(3) Neurophysiology: principles of neural transmission Flashcards
What’s a Multipolar neuron?
many dendrites, one axon
What’s a Bipolar neuron?
one dendrite at one end, one axon at another end
What’s a Unipolar neuron?
one branch leaves cell body, spreads in two directions
Neurons transmit signals by the flow of ions…
in and out of cells
Atoms consist of a nucleus (protons and neutrons) orbited by…
electrons
- Each proton has one …
- Each electron has one …
positive electrical charge (1+)
negative electrical charge (1-)
The number of electrons =
the number of protons – positive and negative charges cancel out – net charge of an atom is zero
Atoms are held together by electrostatic force:
- Opposite charges attract one another
- Same charges repel one another
What is a ion?
Ions are atoms or molecules that have lost or gained one or more electrons
What is a cation?
Ions that have lost electrons are positively charged (cations)
What is an anion?
Ions that have gained electrons are negatively charged (anions)
Solid substances made of ions are known as …
salts – these always contain equal numbers of positive and negative charges (e.g. NaCl – table salt: Na+Cl-)
When salts are dissolved in water…
positive and negative ions separate and move about freely
-However, the ions are still influenced by electrostatic forces
What does Sodium (Na+) generate?
generating action potentials (nerve impulses)
What does Potassium (K+) maintain?
maintaining resting potential
What does Chloride ions (Cl) supress?
supressing action potentials
What do Proteins (An) maintain?
maintaining resting potential
Where is Sodium (Na+) located?
mainly outside neurons (extracellular)
Where is Potassium (K+) located?
mainly inside neurons (intracellular)
Where is Calcium (Ca2+) located?
(almost) exclusively extracellular
Where is Chloride ions (Cl-) located?
mostly extracellular
Where are Proteins (An-) located?
mostly intracellular
What is diffusion?
- The movement of particles (atoms, ions, molecules) in gas or liquid from regions of high to low concentration
- Diffusion is caused by the random movement of particles
What does the speed of diffusion depend on?
The speed of diffusion depends on the temperature, the size of the particles, and how difficult it is for the particles to travel through the liquid (viscosity)
If a membrane allows all particles to diffuse through it is
‘fully permeable’
-If a membrane only allows some particles to diffuse through but not others it is
semipermeable or selectively permeable
Semi permeability can arise if a membrane contains pores that are
too small for large particles (such as proteins) but large enough for small particles to cross it
Neuronal cell membranes contain ‘pores’ made up of large proteins that allow certain ions to pass through the membrane, which are called…
ion channels
Ion channels are selective: they only allow one type of ion through , meaning…
Because of ion channels, neuronal cell membranes are semi-permeable: they only allow some types of ions to diffuse through
What do ion channels look like?
- Very large proteins
- Highly conserved in evolution – similar in humans and bacteria!
- Usually made up of several identical parts (“subunits”)
- Only a small part of the channel sits within the membrane
Why does membrane potential occur?
- Difference in ionic concentrations between the inside and outside of the neuron
- Ion channels in the neuronal cell membrane that only allow certain ions to pass in and out of the neuron
How can membrane potential be measured?
The membrane potential can be measured using tiny electrodes
All charged particles (such as ions) are surrounded by an
electric field
The strength of this electric field is the
electric potential
Voltage is measured in
volts (V)
What is resting potential?
The electric potential of the cell membrane of a neuron at rest (not signalling)
-The inside is more negative than outside (negative potential)
What is resting membrane potential?
- When neurons are at ‘rest’, there is slightly more negative charge inside the neuron than outside the neuron
- This creates a difference in electric potential – a voltage – across the neuronal cell membrane
What is a membranes resting potential?
The resting potential is about -70 millivolts (mV)
Why Do Neurons have a Resting Potential (and why is it negative??)
- When neurons are at ‘rest’, only potassium ions (K+) can freely cross the cell membrane (because only K+ ion channels are open)
- Because the concentration of K+ ions is higher inside the cell than outside, some K+ ions leave the cell by diffusion
- Negative protein ions (An-) cannot cross the membrane and remain inside the cell – leaving more negative than positive ions inside
- This causes a negative charge to build up inside the neuron
What Determines the Resting Potential?
- Determined by 2 opposing forces:
1. Concentration difference (concentration gradient) - Drives K+ out of the cell
- This makes the inside more negative
2. Electrostatic force (electrostatic gradient) - Because opposite charges attract, K+ is drawn back into to the negative inside of the cell
- When these forces are balanced, equal numbers of K+ ions leave and enter the cell
Why is There More K+ Inside the Cell?
