U6 - Nerve Impulses & Synapses

Why are there many dendrites at the ends of neurones?

To create a larger surface area for receiving action potentials

What is the role of the cell body in the centre of the dendrites?

Contains the usual organelles (lots of RER and mitochondria for the production of proteins and neurotransmitters)

What are Schwann cells?

• Cells along the axon

• Provide protection of the axon and electrical insulation for the axon

• Contains myelin sheath

What are the nodes of Ranvier?

Gaps between the Schwann cells, where there is no myelin sheath

What is saltatory conduction?

When action potentials jump from one node of ranvier to another

What makes neurones excitable?

1. They have a resting potential

2. They have protein transporters

   • remember phospholipid bilayers are impermeable to charged ions Na⁺ and K⁺

What are the 3 types of protein transporters?

1. Na⁺ / K⁺ pump

2. Open Na⁺ and K⁺ channels

3. Voltage - geated channels (VgC)

How does an Na⁺ / K⁺ pump work?

• Works 24 / 7

• All over the neurones

• Active transport (using ATP) → 3Na⁺ out and 2K⁺ in

How do open Na⁺ and K⁺ channels work?

• All over the neurone

• More K⁺ channels taking potassium out than Na⁺ channels bringing sodium in, so the membrane is more permeable to K⁺ than Na⁺

How do V-gated channels work?

• Sensitive to the charge around them

• All over the axon → especially on the axon hillock → VgCNa⁺ are particularly dense here

What is stage one of creating and maintaining a resting potential?

• Equal concentration of K⁺ and Na⁺ inside and outside the axon

• So, no membrane potential = 0mV

  • if no transport is occuring

What is stage two of creating and maintaining a resting potential?

• The Na⁺ / K⁺ pump uses active transport to move 3Na⁺out and 2K⁺ into the axon

• So, there is an increased conc. of K⁺ and a decreased conc. of Na⁺ inside the axon

• So, overall, there is a decreased number of positively charged ions inside the axon

• DECREASE IN MEMBRANE POTENTIAL (-10mV)

What is stage three of creating and maintaining a resting potential?

• K⁺ diffuses out of the axonby facilitated diffusion, down an electrochemical gradient

• Na⁺diffuses into the aconalso by facilitated diffusion, down an electrochemical gradient

• Axon membrane is far more permeable to K⁺ than Na⁺

• Decreased concentration of positive ions inside the axon

• -65mV = RESTING POTENTIAL

What are the essential factors for creating and maintaining a resting potential?

1. Na⁺ / K⁺ pump actively transports 3Na⁺ out of the axon and 2K⁺ into the axon (using ATP)

2. Axon is more permeable (more channel proteins) to K⁺ , so more K⁺ diffuses out of the axon than Na⁺ diffuses in

What are the key principles of the generator potential?

• A generator potential is a small depolarisation of a neurone’s membrane potential, causing a deviation from the resting potential (-65mV)

• Generator potentials occur at receptor cells or sensory nerve endings

• They are caused by energy transduction, where a receptor detects a stimulus in one energy form and uses this enegy to open gated Na⁺ channels in the neurones membrane

• If a generator potential causes a large enough depolarisation of the membrane, (usually above -50mV) then it will trigger an ACTION POTENTIAL

• Generator potential below this threshold value will not trigger an action potential. (ALL - OR - NOTHING Law)

What is stage 1 of any generator potential?

• A sensory nerve ending is at resting potential (-65mV)

• The membrane contains voltage-gated Na⁺ channels which ONLY OPEN when the receptor detects an energy change (stimulus)

What happens when there is a small stimulus causing a generator potential?

1. Receptor detects a small energy change (stimulus)

2. Some v-gated Na⁺ channels OPEN, so some Na⁺ diffuses in, down the electrochemical gradient by facilitated diffusion

   • so: membrane potential = slightly less negative (-60mV)

   • but: does not reach the threshold value (-50mV)

3. So: other v-gated Na⁺ channels do not open → so no action potential is sent

What happens when there is an above threshold stimulus causing a generator potential?

1. Receptor detects a large enrgy change (stimulus)

2. Many v-gated Na⁺ channels open

   • lots of Na⁺ diffuses in

   • big depolarisation of membrane (less negative)

3. This causes more v-gated Na⁺ channels to open

   • greater influx of Na⁺ = POSITIVE FEEDBACK

   • so this reaches or exceeds threshold value of -50mV and an ACTION POTENTIAL is sent

What are the key principles of action potentials?

• An action potential is a self-propegating wave of electrical disturbance (depolarisation) that is transmitted along the CSM of a neurone

• The nerve impulse generated does not travel like electricity in a wire, but rather a series of very rapid, temporary reversals of the membrane potentials at each point along the membrane

• An action potential relies on the opening and closing of voltage-gated ion channels, and the subsequent movement of Na⁺ and K⁺ ions

• All action potentials are the same size no matter how large the initiating stimulus is

What is stage 1 of the action potential?

RESTING POTENTIAL

• Na⁺ / K⁺ pump operating

• V-gated channels closed

• -65mV

What is stage 2 of the action potential?

DEPOLARISATION

• 2A → generator potential (-50mV) → influx of Na⁺

• 2B → depolarisation (0mV) → open Na⁺ v-gated channels → positive feedback → big influx Na⁺

• 2C → Na⁺ equilibrium is reached at +40mV → Na⁺ VgC close, K⁺ VgC open → K⁺ diffuses out

What is stage 3 of the action potential?

