Neuroembryology & Functional Organisation of the Nervous System – Lecture 2

Learning Outcomes

• By the end of this lecture the student should be able to:
  • Describe the essential functions and divisions of the nervous system.
  • Differentiate between sensory (afferent), motor (efferent) and autonomic pathways.
  • Outline early nervous-system embryology and recognise major congenital disorders (e.g.
  • Anencephaly
  • Spina bifida
  • Foetal Alcohol Syndrome – FAS).
  • Classify neurons on the basis of morphology (anaxonic, bipolar, unipolar, multipolar).
  • Identify neuroglial cells (both CNS & PNS) and link structure to function.

Key Topics Covered

• Embryological development of the nervous system.
• Congenital malformations of CNS embryogenesis.
• Major organs and gross subdivisions of the nervous system.
• Functional pathways (sensory, motor, autonomic).
• Neuronal morphology & functional classifications.
• Neuroglia – types, locations, roles.

Embryonic Development of the Nervous System

• Early onset: human NS starts very early – during the gastrulation phase.
• Gastrulation recap – inward migration of cells → formation of $3$ germ layers:
  • Ectoderm (surface): gives rise to skin & nervous system.
  • Mesoderm (middle): forms muscle, bone, most organs.
  • Endoderm (deepest): develops into gut & associated viscera.
• Notochord formation converts the embryo from a circular to an axial organisation – serves as the inductive signal for nervous-system development.

Neurulation (week $3$)

• Definition: transformation of the neural plate → neural tube.
• Chronology (typical Carnegie days):
  • $19$ days – neural plate evident.
  • $20$ days – neural groove & folds begin.
  • $22$ days – tube closes centrally; neural crest delineated.
  • $26$ days – anterior neuropore (cranial) closes ≤ day $26$.
  • $27$ days – posterior neuropore (caudal) closes ≈ day $27$–$28$.
• Failure of timely closure ⇒ congenital malformations.

Fate of Derivatives

• Neural tube → CNS; internal fluid cavity (neurocoel) later expands to the ventricular system.
• Neural crest → PNS (dorsal-root ganglia, autonomic ganglia, Schwann cells, melanocytes, etc.).
• Somites (mesoderm) subdivide into:
  • Dermatome – future dermis.
  • Myotome – skeletal muscle.
  • Sclerotome – vertebrae.

Brain Vesicle Formation & Ventricular System

• Cephalic end of neural tube balloons into $3$ primary brain vesicles → later $5$ secondary vesicles → adult brain regions.
• Neurocoel expansion forms the ventricles:
  • Lateral ventricles (in cerebral hemispheres) → connect via interventricular foramina to …
  • Third ventricle (in diencephalon) → via cerebral aqueduct to …
  • Fourth ventricle (between pons & cerebellum) → continuous with central canal of spinal cord.

Common Disorders of CNS Embryogenesis

• Spina Bifida (dysraphism)
  • Broad spectrum where caudal neural tube fails to close.
  • Severity ranges: occulta < meningocele < myelomeningocele.
  • Clinical hallmark: portion of spinal cord +/- meninges exposed on body surface.
• Anencephaly
  • Cranial neuropore fails to close → absence of major portions of brain, skull, scalp.
  • Usually incompatible with life; prenatal diagnosis critical.
• Foetal Alcohol Syndrome (FAS)
  • Alcohol = potent CNS teratogen & depressant; crosses placenta.
  • Acts on reticular formation, spinal cord, cerebellum, cerebral cortex & neurotransmitter systems.
  • Characteristic triad:
  • Growth retardation (microcephaly).
  • Facial dysmorphisms (e.g. thin upper lip, smooth philtrum, flat mid-face, epicanthal folds, low nasal bridge, micrognathia, minor ear anomalies).
  • CNS dysfunction – permanent intellectual disability, hyperactivity, tremors, poor coordination.
  • NO safe alcohol level established during pregnancy → public-health relevance.

Organs & Gross Divisions of the Nervous System

• Central Nervous System (CNS)
  • Brain + spinal cord.
  • Contains neural tissue, connective tissue & vasculature.
  • Core functions: process & integrate sensory data, issue motor commands, host higher mental functions (intelligence, memory, learning, emotion).
• Peripheral Nervous System (PNS)
  • All neural tissue outside CNS: cranial nerves, spinal nerves, peripheral ganglia, special-sense receptors.
  • Functions: deliver sensory information → CNS; convey motor commands → effectors.

Functional Organisation

• Afferent (sensory) division – brings input to CNS.
• Efferent (motor) division – carries output from CNS to effectors.
  • Sub-divided into:
  • Somatic Nervous System (SNS) – voluntary & reflex control of skeletal muscle.
  • Autonomic Nervous System (ANS) – involuntary regulation of smooth & cardiac muscle, glands, adipose.
    • Sympathetic – "fight/flight", energy mobilisation.
    • Parasympathetic – "rest/digest", energy conservation & homeostasis.
    • (Enteric NS often considered a third ANS branch controlling gut.)
• Mnemonic SAME: Sensory Afferent | Motor Efferent.

Overall Nervous-System Functions

1. Sensory input – detect internal & external changes via receptors.
2. Integration – CNS interprets & decides appropriate response.
3. Motor output – activation of effector organs to elicit response.

Cerebral Cortical Functional Areas (overview)

• Primary motor cortex – execution of voluntary movement.
• Premotor cortex – motor planning.
• Broca’s area – motor speech production.
• Primary somatosensory cortex – tactile & proprioceptive input.
• Somatic sensory association – interpretation/meaning.
• Wernicke’s area – sensory language comprehension.
• Primary visual & visual association – vision processing.
• Primary auditory & auditory association – hearing.
• Taste (gustatory) area – insular cortex.
  (Diagram referenced but not testable in detail.)

