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FSU / Science / PET 3322 / Why does na+ enter the cell during the action potential?

Why does na+ enter the cell during the action potential?

Why does na+ enter the cell during the action potential?

Description

School: Florida State University
Department: Science
Course: Functional Anatomy and Physiology I
Professor: Arturo figueroa-galvez
Term: Summer 2016
Tags: anatomy and Physiology
Cost: 25
Name: PET3322- Unit 2, Week 5
Description: These are class notes from T.Th. the week of Sept. 29th. Material covers intro to nervous system.
Uploaded: 10/01/2016
14 Pages 257 Views 4 Unlocks
Reviews


9/27/2016 TUESDAY UNIT 2- NERVOUS SYSTEM (LECTURE 1)


Why does na+ enter the cell during the action potential?



∙ Exam 1 Average- 82.58

∙ Make sure that the mechanisms for plasma membrane crossings is crystal clear ASAP! If  questions, Thursday's lecture will briefly review them.

∙ Don't memorize, understand...the information in class is more important than the  information in the text and in the PowerPoints (in terms of info. on the test). ∙ Exam 2 will cover CNS and half of the PNS (other half in Exam 3 with endocrine,  cardiovasuclar)

FUNCTIONS OF THE NERVOUS SYSTEM

∙ neurons- cell that has all the regular structures, but also perogation that names the  function of the neurons- to transmit electrical signal from one place to another to  stimulate distal cells

∙ neurotransmitter has to bind to a receptor in the cell (soma in the neuron and small  perogation)


Does facilitated diffusion move from high to low concentration?



1. Receiving sensory input. Monitor internal and external stimuli Don't forget about the age old question of What is the sociological imagination conceptand how do we use it to study social problems?

2. Integrating information. Brain and spinal cord process sensory input and initiate  responses

o CNS

3. Controlling muscles and glands

o skeletal muscles specifically

1. Maintaining homeostasis. Regulate and coordinate physiology

2. Establishing and maintaining mental activity. Consciousness, thinking, memory,  emotion 

∙ CNS = brain and spinal cord Don't forget about the age old question of What is the meaning of the leaf area index (lai)?

∙ PNS = cranial nerves (brain, 12) and spinal nerves (nerves that are connected to the  spinal column, 31)


When the inside of the cell becomes positive it is called?



If you want to learn more check out What is the difference between the old and new definition of mass media?

∙ What is a nerve? A bundle of axons and their sheaths that connects CNS to sensory  receptors, muscles, and glands...sheaths are in the PNS and connect to the CNS ∙ Peripheral nervous system

o Sensory (afferent): sensations/sensitive...pain, smell, vision, etc.; origin of these  sensations are the receptors in our bodies; transmits action potentials from  receptors to CNS We also discuss several other topics like What is magnetic resonance imaging used for?

 somatic fibers: impulses from skin, skeletal muscles, and joints to the brain  visceral fibers: impulses from visceral organs to the brain

 dorsal = back, posterior

 ventral = front, anterior

 Example: from sensory receptor in finger (somatic fibers) touching hot  stovetop to brain to tell brain it's hot (CNS)...

o Motor (efferent): means motion; stimulate contraction of the skeletal muscle;  transmit action potentials from CNS to effectors

 Example: ...brain (CNS) tells muscles in arm to move finger away (action  potential fires to cause skeletal contraction)

 voluntary

 synapse: junction (place of meeting) of a nerve cell with another cell  neuromuscular junction is a synapse between a neuron and skeletal muscle cellDon't forget about the age old question of What is the importance of the central dogma?

Dendritic spine

Mitochondrion Golgi apparatus Nucleolus

Nucleus

Nissl bodies

Axon

Trigger

Initial hillock

zone

segment

Dendrites

Neuron

cell

body

Axon

∙ Neurons: functional unit of nervous  system that receive stimuli and  

transmit action potentials; electrical  excitability; more mass in soma, less  mass in axons

o cell body/soma: single nucleus  

with prominent nucleolus

 nissl  

substance/chromatophilic

substance/ rough ER- site  

of protein synthesis

o dendrites: input

 short, often highly  

branches

 receptors

 sendritic spines- little  

protuberance where  

axons synapse with  

dendrite

o axons: output

 can branch to form  

collaterals

 initial segment

beginning of axon

 trigger zone- site where  

action potentials are  

generated; axon hillock  

and part of an axon  

nearest cell body

 synaptic vesicles

 presynaptic terminals

what causes the muscle  

contraction (if

Don't forget about the age old question of Does the economy of the united states affect world markets?

