Basic Buliding Blocks And Axonal Neurotransmission Flashcards
What is a neuron?
Basic cellular unit of the nervous system
Different range with different functions
Basic components
Dendrites
Cell body/soma
Axon
Presynaptic terminals
Connections between neurones
Axonal transmission
Synaptic transmission
Axonal transmission
Transmission of information from location A to B
Synaptic transmission
Integration/processing of information and transmission between neurones
The neurones resting potential
The inside of the neurone has a negative electrical charge because of the ions within the neurone
-70mV
Reached using Na+/K+ ATP pump
Neuronal cell membrane semi permeable
Some substances which are electrically charged (+ve or –ve) cross readily – potassium (K+) and chloride (Cl-)
Some cross with difficulty – sodium (Na+)
Some not at all – large organic proteins (-ve charge)
What are the forces determining distribution of charged ions?
Diffusion
Electrostatic attraction/repulsion
Sodium potassium pump
Slide 11
The action potential
Neurone fires- a sudden pulse where the -ve resting potential is temporarily reversed
All or nothing process to transmit information
What events occur in the action potential?
Depolarization and threshold- voltage gated sodium ion channels open, allowing Na+ into the axon cytoplasm
Reversal of membrane potential
Repolarization of resting potential- voltage gated K+ ions open, allowing more K+ to diffuse out than Na+ in
Refractory period- Limits the no. of AP an excitable membrane can produce in a given time - so APs can be separated
-Absolute refractory period = during repolarisation - neuron cannot generate new AP
-Relative refractory period = hyperpolarisation - neuron can generate new AP if stimulus = larger than one previously
Membrane permeability changes
The membrane potential remains in this resting ‘stable’ state until something disturbs the balance
Neurotransmitter initiate such changes at the dendrites of neurones
Neurotransmitters activate receptors on dendrites / soma
Receptors open ion channels
Ions cross plasma membrane, changing the membrane potential
The potential changes spread through the cell
If the potential changes felt at the axon hillock are positive (+mV), and large enough, an action potential is triggered
Depolarisation/Hyperpolarisation
Depolarise -more +ve V
Polarised at -70mV= RMP
Hyperpolarise -more -ve V
Excitably neurotransmitters depolarise the cell membrane causing them to push towards the membrane
This increases probability of an action potential being evoked
This causes an excitably post synaptic potential
Inhibitory neurotransmitters hyperpolarise the cell membrane decreasing the probability of an action potential being evoked
This causes an inhibitory post synaptic potential (ISPS)
An action potential will be evoked if the membrane potential is depolarised beyond the threshold of excitation
Postsynaptic potentials
Voltage changes spread away (decrementally) from point of origin (Passive Conduction)
Whether AP is generated depends on what reaches the axon hillock
Excitatory post synaptic potential (EPSP).
Inhibitory post synaptic potential
Spatial vs Temporal summation
Spatial summation occurs when several weak signals from different locations are converted into a single larger one
Temporal summation converts a rapid series of weak pulses from a single source into one large signal
The action potential
EPSPs begin to depolarise cell membrane
Threshold ~ -60mV (varies from cell to cell)
When reached Na+ channels open (Na+ rushes in) and polarity reverses to +30 inside
Membrane potential reverses with the inside going positive
…at which point voltage-gated Na+ channels close and K+ channels open (K+ rushes out)
…which restores resting membrane potential
What are voltage changes caused by?
The opening or closing of ion channels
In the cell membrane there are channels which are opened by voltage changes voltage changes control the ion channels which control the voltage changes
The action potential is therefore self perpetuating
When triggered at axon hillock, the action potential will travel along the entire axon
Initiation and propagation of the action potential
Slide 21
How is Axonal conduction sped up?
By myelination
Myelin comes from oligodendrocytes in the CNS and from Schwann cells in the PNS
Saltatory conduction
Decremental (reduced) conduction between nodes (but re-boosted each time)
But very fast along axon
Most CNS neurones
Axonal transmission
Transmission of information from location A to B
Synaptic transmission
Integration/ processing of information
Novichok
Disrupts normal synaptic neurotransmission for neurotransmitter acetylcholine
What occurs when the action potential reaches the terminal buttons?
Calcium ion channels open when action potential reaches pre-synaptic terminal
Ca++ ions cause vesicles to move to release sites – fuse with the cell membrane – and discharge their contents
Transmitter substance diffuses across synaptic cleft
Attach to receptor sites on post-synaptic membrane
What happens to the neurotransmitter then?
Continually trying to excite the next neurone
Would remain active in synapse if it wasn’t for:
-enzymatic degradation
-reuptake
Acetylcholinesterase is the enzyme that breaks down ACh
What does ACh do?
It is the key neurotransmitter at the neuromuscular junction – it activates muscles
Not just skeletal muscles (for voluntary movement), also heart, respiratory muscles, gastrointestinal tract, eye muscles, muscles around blood vessels
5 funds,mental processes of synaptic transmission
Manufacture – intracellular biochemical processes
Storage – vesicles
Release – by action potential
Interact with post-synaptic receptors – diffusion across the synapse
Inactivation – break down or re-uptake
Slide 42
Fast neurotransmitters
Acetylcholine (ACh)
Glutamate (GLU)
Gamma-aminobutyric acid (GABA)
Neuromodulators- slow
Dopamine (DA)
Noradrenalin (NA) (norepenephrine)
Serotonin (5HT) (5-hydroxytryptamine)
Acetylcholine
Transmitter at the neuromuscular junction, also used widely in brain and spinal cord
Noradrenaline
Transmitter in peripheral (Heart) and CNS
Dopamine
Important transmitter in basal ganglia
Serotonin
Involved in many processes in brain no actual function
Diverging projections in brain- innervation many structures
GABA
Main inhibitory transmitter
