Ch. 11, Part 2: Action Potentials, Synapses

At resting membrane potential, the ______ concentration is higher outside the cell, and the ______ concentration is higher inside the cell.

-Na+ -K+

True or False: (In the relative refractory period) An exceptionally strong stimulus can reopen the Na+ channels that have already returned to their resting state and generate another AP.

True- Strong stimuli trigger more frequent APs by intruding into the relative refractory period.

Saltatory Conduction

-APs are triggered only at the gaps -electrical signal appears to jump from gap to gap along the axon -about 30 times faster than continuous conduction

Relative refractory period

-Follows the absolute refractory period -During this period, most Na+ channels have returned to their resting state, some K+ channels are still open, and repolarization is occurring. -axon's threshold for AP generation is substantially elevated, so a stimulus that would normally generate an AP is no longer sufficient

Repolarization (in generating an action potential)

-Na+ channels are inactivating, and K+ channels open -explosively rising phase of the AP persists for about 1 ms -self-limiting bc inactivation gates of Na+ channels begin to close at this point -membrane permeability to Na+ declines -AP spike stops rising -slow voltage-gated K+ channels open and K+ rushes out of cell -restores internal negativity of resting neuron

Depolarization (in generating an action potential)

-Na+ channels open -as local currents depolarize axon membrane, voltage-gated sodium channels open and Na+ rushes into cell -influx depolarizes local patch of membrane, opening more Na+ channels -interior of cell becomes progressively less negative -reaches critical level called threshold rapid depolarization and polarity reversal produces sharp upwards spike of AP -membrane potential and membrane permeability establish a positive feedback cycle

Step 1 (info trans. across Chem. synapse): Action potential arrives at axon terminal.

-Neruotransmission begins with arrival of an AP at the presynaptic axon terminal

Hyperpolarization (in generating an action potential)

-Some K+ channels remain open, and Na+ channels reset -period of increased K+ permeability lasts longer than needed to restore resting state -excessive K+ efflux before the K+ channels close causes a hyperpolarization -shown as a slight dip on an AP curve -Na+ channels begin to reset to original position by changing shape to reopen their inactivation gates & close activation gates

Step 5 (info trans. across Chem. synapse): Binding of NT opens ion channels, creating graded potentials.

-When NT binds to receptor protein, receptor changes its shape, which opens ion channels and creates graded potentials -depending on receptor protein to which NT binds and type of channel the receptor controls, the postsynaptic neuron may be excited or inhibited

Degree of myelination

-action potentials propagate because they are regenerated by voltage-gated channels in the membrane -the presence of a myelin sheath dramatically increases the rate of AP propagation

Resting state (in generating an action potential)

-all gated Na+ and K+ channels are closed -only leakage channels are open, maintaining Em -each Na+ channel has 2 gates, activation and inactivation *each gate must be open for Na+ to enter *closing of either gate closes channel

Multiple Sclerosis

-autoimmune disease that is a result of the immune system's attack on myelin proteins and affects mostly young adults -reduces myelin sheaths in CNS to nonfunctional hardened lesions called scleroses -axons themselves are not damaged and growing numbers of Na+ channels appear spontaneously in the demyelinated fibers -high blood levels of vit D may reduce risk for this?

The B and C fiber groups include what parts of the body?

-autonomic NS motor fibers serving visceral organs -visceral sensory fibers -smaller somatic sensory fibers that transmit sensory impulses from the skin

Step 6 (info trans. across Chem. synapse): Neurotransmitter effects are terminated.

-binding of a NT to its receptor is reversible -as long as its bound to a post synaptic receptor, a NT continues to affect membrane permeability and block reception of additional signals from presynaptic neurons -effects of NT last a few ms before being terminated (by reuptake, degradation, or diffusion)

reuptake

-by astrocytes or presynaptic terminal -where NT is stored or destroyed by enzymes with norepinephrine

degradation

-by enzymes associated with postsynaptic membrane or present in synaptic cleft, as with acetylcholine

Step 2 (info trans. across Chem. synapse): Voltage-gated Ca2+ channels open and Ca2+ enters axon terminal.

