Test 3- Ch. 11 Continued

trigger zone

threshold graded current at ______ causes action potentials

continuous conduction

• action potential in one site causes action potential at the next location.

• cannot go backwards because initial action potential site is depolarized due to Na+ flooding in through voltage gates as Na+ goes down the axon yielding one-way conduction of impulse

• occurs in unmyelinated axons

saltatory conduction

myelinated axons employ ______

• Na+ must go through the myelin sheath before the node of ranvier where voltage gates are located.

• think jumping

AP propogation in an unmyelinated axon

1. when an action potential is produced, the inside of the membrane becomes more positive than the outside. On the outside of the membrane, Na+ from the adjacent area is attracted to the negative charges at the site of the AP. once inside the cell, Na+ diffuses away from the entry point, and this diffusion is called a LOCAL CURRENT

2. as a result of the local current, the part of the membrane immediately adjacent to the AP depolarizes

3. if an action potential is intiated at one end of an axon, it is propagated in one direction down the axon. the absolute refractory period ensures one-way propogation of an action potential because it prevents the local current from stimulating the production of an action potential in the reverse direction

local current

• step 1 in AP propogation in an unmyelinated acon

• when an action potential is produced, the inside of the membrane becomes more positive than the outside. on the outside of the membrane, Na+ from the adjacent area is attracted to the negative charges at the site of the action potential. Once inside the cell, Na+ diffuses away from the entry points. This diffusion of Na+ is called a ______

depolarizes

• step 2 in AP propagation in an unmyelinated axon

• as a result of the local current, the part of the membrane immediately adjacent to the action potential ______

absolute refractory period

• step 3 of AP propagation in an unmeylinated axon

• if an action potential is initiated at one end of an axon, it is propagated in one direction down the axon. The ______ ensures one-way propogation of an AP because it prevents the local current from stimulating the production of an AP in the reverse direction.

saltatory conduction steps

1. an action potential at a node of ranvier generates local currents. The lipids within the membranes of the myelin sheath act as a layer of insulation, forcing the local currents to flow from one node of Ranvier to the next

2. voltage-gated Na+ channels are highly concentrated at the node of Ranvier. Therefore, the local current quickly flows to the next node and stimulates the voltage-gated Na+ channels to open, resulting in the production of an action potential

3. the action potential seems to jump from node to node, increasing the speed of conduction

insultation

• step 1 in saltatory conduction

• an action potential at a node of Ranvier generates local currents. The lipids within the membranes of the myelin sheath acts as a layer of ______, forcing the local currents to flow from one node of Ranvier to the next

highly

• step 2 in saltatory conduction

• voltage-gated Na+ channels are _____ concentrated at the nodes of Ranvier. Therefore, the local current quickly flows to the next node and stimulates the voltage-gated Na+ channels to open, resulting in the production of an action potential

increasing

• step 3 of saltatory conduction

• the action potential seems to jump from node to node, ______ the speed of conduction

Speed of Conduction

• faster in myelinated axons than in non-myelinated due to jumping from node to node

• in myelinated, speed is affected by thickness of myelin sheath (Type A-C)

• diameter of axons: large-diameter conducts more rapidly small-diameter. Large have greater surface area and more voltage-gated Na_ channels

Type A

• large-diameter, myelinated

• conduct at 15-120 m/s

• motor neurons supplying skeletal and most sensory neurons

• keep you alive during emergencies

• more space for Na+ to move, so it’s faster

Type B

• nedium-diameter, lightly myelinated

• condusct at 3-15 m/s

• part of ANS

• same amount of Na+ as other types, but less room to move Na+ inside so it’s slower

Type C

• small-diameter, unmyelinated

• conduct at 2 or less m/s

• part of ANS

Synapse

• junction gap between 2 cells

• site where action potentials in one cell cause action potential in another

• types of cells in synapse: presynaptic and postsynaptic

types of synapses

1. electrical

2. chemical

presynaptic (presynapse)

cell that transmits signal toward the synapse

postsynapstic (postsynapse)

target cell receiving the signal

electrical synapses

• cells connected by gap junctions that allow graded current (Na+) to flow between adjacent cells

  • connexons: protein tubes in cell membrane

• found in cardiac muscle and many types of smooth muscle

• important where contractile activity among a group of cells occurs

example of electrical synapse

due to gap junctions, sodium can zip through the heart with no diffusion so the whole heart can contract at one time

Connexon

protein tube in cell membrane

Electrical synapse Explained

1. an action potential in one cell produces a local current as Na+ from once cell flows to the adjacent part of the plasma membrane and through the gap junction

2. the movement of Na+ generates an action potential in the adjacent cell almost as if the 2 cells had the samem membrane.

