Organization and Function of the Central Nervous System

Posted on Aug 23, 2024 in Biology

Central Nervous System (CNS)

Brain and Spinal Cord

Cephalization

  • Evolutionary development of the anterior CNS
  • Increased number of neurons in the head
  • Highest level reached in the human brain

Embryonic Development

  • Brain and spinal cord begin as a neural tube
  • Three primary vesicles form at the anterior end:
    • Prosencephalon (forebrain)
    • Mesencephalon (midbrain)
    • Rhombencephalon (hindbrain)
  • Posterior end becomes the spinal cord

Embryonic Development: Five Secondary Brain Vesicles

  • Forebrain (prosencephalon) divides into:
    • Telencephalon
    • Diencephalon
  • Midbrain (mesencephalon) remains undivided
  • Hindbrain (rhombencephalon) divides into:
    • Metencephalon
    • Myelencephalon

Embryonic Development: Differentiation of Brain Regions

  • Telencephalon: Cerebral hemispheres
  • Diencephalon: Epithalamus, thalamus, hypothalamus, retina
  • Mesencephalon: Midbrain
  • Metencephalon: Pons and cerebellum
  • Myelencephalon: Medulla oblongata
  • Central cavity of the neural tube: Ventricles

Embryonic Development: Brain Folding

  • Brain grows faster than the membranous skull
  • Folds to occupy available space
  • Forebrain moves toward the brain stem (midbrain, pons, medulla oblongata)
  • Cerebral hemispheres double back and envelop the diencephalon and midbrain while creasing and folding to increase surface area

Regions and Organization

Adult Brain Regions

  • Cerebral hemispheres
  • Diencephalon
  • Brain stem (midbrain, pons, and medulla)
  • Cerebellum

Spinal Cord

  • Central cavity surrounded by gray matter
  • External white matter composed of myelinated fiber tracts

Brain

  • Similar pattern to the spinal cord with additional areas of gray matter
  • Cerebral hemispheres and cerebellum:
    • Outer gray matter called the cortex
  • Cortex disappears in the brain stem
  • Scattered gray matter nuclei amid white matter

Ventricles of the Brain

  • Filled with cerebrospinal fluid (CSF)
  • Lined by ependymal cells
  • Connected to one another and to the central canal of the spinal cord
  • Lateral ventricles connected to the third ventricle via the interventricular foramen
  • Third ventricle connected to the fourth ventricle via the cerebral aqueduct
  • Third ventricle located in the diencephalon
  • Fourth ventricle located in the hindbrain

Cerebral Hemispheres

Surface Markings

  • Ridges (gyri)
  • Shallow grooves (sulci)
  • Deep grooves (fissures)
  • Longitudinal fissure: Separates the two hemispheres
  • Transverse cerebral fissure: Separates the cerebrum and cerebellum

Lobes

  • Frontal
  • Parietal
  • Temporal
  • Occipital
  • Insula

Important Sulci and Gyri

  • Central sulcus: Separates the precentral gyrus of the frontal lobe and the postcentral gyrus of the parietal lobe
  • Parieto-occipital sulcus: Separates the occipital and parietal lobes
  • Lateral sulcus: Outlines the temporal lobes

Basic Regions

  • Cerebral cortex (gray matter) superficially
  • White matter internally
  • Basal nuclei deep within the white matter

Cerebral Cortex

  • Thin (2–4 mm) superficial layer of gray matter
  • 40% of the brain’s mass
  • Site of conscious mind: awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding

General Considerations

  • Three types of functional areas:
    • Motor areas: Control voluntary movement
    • Sensory areas: Conscious awareness of sensation
    • Association areas: Integrate diverse information
  • Each hemisphere is concerned with the contralateral side of the body
  • Lateralization of cortical function in the hemispheres
  • Conscious behavior involves the entire cortex in some way

Motor Areas of the Cerebral Cortex

  • Located in the frontal lobe; control voluntary movement
  • Primary (somatic) motor cortex in the precentral gyrus
  • Premotor cortex anterior to the precentral gyrus
  • Broca’s area anterior to the inferior premotor area
  • Frontal eye field within and anterior to the premotor cortex; superior to Broca’s area

Primary Motor Cortex

  • Large pyramidal cells of the precentral gyri
  • Long axons form the pyramidal (corticospinal) tracts of the spinal cord
  • Allows conscious control of precise, skilled, skeletal muscle movements
  • Motor homunculus: Upside-down caricatures represent contralateral motor innervation of body regions

Premotor Cortex

  • Helps plan movements; staging area for skilled motor activities
  • Controls learned, repetitious, or patterned motor skills
  • Coordinates simultaneous or sequential actions
  • Controls voluntary actions that depend on sensory feedback

Broca’s Area

  • Present in one hemisphere (usually the left)
  • Motor speech area that directs muscle activities associated with speech production