Myelin sheath formed by

Schwann cell Collateral axon

Schwann cell Node of Ranvier

Presynaptic terminals

∙ Color in the brain tissue is produced by density- more dense = darker, less dense =  lighter

o gray matter: unmyleinated axons, cell bodies, dendrites, neurogilia; outer cortex o white matter: inside of brain, outside of spinal cord

∙ Action potentials: (1) sensory receptors- PNS via action potentials, (2) sensory neurons PNS on dorsal spinal cord, (3) interneuron- axon terminals with vessicles with  neurotransmitters at presynaptic neuron, (4) postsynaptic neuron- send signal to  cortex/thalamus, (5) upper motor neuron- send signal to muscles in hand, (6) presynaptic central motor neuron- synapse in motor neurons beforehand motion, motor because  movement, central because CNS in spine, (7) neurotransmitters will release and bind to  receptors in post synaptic neuron, (8) stimulation causes motion across skeletal muscles  (neuromuscular junction)

∙ Resting membrane potentials (electrical charge):

o Gated ion channels- for facilitated diffusion of sodium

 charge inside resting cell is negative caused by the proteins in the  cytoplasm

 charge of membrane outside the cell is positive produced by a high  concentration of sodium

 resting membrane potentials for each cell varies

o Voltage gated channels- for potassium

o polarization- electrical charge becomes either positive or negative; usually caused by sodium...moving from high to low = facilitated diffusion by using gated channel proteins after Ach (comes from axon terminal) opens the gate

o depolarization- potential difference becomes smaller/less polar; more positive  Depolarization- If extracellular concentration of K+ increases: less  gradient between inside and outside.  

o hyperpolarization- potential difference becomes greater/more polar; more  negative

 Hyperpolarization- If extracellular ion concentration decreases: steeper  gradient between inside and outside.  

+35

Depolarization 0

Repolarization

)

V

Threshold (m

–70

Graded

Afterpotential Time (ms)

potential

∙ All-or-none principle. No matter how strong the stimulus, as long as it is greater than  threshold, then action potential will occur.

Know this slide (34, Nervous System Slides):

9/29/2016 THURSDAY UNIT 2- NERVOUS SYSTEM (LECTURE 2) Knowing the mechanisms is crucial for Exams 2-4!!!!

ESTABLISHING MEMBRANE POTENTIAL

(From Tuesday)

∙ Negative ions along inside of cell membrane & positive ions along outside ∙ potential energy difference at rest is –70 mV  

∙ Resting potential exists because:

1. concentration of ions different inside & outside

 extracellular fluid rich in Na+ and Cl– 

 cytosol full of K+, organic phosphate & amino acids

2. membrane permeability differs for Na+ and K+ 

 50-100 greater permeability for K+ 

 inward flow of Na+ can’t keep up with outward flow of K+ 

 Na+/K+ pump removes Na+ as fast as it leaks in

∙ -50 to -100 mV in all cells in the body; depends on excitability (capacity to respond) of  individual cells

NEURONS

∙ The sodium outside the cell gives the positive charge outside the cell, and the negative  charge inside the cell varies based on gated channels for facilitated diffusion, and we  need a neurotransmitter to open the gate

∙ Normal polarity at rest is NEGATIVE

∙ inside of the cell will change when sodium diffuses into the cell, so Na+ neutralizes the  charge (think of a linebacker tackling the quarterback in a game...linebacker (positive  charge) neutralizes (tackles) the quarterback (negative charge) )

∙ Once the charge is neutralized, then threshold can be reached. Sometimes this take  more than one sodium ion. Once the threshold is passed (caused by an increase in  positive charge), then an action potential occurs.  

