1. Nervous System

Nervous System: Core Purpose & Fast Facts

• Communication network of the body; uses electrical signals (action potentials) to coordinate sensation, movement, thought, emotion & speech.

• Example of speed: touch a hot surface → hand retracts almost instantly due to rapid signal transmission.

Structural Divisions

• Central Nervous System (CNS)

  • Brain

  • Spinal cord

• Peripheral Nervous System (PNS)

  • All neural tissue outside brain/spinal cord: cranial nerves + spinal nerves

Functional Pathway (3-Step Flow)

• 1. Sensory (Afferent) Division – “A for Approaching/Arriving”

  • Receptors detect internal state (hunger, bladder distension) & external stimuli (temperature, pain, vibration, light, sound).

  • Sends incoming data → CNS.

• 2. Integration (Interneurons)

  • Located exclusively in CNS.

  • Interpret sensory data, decide on appropriate response.

• 3. Motor (Efferent) Division – “E for Exit”

  • Carries commands from CNS → effector organs (muscles & glands).

Motor Sub-Divisions

• Somatic Nervous System (SNS)

  • Voluntary control of skeletal muscles (e.g., waving a hand).

• Autonomic Nervous System (ANS) – involuntary, organ regulation

  • Sympathetic (“fight or flight” – stress/danger mode).

  • Parasympathetic (“rest & digest” – normal peaceful state).

Histology: Cell Types

• Two major categories

  • Neurons – excitable, perform information processing.

  • Neuroglia (glial cells) – “glue”; support & protect neurons. 6 types (4 CNS, 2 PNS).

Neuron Anatomy

• Cell body (soma) – nucleus + organelles.

• Dendrites – short, branched; receive signals → soma.

• Axon – long projection; conducts signals away from soma.

Neuron Classification

• By function

  • Sensory (afferent)

  • Motor (efferent)

  • Interneurons (association)

• By structure (number of poles/axons)

  • Multipolar – many dendrites + 1 axon (most motor neurons).

  • Bipolar – 1 dendrite & 1 axon (retina, olfactory epithelium).

  • Anaxonic – only dendrites, no axon; rare.

  • Pseudo-unipolar – single short process splits into two branches; common in sensory ganglia.

Neuroglia of the CNS (4)

• Astrocytes – star-shaped; wrap capillaries, form part of blood-brain barrier; regulate nutrient/waste exchange.

• Microglia – small phagocytes; remove debris & pathogens (immune defense).

• Ependymal cells – ciliated cuboidal lining brain ventricles & spinal canal; produce & circulate cerebrospinal fluid (CSF).

• Oligodendrocytes – few extensions; myelinate CNS axons by wrapping multiple segments with myelin sheath.

Neuroglia of the PNS (2)

• Schwann cells – myelinate PNS axons (functional analog of oligodendrocytes).

• Satellite cells – surround neuron cell bodies in PNS ganglia; regulate environment (analog of astrocytes).

Myelin Sheath & Nodes of Ranvier

• Myelin = lipid-rich insulation → speeds conduction.

• Nodes of Ranvier – gaps between myelin segments.

• Action potential jumps node-to-node (saltatory conduction) → greatly increases speed.

Neurophysiology

Resting Membrane Potential (RMP)

• Typical value V_m \approx -70\;\text{mV}.

• Established by:

  • High \text{[Na^+]} outside, high \text{[K^+]} inside.

  • Large intracellular negative proteins (PTN–).

  • Creates chemical & electrical gradients: outside ≈ positive, inside ≈ negative.

Ion Channels

• Voltage-gated – open/close with membrane voltage changes.

• Chemically (ligand)-gated – open/close when neurotransmitter/hormone binds.

Action Potential (AP) Phases

1. Depolarization

   • Stimulus opens \text{Na^+} channels → \text{Na^+} influx.

   • Membrane potential becomes less negative.

   • Threshold at \approx -55\;\text{mV} triggers full AP spike.

2. Repolarization

   • \text{K^+} channels open → \text{K^+} efflux.

   • Returns toward RMP.

3. Hyperpolarization (Overshoot)

   • \text{K^+} outflow briefly drives V_m below -70\;\text{mV}.

   • Na^+/K^+ ATPase restores ion distributions → RMP.

AP Propagation

• Reversal of polarity travels along axon toward terminals.

• Myelinated fibers: saltatory conduction (fast).

• Unmyelinated fibers: continuous conduction (slow).

Synapses

• Presynaptic neuron – delivers AP to terminal.

• Synaptic vesicles – contain neurotransmitter (e.g., acetylcholine).

• Synaptic cleft – fluid-filled gap.

• Postsynaptic neuron – has receptor channels.

• Sequence:

  1. AP arrives → \text{Ca^{2+}} influx → vesicle fusion.

  2. Neurotransmitter released → diffuses across cleft.

  3. Binds receptors → ion channels open.

  4. Generates postsynaptic potential (PSP):

  • Excitatory PSP (EPSP) – depolarizes toward threshold.

  • Inhibitory PSP (IPSP) – hyperpolarizes away from threshold.

  1. Neurotransmitter removed by enzymes or reuptake to terminate signal.

Summation & Threshold Dynamics

• Single sub-threshold EPSP dies out.

• Temporal or spatial summation of EPSPs can reach threshold → new AP.

• EPSP + IPSP may cancel each other.

Factors Affecting Conduction Velocity

• Axon diameter: larger → faster.

• Myelination: present → saltatory & rapid; absent → slow.

• Chemical modifiers: alcohol, sedatives, anesthetics ↓ \text{Na^+} permeability → slow conduction.

• Blood flow/temperature: reduced perfusion or cold → slower signals, tingling/numbness.

Real-World Connections & Examples

• Touching hot stove: afferent → CNS → efferent motor → withdraw hand.

• Fight-or-flight activation (sympathetic) during danger vs digestive calm (parasympathetic) at rest.

• Blood-brain barrier importance: astrocytes restrict toxins (e.g., alcohol) from damaging neurons.

Key Numerical & Term Recap

• Resting potential \approx -70\;\text{mV}; Threshold \approx -55\;\text{mV}.

• Depolarization = \text{Na^+} in; Repolarization = \text{K^+} out.

• Nodes of Ranvier enable saltatory conduction.

• Six neuroglial types: 4 CNS (astrocytes, microglia, ependymal, oligodendrocytes), 2 PNS (Schwann, satellite).

• Structural neuron types: multipolar, bipolar, anaxonic, pseudo-unipolar.