1014msc resting membrane potential and action potential

nervous system

body’s master communication and control system

• detects changes/stimuli

• processes and integrates information

• produces a response by activating effectors

Central nervous system

• brain

• spinal cord

• main site of information processing

peripheral nervous system

• nerves outside of the CNS

• carries information to and from the CNS

afferent

arrives

• sensory information travels towards the CNS

• activates effectors

efferent

exits

• information travels away from the CNS

• activates effectors

sensory neurons

carry information from sensory receptors ——>CNS

motor neurons

carry information CNS——→effectors

interneurons

found within the CNS and involved in processing/integrating information

cell body

• contains nucleus

• rough er

• mitochondria

• golgi apparatus

• maintains the cells

• protein synthesis

• packages proteins for transport

dendrites

• highly branched

• receive incoming signals

• have spines where synapses can occur

• increased surface area allows many inputs from other neurons

axon

• carries action potential

• APs travel along the axon toward the terminals

• very rapid signalling occurs along the axon’s surface

axon terminals

the secretory region of the neuron

presynaptic cell

sends the signal

postsynaptic cell

receives the signal

glial cells

• support neurons

• communicates with neurons and each other

• can produce myelin around axons

myelin sheath

a layer of phospholipid insulation around some axons

CNS: oligodendrocytes

PNS: schwann cells

nodes of ranvier

exposed regions between myelin segments

• the AP effectively jumps from node to node producing much faster transmission, this is called the saltatory conduction

resting membrane potential

the difference in electrical charge maintained across a cell membrane

• -70mV

• the inside of the cell is negative relative to the outside

• the charge is localized mainly at the membrane, rather than the entire inside of the cell being massively negative

polarized membrane

a membrane with a charge difference across it

• outside = slightly positive

• inside = slightly negative

chemical disequilibrium

ions are unevenly distributed across the membrane

ex. K+ high inside —→low outside

Na+ low inside —→high outside

these concentration differences are maintained using active transport which requires ATP

Na+/K+ ATPase pump

• uses ATP

• operates continuously

• helps maintain Na+ and K+ concentration gradients

3Na out——>2K+ in

electrical disequilibrium

• ions are unevenly distributed

• negatively charged proteins are trapped inside

• the membrane separates the charges

the result:

EFC—→slightly positive

ICF——>slightly negative

creates electrical gradient

electrochemical gradient

1. chemical gradient

difference in concentration

2. electrical gradient

attraction/repulsion between charges

together form this gradient

nernst equation

calculates the equilibrium potential of an individual ion

K+ equilibrium potential -90mV

hyperkalemia

an abnormally high concentration of K+ in the ECF

action potential

an electrical signal that travels along the axon

allow neurons to transmit information over long distances

occur in excitable tissues, including:

• nervous tissue

• muscle tissue

repolarization

the membrane returns toward the RMP after being depolarized

hyperpolarization

the membrane becomes more negative than the RMP

graded/sub threshold potentials

a weak stimulus may cause a small amount of Na+ to enter

mechanically gated

opened by mechanical stimuli

ex. stretch

chemically/receptor gated

opened when a chemical signal binds to a receptor

voltage-gated

pened in response to changes in membrane voltage

AP step 1

resting state

• RMP -70mV

• voltage gated Na+ channels=closed

• voltage gated K+ channels= closed

• leakage channels remain open

membrane is polarized

AP step 2

depolarization

a stimulus causes the membrane to reach threshold

• voltage- gated Na+ channels open

• Na+ rapidly moves into the cell

• membrane reaches approx. +30mV

AP step 3

repolarization

• Na+ channels become inactivated

• voltage- gated K+ channels open

• K+ moves out of the cell

AP step 4

hyperpolarization

• K+ channels close slowly

• K+ continues leaving the cell even after the membrane reaches its RMP

• inside becomes temporarily more negative than RMP

Forward travel of AP

• once Na+ channels have opened, they become temporarily inactivated

• that region of the membrane cannot immediately generate another AP

• therefore the AP cannot travel backward into the recently activated region

instead:

it moves forward to the nect section of axon

all-or-nothing principle

an AP either happens completely or doesn’t happen

• if threshold isn’t reached: no AP

• if threshold is reached: full AP

• increasing stimulus strength does not increase AP amplitude

summation

individual graded potentials may be too weak to reach threshold

refractory periods

for approx. 1 ms:

• no second AP can occur

• even an extremely strong stimulus cannot trigger another AP

• BECAUSE Na+ channels are temporarily inactivated

action potential propagation

Once an AP begins:

1. Voltage-gated Na⁺ channels open

2. Na⁺ enters

3. Positive charge spreads to the adjacent section

4. Adjacent membrane depolarizes

5. New Na⁺ channels open

6. The process repeats

Meanwhile, the previous section becomes refractory.

Therefore, the AP moves forward along the axon

conduction velocity

the speed of AP transmission depends on:

1. axon diameter

larger diameter—→faster conduction

2. myelination

more myelination—→faster conduction

the AP jumps between nodes of ranvier

multiple sclerosis

autoimmune demyelinating disease

• myelin sheath is damaged

• slower signal transmission

associated with:

• muscle weakness

• fatigue

• difficulty walking

• potential eventual loss of vision

local anesthetics- lignocaine

block the opening of:

• voltage- gated Na+ channels

• less Na+ enters the neuron

therefore:

• decreased Na+ entry

• decreased propagation

• decreased pain transmission