Anatomy Test 4 Ch 11 pt 2typ

more about step 1 of chem synapses

action potential arrives at axon terminal -Neurotransmission begins with the arrival of an AP at the presynaptic axon terminal.

all-or-none phenomenon

an action potential occurring only if enough Na+ enters the cell and threshold is achieved -It either happens completely or doesn't happen at all

Differences in plasma membrane permeability

-At rest: membrane is impermeable to the large anionic proteins, slightly permeable to sodium, 25x tmore permeable to potassium than to sodium, and permeable to chloride ions. -at resting membrane potential, the negative interior of the cell is due to a much greater ability for K to diffuse out of the cell than for Na to diffuse into the cell

responsibilites of the absolute refractory period

-Ensures that each AP is a separate, all-or-none event -Enforces one-way transmission of the AP: Since the area where the AP originated just made an AP, the Na channels in that area are inactivated and no new AP is generated there. For this reason, the AP propagates away from its point of origin.

changes in membrane potential can produce two types of signals

-Graded potentials: usually incoming signals operating over short distances that have variable (graded) strength -Action potentials: long-distance signals of axons that always have the same strength

neurotransmitter effects are terminated by

-Reuptake: by astrocytes or the presynaptic terminal, where the neurotransmitter is stored or destroyed by enzymes (norepinephrine) -Degradation: by enzymes associated with the postsynaptic membrane or present in the synaptic cleft (acetylcholine) -Diffusion: away from the synapse

differences in ionic composition

-The cell cytosol contains a lower concentration of Na+ and a higher concentration of K+ than the extracellular fluid -Negatively charged (anionic) proteins help to balance the positive charges of intracellular cations (primarily K+) -In the extracellular fluid, the positive charges of Na+ and other cations are balanced chiefly by Cl -potassium (K+) plays the most important role in generating the membrane potential.

concentration vs electrical gradient

-concentration gradient: ions move along chemical concentration gradients from an area of their higher concentration to an area of lower concentration -electrical gradient: Ions move toward an area of opposite electrical charge.

Two factors generate the resting membrane potential

-differences in the ionic composition of the intracellular and extracellular fluids -differences in the plasma membrane's permeability to those ions

overview of nerve fibers

-group a: are mostly somatic sensory and motor fibers, has largest diameter, thick myelin sheaths, and conduct impulses at speeds up to 150 m/s -group b: lightly myelinated fibers of intermedi-ate diameter. They transmit impulses at an average rate of 15 m/s -group c: have the smallest diameter. They are non-myelinated, so they are incapable of saltatory conduction and conduct impulses at a leisurely pace—1 m/s

Information Transfer across Chemical Synapses

1. Action potential arrives at axon terminal 2. Voltage-gated Ca++ channels open and Ca++ enters the axon terminal 3. Ca++ entry causes synaptic vesicles to release neurotransmitter by exocytosis 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane 5. Binding of neurotransmitter opens/closes ion channels 6. Neurotransmitter effects are terminated

the spread of a graded potential

1. positive ions (mostly K) inside the cell move away from the depolarized area and accumulate on the neighboring membrane areas, where they neutralize negative ions. 2. Na+ on the outside of the membrane move toward the depolarized region, which is momentarily less positive 3. the inside becomes less negative and the outside becomes less positive 4. The depolarization spreads and the flow of current to adjacent membrane areas changes the membrane potential there as well 5. Since the plasma membrane is permeable, the current dies out within a few millimeters of its origin, decays and is said to be decremental

generating an action potential steps (Red Dogs Really Happy)

1. resting state: all gated Na+ and K+ channels are closed 2. Depolarization: Na+ channels open 3. Repolarization: Na+ channels are inactivating, and K+ channels open 4. Hyperpolarization: some K+ channels remain open, and Na+ channels reset

relative refractory period

A period after firing when a neuron is returning to its normal polarized state and will fire again only if the incoming message is much stronger than usual -most Na channels have returned to their resting state, some K channels are still open, and repolarization is occurring -the axon's threshold for AP generation is more elevated, so a stimulus that would normally generate an AP is not enough -An exceptionally strong stimulus can reopen the Na channels and make another AP

more about step 1: resting state

All gated Na+ and K+ channels are closed and only the leakage channels are open, maintaining resting membrane potential -Each Na+ channel has two gates: (more about this on slide 23) + a voltage-sensitive activation gate that is closed at rest and responds to depolarization by opening + an inactivation gate that blocks the channel once it is open. depolarization opens and then inactivates sodium channels. -Both gates must be open for Na to enter, but the closing of either gate effectively closes the channel -each voltage-gated potassium channel has a single gate that closed in the resting state and opens slowly in response to depolarization.

propagation of an action potential (overview)

An action potential is conducted along the axon as it depolarizes consecutive sections of the membrane -AP is caused by the entrance of Na towards an area that is more negative -This causes currents to depolarize adjacent membrane areas in the forward direction (away from the origin of the nerve impulse), which opens voltage-gated channels and triggers an AP there -this process repeats and is self propagated aka like dominos falling -After depolarization, each segment of the axon membrane repolarizes, restoring the resting membrane potential in that region.

