NEUROPHYSIOLOGY + NEUROSECRETION Flashcards
define neurophysiology
Study of neuronal function
what does a neuron consist of?
a cell body, from which extends highly branched nerve processes (dendrites) and a single axon.
what are dendrites covered by?
dendritic spine that form connections or synapses with the dendritic spines of other neighbouring neurons.
where do axons arise from and branch into?
arise from axon hillock
and will branch forming telodendria
what are telodendria terminated by?
by small swellings called presynaptic terminals/boutons.
what do presynaptic terminals contain?
many small vesicles which contain chemicals messengers, or neurotransmitters.
where are vesicles synthesised from?
synthesised in the nerve cell body and transported down the axon by a process of axonal transport
what is resting membrane potential?
Inside of a nerve cell has a -ve potential in relation to the +ve electrical potential of the extracellular fluid
how is the electrical gradient established?
Cells pump Na from their interior, into the extracellular fluid, exchanging Na for K. Hence, high extracellular conc of Na and high intracellular conc of K.
why does K move out the cell?
Memb impermeable to Na, and permeable to K.
So K will enter the cell following the electrical gradient, but will also move out of the cell along its own conc gradient
how does the cell become polarised?
When these 2 forces are balanced (Na + K), the cell reaches the resting memb potential, and the cell is described as being polarised.
what value is resting membrane potential?
maintained at about -70 to -90mV
how do Na channels open?
Na channels are normally closed, but can be opened by localised changes in the resting membrane potential (voltage-gated), or by neurotransmitter activation (ligand-gated).
what happens during an action potential?
Na channels open, Na enters the cell, depolarising it.
This is opposed by the Na/K exchange pump, and by the opening of voltage-gated K channels.
how does the membrane return to its resting state?
Na channels are only open for a short time so the memb potential is soon returned to the resting state.
how is an action potential conducted?
Axon is a poor conductor. The AP sets up small internal electric currents called the intrinsic/local currents.
how do action potentials keep getting initiated along the membrane?
local currents are not conducted far along the axon but they can be large enough to open downstream voltage-gated Na channels so initiating another AP
what is the refractory period?
once a voltage-gated Na channel has opened and closed, it goes though a period of inactivation. This inactivation/refractory period prevents the back flow of excitation.
how is conduction enhanced?
Conduction velocity in mammals is enhanced by myelination of axons.
what is the myelin sheath formed by?
schwann cells
what do schwann cells do?
They wrap around the axons to form a sheath leaving a short stretch of unmyelinated axon (the node of Ranvier) before the next cell.
how does the myelin sheath function?
by increasing the conductivity of the axon, increasing the conduction of the intrinsic currents
how does the myelin sheath increase conduction velocity?
by allowing the activation of voltage-gated channels much further along the axon
what do local anaesthetics target?
voltage-gated ion channels
what do local anaesthetics have a high affinity for?
for Na ion channels in their open state
what effect does opening more channels have?
local anaesthetic has a greater effect
what does the local anaesthetic do?
Maintains ion channel in the inactivated state
this means the cell cannot be re-stimulated
what is a synapse?
Junction between a neuron and a target cell
what is an electrical synapse?
Transmission via pores – gap junctions.
Allows rapid communication between neurons
where are electrical synapses found?
found between smooth muscle cells and in cardiac muscle
what does a chemical synapse consist of?
a pre-synaptic memb at the presynaptic terminal of an axon, and a post-synaptic memb
what is a synaptic cleft?
separates the pre- and post-synaptic memb
what happens in the chemical synapse?
An AP in the presynaptic terminus activates the release of neurotransmitter molecules into the synaptic cleft that bind to receptors, opening ligand-gated Na channels, initiating a postsynaptic potential. If large enough, the postsynaptic potential will elicit an AP in the target neuron.
give examples of amino acid neurotransmitters
glutamate, GABA, glycine
give examples of monoamine neurotransmitters
acetylcholine, dopamine, noradrenaline, adrenaline, serotonin, histamine
give examples of purine neurotransmitters
adenosine, ATP
give examples of peptide neurotransmitters
substance P, vasoactive intestinal peptide, somatostatin, cholecystokinin, beta-endorphin, met-enkephalin
give examples of gas neurotransmitters
nitric oxide, hydrogen sulphide, carbon monoxide
what happens once an action potential reaches the synaptic bouton?
Ca channels open and ca enters the bouton
Ca detected by a ca-sensing protein, synaptotagmin.
what does synaptotagmin do?
promotes the formation of a SNARE complex between the SNARE protein synaptobrevin (embedded in the vesicle memb) and the SNARE proteins syntaxin and SNAP-25 (associated with the presynaptic memb).
how is the neurotransmitter release?
Vesicle memb and presynaptic memb fuse and neurotransmitter is released at active zone. Diffuses across synapse.
what happens when the neurotransmitter is released?
binds to receptor (types 1 or 2) and is broken down and recycled back into the neuron.
what is a neuropeptide?
n
what is a vesicle?
has a dense core and can be released from any location of the terminal memb
how is a neuropeptide released?
Requires higher conc of ca – cell firing rate is higher.
More than one neuropeptide can be released from a single synapse, co-transmission
how is neurosecretion controlled?
Mediated by autoreceptors (monitors self secretion) and heteroreceptors (for transmission from other synapses).
what are sub-threshold potentials?
Potentials induced by neurotransmitter release at a single synapse are often too small to trigger a full AP
what are the 2 types of sub-threshold potentials?
inhibitory postsynaptic potentials (IPSPs), or excitatory postsynaptic potentials (EPSPs)
what are the 3 ways neurotransmitters can be inactivated?
reuptake, enzymes, diffusion
how does reuptake cause neurotransmitters to become inactivated
by secondary active transport systems driven by ion gradient – these tend to have high affinities.
how do enzymes cause neurotransmitters to become inactivated?
Act broken down by AChE into acetate and choline. Choline removed by transporter system and reused to synthesis more ACh. AChE can be blocked by organophosphates.
how does diffusion cause neurotransmitters to become inactivated?
route for peptides, but peptides are large neurotransmitters and diffuse slowly, so they have prolonged effects.