-depolarization of membrane by AP opens not only Na+ channels but voltage-gated Ca2+ channels -during brief time Ca2+ channels are open, Ca2+ floods down its electrochemical gradient from ECF into the terminal

synaptic cleft

-fluid filled space approximately 30-50nm that separates the presynaptic and postsynaptic membranes

Axon diameter

-larger axons conduct more rapidly becuase they offer less resistance to the flow of local currents, bringing adjacent areas of the membrane to threshold more quickly

electrical synapses

-less common than chemical synapses -consist of gap junctions like those found between certain other body cells -connexons connect cytoplasm of adjacent neurons and allows ions and small molecules to flow directly from one neuron to the next -neurons are electrically coupled so transmission is very rapid -think of a doorway

Group B (nerve) fibers

-lightly myelinated fibers of intermediate diameter -transmit impulses at average rate of 15 m/s (30mph)

Chemical synapses

-most common type of synapse -specialized to allow release and reception of NT -made up of 2 parts (axon terminal and receptor region)

diffusion

-movement away from the synapse

Group C (nerve) fibers

-smallest diameter -nonmyelinated -incapable of saltatory conduction and conduct impulses at 1 m/s (2mph) or less

Group A (nerve) fibers

-somatic sensory and motor fibers serving the skin, skeletal muscles, and joints -have largest diameter -thick myelin sheaths -conduct impulses at speeds up to 150 m/s (over 300mph)

Step 3 (info trans. across Chem. synapse): Ca2+ entry causes synaptic vesicles to release NT by exocytosis.

-surge of Ca2+ into axon terminal acts as intracellular messenger -Ca2+-sensing protein (synaptotagmin) binds Ca2+ and interacts with SNARE proteins that control membrane fusion -synaptic vesicles fuse with axon membrane and empty contents by exocytosis into synaptic cleft -Ca2+ quickly removed from terminal: either taken up into mitochondria or ejected from neuron by Ca2+ pump -the higher the impulse frequency reaching the presynaptic terminal, the greater the number of synaptic vesicles that fuse and spill, the greater the effect

Stimulus intensity is coded for by:

-the number of impulses per second -in other words, the frequency of action potentials -not coded by increases in strength or amplitude of the individuals APs

synaptic vesicle

-tiny membrane bound sac containing thousands of neurotransmitter molecules

Outline the steps of information transfer across chemical synapses.

1) action potential arrives at axon terminal 2) voltage-gated Ca2+ channels open and Ca2+ enters axon terminal 3) Ca2+ entry causes synaptic vesicles to release NT by exocytosis 4) NT diffuses across the synaptic cleft and binds to specific receptors on postsynaptic membrane 5) binding of NT opens ion channels, creating graded potentials 6) NT effects are terminated

List the steps in generating an action potential

1) resting state 2) depolarization 3) repolarization 4) hyperpolarization

True or False: When a patch of neuron membrane is generating an AP and its voltage-gated sodium channel are open, the neuron can only respond to another stimulus if it has a higher intensity than the AP that is currently being generated. (in an absolute refractory period)

False: the neuron cannot respond to another stimulus, no matter how strong it is.

True or False: Local depolarization are graded potentials and their magnitude decreases when stimuli become more intense.

False: their magnitude is directly proportional to the intensity of the stimuli, so therefore their magnitude would increase with increasing intensity of the stimuli.

All-or-none phemonmenon

In regards to AP, it either happens completely or not at all.

Once initiated, an AP is self-propagating, which means what? (in non-myelinated axons)

It will continue along the axon at a constant velocity.

Absolute refractory period

The time period from the opening of the Na+ channels until the Na+ channels begin to reset to their original resting state.

True or False: Only small amounts of sodium and potassium cross the membrane in the generation of action potentials.

True: The Na+ influx required to reach threshold produces only a 0.012% change in intracellular Na+ concentration.

The rate of impulse propagation depends on which two factors?

axon diameter and degree of myelination

axosomatic synapses

between axon endings of one neuron and the cell body of another neuron

The threshold is the membrane potential at which the outward current created by K+ movement is _____________ the inward current created by Na+ movement.

exactly equal to

Synapse

junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell

Synaptic delay

lasts 0.3-0.5 ms, making transmission across the chemical synapse the rate-limiting step of neural transmission -helps explain why transmission along neural pathways involving only 2 or 3 neurons occurs rapidly, but transmission along multisynaptic pathways typical of higher mental functioning occurs slower

presynaptic neuron

neuron conducting impulses toward the synapse

postsynaptic neuron

neuron transmitting the electrical signal away from the synapse

After repolarization in generating an action potential, the ________________ redistributes the ions.

sodium potassium pump

Subthreshold stimuli vs. threshold stimuli

subthreshold: produce depolarizations that are not translated into nerve impulses threshold: produce depolarizing currents that push the membrane potential toward and beyond the threshold voltage

axodendritic synapses

synapses between the axon endings of one neuron and the dendrites of other neurons