Other Types of Synapses (pic on slide 46)

Axoaxonic, axosomatic, axoaxonal, Dendrodendritic, Dendrosomatic -axodendritic: Synapses between the axon endings of one neuron and the dendrites of other neurons -axoaxonic: synapses between axons -axosomatic: synapses between axon endings of one neuron and the cell body (soma) -dendrodendrtic: between dendrites -dendrosomatic: between dendrites and cell bodies

more about step 5

Binding of neurotransmitter opens ion channels, creating graded potentials -When a neurotransmitter binds to the receptor protein, this receptor changes its shape which opens ion channels and creates graded potentials -Depending on the receptor protein to which the neurotransmitter binds and the type of channel the receptor controls, the postsynaptic neuron may be either excited or inhibited.

more about step 3

Ca2 entry causes synaptic vesicles to release neurotrans-mitter by exocytosis -The surge of Ca2 into the axon terminal acts as an intracellular messenger. A Ca21-sensing protein (synaptotagmin) on the vesicle binds Ca21 and interacts with the SNARE proteins that control membrane fusion. -synaptic vesicles fuse with the axon membrane and empty their contents by exocytosis into the synaptic cleft -Ca2 is then quickly removed from the terminal—either taken up into the mitochondria or ejected from the neuron by an active Ca2 pump.

different names of graded potentials

Graded potentials are given different names, depending on where they occur and the functions they perform -can be called receptor potential or post synaptic potentials -Receptor potential/generator potential: produced when a sensory receptor is excited by its stimulus (e.g., light, pressure, chemicals) -Postsynaptic potential: is produced when the stimulus is a neurotransmitter released by another neuron.

nerve impulse

In a neuron, an AP is also called a nerve impulse -is typically generated only in axons -A neuron generates a nerve impulse only when adequately stimulated -The stimulus changes the permeability of the neuron's membrane by opening specific voltage-gated channels on the axon (no voltage-gated channels means no AP) -Voltage-gated channels open and close in response to changes in the membrane potential. They are initially activated by local currents (graded potentials) that spread toward the axon

what causes the action potential

Na permeability rises to such an extent that entering sodium ions exceed the outward movement of K, establishing the positive feedback cycle and generating an AP -The critical factor is the total amount of current that flows through the membrane during a stimulus -Strong stimuli: depolarize the membrane to threshold quickly -Weaker stimuli: must be applied for longer periods to provide enough current flow -Weak stimuli: don't trigger an AP because the local current flows they produce are so slight that they dissipate before threshold is reached

more about step 3 Repolarization (pic on slide 27)

Na+ channels are inactivating, and voltage-gated K+ channels open restoring the cell back to its normal internal negativity -the rise of the action potential only lasts 1ms and is self limiting bc the na channels begin to close -Na+ declines to resting levels, and the net influx of Na+ stops completely which causes the AP spike to stop rising -As Na+ entry declines, the slow voltage-gated K+ channels open and K rushes out of the cell, following its electrochem gradient

more about step 6

Neurotransmitter effects are terminated -Binding of a neurotransmitter to its receptor is reversible -As long as it is bound to a postsynaptic receptor, a neurotransmitter cont to affect membrane permeability and block reception of additional signals from presynaptic neurons

Presynaptic and postsynaptic neurons

Presynaptic: The neuron that carries the impulse towards the synapse Postsynaptic: The neuron that carries the impulse away from the synapse -the presynaptic neuron sends the information, and the postsynaptic neuron receives the information -most neurons function as both

repolarization

Return of the cell to resting state aka resting membrane potential -caused by reentry of potassium into the cell while sodium exits the cell.

more about step 4 Hyperpolarization (pic on slide 29)

