Central Nervous System: Structure and Function

Central Nervous System (CNS)

Overview

The Central Nervous System (CNS) consists of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly. The CNS is the primary control center for the body’s functions and behaviors.

• Cephalization: The evolutionary development of the anterior portion of the CNS, leading to an increased number of neurons in the head region.

• Brain: Composed of wrinkled, pinkish gray tissue. Major parts include the cerebral hemispheres, cerebellum, and brain stem.

Developmental Anatomy of the Brain

Primary Brain Vesicles

During embryonic development, the neural tube expands and constricts to form three primary brain vesicles:

• Forebrain (prosencephalon)

• Midbrain (mesencephalon)

• Hindbrain (rhombencephalon)

Secondary Brain Vesicles

These vesicles form from the primary vesicles:

• Telencephalon and diencephalon from the forebrain

• Mesencephalon remains as the midbrain

• Metencephalon and myelencephalon from the hindbrain

Adult Brain Structures

• Cerebrum: cerebral cortex, white matter, basal nuclei

• Diencephalon: thalamus, hypothalamus, epithalamus

• Brain stem: midbrain, pons, medulla oblongata

• Cerebellum

Adult Neural Canal Regions

• Structures derived from the neural canal include the lateral ventricles, third ventricle, cerebral aqueduct, and fourth ventricle.

Basic Pattern of the Central Nervous System

Spinal Cord

• Surrounded by a gray matter core

• External white matter composed of myelinated fiber tracts

Brain

• Additional areas of gray matter are present, including nuclei in the cortex and cerebellum.

Ventricles of the Brain

The brain contains fluid-filled cavities called ventricles, which arise from expansion of the lumen of the neural tube.

• Lateral ventricles (C-shaped)

• Third ventricle (in diencephalon)

• Fourth ventricle (in hindbrain, dorsal to pons and medulla)

Cerebral Hemispheres

Structure

• Form the superior part of the brain and make up 83% of its mass

• Contain ridges (gyri) and shallow grooves (sulci)

• Deep grooves (fissures) separate major brain regions

• Three basic regions: cortex (gray matter), white matter, basal nuclei

Major Lobes, Gyri, and Sulci

• Deep sulci divide the hemispheres into five lobes: frontal, parietal, temporal, occipital, and insula

• Central sulcus separates the frontal and parietal lobes

• Parieto-occipital sulcus separates the parietal and occipital lobes

• Lateral sulcus separates the parietal and temporal lobes

Cerebral Cortex

Structure and Function

• Superficial gray matter; accounts for roughly 40% of the mass of the brain

• Enables sensation, communication, memory, understanding, and voluntary movements

• Each hemisphere acts contralaterally (controls the opposite side of the body)

• Hemispheres are not equal in function

• No functional area acts alone; conscious behavior involves the entire cortex

Functional Areas of the Cerebral Cortex

• Motor areas: control voluntary movement

• Sensory areas: conscious awareness of sensation

• Association areas: integrate diverse information

Cerebral Cortex Motor Areas

• Primary motor cortex: located in the precentral gyrus; controls precise, skilled, voluntary movements

• Premotor cortex: anterior to the precentral gyrus; involved in planning movements

• Broca’s area: motor speech area; directs muscles of the tongue

• Frontal eye field: controls voluntary eye movement

Sensory Areas

• Primary somatosensory cortex: receives information from skin and skeletal muscles; exhibits spatial discrimination

• Somatosensory association cortex: integrates sensory information

• Visual areas: primary visual cortex and visual association area

• Auditory areas: primary auditory cortex and auditory association area

• Other areas: olfactory, gustatory, vestibular cortices

Association Areas

• Prefrontal cortex: involved in judgment, reasoning, persistence, and conscience

• Language areas: include Broca’s area and Wernicke’s area

• General (common) interpretation area: integrates signals from multiple sensory modalities

• Visceral association area: perception of visceral sensations

Lateralization of Cortical Function

• Each hemisphere has abilities not shared with its partner

• Cerebral dominance: designates the hemisphere dominant for language

• Left hemisphere: controls language, math, logic

• Right hemisphere: controls visual-spatial skills, emotion, artistic skills

Cerebral White Matter

Structure and Function

• Composed of myelinated fibers and their tracts

• Responsible for communication between cerebral areas, CNS centers, and areas of the cerebrum

• Types of fibers:

  • Commissural fibers: connect corresponding gray areas of the two hemispheres

  • Association fibers: connect different parts of the same hemisphere

  • Projection fibers: enter the hemispheres from lower brain or cord centers

Basal Nuclei

Structure and Function

• Masses of gray matter found deep within the cortical white matter

• Composed of three parts: caudate nucleus, putamen, globus pallidus

• Functions include:

  • Influence muscular activity

  • Regulate intensity of slow or stereotyped movements

  • Inhibit antagonistic and unnecessary movement

Diencephalon

Structure

• Consists of three paired structures: thalamus, hypothalamus, epithalamus

• Encloses the third ventricle

Thalamus

• Superolateral walls of the third ventricle

• Connected by the interthalamic adhesion

• Composes 80% of the diencephalon

• Contains several groups of nuclei

• Major relay station for sensory impulses

Thalamic Function

• All senses (except olfaction) converge and synapse in the thalamus

• Impulses of similar function are sorted out, edited, and relayed as a group

• Plays a key role in mediating sensation, motor activities, cortical arousal, learning, and memory

