Synapse and Neurotransmission (Physiology)

Question 1

Describe the basic structure of the synapse

Answer 1

• Consists of a presynaptic neurone and a post-synaptic neurone separated by a gap called the synaptic cleft.
• The axon terminal of the presynaptic neurone contains synaptic vesicles, many of which are docked at release sites on the plasma membrane, ready to release neurotransmitter on demand.
• A reserve pool of synaptic vesicles is located further away from the membrane, tethered to the cytoskeleton.
• The membrane of postsynaptic neurone has receptors and ligand-gated ion channels.

Question 2

Explain why the synaptic structure is needed

Answer 2

Ensures that the nerve signal only travels in one direction as only the presynaptic neurone contains the synaptic vesicles with neurotransmitter and only the postsynaptic neurone has receptors and ligand-gated ion channels.

Question 3

Describe the major neurotransmitters used at synapses

Answer 3

• Acetylcholine is formed from acetic acid (acetate) and choline.
• Amino acids e.g. glycine, glutamate, and gamma-aminobutyric acid (GABA).
• Monoamines which are from amino acids by removing the COOH group e.g. epinephrine, norepinephrine, dopamine, histamine, and serotonin.
• Purines e.g. adenosine and ATP.
• Gases e.g. nitric oxide and carbon monoxide. They are synthesised as needed, not stored in synaptic vesicles. Also diffuse into the postsynaptic neurone instead of binding with receptors on the membrane.
• Neuropeptides are chains of 2-40 amino acids e.g. cholecystokinin (CCK) and endorphins. They are stored in secretory granules which are twice as large as synaptic vesicles.

Question 4

Describe why the same neurotransmitter may have opposing effects on different tissues

Answer 4

For example

• Acetylcholine decreases cardiac muscle but increases skeletal muscle contraction.
• Even though there are acetylcholine receptors in both tissues, they are of different types.
• Cardiac muscle expresses the muscarinic type of acetylcholine receptor and skeletal muscle expresses the nicotinic type.
• Activation of each receptor results in different cellular events occurring as they are linked to different subcellular effectors.

Question 5

Explain the difference between excitatory and inhibitory postsynaptic potentials

Answer 5

• If a neurotransmitter causes a depolarisation it is referred to as an excitatory postsynaptic potential, since it brings the membrane potential closer to the threshold for an action potential. If the threshold is exceeded an action potential is evoked.
• If a neurotransmitter causes a hyperpolarisation it is referred to as an inhibitory postsynaptic potential, since it brings the membrane potential away from the threshold for an action potential. This makes it less likely for an action potential to be evoked.

Question 6

Describe synaptic transmission

Answer 6

For an excitatory action potential

• The arrival of a nerve signal at the axon terminal opens voltage-gated calcium channels.
• Calcium enters the axon terminal and causes exocytosis of the synaptic vesicles which release acetylcholine into the synaptic cleft.
• The acetylcholine diffuses across the synaptic cleft and binds to ligand-gated channels on the postsynaptic neurone.
• These channels open, allowing sodium to enter the cell and potassium to leave.
• As sodium enters, it spreads along the inside of the membrane and depolarises it, producing a postsynaptic potential.
• If the postsynaptic potential is strong enough and persistent enough, it opens voltage-gated ions in the trigger zone and causes an action potential.

For inhibitory action potentials

• Amino acid neurotransmitters such as GABA work by the same mechanism as acetylcholine.
• The release of GABA and binding of it to its receptor causes chloride channels to open.
• The chloride ions enter the cell and makes the inside of the cell even more negative than the resting membrane potential (hyperpolarisation). Therefore inhibiting the formation of an action potential.

Question 7

Describe summation

Answer 7

• It is the process of adding up excitatory and inhibitory postsynaptic potentials and responding to their net effect.
• It occurs in the trigger zone.
• Temporal summation occurs when a single synapse generates excitatory postsynaptic potentials so quickly that each is generated before the previous one fades. This allows the excitatory postsynaptic potentials to add up over time to a threshold potential that evokes an action potential.
• Spatial summation occurs when excitatory postsynaptic potentials from several synapses add up to the threshold at the axon hillock.
• Any one synapse may generate only a weak signal, but several acting together can bring the axon hillock to threshold. The presynaptic neurones therefore collaborate to evoke an action potential in the postsynaptic neurone.

Question 8

Describe signal termination

Answer 8

• There are three ways in which neurotransmitter can be removed from the synaptic cleft.
• Neurotransmitter degradation - An enzyme in the synaptic cleft breaks the neurotransmitter down into fragments that have no stimulatory effect on the postsynaptic neurone e.g. acetylcholinesterase breaks down acetylcholine into acetate and choline.
• Reuptake - A neurotransmitter or its breakdown products are reabsorbed by transport proteins in the axon terminal, removing them from the synaptic cleft and ending their stimulatory effect.
• Diffusion - Neurotransmitters of their breakdown products diffuse away from the synapse into the nearby extracellular fluid.

Question 9

What are inotropic receptors

Answer 9

• Ligand-gated receptors.
• Cause rapid opening of ion channels resulting in depolarisation or hyperpolarisation of the postsynaptic membrane.
• Mediate fast ionic synaptic responses.

Question 10

What are metabotropic receptors

Answer 10

• G-protein receptors
• Initiate a wide range of cellular responses.
• Mediate slow biochemical synaptic responses.