Some K1 channels remain open, and Na channels reset -The period of increased K permeability lasts longer than needed to restore the restng state -bc of this, before the potassium channels close, a hyperpolarization is seen on the AP curve as a slight dip following the spike -At the end of this phase, the Na channels have reset to their original position by changing shape to reopen their inactivation gates and close their activation gates

electrical synapses

Synapses that transmit information via the direct flow of electrical current at gap junctions -less common than chemical synapses, consists of gap junctions -neurons are electrically coupled, and transmission across these synapses is very rapid -impulses can be uni or bi directional -electrical synapses are found in parts of the brain responsible for eye mvmt, the axoaxonal synapses in the hippocampus and in embryonic nervous tissue -As the nervous system develops, chemical synapses replace some electrical synapses and become the vast majority of all synapses.

threshhold and depolarization

The depolarization must reach threshold values if an axon is to fire and produce an action potential

resting membrane potential

The potential difference in a resting neuron -is usually -70mv ,the minus sign indicates that the cytoplasmic side (inside) of the membrane is negatively charged relative to the outside -the value can vary from -40mV to -90mV -resting potential exists only across the membrane; the solutions inside and outside the cell are electrically neutral

degree of myelination

The presence of a myelin sheath increases the speed of propagation -lightly myelinated fibers conduct more slowly than heavily myelinated fibers.

more about step 2 Depolarization (pic on slide 25)

Voltage-gated Na channels open, K= gates close and Na rushes into the cell. -This influx of positive charge depolarizes that local patch of membrane further, opening more Na channels until all Na channels are opened so the cell interior becomes progressively less negative. -once all channels are open the membrane potential becomes less and less negative and then overshoots to about +30 mV -rapid depolarization and polarity reversal produces the sharp upward spike of the action potential

more about step 2

Voltage-gated Ca2 channels open and Ca2 enters the axon terminal -Depolarization of the membrane by the action potential opens not only Na channels but voltage-gated Ca2 channels as well -During the brief time the Ca2 channels are open, Ca2 floods down its electrochemical gradient from the extracellular fluid into the terminal.

action potential (AP)

a brief reversal of membrane potential with a total amplitude (change in voltage) of about 100 mV (from 270 mV to 130 mV) -is the principal way neurons send signals over long distances -Only cells with excitable membranes (neurons and muscle cells) can generate action potentials. -Depolarization is fol-lowed by repolarization and often a short period of hyperpolarization -Unlike graded potentials, action potentials do not decay with distance.

2 ways to change membrane potential

a change in membrane potential can be produced by -anything that alters ion concentrations on the two sides of the membrane -anything that changes membrane permeability to any ion (permeability changes are important for transferring info) Note: depolarization and hyperpolarization describe changes in membrane potential relative to resting membrane potential.

depolarization

a decrease in membrane potential -The inside of the membrane becomes less negative (moves closer to zero) or more positive than the resting potential -ex: -70 mV to -65 mV is a depolarization

synapse

a junction that mediates infor-mation transfer from one neuron to the next or from a neuron to an effector cel -they transmit signals between neurons -Synaptic cleft: The narrow gap that separates the presynaptic neuron from the postsynaptic cell.

hyperpolarization

an increase in membrane potential -The inside of the membrane becomes more negative (moves farther from zero) than the resting potential -ex: -70mv to -75mv

chemical synapse

are specialized to allow the release and reception of neurotransmitters, chemical synapses convert the electrical signals to chemical signals (neurotransmitters) that travel across the synapse to the postsynaptic cells, where they are converted back into electrical signals -the most common type of synapse -made up of two parts: axon terminal of the presynaptic neuron that contains synaptic vessicles and receptor region on the postsynaptic neuron's membrane -chemical synapses prevent a nerve impulse from being directly transmitted from one neuron to another -an impulse is transmitted via a chemical event that depends on the release, diffusion, and receptor binding of neurotransmitter molecules and results in unidirectional communication between neurons

types of gated channels

chemically gated, voltage gated, mechanically gated -chemically gated: also called ligand gated channels, channels that open when the appropriate chemical or neurotransmitter binds -voltage gated: open and close in response to changes in the membrane potential -mechanically gated: open in response to physical deformation of the receptor

nerve fibers

classified according to diameter, degree of myelination, and conduction speed. -consists of group a, b and C fibers

action potential can be propogated in 1 of 2 ways

continous conduction and salatory conduction -continous conduction (is slow): how action potential propagation occurs in nonmyelinated axons bc the voltage-gated channels in the membrane are right next to each other -Salatory conduction: how action potential propagation occurs in myelinated axons, the current is maintained and moves rapidly to the next myelin sheath gap where it triggers another AP + is called salatory bc the electrical signal appears to jump from gap to gap + occurs 30x faster than cont conduction

electrochemical gradient

determines the direction that the ions move in -two components of electrochem gradients: concentration and electrical gradient

