The Central Nervous System: Structure and Function

The 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 controls vital functions such as heart rate, respiratory rate, blood pressure, and is the center for intelligence, memory, emotion, and behavior.

• Rostral: Toward the nose (anterior in the brain)

• Caudal: Toward the tail (posterior in the brain)

Basic Parts and Organization of the Brain

Major Regions

The brain is divided into four main regions: the brain stem, cerebellum, diencephalon, and cerebrum. Each region has specialized functions and structures.

• Brain Stem: Includes the midbrain, pons, and medulla oblongata

• Cerebellum: Coordinates movement and balance

• Diencephalon: Contains the thalamus, hypothalamus, and epithalamus

• Cerebrum: Largest part, responsible for higher brain functions

Brain Functions

• Controls autonomic nervous system and endocrine system

• Innervates the head via cranial nerves

• Performs complex neural functions: intelligence, consciousness, memory, sensory-motor integration, emotion, behavior, and socialization

Ventricles of the Brain

Structure and Function

The brain contains four interconnected ventricles filled with cerebrospinal fluid (CSF). These ventricles are lined with ependymal cells and are continuous with the central canal of the spinal cord.

• Lateral Ventricles: Located in the cerebral hemispheres; horseshoe-shaped

• Third Ventricle: Located in the diencephalon; connected to lateral ventricles by the interventricular foramen

• Cerebral Aqueduct: Connects the third and fourth ventricles

• Fourth Ventricle: Located in the hindbrain; connects to the central canal of the spinal cord

Distribution of Gray and White Matter

Organization in the CNS

Gray matter consists of neuronal cell bodies, while white matter is composed of axons. In the brain, gray matter forms the cortex and deep nuclei, while white matter lies beneath the cortex. In the spinal cord, gray matter is central and white matter is external.

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

• White Matter: Contains myelinated axons for communication

The Brain Stem

Structure and Function

The brain stem includes the midbrain, pons, and medulla oblongata. It acts as a passageway for all fiber tracts between the cerebrum and spinal cord and is involved in the innervation of the face and head. Ten of the twelve cranial nerves attach to the brain stem.

• Produces automatic behaviors necessary for survival

• Integrates auditory and visual reflexes

Medulla Oblongata

• Continuous with the spinal cord

• Contains pyramids (motor tracts) and inferior olivary nucleus

• Four cranial nerves attach here: VIII, IX, X, XII

• Contains centers for cardiac, vasomotor, and respiratory control

Pons

• Bridge between midbrain and medulla oblongata

• Contains nuclei for cranial nerves V, VI, VII

• Connects cerebral cortex and cerebellum via middle cerebellar peduncles

Midbrain

• Located between diencephalon and pons

• Contains cerebral aqueduct and peduncles

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

The Cerebellum

Structure and Function

The cerebellum is located dorsal to the pons and medulla. It coordinates body movements, maintains equilibrium, and is involved in learning motor skills and cognitive functions such as language and problem-solving.

• Two hemispheres, each with anterior, posterior, and flocculonodular lobes

• Surface folded into folia

• Composed of cortex (gray matter), arbor vitae (white matter), and deep nuclei

The Diencephalon

Major Components

The diencephalon forms the central core of the forebrain and is surrounded by the cerebral hemispheres. It consists of the thalamus, hypothalamus, and epithalamus, and borders the third ventricle.

• Thalamus: Relay station for sensory information to the cerebral cortex

• Hypothalamus: Main visceral control center; regulates ANS, emotions, temperature, hunger, thirst, sleep-wake cycles, and endocrine system

• Epithalamus: Contains pineal gland, which secretes melatonin and regulates circadian rhythms

The Cerebrum

Structure

The cerebrum accounts for 83% of brain mass and is divided into lobes by deep fissures and sulci. The main lobes are frontal, parietal, occipital, temporal, and insula (deep within the lateral sulcus).

• Gyri: Twisted ridges between sulci

• Sulci: Grooves on the surface

• Fissures: Deep grooves separating major regions

Protection of the Brain

Protective Structures

The brain is protected by the skull, meninges, cerebrospinal fluid, and the blood-brain barrier.

• Meninges: Three layers—dura mater (strongest, two layers), arachnoid mater (beneath dura, contains villi for CSF absorption), pia mater (delicate, clings to brain surface)

• Cerebrospinal Fluid (CSF): Cushions, nourishes, and removes waste from the brain and spinal cord; formed in choroid plexuses

• Blood-Brain Barrier: Prevents most toxins from entering the brain, but allows passage of essential nutrients and some substances like alcohol and nicotine

The Spinal Cord

Structure and Function

The spinal cord provides a two-way conduction pathway between the brain and body and is a major center for reflexes. It extends from the foramen magnum to the level of vertebra L1 or L2.

• Conus Medullaris: Inferior end of the spinal cord

• Filum Terminale: Connective tissue attaching to the coccyx

• Cervical and Lumbar Enlargements: Where nerves for limbs arise

• Cauda Equina: Collection of spinal nerve roots

Gray and White Matter

• Gray Matter: Central, shaped like an "H"; contains cell bodies and interneurons

• White Matter: External; contains ascending, descending, and commissural fibers

Disorders of the Central Nervous System

Spinal Cord Damage

• Paralysis: Loss of motor function

• Parasthesia: Loss of sensation

• Paraplegia: Paralysis of lower limbs (injury between T1 and L2)

• Quadriplegia: Paralysis of all four limbs (injury in cervical region)

Brain Dysfunction

• Cerebrovascular Accident (Stroke): Blockage or interruption of blood flow to a brain region

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

Congenital Malformations

• Anencephaly: Absence of cerebrum and cerebellum

• Spina Bifida: Absence of vertebral lamina

• Cerebral Palsy: Poor control of voluntary muscles due to cerebral cortex damage