∙ Conditions:

1. greater concentration of sodium outside the cell than inside (facilitated diffusion =  high to low)

2. open sodium gate (via help of a neuotransmitter)

3. to start diffusion, the inside of the cell has to be negative to attract sodium ∙ Now that there is a positive charge inside the cell, homeostasis must be maintained so  the positivity inside the cell becomes resting again...negative.

∙ Sodium diffusion opens the voltage gate for potassium to enter (end of depolarization) NOW IT'S THE POTASSIUM CHANNEL

∙ inside of the cell is positive

∙ potassium is positive

∙ potassium is leaving the cell Re-Polarization

∙ positivity starts decreasing

∙ the cell becomes more negative

∙ The goal is to reach the -70mV = homeostasis

o this is homeostasis to the electrical charge only, but we also have to pay  attention to the homeostasis of sodium and potassium as well...so now we  activate the sodium postassium pump

 ATP pump take 3 sodium out and puts 2 potassium in

 the cell is giving three and getting two...negative balance

 the diffusion of potassium through the channel causes the negativity to  increase (inside is more negative than the outside at rest...resting  

potential)

∙ more = hyper...hyperpolarization, hyperpolarity created by the  

sodium potassium pump

 then homeostasis is reached after the negativity decreases a little ∙ Is hyperpolarization the only electrical event in the cell?

∙ No. This is part of the re-polarization which is part of the stimulation; when we re polarize, we stimulate and activate the cell. Hyperpolarization is the consequence of  trying to restore the negative charge to the cell, and is part of the process.

o Hyperpolarization = more negative than the resting membrane potential ∙ The movement of the action potential is called propagation

∙ Number of potentials produced per unit of time to a stimulus (simply memorize these...) o Threshold stimulus: causes a graded potential that is great enough to initiate  an action potential.

o Subthreshold stimulus: does not cause a graded potential that is great enough to initiate an action potential.

∙ Myelinated sheaths increase the speed of some neurons. It is a phospholipid that allows  for a faster propagation for the action potential.  

∙ Un-myelinated axons are just regularly transmitted propagations of action potentials o Peripheral = soma of that neuron is in the spinal cord (even this is in the  CNS...spinal cord and brain...the soma are outside of the spinal cord, so it's still  peripheral)

o Neuromuscular junction = communication between muscles and the neurons ∙ Rate of impulse propagation is determined by:

o Axon diameter – the larger the diameter, the faster the impulse

o Presence of a myelin sheath – myelination dramatically increases impulse speed ∙ Synapse- the interaction between two neurons; site where action potentials in one cell  cause action potentials in another cell

o chemical synapses- one-way information transfer from a presynaptic neuron to a  postsynaptic neuron

 axodendritic: from axon to dendrite

 axosomatic: from axon to cell body

o Types of cells in synapse:

 presynaptic releases neurotransmitters

 postsynaptic elicits the response

Picture below is a confusing summary but may help some people (see slide #43 on  Neurotransmitter Lecture PP):

Closed activation

Voltage­gated

gate

Na+ 

Voltage­gated

)

Na+ channel Extracellular

+35 V

Resting membrane 

potential. Na+ channels  (pink) and most, but not all, K+ channels (purple) are closed. 

K+ channel

fluid

m

(

l

a

i

t

n

e

t

o

p

e n

0

Threshold

–70 a

The outside of the plasma membrane is positively Open

charged compared to the

r

b

m

eM

Resting

membrane potential ime (ms) T

inside.

inactivation

gate

Cytoplasm

Open

K+ 

)

Depolarization

activation

Na+ 

+35 V

Depolarization.

Na+ channels open. K+ 

channels begin to open. Depolarization results 

because the inward movement 

gate

Na+ 

Na+ channels open.

m

(

l

a

i

t

n

e

t

o

p

e n

0

Local

potential

Threshold

–70 a

of Na+ makes the inside of the

r

bm

Resting

membrane potential

membrane more positive.

Na Time (ms) + diffuse

e

M

into cell.

Closed

K+ diffuse

)

+35 V

Repolarization

Repolarization.  Na+ 3 

activation gate

out of cell.

m

(l

a

i

t

n

0

Resting

membrane

channels close and additional 

e

t

o

potential

K+ channels open. Na+ 

movement into the cell stops, and K+ movement out of the cell increases, causing repolarization.

