Neurons: Cellular and Network Properties

Organization of the Nervous System

The nervous system is divided into central and peripheral components, each with specialized functions and subdivisions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integrating and processing information.

• Peripheral Nervous System (PNS): Composed of nerves outside the CNS; transmits sensory and motor signals.

• Sensory (afferent) neurons: Carry signals from sensory receptors to the CNS.

• Efferent neurons: Transmit information from the CNS to effectors (muscles and glands).

• Somatic motor division: Controls voluntary skeletal muscle movement.

• Autonomic division: Regulates involuntary functions; includes:

  • Sympathetic branch: "Fight or flight" responses; increases alertness and metabolic activity.

  • Parasympathetic branch: "Rest and digest" responses; promotes relaxation and digestion.

  • Enteric nervous system: Network of neurons in the digestive tract walls.

Structure and Classification of Neurons

Neurons are the functional units of the nervous system, specialized for communication.

• Cell body: Contains nucleus and organelles; control center for cellular functions.

• Axon: Transmits outgoing signals from the cell body to target cells.

• Dendrites: Receive incoming signals from other cells.

• Classification by function:

  • Sensory (afferent) neurons: Carry information from receptors to CNS.

  • Interneurons: Confined to CNS; facilitate communication between neurons.

  • Efferent neurons: Transmit instructions to muscles and glands.

• Collaterals: Branches from the axon.

• Axon terminals: Enlarged endings that store and release neurotransmitters.

• Varicosities: Enlarged regions along autonomic axons for neurotransmitter release.

• Nerves: Bundles of axons; classified as sensory, motor, or mixed.

• Axon hillock: Specialized region where axon originates; site of action potential initiation.

• Axonal transport: Movement of proteins and organelles between cell body and axon terminals.

  • Slow axonal transport: Moves soluble and cytoskeletal proteins slowly.

  • Fast axonal transport: Rapid movement of substances in both directions.

  • Anterograde: Toward axon terminals.

  • Retrograde: Toward cell body.

Synapses and Cell-to-Cell Communication

Synapses are specialized junctions for communication between neurons and their targets.

• Synapse: Region where axon terminal meets target cell.

• Presynaptic cell: Delivers signal.

• Postsynaptic cell: Receives signal.

• Synaptic cleft: Narrow space between presynaptic and postsynaptic cells.

Glial Cells: Support and Function

Glial cells provide structural and biochemical support to neurons.

• PNS:

  • Schwann cells: Form myelin; insulate axons and speed signal transmission.

  • Nodes of Ranvier: Gaps between Schwann cells; facilitate rapid signal transmission.

  • Satellite cells: Supportive capsules around cell bodies in ganglia.

• CNS:

  • Oligodendrocytes: Form myelin around multiple axons.

  • Astrocytes: Highly branched; regulate synaptic activity, provide substrates for ATP, maintain homeostasis, and form blood-brain barrier.

  • Microglial cells: Specialized immune cells; remove damaged cells and invaders.

  • Ependymal cells: Create blood-brain barrier; source of neural stem cells.

Electrical Properties of Neurons

Neurons generate electrical signals through ion movement across membranes.

• Resting membrane potential (RMP): Difference in charge across membrane; inside is more negative (more K+), outside is more positive (more Na+).

• Depolarization: Inside becomes more positive as Na+ enters.

• Repolarization: Inside returns to negative as K+ leaves.

• Hyperpolarization: Inside becomes more negative than RMP; extra K+ leaves.

Graded Potentials

Graded potentials are variable-strength signals occurring at dendrites and cell body.

• Strength varies with stimulus.

• Lose strength as they move away from stimulus site due to current leak and cytoplasmic resistance.

• Strong graded potentials reaching the trigger zone (axon hillock) may initiate action potentials.

• Excitatory: Depolarizing.

• Inhibitory: Hyperpolarizing.

• Subthreshold: Not strong enough to trigger action potential.

• Suprathreshold: Strong enough to trigger action potential.

• Excitability: Ability to respond to stimulus and fire action potential.

Action Potentials

Action potentials are uniform, all-or-none electrical signals traveling down the axon.

• Initiated at trigger zone if threshold is reached.

• Do not lose strength as they travel.

• Na+ channels have two gates: activation (closed at rest) and inactivation (open at rest).

• Depolarization opens activation gates, allowing Na+ influx; positive feedback loop.

