powerpoint presentation: the nerve impulse

THE NERVE IMPULSE

© 2016 Paul Billiet ODWS

http://www.saburchill.com/IBbiology/bio_hp.html

Cells and membrane potentials

All animal cells generate a small voltage across their membranes

There are a large amount of small organic molecules in the cytoplasm (e.g. amino acids)

To balance this, animal cells pump Na+ out of the cells

This regulates osmosis but it leaves a large number of organic molecules

These organic molecules are overall negatively charged (anions) in the cytoplasm

Thus the cell has a potential difference (voltage) across its membrane.



Experiments on the neuron of a giant squid

Ion

Concentration /mmol kg-1 water

Axoplasm (the cytoplasm

in an axon)

Blood plasma

Sea water

K+ 400 20 10

Na+ 50 440 460

Cl- 120 560 540

Organic anions

(-ve ions)360 - -



The neuron

www.lab.anhb.uwa.edu.au/.../Nervous/Nervous.htm

www.biologymad.com/.../nervoussystemintro.htm


http://www.lab.anhb.uwa.edu.au/.../Nervous/Nervous.htm

http://www.biologymad.com/.../nervoussystemintro.htm


The neuron

Dendrites

Myelin sheath

Schwann cell Nucleus of Schwann cell

Axon

Nodes of Ranvier

Terminal dendrites

Cell body



Neurons

Neurons, like other cells, are more negatively charged inside than outside

This results in a membrane potential of about – 70 milliVolts

This is called the resting potential of the neuron.



Potassium & Sodium Ions

The two important ions: K+ and Na+

Both are positively charged ions Na+ ions move more slowly across the

membrane than K+ or Cl- ions The Na+ ion is smaller than the K+ ion

(Na+ has a larger coating of water molecules giving it a bigger diameter)

This makes the plasma membrane 25 times more permeable to K+ than Na+.



Potassium & Sodium Ions K+ ions leak out a little from K+ ion pores

cell is negative inside pulling K+ inbut there is a very high concentration of K+ inside pulling K+ out

K+ has to be actively pumped inwards a bit The resting potential of the neuron is almost at

the equilibrium for K+ ions K+ leak out a bit and need pumping in Na+ ions, however, are actively pumped out

and kept out.© 2016 Paul Billiet ODWS


A coupled Na+-K+ pump

coupled ion

pump

plasma membrane

K+

Na+

K+

Na+

Cytoplasm ECF



Getting excited! The neuron’s membrane at rest is more negative

inside than outside The neuron is said to be polarised Neurons are excitable cells Neurons are excited when their membranes

become depolarised.



Depolarisation

Depolarising membranes may be achieved by:a stimulus arriving at a receptor cell (e.g. vibration of a hair cell in the ear)a chemical fitting into a receptor site (e.g. a neurotransmitter)a nerve impulse travelling down a neuron.



Nerve impulses

Nerve impulses are self-propagating like a trail of gunpowder

Localised currents in the ions occur just ahead of the impulse causing localised depolarisation

Nerve impulses are not like electrical signals travelling down a wire.



The action potential The action potential is the state of the neuron

membrane when a nerve impulse passes by.



The action potential Localised currents cause Na+ channels to flip

open Voltage-gated Na+ channels As Na+ moves into the cell, more and more

Na+ channels open A small change in the membrane permeability

to Na+ results in a big change in membrane potential

The volume of the axon is minute compared to the volume of the extracellular fluid.



Time

-70

-55

0

+35

Threshold

mV

Resting potential Action potential

More Na+ channels open Na+ floods into neuron

Na+ voltage-gated channels open



All-or-nothing Na+s move in, the cell it will become more

positive Ion pumps resist the change in the membrane

potential If it rises by 15mV and the pumps cannot

restore the equilibrium Na+ floods in and neuron is depolarised Nerve impulses all look the same, there are

not big ones and little ones This is the all-or-nothing law.



The threshold

–55mV represents the threshold potential Beyond this we get a full action potential The membrane potential rises to +35mV this is

the peak of the action potential The cells are almost at the equilibrium for Na+

ions.



-70

-55

0

+35

Threshold

mV

Time

Resting potential Action potential

Na+ channels close and K+ channels open, K+ floods out of neuron

Resting potential© 2016 Paul Billiet ODWS


Potassium takes over Na+ moves in passively until it reaches

equilibrium At the same time K+ permeability increases as

voltage-gated K+ channels open – K+ channels are a bit slower to respond to the

depolarisation than the Na+ channels K+ ions move out The cell becomes negative inside with respect to

outside again The membrane potential falls The cell become repolarised.



Potassium ion channel

OPEN CLOSED

http://pdb101.rcsb.org/motm/38

Hyperpolarisation The membrane potential falls below the resting

potential of –70mV It is said to be hyperpolarised The axon is negative inside but the ion

concentration is not the same Gradually active pumping of the ions (K+ in

and Na+ out) restores the resting potential During this period no impulses can pass along

that part of the membrane This is called the refractory period.



-70

-55

0

+35

Threshold

Time

mV

Resting potential Resting potential

Action potential

Active pumping of Na+ out and K+ in during the refractory period

Hyperpolarisation of the membrane



The neuron

Dendrites

Myelin sheath

Schwann cell Nucleus of Schwann cell

Axon

Nodes of Ranvier

Terminal dendrites

Cell body



Myelinated neurones

Non-myelinated neuron

Myelinated neuron

In myelinated neurons the cell membrane of the Schwann cell wraps around the axon many times (myelin sheet).


Saltation

• No depolarisation occurs under the myelin• Depolarisation only happens at the nodes

(0.5μm)• All the Na+ channels are concentrated at the

nodes.© 2016 Paul Billiet ODWS

Saltation

An impulse is triggered by local currents that depolarise the next bit of the membrane

In myelinated nerves the triggering jumps from one node to the next

Much quicker than depolarising all the membrane along the whole axon.


Grey matter and White matterWhite matter = myelinated for long distance transmissionGrey matter = non-myelinated for short distance transmission


powerpoint presentation: the nerve impulse

Documents