B2- nerves

Describe the structure of a myelinated motor neurone

describe the function of the following in a motor neuron:

1. cell body

2. dendrites

3. axon

4. Schwann cells

5. Myelin sheath

6. Node of Ranvier

7. terminal end branch

1. nucleus and cytoplasm- produces proteins and neurotransmitters

2. carry nerve impulses towards cell body

3. carries nerve impulses away from cell body

4. produce myelin

5. myelin insulate axon increasing speed of transmission of nerve impulse

6. gap between Schwann cells where myelin sheath absent

7. connect neurone to effector

describe resting potential

inside of axon has a negative charge relative to outside ( more positive ions outside compared to inside )

Explain how a resting potential is established across the axon membrane in a neurone

1. Na⁺/K⁺ pump actively transports:

• (3) Na⁺ diffuse out of axon and (2) K⁺ diffuse into axon

2. creating an electrochemical gradient:

• higher K⁺ conc inside AND higher Na⁺ conc outside

3. Differential membrane permeability:

• More permeable to K⁺ → move out by facilitated diffusion via K⁺ channels

• Less permeable to Na⁺ (closed voltage-gated Na⁺ channels)

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

1. STIMULUS

• Na⁺ channels open; membrane permeability to Na⁺ increases

• Na⁺ diffuse into axon down electrochemical gradient (causing depolarisation)

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

2. DEPOLARISATION

• if threshold potential reached, an action potential is generated

• Na⁺ diffuses into axon

• depolarisation- inside of axon becomes less negative

• as more voltage-gated Na⁺ channels open→ positive feedback effect

• so more Na⁺ diffuse in rapidly

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

3. repolarisation

• voltage-gated Na⁺ channels close

• voltage-gated K⁺ channels open; K⁺ diffuse out of axon

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

4. hyperpolarisation

• K⁺ channels slow to close so theres a slight overshoot- too many K⁺ diffuse out

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential

5. resting potential

• restored by Na⁺/K⁺ pump

Draw / label a graph showing an action potential

describe the all or nothing principle

• for an action potential to be produced, depolarisation must exceed threshold potential

• action potentials produced are always same magnitude / size / peak at same potential

• bigger stimuli instead increase frequency of action potentials

explain how the passage of an action potential along non myelinated axons result in nerve impulses

• action potential passes as a wave of depolarisation

• influx of Na⁺ in one region increase permeability of adjoining region to Na⁺ by causing voltage-gated Na+ channels to open so adjoining region depolarises

explain how the passage of an action potential along myelinated axons result in nerve impulses

• myelination provides electrical insulation

• depolarisation of axon at nodes of Ranvier only

• resulting in saltatory conduction

• so there is no need for depolarisation along whole length of axon

suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement

1. less / no saltatory conduction; depolarisation occurs along whole length of axon

• so nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction

2. ions / depolarisation may pass / leak to other neurones

• causing wrong muscle fibres to contract

describe the nature of the refractory period

• time taken to restore axon to resting potential when no further action potential can be generated

• as Na⁺ channels are inactive / won’t open

explain the importance of the refractory period

1. ensures discrete impulses are produced

2. limits frequency of impulse transmission at a certain intensity ( prevents over reaction to stimulus )

• higher intensity of stimulus causes higher frequency of action potentials

• but only up to a certain intensity

3. also ensures action potentials travel in one direction

name 3 factors that affect the speed of conductance

1. myelination

2. axon diameter

3. temperature

explain how myelination affects the speed of conductance

• depolarisation at Nodes of Ranvier only→ saltatory conduction

• impulse doesn’t travel whole length of axon

describe the structure of a synapse

what are cholinergic synapses?

synapses that use neurotransmitter acetylcholine (ACh)

describe transmission across a cholinergic synapse

At pre synaptic neurone:

1. depolarisation of pre-synaptic membranes causes opening of voltage-gated Ca²⁺ channels

• Ca²⁺ diffuse into pre-synaptic knob

2. causing vesicles containing ACh to move and fuse with pre-synaptic membrane

• releasing ACh into the synaptic cleft (by exocytosis)

At post synaptic neurone:

3. ACh diffuses across synaptic cleft to bind to specific receptors on post synaptic membrane

4. causing Na⁺ channels to open

• Na⁺ diffuse into post-synaptic knob causing depolarisation

• if threshold is met, an action potential is initiated

explain what happens to acetylcholine after synaptic transmission

• it is hydrolysed by acetylcholinesterase

• products are reabsorbed by the presynaptic neurone

• to stop overstimulation- if not removed it would keep binding to receptors, causing depolarisation

explain how synapses result in unidirectional nerve impulses

• neurotransmitter only released from pre-synaptic neurone

• receptors only on post-synaptic membrane

explain how axon diameter affects the speed of conductance

bigger diameter means less resistance to flow of ions in cytoplasm

explain how temperature affects the speed of conductance

• increase rate of diffusion of Na⁺ and K⁺ as more kinetic energy

• but proteins / enzymes could denature at a certain temperature