• Inactivation gates close after delay, stopping Na+ influx.

• Repolarization: K+ leaves, Na+ gates reset.

• Only a small percentage of ions move during each event.

Refractory Periods

Refractory periods limit the frequency of action potentials.

• Absolute refractory period: No action potential can be generated, regardless of stimulus strength.

• Relative refractory period: Action potential possible with stronger-than-normal stimulus.

Conduction Velocity

Action potential conduction speed depends on axon size and myelination.

• Large axons conduct faster due to less friction.

• Myelinated axons prevent current leak; nodes of Ranvier allow rapid signal transmission (saltatory conduction).

• Demyelinating diseases (e.g., multiple sclerosis) impair conduction.

Chemical Factors Affecting Electrical Activity

• Neurotoxins: Block Na+ channels (e.g., local anesthetics).

• Hyperkalemia: Increased K+ lowers threshold, increasing excitability.

• Hypokalemia: Decreased K+ causes hyperpolarization, reducing excitability and causing muscle weakness.

Types of Synapses

• Electrical synapses: Gap junctions; bidirectional; found in CNS, cardiac, and smooth muscle.

• Chemical synapses: Use neurotransmitters for communication.

Neurotransmitters and Receptors

Neurotransmitters are chemical messengers released at synapses.

• Acetylcholine: Acts on cholinergic receptors (nicotinic and muscarinic).

• Amines: Serotonin, histamine, dopamine, norepinephrine, epinephrine.

• Amino acids: Glutamate, aspartate, GABA.

• Peptides: Substance P, opioid peptides, CCK, vasopressin, ANP.

• Purines: Adenosine, AMP, ATP.

• Gases: Nitric oxide, carbon monoxide, hydrogen sulfide.

• Lipids: Eicosanoids; endogenous ligands for cannabinoid receptors (CB1, CB2).

Neurotransmitter Release and Termination

• Synaptic vesicles: Store neurotransmitters in axon terminals.

• Synthesis: Large peptides made in cell body, transported to terminals; small neurotransmitters made in terminals.

• Release: Triggered by Ca2+ influx; vesicle fusion with membrane releases neurotransmitter.

• Kiss-and-run pathway: Vesicle forms fusion pore, releases neurotransmitter through small channel.

• Termination: Neurotransmitter effects end by diffusion, enzyme inactivation, or reuptake.

• Stronger stimuli (more action potentials per second) release more neurotransmitter.

Integration of Neural Information

• Divergence: One neuron branches to multiple targets.

• Convergence: Multiple neurons combine to fewer targets.

• Synaptic plasticity: Ability to change synaptic activity.

• Post-synaptic responses: Can be slow (growth, memory) or fast (neuromuscular junction).

• Excitatory postsynaptic potential (EPSP): Depolarizing.

• Inhibitory postsynaptic potential (IPSP): Hyperpolarizing.

• Spatial summation: Multiple graded potentials from different locations combine.

• Temporal summation: Graded potentials arriving simultaneously are summed.

• Synaptic activity can be modified by neurotransmitter availability.

• Long-term potentiation: Strengthens synapses; mechanism for learning and memory.

• Long-term depression: Weakens synapses.

Key Terms and Definitions

• Neuron: Functional unit of the nervous system.

• Action potential: Rapid, uniform electrical signal traveling down axon.

• Graded potential: Variable-strength signal at dendrites/cell body.

• Synapse: Junction between neurons or neuron and target cell.

• Neurotransmitter: Chemical messenger released at synapse.

• Glial cell: Supportive cell in nervous system.

Table: Types of Glial Cells and Their Functions

Table: Types of Neurotransmitters

Key Equations

• Resting Membrane Potential:

• Additional info: This is the simplified Nernst equation for potassium ions.

• Ohm's Law for Membrane Current:

• Where is current, is conductance, is membrane potential, and is equilibrium potential for the ion.

Example: Saltatory Conduction

In myelinated axons, action potentials "jump" from node to node, greatly increasing conduction speed. This process is called saltatory conduction and is essential for rapid communication in the nervous system.

Example: Multiple Sclerosis

Multiple sclerosis is a demyelinating disease where loss of myelin impairs neural signaling, leading to muscle weakness, vision problems, and other neurological symptoms.

Summary Table: Graded vs. Action Potentials

Additional info: Academic context and examples were added to clarify concepts and provide self-contained explanations.