StemUp: OCR A A level Biology 5.1.3 Neuronal communication

How do sensory receptors convert different types of stimuli into nerve impulses? (3)

- Act as transducers

- Converting various forms of energy (e.g., light, chemical, mechanical)

- Into nerve impulses (electrical impulses)

What happens to a sensory receptor in its resting state? (4)

1. There is a difference in charge between the inside and outside of the cell

2. Created by ion pumps and ion channels

3. Resulting in a potential difference across the membrane

4. Known as the resting potential

What is a generator potential, and how is it triggered? (3)

- A change in the potential difference caused by a stimulus

- It is triggered when a stimulus excites the cell membrane, making it more permeable

- Allowing ions to move in and out of the cell.

How does the strength of a stimulus affect the generator potential? (3)

- The bigger the stimulus

- The more ions move, resulting in a larger change in potential difference

- Producing a bigger generator potential

What happens if the generator potential reaches the threshold? (2)

1. If the generator potential is big enough to reach the threshold potential

2. It triggers an action potential (nerve impulse) along a neurone

What happens if the stimulus is too weak? (3)

1. If the stimulus is too weak

2. The generator potential won't reach the threshold

3. No action potential is triggered

What type of stimuli do Pacinian corpuscles detect? (2)

- Mechanoreceptors that detect mechanical stimuli

- Such as pressure and vibrations

How do Pacinian corpuscles generate a nerve impulse? (4)

1. When stimulated, the lamellae in the corpuscle are deformed and press on the sensory nerve ending

2. This deforms stretch-mediated sodium channels in the sensory neurone's membrane

3. Sodium ions diffuse into the cell, depolarising the membrane and creating a generator potential.

4. If the generator potential reaches the threshold, an action potential is triggered and transmitted to the CNS

What are some other types of sensory receptors? (3)

- Chemoreceptors (e.g., olfactory receptors in the nose)

- Thermoreceptors (e.g., end-bulbs of Krause in mucous membranes of the tongue)

- Photoreceptors (e.g., cone and rod cells in the eye)

What are the key components of all neurones? (4)

- A cell body with a nucleus, cytoplasm, and organelles

- E.g., large amounts of endoplasmic reticulum and mitochondria for neurotransmitter production

- Dendrons (short extensions from the cell body) and dendrites (smaller branches) carry nerve impulses towards the cell body

- Axons carry nerve impulses away from the cell body

What are the characteristics of sensory neurones? (3)

- Short dendrites.

- One long dendron carries nerve impulses from receptor cells to the cell body

- One short axon carries nerve impulses from the cell body to the CNS

What are the characteristics of relay neurones? (2)

- Many short dendrites carry nerve impulses from sensory neurones to the cell body

- Many short axons carry nerve impulses from the cell body to motor neurones

What are the characteristics of motor neurones? (2)

- Many short dendrites carry nerve impulses from the CNS to the cell body

- One long axon carries nerve impulses from the cell body to effector cells

What is the typical pathway for a nervous response? (5)

Receptor → Sensory Neurone → Relay Neurone → Motor Neurone → Effector Cell

What is the myelin sheath, and what is it made of? (3)

- An electrical insulator

- Made of Schwann cells

- Which grow around the axon many times to form layers of the phospholipid bilayer

What are the nodes of Ranvier, and why are they important in myelinated neurones? (4)

- Gaps between Schwann cells

- Where sodium ion channels are concentrated

- Depolarisation only occurs at the nodes

- Allowing the impulse to jump from node to node (saltatory conduction)

What is saltatory conduction, and why is it faster than continuous transmission? (2)

- Saltatory conduction is when the impulse jumps from node to node

- Which is much faster than continuous transmission along the axon

How do impulses travel in non-myelinated neurones? (3)

- In non-myelinated neurones

- The impulse is transmitted continuously along the nerve fibre

- Making it slower than in myelinated neurones

Draw and label a diagram of the structure of a typical neurone (8)

What is the resting potential in a neurone? (3)

- The potential difference across a neurone's membrane when it is not transmitting an impulse

- With the outside of the membrane being more positively charged than the inside

- It is approximately -70mV

How is the resting potential established and maintained? (3)

1. The sodium-potassium pump actively transports 3 Na⁺ out of the axon for every 2 K⁺ in, creating a sodium electrochemical gradient

2. The membrane is not very permeable to Na⁺, preventing them from diffusing back in

3. The membrane is permeable to K⁺, which diffuses back out through potassium leak channels, making the outside more positively charged

What happens when a stimulus triggers an action potential? (5)

1. Voltage-gated Na⁺ channels open, and Na⁺ diffuses into the axon, making the inside less negative

2. If the threshold potential is reached (around -55mV), more Na⁺ channels open, causing rapid depolarisation (positive feedback)

