Neurons, excitatory and inhibitory transmission in the CNS Flashcards
Name the regions of the neuron
Dendrites, cell body (soma), axon hillock and initial segment, axon & synapse
What is the function of dendrites?
They receive input from other neurons and convey graded electrical signals passively to the soma
What does the cell body (soma) of a neuron do?
The soma is the synthetic and metabolic centre. It contains the nucleus, ribosomes, mitochondria and endoplasmic reticulum. The function of the soma is to integrate incoming electrical signals that are conducted passively to the axon hillock
What does the axon hillock and initial segment do?
This is the site of initiation of the ‘all or none’ action potential
What does the axon do?
It conducts output signals as action potentials to the presynaptic terminal and mediates transport of materials between the soma and presynaptic terminal and vice versa by slow and fast axonal transport
What is the synapse’s function?
Point of chemical communication between neurons
There are 4 different types of neurons which are classified according to shape (no. and conformation of processes arising from the soma). Name these 4 types of neurons and give a brief description of each
Unipolar - one neurite Pseudounipolar - one neurite that bifurcates Bipolar - two neurites Multipolar - 3 or more neurites
What are the 4 functional regions of neurons?
Input, integrative, conductile and output
Describe phase 0 of an action potential
A triggering event occurs that depolarizes the cell body. This signal comes from other cells connecting to the neuron, and it causes positively charged ions to flow into the cell body. Positive ions still flow into the cell to depolarize it, but these ions pass through channels that open in response to a neurotransmitter, binding to the channel. These incoming ions bring the membrane potential closer to 0, which is known as depolarization. If the cell body gets positive enough that it can trigger the voltage-gated sodium channels found in the axon, then the action potential will be sent
Describe phase 1 of an action potential
Depolarization makes the cell less polar (membrane potential gets smaller as ions quickly begin to equalize the concentration gradients) . Voltage-gated sodium channels at the part of the axon closest to the cell body activate, thanks to the recently depolarized cell body. This lets positively charged sodium ions flow into the negatively charged axon, and depolarize the surrounding axon.
Describe phase 2 of an action potential
Repolarization - brings the cell back to resting potential. The inactivation gates of the sodium channels close, stopping the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell than out, so when these channels open, more potassium exits than comes in. This means the cell loses positively charged ions, and returns back toward its resting state.
Describe stage 3 of an action potential
Hyperpolarization - makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyperpolarizes, or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to reestablish the resting state.
What is an absolute refractory period in an action potential?
During this time it is absolutely impossible to send another action potential. The inactivation gates of the sodium channels lock shut for a time, so that no sodium will pass through. No sodium means no depolarization, which means no action potential. Absolute refractory periods help direct the action potential down the axon, because only channels further downstream can open and let in depolarizing ions.
What is a relative refractory period in an action potential?
During this time, it is really hard to send an action potential. This is the period after the absolute refractory period, when the inactivation gates are open again. The cell is still hyperpolarized after sending an action potential so it would take even more positive ions than usual to reach the appropriate depolarization potential. This means that the initial triggering event would have to be bigger than normal in order to send more action potentials along. Relative refractory periods can help us figure how intense a stimulus is - cells in your retina will send signals faster in bright light than in dim light, because the trigger is stronger.
Why do passive signals not spread far from their site of origin?
The nerve cell membrane is ‘leaky’ (i.e. not a perfect insulator): passive signals do not spread far from their site of origin due to current loss across the membrane accompanied by a reduced change in potential (imagine a leaky garden hose as an analogy for the axon)
Describe the change in voltage for a given current in terms of Ohm’s law
Current (im) leaks back to extracellular space across the membrane resistance (rm) generating a potential change (DVm). Note that for a given current (im), DVm increases linearly with rm in accordance with Ohm’s Law (V = IR)
What happens to the membrane potential, in the absence of an AP, as the signal travels further?
The membrane potential change (in the absence of an action potential) is a passive process that decays exponentially with distance. λ = length constant.