Frontal Eye Field

  • Controls voluntary eye movements

Sensory Areas of the Cerebral Cortex

  • Primary somatosensory cortex
  • Somatosensory association cortex
  • Visual areas
  • Auditory areas
  • Vestibular cortex
  • Olfactory cortex
  • Gustatory cortex
  • Visceral sensory area

Primary Somatosensory Cortex

  • In the postcentral gyri of the parietal lobe
  • Receives general sensory information from the skin, and proprioceptors of skeletal muscle, joints, and tendons
  • Capable of spatial discrimination: Identification of the body region being stimulated
  • Somatosensory homunculus: Upside-down caricatures represent contralateral sensory input from body regions

Somatosensory Association Cortex

  • Posterior to the primary somatosensory cortex
  • Integrates sensory input from the primary somatosensory cortex for understanding of an object
  • Determines size, texture, and relationship of parts of objects being felt

Visual Areas

  • Primary visual (striate) cortex:
    • Extreme posterior tip of the occipital lobe
    • Receives visual information from the retinas
  • Visual association area:
    • Surrounds the primary visual cortex
    • Uses past visual experiences to interpret visual stimuli (e.g., color, form, and movement)
    • Example: Ability to recognize faces
    • Complex processing involves the entire posterior half of the cerebral hemispheres

Auditory Areas

  • Primary auditory cortex:
    • Superior margin of the temporal lobes
    • Interprets information from the inner ear as pitch, loudness, and location
  • Auditory association area:
    • Located posterior to the primary auditory cortex
    • Stores memories of sounds and permits perception of a sound stimulus

Vestibular Cortex

  • Posterior part of the insula and adjacent parietal cortex
  • Responsible for conscious awareness of balance (position of the head in space)

Olfactory Cortex

  • Primary olfactory (smell) cortex:
    • Medial aspect of the temporal lobes (in the piriform lobes)
    • Region of conscious awareness of odors

Gustatory Cortex

  • In the insula just deep to the temporal lobe
  • Involved in the perception of taste

Visceral Sensory Area

  • Posterior to the gustatory cortex
  • Conscious perception of visceral sensations, e.g., upset stomach or full bladder

Multimodal Association Areas

  • Receive inputs from multiple sensory areas
  • Send outputs to multiple areas, including the premotor cortex
  • Allows meaning to be assigned to information received, store it in memory, tie it to previous experience, and decide on actions
  • Sensations, thoughts, and emotions become conscious – makes us who we are

Three Broad Multimodal Association Areas

  • Anterior association area (prefrontal cortex)
  • Posterior association area
  • Limbic association area

Anterior Association Area (Prefrontal Cortex)

  • Most complicated cortical region
  • Involved with intellect, cognition, recall, and personality
  • Contains working memory needed for abstract ideas, judgment, reasoning, persistence, and planning
  • Development depends on feedback from the social environment

Posterior Association Area

  • Large region in the temporal, parietal, and occipital lobes
  • Plays a role in recognizing patterns and faces and localizing us in space
  • Involved in understanding written and spoken language (Wernicke’s area)

Limbic Association Area

  • Part of the limbic system
  • Involves the cingulate gyrus, parahippocampal gyrus, and hippocampus
  • Provides emotional impact that makes a scene important and helps establish memories

Lateralization of Cortical Function

  • Hemispheres are almost identical
  • Lateralization: Division of labor between hemispheres
  • Cerebral dominance: Hemisphere dominant for language (left hemisphere in 90% of people)

Left Hemisphere

  • Controls language, math, and logic

Right Hemisphere

  • Controls visual-spatial skills, intuition, emotion, and artistic and musical skills
  • Hemispheres communicate almost instantaneously via fiber tracts and functional integration

Functions of Basal Nuclei

  • Influence muscle movements
  • Play a role in cognition and emotion
  • Regulate the intensity of slow or stereotyped movements
  • Filter out incorrect/inappropriate responses
  • Inhibit antagonistic/unnecessary movements

Diencephalon

  • Three paired structures:
    • Thalamus
    • Hypothalamus
    • Epithalamus
  • Encloses the third ventricle

Thalamus

  • 80% of the diencephalon
  • Superolateral walls of the third ventricle
  • Bilateral nuclei connected by the interthalamic adhesion (intermediate mass)
  • Contains several nuclei, named for their location
  • Nuclei project and receive fibers from the cerebral cortex

Thalamic Function

  • Gateway to the cerebral cortex
  • Sorts, edits, and relays ascending input
  • Relays impulses from the hypothalamus for regulation of emotion and visceral function
  • Relays impulses from the cerebellum and basal nuclei to help direct the motor cortices
  • Relays impulses for memory or sensory integration
  • Mediates sensation, motor activities, cortical arousal, learning, and memory

Hypothalamus

  • Forms the inferolateral walls of the third ventricle
  • Contains many nuclei, including:
    • Mammillary bodies: Paired anterior nuclei; olfactory relay stations
  • Infundibulum: Stalk that connects to the pituitary gland

