48 Neurons, Synapses and Signaling Flashcards
What cells support the neutrons?
Ganglia.
What is the nervous system divided into?
The central nervous system (CNS) and the peripheral nervous system.
What are the basic types of neurons?
Sensory neurons, interneurons and motor neurons
What are sensory neurons?
Neurons that transmit information from SENSORS that detect internal and external stimuli.
(note that the sensory neurone does not actually detect the stimulus, it simply relays it from the sensor)
What are some example of external stimuli relayed by sensory neurons?
Light, sound, touch, heat, smells taste.
What are some example of internal stimuli relayed by sensory neurons?
Blood pressure, blood carbon dioxide levels and also muscle tension.
What are processing centres in the brain called?
Ganglia.
What is the basic pattern of flow of information through the various types of neurons?
Sensory neurons of the PNS relay the “stimulus” from the sensor to the CNS. When it reaches the CNS is is integrated with other stimuli and processed by interneurons of the brain.
The processed output of the internreurons is relayed through the PNS by “motor neurons’ that trigger a response in an “effector” i.e. muscle.
What is the structure of a neuron?
Fine projections called ‘dendrites’ receive electrical input and carry them to the “cell body”, which contains a ’nucleus’.
Any outputs are sent through the ‘axon hillock’ (pint between cell body and axon) which initiates the signal that will propagate down the axon.
The axon ends by branching out, leading to many “synaptic terminals”
How does the structure of the neuron differ between types of neurons?
Interneurons have a vast number of highly branched dendrites and thus can receive input from up to 100,000 other neurons. They also have many axons.
Both sensory neurons and motor neurons have a few axons. Both have relatively few dendrites although motor neutrons tend to have a few more which are also longer.
Sensory neurons are unique in that they have their cell body partly along the axon.
Where do the inputs to the dendrite come?
Either from the axons of other neurons or from sensory receptors.
How does a signal pass between the axon and a dendrite?
This connection is called synapse and the gap between the two neutrons is called the “synaptic cleft”
There is a small gap between the “presynaptic neuron” and the “postsynaptic neuron”.
Upon being triggered by an action potential the “presynaptic neuron” releases neurotransmitters that diffuse across the gap (called the “synaptic cleft” ) and trigger the continuation of the signal when they reach the “postsynaptic neuron”
What functions doe glia perform?
They nourish neurons, insulate the axons and regulate the extracellular fluid that surrounds the neurons.
How are signals carried along the axons of neurons?
As action potentials.
What are action potentials?
A wave of depolarisation that travels down the axon due to coordinated diffusion of ions into and out of the neurons.
Through what do ions move through during action potentials?
“Gated ion channels”
What happens to membrane potential as a signal is received at a cell body?
If the signal is a EPSP (excitatory post-synaptic potential) the membrane potential will “depolarise” Since the “resting potential” is negative this will increase the voltage.
If the signal is an IPSP (inhibitory post-synaptic potential) it will “hyperpolarise” the membrane and thus bring is farther from the threshold.
What conditions lead to the initiation of an action potential?
The sum of the depolarisations by EPSPs and of the hyperpolarisations by IPSPs must exceed the voltage “threshold.”
If this threshold is met an action potential is triggered.
What does “membrane potential” refer to?
The charge difference (voltage) across the membrane due to the difference in ion concentration.
What is the baseline “membrane potential” of the neuron called?
Its “resting potential”
What is the voltage of a typically neuron’s resting potential?
-60 to -80 mV
What does the negative resting potential indicate?
That there is a net negative charge inside the cell.
How does the resting potential arise?
Sodium-potassium pumps transport Na+ out of the cell and move K+ into the cell. This would lead to a net neutral charge.
The potential is negative because few sodium channels are open and thus very few sodium ions can diffuse back into the cell. Conversely many potassium ions are open at rest and thus allow potassium ions to diffuse out of the cell.
Therefore there is a net movement of potassium ions and thus positive charges out of the cell so it becomes more negative.
What form of transport are potassium channels and sodium channels?
They passively transport the ions through ‘facilitated diffusion’
Besides sodium and potassium, what contributes to the membrane potential?
The extracellular fluid has a significantly higher concentration of Chloride ions which actually make the cell less negative.
