Chapter 4

neural and hormonal communication

\[Na+]o \________ [Na+]i

\>

\[K+]o \__________ [K+]i

<

the uneven distribution of Na+ and K+ across the plasma membrane causes all cells to have what

a negative membrane potential (inside more negative than outside)

plasma membrane is \__________

polarized

polarized

potential is much lower than 0 mV

neurons and muscle cells are \____________ because they produce \____________________

excitable
electrical signals

electrical signals

deviations from the resting potential

electrical signals can do what 3 things

1. depolarized
2. repolarized
3. hyperpolarized

what causes an electrical signal

changes in membrane permeability due to channel proteins

K+ and Na+ flow through \__________ channels

leak

Electrical signals arise from ion flowing through \_________ channels

gated

gated channels

may be open or closed

leak channels

channels that are always open

how do gated channels open

in response to a stimulus

4 types of gated channels

1. voltage gated
2. chemically gated
3. mechanically gated
4. thermally gated

what are the 2 basic types of electrical signals

graded potentials and action potentials

graded potentials travel \________ distances

short

action potentials travel \_______ distances

long

how do graded potentials spread across membranes

through passive current flow

what is the process of passive current flow

1. gated Na+ channel originally close; Na+ and K+ leak channels open
2. stimulus opens gated channel, Na+ rushed into cell --> membrane near open Na+ channel becomes depolarized (less negative)
3. inward rushing Na+ repels K+ already inside --> K+ flows out through neighboring K+ leak channels

what are the 3 properties of graded potentials

1. membrane potential dies off quickly away from the site of activated gated channels (short distance signal)
2. stronger stimulus --\> larger graded potentials
weaker stimulus --\> smaller graded potentials
3. duration of graded potential lasts as long as stimulus duration

why does the membrane potential die off quickly away from the site of activated gated channel

due to ionic current leaking through leak channels in neighboring membrane

why does a stronger stimulus activate more gated channels

more current flow through membrane
greater depolarization

why does the duration of graded potential last as long as stimulus duration

gated channels remain open as long as stimulus is present

action potential

very large, transient depolarization of membrane cause by coordinated activity of voltage-gated Na+ and K+ channels

voltage-gated Na+ channel process

1. at membrane potentials below threshold --\> channel closed
2. as membrane potential despolarizes above threshold:
a. activation gate quickly opens --\> channel quickly opens
b. inactivation gate slowly swing shut --\> channel slowly closes

voltage-gated K+ channel process

1. at membrane potentials below threshold --\> channels closed
2. as membrane potential depolarizes above threshold --\> channels slowly opens
3. as membrane potential repolarizes below threshold --\> channel slowly closes

what prevents multiple action potentials from eliminating Na+ and K+ concentration gradients over time?

constant action of Na+/K+ pump

timeline of the action potential

1. Na+ channel and K+ channel are closed and depolarizing triggering even occurs
2. Na+ channel opens and is activated
3. Na+ influx
4. K+ channel opens
5. K+ efflux
6. Na+ channel reset to closed but capable of opening
7. after hyperpolarization
8. K+ channel closes

gated channels in dendrites only produce \____________ potentials

graded

voltage-gated channels in axon produce \_________ potentials

action

dendrites

short, numerous extensions from cell body of neuron that receive electrical signals from other neurons

axon

very long, single extension from cell body of neuron that transmits action potential to other neurons

axon hillock

initial portion of axon that leaves cell body

why does the action potential travel down the axon

voltage-gated channels lead to self-perpetuation of action potential

how do action potentials travel down the axon

1. gated channels in dendrites create graded potentials
2. if potential exceeds threshold at axon hillock, voltage-gated Na+ and K+ channels are activated (influx of Na+ depolarizes membrane of axon hillock [beginning of action potential])
3. depolarization spreads to adjacent membrane in axon --\> activates new voltage-gated Na+ and K+ channels (action potential created at adjacent membrane of axon)
4. process repeats for each adjacent segment of axonal membrane

refractory period

time after threshold during which another action potential cannot occur

refractory periods are due to what 2 things

1. inactivation gate of voltage-gated Na+ channel is shut and slow to reopen
2. activation gate of voltage-gated K+ channel is open and slow to close

what is the consequence of refractory period

action potential only moves in direction away from cell body

axon terminals

branches at the end of the axon

what do axon terminals contact

dendrites of other neurons

action potential travels down axon to \____________

synapses

synapses

junctions between neurons through which signal is passed from one neuron to another

