11. Neurons, Synapses And Signaling Mechanisms Flashcards
What are ion channels?
Specialised proteins in a cell membrane that regulate the passage of ions down a concentration gradient.
What type of transport are ion channels?
Passive transport.
What are gated ion channels?
A type of ion channels that controls the flow of ions across a cell membranes in response to specific stimuli.
What are 3 types of stimulus that control gated ion channels?
- Changes in voltage (neurons).
- Ligand binding.
- Mechanical forces.
What is a ligand?
Any molecule or atom that binds reversibly to a protein.
The opening and closing of gated ion channels alters…?
- The permeability of the membrane to particular ions.
- Subsequently the altering of the membrane potential.
When is resting potential achieved?
When the cell membrane reaches the electrochemical equilibrium potential.
What are voltage-gated ion channels?
Ion channels that open and close based on stimuli associated with voltage changes across the plasma membrane.
What 2 types of cells are associated with voltage gated ion channels?
- Neurons.
- Muscle cells.
After activation, voltage gated ion channels have an…?
Inactivation period that prevents the channel from responding to repeated stimulus.
Why is there an inactivation period in voltage gated ion channels?
To prevent triggering of the channel from repeated stimuli.
A self-regulatory process.
What is depolarisation?
A change in cell membrane potential such that the inside of the membrane is made less negative to the outside.
There is a decrease in polarity difference
What is hyperpolarisation?
A change in cell membrane potential such that the inside of the membrane becomes more negative relative to the outside.
There is an increase in polarity difference
What is a graded potential?
A shift in cell membrane potential in response to hyperpolarisation or depolarisation.
The larger the stimulus the greater the graded potential
What is induced by a graded potential?
A small electrical current that dissipates as it flows along the membrane.
Graded potentials decay with time and distance
When are action potentials generated?
When depolarisation reaches a specific threshold, shifting the membrane potential sufficiently.
What type of graded potential decreases the chance of an action potential?
A hyperpolarisation graded potential.
What occurs to sodium channels to reach the action potential threshold?
- Voltage gated sodium channels open, enabling the flow of Na+ into the cell.
- The influx of Na+ causes further depolarisation.
- A positive feedback loop occurs as this depolarisation causes even more sodium channels to open.
What is the approximate threshold value for action potentials in the neurons of many mammals?
About -55mV (milliVolts)
How does stimulus strength affect an action potential?
An action potential is all or nothing - it does not have different levels of strength according to the stimulus.
Rather, the existence of an action potential is based entirely on the threshold value.
Action potentials spread along…?
Axons.
What are the 6 phases of triggering an action potential?
- Resting state. Gated Na+ and K+ channels are closed.
- Depolarisation opens some Na+ channels.
- Rising phase of action potential. Depolarisation causes more Na+ gated channels to open, starting a positive feedback loop as the threshold is reached.
- Falling phase of action potential. Peak action potential causes a signal to be transmitted. Na+ channel close while K+ channels open.
- Undershoot. Hyperpolarisation makes the cell more negative.
- Cell returns to its resting potential.
What is the refractory period in neuron gated ion channels?
The downtime between one action potential and another, where a hyperpolarised membrane will not respond to any more stimulus.
What is the neuron refractory period linked with?
The temporary blockage/inactivation of sodium channels.
What is the function of an action potential?
An action potential acts as a nerve signal, being conducted along an axon and transmitted to the target tissue.
What is the process of action potentials conveying information between and along excitable cells?
Conduction.
Upon stimulation, action potentials will be…?
Stimulated, inhibited, or modulated in some way.
The magnitude and duration of an action potential are…?
The same at each position along the axon.
What quality of an action potential is directly proportional to the strength of its stimulus?
The frequency.
E.g. a loud noise (strong stimulus) causes more frequent action potentials in auditory neurons
What is the main purpose of the refractory period preventing continued stimulus activation?
To prevent action potential reversal, allowing signals to propagate in only one direction along the axon.
Where are voltage gated sodium channels found on the axon?