3. as a result, action potentials are conducted rapidly between cells, allowing the cells to be synchronized

chemical synapse components

1. presynaptic terminal- end of the presynaptic cell axon

2. synaptic cleft- space between the presynaptic terminal and postsynaptic membrane

3. postsynaptic membrane- membrane of other neurons, muscle cells, or glial cells

neurotransmitters

in chemical synapses _______ are released by action potentials in presynaptic terminal

how neurotransmitters work in chemical synapses

1. synaptic vesicles: action potential cause calcium to enter the cell that causes neurotransmitters to be released from vesicles

2. diffusion of neurotransmitters across the synapse

3. Postsynaptic membrane: when neurotransmitters bind to the receptor, ligand-gated ion channels open

step 1 in chemical synapses

action potentials arriving at the presynaptic terminal causes voltage-gated calcium channels to open

step 2 in chemical synapses

Ca2+ diffuses into the cell and causes synaptic vesicles to release neurotransmitter molecules. Most neurophysiologists agree that synaptic vesicle membranes have a calcium sensor, such as synaptotagmin.

Once calcium binds to the vesicle, the vesicle interacts with membrane-trafficking proteins proteins that guide the vesicle to the presynaptic terminal. The vesicle fuses with the membrane of the presynaptic terminal and releases its contents into the synaptic cleft

step 3 in chemical synapse

neurotransmitter molecules diffuse from the presynaptic terminal across the synaptic cleft

step 4 in chemical synapse

neurotransmitter molecules bind reversibly to specific receptors, such as ligand-gated ion channels, in the postsynaptic membrane. Depending on the ion channel type, this binding produces a depolarization or hyperpolarizing graded potential in the postsynaptic membrane.

Ex) the binding of the neurotransmitter to ligand-gated Na+ channels causes the gates to open, allowing Na+ to diffuse into the postsynaptic cleft. If the resulting depolarizing graded potential reaches the threshold, an action potential is produced.

• on the other hand, the opening of K+ and Cl- channels result in a hyperpolarizing graded potential

neurotransmitter removal

• method depends on neurotransmitter/synapse

• neurotransmitters have short-term effects in postsynaptic membranes because they are rapidly destroyed or removed from the synaptic cleft

Removal of Neurotransmitter from Synaptic Cleft: ACh

1. acetylcholine molecules are released from their receptors

2. acetylcholinesterase splits acetylcholine into choline and acetic acid, which prevents acetylcholine from binding to its receptors again

3. choline is taken up by the presynaptic terminal and combines with acetyl-CoA to reform acetylcholine

4. other acetylcholine molecules diffuse into extracellular fluid away from the synaptic cleft

released

acetylcholine molecules are _______ from their receptors

• step 1 to removing ACh from synaptic cleft

acetylcholinesterase

_______ splits acetylcholine into chlorine and acetic acid, which prevents acetylcholine from binding to its receptors again

• step 2 in removing ACh from synaptic cleft

reform

choline is taken up by the presynaptic terminal and combines with acetyl-CoA to _____ acetylcholine

• choline is transported with a sodium molecule, so it uses a symporter (2 molecules in 1 direction at the same time)

• step 3 in removing ACh from synaptic cleft

extracellular fluid

other acetylcholine molecules diffuse into _______ away from the synaptic cleft

• step 4 in removing ACh in synaptic cleft

Removal of Neurotransmitter from Synaptic Cleft: NE

1. norepinephrine is released into the synaptic cleft

2. norepinephrine is taken up by the presynaptic terminal, which prevents NE from again binding to the receptor

3. norepinephrine is repackaged into synaptic vesicles

4. the enzyme monoamine oxidase (MAO) breaks down some of the NE molecule

terminal

• NE uses the same symporter, but no enzyme like ACH to break it down into 2

• instead, norepinephrine goes back up into the presynaptic ______, but not all of it is used and some escapes and is used as a waste product

Receptor Molecules in Synapses

• neurotransmitter only “fits” into one receptor

• neurotransmitter affects only the cells with receptors for the neurotransmitter

• neurotransmitters are excitatory in some cells and inhibitory in others

• some neurotransmitters (for ex, Norepinephrine) can also attach to the presynaptic terminal and modulate its own release

body

our ____ releases norepinephrine, which increases your BP and HR but too much will kill you, so NE will modulate itself to stop by attaching to its own presynaptic terminal

graded potential

stimulation of neurons initiates a _______ in the cell body. This is often the result of opening of ligand-gated channels. If the degree of stimulation is great enough, the _____ reaches a threshold

Depolarization at axon hillock

when the membrane potential reaches threshold, voltage-gated Na+ channels open allowing an influx of Na+. This results in rapid ______

repolarization

Na+ channels become inactive and K+ channels open, allowing K+ to move out of the cell so cell can become negatively charged again. This results in the _____ of the plasma membrane

action potentail propagation

the action potential at the axon hillock results in local currents, which bring the next area of the axon to threshold. This leads to the ________ along the axon toward the presynaptic terminal

synaptic communication

an action potential at the presynaptic terminal results in the release of neurotransmitters that will alter the activity of the postsynaptic cleft

neurotransmitters and neuromodulators

• chemical messangers secreted by neurons

• neurons can secrete more than one typer

• at least 100 types

criteria to be a neurotransmitter

1. must be synthesized by a neuron and stored within synaptic vesicles in the presynaptic terminals

