5.3: Neuronal communication

What is a sensory receptor?

A specialised cell or structure that detects a specific stimulus and converts it into an action potential (electrical signal)

What are sensory receptors?

transducers

What are transducers?

a cell that converts one form of energy into another - in this case to an electrical impulse

Do sensory receptors respond to all stimuli?

No, each sensory receptor is specific to one type of stimulus

What are Pacinian corpuscles?

a pressure sensor that detects changes in pressure on the skin

What happens when pressure is applied to a Pacinian corpuscle?

• Pressure is applied to the skin

• Rings of connective tissue (lamellae) deform

• This stretches the sensory neurone membrane

• Stretch-mediated sodium channels open

• Na⁺ ions diffuse in

• A generator potential is produced

• If the generator potential reaches threshold:

  • An action potential is triggered

• Action potential travels along the sensory neurone

What are the key differences between a generator potential and an action potential?

Generator potential	Action potential
Local change	Travels
Graded (size varies - depends on stimulus strength)	All-or-nothing: once threshold is reached, always the same size
Depends on stimulus strength	Same size every time
Can summate	Cannot summate

How does stimulus strength affect generator potential?

Stronger stimulus produces a larger generator potential

How does stimulus strength affect action potentials?

Stronger stimulus increases the frequency of action potentials, not their size

What do the sodium potassium pumps in the membrane do?

actively pump sodium ions out of the cell and potassium ions into the cell. 3 sodium ions are pumped out for every 2 potassium ions pumped into the cell

What is the purpose of the sodium potassium pumps doing this?

when the channel proteins are all closed, they work to create a concentration gradient. The conc of sodium ions outside the cell increases, while the conc of potassium ions inside the cell increases. The membrane is more permeable to potassium ions, so some of these leak out of the cell. The membrane is less permeable to sodium ions, so few of these are able to leak into the cell

What is the result of these ionic movements?

a potential gradient across the cell membrane. The cell is negatively charged inside compared with outside. This negative potential is enhanced by the presence of negatively charged anions inside the cell

Define polarised

when the cell membrane is more negatively charged inside compared with the outside

How is a nerve impulse created?

by altering the permeability of the nerve cell membrane to sodium ions. This is achieved by opening sodium ion channels. This means that sodium ions can move down conc gradient into the cell. The movement of ions creates a potential difference (charge) across the membrane. The inside of the cell becomes less negative than the outside than usual (depolarisation)

Define depolarisation

where the inside of a cell membrane becomes less negatively charged compared with the outside

What happens if a small stimulus is detected compared to a larger stimulus?

if a small one is detected, only a few sodium channels will open.

What are the 3 types of neurons?

sensory, relay and motor

What is the sensory neuron?

they carry the action potential from a sensory receptor to the CNS

What is the structure of sensory neurons?

• One long dendron (from receptor to cell body)

• Cell body partway along the neurone

• Short axon (from cell body to CNS)

• Many dendrites at the receptor end and they are short, they don’t connect to the cell body

• myelin sheath and nodes of Ranvier present

What is the relay neuron?

they connect sensory and motor neurons

What is the structure of relay neurons?

• short axon

• short dendrites - many of them

• found in the CNS

• no myelin sheath

• no dendron

What is the motor neuron?

they carry an action potential from the CNS to an effector such as a muscle or gland

What is the structure of the motor neuron?

• no dendrons

• cell body at the end of the neuron and int he CNS

• long axon and short dendrites (many)

• dendrites are directly connected to the cell body

What is myelin?

A fatty insulating layer surrounding the axon of some neurones

Which cells form the myelin sheath?

Schwann cells

What is the function of myelin?

To increase the speed of nerve impulse transmission

What are the gaps in the myelin sheath called?

Nodes of Ranvier

Why do myelinated neurones transmit impulses faster?

Myelin acts as an insulator and prevents loss of ions

What type of conduction occurs in myelinated neurones?

Saltatory conduction

What does saltatory conduction mean?

The action potential jumps from node to node

Compare conduction in myelinated and non-myelinated neurones.

