbrain, pons, and medulla oblongata

• Cerebellum

Example: The cerebellum is essential for coordination and balance.

Spinal Cord

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

• Gray matter: Contains neuron cell bodies and nonmyelinated neurons

• White matter: Contains myelinated and some nonmyelinated axons

Example: Damage to the spinal cord can result in loss of sensation or motor function below the injury site.

Brain Stem

The brain stem connects the brain to the spinal cord and contains additional gray matter nuclei within white matter.

• Includes midbrain, pons, and medulla oblongata

• Controls vital functions such as breathing and heart rate

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 areas of gray matter nuclei 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

• Three openings connect the fourth ventricle to the subarachnoid space surrounding the brain

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, grooves, and deep fissures.

• Gyri: Ridges

• Sulci: Shallow grooves

• Fissures: Deep grooves

• Longitudinal fissure: Separates the two hemispheres

• Transverse cerebral fissure: Separates cerebrum from cerebellum

Lobes, Sulci, and Fissures

Each hemisphere is divided into five lobes by sulci and fissures:

• Frontal lobe

• Parietal lobe

• Temporal lobe

• Occipital lobe

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

Regions of the Cerebral Hemispheres

1. Cerebral Cortex

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 Areas:

• Motor areas: Control voluntary movement

• Sensory areas: Conscious awareness of sensation

• Association areas: Integrate diverse information

Lateralization: Specialization of cortical function can occur in only one hemisphere. Each hemisphere controls the contralateral (opposite) side of the body.

Functional and Structural Areas of the Cerebral Cortex

The cortex contains distinct areas for motor, sensory, and association functions, mapped to specific regions.

Motor Areas

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

• Premotor cortex: Plans movements and coordinates learned motor skills

• Broca's area: Motor speech area; directs muscles of 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)

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), deficits in fine motor control, and may require retraining of neurons

Somatosensory Areas

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

• Somatosensory homunculus: Represents contralateral sensory input from body regions

• Somatosensory association cortex: Integrates sensory input for object understanding

Visual Areas

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

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

Auditory and Vestibular Areas

• Primary auditory cortex: Interprets information from inner ear (pitch, loudness, 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

• Allow meaning to be given to information, store it in memory, relate to previous experience, and decide on actions

• Broadly divided into anterior association area (prefrontal cortex), posterior association area, and limbic association area

Anterior Association Area (Prefrontal Cortex)

• Most complicated cortical region

• Involved with intellect, cognition, recall, and personality

• Contains working memory for abstract ideas, judgment, reasoning, persistence, and planning

• Development depends on social feedback

Posterior and Limbic Association Areas

• Posterior association area: Large region in temporal, parietal, and occipital lobes; involved in pattern recognition, spatial localization, 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 hemisphere dominant for language (usually left hemisphere)

• Left hemisphere: Controls language, math, logic

• Right hemisphere: Visual-spatial skills, intuition, emotion, artistic and musical skills

• Hemispheres communicate via fiber tracts and functional integration

2. Cerebral White Matter

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

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

3. Basal Nuclei (Ganglia)

The basal nuclei are deep within the white matter of each hemisphere and play a crucial role in motor control, cognition, and emotion.

• Include caudate nucleus, putamen, and globus pallidus

• Functions:

  • Influence muscle movements

  • Regulate intensity of slow or stereotyped movement

  • Filter out incorrect/inappropriate responses

  • Inhibit antagonistic/unnecessary movements

  • Play a role in cognition and emotion

• Disorders: Parkinson's disease and Huntington's disease are associated with basal nuclei dysfunction

Discussion and Application

• Damage to specific regions of the cerebral cortex (e.g., stroke) can result in loss of specific functions, such as hearing if the temporal lobe is affected.

• The facial region of the primary motor cortex is larger than that for the upper back due to finer motor control required for facial muscles.

• Basal nuclei and cerebral white matter differ: white matter is myelinated axons, basal nuclei are cell bodies.

Region	Main Function	Key Structures
Cerebral Cortex	Conscious thought, sensory perception, voluntary movement	Motor, sensory, association areas
Cerebral White Matter	Communication between brain regions	Association, commissural, projection fibers
Basal Nuclei	Motor control, cognition, emotion	Caudate nucleus, putamen, globus pallidus

Additional info: The notes above expand on the original slides by providing definitions, examples, and clinical relevance for each major structure and function described.