neuronal transmission

lecture 3 & 4

CNS

brain and spinal cord

PNS

all other nerves

sensory neurons

info from the body

interneurons

link sensory and motor neurons

motor neurons

info to the body

neurons

do all info processing and transmitting

• different types, shapes, sizes - specialised

soma (cell body)

contains nucleus

dendrites

receives messages from other neurons

axon

carries info (action potential) from soma to terminal buttons

myelin sheath

wraps around axon (insulation)

terminal buttons

end of axon branches

glial cells (glia)

• astrocytes

• oligodendrocytes

• microglia

astrocytes

• structural support - holds neurons in place

• provides nutrients - receive glucose from capillaries, turns it into lactate which goes into neurons

oligodendrocytes

produce myelin sheath (similar to schwan cells)

microglia

• smallest

• clear dead/dying neurons

• get rid of unwanted cells and repairs damaged cells

node of ranvier

naked axon

tranmission within neurons

electrical process - movement of ions across cell membrane

cells electrical charge

• more negative on inside than outside - results in resting potential (store of energy)

• neurons can reverse their electrical charge

cell membrane

all cells covered in a membrane

two layers of phospholipid molecules

phospholipid molecules

• head of molecule is phosphate (hydrophilic, - attracted to each other and water, point outwards), tail is fatty acid (hydrophobic - points inwards and toward each other)

• ion channels (green) let in ions from outside of cell vice versa

ions

cations = positively charged

anions = negatively charged

intercellular fluid (inside neuron) contains K+ and anions (A-)

extracellular fluid (outside neuron) contains sodium (Na+) and chloride (Cl-)

brains floating in sea water

membrane potential

difference in electrical potential inside and outside of the cell

• balanced by diffusion (molecules - high to low conc) + electrostatic pressure (attraction or repulsion of particles depending on their charge)

organic anions A-

• concentrated inside the cell

• cannot cross the membrane

potassium ions K+

• concentrated inside the cell

• through diffusion it wants to move out

• electrostatic - attracted to the inside (-)

• forces balance so they K+ stays put

chloride ions Cl-

• concentrated outside the cell

• through diffusion they want to move in

• electrostatic - repelled from negative inside

• forces balance so Cl- stays put

sodium ions Na+

• more concentrated outside of the cell

• through diffusion they want to move in

• electrostatic - attracted to - inside

• both force Na+ into cell

how are Na+ kept under control

sodium-potassium pumps - constantly pumps out 3x Na+ and pumps in 2 K+

what is the resting potential of a neuron

-70 mV

inside of cell

negative

outside of cell

positive

why is it important to maintain resting potential

so neuron can respond rapidly to a stimulus

action potential

• rapid change/reversal in the membrane potential

• electrical impulse that travels down the soma to terminal buttons and tells them to release a neurotransmitter

• negative - positive - negative

• all on none (fires or doesn't fire)

• size of AP stays same throughout the axon to the terminal buttons

depolarisation

decrease from normal resting potential (brings membrane closer to 0 - more positive)

hyperpolarisation

increases relative to resting potential (more negative)

neuronal transmission process (across cells in membrane)

1. Na+ channels open, Na+ enters cell (only when threshold)

2. K+ channels open, K+ leaves cell

3. Na+ channels become refractory (no more Na+ enters, can’t open for a while)

4. K+ continues leaving cell, causing membrane potential to return to resting level

5. K+ channels close, Na+ channels reset

6. Extra K+ outside diffuses away

what is the threshold of excitation

-50mV

what is the highest point of depolarisation

+40mV

propagation

action potential is transmitted via propagation - regenerated at points along the axon due to entry of Na+ at the neighbouring point

• 1 channel opens, so neighbouring ones do, but this only goes 1 way until they become refractory (like a chain of dominos)

saltatory conduction

myelin sheath

• action potential regenerated along the axon at nodes of ranvier (points between myelin sheath)

saltatory conduction benefits

• fast conduction

• more energy efficient - pumps use energy (but they’re only at nodes of ranvier)

transmission between neurons

• neurons send messages via synaptic transmission

• neurotransmitters are released from 1 neuron and attach to another, initiating a reaction resulting in postsynaptic potentials

