5.1.3 Neuronal communications

Question 1

What is the primary function of neurones in the body?

Answer 1

Neurones transmit electrical impulses rapidly, enabling quick responses to environmental changes.

Question 2

What are the three main structural components of a mammalian neurone?

Answer 2

Cell body
Dendrons/dendrites
Axon

Question 3

What is the function of the cell body in a neurone?

Answer 3

Contains the nucelus and organelles like mitochondria ER
These organelles are vital for the production of neurotransmitters.

Question 4

What is the role of dendrons and dendrites in neurones?

Answer 4

Dendrons branch into dendrites (highly branched)
These receive nerve impulses from many other neurones and transmit them towards the cell body.

Question 5

What does the axon do in a neurone?

Answer 5

A singular, long nerve fibre responsible for carrying impulses away from the cell body to other neurones or effectors.

Question 6

What cells produce the myelin sheath around some axons?

Answer 6

Schwann cells

Question 7

What is the function of the myelin sheath?

Answer 7

Act as an electrical insulator that prevents the passage of ions into or out of the axon at the region it covers.

Question 8

How do impulses travel along myelinated axons?

Answer 8

Via Saltatory conduction - the electrical impulse ‘jump’ between nodes of Ranvier, increasing transmission speed.

Question 9

What are the three types of neurones?

Answer 9

Sensory neurones
Relay (intermediate) neurones
Motor neurones

Question 10

What is the structure and function of sensory neurones?

Answer 10

F: Carries electrical impulses from sensory receptors to the CNS
S: One axon and one dendron, with the dendron leading into several smaller dendrites
Schwann cells form myelin sheath.

Question 11

What is the function and structure of relay neurones?

Answer 11

F: Carries impulses within the CNS, between other sensory and motor neurones
S: Shorter neurones, unmyelinated, highly branched axons and dendrites

Question 12

What is the function and structure of motor neurones?

Answer 12

F: Carries impulses from the CNS to effectors
S: One long axon and multiple highly branched dendrites
Large cell body at one end, myelinated

Question 13

What is the typical pathway of a nerve impulse through the nervous system?

Answer 13

Receptor → Sensory neurone → Relay neurone → Motor neurone → Effector

Question 14

What is a sensory receptor?

Answer 14

Specialised cells that detect stimuli from the environment

Question 15

What is the role of a sensory receptor as a transducer?

Answer 15

It converts stimulus energy (e.g., light, heat) into an electrical signal (nerve impulse).

Question 16

Describe the four types of receptors found in the body.

Answer 16

• Photoreceptors - detect light energy; located in the eyes
• Chemoreceptors - detect chemicals; loacted in nose, tongue and blood vessels (carotid artery & aorta)
• Mechanoreceptors - detect changes in pressure and movement; located in the skin, muscles and inner ear
• Thermoreceptors - detect changes in temperature; located in the skin.

Question 17

What is the resting potential in a receptor cell?

Answer 17

At rest, the receptor cell surface membrane has a volatge due to differences in ion concentration inside and outside the cell.

Question 18

What are the stages of receptor cell function?

Answer 18

1. When a stimulus is detected, the cell surface membrane becomes more permeable, allowing more ions to flow in and out
2. This alters the membrane’s potential difference, creating a receptor potential.
3. A larger stimulus results in a bigger change in voltage, producing a larger receptor potential.
4. If the receptor potential reaches a threshold value, it triggers an action potential.

Question 19

What is a Pacinian corpuscle?

Answer 19

A mechanoreceptor in the skin that detect pressure and vibrations

Question 20

What is the structure of a Pacinian corpuscle?

Answer 20

Surround the end of a sensory neurone’s dendron in layers of connective tissue called lamallae.

Question 21

What happens when the pacinian corpuscles are stimulated?

Answer 21

1. The lamallae deform, pressing on the sensory neurone’s dendron
2. This stretches the neurone’s membrane, cauding it to change shape.
3. The stretch-mediated sodium ion channels in the membrane open, increasing its permeability to Na+.
4. Na+ diffuses into the neurone, depolarising it and resulting in a potential difference.
5. If this potential difference reaches the threshold, an AP is triggered.

Question 22

What is the resting potential of a neurone?

Answer 22

-70 mV
The difference in voltage across the membrane when the neurone is at rest.

Question 23

How is the resting potential achieved?

Answer 23

• Sodium-potassium pumps - Actively transports 3 Na⁺ out and 2 K⁺ in.
• Potassium ion channels - Allow K⁺ to diffuse out of the neurone, further contributing to a negative inside charge.
• Sodium ion channels - Closed, preventing Na⁺ from entering or exiting the cell.

Question 24

What happens to ion distribution during resting potential?

Answer 24

The outside becomes more positive than the inside, leading to a polarised membrane.

Question 25

Describe the stages of the generation of an **action potential**. (six stages)

Answer 25

1. Resting potential - The mebrane is at rest and polarised at around **-70 mV** 2. Stimulus - **Na⁺ ion channels open**, so more Na⁺ flows into the axon making the inside **less negative** 3. Depolarisation - If the threshold potential of -**55 mV** is reached, **Voltage-gated Na⁺ channels open** causing an influx of Na⁺. 4. Repolarisation - At around **+30 mV**, Na⁺ channels close and **Voltage-gated K⁺ channels open**, allowing K⁺ to flow out of the axon, down their conc. gradient. 5. Hyperpolarisation - An **excess** of K⁺ leaves the axon, dropping the potentials below the -70 mV resting level. The K⁺ ion channels are **slow to close** 6. Refractory period - **Sodium-potassium pumps** slowly restore the resting potential.

