1.2 B Receptive Region and Graded Potential

Dendrites

short unmyelinated processes branching off cell body

receive input and transfers these signals to cell body

cell body

aka soma

contains nucleus and organelles

receives signals and transfers them towards the axon hillock region

axon

conducts action potentials from axon hillock region to the axon terminal

may be myelinated by neuroglial cells

neurotransmitters are released from the axon terminal synaptic knob region in response to action potentials reaching this area

axon hilock

contains the initial segment where summation of inputs occurs to determine if threshold is met

if threshold is met an action potential in initiated in this region and conducted down the axon

are dendrites myelinated or unmyelinated

unmyelinated

do dendrites receive or provide a response

receive input

what is at the end of axons

synaptic knobs

receptive/receiving segment

binding of neurotransmitters released from presynaptic neurons

production of graded potentials

• can be excitatory or inhibitory

initial segment

in between condductive segment and receptive segment

summation of graded potentials

initiation of action potential

conductive segment

voltage gated channels

propagation of action potential

transmissive segment

action potential causes the release of neurotransmitter

neurons at rest

ions are unevenly distributed across the plasma membrane

• due to action of pumps

sodium leak channels are always open

potassium leak channels are always open

how are the concentrations of ions unevenly distributed across plasma membrane in resting neurons

na+, cl- and ca+ are at higher extracellular concentration

k+ higher intercellular concentration

resting membrane potential rmp

-70mv in most neurons

this is an electrical charge difference from inside and outside of cell

how is rmp maintaijned

througgh leak channels and pump action

where are k+ and na+ leak channels located

all regions of the neuron

is there a greater # of k+ or na+ leak channels… hjow does this affect rmp

greater # of k+

more k+ is leaked than na+ is gained

results in net negative charge…. -70mv

Na+ and k+ pumps

• unequal movement of ions… 3 positive charges out 2 positive in,,,, net negative

  • maintains concentration gradient for these ions

how much energy does the na+ and k+ account for

75%

receptive segment

at post synaptic membrane

• change in membrane potential due to opening of chemically gated channels

• small localized change in membrane potential

• signals vary in both direction of change and strength of signal

  • signals are multidirectional and weaken with distance

chemically gated cation channels

na+ comes in

k+ goes out

more na+ comes in than k+ goes out

DEPOLARIZE - makes the inner membrane more positive

depolarization

make more positive

chemically gated potassium

net efflux of k+

HYPERPOLARIZATION - makes inner membrane more negative

hyperpolarization

make more negative

efflux

leave

chemically gated chloride channel cl-

net influx of cl-

hyperpolarization

Postsynaptic potentials

graded potentials that change the membrane potential

rest in either depolarization or hyperpolarization of the membrane

signals vary in strength and decrease with distance and over time

depolarization postsynaptic potentials

ion movement due to open receptors causes the inside to be relatively more positive (+)

hyperpolarization

ion movement due to open receptors causes the isde to be relatively more negative (-)

steps in generation of an excitatory postsynaptic potential EPSP

1. excitatory neurotransmitter is released from the synaptic knob of a presynaptic knob of a neuron and diffuses across the synaptic cleft

2. the neurotransmitter binds to a chemically gated cation channel causing them to open resulting in a net influx of sodium into the neuron

3. EPSP is established as the inner face of the plasma membrane becomes depolarized…. more positive

   1. moves towards the threshold to send out an action potential

4. EPSP weakens as it moves from where it was initiated towards the initial segment where summation occurs

what does the presence of calcium ions cause

synaptic cesivles to bind to the membrane allowing neurotransmitters to be released

• voltage gated calcium must travel across synaptic cleft

steps in the generation of an inhibitory post synaptic potential (IPSP)q

1. inhibitory neurotransmitter is released from the synaptic knob

2. neurotransmitter binds chemically gated k+ channels or chemically gated cl- channels causing them to open

   1. allows for specific ions to flow through the gradient.

   2. results in inner face of membrane being more NEGATIVE

3. IPSP is established as the inner face of the plasma membrane and hyperpolarizes (becomes more NEGATIVE)

4. IPSP weakens as it moves from initiated point towards the initial segment where summation occurs

Temporal summation

SAME presynaptic neuron initiates postsynpatic potentials rapidly within a narrow period of time

not as effective as spatial summation

ONLY APPLIES TO A SINGLE SYNAPSE

Spatial summation

DIFFERENT presynaptic neurons initiate postsynaptic potentials within a narrow period of time

APPLIES TO 2 OR MORE SYNAPSES OCCURING SIMULTANESOUSLY

summation

cumalative impact of all simultanesously received graded potentials upon the intial segment

threshold

• 15 mV from RMP

  • determines if action potential is initiated

action potential

all or none event

generated and propagated down axon without any loss in intensity

voltage gated na+ channel

immediate

na+ influx

depolarization (+)

voltage gated k+

delayed

k+ efflux

hyperpolarization