Anatomy and Physiology Neuron Anatomy, Action Potential, Synapses PDF

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Biol 2401 Ian Lian...

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Anatomy and Physiology 11/13 Graded potentials (also called local potentials) ● changes in membrane potential that cannot spread far from site of stimulation; caused by stimuli that open gated ion channels ● example: chemically gated sodium ion channels 1. at resting membrane potential, chemically gated sodium channels are all closed action potential- propagated changed in the membrane potential that affect the entire excitable membrane ● a neuron receives graded potentials at its dendrites/cell body ● action potentials at axon terminals release neurotransmitter ● action potentials allow long-range communication between cell body and axon terminals Steps in action potential generation ● resting membrane potential- voltage-gated sodium and potassium channels are both closed 1. Depolarization to threshold ● initial stimulus- graded depolarization large enough to open voltage-gated sodium channels ● threshold- membrane potential at which channels open ○ about -60 mV 1. Activation of sodium channels- rapid depolarization ● sodium channel activation gates open; sodium ions rush in ● rapid depolarization- membrane potential goes from -60mV to a positive value all or none principle: ● any stimulus either triggers an action potential or doe snot ● all stimuli that bring a membrane to threshold result in identical action potentials 1. ● ●

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inactivation of sodium ion channels; activation of potassium ion channels as membrane potential approaches +30 mV, inactivation gates of sodium channels close (sodium channel inactivation) at same time, voltage-gated potassium channels open ○ potassium ions leave cell, and membrane potential shifts back towards resting membrane potential ■ potassium ions leave cell, and membrane potential, shifts back towards resting membrane potential (repolarization starts) Potassium ions channels close sodium channels shift from inactivated to closed (but capable of opening) as membrane potential returns to near threshold as membrane potential approaches normal resting potential (-70 mV), voltage-gated potassium channels begin closing ○ potassium leaves until all gates close; brief hyperpolarization)

Refractory periods of an action potential ● Absolute refractory period ○ time during which the membrane cannot respond to any further stimulation ● Relative refractory period ○ time during which the membrane can respond, but only to a stimulus that is stronger than normal Action potential propagation

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action potentials are generated at initial segment of axon action potential at one site triggers action potential at adjacent site; process continues to end of axon (propagation)

Two types of propagation 1. Continous prorogation- action potential appears to move step by step through entire axon ○ occurs in unmyelinated axons ○ slower- depolarization moves at about 1m/sec 2. Saltatory propagation- in myelinated axons; depolarizes only at nodes ● skips internodes because ions can’t cross membrane where there is myelin ● faster than continuous; speed varies with axon diameter Synapse- location where information is transferred from a neuron to another neuron or to effector cell ● if neurons are communicating, synapse involves two neurons- presynaptic neuron and postsynaptic neuron Two types of synapses 1. Chemical synapse 2. Electrical synapse Chemical synapse ● most abundant type of synapse ○ all synapses between neurons and other types of cells ○ most synapses between neurons ● rely on neurotransmitter release ● synapses that release acetylcholine (ACh) are categorized as cholinergic synapses (most common type)

Events at a cholinergic synapse ● Step 1. Sxon terminal depolarized by arriving action potential ● Step 2. Depolarization open voltage-gated calcium channels ○ calcium ions rush into axon terminal ○ triggers synaptic vesicles to realize ACh (exocytosis) into synaptic cleft ○ calcium ions are quickly removed, ending release of ACh ● Step 3. ACh diffuses across synaptic cleft :binds to chemically gated Na+ channel receptors on postsynaptic membrane ○ more ACh= more channels open ○ more Na+ enters= greater depolarization ○ if threshold met, initiates action potential ● Step 4 Effects on the postsynaptic membrane are temporary ○ Acetylcholinesterase (AChE) i synaptic cleft breaks down bounds ACh within 20 sec ○ other Ach molecules diffuse away from binding sites Synaptic figure ● neurotransmitters usually reabsorbed and recycled ● after extended stimulation, supply of neurotransmitters may not keep up with demand ● synapse unable to function until ACh replenished ● inability to function= synaptic fatigue Synaptic delay-time lag between arrival of action potential at axon terminal and effect on postsynaptic membrane ● usually .2 to .5 sec ● cumulative delays many be considerable

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reflexes involve few synapses in order to minimize delay

Electrical synapses ● presynaptic and postsynaptic membranes joined by gap junctions ● changes in membrane potential of one cell produce local currents in adjacent cell, as if sharing common membrane ● rare ● no variability in response of postsynaptic cell Postsynaptic potentials ● graded potentials in postsynaptic membrane in response to a neurotransmitter Two Types 1. Excitatory postsynaptic potential (EPSP) 2. Inhibitory postsynaptic potential (IPSP) 1. ● ●

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Excitatory postsynaptic potential (EPSP) graded depolarization shifts membrane potential closer to threshold ○ membrane is facilitated- more facilitation means less additional stimulus is needed to trigger action potential Inhibitory postsynaptic potential (IPSP) graded hyperpolarization shifts membrane potential farther away from threshold ○ membrane is inhibited- larger- than- normal stimulus is needed to trigger action potential

Summation- integration of effects of graded potentials ● collective effects of both ESPS’s and ISPS’s ● example: arrival of two different neurotransmitters at same time opens two different sets of ion channels ○ net effect may be no change in membrane potential Integration of information ● single neuron may receive information from thousands of synapses (excitatory/inhibitory) ● axon hillock integrates all stimulus; determines rate of action potential generation at initial segment ○ is closest to initial segment where action potential starts ○ threshold at axon hillock lower than elsewhere on cell bod Types of summation - individual ESPS or ISPS has small effect on membrane potential single ESPSs will not use as action potential, but they can combine through summation ● two types 1. Temporal summation 2. Spatial summation 1. ● ● ● ●

Temporal summation (tempos, time) when single synapse is stimulated repeatedly under maximum stimulation, action potential can reach synapse each millisecond each new action potential causes relate of more ACh into the synaptic cleft more ACh= more depolarization, possible to threshold

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Spatial summation

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multiple synapses active at the same time degree of depolarization depends on: Number of active excitatory synapses Their distance from initial segment action potential is generated if membrane reaches threshold...