REPOLARISATION

• VgCK⁺ stays open and K⁺ continues to diffuse out of the axon (inside becomes negative)

What is stage 4 of the action potential?

HYPERPOLARISATION

• VgCK⁺ are slow to close so K⁺ equilibrium is reached (-90mV)

What is stage 5 of the action potential?

RESTING POTENTIAL IS RESTORED

• -65mV

• v-gated channels both closed

• Na⁺ / K⁺ pump

What is the general passage of action potentials along a neurone?

• Action potentials mover very rapidly along the membrane of a neurone’s axon (or dendrite)

• Only one generator potential is required → the movement of the action potential then occurs ‘automatically’

• The key stage here is the depolarisation of the membrane at the initiation point: this creates localised currents that result in the depolarisation (and subsequent initiation of a new AP) slightly further along the membrane

• This process repeats all along the membrane as a travelling wave of depolarisation until it reaches a synapse

• The part of the membrane behind this wave of depolarisation undergoes repolarisation (back to resting potential), ready to transmit the next action potential → after a short delay (called the REFRACTORY PERIOD)

What is the importance of the refractory period?

• Promotes separate impulses (no overlap)

• Creates a time delay

• Limits number of AP / minute so no ‘overload’ of neurone

What is the passage of action potentials in myelinated neurones?

• Myelinated neurones have axons surrounded by a mylin sheath (produced by schwann cells wrapped around the axon)

• Myelin is a mixture of lipids and acts as insulation. There are no v-gated channels in the axon membrane underneath the myelin

• Action potentials cna only occur at gaps between myelin sheaths called nodes of ranvier

• Localised currents stretch all the way between nodes of ranvier → speeding up transsmission of nerve impulse by roughly 3 times

• The impulse ‘jumps’ between nodes of ranvier → SALTATORY CONDUCTION

What 3 factors affect the speed of the nerve impulse?

1. The myelin sheath

2. The diameter of the axon

3. Temperature

How does the myelin sheath affect the speed of the nerve impulse?

• Causes nerve impulses to jump from one node of Ranvier to another called saltatory conduction

• This increases the speed of nervous transmission

How does the diameter of the axon affect the speed of the nerve impulse?

The greater the diameter of the axon, the faster the speed of nerve impulses as there is less leakage of ions from larger axons, so easier to maintain the membrane potential

How does the temperatur affect the speed of the nerve impulse?

• Increased temperature = increased speed of nerve impulses (until a point → when membrane proteins denature)

• Increased temperature = increased rate of diffusion of Na⁺ and K⁺ ions (more kinetic energy)

• Increased temperature = increased rate of respiration = increased ATP production for Na⁺ / K⁺ pump

What are the key principles of synapses?

• Synapses occur between the axon of a neurone and either (a) a dendrite (or cell body) of another neurone or (b) an effector cell (muscle or gland)

• An action potential cannot cross the short synaptic cleft by itself

• Instead, chemical neurotransmitters carry the message across the synaps and initiate a new action potential. This movement is UNIDIRECTIONAL (one way only) → neurotransmitters are only produced in the presynaptic neurone

• Over 40 different neurotransmitters exist. Most common are acetylcholine (ACh) and noradrenaline

• A synapse consists of the presynaptic knob, the synaptic cleft and the postsynaptic knob

Where are the synaptic vesicles containing ACh?

In the presynaptic knob

Where are the VgCa²⁺ located?

Across the membrane of the presynaptic knob

Where are the Ligand-gated Na⁺ channels located?

Across the membrane of the postsynaptic knob

What do Ligand-gated Na⁺ channels do?

• Opens when ACh bind to a receptor site

• Binding causes conformational change in protein (tertiary structure) and Na⁺ enters

How does a synapse ensure unidirectionality of an impulse?

• Neurotransmitters are only produced in the presynaptic knob

• Ligand gated Na⁺ channels are only found in the post-synaptic membrane

What is a cholinergic synapse?

A synapse that uses ACh as a neurotransmitter (common in vertebrates)

What is stage 1 of how an impulse crosses a synapse?

Na⁺enters through v-gated channels when AP aarrives. Depolarisation of the membrane causes VgCa²⁺channels to open

What is stage 2 of how an impulse crosses a synapse?

Ca²⁺enters via facilitated diffusion, which causes the cesicles (containinf ACh) to move towards presynaptic membrane (using ATP)

What is stage 3 of how an impulse crosses a synapse?

Vesicles fuse with presynaptic membrane and release ACh into the synaptic cleft (exocytosis)

What is stage 4 of how an impulse crosses a synapse?

ACh diffuses across the synaptic cleft towards the postsynaptic membrane

What is stage 5 of how an impulse crosses a synapse?

ACh binds to receptors on ligand-gated Na⁺ channels on the postsynaptic membrane → channels open and sodium diffuses in

What is stage 6 of how an impulse crosses a synapse?

• Depolarisation of the postsynaptic membrane → so VgNa⁺ channels open → Na⁺ diffuses in = new AP initiated

• So long as enough ACh has bound, so enough Na⁺ enters to reach threshold value (-50mV)

What is stage 1 of the recycling of neurotransmitters?

Enzyme acetylcholinesterase (AChE) binds to the bound ACh → hydrolyses ACh into acetyl and choline, which means it gets released from receptors and ligand-gated channels close

What is stage 2 of the recycling of neurotransmitters?

Choline is reabsorbed into the presynaptic knob and recombined with acetyl in SER (using ATP)

What is stage 3 of the recycling of neurotransmitters?

ACh is packaged into vesicles for future use