A: Central Nervous System: Part 1

Anatomy of the Central Nervous System

Overview

The central nervous system (CNS) is composed of the brain and spinal cord, serving as the primary control center for the body. It processes sensory information, coordinates movement, and is responsible for higher-order functions such as reasoning, memory, and emotion.

Major Divisions of the Brain

Brain Subdivisions

• Cerebrum: Largest part, responsible for thought, voluntary movement, and sensory processing.

• Diencephalon: Includes thalamus, hypothalamus, and epithalamus; involved in sensory relay and homeostasis.

• Brainstem: Composed of midbrain, pons, and medulla oblongata; controls automatic functions and connects brain to spinal cord.

• Cerebellum: Coordinates movement and balance.

Cerebrum

Structure and Organization

• Cerebral Hemispheres: Divided by the median longitudinal fissure into right and left hemispheres.

• Gyri (ridges) and Sulci (grooves): Increase surface area for cortical neurons.

• Each hemisphere is divided into four lobes: frontal, parietal, occipital, and temporal.

Functions of Cerebral Lobes

Lobe	Important Regions	Function
Frontal	Prefrontal cortex, Motor cortex, Broca's area	Controls voluntary movement, reasoning, planning, conscience, impulse control, and speech production
Parietal	Somatosensory cortex	Processes sensory input, spatial orientation, and perception
Occipital	Visual cortex	Visual processing
Temporal	Auditory cortex	Auditory processing, language comprehension, memory

Three Fundamental Regions of the Cerebral Hemisphere

• Cortex (gray matter): Superficial layer, site of conscious mind.

• White matter: Internal, composed of myelinated fibers for communication.

• Basal nuclei: Deep gray matter involved in movement regulation.

Cerebral Cortex

• 2–4 mm thick, accounts for ~40% of brain mass.

• Responsible for higher-order functions: memory, language, reasoning.

• Functional areas:

  • Motor areas (frontal lobes)

  • Sensory areas (parietal, occipital, temporal lobes)

  • Association areas (all lobes)

• Contralateral control: Each hemisphere controls the opposite side of the body.

• Lateralization: Hemispheres are symmetrical but not functionally identical.

Cerebral White Matter

• Located deep to the cortex; composed of myelinated axons.

• Responsible for communication between brain regions and with the spinal cord.

• Internal capsule: Major pathway for information to and from the cortex.

Basal Nuclei

• Subcortical nuclei: caudate, putamen, globus pallidus.

• Regulate initiation and intensity of movement; inhibit unnecessary movements.

• Disorders: Huntington's disease (excess movement), Parkinson's disease (reduced movement).

Diencephalon

Location and Components

• Situated between cerebrum and brainstem.

• Includes thalamus, hypothalamus, and epithalamus.

Thalamus

• Bilateral, egg-shaped nuclei forming the superolateral walls of the third ventricle.

• Acts as a relay station for sensory information to the cerebral cortex.

• Sorts and edits incoming signals; all sensory input (except olfaction) passes through thalamic nuclei.

Hypothalamus

• Located below the thalamus; forms the inferolateral walls of the third ventricle.

• Major visceral control center; critical for homeostasis.

• Regulates pituitary hormone secretion.

Center/Role	Description
Autonomic control	Regulates blood pressure, heart rate, digestive tract motility, pupil size
Emotional response	Involved in pleasure, fear, rage, biological rhythms
Body temperature	Monitors and initiates cooling or heat-generating actions
Food intake	Regulates hunger and satiety
Water balance/thirst	Controls ADH release for water retention
Sleep-wake cycles	Regulates circadian rhythms
Endocrine system	Controls pituitary hormone secretion

Epithalamus

• Dorsal part of diencephalon; forms roof of third ventricle.

• Pineal gland secretes melatonin, regulating sleep-wake cycles.

Brainstem

Structure and Function

• Connects brain to spinal cord; accounts for ~3% of brain mass.

• Composed of midbrain, pons, and medulla oblongata.

• Controls automatic behaviors necessary for survival.

Midbrain

• Located between diencephalon and pons.

• Cerebral aqueduct connects third and fourth ventricles.

• Periaqueductal gray matter involved in pain suppression and autonomic pathways.

• Contains nuclei for cranial nerves III (oculomotor) and IV (trochlear).

• Corpora quadrigemina: Superior colliculi (visual reflexes), inferior colliculi (auditory reflexes).

