Signal Transduction Mechanisms: I Electrical and Synaptic Signaling in Neurons

Cells of the nervous system

• neurons

• Glial cells

neurons

send and receive electrical impulses (nerve impulses)

Glial cells

encompass a variety of cell types

What are the types of neurons

• sensory

• motor

• inter-

sensory neurons

diverse group of cells specialized for the detection of stimuli

motor neurons

transmit signals from the CNS to the muscles and glands with which they make connections (innervate)

interneurons

process signals and transmit information between parts of the nervous system

Types of Glial Cells

• microglia

• oligodendrites

• schwann

• astrocytes

Microglia

fight infections and remove debris

Oligodendrites + Schwann cells

form the myelin sheath around neaurons of the CNS and peripheral nerves

Astrocytes

control access of blood-borne components into the extracellular fluid around the nerve cells, this forms the blood-brain barrier

The cell body of a neuron is similar to that of…

other cells, it includes the nucelus and other endomembrane components

processes

branches that neurons contain

dendrites

processes that receive signals

axons

processes that conduct signals

axoplasm

cytosol within an axon

many vertebrate axons are surrounded by…

a discontinuous myelin sheath

The sheath insulates the segments of axon separating…

the nodes of Ranvier

nerve

a tissue composed of bundles of axons

synapse

junction between a nerve cell, gland, or muscle cell

A neuron + a synapse are…

a one-way street

synaptic terminal

of one axon, it passes information across the synapse in the form of chemical messengers

neurotransmitters

chemical messengers that travel through the synaptic terminal

postsynaptic cell

cell receiving the signal

Neuron function is based on…

the movement of ions across the cell membrane

What are the mechanisms where ions are moved across the membrane?

1. Ion pumps

2. ion channels

ion pumps

use energy (ATP) to move ions against their concentration gradients

ion channels

move ions along their concentration

• full function subject to activation

ion channels have an affect on what?

ion concentration, it also depends on the relative number of them as well

What do ion pumps and channels establish?

the resting potential of a neuron

membrane potential

voltage across its plasma membrane

resting potential

the membrane potential of a neuron not sending signals

changes in membrande potential at as

signals, they transmit and process information

At resting potential, the concentration of what is highest inside the cell?

K⁺ while the concentration of Na⁺ is highest outside the cell

Sodium-potassium pumps the the energy of ATP to…

maintain K⁺ and Na⁺ gradients across the plasma membrane

What do concentration gradients represent?

chemical potential energy

The opening of ion channels in the plasma membrane do what?

convert chemical potential to electrical potential

A neuron at resting potential contains many…

open K⁺ channels (leak channels), K⁺ diffuses out of the cell, Na⁺ mostly remain closed

What is major source of membrane potential

the resulting buildup of negative charge within the neuron

Loss of K⁺ ions leads to a

negative charge inside the cell

resting cells normally have what type of membrane potential?

negative membrane potential

What does the Na+/K+ pump do?

continually pumps sodium ions out of the cell to compensate for the small amount of leakage of sodium into the cell, potassium is carried inward at the same time

How many sodium are transported outward for how many potassium?

3 sodiums out for every 2 potassium in

K⁺ diffuse out, which…

makes the membrane potential more negative

Na+ flow into the cell, which..

drives the potential in the positive direction, which causes depolarization

Cl- tends to diffuse into the cell, but is….

repelled by the negative membrane potential, so it enters along with positive ions

Increased membrane permeability to Cl- does what?

decreases excitability

Increasing the membrane permeability to chloride was what effects that decrease neuronal excitability?

• net entry of chloride ions causes hyperpolarization

• when the membrane becomes permeable to sodium, some chloride will also enter

Almost all cells establish resting potentials that depend on…

ion gradients and ion permeability

An electrically excitable cell will respond to stimuli with an..

action potential

What is an action potential?

a rapid set of changes in membrane potential

During the action potential, the membrane potential changes from…

negative to positive and then back again in a very short time

Voltage-gated ion channels do what?

respond to changes in the voltage across a membrane

Voltage-gated Na+ and K+ channels are responsible for what?

