Transduction and Synapses

Question 1

What is an action potential?

What is graded potential?

What causes membrane permeability change?

Answer 1

• all or nothing event involving voltage gated channels
• propagates along a nerve by depolarisation up to threshold

Graded potential:

• local change in membrane potential

Triggering event:

• receptor: action of stimulus - mechanical, chemical or thermal gated channel
• synaptic: post synaptic membrane

Temperature causes membrane permeability change by receptors

Question 2

Graded potentials can take place at receptors, what is this process?

Answer 2

• Transduction at the nerve ending: conversion of stimulus energy (mechanical) to electrical energy via specific channels
• mechanically sensitive ion channels
• thermal

Question 3

What is a sub threshold depolarisation?

What happens if stimulus is increased?

Increased more?

Unlike an action potential, what is related to a graded potential?

Answer 3

Sub threshold depolarisation: a small stimulus which does not reach threshold potential will be detected by mechanical receptor but not enough to deliver an action potential, and be detected by the nervous system

• increased stimulus: a graded potential again, but not adequate to reach threshold depolarisation
• adequate stimulus: membrane potential change that will propagate through receptor, and reaches threshold so will initiate action potential along the nerve

The size of the stimulus is related to a graded potential, once threshold is reached, an action potential will be fired

Question 4

What is a synapse?

Answer 4

A one-way relay station from a pre-synaptic to post-synaptic state

• many drugs act as synapses e.g. analgesics
• plasticity - ability to change e.g. nerve damage causes rewiring

Normally chemical - neurotransmitters

• glutamate
• GABA
• acetylcholine
• serotonin
• noradrenaline

Question 5

How does an action potential cross a synpase?

Answer 5

• AP propagated to a synapse
• triggers opening of voltage-gated calcium channels which allow influx of Ca into pre-synaptic nerve ending
• Pre-synaptic nerve contains many synaptic vesicles and fuse with membrane in response to Ca and release their contents (neurotransmitters)
• neurotransmitters diffuse across synaptic cleft and bind with receptor channels on post-synaptic membrane (chemically gated ion channels)
• let sodium in and causes depolarisation
• if reaches threshold, AP propagates, if not, signal will stop

Question 6

What is temporal summation?

What is spacial summation?

Answer 6

Temporal summation: two action potentials in pre-synaptic nerve arriving close together causes a graded potential at the post-synaptic nerve that add together in time to reach threshold

Spacial summation: single AP not sufficient to bring post-synaptic nerve to threshold so 2 graded potentials add on top of each other and brings post-synaptic nerve to threshold

Question 7

Answer 7

Question 8

What is post synaptic inhibition?

What is pre-synaptic inhibition?

Answer 8

• Inhibitory post synaptic potentials
• hyperpolarisation of post synaptic nerve
  e. g. GABA

Pre-synaptic inhibition:

• no direct effect on post synaptic membrane
• results in decreased neurotransmitter release from presynaptic terminal
• axo-axonal synapse