Chapter 4 Neural conduction and synaptic transmission How neurons send and receive signals 1 Resting membrane potential work membrane potential is the difference in electric charge between the inside and outside of the cell a To record a neuron s membrane potential it is necessary to position the tip one electrode inside than neuron electrodes called microelectrodes and in the tip of another outside in the extracellular fluid b The neurons resting potential is about 70 mV charge the neuron is said to be polarized when there are more sodium ions outside the south of insight and more potassium ions inside the outside i Unequal distribution of Na and K ions is maintained through specialist pores called ion channels in a neutral membrane through which ions can pass ii Sodium ions want to move down concentration gradient are attracted by by negative internal charge and want to get into the cell but cannot the sodium I am channels and resting neurons are closed 1 To maintain charge cell continuously exchanges 3 Na ions inside for 2 K ions outside via transporters called sodium potassium pumps 2 Transporters are mechanisms in the membrane of the cell actively transport ions or molecules across the membrane 2 Generation and conduction of postsynaptic potentials a When neurons fire they released from their terminal buttons chemicals called neurotransmitters which diffuse across the synaptic cleft and interact with specialized receptor molecules on the receptive membrane of the next neurons in the circuit i Either depolarize the receptive membrane decreased resting membrane potential to 67 mV or hyperpolarize the membrane increased resting membrane potential to 72 mV ii Postsynaptic depolarizations are called excitatory postsynaptic potentials EPSPs and increase the likelihood that the new run will fire while postsynaptic hyperpolarization are called inhibitory postsynaptic potentials IPSPs and decrease the likelihood that the neuronal fire 1 Both IPSP is in EPSPs are grated responses meeting amplitude of the response is proportional to the intensity of the signal that initiated it 2 Transmission of postsynaptic potentials is almost instantaneous although the duration of EPSPs and IPSPs varies and the signal is decremental growing fainter with distance 3 Integration of postsynaptic potentials and generation of action potentials a Postsynaptic potentials created in a single synapse typically have little effect on the firing of the postsynaptic neuron typically covered in thousands of synapses firing or not depends on balance between excitatory inhibitory signals reaching the axon b Once believed that action potentials generate at the Axon Hillock is the junction between the cell body and axon but really come from the axonal initial segment the adjacent section of the axon c If some of depolarization hyperpolarization screeching axon initial segment at any time is sufficient to depolarize the membrane to a threshold of excitement usually about 65 mV an action potential is generated i An action potential is a massive with momentary 1 ms reversal of the membrane potential from 70 mV to 50 mV and it s not the greatest potential but all are nothing response where Magnum do not related to intensity of stimuli that elicited it ii 4 Integration is the adding combining a number of individual signals into one overall signal over space spatial summation where local EPSPs IPSP is some cancel out or over time temporal summation where postsynaptic potentials produced in rapid succession some to form a general signal Conduction of action potentials a Action potentials are produced conducted along the axon through the actions of voltage activated ion channels ion channels that open closed in response to changes in the level of the membrane potential b When membrane potential of axon is depolarized to the threshold of excitation by an EPSP voltage activated sodium channels in the axon membrane open wide and Na ions rush in membrane potential driven up from 70 mV to 50 mV i Rapid influx of sodium ions triggers opening of voltage activated potassium channels driving K ions out of the cell ii After about 1msec sodium channels closes end of the rising phase of the action potential beginning of repolarization by continued ex flux of potassium ions iii Once repolarization achieved potassium channels close but do so too slowly leaving self slightly hyperpolarized larger activation needed to again reach threshold of activation c Absolute refractory period is a 1 to 2 msec period after initiation of action potential during which it isn t possible to elicit the second one i Relative refractory period is period during which it is possible to fire again but only by applying higher than normal levels of stimulation ii Responsible for action potentials normally traveling along axons in only one direction and that the rate of neural firing is related to the intensity the stimulation d The conduction of action potentials along taxon is not decremental does not grow weaker S it travels along axonal membrane and action potentials are conducted more slowly than postsynaptic potentials i Due to IPSP s EPSPs being passive where axonal conduction of action potentials is largely active as there re many iron channels on the axonal membrane to open e Antidromic conduction is went electrical stimulation of sufficient intensity is applies to the terminal and axon and action potential is generated that travel along the axon back to the cell body i Orthodromic conduction is the normal axonal conduction in the nuptial direction from cell body to terminal buttons f Ions can pass through the axonal membrane of myelinated axons only at the Nodes of Ranvier the gaps between adjacent smiling segments axonal sodium channels are concentrated here i when action potential generated in myelinated axon signal conducted passively jumps from node of Ranvier to node of Ranvier diminishing the time it reaches the node but still strong enough to open the voltage activated sodium channels and generate another full blown action potential 1 Myelination increases the speed of axonal conduction conduction along myelinated segments is passive occurs instantaneously so the signal jumps from node to node with only slight delay at each node 2 Transmission of action potentials and myelinated axons it s called salutatory conduction ii Conduction fastest in large diameter axons that are myelinated like mammalian motor neurons that can travel 60 m s compared to 1