- Because of the sodium-potassium pump (Na+/K+ pump)
- An ion channel that pumps Na+ out of the cell and K+ into the cell
- Because this is against the concentration gradients of Na+ and K+ it requires energy
How does the Na+/K+ Pump Work?
- The Na+/K+ pump uses energy from ATP to pump Na+ out of the cell and K+ into the cell
- Pumps 3 Na+ out for every 2 K+ in
- Most of the brain’s energy consumption is used to fuel this pump!
What is Hyperpolarization?
= increase in resting potential (more negative)
What is Depolarization?
= decrease in resting potential (less negative)
What does hyperpolarization and depolarization?
- Spread passively along membrane and diminish in size with distance
- Depolarization and hyperpolarization counteract each other
- The membrane potential is the sum of the depolarization and hyperpolarization
If depolarization exceeds a threshold, what occurs?
an action potential (AP) occurs
What does an action potential do?
- Sudden and brief (0.5-2ms)
- Momentarily reserves membrane potential
- Repolarizes quickly and overshoots
- Magnitude is fixed: all or nothing response
What are nerve impulses?
A complex cascade of opening and closing of voltage gated ion channels that given the influx and outflux of Na+ and K+ ions
What are action potentials initiated by?
APs are initiated by the voltage-gated Na+ channel:
- Closed at resting potential
- Begin to open when membrane is depolarized to ~ -40mV
- Na+ ions diffuse from outside the cell (high concentration) to the inside (low concentration)
- This further depolarizes the cell, causing more channels to open
- Causes rapid increase in membrane potential from -70 mV to +40 mV
How Does Voltage-Gated Channels Work?
- Contain a voltage sensor “paddle” – a protein structure that changes shape depending on membrane potential
- The change in shape causes the channel to open or close
- Different types of voltage-gated channels open or close at different membrane potentials
How does an AP return to resting potentional?
When membrane potential reaches ~ +40 mV, voltage-gated Na+ channels close and become “inactivated”
-At the same time, voltage-gated K+ channels open
-No more Na+ ions can enter the cell
-K+ ions now diffuse from inside the cell (high concentration) to the outside (low concentration), reducing the positive charge inside
-This causes the membrane potential to become negative again
The membrane potential briefly becomes hyperpolarized – more negative than the resting potential – before returning to normal
What is a Refractory Period?
- The period immediately after an AP during which another AP cannot be elicited
- due to the inactivation of the voltage-gated Na+ channel
- Like re-flushing a toilet
If an AP depolarizes the neighbouring membrane region beyond threshold, it sets off an…
AP in the neighbouring region
What happens to an AP after it depolarizes the neighbouring membrane?
This AP in turn depolarizes the membrane in its neighbouring region – which in turn sets off an AP in that region, which depolarizes the next region – and so on (a chain reaction)
-The result is a wave of depolarization that spreads along the axon
Action potentials only spread in one direction – from the …
cell body to the axon terminals.
Because of the refractory period: …
the period immediately after an AP during which another AP cannot be elicited
Once the membrane is depolarized, Na+ channels become inactivated – meaning…
they remain closed for a period regardless of membrane potential – no new AP can occur
- So the AP can only spread to a region of membrane that has not recently undergone an AP
- This causes the AP to travel in one direction only
Myelinated axon:
ion flow only at Ranvier nodes
-Impulses leap from node to node:
saltatory conduction
What is Multiple Sclerosis?
- Demyelinating disease – breaks down myelin sheaths – prevents saltatory conduction
- Nerve impulses can no longer be transmitted effectively
- Loss of muscle control, loss of sensation, problems with coordination, visuals problems, loss of bladder control, cognitive impairments
Fugu livers contain
tetrodotoxin (TTX), a neurotoxin
What does TTX do?
- TTX blocks the opening of voltage gated Na+ channels
- Because Na+ can no longer enter the axon, action potentials cannot be generated
- Breathing depends on active nerve impulses to muscles in thorax
- So victims can’t breathe – death from asphyxiation…
- However, victims remain conscious…
- No antidote – requires artificial respiration until toxin metabolised
How does anaesthetics work?
- Most local anaesthetics block voltage gated Na+ channels – like TTX but much less effective
- Action potentials cannot be transmitted
- Signals from pain receptors cannot reach the brain – no sensation
-Signals (AP) are transmitted from one neuron to another at structures called synapses, can be transmitted in two ways
- Signals can be transmitted in two ways:
- Chemically: through neurotransmitters
- Electrically: through simple conduction
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