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

The Central Nervous System

Overview and Directional Terms

The Central Nervous System (CNS) consists of the brain and spinal cord, serving as the main control center for the body. Unique directional terms are used in CNS anatomy:

• Rostral: Toward the nose

• Caudal: Toward the tail

The Brain

Functions of the Brain

The brain regulates vital functions and complex behaviors:

• Controls: Heart rate, respiratory rate, blood pressure

• Regulates: Autonomic nervous system and endocrine system

• Innervates: Head via cranial nerves

• Performs: Intelligence, consciousness, memory, sensory-motor integration, emotion, behavior, and socialization

Basic Parts and Organization

The brain is divided into four main regions:

• Brain stem: Midbrain, pons, medulla oblongata

• Cerebellum

• Diencephalon: Thalamus, hypothalamus, epithalamus

• Cerebrum: Two cerebral hemispheres

Ventricles of the Brain

Structure and Function

Brain ventricles are expansions of the central cavity, filled with cerebrospinal fluid (CSF) and lined with ependymal cells. They are continuous with each other and the central canal of the spinal cord.

• Lateral ventricles: Located in cerebral hemispheres, horseshoe-shaped

• Third ventricle: In the diencephalon, connected to lateral ventricles by interventricular foramen

• Cerebral aqueduct: Connects third and fourth ventricles

• Fourth ventricle: In the hindbrain, connects to central canal of spinal cord

Distribution of Gray and White Matter

Organization in the CNS

The CNS is organized with centrally located gray matter (neuronal cell bodies) and externally located white matter (axons). An additional layer of gray matter, called the cortex, is found in the cerebrum and cerebellum.

• Gray matter: Contains neuronal cell bodies

• White matter: Contains axons

• Cortex: Outer layer of gray matter in cerebrum and cerebellum

The Brain Stem

Structure and Functions

The brain stem includes the midbrain, pons, and medulla oblongata. It serves as a passageway for fiber tracts between the cerebrum and spinal cord, innervates the face and head, produces automatic behaviors necessary for survival, and integrates auditory and visual reflexes.

Medulla Oblongata

• Location: Most caudal part of the brain stem, continuous with the spinal cord

• Pyramids: Major fiber tracts on ventral surface; decussation (crossing over) of motor tracts occurs here

• Cranial nerves: VIII (vestibulocochlear), IX (glossopharyngeal), X (vagus), XII (hypoglossal) attach to the medulla

• Reticular formation: Core contains nuclei influencing autonomic functions (cardiac, vasomotor, respiratory centers)

Pons

• Bridge: Connects midbrain and medulla oblongata

• Cranial nerve nuclei: V (trigeminal), VI (abducens), VII (facial)

Midbrain

• Location: Between diencephalon and pons

• Cerebral aqueduct: Central cavity of the midbrain

• Cerebral peduncles: Contain pyramidal tracts

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

• Substantia nigra: Pigmented nuclei linked to basal nuclei, associated with Parkinson’s disease

• Red nucleus: Largest nucleus of the reticular formation

The Cerebellum

Structure and Function

The cerebellum is located dorsal to the pons and medulla. It smooths and coordinates body movements and helps maintain equilibrium.

• Hemispheres: Two, each subdivided into anterior, posterior, and flocculonodular lobes

• Folia: Surface ridges separated by fissures

• Regions: Cortex (gray matter), arbor vitae (internal white matter), deep cerebellar nuclei (gray matter)

Coordination of Movement

• Receives information on equilibrium, current movements, and from the cerebral cortex

• Compares intended movement with body position

• Sends instructions to the cerebral cortex to adjust motor commands

Higher Cognitive Functions

• Learning new motor skills

• Participates in cognition (language, problem solving, task planning)

The Diencephalon

Structure and Components

The diencephalon forms the central core of the forebrain and is composed of three paired structures: thalamus, hypothalamus, and epithalamus. It borders the third ventricle and is primarily gray matter.

Thalamus

• Makes up 80% of the diencephalon

• Acts as a relay station for incoming sensory messages

• All communication with the cerebral cortex passes through thalamic nuclei (except olfactory information)

• Organizes and amplifies or tones down signals

Hypothalamus

• Located between optic chiasm and mammillary bodies

• Pituitary gland projects inferiorly

• Main visceral control center of the body

• Functions: control of ANS, emotional responses, body temperature, hunger/thirst, behavior, sleep-wake cycles, endocrine system, memory formation

Epithalamus

• Forms part of the roof of the third ventricle

• Includes the pineal gland, which secretes melatonin and helps regulate circadian rhythm

The Cerebrum

Structure and Surface Features

The cerebrum accounts for 83% of brain mass. Deep grooves called fissures separate major regions, while sulci (grooves) and gyri (ridges) mark the surface. Deeper sulci divide the cerebrum into lobes named for the overlying skull bones: frontal, parietal, occipital, temporal, and insula.