In the nodes of Ranvier, where they have direct access to the extracellular fluid.
Where are action potentials generated due to the location of sodium ion channels?
Only at the nodes of Ranvier.
Each current generated then travels within the insulated axon to the next node.
What are electrical synapses?
Gap junctions that allow electrical currents to flow directly from one neuron to another.
What is the more common type of synapse?
The chemical synapse.
How do chemical synapses function?
They release chemical neurotransmitters from the presynaptic neuron into the synapse to reach the postsynaptic neuron.
How are neurotransmitters packaged?
They are packaged into synaptic vesicles at the axon terminals (+ dendrites).
What are the 4 steps of an action potential initiated chemical neurotransmission across the synapse?
- Action potential depolarises the plasma membrane at the synaptic terminal, opening voltage gates that allow Ca2+ to diffuse in.
- Increased Calcium ions cause the synaptic vesicles to fuse with the terminal membrane and release the neurotransmitters through exocytosis.
- Neurotransmitters cross the synapse before binding to and activating ligan-gated ion channels (receptors) in the postsynaptic membrane of the receiving neuron.
- This results in a graded potential of the postsynaptic neuron (localised depolarisation or hyperpolarisation).
How would removal of calcium affect neuron communication?
- The creation and transmission of the action potential would be unaffected.
- The transmission of the action potential between neurons would be negatively affected.
What is an excitatory postsynaptic potential (EPSP)?
A postsynapse potential that brings the neuron’s membrane potential closer to its firing threshold.
What is inhibitor postsynaptic potential?
A postsynapse potential that brings the neuron’s membrane potential further from its firing threshold.
What is summation (neurons)?
The result of incoming EPSP and IPSP.
What is the Axon hillock?
A specialised part of the cell body of a neuron that connects to the axon.
Where does the summation, determining threshold breach in a neuron, occur?
In the axon hillock.
When is the threshold of a neuron breached during summation?
When the EPSPs are strong enough to overcome the IPSPs.
Can a single neuron receive multiple signal inputs?
Yes.
Both excitatory and inhibitory inputs from multiple neurons can be received.
How are neurotransmitters disposed of after completing their signalling?
In some combination of the following:
1. Diffuse away from the synapse.
2. Direct degradation by enzymatic hydrolysis at the synapse.
3. Reuptake at the presynaptic neuron - repackaged and reused.
What is the complexity of the nervous system of a cnidarian (e.g. hydra)?
Nerve cells form a decentralised nerve net.
What is the complexity of nervous system of an echinoderm (e.g. starfish)?
Nerve cells are bundled into fibres called nerves.
What is the complexity of the nervous system of bilateral animals like the planarians (flatworms)?
Neurons cluster into an anterior brain that processes information.
What is the complexity of the nervous system of arthropods?
In addition to a brain, they have peripheral ganglia located along the ventral nerve cord.
What is the complexity of the nervous system of molluscs?
Highly developed with complex brains containing millions of neurons.
What is the complexity of the nervous system of vertebrates?
The brain and spinal cord comprise the CNS, with neurons in the rest of the body forming the PNS.
What is the nervous system complexity of porifera?
Sponges have no nervous system and no nerves.
What 2 types of structures respond to a stimulus sent from the CNS?
- Glands.
- Muscles.
What is vasopressin?
A hormone of the posterior pituitary.
Also known as antidiuretic hormone (ADH).
What stimulates the release of vasopressin (ADH)? 3 steps
- Chemoreceptors detect fluctuations in CO2 and O2, as well flucations in blood pH.
- Chemoreceptors transmit information as action potential to the CNS.
- CNS sends action potential to the hypothalamus to stimulate release of vasopressin.
Sensory information travels to the CNS as…?
Motor commands travel from the CNS as…?
Both as action potentials!
What stimuli causes mechanoreceptors to trigger an action potential?
- Sound.
- Touch.
- Motion.
What stimuli causes chemoreceptors to trigger an action potential?
- Solutes.
- Tastes.
- Smells.