Nervous System – Comprehensive Study Notes

Macroscopic Organization of the Nervous System

• Two primary anatomical divisions

  • Central Nervous System (CNS)
  • Brain and spinal cord (plus the receptors of the special sense organs—eyes, ears, etc.—because their afferent fibers enter directly)
  • Peripheral Nervous System (PNS)
  • “Everything else”: all nervous tissue outside the brain & spinal cord

• Receptor vs. Effector vocabulary

  • Receptor – structure sending afferent (incoming) signals \rightarrow CNS
  • Effector – structure receiving efferent (outgoing) signals \leftarrow CNS

• Cerebrospinal Fluid (CSF)

  • Produced by the choroid plexus (know for exam!)

Central Nervous System (CNS)

• Brain – primary processing center
  • Interprets sensory data from special senses
  • Initiates motor output
  • Seat of higher cognitive functions (reasoning, memory, emotions)
• Spinal Cord – main conduction pathway
  • Relays information between brain and periphery

Peripheral Nervous System (PNS) – Structural & Functional Layout

• Structural subdivisions

  • Somatic Nervous System (SNS)
  • Autonomic Nervous System (ANS)
  • Enteric Nervous System (ENS)

• Functional divisions within the PNS

  • Afferent (sensory) division – moves information to the CNS
  • Originates from receptors in peripheral tissues & organs
  • Efferent (motor) division – carries commands from the CNS
  • Targets muscles, glands, adipose tissue (collectively called effectors)

Somatic Nervous System (SNS)

• Controls skeletal muscle contraction
• Two modes of activity
  • Voluntary – conscious movements: “thinking about moving”
  • Involuntary – unconscious skeletal events
  • Reflexes (e.g., stretch reflex – most tested)
  • Fasciculations (spontaneous twitches)

Autonomic Nervous System (ANS)

• Controls subconscious visceral functions
  • Smooth muscle, cardiac muscle, glandular activity
  • Core examples: heartbeat regulation, hormonal secretion, digestion
• Two opposing divisions
  • Sympathetic – “fight-or-flight”
  • Dominant hormone: adrenaline (epinephrine)
  • Parasympathetic – “rest-and-digest”
  • Dominant hormone: noradrenaline (norepinephrine)

Enteric Nervous System (ENS)

• Extensive neural network embedded in GI tract wall
• Can function independently of CNS, but is modulated by the ANS
• Orchestrates motility, secretion, and blood-flow for digestion

Sensory Receptor Taxonomy (PNS – Afferent Side)

• Exteroceptors – body surface
  • Touch, temperature, nociceptors (pain)
• Interoceptors – internal organs & tissues
  • Monitor BP, pH, \text{O}_2 levels, visceral pain
• Proprioceptors – skeletal muscles & joints
  • Inform the CNS of body position and movement
• Physical form variety
  • May be neurons or specialized neuroglia
  • Range from single cells (Pain fibers) to complex organs (eyes, ears)

Motor Pathways & Autonomic Ganglia (PNS – Efferent Side)

• Effectors – target organs that react to motor commands
• Ganglion – cluster of neuronal cell bodies acting as a relay station
  • Autonomic ganglia bridge preganglionic and postganglionic neurons
• Neurotransmitter profile
  • Preganglionic fibers: \text{ACh} (acetylcholine)
  • Postganglionic fibers
  • Sympathetic – \text{ACh} or \text{NE} (norepinephrine)
  • Parasympathetic – \text{ACh}

Nervous Tissue – Cellular Players

Neurons (basic functional units)

• Communicate via

  • Electrical signals – action potentials (within neuron)
  • Chemical signals – neurotransmitters (across synapse)

• Structural parts

  • Cell body (soma)
  • Nucleus & nucleolus (ribosome production)
  • Perikaryon – cytoplasm
  • Mitochondria – produce 32\text{–}36 ATP / glucose
  • RER + ribosomes – synthesize proteins/peptides
  • Dendrites – receive chemical input and convert to electrical change
  • Axon – propagates action potentials away from soma
  • Axolemma – plasma membrane of axon
  • Axoplasm – cytoplasm of axon
  • Initial segment (base) – where AP is first generated
  • Axon hillock – decision point; exhibits all-or-none principle
  • Telodendria – terminal axon branches
  • Axon terminals (synaptic boutons) – release neurotransmitter into synapse

• Myelin sheath

  • Acts as electrical insulation; Multiple Sclerosis (MS) results from its destruction

• Neuron morphologic classes (know locations)

  • Anaxonic – brain, special senses (retina, olfactory bulb)
  • Bipolar – classic special-sense afferents (retina, inner ear)
  • Unipolar – majority of PNS sensory neurons (touch, pressure, pain, temperature)
  • Multipolar – motor neurons controlling skeletal muscle (very common)

Neuroglia (supporting cells)

• CNS types (4)
  • Astrocytes – blood–brain barrier; ionic regulation
  • Microglia – immune defense
  • Oligodendrocytes – myelinate CNS axons
  • Ependymal cells – line ventricles; produce CSF (with choroid plexus)
• PNS types (2)
  • Satellite cells – regulate micro-environment of PNS ganglia
  • Schwann cells – myelinate PNS axons

Neurophysiology Fundamentals

Membrane Potential

• Resting membrane potential V_{rest} = -70\,\text{mV} (inside more negative than outside)
• Ion contributors: \text{Na}^+,\; \text{K}^+,\; \text{Ca}^{2+},\; \text{Cl}^-
• Maintained by Na(^+)/K(^+)-ATPase
  • Pumps 3 \text{Na}^+ out / 2 \text{K}^+ in per cycle
  • Energetically expensive but crucial for excitability

Action Potentials (APs)

• Rapid, stereotyped electrical impulses along axon surface
• Initiation site: initial segment + axon hillock
• Threshold concept – must reach critical voltage; otherwise no AP (all-or-none)
• Propagation – self-reinforcing wave to the axon terminals, triggering neurotransmitter release

Ion Channels – Gating Mechanisms

• Chemically gated (ligand-gated) channels
  • Open when specific ligand (cation or anion) binds
  • Predominant on dendrites & soma, especially at synapses
• Voltage-gated channels
  • Respond to membrane potential changes (depolarization, repolarization, hyperpolarization)
  • Abundant along axons
• Mechanically gated channels
  • Respond to physical deformation (vibration, pressure, stretch)
  • Examples: hair cells in inner ear translating sound waves

Clinical & Conceptual Connections

• Multiple Sclerosis (MS) – autoimmune destruction of myelin in CNS \rightarrow slowed/blocked AP conduction
• Stretch Reflex – prototypical somatic reflex; used in neurological exams to assess spinal cord integrity
• Fight-or-Flight vs. Rest-and-Digest – complementary ANS states; imbalance manifests in anxiety disorders, cardiac issues, digestive problems
• Pain (nociception) – mediated by nociceptors; basis for analgesic pharmacology targeting Na(^+) or Ca(^{2+}) channels
• Enteric Autonomy – ENS dysfunction leads to motility disorders (e.g., irritable bowel syndrome) yet can proceed independently during spinal injury