Myelinated neurones use saltatory conduction and are faster; non-myelinated neurones conduct continuously and are slower

What are the advantages of myelination?

• can transmit action potential much more quickly

• carry action potentials over long distances

What is the resting potential?

electrical potential difference across the membrane of a neurone when it is not transmitting an impulse, typically about –60 mV

What are the main factors that create the resting potential?

• Sodium–potassium pump (Na⁺/K⁺ pump)

• Potassium ion diffusion (leak channels)

• Membrane permeability differences

What does the sodium–potassium pump do?

It actively transports:

• 3 Na⁺ ions out of the neurone

• 2 K⁺ ions into the neurone
  → using ATP

How does the Na⁺/K⁺ pump contribute to resting potential?

Makes the inside of the neurone more negative

Why does K⁺ diffuse (leak) out of the neurone?

• High concentration inside (due to pump)

• Membrane is more permeable to K⁺ than Na⁺

What happens to the membrane at resting potential?

it is polarised

What is a generator potential?

graded depolarisation in a sensory receptor produced in response to a stimulus

What happens to ion channels when a stimulus is applied?

few sodium ion channels open in the receptor membrane allowing Na+ into the cell and produce a small depolarisation (generator potential)

What must happen for an action potential to be triggered?

The generator potential must reach the threshold level

What is an action potential?

a rapid, temporary reversal of membrane potential that travels along a neurone

What must happen before an action potential is triggered?

The membrane must be depolarised to threshold (≈ –50 mV)

What happens at threshold?

Voltage-gated Na⁺ channels open causing sodium ions to diffuse into the cell making it less negative (membrane depolarises : -50mV)

What is positive feedback in an action potential?

• Initial Na⁺ entry causes further depolarisation

• This opens more Na⁺ channels

• → even more Na⁺ enters

What is the peak of an action potential?

Around +40 mV, when the inside becomes positively charged compared to outside

What happens to Na⁺ and K⁺ channels at the peak?

they close and voltage-gated K⁺ channels open

What happens during repolarisation?

• Voltage-gated K⁺ channels open

• K⁺ diffuses out of the neurone

• Membrane becomes negative again

What causes hyperpolarisation?

• K⁺ channels close slowly

• Too much K⁺ leaves

• Membrane becomes more negative than resting potential

What is the refractory period?

A period where the neurone cannot (or is less likely to) generate another action potential

What happens during the refractory period?

Various ion pumps(sodium/potassium) and channels work together to restore the membrane back to the resting potential

Why is the refractory period important?

• Ensures one-way transmission of impulses

• Prevents overlap of signals

• allows cell to recover after an action potential

What does an action potential graph look like?

What is the all-or-nothing principle?

An action potential is only generated if the threshold is reached, and when it is, it is always the same size regardless of stimulus strength.

Does a stronger stimulus produce a bigger action potential?

❌ No — action potentials are always the same size
✔ Stronger stimulus → higher frequency of action potentials

How is a higher frequency made?

higher intensity stimulus - more sodium channels are open - more generator potentials - more frequent action potentials entering CNS

What are local currents?

the movement of ions that spread depolarisation to adjacent regions of a neurone, helping spread an action potential

How do local currents spread depolarisation?

• Na⁺ enters the neurone during depolarisation

  • Positive ions flows sideways inside the axon (down conc gradient)

  • Moves to adjacent resting regions

  • Causes them to depolarise

What happens in the next section of membrane?

• Depolarisation reaches threshold

• Voltage-gated Na⁺ channels open

• A new action potential is generated

Why does the action potential only move in one direction?

because the previous region is in the refractory period:

• Na⁺ channels are inactivated

• Cannot be depolarised again immediately

What are the 3 factors affecting the speed of transmission of an action potential?

1. myelination

2. axon diameter

3. temperature

How does myelination affect the speed of transmission?

• myelin sheath allows for saltatory conduction

• faster

How does axon diameter affect the speed of transmission?

A larger axon diameter means there is less resistance to ion flow, so the wave of depolarisation travels faster along the axon.

How does temperature affect the speed of transmission?

