Lecture 3 - The synapse

Question 1

What 3 factors determine the electrical activity of the cell?

Answer 1

1. how well the ions move across the cell membrane
   -> very permeable to K+ but not Na+
2. why ions move across the cell membrane
3. when the ions move

Question 2

Why do ions move across the cell membrane?

Answer 2

• K+ moves into the cell to neutralise big A-
• But K+ wants to move out of the cell down the concentration gradient
• Less K+ outside cell
• When the force of the electrical attraction of K+ into the cell is the same as the force of the concentration attraction of K+ out of the cell, we have equilibrium
• But the inside of the cell is still at -65mV with respect to the outside

Question 3

When do the ions move?

Answer 3

• If cells are depolarised (made less -, more +)
• Na+ channels open
• Na+ rushes in, as it is trying to neutralise the big A- and rush down its concentration gradient
• Na+ much more concentrated outside cell
• this makes the inside of the cell more positive
• at +40mV, the Na+ gates close and voltage gated k+ channels open
• K+ rushes out down the concentration gradient
• k+ much more concentrated inside cell
• because + is leaving, the inside of the cell becomes more - again

Question 4

What is a synapse?

Answer 4

• the gap between 2 neurons
• electrical signal needs to convert to chemical to communicate AP to next neuron
• the synapse is formed by the termination of an axon from one neuron onto the dendrite of another neuron

Question 5

What happens at the synapse?

Answer 5

• Voltage-gated Ca channels open when AP reaches the pre-synaptic terminal
• Synaptic cleft is rich in Ca so Ca moves into the presynaptic terminal and binds to vesicles
• Vesicles fuse with pre-synaptic membrane and release neurotransmitters into the synapse
• NT bind to postsynaptic ion channels which can have an excitatory or inhibitory effect

Question 6

Synpases that cause movement?

Answer 6

• there are specialized synapses that form between neurons and muscles (neuro-muscular junction), these cause muscles to contract and allow us to move
• the axons of neurons that activate muscles are located in the spinal cord
• ALS (Amyotrophic Lateral Sclerosis)is a disease that affects these neurons, this disease gradually restricts movement and eventually results in death

Question 7

Sea slugs and calcium?

Answer 7

• The sea snail, Aplysia, withdraws its gills in a defensive manner when confronted with a jet of water
• However, with repeated stimulation it learns that the water jet is not harmful and shows a weakened response (habituation/adaptation)
• Recent studies suggest that this process can result from a reduction in the influx of calcium at the axon’s pre-synaptic terminal and the release of less neurotransmitter

Question 8

What are NT receptors?

Answer 8

proteins that are found on dendrites

Question 9

Iontropic NT receptors?

Answer 9

• they are receptors composed of two parts, 1 part binds neurotransmitter, the other part is an ion channel
• when a neurotransmitter binds to a receptor, the ion channel opens allowing ions into the cell
• these receptors are selective to certain types of neurotransmitters – they work like a lock and key

Question 10

Metabotropic NT receptors?

Answer 10

• influence ion channels indirectly
• when a neurotransmitter binds to the binding site, the α subunit detaches and causes the ion channel to open allowing ions to pass through
• they influence function more slowly and their effect is longer than ionotropic NT receptors

Question 11

Excitatory NT?

Answer 11

• released at type I synapse
• bind to receptors that cause an influx of positive ions (Na+)

Question 12

Inhibitory NT?

Answer 12

• released at type II synapses
• cause an influx of negative ions (Cl-)

Question 13

NT removal and inactivation?

Answer 13

• Following their release neurotransmitters need to be removed or inactivated
• If neurotransmitters are not removed or inactivated prolonged activation can occur

Question 14

What is glutamate?

Answer 14

• the brains major excitatory NT
• forms links between neurons that are the basis of learning and memory

Question 15

What is GABA?

Answer 15

the brains main inhibitory NT

Question 16

What is dopamine involved in?

Answer 16

movement control and reward circuits

Question 17

What is serotonin?

Answer 17

a feel good chemical, has an effect on mood and anxiety

Question 18

What is acetylcholine?

Answer 18

the NT used at the neuro-muscular junction

Question 19

Parkinson’s disease?

Answer 19

• results from a loss of dopaminergic neurons in the brain stem
• the drug levy-dopa mimics the actions of dopamine and can relieve symptoms
• however a side effect = schizophrenia

Question 20

What toxins poison ion channels?

Answer 20

• pufferfish produce tetrodotoxin = inactivation of Na+ channels
• K+ channels are inactivated by toxins from wasps and bees

Question 21

What are some toxins that affect transmitter release?

Answer 21

• α -latrotoxin is released by the black widow spider, it causes a massive release of NT (Acetylcholine) at the nerve-muscular junction that results in paralysis
• Botulinum toxin works by stopping the release of excitatory neurotransmitters (Acetylcholine) at the neuro-muscular junction = prevents the contraction of the affected muscles
• The tetanus toxin works by preventing inhibitory neurotransmitters to be released in the spinal cord
• This causes hyperactivity of the muscles

Question 22

What toxins block NT receptors?

Answer 22

• Poisonous plants and venomous animals are widespread in nature
• Many of these toxins bind to neurotransmitter receptors
• E.g, αBungarotoxin which is found in the venom of the branded krait blocks (a snake) neurotransmitter receptors on the nerve-muscle junction, this prevents the victim from making an escape

Question 23

Psychoactive drugs?

Answer 23

• They mimic the effect of NT’s by binding directly to NT receptors
• LSD and psilocybe (mushrooms) mimic the effect of serotonin (called agonists)
• Alcohol stimulates GABA receptors (acting as an agonist) increasing the effect of this inhibitory NT leading to a sedative like effect
  -> alcohol also acts as an agonist as it blocks glutamate receptors which are involved in memory formation

Question 24

What drugs affect the uptake of NT?

Answer 24

• Cocaine:
  -> prevents the reuptake of dopamine
  -> giving rise to the feeling of arousal
• Prozac:
  -> blocks the re-uptake of serotonin
  -> gives rise to a feeling of well-being

Question 25

Why do people become addicted to drugs?

Answer 25

• Many theories suggest an association between drug taking and reward circuits in the brain
  -> The reward system is used to give pleasurable feelings when we do things that keep us alive
• The release of the neurotransmitter dopamine is thought to be important in reward circuits
• Many highly addictive drugs (cocaine, heroin, nicotine) activate this system in the brain