Chapter 12: The Central Nervous System

Overview of the Central Nervous System (CNS)

The CNS, composed of the brain and spinal cord, is responsible for integrating sensory information, coordinating movement, and maintaining homeostasis. It works closely with the peripheral nervous system (PNS) to perform sensory, integrative, and motor functions.

• Sensory and motor functions are performed by the PNS, while integrative functions are carried out by the CNS.

• The brain is divided into four main regions: cerebrum, diencephalon, cerebellum, and brainstem.

• The spinal cord serves as a relay and processing station, connecting the brain to the rest of the body.

• White matter consists of myelinated axons; gray matter contains neuron cell bodies, dendrites, and unmyelinated axons.

Development of the CNS

The CNS develops from the neural tube, which forms primary and secondary brain vesicles that give rise to the mature brain's divisions.

• Primary vesicles: forebrain, midbrain, hindbrain

• Secondary vesicles: telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon

• The ventricles of the brain and the central canal of the spinal cord arise from the neural tube cavity.

The Brain

Cerebrum

The cerebrum is the largest part of the brain, responsible for higher mental functions such as learning, memory, personality, and voluntary movement.

• Divided into right and left cerebral hemispheres, separated by the longitudinal fissure.

• Surface features include gyri (ridges) and sulci (shallow grooves), which increase surface area.

• Divided into five lobes: frontal, parietal, temporal, occipital, and insula.

Cerebral Cortex (Gray Matter)

• Neocortex: Six-layered structure responsible for conscious processes (e.g., planning, interpreting sensory input).

• Three main functional areas:

  • Primary motor cortex: Plans and executes movement (precentral gyrus, frontal lobe).

  • Primary sensory cortices: Receive and process sensory input (e.g., S1 in postcentral gyrus).

  • Association areas: Integrate different types of stimuli.

• Motor cortices: Upper motor neurons in the frontal lobe control voluntary movement; lower motor neurons in the PNS execute movement.

• Sensory cortices: Include somatosensory, visual, auditory, gustatory, vestibular, and olfactory areas.

• Language areas: Broca's area (speech production), Wernicke's area (language comprehension).

• Prefrontal cortex: Involved in behavior, personality, learning, memory, and psychological state.

Basal Nuclei (Gray Matter)

• Clusters of neuron cell bodies deep within the hemispheres: caudate nucleus, putamen, globus pallidus.

• Regulate voluntary movement and inhibit involuntary movement.

• Disorders can affect movement, behavior, and cognition.

Cerebral White Matter

• Commissural fibers: Connect right and left hemispheres (e.g., corpus callosum).

• Projection fibers: Connect cortex with other brain areas and spinal cord (e.g., corona radiata, internal capsule).

• Association fibers: Connect gyri within a single hemisphere.

Limbic System

• Includes the limbic lobe, hippocampus (memory), amygdala (emotion), and fornix (white matter tract).

• Functions in memory, learning, emotion, and behavior.

Diencephalon

The diencephalon is the central core of the brain, consisting of the thalamus, hypothalamus, epithalamus, and subthalamus.

• Thalamus: Main relay station for sensory information to the cortex; regulates cortical activity.

• Hypothalamus: Maintains homeostasis, controls the autonomic nervous system (ANS), regulates endocrine function via the pituitary gland, and influences behavior and memory.

• Epithalamus: Contains the pineal gland (secretes melatonin).

• Subthalamus: Works with basal nuclei to control movement.

Cerebellum

The cerebellum coordinates ongoing movement and reduces motor error. It consists of two hemispheres connected by the vermis and is divided into anterior, posterior, and flocculonodular lobes.

• Surface features: folia (ridges), arbor vitae (tree-like white matter).

• Connected to the brainstem by cerebellar peduncles.

Brainstem

The brainstem connects the brain and spinal cord and controls vital functions such as heart rate, breathing, and reflexes. It consists of the midbrain, pons, and medulla oblongata.

• Midbrain: Contains nuclei for movement, visual and auditory reflexes (e.g., superior and inferior colliculi), and the substantia nigra.

• Pons: Regulates movement, breathing, sleep, and arousal.

• Medulla oblongata: Contains tracts for motor control (corticospinal), sensory nuclei, and centers for cardiovascular and respiratory regulation.

• Reticular formation: Network of nuclei involved in arousal, sleep-wake cycle, pain, and autonomic functions.

Homeostasis and Higher Mental Functions

Homeostasis

• Maintained by the nervous and endocrine systems.

