Essential idea: Neurons transmit the message, synapses modulate the message.

6.5 Neurons and synapses

By Chris Paine

https://bioknowledgy.weebly.com/

The image shows a tiny segment of a human brain the lines show neurons and the dots show synapses. The image is intended to illustrate both the how complex even a small mammal's brain is and additionally how important the synapses between neurons are; it is the synapses that drive communication and conscious thought. With the exception of the memory centre the number of cells in the human brain does not increase after birth, what increase is the number of connections and hence synapses between neurons.

http://med.stanford.edu/mcp/_jcr_content/hero/hero_banner/images/imageSlide8.img.620.high.jpg

UnderstandingsStatement Guidance

6.5.U1 Neurons transmit electrical impulses.The details of structure of different types of

neuron are not needed.

6.5.U2 The myelination of nerve fibres allows for

saltatory conduction.

6.5.U3

Neurons pump sodium and potassium ions

across their membranes to generate a resting

potential.

6.5.U4 An action potential consists of depolarization

and repolarization of the neuron.

6.5.U5 Nerve impulses are action potentials propagated

along the axons of neurons.

6.5.U6

Propagation of nerve impulses is the result of

local currents that cause each successive part of

the axon to reach the threshold potential.

6.5.U7 Synapses are junctions between neurons and

between neurons and receptor or effector cells.

Only chemical synapses are required, not

electrical, and they can simply be referred to

as synapses.

6.5.U8 When presynaptic neurons are depolarized they

release a neurotransmitter into the synapse.

6.5.U9 A nerve impulse is only initiated if the threshold

potential is reached.

Applications and SkillsStatement Guidance

6.5.A1 Secretion and reabsorption of acetylcholine by

neurons at synapses.

6.5.A2

Blocking of synaptic transmission at cholinergic

synapses in insects by binding of neonicotinoid

pesticides to acetylcholine receptors.

6.5.S1 Analysis of oscilloscope traces showing resting

potentials and action potentials.

6.5.U1 Neurons transmit electrical impulses.

Schwan cell

6.5.U5 Nerve impulses are action potentials propagated along the axons of neurons.

http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf

6.5.S1 Analysis of oscilloscope traces showing resting potentials and action potentials.

https://phet.colorado.edu/en/simulation/neuron

Use the PhET simulation to build an understanding of resting and action potentials and how they relate to the voltage changes in the axon membrane.

The neuron lab worksheet activity acts as a guide for the investigation:https://phet.colorado.edu/en/contributions/view/3608

Investigate how neurons generate electrical impulses

6.5.U3 Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

Plasma membrane is 50 times more permeable to K+ ions than Na+

n.b. proteins inside the nerve fiber are negatively charged which increases the charge imbalance.

-70mV

6.5.U4 An action potential consists of depolarization and repolarization of the neuron.

The sodium-potassium pump (Na+/K+ pump)maintains the electrochemical gradient of the resting potential. Some K+ leaks out of the neuron (making the membrane potential negative, -70mv).

1 In response to a stimulus (e.g. change in membrane potential) in an adjacent section of the neuron some voltage gated Na+ channels open and sodium enters the neuron by diffusion. If a sufficient change in membrane potential is achieved (threshold potential) all the voltage gated Na+ channels open. The entry of Na+ causes the membrane potential to become positive (depolarisation)

http://www.ib.bioninja.com.au/_Media/action_potential_med.jpeg

2

Action potential is the reversal (depolarization) and restoration (repolarization) of the membrane potential as an impulse travels along it.

Action potential is the reversal (depolarization) and restoration (repolarization) of the membrane potential as an impulse travels along it.

6.5.U4 An action potential consists of depolarization and repolarization of the neuron.

The depolarisation of the membrane potential causes the voltage gated Na+ channels to close and the voltage gated K+ channels open. K+ diffuses out of the neuron rapidly and the membrane potential becomes negative again (repolarisation)

3

http://www.ib.bioninja.com.au/_Media/action_potential_med.jpeg

4 Before the neuron is ready to propagate another impulse the distribution of Na+ (out) and K+ (in) needs to be reset by the Na+/K+ pump, returning the neuron to resting potential. This enforced rest (refractory period) ensures impulses can only travel in a single direction.

Action potential is the reversal (depolarization) and restoration (repolarization) of the membrane potential as an impulse travels along it.