Two types of synapses

electrical and chemical

types of ion channels

leakage channels and gated channels -leakage channels: membrane channels that are always open -gated channels: Part of the protein forms a molecular "gate" that changes shape to open and close the channel in response to specific signals

subthreshold and threshold stimulus

local depolarizations are graded potentials and their magnitude increases when stimuli become more intense -subthreshold stimuli: Brief weak stimuli that produce subthreshold depolarizations that are not translated into nerve impulses -stronger threshold stimuli: produce depolarizing currents that push the membrane potential toward and beyond the threshold voltage.

positive feedback cycle

membrane potential can depend on membrane permeability and membrane permeability can depend on membrane potential -both statements can be true because these two relationships establish a positive feedback cycle: Increasing Na+ permeability due to increased channel openings leads to greater depolarization, which increases Na permeability. -The positive feedback cycle is responsible for the rising (depolarizing) phase of an action potential

propagation of a nerve impulse

propagation of a nerve impulse is more accurate, because the AP is regenerated anew by the voltage-gated channels at each membrane patch, and every sub-sequent AP is identical to the one that was generated initially. Without voltage-gated channels, propagation cannot occur

Sodium-Postassium Pump

pumps 3 sodium ions out of the cell for every 2 potassium ions pumped in -keeps the cell from having equal concentrations of Na and K inside and outside the cell -it stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium

repolarization definition 2

restoring the internal negativity of a resting neuron -Both the abrupt decline in Na permeability and the increased permeability to K contribute to repolarization -restores resting electrical conditions, Does not restore the resting ionic conditions

graded potentials

short-lived, localized changes in membrane potential, usually in dendrites or the cell body that cause current flows that decrease in magnitude with distance -can be depolarizations or hyperpolarizations -are called "graded" because their magnitude varies directly with stimulus strength. The stronger the stimulus, the more the voltage changes and the farther the current flows. -are triggered by a stimulus in the neuron's environment that opens gated ion channels.

axon diameter

the larger the axon's diameter, the faster it conducts impulses -Larger axons conduct faster bc they offer less resistance to the flow of local currents, bringing adjacent areas of the membrane to threshold more quickly

threshold (happens during step 2)

the level of stimulation required to trigger a neural impulse -When depolarization reaches a critical level called threshold (often between -55 and -50 mV), depo-larization becomes self-generating, urged on by positive feedback. -more na channels will keep on opening until all of them are open and reaches +30mV

conduction velocity

the speed at which an action potential is propagated along the length of an axon -depends on two things: axon diameter and degree of myelination

more about nerve impulses aka action potential in neurons

the transition from local graded potential to long-distance action potential takes place at the initial segment of the axon - In sensory neurons, the action potential is generated by the peripheral process

what determines the threshold point

threshold is the membrane potential at which the outward current created by K movement is exactly equal to the inward current created by Na mvmt -Threshold is reached when: the membrane has been depolarized by 15 to 20 mV from the resting value bc this is an unstable equilibrium state -If one more Na enters, further depolarization occurs, opening more Na channels and allowing more Na to enter -If one more K leaves, the membrane potential is driven away from threshold, Na channels close, and K continues to leave until the potential returns to its resting value.

absolute refractory period

time during which another action potential is impossible -when a patch of neuron membrane is generating an AP and its voltage-gated sodium channels are open, the neuron cannot respond to another stimulus no matter how strong -period begins with the open-ing of the Na1 channels and ends when the Na1 channels begin to reset to their original resting state

propagation of a action potential at 0ms

• Na+ influx causes a patch of the axonal membrane to depolarize • Positive ions in the axoplasm (cytoplasm of the axon) move toward the polarized (negative) portion of the membrane

propogation of an action potential at 2ms

•Ions of the ECF move toward the area of greatest (-)charge •A current is created that depolarizes the adjacent membrane in a forward direction •The impulse propagates away from its point of origin

more about salatory conduction

•Myelin sheaths insulate and prevent leakage of charge •Current passes through a myelinated axon only at the nodes of Ranvier •Voltage-gated Na+ channels are concentrated @ nodes •AP generated only @ nodes

propagation of an action potential at 4ms

•The action potential moves away from the stimulus •When Na+ gates are closing, K+ gates are open and create a current flow