5 - MZH - Homeostasis 7 - Neuronal Communication

Question 1

What does the nervous system do?

• What type of response does it provide?
• Give 4 examples

Answer 1

Provides rapid responses to changes in both the internal and external environment

Question 2

What does a general nervous pathway look like?

What does each stage do?

Answer 2

Question 3

Define Stimulus, Receptor and Effector

Answer 3

Stimulus = Detectable change in the external or internal environment of an organism

Receptor = A cell, or protein on a CSM that detects specific stimuli

Effector = A muscle or gland which carries a body’s response to a stimulus

Question 4

What are sensory receptors?

What are transducers?

Answer 4

Sensory receptor = Specialised cells that respond to stimuli. Most sensory receptors are known as transducers - they convert one form of energy into another.

In this case the transducers are converting energy to electrical impulses/ action potentials.

Question 5

Give 4 examples of:

Stimulus - Sensory receptor - Energy change involved (what brings abou this every transfer change?)

Answer 5

Transducers bring about this transfer of energy

Question 6

What are Pacinian Corpuscles?

Structure + Function?

Answer 6

Pacinian corpuscles = (type of mechanoreceptor) which detects changes in pressure on the skin and joints.

NOTE: NOT constant pressure.

Structure:

• Each pacinian corpuscle consists of a single nerve fibre of a dendrite of a sensory neurone wrapped around by layers of a membrane called lamellae.
• Layers of connective tissue are seperated by a jelly/ viscous gel like material.

Function:

1. Pressure applied to skin deforms the lamaellae which pushes up against the dendrite triggering a generator potential.
2. Stretch in membrane opens voltage gated Na⁺ ion channels. Inflow of Na⁺ ions raises membrane potential - depolarises it.
3. The more intense the stimuli the larger the generator potential.
   4.

Question 7

Resting potential = ?

Answer 7

Resting potential = -70mV

Question 8

Structure of a neurone:

Feature: Large cell body

Decription?

Answer 8

Contains the nucleus and many of the cell organelles

Question 9

Structure of a neurone:

Feature: Dendrons

Decription?

Answer 9

Short extensions of the cell body which transmit impulses towards the cell body.

Question 10

Structure of a neurone:

Feature: Dendrites

Decription?

Answer 10

• Each dendron has smaller extensions called dendrites
• Dendrites are stimulated by other neurones

Question 11

Structure of a neurone:

Feature: Axon

Decription?

Answer 11

• It’s a single entension of the cell body
• Can be up to 1m long
• Always transmits nerve impulses away from cell body
• Ends in series of synaptic knobs

Question 12

Structure of a neurone:

Feature: Myelin

Decription?

Answer 12

• In mammals many axons are surrounded by a sheath of fatty material = Myelin sheath
• Myelin enables the neurones to conduct action potentials rapidly

Question 13

Structure of a neurone:

Feature: Schwann cell

Decription?

Answer 13

Specialised cells which produce myelin

Question 14

Structure of a neurone:

Feature: Node of Ranvier

Decription?

Answer 14

• Small gaps between the myelin sheath where bare membrane is exposed.
• Nerve impulses travel along in a series of jumps from one node to the next - Advantage of myelination.
• This jumping is called Salutatory conduction

Question 15

Name the different types of neurones (3)

What do they do?

Answer 15

Sensory neurone = Carries action potential from a sensory receptor → CNS

Motor neurones = Carries action potentials from the CNS → effector e.g. muscle or gland

Relay neurones = Connects the sensory and motor neurones together

Question 16

Compare the structure of Sensory and motor neurones:

• Cell body
• Dendrites
• Axon
• Dendron
• Myelinated

Answer 16

Question 17

Define resting potential and give it’s value

Answer 17

Resting potential = Charge difference across a neurone membrane that must be set up before action potentials can be generated

-70mV

Question 18

How is the resting potential generated and maintained?

Answer 18

Resting potential = -70mV

There is an imbalance of Na⁺ and K⁺ across the membrane. More Na⁺ on the outside and more K⁺ inside neurone.

1. Imbalance of ions is maintaned by a Na⁺/K⁺ pump.
2. Na⁺/K⁺ pump uses ATP for active transport to remove Na⁺ and take up K⁺.
3. Ratio of 3Na⁺ out : 2K⁺ in
4. Both ions can diffuse back down their electrochemical gradient through open channel proteins. NOTE: There’s many more K⁺ channel protein than Na⁺ channel proteins.
5. ∴ Many K⁺ that has been pumped into neurone will leave through their open channel proteins but very few Na⁺ can return back to neurone once pumped out. This further reinforces the imbalance of ions first established by Na⁺/K⁺ pump

Question 19

At what potential difference is the membrane considered to be polarised?

