nerve impulse
- wave of altered charge across nerve cell membrane that sweeps along axon
- depolarisation/action potential
action potential
- at rest, there is a difference in ion concentration between inside and outside of cell
- maintained by ion pumps in plasma membrane
- electrical potential difference across neuron plasma membrane approx -70mV
sodium ions and potassium ions
when nerve cell is stimulated, action potential is generated
Na+ ions channel open: positively charged Na+ ions flow into cell
K+ ion channels open in response to the depolarisation
Na+ channels close
Positively charged K+ ions flow out of the cell
K+ channels close
Na+ channels rest
return to rest
action potential moves along axon
one way travel: refractory period = time immediately following AP when new AP cannot be initiated in same area of membrane
how does an action potential start, dendrites
multiple inputs from multiple dendrites
- excitatory pre-synaptic potentials (EPSPs)
- inhibitory pre-synaptic potentials (IPSPs)
- vary in magnitude
how does an action potential start, summation
summation of changes in membrane potential.
- EPSP increases chance of AP initiation, IPSP decreases chance.
- if overall membrane potential reaches threshold voltage, Na+ channels open and AP initiated
summation
spatial summation- summation of inputs from different areas of cell eg. from different dendrites
temporal summation- input occurs multiple times from the same are/dendrite
- repeated inputs in short time period- summation
- summation -> APs generated more or less frequently
AP magnitude
- is always the same
- if total voltage at axon hillock exceeds the threshold potential, an AP is generated and sent along the axon. if not, no AP is generated
- AP frequency, not magnitude, can vary
- signal intensity is encoded by AP frequency
signalling between nerve cells
- neural pathways can contain multiple nerve cells eg. reflex arc
- signals passes along axon of each nerve cell as action potential
- when AP reaches end of neuron, it must cross synapse (GAP)-> AP set up in next neuron
neurotransmitter
- chemical signals: transmit signals across synapse
- released when AP reaches pre-synaptic neuron termini
- bind to receptors on dendrites of post-synaptic neuron
neurotranmitter function
- different NT’s associated with different nervous system function (eg. memory, movement, emotions)
- differs locations in nervous system
- excitatory or inhibitory: promote or inhibit formation of AP in receiving neuron
- many drugs act on neurotransmitter signalling
amino acids and derivatives
- glutamate: main excitatory neurotransmitter in CNS
- GABA: main inhibitory neurotransmitter in CNS
catecholamines (monoamines)
- derived from Tyr
- dopamine/serotonin
acetylcholine
-derived from choline
- neuromuscular junction
peptides
eg substance P, endorphins
GABA binding to GABA-A receptors
- GABA-A receptors is an ion channel receptor ‘ionotropic’
- binding of ligand GABA to receptor causes conformational change that opens ion channel ‘ligand-gated’
- ion moves through channel along concentration gradient
GABA-A receptor
-chloride ion channel
- channel opened by GABA binding, Cl- flows into cell, hyper polarisation -> harder to reach threshold voltage for AP formation in post-synaptic cell
- benzodiazepines, ethanol, some anaesthetics, potentiate GABA activation of GABA-AR, so more inhibition of APs- sedative action
what is a neuromuscular junction
NMJ = junction between motor neuron (nerve that controls muscle) and muscle cell
-similar to synapse between 2 neurons
acetylcholine “switching off’
-nAChR- sodium ion channel receptor
ACh binding allows Na+ entry
Na+ influx- depolarisation of muscle cell membrane
depolarisation initiates contraction
acetylcholinesterase I cleft removes ACh “switches off”
myasthenia gravis
muscle weakness
-autoimmune antibodies attack ACh receptors at neuromuscular junction
- treatment: AChE inhibitors used to enhance NMJ transmission
electrical signalling via gap junctions
connexion proteins form connection with channel
- ions carry charge from one cell to another (diffusion)
cardiac myocytes
- heartbeat- electrical impulse travelling across heart tissue causes contraction of cardiac muscle
- specialised cells with neutron-like properties spread throughout heart eg. purkinje fibres
- atria first, then ventricles, ion flow between adjacent muscle cells (myocytes)