Conversely the cell is made more negative due to the large anion inside it such as proteins.
What does A- (superscript -) refer to?
Large anions i.e. proteins inside the cell.
Where are more Cl- ions seen: inside the neuron or in the extracellular fluid? What is the consequence of this?
More are in the extracellular fluid which actually makes the cell less negative.
How can the membrane potential be predicted?
Assuming all the ions have a charge of 1+ or 1- Nernst’s equation can be used so that:
Eion = 63 mV (log (ion concentration outside / ion concentration inside) )
Where Eion (E subscript ion) is the ion’s equilibrium potential (in mV) and the ion concentration are measured in moles/millimoles.
What does equilibrium potential refer to?
The theoretical membrane potential of the ions when they reach equilibrium i.e. no net flow.
How do signals travel down dendrites?
As “graded potentials” i.e. electrotonic potentials.
Unlike action potentials which are ‘all-or-nothing’ these have a variety of strengths and thus can be used to integrate the EPSPs and IPSPs of the various dendrites as they recombine.
How are graded potentials formed and what property does this yield?
Graded potentials are formed as ion move into or out of the dendrite. These ions then diffuse through the dendrite, leading to a increase in membrane,
However the further the ions diffuse the more the signal weakness and thus dendrites with synapses closer to cell body initiate stronger IPSPs and EPSPs.
How can an action potential portray the strength off a signal?
An action potential, unlike a graded potential, is an all or nothing response and thus the magnitude of the signal can not be encoded in the strength of the action potential.
To encode quantitative information either many separate action potentials must be sent rapidly or many fibres carry the same signal to the same place.
What structure leads to the conduction of action potentials?
Voltage-gated ion channels.
What conditions trigger an action potential?
The sum of the EPSPs and IPSPs received by the cell body must exceed a ’threshold’
What is a typical threshold for an action potential to be triggered?
-55 mV
What happens during an action potential?
Resting State: The gates Na+ and K+ channels are closed. The undated channels maintain the resting potential.
Depolarisation: A stimulus opens some Sodium channels at the postsynaptic neuron. The influx of sodium ions depolarises the membrane of the cell body through a graded potential. If the depolarisation reaches the threshold an action potential is generated.
Rising phase: The depolarisation opens the voltage gates sodium channels whereas the potassium gates remain closed. This causes a further influx of sodium ions into the cell and thus the membrane potential increases rapidly.
Falling phase: As the voltage increases the sodium channels become inactivated as the potassium channels open. This causes K+ ions to leave the neuron and thus the membrane depolarises. out of the
Undershoot: The sodium channels close, but some potassium channels are still open causing the membrane potential to drop below resting potential. These potassium channels close and the sodium channels become unblocked (though still closed), the membrane returns to its resting state.
What does the period in which the neuron is unable to fire called?
The refractory period.
What causes the refractory period of the neuron?
During the falling phase and undershoot of the action potential the sodium channels are not only closed but also blocked by “inactivation loops.” Therefore the sodium channels can’t open until these loops have been removed. Because the sodium ion channels are needed to initiate the action potential the time in which these channels are blocked, called the ‘refractory period’, does not allow any new action potentials to form.
(note that this refractory period is not due to the cell running out of ions etc.)
How is volume portrayed through action potentials?
As the sound gets louder the ear sends action potentials at a greater frequency.
Where is the action potential initiated?
Generally the axon hillock.
Why is the refractory period important to the proper operation of the neuron?
It ensures that the action potential only travels in one direction.
Why do action potentials travel in one direction?
An action potential is generated as Na+ flows inward across the membrane at one location.
The depolarization of the action potential spreads to the neighboring region of the membrane, reinitiating the action potential there. Behind this this region, the membrane is repolarizing as K+ flows outward.
As the region behind the front of the action potential is depolarising it is in its refractory period. Therefore it can not carry an action potential.
Therefore because the region behind the action potential is in its ‘refractory period’ the action potential can not travel backwards and depolarise the cell body.
What are the major factors in deterring the rate of an action potential down an axon?
The diameter of the axon (thicker=faster) and whether or not the axon is ‘myelinated’ (surrounded by a myelin sheath.)
