Synapses

What are synapses and explain the types

Synapses seperate neurons and allow the electrical impulse to be carried from one neuron to the next

types

1. Electrical synapse - the gap is small enough for the impulse to jump and travel directly from one neuron to the next

2. chemical synapse - gap is too big so chemical neurotransmitters transmit the impulse

explain chemical synaptic transmission

The pre-synaptic neuron end bulb alters its membrane permeability so that voltage-dependent calcium channels open and calcium diffuses in.

This stimulates vesicles containing neurotransmitters to diffuse across the presynaptic neuron, fuses with the membrane and by exocytosis (energy required so lots of mitochondria) release the neurotransmitter into the cleft

They diffuse across the cleft and then bind to intrinsic protein receptors with two binding sites. The protein then undergoes co-operative binding and changes shape

This stimulates voltage-dependent calcium channels to open in the post SN and calcium enters. This depolarises the PSN and if it reaches enough voltage for depolarisation it will fire the action potential otherwise it won’t.

How to prevent multiple action potentials

1. calcium is actively transported out of the end bulb

2. hydrolysis by enzymes of neurotransmitter eg. acetylcholinesterase to acetylcholine into ethanoic acid and choline which is reuptakes and choline with acetyl coA forms AC again.

3. reuptake of neurotransmitter

Why do neurones transmit in one direction

1. repolarisation occurs behind so no action potential can be fired

2. receptors are only on pre SN

3. Neurotransmitters are only on pre SN

what are the properties of synapses and explain how an action potential can be built up

synapses:

1. transmit in one direction

2. transmit info between neurons

3. control and filter minor stimuli as only when the threshold voltage is reached of +40mv will an action potential be induced otherwsie it won’t - all or nothing principle - means the brain isn’t otherloaded with info.

4. because the action potential is always +40mv - an action potential only increases frequency

The action potential can be built up by

1. temporal resolution - lots of depolarizations over time

2. spatial resolution - several pre SN lead to the same post SN

What are drugs and how do many of them of them work

Drugs = substances that have an physiological effect on the body when inside in body

Many act as synapses and effect functions of neurotransmitters which can have an effect on the neurological pathway and nervous system

Explain one of the types of drugs’ effect on the nervous system (2)

2. stimulants - they increase nervous impulses by creating more action potentials

One way they do this is through mimicking neurotransmitters eg. Nicotine and AC have the nitrogen atom structure so N can bind to the receptors as well which means more action potentials. It also isn’t broken down so more nicotine is available. This can lead to addiction and drug tolerance as the body becomes used to the drug so the nervous system can’t function without it. - withdrawl and made worse with happiness symptoms from dopamine

Another way is through inhibiting hydrolysis of neurotransmitter. Eg. organo phosphate (ester of phosphoric acid) inhibits AC estrase so AC isnt broken down and action potentials are constantly fired. Its found in insecticides and herbicides and nerve gas (inhibits enzymes at neuromuscular junctions causing uncontrollable muscle reflexes)

Explain another 2 types of drugs

1. sedative drugs - decrease action potentials

2. psychoactive drugs - effect Central NS so the brains functioning of perception. memory and mood are effected - cxan lead to addiction

What are the factors affecting the speed of the nervous impulse

1. temperature - the molecules have more kinetic energy so they move faster ca+

2. diameter of axon - larger diameter = higher SA:V so more diffusion of ca+

3. Myelination causing saltatory conduction - The axon is myelinated with myelin sheath that electrically insulated the neoron so that less electrical voltage change occurs so that ca+ ion voltage channels close and only open at gaps in myelination - Nodes of Ranvier. So the electrical impulse appears to jump