Overview of Central Nervous System (CNS) Functions

Introduction to CNS Functions

The central nervous system (CNS) is responsible for integrating sensory information, making decisions, and coordinating motor output. It works in conjunction with the peripheral nervous system (PNS), which handles sensory and motor functions outside the CNS.

• Sensory Functions: Detection of sensations both inside and outside the body.

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

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

Basic Structure of the Brain and Spinal Cord

Major Regions of the Brain

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

• Cerebrum: Largest part, interprets sensory information, controls higher mental functions, divided into right and left hemispheres.

• Cerebellum: Second largest, coordinates movement, evaluates sensory input, and maintains timing.

• Brainstem: Relays information between the spinal cord and higher brain regions; includes the midbrain, pons, and medulla oblongata.

• Diencephalon: Contains the thalamus, hypothalamus, and epithalamus; involved in sensory relay, homeostasis, and endocrine integration.

Gray Matter and White Matter

The brain consists of gray matter (neuron cell bodies, dendrites, and synapses) and white matter (myelinated axons). The cerebral cortex is a surface layer of gray matter, while white matter lies deeper.

• Gyri: Elevated ridges on the brain surface.

• Sulci: Shallow depressions between gyri.

• Fissures: Deep grooves separating major brain regions.

Protection and Support of the Brain

The brain is protected physically by the skull, cranial meninges, and cerebrospinal fluid (CSF), and biochemically by the blood-brain barrier.

• Cranial Meninges: Three connective tissue layers—dura mater, arachnoid mater, and pia mater—surround and protect the brain.

• Cerebrospinal Fluid (CSF): Cushions the brain, provides buoyancy, and transports nutrients and waste.

• Blood-Brain Barrier: Selectively isolates the brain from blood-borne substances.

Cranial Meninges

Layers of the Cranial Meninges

The cranial meninges are three connective tissue layers that protect the brain and are continuous with the spinal meninges.

• Dura Mater: Outermost, tough, fibrous layer; has periosteal and meningeal layers with venous sinuses between them.

• Arachnoid Mater: Middle, smooth layer; does not dip into brain crevices; subarachnoid space beneath contains CSF.

• Pia Mater: Innermost, delicate layer; closely follows brain contours and enters sulci.

Dural Folds and Sinuses

Dural folds are extensions of the dura mater that stabilize and support the brain. Major folds include the falx cerebri, tentorium cerebelli, and falx cerebelli. Venous sinuses within these folds drain blood from the brain.

Cerebrospinal Fluid (CSF) and Ventricles

Production and Circulation of CSF

CSF is produced by ependymal cells in the choroid plexus, circulates through the ventricles, and bathes the CNS. It is reabsorbed into the venous system via arachnoid granulations.

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

• Composition: Water, ions, glucose, some white blood cells, little protein.

Blood Supply and Barriers of the Brain

Blood Supply

The brain receives nutrients and oxygen via the internal carotid and vertebral arteries and is drained by the internal jugular and vertebral veins. Despite being only 2% of body mass, the brain consumes 20% of the body's oxygen and glucose.

Blood-Brain Barrier (BBB) and Blood-CSF Barrier

The BBB is formed by tight junctions between endothelial cells, restricting passage of substances. Astrocytes regulate its permeability. The blood-CSF barrier is formed by specialized ependymal cells in the choroid plexus.

• Lipid-soluble substances can diffuse across the BBB; water and ions require channels or active transport.

• Breaks in the BBB occur in regions like the hypothalamus, pituitary gland, pineal gland, and choroid plexus for hormone secretion and CSF production.

Major Brain Regions and Their Functions

Cerebrum

The cerebrum is the largest brain region, responsible for conscious thought, intellectual functions, and voluntary motor control. It is divided into lobes by sulci and fissures.

• Lobes: Frontal, parietal, temporal, occipital, and insula.

• Gray Matter: Outer cerebral cortex and basal nuclei.

• White Matter: Association, commissural, and projection fibers connect different brain regions.

Functional Areas of the Cerebral Cortex

• Primary Motor Cortex: Initiates voluntary movements (precentral gyrus).

• Premotor Area: Plans movements.

• Primary Somatosensory Cortex: Receives sensory input (postcentral gyrus).

• Association Areas: Interpret sensory information.

• Special Sensory Cortices: Visual (occipital), auditory (temporal), olfactory (temporal), gustatory (insula).

• Integrative Areas: Wernicke’s area (language comprehension), Broca’s area (speech production).

Limbic System

The limbic system is a functional grouping involved in emotion, motivation, and memory. Key structures include the hippocampus (memory formation), amygdala (emotion), and parts of the thalamus and hypothalamus.

Diencephalon

Thalamus

The thalamus relays and processes sensory information to the cerebrum and is involved in emotion and memory as part of the limbic system.

Hypothalamus

The hypothalamus is the major control center for the endocrine system and autonomic nervous system (ANS). It regulates hormone production, thermoregulation, hunger, thirst, and emotional responses.

Pituitary Gland

The pituitary gland is a major endocrine organ controlled by the hypothalamus, forming the hypothalamic-pituitary axis.

Brainstem

Midbrain (Mesencephalon)

The midbrain is involved in visual and auditory reflexes, motor output, and maintaining consciousness. The substantia nigra inhibits unwanted movements.

Pons

The pons connects the cerebellum to the brainstem and is involved in sensory and motor functions, sleep, respiration, and posture. It contains nuclei for cranial nerves V–VIII.

Medulla Oblongata

The medulla oblongata connects the brain to the spinal cord and regulates autonomic functions such as heart rate, blood pressure, and digestion. It contains nuclei for cranial nerves VIII–XII and reflex centers for cardiovascular and respiratory control.

Cerebellum

Structure and Function

The cerebellum coordinates movement, adjusts postural muscles, and compares intended with actual movement to reduce motor error. It also plays roles in sensory evaluation, timing, and impulse control.

• Folia: Folds of the cerebellar cortex.

• Arbor Vitae: Internal white matter.

• Purkinje Cells: Large, branched neurons in the cortex.

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: Tapered end of the spinal cord.

• Filum Terminale: Filamentous continuation anchoring the cord.

• Cauda Equina: Bundle of lumbar, sacral, and coccygeal nerve roots.

Spinal Nerves and Meninges

• 31 pairs of spinal nerves arise from the cord, each formed by merging anterior (motor) and posterior (sensory) roots.

• Spinal Meninges: Dura mater (outer), arachnoid mater (middle), pia mater (inner) protect the cord and contain CSF.

Internal Structure

The spinal cord has a central canal filled with CSF, surrounded by gray matter (butterfly-shaped) and white matter (tracts).

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

• White Matter: Ascending (sensory) and descending (motor) tracts.

Summary Table: Major Brain Divisions and Functions