Key Area 4
Nerve impulse transmission
Starter
Let's start with this quick Quizlet game. Simply match the 2 cards that are connected - but do it quickly! Click here to access it directly.
Communication between neurons
As we have stated before, the resting membrane potential is a state where there is no net flow of ions across the membrane. In neurons, the resting potential is generated and maintained by the action of the Na/K pump, removing 3 positive sodium ions from the cell and allowing 2 positive potassium ions into the cell. The resting membrane potential of a neuron is around -70 mV.
Task 65
Armani burst into the room. "Alright miss, wit we dain eh day?".
"We're starting to think more about nerve transmission as a form of cellular communication", replied Dr McRobbie, "Armani, grab your notes and try to remember the structure of a neuron and a synapse".
On the board, Dr McRobbie had asked the class to make sketches of "the basic structure of a neuron" and "a close-up view of a synapse".
"When you have finished that, I would like to explain what must happen for the nerve impulse in a pre-synaptic neuron to be transmitted to a post-synaptic neuron".
Armani scoffed - "ye must be joking Miss, I cannae remember wit I 'ad fer ma breakfast never mind that!".
"I thought you might say that - here is a video (right) to help jog your memory".
Watch the video shown opposite and respond to these 3 tasks in your own notes.
Suggested answers are available here.
Generation of a nerve impulse
The transmission of a nerve impulse requires changes in the membrane potential of the neuron's plasma membrane. An action potential is a wave of electrical excitation along a neuron's plasma membrane. Neurotransmitters initiate a response by binding to their receptors at a synapse. Neurotransmitter receptors are ligand-gated ion channels.
Neurotransmitters bind their receptor
Neurotransmission first relies on a neurotransmitter-containing vesicle fusing with the membrane of the pre-synaptic neuron. The neurotransmitter is released into the synaptic cleft and diffuses across until it binds with receptors on the post-synaptic cleft membrane. These receptors are ligand-gated ion channels.
Ligand-gated ion channels now open.
Once the neurotransmitter has bound to its receptor (ligand-gated ion channel), the ion channel opens and positively-charged ions flood into the cell. This has the effect of "depolarising" the membrane - Depolarisation is a change in the membrane potential to a less negative value inside the cell, compared to outside and often approached ~0mV at this stage.
Voltage-gated ion channels open
If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, the opening of voltage-gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient. This leads to a rapid and large change in the membrane potential (increasing to between 0mV and +70mV).
Restoration of resting membrane potential
A short time after opening, the sodium channels become inactivated. Voltage-gated potassuim channels then open (channel on the left) to allow potassium to move out of the cell. This removes positive charges from the inside of the cell and starts to bring the resting potential closer to resting values of approx. -70mV.
Task 66
Explain the meaning of "depolarisation". What is the resting potential of a neuron and how is this maintained - can you include a suitable diagram to support your answer?
Suggested answers are available here.
Restoration of the membrane potential
Restoration of the resting membrane potential allows the inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response to depolarisation of the membrane. Ion concentration gradients are re-established by the Na/K pump, which actively transports excess ions in and out of the cell back to resting potential levels. Following repolarisation, the sodium and potassium ion concentration gradients are reduced.
Nerve transmission overview
Ultimately, the inactivation of sodium channels and the opening of potassium channels restores the resting membrane potential.
This graph shows the resting membrane potential maintained by the Na/K pump (1); a stimulus (neurotransmitter) results in a change in membrane potential due to the opening of sodium channels (2). Voltage-gated sodium channels then open (3) at the peak of the action potential. The change in voltage now triggers the opening of voltage-gated potassium ions (4), which allows positive ions to flood out of the cell. Sodium gated channels now shut. This helps restore the resting membrane potential of approx -70mV (6). Ultimately, the Na/K pump maintains and restores the resting membrane potential again of -70mV (7).
Task 67
Sketch a graph to show the changes in membrane potential during impulse transmission along a nerve axon. Explain the effect of ligand-gated and voltage-gated ion channels on the membrane potential of a neuron during neurotransmission.
Suggested answers are available here.
Waves of Depolarisation
Depolarisation of a patch of membrane causes neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened. This allows the impulse to spread along the full neuron axon.
When the action potential reaches the end of the neuron, it causes vesicles containing neurotransmitter to fuse with the membrane - this releases neurotransmitter, which stimulates a response in a connecting cell. And the cycle continues!
