Chapter 48 | Neurons, Synapses, and Signaling

The Nervous System

Information processing

• CNS: Integrates information

• PNS: Transmits information

• Basic unit: Neuron

Information processing

1. Sensory input (external/internal) via sensory neurons (PNS)

2. Integration of information in the CNS (via interneurons)

3. Response (external/internal) via motor neurons (PNS)

Nerves

Neurons bundled at axons

Ganglia

Neurons clustered at their cell bodies

Neuron

Consists of the cell body, dendrites, axon, axon terminal, synapse, & glia

Cell body

Contains majority of organelles, nucleus, & cytoplasm

Dendrites

Extensions of the cell body; functions to receive information

Axon

Large extension from the cell body, transmits information in the form of electrical signals from dendrites/cell body to the axon terminal

Axon terminal

Branched end of axon transmitting information to the next cell (e.g. neuron, gland, organ, muscle)

Synapse

Where information is transmitted through the end of one cell to the beginning of another cell

Glia

Support cells of the nervous system

Neuron transmission

• Between neurons: signal is chemical (via neurotransmitters)

• Within a neuron: signal is electrical (via ion movement)

Resting state

• Na⁺ high outside, K⁺ high inside the neuron

• Maintained by Na⁺/K⁺ pump

• Creates charge gradient:
  • Outside = more positive, inside = more negative
  • Resting membrane potential = -70 mV

• All channels are closed

Electrochemical gradient

A gradient of electrochemical potential, usually for an ion that can move across a membrane

Membrane potential

Measure of the charge gradient across the plasma membrane

Resting potential

The electrical potential difference (voltage) across the cell membrane of a non-excitable cell when it is not stimulated or actively transmitting signals

Na⁺/ K⁺ pump

• Transmembrane protein that facilitates the active transport of Na⁺ and K⁺ ions across the cell membrane

• Pumps 3 Na⁺ out, 2 K⁺ in

Depolarization

• Presynaptic cell releases neurotransmitters via exocytosis

• Neurotransmitter diffuses across synapse, binds to receptors on receiving neuron

• Excitatory receptors = ligand-gated Na⁺ channels:
  • Neurotransmitter = ligand
  • Binding opens channel → Na⁺ floods in

Neurotransmitter

A chemical messenger that transmits signals between neurons. Made from amino acids or peptides

Channel proteins

Transmembrane proteins that facilitate the transport of specific ions or molecules across the cell membrane

Rising phase of the action potential

• If threshold is reached → voltage-gated Na⁺ channels open

• Na⁺ floods in → causes depolarization

• Depolarization triggers nearby Na⁺ channels to open

• Action potential spreads down neuron = signal transmission

• This is the "firing" of the neuron

Threshold potential

The critical membrane voltage that a neuron must reach to trigger an action potential (where the membrane is less negative enough)

Voltage-gated channels

Channels that open when there is a change in charge

Falling phase of the membrane potential

• Na⁺ channels close

• K⁺ channels open

• K⁺ exits the cell → inside becomes negative again; Repolarization

Repolarization of the membrane

The process where a cell membrane's electrical charge returns to its resting state after a period of depolarization

Undershoot / Hyperpolarization

Too many K⁺ leaving the cell, causing the membrane potential to become more negative than the resting potential

Falling phase → Resting potential

Na⁺ & K⁺ gradient is re-established using the Na⁺/K⁺ pump

Refractory period

The time taken to re-establish resting potential

At axon terminal

• Action potential opens voltage-gated Ca²⁺ channels

• Ca²⁺ enters the neuron, binds to vesicles with neurotransmitters

• Triggers neurotransmitter release into the synapse

• Signal continues to the next neuron

Excitatory Neurotransmitters

• Binds to ligand-gated channels on the post-synaptic neuron, causing depolarization and moves the membrane closer to the threshold potential

• Creates an Excitatory Post-Synaptic Potential (EPSP)

**Note: No threshold potential = no action potential (even if there is EPSP)

Inhibitory Neurotransmitters

• Binds to ligand-gated channels on the postsynaptic neuron, causing hyperpolarization by opening Cl^- channels and inhibiting firing of the neuron

• Creates an Inhibitory Post-Synaptic Potential (IPSP)

Summation

Sum of all EPSPs & IPSPs that either reach threshold or not

Temporal summation

All EPSPs / IPSPs either reaching threshold or not in a period of time

Spatial summation

All EPSPs / IPSPs in a certain location in close proximity

After the neuron fires / is inhibited

• Enzymes break down neurotransmitters, chemically changing them so they can no longer bind to receptors

OR

• Neurotransmitters are taken back into the pre-synaptic neuron via passive transporters or reuptake channels

Myelin sheath

Wrapped around the axon of a neuron, an insulating coat of cell membranes from Schwann cells

Nodes of Ranvier

Exposed areas of the axon that are not myelinated, where action potentials are generated

Saltatory Conduction

An action potential is regenerated at each node [of Ranvier], appearing to jump along the axon from node to node