K+ channels

K+ 

p

e

n

–70

a

r

b

m

e

K+ 

Threshold

Time (ms)

Closed

inactivation

open.

gate

Activation

Na+ channels close.

Na+ channel

M

)

+35 V

End of repolarization and

afterpotential. Voltage­gated Na+ 

gate closed

m

(l

a

i

t

n

e

t

0

channels are closed. Closure of the activation gates and opening of the inactivation gates reestablish the resting condition for Na+ channels  (see step 1).  Diffusion of K+ through voltage­gated channels produces the afterpotential.

K+ 

K+ channels

o

p

e

n

–70

a

r

b

m

e

M

K+ 

K+ channels

Threshold

Time (ms)

Inactivation open.

gate open

open.

)

+35 V

K+ channels

K+ channels

m

(l

0

Resting membrane potential.

The resting membrane potential is reestablished after the voltage­gated K+ channels close.

closed. Na+ channel closed.

a

i

t

n

e

t

o

p

e

n

–70

a

r

b

m

e

M

Threshold

Time (ms)

∙ neurotransmitters produced in soma and stored in the presynaptic terminals to cause  action potentials

∙ Exocytosis- takes the vesicles to transport outside the cell

∙ Facilitated diffusion happens when increasing Ca concentration enters, sodium diffuses  out, and depolarization occurs:

1

Presynaptic neuron

Nerve impulse

2

Ca2+ 

Synaptic end bulb

Ca2+ 

Voltage­gated Ca2+ channel Cytoplasm

Synaptic cleft

3

Ca2+ 

Synaptic

vesicles

Na+ 

Neurotransmitter

Neurotransmitter 4

receptor

Ligand­gated

5

Ligand­gated channel open

channel closed Postsynaptic neuron 6 7

Postsynaptic

potential

Nerv e 

impu lse

∙ ACh is really the only neurotransmitter needed for the test (norepinephrine will be on  exam 3)

o Enzymatic degradation:

 ACh: acetylcholinesterase splits ACh into acetic acid and choline. Choline  recycled within presynaptic neuron.

 2 components: (1) acetate & (2) choline

 together they stimulate the receptor

 if they are separated, there would be no effect (see step 3)

 if ACh is inactive, the response is also inactivated in the cell

 ACh produced in the soma

o this is the same concept as the gated parking lots (unit 1 notes)

∙ Neurotransmitters are excitatory in some cells and inhibitory in others o a depolarizing postsynaptic potential is called an EPSP

 it results from the opening of ligand-gated Na+ channels

o an inhibitory postsynaptic potential is called an IPSP

 it results from the opening of ligand-gated Cl– or K+ channels

 it causes the postsynaptic cell to become more negative or  

hyperpolarized

o the postsynaptic cell is less likely to reach threshold

∙ In the heart muscle, ACh is inhibitory

o When we excite the cell, the resting membrane becomes less negative/ more  positive until an action potential is stimulated after the threshold is passed  (depolarization --> repolarization (hyperpolarization) )

o in the heart, hyperpolarization occurs, but this is different!

 this is the ONLY electrical event, causing inhibition of the cell...in this case  the cell is the heart, and if the heart is inhibited, then it will contract with  less force, the heart rate will decrease

o potassium leaves, chlorine enters

∙ In skeletal muscle, ACh is inhibitory, but the receptor is different

o depends on the receptor

9/29/2016 THURSDAY UNIT 2- NERVOUS SYSTEM (LECTURE 2) Knowing the mechanisms is crucial for Exams 2-4!!!!

ESTABLISHING MEMBRANE POTENTIAL

(From Tuesday)

∙ Negative ions along inside of cell membrane & positive ions along outside ∙ potential energy difference at rest is –70 mV  

∙ Resting potential exists because:

1. concentration of ions different inside & outside

 extracellular fluid rich in Na+ and Cl– 

 cytosol full of K+, organic phosphate & amino acids

2. membrane permeability differs for Na+ and K+ 

 50-100 greater permeability for K+ 

 inward flow of Na+ can’t keep up with outward flow of K+ 

 Na+/K+ pump removes Na+ as fast as it leaks in

∙ -50 to -100 mV in all cells in the body; depends on excitability (capacity to respond) of  individual cells

