Cells and Organization of the Nervous System

Cells of the Central/Peripheral Nervous System

• Organization of the Central Nervous System (CNS) and Peripheral Nervous System (PNS)

  • CNS: Brain and spinal cord
  • PNS: Cranial/spinal nerves and receptors
  • Somatic nervous system: Sensory neurons (skin, muscles, joints)
  • Autonomic nervous system: Sympathetic, parasympathetic, enteric (involuntary organ control)

Neurons: Basic Functional Units

• Derived from neurons and neuroglial cells

• Structure:

  • Cell body (perikaryon)
  • Dendrites (anterograde impulse conduction)
  • Axon (conducts impulse away from cell body; may branch distally)

• Neuron types by shape:

  • Unipolar (in vertebrates only)
  • Bipolar (retina, ear, olfactory mucosa)
  • Pseudounipolar (dorsal root ganglia, cranial ganglia → sensory bypass cell body)
  • Multipolar (majority of CNS neurons → multiple dendrites, one axon)

Gray Matter vs. White Matter

• Gray Matter: Neuron cell bodies (CNS)
• White Matter: Myelinated axons
• Peripheral ganglia (cranial, spinal, autonomic)

Functional Classification of Neurons

• Motor neurons (multipolar) → innervate muscles/glands
• Sensory neurons (pseudounipolar) → receive various sensory inputs
• Interneurons → connect adjacent neurons

Neuron Membrane and Organelles

• Bilaminar lipoprotein membrane (phospholipids → hydrophobic inside, hydrophilic outside)
• Integral proteins → ionic pumps, ion channels, enzymes, receptors
• Organelles: Nucleus, mitochondria, ribosomes, ER, lysosomes, Golgi
• Neurotubules/neurofilaments → support and transport

Neuroglial (Glial) Cells

• Glial cells > neurons in number

• No axons/dendrites → do not conduct impulses

• Functions:

  • Maintain ionic environment
  • Modulate conduction
  • Regulate neurotransmitter reuptake
  • Repair after injury

Types of Glial Cells

• Astrocytes

  • Most abundant glial cell
  • Support, metabolic regulation, repair
  • Fibrous (white matter) and protoplasmic (gray matter)
  • Perivascular feet → contribute to blood-brain barrier

• Oligodendrocytes

  • Fewer branches → myelinate CNS axons (can myelinate multiple axons)
  • Cannot regenerate after injury

• Schwann Cells (PNS)

  • Myelinate only one axon
  • Form Node of Ranvier → saltatory conduction → faster impulse transmission
  • Wallerian degeneration → Schwann cells guide axonal regeneration

• Microglial Cells

  • Smallest glia → CNS macrophages
  • Activated at injury sites → phagocytosis

• Ependymal Cells

  • Line ventricles and spinal canal
  • Form choroid plexus → secrete CSF

  Functions of Glial Cells

  • Astrocytes: Support, metabolic and nutritive functions
  • Ependymal cells: Probable role in cerebrospinal fluid production
  • Microglia: Phagocytosis
  • Oligodendrocytes: Insulation (form myelin sheath in the brain and spinal cord)
  • Schwann cells: Insulation (form myelin sheath in the peripheral nerves)

Blood-Brain Barrier (BBB)

• Overview and Function

  • Discovered after IV dye stained all tissues except brain and spinal cord
  • Isolates CNS extracellular compartment from blood (intravascular space)
  • Vital for CNS homeostasis and neuronal function

• Anatomy and Structure

  • Formed by:

    • Microvascular endothelial cells (tight junctions, no fenestrae, low pinocytosis)
    • Pericytes (mural cells)
    • Capillary basement membrane
    • Neuroglial membrane
    • Glial podocytes (astrocyte projections)

  • Highly selective barrier → limits water-soluble molecule passage

• Circumventricular Organs (No BBB)

  • Allow neurosecretory exchange with blood

  • Include:

    • Area postrema
    • Posterior pituitary gland
    • Pineal gland
    • Choroid plexus
    • Parts of hypothalamus

