Fundamentals of the Nervous System and Nervous Tissue: Key Concepts and Study Guide

Chapter 11 – Fundamentals of the Nervous System and Nervous Tissue

Definitions and Key Terms

This chapter introduces essential terminology for understanding the structure and function of the nervous system and nervous tissue. Mastery of these terms is foundational for further study in neuroanatomy and neurophysiology.

• Neuron: The basic functional unit of the nervous system, specialized for transmitting electrical impulses.

• Neuroglia (Glial cells): Supportive cells in the nervous system that provide structural and metabolic support to neurons.

• Action Potential: A rapid change in membrane potential that propagates along the axon of a neuron, enabling communication.

• Synapse: The junction between two neurons where information is transmitted via neurotransmitters.

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

• Peripheral Nervous System (PNS): Consists of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

• Autonomic Nervous System (ANS): A division of the PNS that controls involuntary bodily functions.

• Resting Membrane Potential: The electrical potential difference across the plasma membrane of a resting neuron, typically around -70 mV.

• Graded Potential: A change in membrane potential that varies in size and does not propagate far from the site of origin.

• Excitatory Postsynaptic Potential (EPSP): A depolarizing graded potential that brings the postsynaptic neuron closer to threshold.

• Inhibitory Postsynaptic Potential (IPSP): A hyperpolarizing graded potential that moves the postsynaptic neuron further from threshold.

• Myelin Sheath: An insulating layer around axons that increases the speed of impulse conduction.

• Saltatory Conduction: The rapid transmission of nerve impulses along myelinated axons, where the impulse jumps from node to node.

• Neurotransmitter: Chemical messengers released at synapses to transmit signals between neurons.

• Voltage-Gated Channel: Ion channels that open or close in response to changes in membrane potential.

• Ligand-Gated Channel: Ion channels that open in response to binding of a chemical messenger (ligand).

• Absolute Refractory Period: The time during which a neuron cannot fire another action potential, regardless of stimulus strength.

• Relative Refractory Period: The period following the absolute refractory period when a stronger-than-normal stimulus is required to initiate another action potential.

Major Topics and Learning Objectives

Nervous System Overview

The nervous system is responsible for rapid communication and control throughout the body. It is divided into central and peripheral components, each with specialized functions.

• Three Functions: Sensory input, integration, and motor output.

• Central Nervous System (CNS): Processes and integrates sensory information; initiates motor output.

• Peripheral Nervous System (PNS): Transmits sensory and motor signals between the CNS and the rest of the body.

• Divisions of the PNS:

  • Motor (Efferent) Division: Carries signals from the CNS to effectors (muscles and glands).

  • Sensory (Afferent) Division: Carries sensory information to the CNS.

  • Somatic Nervous System: Controls voluntary movements.

  • Autonomic Nervous System (ANS): Controls involuntary functions; subdivided into sympathetic and parasympathetic divisions.

Cells of the Nervous System

The nervous system contains two main types of cells: neurons and neuroglia. Neuroglia support and protect neurons, while neurons transmit electrical signals.

• Astrocytes: Star-shaped glial cells in the CNS; maintain the blood-brain barrier and regulate ion balance.

• Microglia: Phagocytic cells that remove debris and pathogens in the CNS.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

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

Neuron Structure

Neurons have specialized structures for receiving, processing, and transmitting information.

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Transmits electrical impulses away from the cell body.

• Axon Hillock: The site where action potentials are initiated.

• Myelin Sheath: Insulates axons and increases conduction speed.

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

• Classification: Neurons can be classified by structure (multipolar, bipolar, unipolar) and function (sensory, motor, interneuron).

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane when the cell is not actively transmitting a signal.

• Typical Value: $-70\ \text{mV}$

• Ion Distribution: Maintained by sodium-potassium pumps and leak channels.

• Key Equation: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for potassium)

• Factors Affecting Potential: Ion concentration gradients, membrane permeability, and active transport.

Graded Potentials

Graded potentials are local changes in membrane potential that vary in magnitude and decay with distance from the stimulus.

• Characteristics: Can be depolarizing or hyperpolarizing; summate to influence action potential initiation.

• Types: EPSPs (excitatory) and IPSPs (inhibitory).

• Stimulus Intensity: Stronger stimuli produce larger graded potentials.

Action Potentials

Action potentials are rapid, all-or-none electrical impulses that travel along axons, enabling long-distance communication.

• Phases: Depolarization, repolarization, and hyperpolarization.

• Key Channels: Voltage-gated sodium and potassium channels.

• Threshold: The membrane potential at which an action potential is triggered (typically around -55 mV).

• Propagation: Action potentials travel unidirectionally along axons.

• Refractory Periods: Absolute and relative refractory periods ensure one-way transmission and limit firing rate.

• Conduction Velocity: Increased by myelination and larger axon diameter.

• Saltatory Conduction: In myelinated axons, action potentials jump between nodes of Ranvier.

• Key Equation: $I = g(V - E)$ (Ohm's law for ionic currents)

Synapses

Synapses are specialized junctions where neurons communicate with other neurons or effector cells.

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

• Electrical Synapses: Allow direct passage of ions via gap junctions (less common in the nervous system).

• Process: Action potential arrives at axon terminal, triggers neurotransmitter release, which binds to receptors on the postsynaptic cell.

• Summation: EPSPs and IPSPs can summate spatially and temporally to influence postsynaptic firing.

Neurotransmitters

Neurotransmitters are chemicals that transmit signals across synapses. Their effects depend on the type of receptor they bind to.

• Types: Acetylcholine, dopamine, serotonin, GABA, glutamate, etc.

• Receptor Types: Channel-linked (ionotropic) and G protein-linked (metabotropic).

• Mechanism: Binding to receptors can open ion channels or activate second messenger pathways.

Neuronal Pathways and Circuits

Neurons are organized into circuits that process information and generate responses.

• Diverging Circuits: One neuron stimulates many others; amplifies signals.

• Converging Circuits: Many neurons converge on a single postsynaptic cell; integrates information.

• Reverberating Circuits: Feedback loops maintain activity; involved in rhythmic processes.

• Parallel After-Discharge Circuits: Multiple pathways converge on a single output; allows for complex processing.

Table: Comparison of CNS and PNS Neuroglia

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier
Microglia	CNS	Phagocytosis of debris and pathogens
Oligodendrocytes	CNS	Form myelin sheaths
Schwann Cells	PNS	Form myelin sheaths
Ependymal Cells	CNS	Produce cerebrospinal fluid

Additional info:

• Some definitions and objectives were inferred and expanded for clarity and completeness.

• Equations and examples were added to provide academic context and support exam preparation.