THE NERVE IMPULSE © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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 - © 2016 Paul Billiet ODWS

The neuron © 2016 Paul Billiet ODWS www.lab.anhb.uwa.edu.au/.../Nervous/Nervous.htm www.biologymad.com/.../nervoussystemintro.htm © 2016 Paul Billiet ODWS

The neuron Dendrites Myelin sheath Schwann cell Nucleus of Schwann cell Axon Nodes of Ranvier Terminal dendrites Cell body © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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+. © 2016 Paul Billiet ODWS

Potassium & Sodium Ions K+ ions leak out a little from K+ ion pores cell is negative inside pulling K+ in but 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 plasma membrane K+ K+ coupled ion pump Na+ Na+ Cytoplasm ECF K+ K+ coupled ion pump Na+ Na+ © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

The action potential The action potential is the state of the neuron membrane when a nerve impulse passes by. © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

Time mV Resting potential Action potential +35 More Na+ channels open Na+ floods into neuron mV Na+ voltage-gated channels open -55 Threshold -70 Resting potential Action potential © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

mV Time Resting potential Action potential Resting potential -70 -55 +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. © 2016 Paul Billiet ODWS

Potassium ion channel OPEN CLOSED

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. © 2016 Paul Billiet ODWS

Time mV Resting potential Action potential -70 -55 +35 Threshold Time mV Resting potential Action potential Hyperpolarisation of the membrane Active pumping of Na+ out and K+ in during the refractory period © 2016 Paul Billiet ODWS

The neuron Dendrites Myelin sheath Schwann cell Nucleus of Schwann cell Axon Nodes of Ranvier Terminal dendrites Cell body © 2016 Paul Billiet ODWS

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). © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

Grey matter and White matter White matter = myelinated for long distance transmission Grey matter = non-myelinated for short distance transmission © 2016 Paul Billiet ODWS