• BBB Development and Disruption

  • Incomplete in newborns → risk of kernicterus (bilirubin crosses BBB → basal ganglia damage)

  • Disruption causes:

    • Traumatic head injury
    • Subarachnoid/intracerebral hemorrhage
    • Cerebral ischemia
    • Disease states (e.g., multiple sclerosis, stroke, tumors)

• Clinical Applications

  • Disruption technique → hyperosmolar intracarotid injection
  • Shrinks endothelial cells → opens tight junctions
  • Allows chemotherapeutic drug delivery for brain malignancies

Cerebral Structures

• Cerebral Hemispheres Overview

  • Largest and most developed brain regions

  • Structures include:

    • Cerebral cortex
    • Hippocampal formation
    • Amygdala
    • Basal ganglia

  • Cerebral cortex → outer 3-mm layer → convoluted (gyri & sulci)

  • Fissures/sulci divide hemispheres/lobes:

    • Medial longitudinal fissure → R/L hemispheres
    • Lateral fissure (Sylvius) and central sulcus (Rolando) → define lobes

• Cerebral Cortex and Lobes

  • Frontal lobe → motor control (precentral gyrus → Brodmann area 4)
  • Parietal lobe → pain, touch, sensory association (postcentral gyrus → Brodmann areas 1, 2, 3, 5, 7)
  • Temporal lobe → auditory cortex (separated by Sylvian fissure)
  • Occipital lobe → visual cortex (calcarine fissure)
  • Corpus callosum → commissural fibers → interconnect hemispheres

• Other Cerebral Structures

  • Basal ganglia → control of movement
  • Amygdala → emotional regulation, pain/appetite response, stress response
  • Hippocampal formation → memory and learning

• Diencephalon

  • Located midline between cerebral hemispheres

  • Includes:

    • Thalamus → sensory relay to cortex

    • Hypothalamus → neurohumoral control (includes mammillary bodies)

    • Epithalamus and subthalamus

    • Thalamus: Acts as a relay station that directs information to various cortical structures

    • Hypothalamus: Primary neurohumoral organ

• Brainstem

  • Composed of:

    • Midbrain, pons, and medulla

  • Reticular activating system → maintains consciousness/arousal

  • Pons → connects medulla and midbrain → contains CN V & VII nuclei

  • Medulla → continues as spinal cord → contains vital centers:

    • Respiratory and cardiovascular control
    • Nuclei of CN VIII, IX, X, XI, XII

• Cerebellum

  • Located below occipital lobe, posterior to pons and medulla

  • Similar structure to cerebral cortex → outer gray matter, inner white matter

  • Divided into 3 functional regions:

    • Flocculonodular lobe (archeocerebellum) → maintains equilibrium
    • Paleocerebellum (anterior lobe, vermis) → regulates muscle tone
    • Neocerebellum (posterior lobe, vermis) → coordinates voluntary movement

  • Integrates CNS and peripheral input → modulates tone, equilibrium, and voluntary movement

Meninges

• Overview

  • Brain and spinal cord are protected by three layers:

    • Dura mater (outermost and thickest)
    • Arachnoid mater (middle, thin, avascular)
    • Pia mater (innermost, thin, adherent)

• Dura Mater

  • Thickest layer → overlies hemispheres and brainstem

  • Two layers:

    • Periosteal layer → adherent to inner cranium
    • Meningeal layer → continuous with spinal dura and nerve perineurium

  • Forms folds:

    • Falx cerebri → separates cerebral hemispheres
    • Tentorium cerebelli → separates occipital lobe from cerebellum

  • Innervation: 1st three cervical roots + trigeminal nerve → pain during awake craniotomy ("behind the eye")

• Arachnoid Mater

  • Thin, avascular, joined to dura mater

  • Subdural space (potential space between dura and arachnoid)

  • Clinical importance →

    • Subdural injection during spinal anesthesia → patchy/asymmetric block
    • Subdural hematoma → bleeding requires surgical intervention

