Nervous System

4.1 Cells of Nervous System

Introduces neurons and glial cells, their structures, functions, and how they form the basic units of the nervous system.

• Neurons
• Soma: contains nucleus, ER, ribosomes
• Dendrites: receive information from other cells
• Axon hillock: integrates signals, initiates action potentials
• Axon: conducts AP; myelinated by:
  • Oligodendrocytes (CNS)
  • Schwann cells (PNS)

• Axon terminal/synaptic bouton: transmits signals to next cell
• Bundled axons:
  • Nerves (PNS): can be sensory, motor, or mixed
  • Tracts (CNS): only one type of info
• Cell body groupings:
  • Ganglia (PNS)
  • Nuclei (CNS)
• Glial cells (neuroglia)
  • Astrocytes: nourish neurons, form blood-brain barrier
  • Ependymal cells: line ventricles, produce CSF
  • Microglia: phagocytic, remove waste
  • Oligodendrocytes/Schwann cells: myelin production

  

4.2 Transmission of Neural Impulses

Covers resting potential, action potential initiation and propagation, ion channels, synaptic transmission, and factors affecting conduction.

• Resting potential
  • $V_m = -70\ \mathrm{mV}$
  • K⁺ leak channels contribute
• Axon hillock
  • AP triggered if threshold reached
  • Summation:
    • Temporal: multiple signals at same time
    • Spatial: multiple signals close in space
• Ion channels & membrane potential
  • Na⁺ influx depolarizes cell (Vm = +35)
  • Relative refractory period prevents immediate reactivation
• Impulse propagation
  • AP depolarizes neighboring regions → Na⁺ influx triggers next AP
  • Inactivation prevents backward conduction
  • Clinical relevance:
    • Local anesthetics block VGNa⁺ channels (pain neurons preferentially affected)
    • Tetrodotoxin (TTX) blocks VGNa⁺ → paralysis (e.g., diaphragm)
• Synaptic transmission
  • Ca²⁺ influx triggers neurotransmitter vesicle fusion with terminal membrane

4.3 Organization of Human Nervous System

Outlines CNS and PNS divisions, reflex circuits, and functional organization of somatic and autonomic systems.

• CNS & PNS
  • CNS: brain and spinal cord
  • PNS: nerve tissue outside CNS
    • 31 spinal nerve pairs, 10–12 cranial nerves
    • Olfactory & optic are CNS outgrowths
  • Spinal cord:
    • White matter deeper than gray matter
    • Protected by vertebral column
    • Dorsal: sensory input
    • Cervical, thoracic, lumbar, sacral regions
• Somatic nervous system
  • Innervates skeletal muscle
• Autonomic nervous system (ANS)
  • Sympathetic: fight or flight
  • Parasympathetic: rest and digest
• Reflexes
  • Reflex arcs control immediate responses
  • Monosynaptic: single synapse (e.g., knee-jerk)
    • Sensory afferent → motor efferent
    • Protects muscles from overstretch
  • Polysynaptic: at least one interneuron
    • Withdrawal reflex requires contralateral limb extension

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