1 neuron connects to…

15,000 other neurons

synapse

junction between 2 neurons (terminal buttons of N1 and membrane of dendrites on N2)

2 membranes

• presynaptic membrane

• postsynaptic membrane

synaptic vesicles

• contain neurotransmitters

• some made in soma and sent down, some made in terminal buttons through recycling

how wide is the synaptic gap

20nm

astrocytes at synapse

• help clearance of neurotransmitters

• take away excess glutamate

• monitor and alter function at the synapse

synaptic transmission

1. action potential passes down

2. Ca channel opens; Ca2+ (more concentrated outside neuron and want to enter) enters

3. vesicles fuse with membrane. pore opens (omega figures)

4. release of neurotransmitter (exocytosis)

5. NT diffuses, binds to post-synaptic membrane

6. post-synaptic ion channels open

7. ions flow in or out = excitatory of inhibitory post synaptic potentials

what does the neurotransmitter do on the post synapse membrane

NT alters membrane potential of post-synaptic membrane - inhibitory or excitatory

how is binding like a lock and key

• NT attaches to a specific binding site of receptor on post synaptic membrane, which opens an ion channel

• PSP depends on which ion channel is opened (changes which ions move in and out)

EPSP

excitatory postsynaptic potentials

(increases chance of AP produced)

IPSP

inhibitory postsynaptic potentials

(decreases chance of AP produced)

what happens if Na+ channel opens

Na+ enters, causing depolarisation (EPSP)

what happens if K+ channel opens

K+ leaves, causing hyperpolarization (IPSP)

what happens if Cl- channel opens

Cl- enters, causing hyperpolarization (IPSP)

what happens if Ca2+ channel opens

Ca2+ enters, which activates the enzyme

2 types of receptors

• inotropic receptors

• metabotropic receptor

ionotropic receptor

contains a binding site and an ion channel

• NT attaches to binding site which opens the channel

• direct way

metabotropic receptor

contains a binding site, but not a direct channel next to them to open

• NT attaches to binding site, which initiates a chain reaction that eventually opens ion channels

• requires energy

• PSPs slower than ionotropic

• indirect way

what is termination

remove excess neurotransmitters so we can have future release and binding

methods of termination

• reuptake

• enzyme deactivation/degradation

(may be a combo of both)

• astrocytes may uptake excess glutamine

reuptake

transmitter is taken back by presynaptic terminal via transporter molecules

enzyme deactivation/degradation

transmitter broken down by an enzyme eg acetylcholinesterase breaks down ach into chlorine and acetic acid

integration

summation of PSPs (excitatory and/or inhibitory) in control of neuron firing, so the neuron can decide whether to fire

do inhibitory PSPs always inhibit behaviour?

NO

inhibition of inhibitory neurons = more likelihood of behaviour

excitation of inhibitory neurons = less likelihood of behaviour

• inhibitory behaviour eg getting up and walking around in your sleep

GABA

• most abundant inhibitory neurotransmitter in CNS (reduces chance of neuronal firing)

• learning chemical

• inhibiting too much can lead to seizures

glutamate

• most abundant excitatory neurotransmitter in CNS

• can bind to number of receptors

• learning and memory

• too much excitation leads to strokes and brain trauma

acetylcholine (ACh)

• found in CNS and PNS, specifically at neuromuscular junctions

• signals passed from neurons into the muscle fiber (muscular contraction)

• also direct attention and neuroplasticity

dopamine

• pleasure chemical

• motor control

• reward, decision making, memory, attention

• plays role in Parkinson's, addiction, sz

seretonin

• calming chemical

• mood regulation, eating, sleep, decision-making

• lack = depression

antagonist

drug that blocks a neurotransmitter

antagonist eg

botulinum toxin (botox) blocks release of acetylcholine and prevents muscle contraction (paralyses muscles)

agonist

drug that mimics a neurotransmitter and enhances synapse function

agonist eg

muscarine (naturally in mushrooms) - imitates acetylcholine