• Higher temperatures accelerate the diffusion of ions, leading to faster depolarisation and faster impulse transmission.

• However, temperatures above 40°C can cause proteins to denature, which results in slower impulse transmission due to membrane damage.

What is saltatory conduction?

the transmission of an action potential along a myelinated neurone where the impulse ‘jumps’ from node to node

Where are action potentials generated in a myelinated neurone?

Only at the nodes of Ranvier (membrane is only depolarised here)

Why don’t action potentials occur in myelinated regions?

• Myelin acts as an electrical insulator

• Prevents ion movement across the membrane

How do local currents behave in saltatory conduction?

• Local currents travel under the myelin sheath - creates a longer localised circuit

• Rapidly carry depolarisation to the next node

What are the advantages of saltatory conduction?

• much faster transmission

• more energy-efficient

• maintains signal strength over long distances

What is the synaptic cleft?

a small gap between 2 neurones

What is a cholinergic synapse?

a synapse that uses acetylcholine as its neurotransmitter

Label this synapse diagram

What are the specialised features of the pre-synaptic bulb?

• many mitochondria

• larger amount of SER - packages neurotransmitter into vesicles

• lots of vesicles containing acetylcholine

• voltage-gated calcium ion channels in the membrane

What happens when an action potential arrives at the synaptic bulb?

it causes the voltage gated Ca²⁺ channels to open which causes calcium ions to diffuse into the presynaptic bulb

What is the role of Ca²⁺ in synaptic transmission?

Ca²⁺ causes synaptic vesicles to move to and fuse with the presynaptic membrane

How is acetylcholine released?

By exocytosis

What happens at the postsynaptic membrane?

acetylcholine diffuses across the synaptic cleft and binds to complementary receptor sites (change shape) on the sodium ion channels

What happens when neurotransmitter binds to receptors?

This opens sodium ion channels in the postsynaptic membrane, leading to the depolarisation of the postsynaptic membrane. If this depolarisation reaches a threshold level, an action potential is triggered in the postsynaptic neurone

What enzyme breaks down acetylcholine?

Acetylcholinesterase

Why must acetylcholine be broken down?

To stop continuous stimulation - if it is left in the synaptic cleft, it will continue to open sodium ion channels and cause action potentials (sodium channels close)

What is the product of the hydrolysing of acetylcholine?

choline and acetate

What happens to the choline and acetate?

recycled - enter the synaptic bulb and are recombined to form acetylcholine using ATP from respiration in the mitochondria. stored in vesicles for future use

What does EPSP stand for?

Excitatory Postsynaptic Potential

What is an EPSP?

A small depolarisation of the postsynaptic membrane that makes it more likely to reach threshold and fire an action potential

What is summation?

occurs when the effects of several excitatory post-synaptic potentials are added together (until it reaches threshold)

What does IPSP stand for?

Inhibitory Postsynaptic Potential

What is an IPSP?

A hyperpolarisation of the postsynaptic membrane that makes it less likely to reach threshold and generate an action potential ; neurotransmitters may bind to post-synaptic membrane and close ion channels

What happens if EPSPs outweigh IPSPs?

• Membrane depolarises to threshold

• Action potential generated

What happens if IPSPs outweigh EPSPs?

• Membrane becomes more negative

• No action potential

What are the two types of summation?

• Temporal summation

• Spatial summation

What is temporal summation?

Multiple impulses from the same synapse arriving close together in time - filters out ‘background’ of low level stimuli

How does temporal summation increase firing?

• Repeated firing by a presynaptic neurone leads to continuous neurotransmitter release.

• An increased amount of neurotransmitter makes it more likely to trigger postsynaptic firing.

What is spatial summation?

Impulses arriving from multiple synapses at the same time (converge on a single postsynaptic neurone or effector cell)

How does spatial summation increase firing?

The combined input of neurotransmitters can trigger postsynaptic firing

How many IPSPs are needed to prevent an action potential?

1

What happens when a synapse runs out of vesicles containing the neurotransmitter?

it is said to be fatigued - the nervous system no longer responds to the stimulus as we have become habituated to it