neurophysiology - from cells to networks

What is A?

Cell membrane

What is B?

Dendrites

What is C?

Cell body (soma)

What is D?

Axon

What is E?

Node of ranvier

What is F?

Oligondrocyte

What is G?

Myelin sheath

What is H?

Synapse

What type of neuronal cell is this ? And what is its function?

Neurone - communication

What is A?

Microglia

What is B?

Oligodendrocyte

What is C?

Protoplasmic

What is D?

Fibrous

What is E?

Blood vessel

What is F?

Perivascular foot

What neuronal cells are this? What is its role ?

Glia

Support functions, immune regulation

What are neurones ?

The basic unit of structure and function in the nervous system

What do neurones do?

• Conduct impulses

• process information

• Sense environmental changes

• Communicate changes to other neurons

• Command body response

Why do neurones have high energy usage ?

Constant need for glucose and oxygen

What does the cell body contain?

Contain nucleus and cellular activity

What are axons ?

Single extension of the neurones providing output

What are dendrites?

Branch like extensions that receive messages from other neurones

What is A?

Function

What is B?

Locality

What is C?

Cell morphology

What is D?

Neurotransmitter

What is A?

Sensory

What is B?

Relay

What is C?

Motor

What do motor neurones do?

Carry impulses away from the brain and spinal cord

What do sensory neurones do?

Carry impulses from inside/outside the body to brain/spinal cord

What are relay neurones ?

Process incoming impulses and pass them onto motor neurones

Each phospholipid has what?

• phosphate “head”

• Two fatty acid tails -

• arranged back to back or form the belayer

What is A?

Hydrophobic

What is B?

Hydrophilic

What is C?

Phosphate

What is D?

Glycerol

What is E?

Saturated fatty acid

What is F?

Unsaturated fatty acid

What are the properties of the phosphate head?

How does this form a bilayer?

• negatively charged

• Polar and hydrophilic - water loving

• Attracted to: Extracellular fluid, intracellular fluid

• Faces outwards on both sides of the membrane

What are functions of the fatty acid tails? How does they form the bilayer?

• uncharged and non polar

• Hydrophobic - water fearing

• Oriented inwards - away from water

• Form the membranes hydrophobic core

What are the fluid environments of the phospholipid bilayer?

• Intracellular fluid (ICF) - fluid inside the cell

• Extracellular fluid (ECF) - fluid outside the cell

• Interstitial fluid (IF) - ECF not contained within blood vessels

How is the phospholipid bilayer organised?

See below

How does the neuronal membrane act as a semipermeable barrier?

• Separates intracellular fluid (ICF) from Extracellular fluid (ECF)

• Allows some substances to cross while restricting others

How are neurons selectively permeable ?

neuronal membranes are:

• highly selective

• Not freely permeable to most ions

Ion movement depends on:

• specific ion channels

• Membrane properties

what are properties of channel proteins ?

• span the membrane

• Contain a central pore

• Provide a hydrophobic pathway through the hydrophobic core

• Allow specific ions to cross the membrane

How are ion channels selective ?

• only certain ions can pass, Selectivity depends on:

• Channel size

• Charge lining the pore

Critical for controlling neuronal excitability

Explain the ligand gated ion channels

• channel is closed at rest

• Neurotransmitter (ligand) bind to receptor site

• Binding opens the ion channel

• Creates a hydrophilic pore through the membrane

• Ions move down their electrochemical gradients

• Na+ / Ca2+ enter, K+ exits

• Ion movement changes membrane potential

• Convert chemical signal → electrical response

Explain how the mechanically gated ion channels enable sensory transduction

• channel closed at rest

• Opens in response to mechanical deformation (stretch, pressure etc)

• Mechanical force causes conformational change in channel protein

• Channel opening creates a hydrophilic pore

• Ions move down their electrochemical gradients

• Na+ enters, K+ exits

• Ion flow alters membrane potential

• Enables sensory transduction

Explain how an action potential is generated and propagated

• channel closed at resting membrane potential

• Membrane depolarisation triggers channel opening

• Opening occurs at threshold voltage (approx 50mv)

• Voltage change causes conformational shift in channel protein

• Channel opens → hydrophilic pore formed

• Ions move down their electrochemical gradients

• Rapid ion flux changes membrane potential

• Essential for action potential generation and propagation

Explain why membrane potential is stable but never static

• channels are not gated by stimuli

• Randomly open and close at rest

• Allow continuous passive ion movement

• Ions move gradually down their concentration gradients

• Most permeable to K+ (some Na+)

• Net K+ efflux dominates

• Establish and maintain the resting membrane potential

• Membrane potential is stable but never static

How can we measure resting membrane potential?

Measured using 2 electrodes:

• reference electrode in Extracellular fluid

• Recording micro electrode inside the neuron

Electrodes connected to a voltmeter

Voltmeter measures electrical potential difference

What is the resting membrane potential?

Voltage difference across the neuronal membrane

At rest what mV is the neuron?

-70mv

Inside of the neuron is what relative to outside?

Negative

What do the results of measuring the resting potential membrane reflect?