they are responsible for the action potential

Ligand-gated ion channels open when a particular molecule…

when a particular molecule binds the channel

Voltage-gated sodium channels can open rapidly in response to…

stimulus and then close again; this is called channel gating

are channels partially open

no, open or closed state is all-or-none

voltage sensor

one alpha helix, S4, that responds to changes in potential

channel inactivation

a second form of closed state, cannot be reopened immediately

Depolarization that brings the membrane to the threshold potential…

initiates an action potential

An action potential is a brief but large…

electrical depolarization and repolarization of the neuronal plasma membrane

What is the action potential caused by?

inward movement of sodium and subsequence outward movement of potassium

Movement of sodium and potassium ions during the action potential is controlled by what?

the opening and closing of voltage-gated channels

propagation

the process where once action potential is initiated, it travels along the membrane AWAY from the origin

resting membrane potential

-60mV

membrane potential rises dramatically to

+40mV

Then it falls slowly to about (undershoot, or hyperpolarization)

-75mV

The action potential stabilizes again at

-60mV

Hodgkin cycle

positive feedback loop where the increased flow of Na+ through the channels → membrane depolarization → more Na channels open → more Na+ influx

Sub-threshold depolarization

when the membrane is depolarized by a small amount, the membrane potential recovers because of K+ movement through leak channes, no action potential occurs in this case

Depolarizing Phase

when the membrane is depolarized past the threshold potential, a significant number of Na+ channels begin activating which shoots membrane potential up rapidly

Repolarizing phase

once the membrane potential spikes, the membrane quickly repolarizes

What is repolarization due to?

inactivation of sodium channels and opening of voltage-gated potassium channels, the sodium channels remain closed until membrane potential is agin negative

Hyperppolarizing phase (undershoot)

At the end of an action potential, the membrane potential temporarily drops below the resting potential because of the increased K+ permeability

The refractory periods

for a few milliseconds after an action potential, it is impossible to trigger a second one

absolute refractory period

when sodium channels are inactivated and cannot open by depolarization

during undershoot, sodium channels can…

open again, but potassium channels too are open

The potassium leak channels and voltage-gated channels being open does what?

drives the membrane potential down, this is well below the threshold for triggering another action potential - relative refractory period

depolarization at one point on the membrane can spread to…

adjacent regions through passive spread of depolarization

As depolarization spreads away from the origin, it …

decreases in magnitude, so signals cannot travel far by this means

in order for action potential to go farther, what happens?

it must be propagated or actively generated along the membrane

incoming signals are. transmitted to a neuron at

points of contact (synapses)

incoming signals depolarize the dendrites, and the depolarization…

spreads passively over the membrane to the base of the axons, the axon hillcok - this is where action potentials are most easily generated

Propagation of an action potential in a nonyelinated nerve cell (1)

stimulation of a resting membrane results in depolarization and an inward rush of Na+

Propagation of an action potential in a nonyelinated nerve cell (2)

membrane polarity is temporarily reversed, and this spreads

Propagation of an action potential in a nonyelinated nerve cell (3)

nearby depolarization is above a threshold and results in an inward movement of Na+

Propagation of an action potential in a nonyelinated nerve cell (4)

The original region on the membrande becomes permeable to K+ ions, which rush out of the cell and return the membrane to its resting state

Propagation of an action potential in a nonyelinated nerve cell (5)

meanwhile, depolarization has spread farther, initiating the same sequence of events there

propagated action potential/nerve impulse

the propagation of action potential, it can move only away from the initial site

The myelin sheath acts like…

an electrical insulator surrounding the axon

what forms the myelin sheath?

concentric layers of membrane that surround many vertebrate axons

What is the myelin sheath formed by?

oligodendrites in the CNS and Schwann cells in the PNS

Myelination does what?

decreases the ability of the neuronal membrane to retain electrical charge

where is the action potential renewed?

nodes of ranvier

nerve impulses can spread farther and faster…

than in the absence of myelination

Nodes of ranvier are spaces closely enough to…

ensure the action potential at one node can trigger one in the next node

action potentials jump from one node to the next, this is…

saltatory propagation, which is more rapid than continuous propagation

electrical synapse

one neuron is connected to a second neuron via gap junctions

ions move through the junctions…

between the cells, there is no delay in transmission

chemical synapse

presynaptic and postsynaptic neurons that are not connected by gap junctions