Cerebral Cortex

• Home of conscious mind

• Enables awareness, voluntary movement, communication, memory, and understanding

• Composed of gray matter; folds triple its size

• Functional regions: motor, sensory, and association areas

Functional Areas of the Cerebral Cortex

Sensory Areas

• Primary sensory cortex for each major sense

• Association areas process sensory information

• Multimodal association areas integrate input from multiple regions

Motor Areas

• Premotor cortex: controls complex movements, plans actions

• Primary motor cortex: controls voluntary motor functions, located in precentral gyrus

• Frontal eye field: controls voluntary eye movement

• Broca’s area: manages speech production

Cerebral White Matter

Types of Tracts

• Commissural fibers: Connect cerebral hemispheres (e.g., corpus callosum)

• Association fibers: Connect different parts of the same hemisphere

• Projection fibers: Run vertically, connecting cortex with lower regions

Deep Gray Matter of the Cerebrum

Basal Nuclei (Basal Ganglia)

• Group of nuclei deep within cerebral white matter

• Includes caudate nucleus, putamen, globus pallidus

• Involved in motor control, start/stop/regulate voluntary movements, select appropriate muscles, estimate passage of time

Functional Brain Systems

Limbic System

• Located in medial aspect of cerebral hemispheres and diencephalon

• Composed of septal nuclei, cingulate gyrus, hippocampal formation, amygdaloid body

• Functions: emotional processing, memory formation, pain interpretation

Reticular Formation

• Runs through central core of brain stem

• Reticular activating system (RAS): maintains consciousness and alertness, functions in sleep/arousal

Protection of the Brain

Meninges

• Cover and protect CNS

• Enclose and protect vessels supplying CNS

• Contain cerebrospinal fluid between pia and arachnoid maters

Dura Mater

• Strongest meninge, two layers (periosteal and meningeal)

• Encloses dural sinuses

• Largest sinus: superior sagittal sinus

• Extends inward to subdivide cranial cavity

Arachnoid Mater

• Located beneath dura mater

• Arachnoid villi allow CSF to pass into dural blood sinuses

Pia Mater

• Delicate connective tissue

• Clings tightly to brain surface, follows cortex convolutions

Cerebrospinal Fluid (CSF)

• Fills hollow cavities of brain and spinal cord

• Provides liquid cushion, nourishes, removes wastes, carries chemical signals

• Formed in choroid plexuses in brain ventricles

Blood-Brain Barrier

• Prevents most blood-borne toxins from entering brain

• Impermeable capillaries, not absolute barrier (nutrients, alcohol, nicotine, anesthetics pass through)

Spinal Cord

Structure and Function

• Spinal nerves attach to it

• Provides two-way conduction pathway

• Major center for reflexes

• Runs through vertebral canal, extends from foramen magnum to L1/L2

Key Features

• Conus medullaris: inferior end

• Filum terminale: connective tissue filament attaching to coccyx

• Cervical/lumbar enlargements: nerves for limbs arise

• Cauda equina: collection of spinal nerve roots

Spinal Cord Segments

• Designated by spinal nerve issuing from it (e.g., T1)

White and Gray Matter

• White matter: outer region, myelinated/nonmyelinated axons, communication between spinal cord and brain

• Gray matter: shaped like "H", contains neuronal cell bodies, divided into somatic/visceral regions

Protection of the Spinal Cord

• Protected by vertebrae, meninges, and CSF

• Meninges: dura mater (single layer), arachnoid mater, pia mater (delicate, extends to coccyx)

Sensory and Motor Pathways in the CNS

Pathways and Tracts

• Multineuron pathways connect brain and body periphery

• Ascending pathways: carry information to CNS

• Descending pathways: carry information away from CNS

Ascending Pathways

• Conduct general somatic sensory impulses

• Chains of first-, second-, and third-order neurons

• Main pathways: dorsal column, spinothalamic, spinocerebellar

Descending Pathways

• Deliver motor instructions from brain to spinal cord

• Direct pathways (pyramidal tracts), indirect motor pathways (tectospinal, vestibulospinal, rubrospinal, reticulospinal)

Disorders of the Central Nervous System

Spinal Cord Damage

• Paralysis: Loss of motor function

• Parasthesia: Loss of sensation

• Paraplegia: Injury between T1 and L2, paralysis of lower limbs

• Quadriplegia: Injury in cervical region, paralysis of all four limbs

Brain Dysfunction

• Cerebrovascular accident (stroke): Blockage/interruption of blood flow to brain region

• Alzheimer’s disease: Progressive degenerative disease leading to dementia

Congenital Malformations

• Anencephaly: Cerebrum and cerebellum absent

• Spina bifida: Absence of vertebral lamina

• Cerebral palsy: Poorly controlled voluntary muscles due to cerebral cortex damage

Postnatal Changes in the Brain

• Brain structures complete development at different times

• Critical periods in learning (e.g., language)

• Some development continues into early 20s

• Decline with age due to changes in neural circuitry and neurotransmitter release

Summary Table: Functions of Major Parts of the Brain

Brain Region	Main Function
Cerebrum	Conscious thought, memory, voluntary movement
Diencephalon	Sensory relay, homeostasis, hormone regulation
Brain Stem	Basic life functions, cranial nerve nuclei, reflexes
Cerebellum	Coordination, balance, motor learning
Spinal Cord	Reflexes, sensory/motor pathway conduction

Additional info: Table entries inferred from textbook context and lecture notes.