NEURONS

∙ The sodium outside the cell gives the positive charge outside the cell, and the negative  charge inside the cell varies based on gated channels for facilitated diffusion, and we  need a neurotransmitter to open the gate

∙ Normal polarity at rest is NEGATIVE

∙ inside of the cell will change when sodium diffuses into the cell, so Na+ neutralizes the  charge (think of a linebacker tackling the quarterback in a game...linebacker (positive  charge) neutralizes (tackles) the quarterback (negative charge) )

∙ Once the charge is neutralized, then threshold can be reached. Sometimes this take  more than one sodium ion. Once the threshold is passed (caused by an increase in  positive charge), then an action potential occurs.  

∙ Conditions:

1. greater concentration of sodium outside the cell than inside (facilitated diffusion =  high to low)

2. open sodium gate (via help of a neuotransmitter)

3. to start diffusion, the inside of the cell has to be negative to attract sodium ∙ Now that there is a positive charge inside the cell, homeostasis must be maintained so  the positivity inside the cell becomes resting again...negative.

∙ Sodium diffusion opens the voltage gate for potassium to enter (end of depolarization) NOW IT'S THE POTASSIUM CHANNEL

∙ inside of the cell is positive

∙ potassium is positive

∙ potassium is leaving the cell Re-Polarization

∙ positivity starts decreasing

∙ the cell becomes more negative

∙ The goal is to reach the -70mV = homeostasis

o this is homeostasis to the electrical charge only, but we also have to pay  attention to the homeostasis of sodium and potassium as well...so now we  activate the sodium postassium pump

 ATP pump take 3 sodium out and puts 2 potassium in

 the cell is giving three and getting two...negative balance

 the diffusion of potassium through the channel causes the negativity to  increase (inside is more negative than the outside at rest...resting  

potential)

∙ more = hyper...hyperpolarization, hyperpolarity created by the  

sodium potassium pump

 then homeostasis is reached after the negativity decreases a little ∙ Is hyperpolarization the only electrical event in the cell?

∙ No. This is part of the re-polarization which is part of the stimulation; when we re polarize, we stimulate and activate the cell. Hyperpolarization is the consequence of  trying to restore the negative charge to the cell, and is part of the process.

o Hyperpolarization = more negative than the resting membrane potential ∙ The movement of the action potential is called propagation

∙ Number of potentials produced per unit of time to a stimulus (simply memorize these...) o Threshold stimulus: causes a graded potential that is great enough to initiate  an action potential.

o Subthreshold stimulus: does not cause a graded potential that is great enough to initiate an action potential.

∙ Myelinated sheaths increase the speed of some neurons. It is a phospholipid that allows  for a faster propagation for the action potential.  

∙ Un-myelinated axons are just regularly transmitted propagations of action potentials o Peripheral = soma of that neuron is in the spinal cord (even this is in the  CNS...spinal cord and brain...the soma are outside of the spinal cord, so it's still  peripheral)

o Neuromuscular junction = communication between muscles and the neurons ∙ Rate of impulse propagation is determined by:

o Axon diameter – the larger the diameter, the faster the impulse

o Presence of a myelin sheath – myelination dramatically increases impulse speed ∙ Synapse- the interaction between two neurons; site where action potentials in one cell  cause action potentials in another cell

o chemical synapses- one-way information transfer from a presynaptic neuron to a  postsynaptic neuron

 axodendritic: from axon to dendrite

 axosomatic: from axon to cell body

o Types of cells in synapse:

 presynaptic releases neurotransmitters

 postsynaptic elicits the response

Picture below is a confusing summary but may help some people (see slide #43 on  Neurotransmitter Lecture PP):

Closed activation

Voltage­gated

gate

Na+ 

Voltage­gated

)

Na+ channel Extracellular

+35 V

Resting membrane 

potential. Na+ channels  (pink) and most, but not all, K+ channels (purple) are closed. 

K+ channel

fluid

m

(

l

a

i

t

n

e

t

o

p

e n

0

Threshold

–70 a

The outside of the plasma membrane is positively Open

charged compared to the

r

b

m

eM

Resting

membrane potential ime (ms) T

inside.

inactivation

gate

Cytoplasm

Open

K+ 

)

Depolarization

activation

Na+ 

+35 V

Depolarization.

