Study Notes: Organization and Function of the Nervous System

Organization of the Nervous System

Main Divisions of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It is divided into several main components:

• Central Nervous System (CNS): Consists of the brain and spinal cord; integrates and processes information.

• Peripheral Nervous System (PNS): 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/glands).

• Somatic Motor Division: Controls voluntary movements via 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.

Cells of the Nervous System

Primary Cell Types

The nervous system contains two main cell types:

• Neurons: Specialized for transmitting electrical signals.

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

Neurons: Structure and Function

• Cell Body (Soma): Contains the nucleus and organelles; acts as the metabolic center.

• Dendrites: Branch-like extensions that receive incoming signals.

• Axon: Long projection that carries outgoing signals to other cells.

Neurons are classified functionally as:

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

• Interneurons: Connect neurons within the CNS.

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

Axonal Transport

• Slow Axonal Transport: Moves cytoskeletal and soluble proteins; rate is slow.

• Fast Axonal Transport: Moves membrane-bound organelles; uses motor proteins and microtubules.

Glial Cells: Support and Myelination

Types of Glial Cells

• In the CNS: Astrocytes, Microglia, Ependymal cells, Oligodendrocytes

• In the PNS: Schwann cells, Satellite cells

Myelin-Forming Glia

• Myelin: A fatty substance that insulates axons, increasing the speed of electrical conduction.

• Oligodendrocytes: Form myelin in the CNS.

• Schwann Cells: Form myelin in the PNS.

Other Glial Cells

• Astrocytes: Maintain the blood-brain barrier, regulate nutrients, and support neurons.

• Microglia: Act as immune cells in the CNS, removing debris and pathogens.

• Ependymal Cells: Line ventricles of the brain and produce cerebrospinal fluid.

• Satellite Cells: Support neurons in the PNS.

Neural Stem Cells

• Neural Stem Cells: Undifferentiated cells capable of generating new neurons and glia; important for repair and regeneration.

Electrical Signals in Neurons

Membrane Potential and Ion Movement

Neurons communicate via changes in membrane potential, which are governed by ion movement across the membrane.

• Nernst Equation: Predicts equilibrium potential for a single ion.

• GHK (Goldman-Hodgkin-Katz) Equation: Predicts membrane potential considering multiple ions.

Nernst Equation:

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

GHK 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 Channels and Permeability

• Mechanically Gated Channels: Open in response to physical deformation.

• Chemically Gated Channels: Open in response to ligand binding.

• Voltage-Gated Channels: Open in response to changes in membrane potential.

Ohm's Law in Neurons

Current flow in neurons follows Ohm's Law:

$I = \frac{V}{R}$

• I: Current

• V: Voltage (potential difference)

• R: Resistance

Graded Potentials and Action Potentials

• Graded Potentials: Local changes in membrane potential; strength decreases with distance.

• Action Potentials: Rapid, uniform electrical signals that travel long distances; all-or-none phenomenon.

Phases of the Action Potential

• Rising Phase: Rapid depolarization due to Na+ influx.

• Falling Phase: Repolarization due to K+ efflux.

• After-Hyperpolarization: Membrane potential becomes more negative than resting.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

Saltatory Conduction

• Myelinated Axons: Action potentials jump between nodes of Ranvier, increasing conduction speed.

Cell-to-Cell Communication in the Nervous System

Synapses: Electrical and Chemical

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

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Neurocrine Molecules

• Neurotransmitter: Fast-acting chemical messenger.

• Neuromodulator: Modifies synaptic transmission.

• Neurohormone: Released into the blood to act on distant targets.

Neurocrine Receptors

• Ionotropic Receptors: Ligand-gated ion channels; fast response.

• GPCRs (G-Protein Coupled Receptors): Activate intracellular signaling cascades; slower, modulatory effects.

Major Neurocrine Molecules Table

Neurocrine class	Neurocrine molecules	Receptor	Subtypes	Receptor category
Acetylcholine	Acetylcholine	Cholinergic	Nicotinic, Muscarinic	Ionotropic, GPCR
Amines	Norepinephrine (NE), Dopamine (DA), Serotonin (5-HT), Histamine (H)	Adrenergic, Dopaminergic, Serotonergic, Histaminergic	Multiple	GPCR
Amino acids	Glutamate, GABA, Glycine	Glutamatergic, GABAergic, Glycinergic	Multiple	Ionotropic, GPCR
Peptides	Substance P, Enkephalins, Endorphins	Peptide receptors	Multiple	GPCR
Purines	Adenosine	Purinergic	Multiple	GPCR
Gases	Nitric oxide (NO)	N/A	N/A	N/A
Lipids	Eicosanoids	Cannabinoid	Multiple	GPCR

Neurotransmitter Synthesis and Release

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

• Release: Ca2+-dependent exocytosis releases neurotransmitters at the synapse interacting with the postsynaptic cell.

• Kiss-and-run pathway: A rapid, transient release of neurotransmitter compared to classic exocytosis.

Termination of Neurotransmitter Activity

• Neurotransmitter activity can be terminated by reuptake, enzymatic degradation, or diffusion away from the synapse.

Divergence and Convergence

• Divergence: One neuron sends signals to multiple targets.

• Convergence: Multiple neurons send signals to a single target.

Synaptic Plasticity

• Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time, affecting learning and memory.

Postsynaptic Responses

• Fast Synaptic Potentials: Quick changes in postsynaptic membrane potential.

• Slow Synaptic Potentials: Longer-lasting changes due to second messenger systems.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic cell.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic cell.

Summation

• Spatial Summation: Multiple signals from different locations combine.

• Temporal Summation: Multiple signals from the same location in rapid succession combine.

Long-Term Potentiation and Depression

• Long-Term Potentiation (LTP): Persistent strengthening of synapses based on recent patterns of activity; important for learning and memory.

• Long-Term Depression (LTD): Persistent weakening of synapses; may be involved in forgetting or synaptic pruning.

Example: The process of learning a new skill involves LTP at synapses in relevant brain regions, while LTD may help remove unused connections.

Additional info: Some explanations and table entries have been expanded for clarity and completeness based on standard academic sources.