Study Guide: The Central Nervous System (CNS) – Structure and Function

The Central Nervous System: Overview

Functions of the Nervous System

The nervous system is responsible for detecting sensations, integrating information, and initiating responses. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS includes the brain and spinal cord, while the PNS consists of nerves outside the CNS.

• Sensory Functions: Detect sensations inside and outside the body.

• Integrative Functions: Decision-making processes, carried out exclusively by the CNS.

• Motor Functions: Stimulation of muscle contractions or gland secretions.

Major Regions of the Brain

Brain Structure and Divisions

The adult human brain contains nearly 97% of the body's neural tissue and is divided into several regions, each with distinct functions.

• Cerebrum: Largest part, interprets sensory information, controls higher mental functions.

• Cerebellum: Coordinates movement, evaluates sensory input, involved in time keeping.

• Brainstem: Relays information, controls basic life-sustaining functions (heart rate, blood pressure, respiration, digestion).

• Diencephalon: Includes thalamus, hypothalamus, epithalamus, and pituitary gland; links cerebrum with brainstem.

Basic Structure of the Brain and Spinal Cord

Cerebrum

The cerebrum is divided into right and left hemispheres by the longitudinal fissure. Its surface is covered by the neural cortex (gray matter), which is highly folded to increase surface area.

• Gyri: Elevated ridges.

• Sulci: Shallow depressions.

• Fissures: Deep grooves.

Cerebellum

The cerebellum is the second largest brain region, divided into hemispheres by the vermis. Its cortex is folded into folia, and white matter forms the arbor vitae.

• Functions: Coordination of movement, sensory evaluation, time keeping.

Brainstem

The brainstem relays information between the spinal cord and higher brain regions. It includes the midbrain, pons, and medulla oblongata.

• Midbrain: Visual and auditory reflexes.

• Pons: Connects cerebellum to brainstem, involved in sensory and motor functions.

• Medulla Oblongata: Regulates autonomic functions.

Brain Protection and Support

Cranial Meninges

The brain is protected by three connective tissue layers called meninges: dura mater, arachnoid mater, and pia mater. These layers stabilize and protect the brain from trauma.

• Dura Mater: Tough, outer layer; contains venous sinuses for blood drainage.

• Arachnoid Mater: Middle, smooth layer; subarachnoid space contains cerebrospinal fluid (CSF).

• Pia Mater: Delicate, inner layer; adheres to brain surface and enters sulci.

Dural Folds

Dural folds are extensions of the dura mater that stabilize and support the brain. The three largest folds are:

• Falx cerebri: Between cerebral hemispheres.

• Tentorium cerebelli: Separates cerebellum and cerebrum.

• Falx cerebelli: Divides cerebellar hemispheres.

The Ventricles and Cerebrospinal Fluid (CSF)

Ventricles of the Brain

The brain contains internal chambers called ventricles, which are filled with cerebrospinal fluid (CSF). CSF circulates around and bathes all exposed surfaces of the CNS.

• Functions of CSF: Cushions neural structures, provides buoyancy, transports nutrients and waste.

• Production: Ependymal cells filter plasma and secrete CSF into ventricles.

• Flow: CSF flows through ventricles, bathes brain surface, and is reabsorbed into venous sinuses.

Blood Supply and Barriers

Blood Supply to the Brain

The brain receives nutrients and oxygen via the internal carotid and vertebral arteries. Blood is drained by internal jugular and vertebral veins.

• High metabolic demand: Brain is 2% of body mass but uses 20% of oxygen and glucose.

Blood-Brain Barrier (BBB)

The BBB isolates CNS neural tissue from general circulation. It is formed by tight junctions between endothelial cells of CNS capillaries, controlled by astrocytes.

• Lipid-soluble compounds: Diffuse across endothelial cells.

• Water and ions: Pass through channels.

• Large water-soluble compounds: Use active transport.

Blood-CSF Barrier

Specialized ependymal cells surround capillaries of the choroid plexus, limiting entry of pathogens from blood to CSF.

The Diencephalon

Structure and Divisions

The diencephalon is located under the cerebrum and links it with the brainstem. It consists of the thalamus, hypothalamus, epithalamus, and pituitary gland.

• Thalamus: Relays and processes sensory information, part of limbic system.

• Hypothalamus: Major control center for endocrine and autonomic nervous system (ANS).

• Pituitary gland: Major endocrine gland, controlled by hypothalamus.

• Epithalamus: Contains pineal gland.

The Brainstem

Midbrain

The midbrain (mesencephalon) is involved in visual and auditory reflexes, motor output, and maintaining consciousness.

Pons

The pons connects the cerebellum to the brainstem and is involved in sensory and motor functions, sleep, respiration, and posture.

Medulla Oblongata

The medulla oblongata connects the brain to the spinal cord and regulates autonomic functions such as heart rate, blood pressure, and digestion.

The Cerebellum

Structure and Function

The cerebellum adjusts postural muscles, coordinates repetitive movements, and compares intended movement with actual performance to reduce motor error.

• Folia: Highly folded neural cortex.

• Arbor vitae: Internal white matter.

• Purkinje cells: Large, branched cells in cerebellar cortex.

The Cerebrum

Structure and Lobes

The cerebrum is the largest part of the brain, divided into five lobes by sulci: frontal, parietal, temporal, occipital, and insula.

• Gray Matter: Outer cerebral cortex and basal nuclei.

• White Matter: Deep to cortex, around basal nuclei.

Functional Areas

Each hemisphere receives sensory information from and sends motor commands to the opposite side of the body (hemispheric lateralization). Major functional areas include:

• Primary Motor Cortex: Plans and executes voluntary movement.

• Primary Sensory Cortices: Process sensory input.

• Special Sensory Cortices: Vision, hearing, smell, taste.

• Association Areas: Interpret sensory information.

• Integrative Areas: Wernicke’s area (general interpretive), Broca’s area (speech center).

Spinal Cord Anatomy

External Structure

The spinal cord is located within the vertebral cavity, extending from the foramen magnum to L1/L2. It serves as a major reflex center and pathway between the brain and periphery.

• Conus medullaris: End of the spinal cord.

• Filum terminale: Filamentous continuation.

• Cauda equina: Nerve roots extending through the subarachnoid space.

Spinal Nerves

There are 31 pairs of spinal nerves. Anterior roots contain motor fibers, posterior roots contain sensory fibers. Nerve roots merge to form spinal nerves, which exit via the intervertebral foramen.

Spinal Meninges

The spinal cord is protected by three layers: dura mater, arachnoid mater, and pia mater. The dura mater forms the spinal dural sac, the arachnoid mater encloses the subarachnoid space, and the pia mater adheres to the cord and forms denticulate ligaments.

Internal Structure

The spinal cord has butterfly-shaped gray matter surrounded by white matter tracts. The central canal is filled with CSF.

• Gray Matter: Anterior horn (motor), posterior horn (sensory), lateral horn (ANS).

• White Matter: Ascending and descending tracts.

Summary Table: Major Brain Divisions and Functions

Brain Division	Function
Cerebrum	Performs higher mental functions, interprets sensory stimuli, plans and initiates movement
Diencephalon	Processes, integrates, and relays information; maintains homeostasis; regulates biological rhythms
Cerebellum	Monitors and coordinates movement
Brainstem	Maintains homeostasis, controls certain reflexes, monitors movement, integrates and relays information

Additional info: Some details and explanations were expanded for clarity and completeness based on standard academic context for college-level anatomy and physiology.