• ion concentration gradients

• Selective membrane permeability

• Leak channel activity

Explain the process of depolarisation into repolarisation into hyper polarisation then returning to rest

threshold reached at approx 55mv

• voltage gated Na+ channels open

• Rapid Na+ influx

• Membrane potential rises to approx 30mv

• 
  

• Na+ channels inactivate

• Voltage gated K+ channels open

• K+ efflux restores negativity

• 
  

• K+ channels remain open briefly

• Membrane potential becomes more negative than rest

• 
  

• K+ channels close

• Leak channels and ion pumps re-establish resting potential

When does absolute refractory period occur ?

During rising phase, overshoot, early falling phase

What causes absolute refractory period and what does it ensure?

Caused as Voltage gated Na+ channels are inactivated

Ensures:

• one way propagation of action potentials

• Maximum firing frequency is limited

When does relative refractory period occur?

Late falling phase, undershoot (Hyper polarisation)

What is the cause of a relative refractory period ?

Stronger than normal stimulus is required

Caused by some Na+ channels being recovered

Why do action potentials start at the axon hillock?

Due to the high proportion of voltage gated Na+ and K+ channels there

This high proportion of voltage gated Na+ and K+ channels continues down the axon to the terminal

What happens to an action potentials after its initiated at the hillock?

Propagates in one direction towards the terminal

What does it mean when an axon has a large diameter?

Large diameter allows more ions to flow through them

Ions have a much harder time flowing through small diameter axons

How does myelin speed up the conduction of an action potentials firing down the axon?

Action potentials are only generated at each node of ranvier

Action potentials ‘skip’ their way down the axon

Action potential regeneration at each node is called saltatory conduction, saltatory conduction in a myelinated axon is significantly faster

What are the benefits of myelination?

• gives action potentials a speed boost

• Gives Na+/K+ pumps a rest

What is synaptic transmission?

The process of information transfer between neurons

What is a chemical and electrical synapse?

Electrical synapse: information can pass between cels in either direction

Chemical synapse: information flow is unidirectional

At an electrical synapse neurons are coupled together by what?

Gap junctions

How can depolarisation spread quickly across gap junctions?

As there is no measurable delay in signal propagation across an electrical synapse

Electrical synapses mediate what?

Important reflexes including the coordination of heart muscle contractions

What are A, B, C and D?

A- electrical signal

B- chemical message

C- electrical signal

D- cell metabolism

What size and speed are amino acid neurotransmitters?

• small and fast acting

What size and speed are amine neurotransmitters?

• intermediate size: both fast and modulatory actions

What size and speed are peptide neurotransmitters?

• large size with slow and specialised actions

Where are small and intermediate sized neurotransmitters made ?

Directly in the axon terminal

What is happening in this image?

1) precursor molecule interacts with an enzyme in the cytoskeleton at the terminal region and is converted inti a neurotransmitter

2) neurotransmitter is packaged and stored in synaptic vesicles

Describe what’s happening in this image

1) precursor peptide is synthesised in the rough ER

2) in the Golgi apparatus the new peptide is sorted, activated and packaged

3) secretory granules “bud” away from the Golgi apparatus

4) the granules are transported to the terminal via axoplasmic transport

Describe how this process leads to endocytosis?

1) vesicle is docked along inside of presynaptic membrane

2) an action potentials depolarises terminal opening voltage-gated Ca2+ channels

3) Ca2+ rapidly floods into the terminal

4) Ca2+ interacts with snare proteins causing the vesicle to fuse with the presynaptic membrane and spill its contents (exocytosis)

5) the vesicle membrane is recycled (endocytosis)

What happens to a receptor when a neurotransmitter binds to it ?

It affects the functioning of that receptor

What happens when there is no neurotransmitter bound to the receptors ?

Then the ion channel remains closed and impermeable to ions

How are certain ions allowed to pass?

The neurotransmitter causes a change in the shape of the subunits that form the channel allowing certain ions to pass

How is an excitatory postsynaptic potential formed (EPSP)

1) neurotransmitter binds to receptors which opens ion channels

2) the influx of Na+ into the postsynaptic cell will depolarise Vm towards threshold

3) this is called an excitatory postsynaptic potential

How is an inhibitory postsynaptic potential (IPSP) formed?

1) neurotransmitter binds to receptors which changes the conformation of the receptors and opens ion channels

2) this influx of Cl- into the postsynaptic cell will repolarise Vm towards rest

3) this is called an inhibitory postsynaptic potential

Describe what is happening in this image

1) acetylcholine (ACH) is released upon the arrival of an action potential

2) ach acts for a short time on postsynaptic receptors before it is degraded by acetylcholinesterase

3) choline is recycled and pumped back into the presynaptic terminal by the choline transporter

A single EPSP is usually too small to make a neuron fire an action potential, what is done about this?

• EPSPs have to be summed together to bring a neuron to firing threshold

• Activity across space and time is additive

How does activation of a single synapse vs activation of ,ultiple inputs generate ?

Single synapse activation generates a small amplitude EPSP

Multiple input activation simultaneously generates a much larger postsynaptic response

How does the image on the right show us how a much larger postsynaptic response can be generated?

Activating the same synapse in rapid succession

What is happening in this image ?

1) the EPSP recorded at the active synapse is large in amplitude

2) EPSP loses much of its “punch” with distance

3) a tonically active inhibitory synapse would drive Cl- inwards and K+ outwards as the EPSP approached