Neurons and Electrical Impulses in the Nervous System Flashcards
what are neurons
nerve cells present in the nervous system that are specialised for communication. they connect with each other and with other excitable cells like muscles and glands. there are various types and their structure is related to their function
what are the different elements to the classical neuron
dendrites, axons and axon terminals
where are inputs received on neurons
the dendrites
where are the outputs produced on neurons
axon terminals
what are the different types of neuron
- purkinje cell
- spinal interneuron
- sensory neuron
- pyramidal neuron
- motor neuron
- bipolar cell
where are purkinje cell neurons found
the cerebellum
where are spinal interneurons found
the spine
where are pyramidal cells found
cerebral cortex
where are motor neurons found
spinal cord
where are bipolar cells found
retina
what is the membrane potential
potential difference that exists across the membrane of all cells
what does resting membrane potential arise from
the separation of charges on either side of the membrane
what is resting membrane potential determined by
diffusion of potassium ions from the cell interior through the potassium channels. the sodium potassium pump also contributes by moving unequal amounts of sodium and potassium
how can membrane potential be altered
by applying an electric current to the cell ie a stimulus. the hyperpolarising current moves the membrane potential further from 0, and a depolarising current moves the membrane potential closer to 0. the membrane potential then returns to the original resting level, and these responses are graded. the amplitude will depend on the size of the stimulus
what value does the membrane potential have to reach in order to have depolarisation
-55mV
what is action potential amplitude independent of
stimulus intensity
what is action potential
a large change in membrane potential, where the membrane potential overshoots zero and reverses polarisation
describe the process of generating action potential
- stimulus applied
- depolarisation
- membrane potential moves toward 0 mV
- membrane potential reaches threshold (-55mV) for opening of voltage gated sodium ion channels
- membrane potential moves rapidly toward 0 mV
- start of the action potential
- upstroke of the action potential is due to entry of sodium ions
- sodium ions diffuse inward through voltage gated channels
- when the membrane potential reaches about +35mV the sodium channels shut and the potassium channels open
- potassium channels are also voltage gated
- potassium leave the cell and this makes membrane potential more negative
- after overshoot, the membrane potential returns to original resting level
give a summary of action potential
- all or none
- independent of stimulus
- at the threshold, the voltage gated sodium channels open and sodium diffuse in which leads to depolarisation
- at the peak the sodium channels close and the voltage gated potassium channels open. potassium ions diffuse out and there is repolarisation
- resting membrane potential
how do local anaesthetics work
they stop nerve conduction by blocking the sodium channels
what is the refractory period
after action potential is intiated, the neuron cannot generate another action potential until the first one has ended. this period of inexcitability is known as the refractory period, and happens due to the inactivation of voltage gated sodium channels. the action potentials cannot add together, so they are all or none events
how is action potential propagated across the neuron
action potential in one section of axon depolarises adjacent resting parts of the axon, so the action potential is regenerated further along the axon, and the action potentials travel along the axon as waves of depolarisation
what does the speed of action potential propagation increase with
the axon diameter
do large axons conduct impulses faster or slower than smaller ones
faster
in what size of axon is rapid conduction achieved
very large
why did vertebrates evolve a means of increasing action potential conduction speed
their speed of action potential propagation is too slow to enable the fast postural reflexes present in vertebrates
what is myelination
wrapping a layer of myelin round an axon
what does myelination do
it increases the action potential conduction speed
what is myelination of axons
myelin is a fatty layer formed by wrapping the membranes of glial cells round the axon, which insulates the axon and improves conduction
what are nodes of ranvier
the intervals that interrupt the sheaths of myelin around axons in myelination
what is saltatory conduction
the action potential jumps along the axon from node to node
what do chemical synapses occur between
- neurons and neurons
- neurons and muscle cells
- neurons and gland cells
what do cells communicate via
synaptic vesicles
what are the events at a chemical synapse
- action potential depolarises presynaptic ending
- an influx of calcium into the presynaptic ending
- vesicles migrate toward the plasma membrane
- release of transmitter into the cleft
- transmitter binds to receptors on post synaptic cells
- leads to changes in post synaptic cells
- inactivation of the transmitter
what are some general methods of inactivation of transmitter
- reuptake into the presynaptic cell
- enzyme destruction
what are some neurotransmitter examples
amino acid derivatives - acetylcholine, dopamine, glutamate, glycine, noradrenaline, serotonin and GABA.