The distance (x) over which current (ii) spreads depends upon membrane resistance (rm) and the axial resistance of the axoplasm (ri) – increasing the ratio rm/ri increases λ
Describe the relationship between passive conduction and action potential velocity
Passive conduction is a factor in the propagation of the action potential (AP)
The longer the length constant (λ) the further the local current spread
Further local current spread increases AP conduction velocity
What are 2 ways of increasing passive current spread?
Decrease ri (possible by increasing axon diameter)
Increase rm (possible by adding an insulating material – myelin – provided by Schwann cells in the PNS and oligodendrocytes in the CNS). Both are types of macroglia
What is the difference in conduction between unmyelinated and myelinated axons?
Conduction in myelinated axons is much faster than in nonmyelinated axons of the same diameter
What is the difference between a Schwann cell and an oligodendrocyte?
Schwann cells are found in the PNS, oligodendrocytes are found in the CNS
There are many Schwann cells surrounding each axon, whereas there is only one oligodendrocyte for many axons
Describe saltatory conduction in myelinated axons
The action potential ‘jumps’ from one node of Ranvier to the next
What do demyelinating disorders do to nerve conduction?
Demyelinating disorders [e.g. multiple sclerosis (CNS) and Guillian-Barré syndrome (PNS)] cause slowing (or even cessation) of nerve conduction
Describe the stages of chemical neurotransmission
- Uptake of precursor
- Synthesis of transmitter
- Storage of transmitter
- Depolarisation by action potential
- Calcium influx through voltage activated Ca channels
- Ca induced release of transmitter (exocytosis)
- Receptor activation
- Enzyme mediated inactivation of transmitter or
- Reuptake of transmitter
Describe the key structural elements of the chemical synapse
Pre and post-synaptic membranes separated by a narrow synaptic cleft
A matrix of fibrous extracellular protein within the cleft that holds the pre- and post-synaptic membranes together
Vesicles within the presynaptic terminal that store the neurotransmitter
Membrane differentiations:
- presynaptically, the active zones around which vesicles cluster
- postsynaptically, the postsynaptic density containing neurotransmitter receptors
What are the morphological types of synapse?
Axodendritic, axosomatic and axoaxonic
What are the functional types of synase?
Inhibitory and excitatory
What is the transmitter in the CNS for an excitatory synapse?
Glutamate
What does glutamate do?
Glutamate activates postsynaptic, cation selective, ionotropic, glutamate receptors generating a local, graded, excitatory (depolarizing) response: the excitatory postsynaptic potential (e.p.s.p.)
What is the transmitter in the CNS for an inhibitory synapse?
Gamma-aminobutyric acid (GABA) or glycine
What does GABA or glycine do?
GABA, or glycine, activates postsynaptic, anion selective, ionotropic, GABAA, or glycine, receptors generating a local, graded, inhibitory (hyperpolarizing) response: the inhibitory postsynaptic potential (i.p.s.p.)
What is spatial summation?
Many inputs converge upon a neurone to determine its output
What is temporal summation?
A single input may modulate output by variation in action potential frequency of that input
Name 3 amino acids that are neurotransmitters
Glutamate, GABA and glycine
Name 4 amines that are neurotransmitters
Dopamine, Serotonin (5-HT), noradrenaline (NA), histamine
Name 4 (out of 8) peptides that are neurotransmitters
Cholecystokinin, dynorphin, enkephalins, neuropeptide Y, somatostatin, substance P, thyrotropin releasing hormone (TRH), vasoactive intestinal polypeptide (VIP)
How are acetylcholine, amino acids and amines released?
Acetylcholine, amino acids and amines are released from synaptic vesicles
How are peptides released?
Peptides are released from secretory vesicles
What do glutamate, GABA, glycine, acetylcholine and serotonin (5-HT) activate?
Glutamate, GABA, glycine, acetylcholine, and 5-HT can activate ionotropic ligand-gated ion channels (LGICs). These mediate fast neurotransmission
What do glutamate, GABA, glycine, acetylcholine and serotonin (5-HT) activate?
All, except glycine, can also activate metabotropic G-protein-coupled receptors. These mediate relatively slow neurotransmission