3. At +40mV, Na⁺ channels close, and voltage-gated K⁺ channels open, allowing K⁺ to diffuse out of the axon, leading to repolarisation

4. Hyperpolarisation occurs as too many K⁺ ions leave the axon, making the membrane more negative than the resting potential

5. The sodium-potassium pump restores the membrane to its resting potential

What is the threshold potential, and why is it important? (3)

- The threshold potential is around -55mV

- If this value is reached, it triggers the opening of more Na⁺ channels and the generation of an action potential

- If the threshold is not reached, no action potential will occur

What is the refractory period? (2)

- A recovery period after an action potential during which the voltage-gated Na⁺ channels cannot open

- Preventing the membrane from being excited again

What is the importance of a refractory period? (2)

- Ensures action potentials are unidirectional (prevents them from travelling backward)

- Ensures action potentials do not overlap and occur as discrete impulses

How does an action potential move along a neurone? (3)

1. Na⁺ ions that enter during depolarisation diffuse along the neurone

2. Causing the next section to reach the threshold potential and trigger a new action potential

3. This creates a wave of depolarisation that moves along the neurone

What is meant by the 'all or nothing' principle? (2)

- Action potentials are all the same size

- A threshold potential has to be reached for an action potential to occur

Draw a diagram of an action potential graph, labelling the axes and stages of an action potential (6)

How does myelination affect the speed of action potentials? (2)

1. Myelination allows for saltatory conduction, where the impulse jumps from node to node

2. This is faster and more energy-efficient than continuous depolarisation along the entire length of the neurone

How does axon diameter affect the speed of action potentials? (3)

- The bigger the axon diameter

- The faster the impulse

- Because there is less resistance to the flow of ions in the cytoplasm

How does temperature affect the speed of action potentials? (3)

- A higher temperature increases the speed of nerve impulses as ions diffuse faster

- However, temperatures above 40°C can cause proteins (e.g., sodium-potassium pump) to become denatured

- Slowing down or stopping the impulse

How does the size of a stimulus affect an action potential? (2)

- The size of a stimulus does not affect the size of an action potential (all-or-nothing principle).

- However, a larger stimulus increases the frequency of action potentials

What is a synapse? (1)

A junction between two neurones or between a neurone and an effector cell

What are excitatory and inhibitory neurotransmitters? (4)

- Excitatory neurotransmitters (e.g., acetylcholine)

- Cause depolarisation of the postsynaptic neurone, potentially triggering an action potential

- Inhibitory neurotransmitters (e.g., GABA)

- Cause hyperpolarisation of the postsynaptic membrane, preventing an action potential from being triggered

What happens when an action potential reaches the synaptic knob? (2)

- Voltage-gated calcium channels open, allowing Ca²⁺ ions to diffuse into the synaptic knob

- The influx of Ca²⁺ causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine (ACh) into the synaptic cleft by exocytosis

What happens after acetylcholine is released into the synaptic cleft? (4)

- ACh diffuses across the synaptic cleft and binds to ACh receptors on the postsynaptic membrane

- Causing sodium ion channels to open

- The influx of Na⁺ ions causes depolarisation of the postsynaptic membrane

- If the threshold is reached, an action potential is generated.

How is the neurotransmitter removed from the synaptic cleft? (3)

1. Acetylcholinesterase breaks down ACh into its products

2. Which are reabsorbed by the presynaptic neurone

3. This prevents continuous stimulation and ensures discrete action potentials

Why must acetylcholine be removed from the synaptic cleft? (2)

- ACh must be removed to prevent continuous firing of action potentials

- To allow for discrete transfer of information across the synapse

What is unidirectionality in synapses? (2)

- Ensures that nerve impulses travel in one direction only

- From the presynaptic membrane to the postsynaptic membrane

How do synapses filter out low-level stimuli? (3)

- Low-level stimuli do not release enough neurotransmitter to open sufficient Na⁺ ion channels

- So the threshold is not reached

- No action potential is produced

What is synaptic divergence, and what is its role? (2)

- When one neurone connects to many neurones

- Allowing information to be dispersed to different parts of the body

What is synaptic convergence, and what is its role? (2)

- When many neurones connect to one neurone

- Allowing information to be amplified

What is summation, and how does it enable a nervous response? (2)

- Occurs when low-level stimuli add together to release enough neurotransmitter

- To produce an action potential in the postsynaptic neurone

What is temporal summation at a synapse? (3)

- Occurs when two or more nerve impulses arrive in quick succession from the same presynaptic neurone

- Resulting in enough neurotransmitter being released

- To reach the threshold and trigger an action potential.

What is spatial summation at a synapse? (3)

- Occurs when multiple presynaptic neurones release neurotransmitter to one postsynaptic neurone

- The combined amount of neurotransmitter is enough to reach the threshold

- To trigger an action potential