• Substantia nigra: Dopaminergic neurons; degeneration leads to Parkinson's disease.

• Red nucleus: Motor coordination.

Pons

• Bulging region between midbrain and medulla.

• Composed of conduction tracts; connects cerebellum to brainstem.

• Pontine nuclei relay signals between motor cortex and cerebellum.

Medulla Oblongata

• Most inferior part of brainstem; forms ventral wall of fourth ventricle.

• Pyramids: Corticospinal tracts; decussation (crossing) results in contralateral control.

• Autonomic reflex centers: cardiovascular, respiratory, and other vital functions.

• Works with hypothalamus for visceral regulation.

Cerebellum

Structure and Function

• Accounts for ~11% of brain mass; located under occipital lobes.

• Coordinates voluntary movements and balance.

• Processes input from motor cortex, brainstem, and sensory receptors.

• Contains two hemispheres, gyri (folia), fissures, and lobes.

• Purkinje cells: Large neurons in cerebellar cortex.

• Flocculonodular lobes: Receive input from inner ear for balance.

• Cerebellar peduncles: Connect cerebellum to brainstem.

• Involved in some cognitive functions (e.g., puzzle solving).

Functional Brain Systems

Limbic System

• Located in cerebral hemispheres and diencephalon.

• Controls emotional responses and memory.

• Amygdala: Recognizes fear, anger; involved in memory.

• Interacts with prefrontal cortex for emotional regulation.

Reticular Formation

• Extends through medulla, pons, and midbrain.

• Reticular Activating System (RAS): Maintains cortical alertness and consciousness.

• Filters repetitive or weak signals; inhibited by sleep centers, alcohol, tranquilizers.

• Contains motor nuclei projecting to spinal cord.

Protection of the CNS

Meninges

• Three connective tissue membranes: dura mater, arachnoid mater, pia mater.

• Dura mater: Outermost, strongest; two layers (periosteal and meningeal).

• Arachnoid mater: Middle layer; subarachnoid space filled with CSF and blood vessels.

• Pia mater: Innermost, delicate, adheres to brain surface.

Cerebrospinal Fluid (CSF)

• Liquid cushion for brain and spinal cord; provides buoyancy and protection.

• Produced by choroid plexuses in ventricles; regulated by ependymal cells.

• Circulates in subarachnoid space; composition differs from plasma (less protein, Ca2+, K+; more Na+, Cl-, H+).

Blood-Brain Barrier (BBB)

• Protective mechanism formed by endothelial tight junctions.

• Regulates passage of substances between blood and CNS.

• Permits essential nutrients; blocks toxins, most drugs.

• Ineffective against fat-soluble substances, O2, CO2.

• Absent in some areas (e.g., vomiting center, hypothalamus).

Ventricular System of the Brain

Ventricles and CSF

• Four interconnected chambers: two lateral ventricles, third ventricle, fourth ventricle.

• Lateral ventricles: C-shaped, one in each hemisphere.

• Third ventricle: Between halves of diencephalon; connected to fourth ventricle by cerebral aqueduct.

• Fourth ventricle: Posterior to pons and medulla; continuous with central canal of spinal cord.

• CSF drains from fourth ventricle into subarachnoid space.

Pathophysiology

Hypothalamic Dysfunction

• Arginine vasopressin (AVP, Antidiuretic hormone [ADH]): Produced in hypothalamus, released from posterior pituitary; regulates water balance and blood pressure.

• Deficiency leads to diabetes insipidus: excessive urine production, dehydration; treated with desmopressin.

Parkinson's Disease

• Degeneration of substantia nigra in midbrain reduces dopamine production.

• Results in motor symptoms: tremor, rigidity, bradykinesia.

Meningitis

• Inflammation of the meninges; symptoms include headache, fever, stiff neck.

• Caused by bacterial, viral, or fungal infections; viral is most common in the US.

• Bacterial meningitis can be rapidly fatal; diagnosed by CSF analysis.

• Common pathogens: Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae.

References and Further Study

• Morton DA, Foreman K, Albertine KH. The Big Picture: Gross Anatomy, 2nd Ed.

• Agur AMR, Dalley AF. Moore's Essential Clinical Anatomy, 7e.

• Marieb EN, Hoehn K. Human Anatomy & Physiology, 8e.

For additional learning, see the provided video links on brain structures, CSF circulation, diencephalon, substantia nigra, and the blood-brain barrier.