Question 26

Why is action potential generation a **positive feedback** mechanism?

Answer 26

The initial Na⁺ influx depolarises the axon membrane, which opens more Na⁺ channels. A greater influx of Na⁺ = further depolarisation of membrane

Question 27

What is the **"all-or-nothing"** principle?

Answer 27

Once the threshold potential is reached, an AP is always triggered regardless of stimulus' strength No partial response and APs are always the same size

Question 28

What is the refractory period?

Answer 28

Recovery phases after an AP During, the neurone's membranes cannot generate another AP.

Question 29

Why can another impulse not be fired during it?

Answer 29

Na⁺ channels are closed - no depolarisation possible.

Question 30

What are three features of the refractory period?

Answer 30

* Ensure action potentials **do not overlap** * **Limits frequency** impulses are transmitted * Guarantees impulses travel in only **one direction**

Question 31

How do impulses move along a neurone?

Answer 31

1. The opening of Na⁺ channels results in **local depolarisation** (+ve ions spread sideways). 2. Adjacent voltage-gated Na⁺ channels open in response 3. Leading to depolarisation of nearby membranes in a **wave of depolarisation**. 4. Areas of the membrane that have just experienced depolarisation are in **refractory period** and remain **unresponsive** while they repolarise 5. This ensures the wave moves in **one direction**, preventing backflow of nerve impulse.

Question 32

What three factors affect the **speed** of transmission of an action potential?

Answer 32

* Myelination * Axon diameter * Temperature

Question 33

What is the effect of **myelination** on impulse speed?

Answer 33

Increases speed of transmission via saltatory conduction APs 'jump' between nodes of Ranvier - faster than continuous depolarisation.

Question 34

What is the effect of **axon diameter** on speed?

Answer 34

**Larger** axon diameter means there is **less resistance to ion flow**, so wave of depolarisation travels **faster** along axon.

Question 35

What is the effect of **temperature** on speed?

Answer 35

**Higher** temperature means **faster diffusion** of ions, leading to **faster** depolarisation BUT temperatures above 40°C can cause channel proteins to **denature** = slower impulse transmission.

Question 36

What is a synapse?

Answer 36

A gap between neurones (or a neurone and effector) where signals are passed using neurotransmitters.

Question 37

Describe the key structures in a synapse. (six structures)

Answer 37

* **Presynaptic neurone** - releases neurotransmitters into the synapse * **Synaptic knob** - at the end of the presynaptic neurone, contains organelles needed for neurotransmitter production. * **Synaptic vesicles** - store neurotransmitters & fuses with membrane to release them * **Synaptic cleft** - gap between presynaptic and postsynaptic neurones' membranes * **Postsynaptic neurone** - receives neurotransmitters and can generate new APs * **Neurotransmitter receptors** - specific on postsynaptic neurone and bind with neurotransmitters.

Question 38

What are **excitatory** synapses?

Answer 38

They **depolarise** the postsynaptic membrane, and may trigger an AP if threshold potential reached. (e.g. Noradrenaline at cardiac synapses)

Question 39

What are inhibitory synapses?

Answer 39

They **hyperpolarise** the postsynaptic membrane, **preventing APs** (e.g. acteycholine at cardiac synapses!!)

Question 40

What is **summation** at synapses⚡?

Answer 40

A process that **combines multiple signals** to help trigger an action potential in the postsynaptic neurone. Two types: Spatial and Temporal

Question 41

What is **Spatial Summation**?

Answer 41

* Multiple presynaptic neurones release **neurotransmitters** at the **same time**. (pre neurones converge on a single post neurone) * These **inputs combine** at one postsynaptic neurone. * If enough neurotransmitter is released overall → threshold is reached → action potential is triggered * **Inhibitory signals** can also be involved and may **cancel** out excitatory ones.

Question 42

What is **Temporal Summation**?

Answer 42

* A **single** presynaptic neurone releases neurotransmitter repeatedly in **quick succession**. * The postsynaptic neurone **adds** up these signals over a short period. * If neurotransmitter builds up enough → threshold is reached → action potential occurs.

Question 43

🟩 What are the steps in synaptic transmission? (seven steps)

Answer 43

1. An **AP arrives** at the presynaptic knob 2. Causes **volatge-gated Ca²⁺ channels to open** & Ca²⁺ enters 3. **Synaptic vesicles**, which contain neurotransmitters, move towards and fuses with presynaptic membrane. 4. Neurotransmitters released into synaptic cleft via **exocytosis**. 5. Neurotransmitters **diffuse across** cleft & **bind to receptors** on postsynaptic membrane, causing receptors to **change shape**. 6. **Na⁺ channels open** in postsynaptic cleft, leading to depolarisation of the postsynaptic membrane. 7. If threshold is reached → action potential is triggered in the postsynaptic neurone.

Question 44

What is a cholinergic synapse?

Answer 44

A synapse that uses **acetylcholine** (**ACh**) as its neurotransmitter. (common in the CNS and neuromuscular junctions).

Question 45

How is acetylcholine (ACh) removed from the synaptic cleft?

Answer 45

1. ACh is broken down by the enzyme **acetylcholinesterase** into choline and ethanoic acid. 2. Products are reabsorbed into **presynaptic knob** via active transport 3. They can then be **recycled** to more ACh 4. ACh is then transported into synaptic vesicles.

Question 46

Why is it important to remove neurotransmitters like ACh from the synaptic cleft?

Answer 46

To **prevent continuous stimulation** and allow the synapse to reset for the next signal.