Neurons (Nerve Cells)

• Basic functional units – communicate via electrical & chemical signals.
• Four major anatomical regions
  1. Cell body (soma) – contains nucleus, Nissl bodies (RER + ribosomes), mitochondria, Golgi, cytoskeleton (neurofilaments/neurotubules → neurofibrils for support).
  2. Dendrites – numerous, receive input.
  • Dendritic spines: tiny projections → major synaptic site (≈ $80$–$90\%$ of surface area).
  1. Axon – single long process conducting impulses away from soma.
  • Begins at axon hillock/initial segment.
  • Ends in telodendria & synaptic terminals.
  1. Synapse – functional junction with another neuron/effector.

Structural Classification

• Anaxonic – small, processes indistinguishable; brain & special sense organs.
• Bipolar – $1$ dendrite + $1$ axon opposite; retina, olfactory epithelium, cochlea.
• Unipolar (pseudounipolar) – fused dendrites & axon; cell body off to side; sensory neurons of PNS.
• Multipolar – many dendrites, $1$ axon; most common in CNS & all skeletal-muscle motor neurons.

Functional Classification

1. Sensory (afferent) neurons – transmit stimuli → CNS.
   • Somatic sensory – external environment & body position.
   • Visceral sensory – internal milieu of organs.
   • Receptor types:
     • Interoceptors – viscera.
     • Exteroceptors – external environment (touch, temp, special senses).
     • Proprioceptors – muscle/joint position.
2. Motor (efferent) neurons – convey commands ← CNS to effectors.
   • Somatic motor vs Autonomic motor.
   • Axons termed efferent fibers.
3. Interneurons (association neurons) – entirely within CNS; integrate sensory & motor, underpin memory, planning, learning.

Neuroglia (Glial Cells)

• Supporting cells; outnumber neurons.

CNS Neuroglia ("EOMA")

1. Ependymal cells
   • Line ventricles & central canal; form ependyma.
   • Produce, circulate (via cilia/microvilli) & monitor CSF.
   • Contain neural stem cells for repair.
2. Oligodendrocytes
   • Produce myelin sheaths around multiple CNS axons.
   • Myelination ↑ conduction speed (saltatory), electrically insulates.
   • Myelinated regions = white matter; lack outer neurilemma.
3. Microglia
   • Small, motile phagocytes derived from mesoderm; remove debris, pathogens.
4. Astrocytes (most abundant)
   • Maintain blood-brain barrier via perivascular "end-feet".
   • Provide structural scaffold; regulate ion & nutrient milieu; guide neuronal development; form scar tissue after injury (gliosis).

PNS Neuroglia

1. Schwann cells
   • Entire cell wraps around a segment of one axon → myelin sheath + neurilemma.
   • Necessary for axonal regeneration in PNS.
   • Also envelop groups of unmyelinated axons.
2. Satellite cells
   • Surround neuronal cell bodies in ganglia; regulate exchange of O$<em>2$, CO$</em>2$, nutrients, neurotransmitters.

Myelination & Nodes of Ranvier

• Internode – myelinated segment.
• Node (of Ranvier) – gap; site of saltatory conduction.
• CNS: one oligodendrocyte → many internodes on different axons.
• PNS: many Schwann cells → sequential internodes along single axon.

Functional Significance

• Myelin ↑ membrane resistance & ↓ capacitance → conduction velocity ↑ ($\uparrow v$) while saving space/energy.
• Demyelinating diseases (e.g. MS, Guillain-Barré) severely impair signalling.

Peripheral Nerve Connective-Tissue Organisation

• Endoneurium – delicate layer around individual axons.
• Perineurium – encloses fascicles (bundles of fibers); diffusion barrier.
• Epineurium – dense sheath surrounding entire nerve; contains vasa nervorum.

Anatomical Terms Recap

• Nucleus – cluster of neuronal somata in CNS.
• Ganglion – cluster of neuronal somata in PNS.
• Tract – bundle of CNS axons sharing origin & function; grouped into columns in spinal cord.
• Nerve – bundle of PNS axons.
• Gray matter – somata, dendrites, unmyelinated axons (colour from Nissl bodies).
• White matter – myelinated axons (colour from myelin lipid).

Integrative Pathways Overview

• Ascending (sensory) pathways – receptor → peripheral nerve → spinal/cranial nucleus → tracts → specific brain centres.
• Descending (motor) pathways – cortical/brainstem centre → tract → lower motor neuron → peripheral nerve → effector.
• Proper coordination relies on interneurons modulating both streams.

Real-World Relevance & Clinical Correlates

• Early embryology knowledge → prenatal screening (ultrasound, AFP levels) & folic-acid supplementation to reduce neural-tube defects.
• Understanding neuroglia vital for tackling tumours (e.g. astrocytomas, oligodendrogliomas) & neuro-immune disorders.
• Myelin physiology underpins treatment strategies for demyelinating diseases & nerve-repair technologies (Schwann-cell grafts, bioengineered conduits).
• Functional divisions (SNS vs ANS) guide pharmacology (sympathomimetics vs parasympatholytics), anaesthesia, and emergency medicine (fight/flight responses).

Quick Revision Nuggets

• Neural tube closes cranially by ≤ $26$ days, caudally by ≈ $27$ days.
• SAME mnemonic – Sensory Afferent Motor Efferent.
• EOMA – CNS glia (Ependymal, Oligodendrocyte, Microglia, Astrocyte).
• Neuronal morphology: Anaxonic, Bipolar, Unipolar, Multipolar ("ABUM").
• ANS: Sympathetic = "thoraco-lumbar, fight/flight" | Parasympathetic = "cranio-sacral, rest/digest".
• Gray vs White matter distinguished primarily by myelin content.