Fundamentals of the Nervous System and Nervous Tissue – Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Overview

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts. This chapter focuses on the cellular and physiological foundations of the nervous system, emphasizing the structure and function of neurons, the generation and propagation of electrical signals, and synaptic communication.

Chapter Objectives

• Identify the different regions of the neuron and associate each region with the functions of reception, propagation, and transmission of nerve impulses.

• Explain the phenomena (diffusion of ions, types of ion channels) responsible for the electrical activity of neurons, including resting membrane potential, graded potential, and action potential.

• Describe the factors that influence propagation of the action potential along an axon.

• Explain the mechanisms of synaptic transmission (synapses, postsynaptic potentials, synaptic integration).

Organization of the Nervous System

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Consists of all neural tissue outside the CNS; subdivided into sensory (afferent) and motor (efferent) divisions.

• Motor Division: Further divided into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, and glands).

• Autonomic Nervous System: Includes sympathetic (fight or flight) and parasympathetic (rest and digest) branches.

Neurons: Structure and Function

Regions of the Neuron

• Dendrites: Receive incoming signals from other neurons; function as the main receptive or input region.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals and is the metabolic center of the neuron.

• Axon: Conducts electrical impulses (action potentials) away from the cell body toward other neurons or effectors; the axon hillock is the trigger zone for action potential initiation.

• Axon Terminals: Release neurotransmitters to communicate with target cells.

Example: Sensory neurons have long dendrites to receive signals from sensory receptors, while motor neurons have long axons to transmit signals to muscles.

Special Characteristics of Neurons

• Excitability: Ability to respond to stimuli and generate electrical signals.

• Longevity: Neurons can last a lifetime.

• Amitotic: Most neurons do not divide after differentiation (exceptions exist).

• High Metabolic Rate: Require continuous supply of oxygen and glucose.

Neuroglia (Glial Cells)

• Astrocytes: Provide structural and metabolic support, regulate extracellular ion balance, and participate in repair and neurovascular coupling.

• Oligodendrocytes: Myelinate axons in the CNS, increasing conduction speed.

• Schwann Cells: Myelinate axons in the PNS.

• Microglia: Act as immune cells in the CNS.

• Ependymal Cells: Line brain ventricles and produce cerebrospinal fluid (CSF).

Electrical Properties of Neurons

Resting Membrane Potential

• Definition: The voltage difference across the plasma membrane of a resting neuron, typically around -70 mV.

• Establishment: Maintained by the Na+/K+ ATPase pump and differential permeability of the membrane to Na+ and K+ ions.

• Key Equation (Nernst Equation):

$E_{ion} = \frac{61}{z} \log \left(\frac{[\text{ion}]_{out}}{[\text{ion}]_{in}}\right)$

• Where $E_{ion}$ is the equilibrium potential for the ion, $z$ is the valence, and $[\text{ion}]_{out}$ and $[\text{ion}]_{in}$ are the extracellular and intracellular concentrations, respectively.

Ion Channels

• Leak Channels: Always open; contribute to resting membrane potential.

• Gated Channels: Open or close in response to specific stimuli (chemical, voltage, or mechanical).

Graded Potentials

• Definition: Localized changes in membrane potential that vary in size and decay with distance.

• Types: Depolarization (membrane potential becomes less negative) and hyperpolarization (more negative).

• Function: Serve as short-distance signals; can summate to trigger action potentials.

Action Potentials

• Definition: Rapid, large, and uniform changes in membrane potential that propagate along axons; the principal means of long-distance neural communication.

• All-or-None Principle: Action potentials occur fully or not at all once threshold is reached (typically around -55 mV).

• Phases: Depolarization (Na+ influx), repolarization (K+ efflux), and hyperpolarization.

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

Propagation of Action Potentials

• Unidirectional: Due to refractory periods, APs travel in one direction along the axon.

• Speed Factors: Increased by larger axon diameter and myelination (saltatory conduction).

Synaptic Transmission

• Synapse: Junction between two neurons or a neuron and an effector cell.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Mechanism: Action potential triggers neurotransmitter release, which binds to receptors on the postsynaptic membrane, generating postsynaptic potentials (excitatory or inhibitory).

• Synaptic Integration: The process by which multiple synaptic potentials combine within one postsynaptic neuron.

Summary Table: Key Differences Between Graded and Action Potentials

Feature	Graded Potential	Action Potential
Amplitude	Varies with stimulus strength	All-or-none, fixed size
Propagation	Decremental (decays with distance)	Non-decremental (self-propagating)
Location	Dendrites, cell body	Axon
Function	Short-distance signaling	Long-distance signaling

Key Terms

• Neuron: Functional unit of the nervous system.

• Action Potential: Rapid, all-or-none electrical signal.

• Resting Membrane Potential: Baseline electrical charge difference across the membrane.

• Synapse: Site of communication between neurons.

• Neurotransmitter: Chemical messenger released at synapses.