Answer 19

Resting potential at -70mV

Question 20

When is a action potential triggered?

Answer 20

• Action potentials begin when a particular stimulus causes the membrane in one part of the neurone to suddenly change its permeability.
• The depolarisation of the membrane triggers a generated potential and if this generated potential is equal to or exceeds the threshold value then a action potential is triggered.

Question 21

State the 5 steps in the events of an action potential

Answer 21

1. Resting potential
2. Depolarisation
3. Repolarisation
4. Hyperpolarisation
5. Resting potential

Do it all over again!

Question 22

Describe the sequence of events that occur when an action potential is triggered (+ use figures)

Answer 22

1. Resting potential - Initial membrane potential = -70mV
2. Depolarisation - A stimulus occus and the voltage gated Na⁺ channels open. Na⁺ diffuse into neurone down their electrochemical gradient. Depolarisation causes more channels to open and ∴ results in further depolarisation.
3. So many Na⁺ diffuse into of neurone that the inside of neurone becomes positive (+40 mV).
4. Repolarisation - Na⁺ channels close and voltage gated K⁺ channels open. K⁺ diffuses down it’s electrochemical gradient out of the neurone making the inside of the membrane become more negative.
5. Hyperpolarisation - So many K⁺ ions rush out of membrane that the membrane potential falls below the resting potential (undershoot).
6. Resting potential - Na⁺/K⁺ pump restores resting potential by pumping 3Na⁺ out for every 2K⁺ in. From hyperpolarisation back to resting potential is called the refactory period.

Question 23

What is the purpose of the refactory period?

Answer 23

Period between hyperpolarisation and restoration of resting potential.

Refactory period is split into 2 parts:

1. Absolute refactory period = Second action potential can’t occur.
2. Relative refactory period = Second action potetnial can occur with a large enough stimulation.

• After an action potential, the neurone cell membrane can’t be excited again straight away
• This is because the ion channels are recovering and they can’t be made open.
• It also acts as a time delay to ensure that action potentials don’t overlap and are unidirectional i.e. only travel in one direction.

Question 24

What are local currents and their purpose?

Answer 24

Local currents allow action potentials to be transmitted as a nerve impulse

Question 25

Describe how the transmission of action potentials occur along a neurone?

Answer 25

1. Action potential at one **Node of Ranvier** and a resting potetnial at the next node causes a **localised circuit of electricity**. 2. This electrical energy opens the voltage-gated Na⁺ channels at the next node ∴ Na⁺ rush into axon caused membrane to be depolarised. K⁺ rush out and a new action potential is generated. 3. Previous node returns to its resting potential due to Na⁺/K⁺ pump. 4. Action potential continues to travel down the neurone in the same fashion.

Question 26

Give 4 distinct characteristics of action potentials

Answer 26

* They're only generated if the stimulus triggers a generated potential greater than a certain threshold value * Action potentials have a all or nothing law where all action potentials are the same size * Strong stimuli cause many action potentials to be generated i.e. increased frequency * They always travel in one direction

Question 27

Comparison betwwen myelinated and non-myelinated neurones: * What neurones are included in each type * Do they have a myelin sheath? * Length of axons and dendrons * Whether neurones are insulated * Transmission speed * Type of transmission involved

Answer 27

Question 28

Advantages of myelination (2) Why is speed not as big of a concern in non-myelinated neurones?

Answer 28

* **Much faster transmission of action potentials** from 2-20ms^-1 in non-myelinated neurones to 100-200ms^-1 in myelinated neurones * Greater speed of transmission ∴ result in a more **rapid response** as in sensory & motor neurones action potentials may have to travel relatively long distances Non myelinated nerves tend to be shorter ∴ an increased speed in transmission is not that important

Question 29

State and explain 2 other factors that may affect the speed of conduction of action potentials aside from myelination

Answer 29

**Temperature** - Higher temperatures will increase the rate of diffusion of ions and thus increase the rate of conduction **Diameter** - An axon with a larger diameter will transmit action potentials faster but this doesn't necessarily apply to all animals

Question 30

What is a **synapse**? It's purpose and what type of synapses are involved with action potentials? Why is it called that?

