Textbook Notes (363,420)
Canada (158,366)
Psychology (4,731)
Chapter 4


4 Pages
Unlock Document

Western University
Psychology 2220A/B
Scott Mac Dougall- Shackleton

NeuralConductionand SynapticTransmission January-12-12 12:00PM - The lizard, a case of parkinson's disease - what was happening in his brain? ○ A small group of nerves called the substantia nigra were dying - these neurons make dopamine which they deliver to the striatum ○ As the substantia nigra is dying, the amount of dopamine delivered to the striatum decreases ○ The striatum helps control movement, but it needs dopamine for that - Dopamine isn't an effective treatment for PD b/c it doesn't readily penetrate the blood-brain barrier ○ L-dopa - the chemical precursor of dopamine - penetrates the BBB and is converted into dopamine once inside the brain -> effective treatment RESTING MEMBRANE POTENTIAL - Membrane potential - the difference in electrical charge between the inside and the outside of a cell - RECORDING THE MEMBRANE POTENTIAL - Requires the tip of one electrode inside (intracellular requires microelectrodes) and one outside the neuron - RESTING MEMBRANE POTENTIAL - Both tips in the extracellular fluid (outside the neuron), voltage diff = 0 - When tip of intracellular electrode is inserted into a neuron, a steady potential of about -70mV is recorded ○ Indicates that the potential inside the resting neuron is about 70 mV less than that outside the neuron - Thus, neuron's resting potential = -70mV (polarized membrane) - IONIC BASIS OF THE RESTING POTENTIAL - The unequal distribution of charge in the resting membrane can be understood in terms of the interaction of four factors: ○ Random motion = high to low concentration gradient ○ Electrostatic pressure - Four ions contribute to the resting potential: ○ Sodium ions (Na+) - higher [] OUTSIDE a resting neuron ○ Potassium ions (K+) - higher [] INSIDE a resting neuron ○ Chloride ions (Cl-) - higher [] OUTSIDE a resting neuron ○ Various negatively charged protein ions - stay inside the neuron - When neurons are at rest, the unequal distribution of cl- ions across the neural membrane is maintained in equilibrium by the balance bw the tendency for cl- ions to move down their concentration gradient into the neuron and the 70mV driving them out - 90mV ofelectrostatic pressure would be required to keep intracellular K+ ions from moving down their concentration gradient and leaving the neuron - The concentration of Na+ ions that exists outside of a resting neuron is such that 50mV of outward pressure would be required to keep Na+ ions from moving down their concentration gradient into the neuron, which is added to the 70mV of electrostatic pressure acting to move them in the samedirection ○ 120mV ofpressure is acting to force Na+ ions into resting neurons - Confirmation of Hodgkin and Huxley's calculations: ○ K+ ions are continuously being driven out of resting neurons by 20mV of pressure and that, despite the high resistance of the cell membrane tothe passage of Na+ ions, those ions are continuously being driven in by the 120mV of pressure - Fig4.2 - the passive and active factors that influence the distribution of na+, k+, and cl- ions across the neural membrane: ○ Passivefactors: continously drive k+ ions out of the resting neuron and na+ ions in ○ Activefactors: k+ ions must be actively pumped in and na+ ions must be actively pumped out to maintain the resting equilibri um  Sodium-potassium pumps - Point of equilibrium of cl- ions inside and outside the cell = at -70mV GENERATION AND CONDUCTION OF POSTSYNAPTIVE POTENTIALS - When neurotransmitters bind to postsynaptic receptors, they typically have one of two effects: ○ May depolarize the receptive membrane (decrease the membrane potential, from -70 to -67 for ex) ○ May hyperpolarize the receptive membrane (increase the membrane potential, -70 to -72) - Graded responses - their amplitudes are proportional to the intensity of the signals that elicit them ○ Excitatory postsynapticpotentials (EPSPs) - postsynaptic depolarizations - increase the chance that a neuron will fire ○ Inhibitory post synapticpotentials (IPSPs) - postsynaptic hyperpolarizations - decrease the chance that a neuron will fire - The transmission of postsynaptic potentials has two important characteristics: 1. Rapid - assumed to be instantaneous 2. Decremental - decrease in amplitude as they travel through the neuron (but they never travel very far along an axon) - Someneurons have a mechanism for amplifying dendritic signals that originate far form their cell bodies INTEGRATION OF POSTSYNAPTIC POTENTIALS AND GENERATIO OF ACTION POTENTIALS - Whether or not a neuron fires depends on the balance between the excitatory and inhibitory signals reaching its axon - if the sum reaches the threshold of excitation (usually -65mV), an AP is generated near the axon hillock ○ Axon hillock - until recently, it was believed that AP's started here (the conical structure at the junction between the cell body and the axon)  But, AP's are actually generated in the adjacent section of the axon - AP lasts for 1 ms - reversal of the membrane potential from about -70 to +50 mV ○ Not a graded response - their magnitude is not related to the intensity of the stimuli that elicit them  It is an all-or-none response - Integration - adding/combining a number of individual signals into one overall signal ○ Neurons integrate in 2 ways: over space and over time 1. Spatial summation - three possible combinations: i. Two simultaneous EPSPs sum to produce a greater EPSP ii. Two simultaneous IPSPs sum to produce a greater IPSP iii. A simultaneous IPSP and EPSP cancel each other out 2. Temporal summation - two possible combinations: i. Two EPSPs elicited in rapid succession sum to produce a larger EPSP ii. Two IPSPs elicited in rapid succession sum to produce a larger IPSP CONDUCTION OF ACTION POTENTIALS - IONIC BASIS OF ACTION POTENTIALS - Voltage-activated ion channels - ion channels that open or close in response to changes in the level of the membrane potential - When the membrane potential of the axon is reduced to the threshold of excitation, the voltage-activated sodium channels open wide, and na+ ions rush in - this drives the membrane from -70 to +50 ○ K+ ions near the membrane are driven out of the cell (b/c of high internal concentration and later b/c of the positive internal charge ○ Na+ channels close after 1ms - marks the end of the rising phase; beginning of repolarization (falling phase) by the continued efflux of K+ ions ○ Once repolarization has been achieved, the K+ channels gradually close - A single AP has little effect on the relative concentrations of various ions inside/outside the neuron - Draw figure 4.6 - REFRACTORY PERIODS - Absoluterefractory period - a brief period of about 1-2 ms after the initiation of an action potential which it is impossible to elicit a second one - Relativerefractory period - follows the absolute ref period - the period during which it is possible to fire the neuron again, but only by applying higher-than-normal levels of stimulation - The refractory period is responsible for two important characteristics of neural activity: 1. It is responsible for the fact that AP's normally travel along axons in only one direction - an AP cannot reverse direction since the portions of anaxon over which an AP has just travelled are left momentarily refractory 2. Responsible for the fact that the rate of neural firing is related to the intensity of stimulation(must be intense in order to surpass the relative refractory) - AXONAL CONDUCTION OF ACTION POTENTIALS - The conduction of AP's along an axon is nondecremental - AP's don't grow weaker as they travel along the axonal membrane - AP's are conducted more slowly than postsynaptic potentials - Reason for these differences is that the conduction of EPSPs and IPSPs is passive; whereas the axonal conduction of action potentials is largely active - Antidromicconduction - if electrical stimulation of sufficient intensity is applied to the terminal end of an axon, an AP will be generated and will travel along the axon back to the cell body - Orthodromicconduction - axonal conduction in the natural direction - from cell body to terminal buttons - CONDUCTION IN MYELINATED AXONS - The axons of many neurons are insulated from the extracellular fluid by segments of fatty tissue called myelin ○ Inmyelinated axons, ions can pass through the axonal membrane only at the nodes of ranvier - the gaps between adjacent myelin segments  Axonal sodium channels are concentrated at the nodes of ranvier - When an AP is generated in a myelinated axon, the signal is conducted passively (instantaneously and decrementally) along the first segment of myelin to the next node of ranvier - unmyelinated = not passive - Myelination increases the speed of axonal conduction - Saltatory conduction -the transmission of action potentials in myelinated axons - THE VELOCITY OF AXONAL CONDUCTION - Conduction is faster in large-diameter axons and myelinated axons ○ Ex. Mammalian motor neurons - max velocity in humans is ~60m/sec; in cats, 100m/sec - CONDUCTION IN NEURONS WITHOUT AXONS - Interneurons - neurons that either lack axons, or have very short axons ○ Do not normally display AP's ○ Conduction in interneurons is usually passive and decremental - THE HODGKIN-HUXLEY MODEL IN PERSPECTIVE - The Hodgkin-Huxley model was based on th
More Less

Related notes for Psychology 2220A/B

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.