• Pia Mater

  • Thin, avascular, adheres directly to brain and spinal cord

  • Subarachnoid space (between pia and arachnoid):

    • Extends to S2-S3 in spinal cord
    • Contains CSF and CNS vasculature
    • Vascular injury → subarachnoid hemorrhage (SAH) and hematoma

• Epidural Space

  • Located above dura → inside spinal canal

  • Contains:

    • Venous plexus
    • Epidural fat → cushions neural structures

  • Skin to epidural space → depth varies (3-8 cm)

Cerebral Spinal Fluid (CSF)

• Overview

  • Found in:

    • Ventricles of the brain
    • Cisterns surrounding the brain
    • Subarachnoid space (brain & spinal cord)

  • Adult total volume ≈ 150 mL

  • Specific gravity: 1.002-1.009

  • pH: ~7.32

  • Functions:

    • Cushions brain and spinal cord
    • Maintains extracellular environment for neurons/glia

• CSF Production

  • Secreted by ependymal cells of choroid plexus

  • Production rate: ~30 mL/hour

  • CSF is isotonic with plasma but not a plasma filtrate

  • Composition differences:

    • ↓ Potassium, calcium, bicarbonate, glucose (vs. plasma)
    • ↑ Sodium, chloride, magnesium (vs. plasma)

  • Entire CSF volume replaced every 3-4 hours

• CSF Flow Pathway

  • Lateral ventricles → foramen of Monro → third ventricle → aqueduct of Sylvius → fourth ventricle → foramen of Magendie (medial) and foramina of Luschka (lateral) → subarachnoid space

• Cisterns

  • Cisterna magna → between medulla and cerebellum → filled with CSF
  • Cisterna pontis and cisterna basalis → additional CSF-filled spaces

• CSF Absorption

  • Drains into venous blood via:

    • Superior sagittal sinus
    • Arachnoid granulations

Spinal Cord

• Overview

  • Extends from medulla at foramen magnum → filum terminale (coccyx)

  • 31 pairs of spinal nerves:

    • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

  • Cervical nerves exit between skull & C1; others exit between vertebrae

  • Cauda equina:

    • Long lumbar/sacral nerve roots → spinal cord ~25 cm shorter than vertebral canal in adults

• Spinal Cord Organization

  • Divided by dorsal (sensory) and ventral (motor) root fibers

  • Gray matter (central, H-shaped):

    • Neuron cell bodies & unmyelinated fibers

  • White matter (peripheral):

    • Fiber tracts → organized sensory (ascending) and motor (descending) pathways

• Gray Matter: Laminae and Horns

  • Divisions:

    • Ventral horns (columns) → motor neurons
    • Dorsal horns (columns) → sensory neurons
    • Intermediolateral horns (T1–L2) → autonomic neurons

  • Rexed Laminae I–X:

    • I–VI → dorsal horn → sensory processing
    • VII–IX → ventral horn → motor neurons/interneurons
    • X → central gray → interneurons

  • Enlargements:

    • C5–C7 → upper extremities
    • L3–S2 → lower extremities

• White Matter: Tracts

  • Dorsal white matter → ascending sensory tracts

  • Lateral/ventral white matter → descending motor tracts

  • Decussation (crossing over) occurs at various levels

  • Types of tracts:

    • Projection tracts → connect brain and spinal cord
    • Association tracts (fasciculi proprii) → intersegmental → important for reflexes

• Peripheral Nerve Structure

  • Meningeal coverings merge with nerve connective tissues:

    • Epineurium → outer covering
    • Perineurium → covers fascicles
    • Endoneurium → surrounds individual axons

• Peripheral Nerve Fiber Classification

  • Classified by diameter and conduction velocity:

    • A alpha fibers → largest → fastest conduction
    • A beta, A gamma, A delta fibers → medium → moderate conduction
    • B fibers → small → slower conduction
    • C fibers → smallest → slowest conduction

