Anatomy and Physiology I Electrical Signals in Neurons Action Potentials The Synapse Instructor:...

Anatomy and Physiology I

Electrical Signals in Neurons

Action Potentials

The Synapse

Instructor: Mary Holman

Electrical Signals in Neurons

• Two basic features of cell membranes of neurons and other excitable cells

Membrane potential

Ion channels

• Allow neurons to communicate using electrical signals

Action potentials

Fig. 10.14c

+

+

––

+

+

––

+

+

–+–

–+–+

–

+–

+–

+ –

+–

+–

+–

+–

–70 mV

Low Na+

Low K+ High K+

High Na+

Na+

K+Pump

The Membrane Potential of the Resting Neuron

-70 millivolts

1 mV= 0.001Volts

Impermeantanions

TriggerZone

Membrane Potential = unequal distribution of + and - ionson either side of a cell membrane

Resting Membrane Potential of the Neuron

From: Principles of Anatomy & Physiology by Tortora & Grabowski

Ion Channels

Gated channels that open and close in response to a stimulus

1. Voltage-gated ion channels as in action potentials

2. Ligand-gated ion channels as in neurotransmitters

3. Mechanically-gated ion channel

Fig. 10.13

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Gate-like Mechanism of Ion Channels

Protein

(b) Channel open(a) Channel closed

Cellmembrane

Fatty acidtailPhosphate head

Stimulus-action potential-chemical-mechanical

ions

Membrane potentialchanges due to move-ment of ions when channels are open

• A cell that has a membrane potential is said to be polarized

• When the ion flow due to the opening of an ion channel produces a more negative membrane potential it is called

hyperpolarized

• If the membrane potential becomes more positive, it is called depolarized

Membrane Polarization

Action Potential• If the membrane is depolarized to

~ -55mV it is said to have reached threshold potential

• When a threshold potential is reached, a nerve impulse or action

potential is generated

Fig. 10.15a


–62 mV

Na+

Na+

Neurotransmitter

Ligand-gatedNa+ channel

Presynapticneuron

Sub-threshold DepolarizationDoes not Result in an Action Potential

Trigger Zone

Fig. 10.15b


–55 mV

Na+Na+

Na+Na+

Na+

Trigger zone

Voltage-gatedNa+ channel

An Action Potential is Generated WhenDepolarization Reaches ~55mV

Ligand-gatedNa+ channel

Fig. 10.16

(a)

Region of depolarization(b)

Region of repolarization

–70

–0

–70

–0

–70

–0K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+


K+ K+ K+ K+ K+

K+

K+

K+ K+

K+ K+

Na+ Na+ Na+

Na+ Na+ Na+

Thresholdstimulus



K+ K+ K+ K+ K+

K+ K+ K+ K+ K+

Na+ Na+ Na+

Na+ Na+ Na+

K+

K+

K+ K+ K+

K+ K+ K+

Na+ channels openK+ channels closed

K+ channels openNa+ channels closed

ActionPotential& NerveMembrane

An Action Potential = Nerve Impulse

• All-or-none response

all impulses are the same strength

more stimuli leads to more impulses

• Refractory Period Absolute vs Relative

period of time between impulses when membrane is unresponsive to

stimuli ~1--30 milliseconds

Fig. 10.17

Milliseconds

10

0

+20

+40

2 3 4 5 6 7 8

Mem

bra

ne

po

ten

tial

(m

illi

volt

s)Action potential

Hyperpolarization

–40

–20

–60

–80

Restingpotential

Resting potentialreestablished


Oscilloscope Recording of an Action Potential

Fig. 10.18


Direction of nerve impulse

+ +

+ +

+

– – – – – – – – –

– – – – –– – – –

– – – – –– – – –

– – – – – – – – –

– – – – – – – – –

– – – – – – – – –

+ + + + + + + +

+ + + + + + + + +

+ +

+ +

++ + + + + + + +

++ + + + + + + +

+ +

+ +

++ + + ++ + + +

++ + + ++ + + +

Region ofaction potential

A Nerve Impulse = moving Zone of Depolarization

• Myelinated nerves conduct action potentials from node to node

• This mode of transmission is called saltatory conduction

Impulse Conduction

Fig. 10.19


+ +

+ ++++

+ +

+ +

+ +

+ +

+ +

+ +

Electric current Nodes Axon

Schwann cells(a)

+++++

++

+ +

+ +

+ +

+ +

+ +

+ +

(b)

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ ++++

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

– –– –

Action potential

Action potential

Action potential

A Nerve Impulse Moving along a Myelinated Axon

The Synapse

• Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron.

• Neurotransmitters are released when the impulse reaches a synaptic knob

Fig. 10.11


Dendrites

Impulse

Impulse

Impulse

Synapticcleft

Axon ofpresynapticneuron

Cell body of postsynaptic neuron

Axon of postsynaptic neuron

Axon ofpresynapticneuron

Synapses ofThree Neurons

Events at the Synaptic Cleft

• Action potential races along axon to the terminal knobs or varicosities.• The change in polarization of the membrane of the knob opens voltage sensitive Ca++ gates & Ca++ floods in.• The increased [Ca++] stimulates vesicles to move to the cell membrane and release neurotransmitters via exocytosis.• Neurotransmitter molecules bind to receptors on post-

synaptic neuron membrane which opens ligand-gated ion channels. Depending on which ion channels are opened, membrane is hyperpolarized or depolarized.

Fig. 10.12a


Mitochondrion

Synaptic knob

Synaptic cleftNeurotransmitter

Axon

Ca+2

Presynaptic neuron

Direction ofnerve impulse

Synapticvesicles

Cell body or dendriteof postsynaptic neuron

Synapticvesicle

Vesicle releasingneurotransmitter

Axonmembrane

Polarizedmembrane

Depolarizedmembrane

Ca+2Ca+2

Activity at a Synaptic Cleft

Fig. 10.12b


Mitochondrion

Synapticvesicle

Synapticcleft

Postsynapticmembrane

© Don W. Fawcett/Photo Researchers, Inc.

A Synaptic Cleft

TEM 37,500x

Neurotransmitters

• Released from vesicles in terminal knobs of presynaptic axons

• React with receptors on membrane of postsynaptic neuron

• Many different molecules act as neuro-transmitters

• Many serious diseases result from

imbalances of neurotransmitters

• Can cause excitatory (EPSP) or inhibitory (IPSP) post-synaptic membrane potentials by opening or closing various ion channels

• Usually the net effect the EPSPs and IPSPs is determined in the region of the trigger zone of the neuron

Actions of Neurotransmitters

Fig. 10.20


Nucleus

Neuroncell body

Presynapticknob

Presynapticaxon

Neurons Receive Multiple Signals Simultaneously

Impulse Processing• Neuron pools in CNS interpret

impulses from many neurons

• Convergence

Multiple axons delivering impulses to the same neuron

• Divergence

An axon that branches and delivers impulses to several neurons

Fig. 10.21a


1 2

3

Impulse Convergence

Fig. 10.21b


4

5

6

Impulse Divergence

Fig. 10A


Ion channel

Nicotine

Outsidenerve cell

Membranelipid bilayer

Insidenerve cell

Addictions

• Some addicting compounds bindto sites that bindintrinsic pain re-lievers (endorphins)

• Others alter bindingof neurotransmittersacting either as anagonist or antagonist

Anatomy and Physiology I Electrical Signals in Neurons Action Potentials The Synapse Instructor:...

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