Structure of Neurons Flashcards
What are the different regions of neurons?
Dendrites, soma (cell body), axon hillock and initial segment, axon, synapse
What is the function of dendrites?
Receive inputs from other neurons and convey graded electrical signals passively to the soma
What is the function of the soma?
Integrates electrical signals that are incoming via passive conduction to the axon hillock
What are some features of the soma?
Synthetic and metabolic centre
Contains nucleus, ribosomes, mitochondria and ER
What is the purpose of the axon hillock and initial segment?
Site of initiation of the all or nothing AP
What is the function of the axon?
Conducts output signals as APs to presynaptic terminal
Mediates transport of materials between soma and presynaptic terminal (anterograde direction) and vice versa
What is the function of the synapse?
Point of chemical communication between neurons or to other cells
What are the different classes of neurons?
Unipolar = one neurite, found in PNS Pseuodunipolar = one neurite that splits Bipolar = two neurites Multipolar = three or more neurites
What are the different functional regions of neurons?
Input region = electrical signals enter
Integrative region = all signals are assimilated
Conductile region = signal conducted away from soma
Output region = signal leaves neuron
Do all neurons have all of the functional units?
Yes, all four regions are present irrespective of function
When is an all or nothing AP generated?
Once threshold is reached
What is the overshoot period during AP generation?
Very brief period when polarity is reversed to inside positive
What do APs allow electrical signals to do?
Allows electrical signals to be conducted over large distances without degrading
What is the difference between passive signals and APs?
Passive signals diminish as they spread but APs have constant amplitude
What is the impact caused by the nerve cell membrane being leaky?
Passive signals don’t spread far due to current loss across the membrane accompanied by a reduced change in potential
What does the distance across which current spreads depend on?
Membrane resistance and axial resistance of axoplasm
What is passive conduction a factor in?
Propagation of the AP
What effect does increasing the length constant have on the local current spread?
It increases the local current spread = this in turn increases AP conduction velocity
How can you improve the spread of passive conduction?
Decrease axial resistance by increasing axon diameter
Increase membrane resistance by adding insulating material
How does insulation impact conduction?
Conduction in myelinated axons is much faster than in unmyelinated axons of the same diameter
What is salatory conduction?
AP jumps from one node of Ranvier to the next = occurs in myelinated axons
What are the steps in synaptic transmission?
Uptake of precursor Synthesis and storage of precursor Depolarisation by AP and Ca2+ influx Ca2+ induced release of transmitter Receptor activation Enzyme mediated inactivation or reuptake of transmitter
What separates pre and post synaptic membranes?
A narrow synaptic cleft
What does the synaptic cleft contain?
Matrix of fibrous extracellular protein that holds the membranes together
Where is neurotransmitter stored that is used for synaptic transmission?
In vesicles within the presynaptic terminal
What are the different structural classes of synapses?
Axodendritic = very common Axosomatic = common Axoaxonic = uncommon
How are synpases classified?
By the location of the presynaptic terminal upon the postsynaptic cell
Can all of the different structural classes of synapse be present on the one axon?
Yes
What are the different physiological classes of synapses?
Excitatory and inhibitory
What are some features of an excitatory synapse?
Most frequently glutamate
Cation selective postsynaptic receptor
Depolarising response
What are some features of inhibitory synapses?
Most often GABA or glycine
Activates postsynaptic anion selective receptor
Hyperpolarising response
Can an inhibitory postsynaptic potential initiate and AP?
No
What does EPSP stand for?
Excitatory postsynaptic potential
What occurs in spatial summation?
Many inputs converge upon neuron to determine its output
What occurs in temporal summation?
Single input may modulate output by variation in AP frequency of that input
Are temporal and spatial summation complementary processes?
Yes
How is direct gating carried out?
By inotrophic receptors = receptor is integral component of molecule that forms channel it controls
What mediated indirect gating?
By activation of metabotropic receptors = receptor and channel it controls are distinct
Is direct or indirect gating faster?
Direct gating is faster
What type of transmission displays both direct and indirect gating?
Cholinergic synaptic transmission in autonomic ganglia
How does cholinergic synaptic transmission display direct gating?
Fast epsp due to activation of nicotinic ACh receptors = channels conduct Na+ and K+
How does cholinergic synaptic transmission display indirect gating?
Slow epsp follows activation of muscarinic ACh receptors = ACh closes K+ channels
What are the features of non-NMDA glutamate receptors?
Bind agonists kainate or AMPA controlling a channel permeable to Na+ and K+
What are the features of NMDA glutamate receptors?
Controls a channel permeable to Na+, Ca2+ and K+
What is the difference between non-NMDA and NMDA glutamate receptors?
Non-NMDA receptors mediate fast excitatory synpatic transmission in CNS, whereas NMDA receptors contributes a slow component to the excitatory synaptic potential
Why do NMDA glutamate receptors promote neurotoxicity?
Have high permeability to Ca2+
How do metabotropic receptors exert an effect since they don’t have a integral ion channel?
By activation of second messenger cascade
