Spinal Reflexes Flashcards
Neural Communication- How is resting membrane achieved?
A nerve will remain inactive when the resting membrane potential is maintained.
Potasssium must be continually regulated for this to happen - RM is achieved when the movement of K out of the cell equals K movement into the cell
What happens as the Synapse?
Depolarisation of presynaptic terminal causes Ca influx.
Ca require for vehicle fusion and release.
[The Action Potential AP in the presynaptic nerve reaches the calcium receptor - this then causes a calcium influx which causes the vesicles to release the neurotransmitters.
Neurotransmitters are proteins, they then randomly hit the receptors on the post synaptic nerve which causes a channel to open up]
What are the seven steps neurotransmitters go through at the synapse?
- NT are synthesised from precursors under the influence of enzymes
- NT molecules are stored in vesicles
- NT molecules that leak from their vesicles are destroyed by enzymes
- Action potentials cause vesicles to fuse with the presynaptic membrane and release their NT molecules into the synapse
- Released NT molecules bind with autoreceptors and inhibit subsequent NT release
- Released NT molecules bind to postsynaptic receptors
- Released NT molecules are deactivated either by reputable or enzymatic degeneration
At they synapse - What is ‘gating’?
In nerve cells, the channels that open to allow ions to flow through are ‘gated’
Switching from closed-open-closed is caused by the binding of a transmitter [ligand-gated], a change of voltage [voltage gated] or phophorylation of the channel protein.
NT influence ligand-gated channels.
NB - What is an IONOTROPIC RECEPTOR [IR]?
When the channel opens at the synapse it causes a flow of positive ions, i.e. Current flow, to change the membrane potential = IR [pertains to ions]
Once a potential difference across the membrane changes in the post-synaptic neurone [often dendrites], this is transmitted along the membrane.
Done by an AP [active process], or passive spread due to flow of current.
Input arriving at dendrites is transmitted passively to Axon Hillock and AP may be initiated: depends on totally excitatory or inhibitory input.
Synaptic Integration - what can influence the transmission between neurones?
The number of connections received by a neurone
The type of NT, receptor and current flow
And the size of the response in postsynaptic cell
Thus, an AP in a presynaptic neurone rarely elicits AP in the post synaptic neurone,
AP in postsynaptic neurone requires integration of all inputs that it receives
Synaptic Integration - What are the 2 types of currents that can change the polarisation of a membranes potential?
- Excitatory postsynaptic potential (EPSP)
- Inhibitory postsynaptic potential (IPSP)
These refer to the channel itself not the orgin of the NT
The type of current depends on the type of ion channel gated by a NT - not the transmitter itself
Which transmitters almost always have excitatory or inhibitor effects?
Excitatory - Glutamate in the SC
Inhibitory - GABA & glycine in the brain.
Synaptic Integration - what is Temporal summation?
Temporal summation of the EPSPs moves the membrane potential closer to the threshold due to synapases of cell bodies often being inhibitory - which makes the neurone less likely to fire
Therefore it needs more impulses [temporal summation] and/or impulses from multiple sources and closer to Axon Hillock [spatial summation]
Remember- there’s no myelin in dendrites or cell bodies, so the amplitude of a potential will decrease with distance
Synaptic Integration - What 2 things does a post synaptic neurone depend on to discharge?
- Amplitude and sign [excitatory/inhibitory] of postsynaptic potential.
- Location of the synapse relative to Axon Hillock.
Remember - a neurone can synapse at the dendrite, cell body or axon.
Second Messengers - what are they?
In addition to IONOTROPIC RECEPTORS, there as also METABOTROPIC RECEPTORS
Indirectly gate ion channels via second messengers [e.g. G protein-coupled]
METABOTROPIC RECEPTORS can open/close ion channels, but act through one or more metabolic steps
Second Messengers - What is a G-protein? NB
As example. - cAMP activates an intermediate molecule - G protein [Guanine nucleotide-binding protein]
G protein stimulates other processes to enhance or, more often, reduce the membrane potential rise.
Second Messengers - What do METABOTROPIC RECEPTORS influence?
- resting membrane potential
- input conductance [how strongly current is drawn through an ion channel]
- voltage threshold
- dendrite/neurone propagation ability
And thus influences = action potential duration & discharge characteristics
Persistent Inward Currents [PICs] - Explain the action of PICs
Dendrites contain METABOTROPIC receptors for serotonin [5-HT] & noradrenaline [NA]
These are called MONOAMINES: their released by presynaptic neurones emanating form the brain stem [and also in the SC itself]
5-HT = Caudal Raphe Nucleus NA = Locus Coeruleus
5HT and NA activation causes second messengers to open voltage gates Ca & Na channels.
This creates long-lasting inward currents called Persistent Inward Currents/ PICs.
Numerous 5HT & NA receptors on dendrites = causes influx of positive charge [PIC] via METABOTROPIC/second messenger system.
Na = very quick Ca = very long lasting effects
PICs - Where do the axons originate that use 5 HT?
Caudal Raphe Nucleus
Activation of this brainstem region increases descending activity in the ‘monoaminergic system’