2. an action potential must stimulate its release (exocytosis) in the synaptic cleft

3. it must bind to a specific receptor on the postsynaptic membrane

4. must evoke a response in the postsynaptic cell

classification of neurotransmitters

_______ based on:

1. chemical structure

2. their effect on the postsynaptic membrane

3. the mechanism of action at their target

variety

neurotransmitters can have an excitaroy or inhibitory effect in a _____ of ways

• ionotropic effect

• metabotropic effect

ionotropic effect

binding to ion channels

metabotropic effect

binding to G-protein-linked receptors

function

the ____ of a particular neurotransmitter is determined by its receptor in the target cell (postsynapse)

Major Chemical classes of Neurotransmitters

1. Acetylcholine (ACh)

2. Biogenic Amines

3. Amino Acids

4. Purines

5. Neuropeptides

6. Gases and Lipids

Acetylcholine (ACh)

best understood and most common

Biogenic amines

catacholamines and indoleamines; examples include serotonin, dopamine, and norepinephrine

amino acids

examples include GABA, glycine and glutamate

Purines

nitrogen-containing compounds derived from nucleic acids; examples include adenosine and ATP

neuropeptides

short chains of amino acids; include Substance P and endorphines

Gases and Lipids

examples include the gasotransmitters nitric oxide (gas) and carbon monoxide (gas), and endocannabinoids (lipid-derived)

Responses at the Postsynaptic Cleft

1. Excitatory postsynaptic potential (EPSP)

2. Inhibitory postsynaptic potentail (IPSP)

excitatory postsynaptic potential (EPSP)

• depolarization occurs and response is stimulatory

• depolarization might reach threshold producing an action potential and cell response

• easier for AP to hit threshold- depolarization occurs due to Na+ rushing in so it helps AP occur

Inhibitory postsynaptic potential (IPSP)

• hyperpolarization and response is inhibitory

• decrease likelihood of action potential by moving membrane potential farther from the threshold

• ex: endorphines are great at stimulating IPSP because the make you happy, helps take pain away so you can function by stoping AP from reaching brain

Neuromodulators

influence likelihood of an action potential being produced in postsynaptic cell

many synaptic types

1. Axoaxonic synapse

2. Presynaptic inhibition

3. Presynaptic facilitation

Axoaxonic Synapses

• axon of one neuron synapses with the presynaptic terminal of another

• common in CNS

Presynaptic inhibition

• reduction in the amount of neurotransmitter released from the presynaptic terminal

• endorphins can inhibit pain sensation

• endorphins kick in and stop AP from continuing to the nect AP/axon

Presynaptic facilitation

• amount of neurotransmitter released from presynaptic terminal increases

• Glutamate facilitating nitric oxide production

• more Na+ gates open, allowing more Na+ to come in

Spatial and Temporal Summation

• a single postsynaptic potential is not sufficient to reach threshold

• many postsynaptic potentials combine in summation at the trigger zone. If threshold is reached, an action potential is triggered

• 2 types of summation:

  • spatial and temporal

Spatial Summation

Action Potentials 1 and 2 cause the production of graded potentials at 2 different dendrites. These graded potentials summate at the trigger zone to produce a graded potential that exceeds threshold, result in an action potential

• add up AP of Na+ already there at the same time to get this

temporal summation

two action potentials arrive in close succession at the presynaptic membrane. the 1st AP causes the production of a graded potential that does not reach threshold at the trigger zone. The 2nd AP results in the production of a second graded potential that summates with the first to reach threshold, resulting in the production of an action potential

• occurs overtime

• ex: gate opens, +1 Na+, gate close, gate open +1 Na+ until AP

produced

an action potential is ____ at the trigger zone when the graded potentials produced as a result of the EPSPs and IPSPs summate to reach

Neuronal Pathways and Circuits

• organization of neurons in CNS varies in complexity

1. serial pathway

2. convergent pathways

3. divergent pathways

4. reverbrating circiut

5. parallel after-discharge circuit

serial pathway

input travels along one pathway

convergent pathways

• many converge and synapse with small number of neurons. Ex: synthesis of data in brain

• multiple neurons of input make 1 outpus.

• ex: multiple neurons in brain make 1 output of grabbing your cup

Divergent pathways

• small number of presynaptic neurons synapse with large number of postsynaptic neurons. For example, important information can be transmitted to many parts of the brain

• 1 input w/ several outputs

• ex: vision but many pathways to process what you see

Reverberating Circuit

• outputs cause reciprocal activation. For example, rhythmic activities such as breathing

• EPSP easy here

• ex) brain sets rhythm (1 signal), heart beats w/ that signal and neurons will stimulate each other until brain says otherwise

Parallel After-Discharge Circuit

• neurons stimulate several neurons in parallel organization, which converge upon a common output cell. Ex: complex data processing in brain

• 1 input runs parallel to the brain and 1 outpus

• ex: reading a question, lots of thinking to the question, output is writing A for the answer to the question