  • Myelination increases conduction velocity

Peripheral Nervous System

• Overview

  • Divided into:

    • Somatic nervous system → voluntary control (skin, muscles, joints)
    • Autonomic nervous system → involuntary control (viscera)

• Somatic Nervous System

  • Sensory neurons → receive input from skin, muscles, tendons, joints

  • Motor fibers:

    • Originate from ventral horn motor neurons
    • Exit spinal cord via ventral root

  • Mixed nerves:

    • Formed by motor and sensory fibers joining together
    • Separate again near target innervation site

• Cranial Nerves

  • Emerge directly from the cranium → innervate head and neck

  • Sensory cranial nerves:

    • Olfactory (I), Optic (II), Vestibulocochlear (VIII)

  • Motor cranial nerves:

    • Oculomotor (III), Trochlear (IV), Abducens (VI)
    • Spinal Accessory (XI), Hypoglossal (XII)

  • Mixed cranial nerves (sensory + motor):

    • Trigeminal (V), Facial (VII), Glossopharyngeal (IX), Vagus (X)

• Autonomic Nervous System (ANS)

  • Controls involuntary visceral functions

  • Divided into:

    • Sympathetic nervous system (SNS) → "fight or flight"
    • Parasympathetic nervous system (PNS) → "rest and digest"
    • Enteric nervous system → GI tract regulation

  • Functional antagonists: SNS vs PNS

  • Two-neuron pathway:

    • Preganglionic neuron → originates in CNS
    • Postganglionic neuron → terminates at effector organ (smooth muscle, cardiac muscle, glands)

  • Autonomic Fiber Origins

    • Brainstem → source of many autonomic fibers (e.g., cranial nerves)

    • Sacral spinal cord → PNS fibers to lower GI and genitourinary systems

    • Fiber Type and Location

      • General somatic afferent CN V, CN VII, CN IX, CN X, all spinal nerves: Information Conveyed: Pain, touch, temperature, pressure, and proprioception from muscles, tendons, and joint capsules Conscious pain sensations
      • General visceral afferent CN V, CN VII, CN IX, CN X, all spinal nerves
        Sight, hearing
      • Special somatic afferent CN II, CN VIII
        Olfaction, taste
      • Special visceral afferent CN I, CN IX, CN X, CN VII (intermediate branch)
        Mastication, facial expressions
        Voluntary muscles (trunk and extremities), extrinsic muscles of eye, muscles of the tongue
        Smooth muscle, cardiac muscle, some glands
      • Special visceral efferent CN V, CN VII, CN IX, CN X, CN XI
      • General somatic efferent CN III, CN IV, CN VI, CN VII, all spinal nerves
      • General visceral efferent CN III, CN VII, CN IX, CN X, spinal nerves T1 through L2 or L3, S2, S3, S4

Sympathetic Nervous System

• Overview

  • Preganglionic neurons:

    • Originate in intermediolateral gray horn (T1–L2/L3) → thoracolumbar origin
    • Exit via ventral nerve root → white rami communicantes

  • Paravertebral ganglia (sympathetic chain):

    • Paired and bilateral along spinal cord
    • Connected → form sympathetic trunks
    • May synapse at entry, ascend/descend, or pass through (forming splanchnic nerves)

• Synapse Pathways

  • Preganglionic fibers → synapse in ganglia or pass to prevertebral ganglia (celiac, superior/inferior mesenteric)

  • Postganglionic fibers:

    • Exit via gray rami communicantes → enter spinal nerve → target tissues

  • SNS Neurotransmitters:

    • Preganglionic → acetylcholine (ACh)
    • Postganglionic → catecholamines (norepinephrine)

  • Cervical Ganglia

    • Superior, medial, inferior cervical ganglia

    • Inferior + T1 ganglion = stellate ganglion

    • Superior cervical ganglion stimulation:

      • Mydriasis (pupil dilation)
      • Ciliary muscle relaxation
      • Vasoconstriction (head)

    • Damage → Horner Syndrome:

