Neurophysiology Conduction, transmission and integration of neural signals Chapter 3

Summary of neural transmission l Resting potential and action potential l signal propagation l ionic bases of electrical potentials l neural integration

Review of neuron l Input zone (dendrites) l Integration zone (axon hillock) l Conduction zone (axon) l Output zone (axon terminals) l Secretory event

Resting potential l Definition of electrical potential Potential energy Electrical potential

Resting potential l Recording and reference electrodes l amplifier l oscilloscope Experimental setup

Resting potential Across the membrane

Changing membrane potential (creating unrest!)

Graded potentials (decremental conduction) stimulator

The action potential -subthreshold -threshold of excitation -overshoot -afterpotential

Propagation of the action potential -non-decremental conduction

The ionic basis of membrane potentials Diffusion gradients -substances move from areas of high concentration to areas of low concentration Electrostatic gradients -substances can carry charge (ions) -substances move towards areas of unlike charge Dynamic equilibrium -gradients can balance one another

The ionic basis of membrane potential A- = protein K+=potassium Na+=sodium Cl-=chloride +cations -anions

Dynamic equilibrium

Dynamic equilibria For some species of ion (such as potassium), the relative concentrations on either side of the membrane are pretty much mathematically predictable (Nernst equation) just based on the balance of electrostatic and concentration gradients. For others (such as sodium) this is not the case.

This can only mean…. The membrane is not equally permeable to all ions. At least….not all the time!

Nerve cell membranes are filled with interesting proteins

The disposition of ions in the resting nerve cell

Refractory periods Note the difference between closed and inactivated sodium channels -sodium channels only open briefly and then cannot open for some period of time (absolute refractory period)

The pufferfish as a neuroscientific weapon About three o’clock in the morning we found ourselves seized with an extraordinary weakness and numbness all over our limbs. I had almost lost the sense of feeling; nor could I distinguish between light and heavy bodies of such as I had strength to move, a quart pot full of water and a feather being the same in my hand….--Captain James Cook--

Conduction of action potentials in unmyelinated axons

Conduction of action potentials in myelinated neurons -faster, cheaper

Consequences of demyelination - Multiple sclerosis

Synopsis of synaptic action Transmitter release receptor action (metabotropic vs. ionotropic receptors) postsynaptic potentials (EPSP’s and IPSP’s)

The synapse

Electron micrograph of a synapse

Synaptic vesicles fuse with presynaptic membrane (movement of vesicles is caused by entry of calcium into presynaptic terminal

Freeze-fracture view of transmitter release

Receptors - a special kind of membrane channel

Ionotropic receptors

Metabotropic receptors -G-protein coupled

Varieties of postsynaptic potentials (PSP’s) l Excitatory (EPSP) l usually when receptor channels admit sodium l Inhibitory (IPSP) l usually when receptor channels admit chloride

Termination of synaptic action 1. Deactivation 2. Re-uptake by transporters

Neural integration Spatial integration -ripples can collide and combine

Neural integration Spatial integration -ripples can go up (excitatory) or down (inhibitory)

Neural integration Spatial integration -ripples can combine

Another look at spatial summation

Neural integration Temporal integration -ripples can combine to make bigger ripples