6.5.U4 An action potential consists of depolarization and repolarization of the neuron.

http://www.ib.bioninja.com.au/_Media/action_potential_med.jpeg

From McGraw Hill: http://goo.gl/tI2MD

http://www.wiley.com/WileyCDA/

http://highered.mheducation.com/olc/dl/120107/bio_d.swf

6.5.U6 Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to

reach the threshold potential.

Propagation of a nerve impulse in un-myelinated axons

http://cnx.org/resources/0d4d8e978090c5adf07cc1661372b69be3496ec6/Figure_35_02_04.png

Cell body

6.5.U4 An action potential consists of depolarization and repolarization of the neuron.

http://www.psych.ualberta.ca/~ITL/ap/ap.htm

http://www.mrothery.co.uk/images/nerveimpulse.swf

More action potential resources:

http://www.sumanasinc.com/webcontent/animations/content/actionpotential.html

http://highered.mheducation.com/olc/dl/120107/anim0013.swf

6.5.U2 The myelination of nerve fibres allows for saltatory conduction.

http://cnx.org/resources/1a264d4943c1148665b7216c649d72ad774fc80b/Figure_35_02_05.jpg

http://antranik.org/wp-content/uploads/2012/04/conduction-in-a-myelinated-nerve-fiber-saltatory-conduction.jpg

myelination and saltatory conduction

*The jump along the axon is actually just the very rapid conduction inside the myelinatedportion of the axon.

As myelin acts as an insulator myelinatedaxons only allow action potentials to occur at the unmyelinated nodes of Ranvier.

This forces the the action potential to jump* from node to node (saltatory conduction).

The result of this is that the impulse travels much more quickly (up to 200 m/s) along myelinated axons compared to unmyelinated axons (2 m/s).

Saltatory conduction from node to node also reduces degradation of the impulse and hence allows the impulse to travel longer distances than impulses in unmyelinated axons.

The myelin sheath also reduces energy expenditure over the axon as the quantity of sodium and potassium ions that need to be pumped to restore resting potential is less than that of a un-myelintated axon

6.5.U7 Synapses are junctions between neurons and between neurons and receptor or effector cells.

To function the nervous system needs to receive input/stimuli and then to coordinate a response to it.

For this to happen impulses need to travel from sensory receptor cells via a series of nerve cells to effectors, which are commonly muscles and glands.

There are junctions between each cell called synapses across which impulses cannot travel.

A special group of molecules called neurotransmitters move across the synapse to effect an impulse in the adjacent cell.

http://outreach.mcb.harvard.edu/animations/synaptic.swf

6.5.U7 Synapses are junctions between neurons and between neurons and receptor or effector cells.

6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9

A nerve impulse is only initiated if the threshold potential is reached.

6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9

A nerve impulse is only initiated if the threshold potential is reached.

6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9

A nerve impulse is only initiated if the threshold potential is reached.

6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9

A nerve impulse is only initiated if the threshold potential is reached.

6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9

A nerve impulse is only initiated if the threshold potential is reached.

6.5.A1 Secretion and reabsorption of acetylcholine by neurons at synapses.

http://faculty.pasadena.edu/dkwon/chap%208_files/images/image61.png

Acetylcholine is a neurotransmitter used in many synapses through the nervous system

One use is at the neuromuscular junction, i.e. it is the molecule that motor neurons release to activate muscles. Interfering with the action of acetylcholine can cause a range of effect from paralysis to convulsions.

6.5.A2 Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides

to acetylcholine receptors.

Nature of science: Cooperation and collaboration between groups of scientists - biologists are contributing to

research into memory and learning. (4.3)

Gero Miesenböck FRS Waynflete Professor of Physiology, WellcomeInvestigator

Martin BoothProfessor of Engineering Science

Tim VogelsSir Henry Dale Fellow (physicsist)

Stephen GoodwinProfessor of Neurogenetics, Wellcome Investigator

Korneel HensGroup Leader (Biochemist)

Scott WaddellProfessor of Neurobiology, Wellcome Trust Senior Research Fellow in Basic Biomedical Sciences

http://www.cncb.ox.ac.uk/team/

Nowadays scientists often work in multidisciplinary teams for example the Centre for Neural Circuits and Behaviour (CNCB)

The aim of the CNCB is to understand how intelligence emerges from the physical interaction of nerve cells.

Studying the brain from this top-down approach to answer such fundamental questions requires techniques and understanding from a range of disciplines.

Bibliography / Acknowledgments

Bob Smullen