Fundamentals of the Nervous System and Nervous Tissue II

Propagation of an Action Potential

The propagation of an action potential (AP) is the process by which the electrical signal travels along the length of a neuron. This ensures rapid communication within the nervous system.

• Propagation allows APs to be transmitted from the origin down the entire axon length toward terminals.

• Na+ influx through voltage-gated channels in one membrane area causes local currents that trigger opening of Na+ channels in adjacent membrane areas.

• Leads to depolarization of that new area, which in turn causes depolarization in the next area.

• Once initiated, an AP is self-propagating.

• In nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes.

• Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there; AP occurs only in a forward direction.

Coding for Stimulus Intensity

The nervous system encodes the strength of a stimulus by the frequency of action potentials, not by their amplitude.

• All action potentials are alike and independent of stimulus intensity.

• The central nervous system (CNS) distinguishes between weak and strong stimuli by the frequency of impulses.

• Frequency is the number of impulses (APs) received per second.

• Higher frequencies indicate stronger stimuli.

Refractory Periods

Refractory periods are times during which a neuron cannot trigger another action potential, ensuring one-way transmission and proper signal separation.

• Absolute refractory period: Time from opening of Na+ channels until resetting of the channels. Ensures each AP is an all-or-none event and enforces one-way transmission.

• Relative refractory period: Follows the absolute period. Most Na+ channels have returned to their resting state, some K+ channels are still open, and repolarization is occurring. Threshold for AP generation is elevated; only exceptionally strong stimuli can stimulate an AP.

Conduction Velocity

The speed at which an action potential travels along an axon depends on axon diameter and degree of myelination.

• APs occur only in axons, not other cell areas.

• Axon diameter: Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction.

• Degree of myelination: Two types of conduction depending on presence or absence of myelin.

  • Continuous conduction: Slow conduction that occurs in nonmyelinated axons.

  • Saltatory conduction: Occurs only in myelinated axons and is about 30 times faster. Myelin sheaths insulate and prevent leakage of charge. APs are generated only at gaps (nodes of Ranvier). Electrical signal appears to jump rapidly from gap to gap.

Propagation in Nonmyelinated vs. Myelinated Axons

• Nonmyelinated axons: Continuous conduction; slow.

• Myelinated axons: Saltatory conduction; fast, APs generated only at nodes of Ranvier.

Multiple Sclerosis (MS)

Multiple sclerosis is an autoimmune disease that affects primarily young adults, characterized by the destruction of myelin sheaths in the CNS.

• Myelin sheaths in CNS are destroyed when immune system attacks myelin.

• Symptoms: Visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence.

• Treatment: Drugs that modify immune system activity; maintaining high blood levels of vitamin D may reduce risk of development.

Classification of Nerve Fibers by Conduction Velocity

Nerve fibers are classified according to diameter, degree of myelination, and speed of conduction.

Chemical Synapse

Structure and Function

Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as muscles or glands.

• Most common type of synapse.

• Composed of two parts:

  • Axon terminal of presynaptic neuron: Contains synaptic vesicles filled with neurotransmitter.

  • Receptor region on postsynaptic neuron: Receives neurotransmitter, usually on dendrite or cell body.

• Synaptic cleft: Fluid-filled space separating the two parts.

• Electrical impulse changed to chemical across synapse, then back into electrical.

Information Transfer Across Synapses

• AP arrives at axon terminal of presynaptic neuron.

• Ca2+ channels open, and Ca2+ enters axon terminal.

• Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis.

• Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.

• Binding of neurotransmitter opens ion channels, creating graded potentials.

• Neurotransmitter effects are terminated by reuptake, degradation, or diffusion away from the synapse.

Postsynaptic Potentials

Types and Effects

Neurotransmitter receptors cause graded potentials that vary in strength based on the amount and time of neurotransmitter binding.

• Excitatory postsynaptic potentials (EPSPs): Depolarizing, bring the postsynaptic cell closer to threshold for firing an AP.

• Inhibitory postsynaptic potentials (IPSPs): Hyperpolarizing, move the postsynaptic cell further from threshold, making an AP less likely.

Summation and Modification of Synaptic Events

• Summation by the postsynaptic neuron can be temporal (rapid firing of impulses) or spatial (multiple inputs at once).

• EPSPs and IPSPs can summate to influence whether an AP will be generated.

Neurotransmitters

Overview

Neurotransmitters are the chemical messengers of the nervous system, responsible for transmitting signals across synapses.

• Over 50 neurotransmitters have been identified.

• Most neurons make two or more neurotransmitters.

• Classified by chemical structure and function.

Neurotransmitters Classified by Structure

Neurotransmitters Classified by Function

• Effects: Excitatory (depolarizing) or inhibitory (hyperpolarizing).

• Action: Direct (bind to and open ion channels) or indirect (act through second messengers).

• Some neurotransmitters have different effects depending on receptor type.

Summary Table: Neurotransmitter Effects

Additional info: Neurotransmitters may also act as paracrine agents, affecting only local cells.