• Plays a role in sleep and states of wakefulness

Hypothalamus

• Caps the brainstem and forms the inferolateral walls of the third ventricle

• Contains several nuclei, including the mammillary bodies

• Main visceral control center of the body

Hypothalamic Function

• Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, and many other visceral activities

• Involved in perception of pleasure, fear, rage

• Controls mechanisms regulating hunger and satiety

• Regulates sleep and the timing of the sleep cycle via the suprachiasmatic nucleus

• Controls endocrine system functions by producing releasing and inhibiting hormones

Endocrine Functions of the Hypothalamus

• Controls secretion of hormones by the anterior pituitary

• Produces ADH and oxytocin at the paraventricular nucleus

Epithalamus

• Forms the roof of the third ventricle

• Contains the pineal gland, which secretes melatonin

• Melatonin is involved in sleep regulation and the sleep-wake cycle

Brain Stem

Midbrain

• Located between the diencephalon and the pons

• Contains cerebral peduncles, corpora quadrigemina, and nuclei for cranial nerves III and IV

Pons

• Located between the midbrain and the medulla oblongata

• Forms part of the anterior wall of the fourth ventricle

• Contains nuclei for cranial nerves V, VI, VII

Medulla Oblongata

• Most inferior part of the brain stem

• Contains nuclei for cranial nerves VIII, IX, X, XI, XII

• Controls vital autonomic functions (cardiac, respiratory, vasomotor centers)

Cerebellum

Structure and Function

• Located dorsal to the pons and medulla

• Provides precise timing and appropriate patterns of skeletal muscle contraction

• Processes input from cerebral motor cortex, proprioceptors, and visual/auditory systems

• Consists of two bilaterally symmetrical hemispheres connected by the vermis

• Contains folia (surface folds), arbor vitae (white matter), and cerebellar peduncles (fiber tracts)

Cerebellar Processing

• Receives impulses of intent to initiate voluntary muscle contraction

• Signals "inform" the cerebellum of the body’s position and momentum

• Cerebellar cortex calculates the best way to perform a movement

• "Blueprint" of coordinated movement is sent to the cerebral motor cortex

Functional Brain Systems

Limbic System

• Structures located on the medial aspects of cerebral hemispheres and diencephalon

• Includes the amygdala, hypothalamus, and parts of the thalamus

• Plays a role in emotions, motivation, and memory

Reticular Formation

• Extends through the central core of the medulla, pons, and midbrain

• Has far-flung axonal connections with hypothalamus, thalamus, cerebellum, and spinal cord

• Governs the brain as a whole

• Functions include arousal, motor control, and regulation of visceral functions

Protection of the Brain

Meninges

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

• Functions:

  • Cover and protect the CNS

  • Protect blood vessels and enclose venous sinuses

  • Contain cerebrospinal fluid (CSF)

  • Form partitions within the skull

Dura Mater

• Strongest meninx; composed of two fibrous connective tissue layers

• Forms dural septa that limit excessive movement of the brain

Arachnoid Mater

• Loose brain covering; separated from dura mater by subdural space

• Subarachnoid space contains CSF and large blood vessels

Pia Mater

• Delicate connective tissue; clings tightly to the brain

Cerebrospinal Fluid (CSF)

• Watery solution similar to blood plasma

• Contains less protein and different ion concentrations than plasma

• Forms a liquid cushion for the CNS

• Prevents the brain from crushing under its own weight

• Protects the CNS from blows and other trauma

• Nourishes the brain and carries chemical signals

Choroid Plexuses

• Form tissue fluid filters; hang from the roof of each ventricle

• Help cleanse CSF by removing wastes

Blood-Brain Barrier

• Helps maintain a stable environment for the brain

• Separates neurons from bloodborne substances

• Allows nutrients to pass freely but restricts passage of harmful substances

• Formed by tight junctions of endothelial cells and astrocytes

Spinal Cord

Structure and Function

• Extends from the foramen magnum to L1 or L2 vertebra

• Provides two-way communication to and from the brain

• Protected by bone, meninges, and CSF

• Anchored to the coccyx by the filum terminale

• Gray matter consists of neuron cell bodies, unmyelinated processes, and neuroglia

• White matter consists of myelinated and unmyelinated nerve fibers

Cross-Sectional Anatomy of the Spinal Cord

• Divided into anterior (ventral), posterior (dorsal), and lateral funiculi

• Posterior median sulcus divides posterior funiculi

Gray Matter and Spinal Roots

• Gray matter consists of soma, unmyelinated processes, and neuroglia

• Central canal connects areas of gray matter

• Posterior (dorsal) horns: interneurons

• Lateral horns: contain sympathetic nerve fibers

Gray Matter Organization

• Four zones: somatic sensory (SS), visceral sensory (VS), visceral motor (VM), somatic motor (SM)

Example: The blood-brain barrier prevents toxins and pathogens from entering the brain, protecting neural tissue from damage.

Additional info: The notes above are expanded and clarified for completeness and academic context, suitable for college-level Anatomy & Physiology students.