Co-Ordination And Control ( Impulses And Synapse)

Question 1

What is a Resting Potential?

Answer 1

The inside of the neurone is negative charged (with respect to the outside). The axon is polarised (due to excess Na+ ions in the outside). This potential occurs when the neurone is NOT transmitting (-70mv).

Question 2

What is an Action Potential?

Answer 2

When neuron Eis stimulated, sodium channels open, allowing Na+ ions to diffuse in, making the inside less negative. If critical potential difference (threshold stimulus) is reached, the neurone becomes depolarised.

Question 3

What is a Refractory Period?

Answer 3

AP followed by a period when the membrane repolarises and recovers it resting potential. The membrane is unexcitbale. Re-polarisation must happen before further action potential can develop.

Question 4

When is the All-For-Nothing Law?

Answer 4

To initiate an AP, a stimulus must exceed the threshold stimulus. AP doesn’t vary in size, the AP will remain constant. It either occurs or not.

Question 5

How are nerve impulses transmitted?

Answer 5

AP is propagated along the dendron/axon, due to depolarisation of one part of the axon which sets up local circuits with areas on either side of it.

Question 6

How do local circuits occur?

Answer 6

Na+ ions are attracted to neighbouring negative region.
- on one side of the membrane is in its refractory period.
- the other unexcitable side is depolarised by local circuits and formation of AP.
This process is respected along the whole neurone.

Question 7

How can the Speed of Nerve Impulse Conduction be increased?

Answer 7

Myelination of the axon
Diameter of the axon

Question 8

How does myelination of the axon increase nerve impulse conduction?

Answer 8

The neurone is insulated by myelin sheath - the AP only occurs in the Nodes of Ranvier, they jump from one to the next. This increases the speed in which the nerve is propagated, this is called saltatory conduction.

Question 9

How does the diameter of the axon increase the speed of nerve impulse conduction?

Answer 9

Some species have non-myelinated axons, with a large diameter, and so a large SA which ions might be moved. Therefore, depolarisation occur more rapidly, increasing the transmission speed.

Question 10

Describe the Excitatory Synapse

Answer 10

• impulse arrives at the synaptic bulb, the membrane becomes permeable to Ca2+ ions
• vesicles move towards the pre-synaptic membrane
• they fuse with the membrane and release acetylcholine (Ach) by exocytosis into the synaptic cleft
• Ach molecules diffuse across the synaptic cleft to the post-synaptic membrane
• Ach molecules attach to specific receptors
• an influx of Na+ ions by diffusion, post-synaptic membrane becomes depolarised so an excitatory post-synaptic potential (EPSP) is generated
• if EPSP reached a threshold intensity then an AP is evoked and an impulse is propagated
• the breakdown of Ach prevents the continues stimulation of impulses. Ach is hydrolysed by acetylcholinestrase (AchE)
• the products diffuse back towards the pre-synaptic membrane and re absorbed into the synaptic bulb and re-synthesised into Ach using ATP from mitochondria

Question 11

Describe and Inhibitory Synapse

Answer 11

The neurotransmitter they release makes it more difficult for EPSP to form. It leads to an influx of negative ions, making the inside even more negative, creating an inhibitory post-synaptic potenzial (IPSP)m which is more negative than the resting potential. This is known as hyperpolaristaion.

Question 12

Why have an inhibitory synapse?

Answer 12

Reduced input of background stimuli that would clutter the nervous activity in the brain and prevent reflux actions.