peptides - substance P, VIP, enkephalins, endorphins
what is VIP
vasoactive intestinal polypeptide
what is GABA
gamma amino butyric acid
what do the effects of transmitters on cells depend on
the type of receptor and second messengers in the post synaptic cell
what happens to the activity of a cell if it is depolarised
increased activity known as excitory
what happens to the activity of a cell if it is hyperpolarised
decreased activity known as inhibitory
what occurs at an excitatory synapse
transmitter causes depolarisation, there is excitatory post synaptic potential which brings the membrane potential nearer to the firing threshold
what occurs at an inhibitory synapse
transmitter causes hyperpolarisation. there is inhibitory post synaptic potential which takes the membrane potential further from the firing threshold
what can neurons receive inputs from
many other neurons
can single post synaptic potentials have an effect on the membrane potential of the post synaptic cell
no they are too small
why is it necessary for excitatory post synaptic potentials to add together or summate
the post synaptic potentials are very small and have little effect on the membrane potential of the post synaptic cell
when will a post synaptic cell fire
if it receives sufficient excitation to drive the membrane potential beyond threshold. this occurs when it receives many excitatory inputs and few inhibitory inputs within a short time period
what is convergence
when each neuron receives many inputs from other cells
what is divergence
when each neuron synapses many other cells
is the neuron more likely to fire if the excitatory inputs predominate
yes
what is a neuromuscular junction
a synapse between a motor nerve and a muscle fibre. it is also referred to as a motor end plate, and the area of contact is greater than in a nerve nerve synapse. one action potential in the nerve leads to muscle contraction.
what is the transmitter at a neuromuscular junction
acetylcholine
what is acetylcholine broken down by
the enzyme acetyl cholinesterase
what are the events at neuromuscular junctions
- the action potential depolarises the motor nerve ending
- an influx of calcium ions into the nerve ending
- vesicles migrate toward the plasma membrane
- release of acetylcholine into the cleft, which then diffuses into the cleft and binds to receptors on post synaptic cells.
- choline and acetate are taken up by the neuron
can drugs affect synaptic activity
yes
how can drugs affect synaptic activity
- synthesis or storage of transmitter
- release of transmitter
- action of transmitter on receptor
- second messenger system
- inactivation of transmitter
how does botox paralyse facial muscles
prevents the release of neurotransmitter from motor nerves
how does the drug beta blocker affect synaptic activity
it blocks beta adrenergic receptors and therefore blocks the release of adrenaline
how does black widow spider venom affect synaptic activity
causes neurons to release excessive signalling molecules
what does the drug curare do
paralyse muscles
how does nicotine affect synaptic activity
increases communication between reward related neurons
how does physostigmine affect synaptic activity
interferes with the metabolism of the neurotransmitter acetylcholine
what is the axon hillock
the start of the axon
what do ligand gated channels respond to
ligands, such as neurotransmitters
what does an influx of positive charge into the cell cause
an excitatory post synaptic potential, which leads to depolarisation
what does an influx of negative charge into the cell cause
inhibitory post synaptic potential, which leads to repolarisation
what can multiple excitatory post synaptic potentials lead to
pushing the membrane potential to a particar threshold value, and this value will depend on the tissue type
what is resting membrane potential charge
negative
why is myelinated axons a faster way for signals to cross
due to saltitory conduction
what is the soma
the cell body
why is there a refractory period
to stop action potentials occuring too closely to each other, or going in the opposite direction back down the axon