what are the 2 types of synapses

electrical and chemical

in electrical synapses, how are 2 neurons joined

by gap junctions

what do electrical signals allow for

direct passage of electrical current - ions flow through connexons

which synapse is more prevelant

chemical synapse

process of chemical neurotransmission

1. action potential reaches axon terminal which specifically contains voltage-gated Ca2+ channels
2. depolarization of axon terminal triggers opening of activation gate of voltage-gated Ca2+ channels --\> Ca2+ floods into cell
3. increased intracellular Ca2+ concentration triggers secretory vesicles to undergo exocytosis at synapse
4. secretory vesicles contain chemical signal (neurotransmitter) which diffuses across synaptic cleft (space between neurons at the synapse)
5. neurotransmitter binds to chemically gated channels on the postsynaptic neuron --\> opens activation gate
6. ions flow through chemically gated channels and create a graded potential in postsynaptic neuron (postsynaptic potential)

what determines the kind of postsynaptic potential that forms

type of neurotransmitter contained in secretory vesicles
type of neurotransmitter-gated channels in postsynaptic membrane

excitatory postsynaptic potential

depolarizes membrane closer to action potential threshold

inhibitory postsynaptic potential

hyperpolarizes membrane away from action potential threshold

what does the type of neurotransmitter-gated channel determine

a. whether EPSP vs. IPSP
b. duration of PSP
c. amplitude of PSP

crossing action potential threshold depends on what?

- the relative timing of inputs
- the number of inputs occurring at the same time
- the balance of excitatory and inhibitory inputs

direct communication

direct physical contact between cells

indirect communication

chemical messenger sent to target cells

what are examples of direct communication

gap junctions
transient direct linkup of cells' surface markers

what are examples of indirect communication

paracrine
neurotransmitter
hormone
neurohormone

paracrine secretion

chemical messenger whose target cells are int he immediate vicinity

hormone secretion

chemical messenger secreted into blood whose target cells are throughout body

neurohormone secretion

hormone released by special neurosecretory neurons

how do chemical messengers activate target cells

depends on whether chemical messenger is lipid-soluble or water-soluble

lipid soluble messengers diffuse...

directly through the plasma membrane

what do lipid soluble messengers activate

intracellular receptors

water soluble messengers

cannot directly pass through plasma membrane

what do water soluble messengers activate

receptors on the extracellular side of the plasma membrane

the binding of the first messenger to receptor leads to either .....

1. opening of a chemically gated receptor channel
2. direct activation of receptor-enzyme complexes
3. direct activation of a G protein, which then produces a second messenger that diffuses intracellularly with many intracellular targets

what do receptor-enzyme complexes cause

phosphorylation of designated intracellular proteins

lipid-soluble chemical messengers often change \___________

gene expression

what is an example of a lipid soluble chemical messenger

steroid hormones

receptor-enzyme complexes bind what?

water-soluble messengers on extracellular surface of plasma membrane

receptor-enzyme complexes either....

a. is a protein kinase itself (type of enzyme)
b. is attached to protein kinases

activation of receptor activates \______________________

protein kinase

protein kinase

enzyme that transfers phosphate group from ATP to particular intracellular protein

how do you get the active form of a protein

phosphorylation

how do you get an inactive form of a protein

dephosphorylation

what is the receptor of tyrosine kinase pathway

a kinase itself

g-protein coupled receptors bind what?

bind water-soluble, first messengers on extracellular surface of plasma membrane

the binding of water soluble, first messengers on the g-protein-coupled receptors lead to what

signal cascade

g protein

membrane-bound, peripheral protein that acts as intermediary signal

effector protein

increases or decreases the amount of second messenger

second messenger

chemical messenger that diffuses throughout intracellular space

what puts an end to the signal cascade

a. first messenger becomes unbound from receptor
b. G protein spontaneously reverts back to inactive state
c. enzymes degrade second messengers and inactivate protein kinases
d. protein phosphatases remove phosphate groups from proteins

protein phosphatases

enzymes that remove phosphate groups form designated proteins

what are the 2 kinds of paracrines

cytokines and eicosanoids

cytokines

protein signal molecules

function of cytokines

mediate inflammation, enhance function of immune cells, influence cell growth during development (growth factors)

cytokines are secreted by?

white blood cells of immune system
endothelial cells lining blood vessels
adipose tissue cells that store fate

eicosanoids

lipid signal molecules

what is the most ubiquitous chemical messenger

eicosanoids

what are the 3 classes of eicosanoids

prostaglandins, thromboxanes, leukotrienes

prostaglandins

dependent on where secreted in the body

thromboxanes

promote blood vessel constriction and stoppage of bleeding

leukotrienes

involved in inflammatory responses

what are the 2 groups of hormones

hydrophilic, lipophilic

hydrophilic hormones include what?

peptide hormones and some amine hormones

lipophilic hormones include what?

steroid hormones and some amine hormones

what is the secretion of hydrophilic hormones

exocytosis from secretory vesicles

what is the secretion of lipophilic hormones

diffusion through membrane immediately after production

what is the transportation in blood like in hydrophilic hormones

mostly free floating

some bound to plasma proteins

what is the transportation in blood like for lipophilic hormones

mostly bound to plasma proteins