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The Nervous System - In-Depth Study Notes

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1. Definition and Functions

1. Sensory - detects stimuli from internal and external environments
2. Integration - analyzes, stores, interprets, and decides upon responses
3. Motor - initiates and regulates voluntary and involuntary responses

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2. Structural Divisions

Central Nervous System (CNS)

• Brain - enclosed in the cranial cavity
• Spinal cord - enclosed in the vertebral canal

Peripheral Nervous System (PNS)

• Cranial nerves (12 pairs)
• Spinal nerves (31 pairs)
• Autonomic nerves
• Ganglia - collections of nerve cell bodies outside the CNS
• Enteric nervous system - intrinsic nervous system of the gut wall
• Specialized sensory and motor nerve endings

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3. Functional Divisions

• Sympathetic division - "fight or flight"
• Parasympathetic division - "rest and digest"
• Enteric division - communicates with CNS via sympathetic and parasympathetic fibers but can function independently

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4. Development (Embryology)

1. ~Day 18-19: Ectoderm thickens to form the neural plate
2. Day 20-21: Neural plate develops a neural groove flanked by neural folds
3. The neural folds fuse in a process called neurulation to form the neural tube (with inner neural canal)
4. Proliferation of neural tube cells generates the vesicles of the brain and ventricular system

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5. Cellular Components

5a. Neurons

Classification of Neurons by Number of Processes

Classification of Neurons by Function

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5b. Glial Cells (Neuroglia)

• Physical support and protection for neurons
• Insulation of nerve cell bodies/processes to facilitate rapid impulse transmission
• Metabolism and maintenance of ionic environment
• Immune defense

CNS Glia (Central Neuroglia)

PNS Glia (Peripheral Neuroglia)

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5c. Myelin and Saltatory Conduction

• Glial cells wrap axons in a phospholipid-based myelin sheath, which acts as insulation and greatly increases conduction speed
• Oligodendrocytes myelinate in the CNS; Schwann cells myelinate in the PNS
• Gaps between myelin segments are called nodes of Ranvier - these have high densities of voltage-gated ion channels
• Action potentials "jump" from node to node (saltatory conduction), enabling rapid signal propagation

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6. Terms of Orientation

• Rostral = toward the nose/forebrain
• Caudal = toward the tail/spinal cord
• Dorsal = toward the back (posterior in spinal cord; superior in brain)
• Ventral = toward the front (anterior in spinal cord; inferior in brain)

• Coronal - divides into anterior and posterior parts
• Sagittal - divides into left and right parts (midsagittal = through midline; parasagittal = just off midline)
• Horizontal (axial/transverse) - divides into superior and inferior parts

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7. Major Anatomical Regions at a Glance

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8. Neural Pathways and Reflex Arcs

• Neural pathways involve multiple neurons linked in chain-like fashion
• Reflex arcs are the simplest neural pathways: sensory neuron → (interneuron) → motor neuron
• In humans, most sensory neurons do not pass directly to the brain; they communicate via synapses with motor neurons in the spinal cord first
• Synapses are specialized contacts between neurons where neurotransmitters are released from presynaptic axon terminals to affect the postsynaptic cell

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Quick Reference Summary

Nervous System
├── CNS (Brain + Spinal Cord)
│   ├── Brain: cerebral hemispheres, diencephalon, brainstem, cerebellum
│   └── Spinal cord: 31 segments, ascending/descending tracts
└── PNS (everything outside CNS)
    ├── Somatic NS - voluntary (skeletal muscle)
    └── Autonomic NS - involuntary
        ├── Sympathetic ("fight or flight")
        ├── Parasympathetic ("rest and digest")
        └── Enteric (gut)

Cells:
├── Neurons (functional units): multipolar, bipolar, pseudounipolar
│   └── Functional: sensory, motor, interneurons (>99.9%)
└── Glia (support): CNS = oligodendrocytes, astrocytes, microglia, ependymal
                    PNS = Schwann cells, satellite cells

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