Overview of Central Nervous System (CNS) Functions

Major Functions of the Nervous System

The nervous system is responsible for detecting stimuli, integrating information, and initiating responses. It is divided into sensory, integrative, and motor functions:

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

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

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

The peripheral nervous system (PNS) handles sensory and motor functions, while the CNS is responsible for integrative functions.

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 regions, each with specialized functions:

• Cerebrum: Largest part; interprets sensory information and controls higher mental functions. Divided into right and left hemispheres by the longitudinal fissure. The surface layer is the cerebral cortex (gray matter), with folds (gyri), shallow depressions (sulci), and deep grooves (fissures).

• Cerebellum: Second largest; coordinates movement, evaluates sensory input, and is involved in timekeeping. Divided into hemispheres by the vermis, with a cortex of gray matter and internal white matter called arbor vitae.

• Brainstem: Relays information between the spinal cord and higher brain regions; controls basic life-sustaining functions. Includes the midbrain, pons, and medulla oblongata. The diencephalon (thalamus, hypothalamus, epithalamus) is functionally associated with the brainstem.

Gray Matter and White Matter

The brain consists of gray matter (neuron cell bodies, dendrites, and synapses) and white matter (myelinated axons). Internal passageways and chambers are filled with cerebrospinal fluid (CSF). As one ascends from the medulla oblongata to the cerebrum, brain functions become more complex.

Protection and Support of the Brain

• Physical Protection: Provided by the bones of the cranium, cranial meninges, and CSF.

• Biochemical Isolation: Maintained by the blood-brain barrier.

Cranial Meninges

Structure and Function

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

• Dura Mater: Outermost, tough layer; consists of periosteal (fused to skull) and meningeal layers, with venous sinuses between them. No epidural space in the cranium.

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

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

Dural Folds

Folds of the inner dura mater stabilize and support the brain:

• 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 interconnected cavities called ventricles, filled with CSF. These include the lateral ventricles, third ventricle, and fourth ventricle.

Functions and Circulation of CSF

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

• Production: CSF is produced by ependymal cells in the choroid plexus, filtered from plasma, and secreted into the ventricles.

• Flow: CSF circulates through the ventricles, exits via apertures, bathes the brain and spinal cord, and is reabsorbed into venous sinuses via arachnoid granulations.

• Volume: About 500 mL of CSF is produced and drained daily.

Blood Supply and Barriers of the Brain

Blood Supply

The brain receives nutrients and oxygen via the internal carotid and vertebral arteries and drains blood via the internal jugular and vertebral veins. Although only 2% of body mass, the brain consumes 20% of the body's oxygen and glucose.

Blood-Brain Barrier (BBB)

The BBB isolates CNS neural tissue from general circulation, formed by tight junctions between endothelial cells. Lipid-soluble substances can diffuse, while water and ions require channels, and larger molecules use active transport. Astrocytes regulate BBB permeability.

Blood-CSF Barrier

Specialized ependymal cells with tight junctions surround capillaries of the choroid plexus, limiting pathogen entry from blood to CSF.

Breaks in the BBB

Certain brain regions (hypothalamus, pituitary gland, pineal gland, choroid plexus) have a less restrictive barrier to allow hormone secretion and CSF production.

The Diencephalon

Major Divisions

The diencephalon is located beneath the cerebrum and connects it to the brainstem. It includes:

• Thalamus: Relays and processes sensory information to the cerebrum; part of the limbic system (emotion, memory).

• Hypothalamus: Major control center for the endocrine system and autonomic nervous system (ANS); regulates hormone production, thermoregulation, hunger, thirst, and emotional responses.

• Epithalamus: Contains the pineal gland, which secretes melatonin.

• Pituitary Gland: Major endocrine gland connected to the hypothalamus via the infundibulum; forms the hypothalamic-pituitary axis.

The Brainstem

Major Components and Functions

• Midbrain (Mesencephalon): Involved in visual and auditory reflexes, motor output, and maintaining consciousness. Contains the substantia nigra, which inhibits unwanted muscle contractions.

• Pons: Connects the cerebellum to the brainstem; involved in sensory and motor functions (hearing, equilibrium, taste, facial sensation, eye movement, chewing, swallowing, facial expressions, saliva and tear secretion, sleep, respiration, posture).

• Medulla Oblongata: Connects the brain to the spinal cord; regulates autonomic functions (heart rate, blood pressure, digestion) and contains nuclei for cranial nerves and reflex centers.

The Cerebellum

Structure and Function

The cerebellum is responsible for coordinating voluntary movements, adjusting postural muscles, and reducing motor error by comparing intended movement with actual performance. It also plays roles in sensory evaluation, timekeeping, emotions, and impulse control.

• Folia: Highly folded surface of the cerebellar cortex (gray matter).

• Arbor Vitae: Internal white matter.

• Purkinje Cells: Large, branched neurons in the cerebellar cortex that receive extensive synaptic input.

The Cerebrum

Structure and Lobes

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

• Frontal Lobe

• Parietal Lobe

• Temporal Lobe

• Occipital Lobe

• Insula (medial to lateral sulcus)

Gray and White Matter

• Gray Matter: Outer cerebral cortex and basal nuclei; involved in conscious processes and intellectual functions.

• White Matter: Deep to the cortex; contains association fibers (within one hemisphere), commissural fibers (between hemispheres, e.g., corpus callosum), and projection fibers (connect cerebrum with lower areas).

Functional Areas of the Cerebral Cortex

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

• Premotor Area: Plans movements for the primary motor cortex.

• Primary Somatosensory Cortex (postcentral gyrus): Receives somatic sensory information (touch, pressure, pain, temperature).

• Somatosensory Association Area: Interprets sensory input.

• Special Sensory Cortices: Visual (occipital lobe), auditory (temporal lobe), olfactory (medial temporal lobe), gustatory (insula), and orbitofrontal cortex (association area for smell and taste).

Integrative Areas

• Wernicke’s Area: General interpretive area for language comprehension.

• Broca’s Area: Speech center, coordinates complex vocalization functions.

• Prefrontal Cortex: Involved in personality, decision-making, and behavior planning.

Limbic System

The limbic system is a functional grouping involved in emotion, motivation, and memory. It includes the limbic lobe, amygdala, hippocampus, thalamus, hypothalamus, and reticular formation. The hippocampus organizes new information into memory, while the cerebral cortex is responsible for long-term storage.

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. The spinal cord ends at the conus medullaris, with the filum terminale and cauda equina extending below.

Spinal Nerves

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

• Spinal nerves exit the vertebral canal via the intervertebral foramina.

Spinal Meninges

• Dura Mater: Outermost, tough, elastic layer; forms the spinal dural sac and dural root sheaths. The epidural space is present only in the spine.

• Arachnoid Mater: Delicate, fibrous membrane enclosing the subarachnoid space (contains CSF). Arachnoid trabeculae connect it to the pia mater.

• Pia Mater: Delicate layer on the spinal cord surface; forms the filum terminale and denticulate ligaments for stabilization.

Internal Structure

• Butterfly-shaped gray matter (anterior, posterior, and lateral horns) surrounded by white matter tracts.

• Central canal filled with CSF; gray commissure connects the two sides.

• White matter organized into ascending (sensory) and descending (motor) tracts.

Summary Table: Major Brain Divisions and Functions