Answer 30

**Synapse** = The junction between 2 neurones or between a neurone and effector * The gap created by the synapse is too wide for electrical impulses to cross ∴ a **chemical neurotransmitter** is needed. * **Cholinergic synapses** are used for the transmission of electrical impulses. It's called so as it uses the neurotransmitter **Acetylcholine (ACh)**

Question 31

Describe the sequence of events that occur during **synaptic transmission** (8)

Answer 31

1. Action potential arrives at the **pre-synaptic bulb** 2. Voltage gated Ca²⁺ gates in the CSM of the pre-synaptic bulb to open. Ca²⁺ enter the pre-synaptic bulb. 3. Vesicles containing the neurotransmitter ACh in the pre-synaptic bulb move to the pre-synaptic membrane and fuse with it releasing ACh into the **synaptic cleft** by exocytosis. 4. ACh diffuses across synaptic cleft and attach to ACh receptor molecule present on post-synaptic membrane. 5. **Neurotransmitter/receptor complex** triggers the opening of Na⁺ gated channels on the CSM of the post-synaptic bulb. 6. If sufficient ions enter the post-synaptic bulb an action potential is generated in the post-synaptic neurone. 7. ACh bound to receptor proteins on the post-synaptic bulb are hydrolysed. *ACh → Choline + Ethanoic acid* - enzyme = **acetylcholine esterase**. Choline and ethanoic acid detach from receptor sites and diffused back to pre-synaptic membrane. 8. Choline and ethanoic acid molecules enter pre-synaptic bulb and are reformed into ACh (uses ATP) which is stored into vesicles for future use.

Question 32

Explain the **all or nothing law** with action potentials

Answer 32

* Once the threshold is reached, an action potential will always fire with the same chance in voltage i.e. amplitude doesn't change. * The stronger the stimuli the greater the number of action action potentials fired. Stronger stimuli = Increased frequency of action potentials (like binary)

Question 33

Synaptic transmission: Unidirectional (3)

Answer 33

* Transmission at a synapse can only flow in one direction * Neurotransmitters are only released from the pre-synaptic membranes * Neurotransmitter receptors are only present on the post-synaptic membranes

Question 34

What is the difference between **synaptic convergence** and **synaptic divergence**?

Answer 34

**Synaptic convergence** = Many pre-synaptic neurones release neurotransmitters to one post-synaptic neurone. MANY TO ONE NOTE: Synaptic convergence allows spatial and temportal summation as well as integration) **Synaptic divergence** = One pre-synaptic neurone release neurotransmitters to many post-synaptic neurones. ONE TO MANY

Question 35

Synaptic transmission: Summation

Answer 35

Weak stimuli leads to not enough neurotransmitter released to reach threshold level ∴ depolarisation of post-synaptic membrane doesn't occur. 2 Different types of summation: 1. **Spatial summation** *(down to convergence)* - ≥2 presynaptic neurones converge and release their neurotransmitters at the same time onto the same post synaptic neurone. * Small amounts of neurotransmitter is released from each presynaptic neurone. They add up to reach the threshold level. 2. **Temporal summation** *(down to divergence)* - ≥2 nerve impulses arrive in qiuck succession from the same presynaptic neurone. * Makes action potential more liekly as more neurotransmitter is released into the synaptic cleft

Question 36

Synaptic transmission: Inhibition

Answer 36

**MIMIC, BLOCK or INHIBIT (in 2 ways)** 1. MINIC - Chemicals with the same shape as the neurotransmitters. These drugs are called "**agonists**" * e.g. *Nicotine* mimics ACh and binds to certain types of cholinergic receptors in brain. 2. BLOCK - Chemicals that block receptors so they can't be activated by neurotransmitters. Fewer receptors (if any) can be activated. * e.g. *Curare* blocks ACh by blocking certain cholinergic receptors at certain neuromuscular junctions, so muscle cells can't be stimulated ∴ muscle becomes paralysed. 3. INHIBIT BY PREVENTING HYDROLYSIS THE NEUROTRANSMITTER - Results in more neurotransmitters in the synaptic cleft to bind to receptors. They remain there for longer. * e.g. *Nerve gases* stop ACh from being hydrolysed in the synaptic cleft. This leads to loss of muscle control. 4. INHIBIT BY RELEASING LESS NEUROTRANSMITTER FROM PRESYNAPTIC NEURONE - So fewer receptors are activated. * e.g. *Opioids* block Ca²⁺ channels in the presunaptic neurone. Means that fewer vesicles fuse with the presynaptic membrane ∴ less neurotransmitter is released.