Hypothalamic Functions

  • Controls the autonomic nervous system (e.g., blood pressure, rate and force of heartbeat, digestive tract motility, pupil size)
  • Controls physical responses to emotions (limbic system)
  • Involved in the perception of pleasure, fear, and rage, and in biological rhythms and drives
  • Regulates body temperature (sweating/shivering)
  • Regulates hunger and satiety in response to nutrient blood levels or hormones
  • Regulates water balance and thirst
  • Regulates sleep-wake cycles via the suprachiasmatic nucleus (biological clock)
  • Controls the endocrine system:
    • Controls secretions of the anterior pituitary gland
    • Produces posterior pituitary hormones

Epithalamus

  • Most dorsal portion of the diencephalon; forms the roof of the third ventricle
  • Contains the pineal gland, which extends from the posterior border and secretes melatonin
  • Melatonin helps regulate the sleep-wake cycle

Brain Stem

  • Three regions:
    • Midbrain
    • Pons
    • Medulla oblongata
  • Similar structure to the spinal cord but contains embedded nuclei within white matter
  • Controls automatic behaviors necessary for survival
  • Contains fiber tracts connecting higher and lower neural centers
  • Nuclei associated with 10 of the 12 pairs of cranial nerves

Midbrain

  • Located between the diencephalon and the pons
  • Contains the cerebral aqueduct, a channel connecting the third and fourth ventricles

Pons

  • Fourth ventricle separates the pons and cerebellum
  • Fibers of the pons:
    • Connect higher brain centers and the spinal cord
    • Relay impulses between the motor cortex and the cerebellum
  • Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)
  • Contains some nuclei of the reticular formation, which help maintain the normal rhythm of breathing

Medulla Oblongata

  • Joins the spinal cord at the foramen magnum
  • Forms part of the ventral wall of the fourth ventricle
  • Contains the inferior olivary nuclei, which relay sensory information from muscles and joints to the cerebellum
  • Cranial nerves VIII, IX, X, and XII are associated with the medulla
  • Contains vestibular nuclei (also found in the pons), which mediate responses that maintain equilibrium
  • Several nuclei (e.g., nucleus cuneatus and nucleus gracilis) relay sensory information

Medulla Oblongata: Functions

  • Autonomic reflex center
  • Functions overlap with the hypothalamus, which relays instructions via the medulla
  • Contains the cardiovascular center:
    • Cardiac center: Adjusts the force and rate of heart contraction
    • Vasomotor center: Adjusts blood vessel diameter for blood pressure regulation
  • Contains respiratory centers:
    • Generate respiratory rhythm
    • Control the rate and depth of breathing (with pontine centers)
  • Additional centers regulate vomiting, hiccupping, swallowing, coughing, and sneezing

Cerebellum

  • 11% of brain mass
  • Dorsal to the pons and medulla
  • Receives input from the cortex, brain stem, and sensory receptors
  • Allows smooth, coordinated movements

Cerebellar Processing of Motor Activity

  • Cerebellum receives impulses from the cerebral cortex of the intent to initiate voluntary muscle contraction
  • Signals from proprioceptors and visual and equilibrium pathways continuously “inform” the cerebellum of the body’s position and momentum
  • Cerebellar cortex calculates the best way to smoothly coordinate muscle contraction
  • “Blueprint” of coordinated movement is sent to the cerebral motor cortex and brain stem nuclei

Cognitive Function of the Cerebellum

  • Plays a role in thinking, language, and emotion
  • May compare actual with expected output and adjust accordingly

Protection of the Brain

  • Bone (skull)
  • Membranes (meninges)
  • Watery cushion (cerebrospinal fluid)
  • Blood-brain barrier

Meninges

  • Cover and protect the CNS
  • Protect blood vessels and enclose venous sinuses
  • Contain cerebrospinal fluid (CSF)
  • Form partitions in the skull
  • Three layers:
    • Dura mater
    • Arachnoid mater
    • Pia mater

Meningitis

Inflammation of meningesCerebrospinal Fluid (CSF)CompositionWatery solution formed from blood plasma Less protein and different ion concentrations than plasmaConstant volumeCerebrospinal Fluid (CSF)FunctionsGives buoyancy to CNS structuresReduces weight by 97%Protects CNS from blows and other traumaNourishes brain and carries chemical signals Blood Brain BarrierHelps maintain stable environment for brain Separates neurons from some bloodborne substancesBlood Brain BarrierCompositionContinuous endothelium of capillary wallsThick basal lamina around capillariesFeet of astrocytesProvide signal to endothelium for formation of tight junctionsBlood Brain Barrier: FunctionsSelective barrierAllows nutrients to move by facilitated diffusionMetabolic wastes, proteins, toxins, most drugs, small nonessential amino acids, K+ deniedAllows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics Absent in some areas, e.g., vomiting center and hypothalamus, where necessary to monitor chemical composition of blood