Summary
Watch this useful animation to summarise the key steps in nerve impulse transmission. Click the pink button below.
Hank Green also summarises this process in this video here:
Task 68
"Time for consolidation, you lot. Read the statements shown about nerve impulse transmission and put them in the correct order", instructed Dr McRobbie in a commanding tone.
"I'm definitely needing some consolidation today Miss. Maybe a whole period of quiet reflection would be good", suggested Big Davie.
"Hmm, good one Big Davie, good one".
Suggested answers are available here.
Now visit SCHOLAR to work through "4.4 Generation of a nerve impulse" to help consolidate this first part of Key Area 4d.
Initiation of a nerve impulse in response to an environmental stimulus
Watch the video opposite to see an eyeball dissection. You might get the opportunity to do this in class or even at home (often you can order eyeballs from your local butcher!)
But otherwise, Ross Exton at the Bristol Science Centre does a superb job!
The Vertebrate Eye
The retina is the area within the eye that detects light and contains 2 types of photoreceptor cells:
Rods
Cones
Try a Google Virtual Tour of the Eye
Task 69
Dr McRobbie had thoroughly grossed them all out by waltzing into the room, swinging a bag of eyeballs. "Oh Miss, that's pure rank, where did you even get those things". Dr McRobbie informed them that she'd ordered them from the butcher in Bridge of Allan.
"Yer jokin Miss - wit, ye just phoned up and said "Aye, gie me some eyebaws!", laughed wee Jonny, swinging on his chair.
"I did just that Jonny, just much more politely", replied Dr McRobbie.
The class couldn't believe it but, nevertheless, assumed their positions behind their dissecting tray and tools. Dr McRobbie displayed a diagram of the vertebrate eye on the board and asked the class to try and identify a number of key structures.
Can you label the diagram of the vertebrate eye in your own notes? Suggested answers are here.
Rod Cells - Low-light sensitivity
In animals, the light-sensitive molecule retinal is combined with a membrane protein, opsin, to form the photoreceptors of the eye.
In rod cells, this retinal-opsin complex is called RHODOPSIN.
As you can see from the diagram opposite, opsin is a largely alpha-helical protein (typical of transmembrane proteins) and retinal is found as a small molecule within this (green stick structure).
Importantly, this retinal molecule is sensitive to light - retinal can absorb a photon of light and, as a consequence, the conformation of rhodopsin changes. This results in photoexcited rhodopsin.
see image left: A cascade of proteins then amplifies the original signal. A single photoexcited rhodopsin molecule activates hundreds of G-protein molecules, called transducin (pink). Each activated transducin molecule activates one molecule of the enzyme phosphodiesterase (PDE, shown in blue).
Each active PDE molecule catalyses the hydrolysis of thousands of cyclic GMP (cGMP) molecules every second.
The reduction in cGMP concentration as a result of its hydrolysis affects the functioning of ion channels in the membrane of rod cells. As a consequence, these ion channels close which affects the membrane potential of rod cells and this triggers a nerve impulses in neurons in the retina (see right).
As you can see in the image opposite, in dark conditions (A), rhodopsin does not become photoactivated. In this state, PDE is not activated and cGMP is not hydrolysed. As a consequence, the ion channel remains open and positive sodium ions continue to enter the cell, maintaining the membrane potential.
However, in the presence of light, this signal cascade fires. Activated transducin stimulates PDE, which hydrolyses cGMP. This ultimately results in the closure of ion channels, which prevents the influx of ions into the cell. As a result, the membrane potential changes and a nerve impulse is fired.
Therefore, a VERY HIGH DEGREE OF AMPLIFICATION results in rod cells being able to respond to low intensities of light.
Watch this bang-on relevant video on visual transduction before moving onto Task 69.
Task 70
The diagram below summarises signal transduction in rod cells of the vertebrate retina. Can you provide a text-based summary to describe what is happening? Suggested answers are available here.
Cone cells
Sensitive to a wider range of wavelengths of light
In cone cells, different forms of opsin combine with retinal to give different photoreceptor proteins.
Each of these proteins have a maximal sensitivity to specific wavelength: red, green, blue (or UV in certain vertebrates). This gives humans their colour vision.
Now visit SCHOLAR to work through 4.5, 4.6, 4.7 and 4.8, shown right, to support your learning of Topic 1, Key area 4.
Time to create your own content for our online class blog and become a published author! Write a piece that showcases your learning of Key Area 4 on Communication & Signalling.
