Study Guide: Nervous System Organization, Neuronal Signaling, and Synaptic Communication

8.1 Organization of the Nervous System

Overview of Nervous System Components

The nervous system is a complex network responsible for coordinating body functions and responding to internal and external stimuli. It is divided into several main components, each with specialized roles.

• Central Nervous System (CNS): Consists of the brain and spinal cord; integrates sensory information and coordinates bodily responses.

• Peripheral Nervous System (PNS): Composed of all neural tissue outside the CNS; connects the CNS to limbs and organs.

• Sensory Neurons (Afferent): Transmit sensory information from receptors to the CNS.

• Efferent Neurons: Carry commands from the CNS to effectors (muscles and glands).

• Somatic Motor Division: Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous Division: Regulates involuntary functions (e.g., heart rate, digestion).

• Sympathetic Branch: Prepares the body for 'fight or flight' responses.

• Parasympathetic Branch: Promotes 'rest and digest' activities.

• Enteric Nervous System: Governs the function of the gastrointestinal tract.

8.2 Cells of the Nervous System

Primary Cell Types

• Neurons: Excitable cells that transmit electrical signals.

• Glial Cells (Neuroglia): Support, protect, and nourish neurons.

Functional Groups of Neurons

• Afferent (Sensory) Neurons: Carry information toward the CNS.

• Interneurons: Connect neurons within the CNS; process information.

• Efferent Neurons: Transmit signals from the CNS to effectors.

Comparison: Afferent neurons detect stimuli, interneurons integrate signals, and efferent neurons elicit responses.

Neuron Structure and Function

• Cell Body (Soma): Contains the nucleus and organelles; the control center of the neuron. Extensive cytoskeleton supports transport and structural integrity.

• Dendrites: Receive incoming signals; highly branched to increase surface area for synaptic input.

• Axons: Conduct outgoing electrical impulses to other cells.

Example: In the CNS, dendrites may be more complex than in the PNS, reflecting the need for greater integration of information.

Axonal Transport

• Slow Axonal Transport: Moves cytoskeletal proteins and enzymes; rate is a few mm/day.

• Fast Axonal Transport: Moves organelles and vesicles; rate is up to 400 mm/day; uses motor proteins and microtubules.

Synapses and Synaptogenesis

• Synapse: Junction between two neurons or a neuron and an effector cell; consists of presynaptic terminal, synaptic cleft, and postsynaptic membrane.

• Synaptogenesis: Embryonic nerve cells form synapses with correct targets; survival depends on successful synaptic connections.

Glial Cells: Types and Functions

• Glia in the CNS: Astrocytes, oligodendrocytes, microglia, ependymal cells.

• Glia in the PNS: Schwann cells, satellite cells.

Myelin-Forming Glia

• Myelin: Lipid-rich sheath that insulates axons, increasing conduction speed.

• Oligodendrocytes: Form myelin in the CNS.

• Schwann Cells: Form myelin in the PNS.

• Nodes of Ranvier: Gaps in myelin sheath where action potentials are regenerated.

Other Glial Cells

• Satellite Cells: Support neuron cell bodies in the PNS.

• Astrocytes: Maintain blood-brain barrier, regulate ion/nutrient balance, support repair.

• Microglia: Act as immune cells in the CNS; remove debris and pathogens.

• Ependymal Cells: Line ventricles of the brain; produce and circulate cerebrospinal fluid (CSF).

Neural Stem Cells

• Neural Stem Cells: Undifferentiated cells capable of generating new neurons and glia; potential for treating neural injuries and degenerative diseases.

8.3 Electrical Signals in Neurons

Membrane Potentials and Ion Movement

• Nernst Equation: Predicts equilibrium potential for a single ion based on concentration gradient and charge.

Equation:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• GHK Equation: Predicts membrane potential considering multiple ions.

Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

• Ion Movement: Driven by concentration and electrical gradients (electrochemical gradient).

• Hyperpolarization: Occurs when ion movement makes the inside of the cell more negative.

Ion Channels and Permeability

• Types of Ion Channels: Mechanically gated, chemically gated, voltage-gated, and leak channels.

• Channel Gating: Channels open/close in response to specific stimuli (e.g., voltage, ligand binding, mechanical force).

Current Flow and Ohm's Law

• Ohm's Law: $I = \frac{V}{R}$, where I = current, V = voltage, R = resistance.

