Fundamentals of the Nervous System and Nervous Tissue – Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Overview

The nervous system is responsible for the rapid transmission of information throughout the body, allowing for coordination of actions, sensation, and cognition. This chapter focuses on the structure and function of neurons, the generation and propagation of electrical signals, and the mechanisms of synaptic transmission.

Major Topics

• Structure and function of neurons

• Electrical properties of neurons (membrane potentials)

• Propagation of action potentials

• Synaptic transmission and integration

Neurons: Structure and Function

Regions of the Neuron

Neurons are the fundamental units of the nervous system, specialized for the reception, propagation, and transmission of nerve impulses.

• Dendrites: Receive incoming signals from other neurons or sensory receptors. They are the primary input region of the neuron.

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

• Axon Hillock: The cone-shaped region where the axon originates; serves as the trigger zone for action potentials.

• Axon: Conducts action potentials away from the cell body toward axon terminals; the main propagation region.

• Axon Terminals: Release neurotransmitters to communicate with target cells (other neurons, muscles, or glands).

Example: Sensory neurons receive information via dendrites, integrate it in the soma, and transmit signals along the axon to the central nervous system.

Special Characteristics of Neurons

• Excitability: Ability to generate and conduct electrical impulses.

• Longevity: Most neurons last a lifetime.

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

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

Electrical Activity of Neurons

Resting Membrane Potential

The resting membrane potential is the voltage difference across the plasma membrane of a resting neuron, typically around -70 mV. It is established by differences in ion concentrations and membrane permeability, primarily to potassium (K+) and sodium (Na+).

• Na+/K+ ATPase Pump: Maintains high K+ inside and high Na+ outside the cell by actively transporting 3 Na+ out and 2 K+ in, using ATP.

• Leak Channels: More K+ leak channels than Na+, making the membrane more permeable to K+.

• Equilibrium Potential: The voltage at which the net flow of a particular ion is zero. Calculated using the Nernst equation:

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

• Resting membrane potential is closest to the equilibrium potential of K+ due to higher permeability.

Types of Ion Channels

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

• Ligand-Gated (Chemically Gated) Channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

• Voltage-Gated Channels: Open or close in response to changes in membrane potential; essential for action potentials.

• Mechanically Gated Channels: Open in response to physical deformation (e.g., touch receptors).

Graded Potentials

Graded potentials are localized changes in membrane potential that vary in magnitude and decay with distance. They occur in dendrites and cell bodies in response to stimuli (e.g., neurotransmitter binding).

• Depolarization: Membrane potential becomes less negative (closer to zero).

• Hyperpolarization: Membrane potential becomes more negative.

• Summation: Graded potentials can add together to reach threshold for action potential initiation.

Action Potentials

Action potentials are rapid, large changes in membrane potential that travel along axons without decreasing in strength. They are the primary means of long-distance neural communication.

• Threshold: The critical level (~-55 mV) that must be reached to trigger an action potential.

• All-or-None Principle: Once threshold is reached, the action potential is always the same size.

• Phases:

  • Depolarization: Voltage-gated Na+ channels open, Na+ influx.

  • Repolarization: Voltage-gated K+ channels open, K+ efflux.

  • Hyperpolarization: K+ channels remain open briefly after repolarization.

• Refractory Periods:

  • Absolute: No new action potential can be generated.

  • Relative: A stronger-than-normal stimulus can initiate another action potential.

Propagation of Action Potentials

Action potentials propagate along the axon by depolarizing adjacent regions of the membrane. Propagation is typically unidirectional due to the refractory period.

• Factors Affecting Speed:

  • Axon diameter: Larger diameter = faster conduction.

  • Myelination: Myelinated axons conduct faster via saltatory conduction (jumping between nodes of Ranvier).

Synaptic Transmission

Mechanisms of Synaptic Transmission

Synapses are specialized junctions where neurons communicate with other neurons, muscles, or glands.

• Presynaptic Neuron: Sends the signal via neurotransmitter release.

• Postsynaptic Neuron: Receives the signal via neurotransmitter receptors.

• Synaptic Cleft: The small gap between pre- and postsynaptic membranes.

• Process:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ influx.

  3. Neurotransmitter vesicles fuse with membrane; neurotransmitter released into synaptic cleft.

  4. Neurotransmitter binds to receptors on postsynaptic membrane, causing graded potentials (excitatory or inhibitory).

Synaptic Integration

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the likelihood of action potential generation.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing the likelihood of action potential generation.

• Summation: EPSPs and IPSPs can summate temporally (over time) or spatially (from multiple synapses) to influence action potential initiation.

Glial Cells

Types and Functions

• Astrocytes: Support neurons, regulate extracellular ion balance, participate in neurotransmitter uptake, and contribute to the blood-brain barrier.

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

• Schwann Cells: Myelinate axons in the PNS.

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

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

Summary Table: Key Differences in Nervous System Divisions

Division	Main Components	Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration and processing of information
Peripheral Nervous System (PNS)	Nerves, Ganglia	Communication between CNS and body
Somatic Nervous System	Somatic motor neurons	Voluntary control of skeletal muscles
Autonomic Nervous System	Sympathetic and Parasympathetic divisions	Involuntary control of smooth muscle, cardiac muscle, and glands

Additional info: These notes are based on the objectives and introductory content for a college-level Anatomy & Physiology course, focusing on the fundamentals of the nervous system and nervous tissue. For more detailed mechanisms and clinical applications, refer to the recommended textbook chapters.