• Hypothalamus: Boss of the ANS, regulates body temperature, feeding, and circadian rhythms (via the suprachiasmatic nucleus).

• Reticular formation: Regulates vital functions (heart rate, blood pressure, ventilation).

• Sleep: Controlled by the hypothalamus and reticular formation; involves circadian rhythms and stages of sleep (NREM and REM).

Higher Mental Functions

• Cognition: Involves processing complex stimuli, planning, and personality (prefrontal cortex).

• Language: Broca's area (production), Wernicke's area (comprehension); both hemispheres contribute to language function.

• Memory: Declarative (facts) and nondeclarative (skills); involves hippocampus, cortex, cerebellum, and basal nuclei.

• Emotion: Involves hypothalamus, amygdala, and limbic system; integrates visceral, somatic, and subjective responses.

Protection of the Brain and Spinal Cord

Cranial Meninges

• Three layers: dura mater (outer, tough), arachnoid mater (middle, elastic), pia mater (inner, delicate).

• Spaces: epidural (potential), subdural (narrow), subarachnoid (contains CSF).

• Dural folds and sinuses (e.g., falx cerebri, tentorium cerebelli) help partition and drain the brain.

Cerebrospinal Fluid (CSF) and Ventricles

• Four ventricles: right and left lateral, third, and fourth ventricles.

• CSF is produced by choroid plexuses, circulates through ventricles and subarachnoid space, and is reabsorbed via arachnoid granulations.

• Functions: cushions brain, maintains temperature, removes waste, provides buoyancy.

• Disorders: hydrocephalus (excess CSF, increased intracranial pressure).

Blood-Brain Barrier

• Formed by tight junctions between endothelial cells, basal lamina, and astrocytes.

• Regulates passage of substances; permeable to water, gases, and lipid-soluble molecules; restricts large/polar molecules.

• Protects brain from toxins and pathogens, but can limit drug delivery.

The Spinal Cord

Structure and Protection

• Extends from foramen magnum to L1-L2 vertebrae; ends as conus medullaris.

• Protected by vertebrae and three meninges (dura, arachnoid, pia).

• Spaces: epidural (contains fat and veins), subdural (potential), subarachnoid (CSF, site for lumbar puncture).

• Denticulate ligaments anchor the cord; filum terminale anchors to coccyx.

• Cauda equina: Bundle of nerve roots below the conus medullaris.

Internal Anatomy

• Gray matter: Butterfly-shaped; anterior horn (somatic motor), posterior horn (sensory), lateral horn (visceral motor, T1-L2).

• White matter: Organized into funiculi (posterior, lateral, anterior), containing ascending (sensory) and descending (motor) tracts.

• Gray commissure: Connects the two sides, surrounds the central canal.

Spinal Tracts

Sensation and Movement

Sensory Pathways

• Stimuli detected by PNS, relayed to CNS for integration and perception.

• General senses: Touch, pain, temperature, proprioception (body position).

• Special senses: Vision, hearing, taste, smell, equilibrium.

• Ascending pathways involve first-order (PNS), second-order (spinal cord/brainstem), and third-order (thalamus) neurons.

• Somatotopy: Specific body regions mapped to specific cortical areas (sensory homunculus).

Motor Pathways

• Voluntary movement planned and executed by upper motor neurons (cortex), local interneurons, and lower motor neurons (PNS).

• Corticospinal tracts: Control muscles below head/neck; decussate in medulla.

• Corticonuclear tracts: Control head/neck muscles via cranial nerves.

• Basal nuclei and substantia nigra: Inhibit inappropriate movements, initiate motion.

• Cerebellum: Modifies upper motor neuron activity to reduce motor error (motor learning).

Summary Table: Major Brain Divisions and Functions

Key Terms and Definitions

• Gyri: Elevated ridges on the brain surface.

• Sulci: Shallow grooves between gyri.

• Fissures: Deep grooves separating brain regions.

• Ventricles: Cavities in the brain filled with CSF.

• Choroid plexus: Structure producing CSF.

• Somatotopy: Mapping of body regions to specific brain areas.

• Homunculus: Visual representation of somatotopy.

• Decussation: Crossing over of nerve fibers (e.g., corticospinal tracts in medulla).

Additional info:

• Some details on neurotransmitters, specific nuclei, and clinical disorders (e.g., hydrocephalus, aphasia) were expanded for clarity.

• Tables were inferred and constructed to summarize tract functions and brain divisions.