Nervous System & Nervous Tissue

Introduction

The nervous system is a complex network responsible for coordinating body activities through electrical and chemical signaling. Nervous tissue, composed of neurons and supporting cells, is fundamental to the system's function. This guide focuses on the mechanisms of graded potentials, action potentials, and synaptic transmission, which are essential for neural communication.

Graded Potentials vs. Action Potentials

Definitions and Locations

• Graded Potentials (GP): Local changes in membrane potential that occur in the cell body and dendrites. They vary in size and decay with distance.

• Action Potentials (AP): Rapid, uniform electrical signals generated at the axon initial segment and propagated along the axon. They are always the same size and do not decay with distance.

Comparison Table

Summation and Signal Integration

• Temporal Summation: Increased frequency of stimuli from a single source can add together to increase the amplitude of graded potentials.

• Spatial Summation: Stimuli from multiple sources can combine to produce a larger graded potential.

• Action Potentials: Do not summate; they are all-or-none events.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Short-distance signaling; depolarization that spreads to axon hillock, moving membrane potential toward threshold for generating an AP.

• Inhibitory Postsynaptic Potential (IPSP): Short-distance signaling; hyperpolarization that spreads to axon hillock, moving membrane potential away from threshold.

Mechanisms of Graded and Action Potentials

Ion Channel Activity

• Graded Potentials: Open chemically gated channels (e.g., Na+, K+, Cl-), allowing simultaneous ion fluxes.

• Action Potentials: Open voltage-gated channels, first Na+ channels (depolarization), then K+ channels (repolarization).

Membrane Potential Changes

• Depolarization: Membrane potential moves toward 0 mV (more positive).

• Hyperpolarization: Membrane potential moves away from 0 mV (more negative).

• Action Potential: Rapid change from approximately -70 mV to +30 to +50 mV, then back to resting potential.

Conduction in Myelinated vs. Unmyelinated Fibers

Types of Conduction

• Continuous Conduction: Occurs in unmyelinated fibers; the action potential propagates along every part of the axon membrane, resulting in slower conduction.

• Saltatory Conduction: Occurs in myelinated fibers; the action potential jumps from one node of Ranvier to the next, greatly increasing conduction velocity.

Nerve Fiber Classifications

Synaptic Transmission

Types of Synapses

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

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; slower but more versatile.

Neurotransmitters

• Definition: Chemical messengers released by neurons that bind to receptors on other neurons or effector cells, stimulating or inhibiting their activity.

• Classification:

  • By chemical structure (e.g., acetylcholine, biogenic amines, amino acids, peptides)

  • By function (excitatory or inhibitory)

• Examples:

  • Acetylcholine (ACh): Excitatory in skeletal muscle, inhibitory in cardiac muscle

  • Norepinephrine (NE): Excitatory or inhibitory depending on receptor

  • ATP: Can be excitatory or inhibitory

  • GABA: Mainly inhibitory

  • Glycine: Mainly inhibitory

Neural Processing: Serial vs. Parallel

Serial Processing

• Information travels along one pathway to a specific destination.

• Each neuron stimulates the next in sequence; produces specific, anticipated responses.

• Example: Spinal reflex arc.

Parallel Processing

• Information is processed simultaneously along several pathways.

• Allows for higher-level mental functioning and complex responses.

• Example: Smelling apple pie may trigger memories and multiple sensory experiences.

Key Equations

• Resting Membrane Potential (RMP):

• Threshold for Action Potential:

Summary Table: Graded vs. Action Potentials

Additional info: These notes expand on the provided slides and images, adding definitions, examples, and context for key concepts in nervous tissue physiology.