• Local Current Flow: Movement of ions within the cytoplasm; affected by membrane resistance and cytoplasmic resistance.

Graded Potentials

• Graded Potentials: Variable-strength signals that decay with distance; initiated by opening of ion channels.

• Trigger Zone: If graded potential reaches threshold at the axon hillock, an action potential is initiated.

Action Potentials

• All-or-None: Action potentials occur fully or not at all; do not lose strength as they travel.

• Phases: Rising phase (depolarization), falling phase (repolarization), after-hyperpolarization.

• Key Ion Channels: Voltage-gated Na+ and K+ channels.

• Refractory Periods: Absolute (no AP possible) and relative (AP possible with strong stimulus).

• Saltatory Conduction: In myelinated axons, APs jump between nodes of Ranvier, increasing speed.

8.4 Cell-to-Cell Communication in the Nervous System

Synapses: Electrical and Chemical

• Electrical Synapses: Direct cytoplasmic connections via gap junctions; rapid signal transmission.

• Chemical Synapses: Use neurotransmitters to transmit signals across synaptic cleft; most common in the CNS.

Neurocrine Signaling

• Neurotransmitter: Fast-acting chemical messenger released at synapses.

• Neuromodulator: Modifies synaptic transmission, often with slower, longer-lasting effects.

• Neurohormone: Released into the blood, affecting distant targets.

Neurocrine Receptors

• Ionotropic Receptors: Ligand-gated ion channels; mediate fast synaptic transmission.

• Metabotropic Receptors (GPCRs): Activate second messenger pathways; slower, modulatory effects.

Major Neurocrine Molecules Table

The following table summarizes major neurocrine classes, molecules, receptors, and categories:

Neurocrine class	Neurocrine molecules	Receptor	Subtypes	Receptor category
Acetylcholine	Acetylcholine	Cholinergic (nicotinic, muscarinic)	Nicotinic, muscarinic	Ion channel (nicotinic), GPCR (muscarinic)
Amines	Norepinephrine (NE)	Adrenergic (alpha, beta)	Alpha, beta	GPCR
	Dopamine (DA)	Dopaminergic	D1, D2, etc.	GPCR
	Serotonin (5-HT)	Serotonergic	Multiple	GPCR, ion channel
Amino acids	Glutamate, GABA, Glycine	Glutamatergic, GABAergic, Glycinergic	NMDA, AMPA, GABAA, GABAB, etc.	Ion channel, GPCR
Peptides	Substance P, enkephalins, endorphins	Peptide receptors	Multiple	GPCR
Purines	Adenosine	Purinergic	P1, P2	GPCR, ion channel
Gases	Nitric oxide (NO)	N/A	N/A	N/A
Lipids	Eicosanoids	Cannabinoid	CB1, CB2	GPCR

Neurotransmitter Synthesis and Release

• Synthesis: Small-molecule neurotransmitters are synthesized in axon terminals; peptide neurotransmitters are synthesized in the cell body and transported to terminals.

• Release: Ca2+-dependent exocytosis releases neurotransmitters into the synaptic cleft.

• Kiss-and-Run Pathway: Vesicle briefly fuses with membrane, releases part of contents, then detaches.

Termination of Neurotransmitter Activity

• Reuptake into presynaptic cell

• Enzymatic degradation in the synaptic cleft

• Diffusion away from the synapse

Neuronal Integration and Synaptic Plasticity

• Divergence: One neuron branches to affect multiple targets.

• Convergence: Multiple neurons synapse onto a single neuron.

• Synaptic Plasticity: The ability of synapses to strengthen or weaken over time, essential for learning and memory.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing likelihood of action potential.

Summation

• Spatial Summation: Multiple simultaneous inputs from different locations.

• Temporal Summation: Multiple inputs from the same source in rapid succession.

Modulation of Synaptic Activity

• Presynaptic Modulation: Alters neurotransmitter release from the presynaptic neuron.

• Postsynaptic Modulation: Alters postsynaptic cell's response to neurotransmitter.

Long-Term Potentiation (LTP) and Depression (LTD)

• LTP: Long-lasting increase in synaptic strength, associated with learning and memory.

• LTD: Long-lasting decrease in synaptic strength, involved in synaptic pruning and adaptation.

Additional info: This guide expands on the reading questions by providing definitions, context, and examples for each major topic, and reconstructs the neurocrine table for clarity and completeness.