      • Miosis (pupil constriction)
      • Ptosis (drooping eyelid → incomplete due to CN III involvement)
      • Anhidrosis (no sweating)

• Thoracic SNS (Upper T1–T5)

  • Innervate heart and lungs:

    • \beta-receptor →↑ HR (chronotropy), ↑ conduction (dromotropy), ↑ contractility (inotropy)
    • \alpha-receptor → coronary vasoconstriction

  • Bronchial:

    • \beta2 → dilation
    • \alpha → mild constriction

• Abdominal and Pelvic SNS (T5–L3)

  • Form greater/lesser splanchnic nerves → prevertebral ganglia (e.g., mesenteric)

  • Postganglionic effects:

    • ↑ Liver glycogenolysis & gluconeogenesis
    • ↓ Pancreatic secretions (acinar + \beta-cells)
    • ↑ Lipolysis
    • ↓ GI tone & motility
    • GI sphincter contraction
    • Urinary smooth muscle relaxation
    • ↑ Renin secretion (kidney)

Parasympathetic Nervous System

• Overview

  • Craniosacral origin:

    • Cranial nerves III, VII, IX, X
    • Sacral spinal cord segments S2–S5

  • Efferent neurons:

    • Preganglionic neurons in midbrain, medulla, and sacral spinal cord

  • Neurotransmitter:

    • Acetylcholine (ACh) released by both pre- and postganglionic fibers

• Ganglia and Synapse Sites

  • Postganglionic neurons located:

    • Near or within target organs

  • Preganglionic axons travel:

    • With vagus nerve (CN X) to thoracic/abdominal organs
    • With pelvic nerves (nervi erigentes) from S3–S4 to pelvic organs

  • Target Organ Innervation

    • Vagus nerve → ~75% of total parasympathetic activity

    • Vagal branches innervate:

      • Bronchioles
      • Heart and coronary arteries
      • Stomach and large intestine (up to left colic flexure)

    • Sacral fibers (S3–S4):

      • Descending colon
      • Genitourinary organs

CNS Vasculature

• General Overview

  • CNS highly dependent on uninterrupted blood flow
  • Brain receives ~15% of cardiac output (≈50 mL/100g/min)
  • Supplies oxygen, glucose, and essential nutrients

• Arterial Circulation

  • Two main systems:

    • Anterior circulation → from internal carotid arteries
    • Posterior circulation → from vertebral arteries

  • Connected via Circle of Willis (arterial anastomoses)

  • Circle of Willis and Major Arteries

    • Gives rise to:

      • Anterior cerebral arteries → medial cerebral hemispheres
      • Middle cerebral arteries → lateral hemispheres
      • Striate arteries (branches) → supply internal capsule (motor tracts)
      • Posterior cerebral arteries → occipital and temporal lobes

    • Communicating arteries:

      • Anterior communicating artery → between anterior cerebral arteries
      • Posterior communicating artery → between middle and posterior cerebral arteries

• Posterior Circulation

  • Vertebral Arteries

    • Vertebral arteries (from subclavian) enter via foramen magnum

    • Unite to form basilar artery near pons

    • Supply:

      • Brainstem, cerebellum, diencephalon
      • Cervical spinal cord
      • Vestibular and auditory systems
      • Posterior cerebral hemispheres

• Venous Drainage

  • Superficial cerebral/cerebellar cortex veins → dural venous sinuses
  • Basal brain veins → great vein of Galen → straight sinus
  • All venous sinuses → drain into internal jugular veins
  • Emissary veins connect scalp veins to dural sinuses

• Spinal Cord Blood Supply

  • Anterior and posterior spinal arteries (from vertebral arteries)

  • Radicular arteries (from segmental vessels: cervical, intercostal, lumbar)

  • Cervical cord → vertebral + radicular arteries

  • Thoracic/lumbar cord → regional radicular arteries

  • Artery of Adamkiewicz:

    • Enters ~T7
    • Critical for lumbosacral spinal cord
    • Injury → risk of paraplegia