Action Potential (L4)

Electrical signals in neurons Neurons are electrically excitable due to the voltage difference across their membrane Communicate with 2 types of electric signals graded potentials that are local membrane change action potentials that can travel long distances

Graded Potentials They are either excitatory (Post Synaptic Potential; EPSP) or Inhibitory Post Synaptic Potential (IPSP) They are localized. occur most often in the dendrites and cell body of a neuron and don’t travel to axons. The signals vary in amplitude (size), depending on the strength of the stimulus. They can summate in one of the two ways: Spatial summation of effects of neurotransmitters released from several end bulbs onto one neuron. Temporal summation neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron

Action Potentials How signals travel from here to here?

Signal transmission at the chemical synapse Action potential reaches end bulb and voltage-gated Ca2+ channels open Ca2+ flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors the more neurotransmitter released the greater the change in potential of the postsynaptic cell Synaptic delay is 0.5 msec One-way information transfer

LENGTH CONSTANT

Three Possible Responses Small EPSP occurs potential reaches -56 mV only An impulse is generated threshold was reached membrane potential of at least -55 mV IPSP occurs membrane hyperpolarized potential drops below -70 mV Copyright 2009 John Wiley & Sons, Inc.

Generation of Action Potentials An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). During an action potential, voltage-gated Na+ and K+ channels open in sequence According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same. A stronger stimulus will not cause a larger impulse. Copyright 2009 John Wiley & Sons, Inc.

VOLTAGE-GATED SODIUM CHANNELS

VOLTAGE-GATED POTASSIUM CHANNELS

Action Potentials

Generation of Action Potentials

IONIC MOVEMENTS RESPONSIBLE FOR ACTION POTENTIAL

Refractory period Period of time during which neuron can not generate another action potential. Absolute refractory period even very strong stimulus will not begin another AP inactivated Na+ channels must return to the resting state before they can be reopened large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible. Relative refractory period a suprathreshold stimulus will be able to start an AP K+ channels are still open, but Na+ channels have closed

Copyright 2009 John Wiley & Sons, Inc. Propagation of an Action Potential in a neuron after it arises at the trigger zone Copyright 2009 John Wiley & Sons, Inc.

1. Conduction of action potential in non-myelinated axons

2. Conduction of action potential in myelinated axons

Continuous versus Saltatory Conduction Continuous conduction (unmyelinated fibers) step-by-step depolarization of each portion of the length of the axolemma Saltatory conduction depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels current carried by ions flows through extracellular fluid from node to node

Factors that affect the speed of propagation Amount of myelination Axon diameter Temperature Copyright 2009 John Wiley & Sons, Inc.

Speed of impulse propagation The propagation speed of a nerve impulse is not related to stimulus strength. larger, myelinated fibers conduct impulses faster due to size & saltatory conduction Fiber types A fibers largest (5-20 microns & 130 m/sec) myelinated somatic sensory & motor to skeletal muscle B fibers medium (2-3 microns & 15 m/sec) myelinated visceral sensory & autonomic preganglionic C fibers smallest (.5-1.5 microns & 2 m/sec) unmyelinated sensory & autonomic motor

Compound Action Potential Each peak was named: alpha – the first to appear; beta - the next, and so on. The first signal to arrive at a distant recording site has travelled the fastest! So each peak represents a set of axons with similar conduction velocity velocity is calculated from the distance between R1 and R3 and the time taken to traverse that distance (distance/time) = velocity (ranges from 0.5 to ~100 ms-1)

Compound Action Potential In a mixed nerve, action potential appears with multiple peaks and is known as a compound action potential. This action potential results from the summation of action potentials of all fibers in the nerve. Its shape is due to the fact that a mixed nerve is made up of a different types of family of fibers with varying speed of conduction. Therefore when all fibers are stimulated, the activity in fast conducting fibers arrives at the recording electrode sooner than the activity in slower fibers. The number and size of the peaks vary with the type of fibers in the particular nerve being studied. The mammalian nerve fibers are divided into A, B, and C groups with further subdividing the A group into ,, and  fibers. A fibers have the thickest covering of myelin and are larger, therefore they conduct faster as fast as 100m/sec. C fibers are small and unmyelinated and so its conduction velocity is much smaller (0.5-2.3m/S).

Encoding of stimulus intensity If action potential is all-or-none phenomenon how do we detect the Stimuli of different intensity ? Why does a light touch feels different than a firmer pressure? 1. The Frequency of firing of action potential 2. The number of sensory neurons recruited

Functions of action potentials Information delivery to CNS carriage of all sensory input to CNS. Block APs in sensory nerves by local anaesthetics. This usually produces analgesia without paralysis. WHY? Because LAs more effective against small diameter (large surface area to volume ratio) C fibers than a-motorneurones. Information encoding The frequency of APs encodes information (remember amplitude cannot change) - Rapid transmission over distance (nerve cell APs) Note: speed of transmission depends on fiber size and whether it is myelinated. Information of lesser importance carried by slowly conducting unmyelinated fibers. In non-nervous tissue APs are the initiators of a range of cellular responses muscle contraction secretion (eg. Adrenalin from chromaffin cells of medulla)