Na+ channels open. K+ 

channels begin to open. Depolarization results 

because the inward movement 

gate

Na+ 

Na+ channels open.

m

(

l

a

i

t

n

e

t

o

p

e n

0

Local

potential

Threshold

–70 a

of Na+ makes the inside of the

r

bm

Resting

membrane potential

membrane more positive.

Na Time (ms) + diffuse

e

M

into cell.

Closed

K+ diffuse

)

+35 V

Repolarization

Repolarization.  Na+ 3 

activation gate

out of cell.

m

(l

a

i

t

n

0

Resting

membrane

channels close and additional 

e

t

o

potential

K+ channels open. Na+ 

movement into the cell stops, and K+ movement out of the cell increases, causing repolarization.

K+ channels

K+ 

p

e

n

–70

a

r

b

m

e

K+ 

Threshold

Time (ms)

Closed

inactivation

open.

gate

Activation

Na+ channels close.

Na+ channel

M

)

+35 V

End of repolarization and

afterpotential. Voltage­gated Na+ 

gate closed

m

(l

a

i

t

n

e

t

0

channels are closed. Closure of the activation gates and opening of the inactivation gates reestablish the resting condition for Na+ channels  (see step 1).  Diffusion of K+ through voltage­gated channels produces the afterpotential.

K+ 

K+ channels

o

p

e

n

–70

a

r

b

m

e

M

K+ 

K+ channels

Threshold

Time (ms)

Inactivation open.

gate open

open.

)

+35 V

K+ channels

K+ channels

m

(l

0

Resting membrane potential.

The resting membrane potential is reestablished after the voltage­gated K+ channels close.

closed. Na+ channel closed.

a

i

t

n

e

t

o

p

e

n

–70

a

r

b

m

e

M

Threshold

Time (ms)

∙ neurotransmitters produced in soma and stored in the presynaptic terminals to cause  action potentials

∙ Exocytosis- takes the vesicles to transport outside the cell

∙ Facilitated diffusion happens when increasing Ca concentration enters, sodium diffuses  out, and depolarization occurs:

1

Presynaptic neuron

Nerve impulse

2

Ca2+ 

Synaptic end bulb

Ca2+ 

Voltage­gated Ca2+ channel Cytoplasm

Synaptic cleft

3

Ca2+ 

Synaptic

vesicles

Na+ 

Neurotransmitter

Neurotransmitter 4

receptor

Ligand­gated

5

Ligand­gated channel open

channel closed Postsynaptic neuron 6 7

Postsynaptic

potential

Nerv e 

impu lse

∙ ACh is really the only neurotransmitter needed for the test (norepinephrine will be on  exam 3)

o Enzymatic degradation:

 ACh: acetylcholinesterase splits ACh into acetic acid and choline. Choline  recycled within presynaptic neuron.

 2 components: (1) acetate & (2) choline

 together they stimulate the receptor

 if they are separated, there would be no effect (see step 3)

 if ACh is inactive, the response is also inactivated in the cell

 ACh produced in the soma

o this is the same concept as the gated parking lots (unit 1 notes)

∙ Neurotransmitters are excitatory in some cells and inhibitory in others o a depolarizing postsynaptic potential is called an EPSP

 it results from the opening of ligand-gated Na+ channels

o an inhibitory postsynaptic potential is called an IPSP

 it results from the opening of ligand-gated Cl– or K+ channels

 it causes the postsynaptic cell to become more negative or  

hyperpolarized

o the postsynaptic cell is less likely to reach threshold

∙ In the heart muscle, ACh is inhibitory

o When we excite the cell, the resting membrane becomes less negative/ more  positive until an action potential is stimulated after the threshold is passed  (depolarization --> repolarization (hyperpolarization) )

o in the heart, hyperpolarization occurs, but this is different!

 this is the ONLY electrical event, causing inhibition of the cell...in this case  the cell is the heart, and if the heart is inhibited, then it will contract with  less force, the heart rate will decrease

o potassium leaves, chlorine enters

∙ In skeletal muscle, ACh is inhibitory, but the receptor is different

o depends on the receptor

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