week 4 how nerves work Flashcards
what is the central nervous system
brain and spinal cord
what is the peripheral nervous system
the sensory and motor nerves
what are the
sub-categories of the peripheral nervous system
somatic nervous system (voluntary) - controls skeletal muscle
autonomic nervous system - controlling things you don’t think about (e.g blood pressure)
what are the ‘hills’ and ‘grooves’ in the brain called
hills are gyrus
grooves are sulcus
how many pairs of cranial nerves are there and what do they do
12 pairs
they take sensory information and direct into the brain or out of the brain
how many pairs of spinal nerves are there
31 pairs
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
where are the sensory cell bodies located
dorsal root ganglion (the bulge before the dorsal root)
what happens in the dorsal horn
the sensory axons continue in region called dorsal horn and there they make synapses with other neurones and eventually control the activity of motor neurones
where are the cell bodies of motor neurones
ventral horn
where are motor axons sent out
sent out through ventral root and then join up with a spinal nerve so you have a mixed spinal nerve going out to its targets in the periphery
what does white matter contain
lots of spinal tracts
- spinal tracts could be taking information up to the brain or sending information down from the brain (telling motor neurones what to do)
- grey matter in middle white matter more on the outside
what bit of a neuron contains the nucleus
cell body
- sometimes called soma
what bit of the neuron cell receives information
dendrites
what part of the neuron triggers axon potential
initial segment
- sometimes called axon hillock
what part of the neuron sends the action potential
axon
what happens at axon (presynaptic) terminals
transmitter is released
what are the three types of neurones
- afferent (sensory) neurones (PNS)
- interneurones (CNS)
- efferent (motor) neurones (PNS)
how do the three types of neurones (sensory, inter, motor) work together
afferent neurone will have sensory receptor out in periphery responding to something like touch
- afferent neurones are a bit weird where their axon runs right past the cell body so the axon comes in at the dorsal horn
- then it will release a neurotransmitter and it’ll act on the interneurones and they’ll decide whether this needs a motor repsonse
- motor neurones have their cell bodies in the ventral horn and they travel out by ventral root
what are glia
non-neuronal cells of the central nervous system
- 90% of cells in the CNS
what are the four types of glia
- astrocytes
- oligodendrocytes
- microglia
- ependymal cells
what do astrocytes do
- type of glia
- maintain the external environment for the neurones
- surround blood vessels and produce the blood brain barrier
what do oligodendrocytes do
- type of glia
- form myelin sheaths in the CNS (Schwann cells in the PNS)
what do microglia do
- type of glia
- phagocytic hoovers mopping up infection
what do ependymal cells do
- type of glia
- produce the cerebrospinal fluid
what are the three types of potentials that neurones use to send electrical signals
action potentials - transmit signals over long distances
graded potentials - decide when an action potential should be fired
resting membrane potential - keeps cell ready to respond
what is usually the resting membrane potential in cells
-70mV
how do we get the resting membrane potential
- difference in voltage created by sodium potassium pump is very little
- resting membrane potential is due to leaky K+ channels
- K+ leaks out through these channels down its concentration gradient and as it leaks it builds an electrical gradient
- an equilibrium is reached when the electrical gradient is equal and opposite to the concentration gradient
- that is the resting membrane potential
what do the Nernst and Goldman-hodgkin-katz (GHK) equations do
Nernst equation predicts the equilibrium potential for a single ion species
Goldman-hodgkin-katz equation predicts the equilibrium potential generated by several ions
where is there a high K+ concentration - inside or outside the cell
inside
where is there a high Na+ concentration - inside or outside the cell
outside
where is there a high Cl- concentration - inside or outside the cell
outside
what is the function of a graded potential
to depolarise the cell to threshold by opening channels
can also hyperpolarise - make more negative
what are some examples of graded potentials
generator potentials - at sensory receptors
postsynaptic potentials - at synapses
endplate potentials - at neuromuscular junction
pacemaker potenitals - in pacemaker tissues
what does it mean by graded potentials are graded
- a small stimulus will open few channels and evoke a small response (cell will depolarise slightly - become a bit more positive)
- a large stimulus will open many channels and evoke a large response (cell will depolarise quite a bit)
what does it mean by graded potentials are decremental
- they get smaller as they travel along the membrane
- think like water leaking out a hose
- your axons are very leaky so graded potentials are only useful over short distances
- this is why graded potentials are also called local potentials
what does it mean by graded potentials can summate
a single neuron will have lots of synapses evoking their own postsynaptic potential, if two occur at the same time they can add together
how do you generate a graded potential
- you open/sometimes close particular ion channels (e.g K+, Na+, Cl-, Ca2+ although Ca2+ has lots of unintended consequences so tend not to use that) (remember to think about concentration gradient AND electrical gradient)
- e.g. if you wanted to hyper polarise the cell you would open K+ channels so it would leak out
how do you hyper polarise (make more negative) postsynaptic potentials
fast IPSP (inhibitory postsynaptic potential) via inotropic receptor - neurotransmitter binds to Cl- channel and it opens very quickly and Cl- comes into cell making it more negative slow IPSP - there is separate receptor and channel, neurotransmitter is released and binds to receptor (metabotropic) then G protein will open channel and in this case it is K+ leaky channel so cell becomes more negative. takes the G protein some time to find its target this is why it's slow
how do you depolarise (make more positive) postsynaptic potentials
fast EPSP (excitatory postsynaptic potential) - neurotransmitter binds to receptor and opens channel. this channels lets through anything with a charge of +1 (so will let some potassium out but Na+ coming through means K+ is near equilibrium anyway) slow EPSP - obvious one is to block leaky K+ channels. neurotransmitter binds to separate metabotropic receptor and G protein closes channels
where do you get a stronger graded potential on a neurone
if you stimulate a receptor on the end of a dendrite you will get a smaller response. (so might not reach action potential) if you stimulated one closer to the initial segment you would get a larger response
what is spatial summation
- in graded potentials
- two different inputs on neuron are stimulated at approximately the same time to give graded potential. this evokes are larger response than if they were just doing it on their own
what is temporal summation
- in graded potential
- the same input is stimulated twice in quick succession to reach threshold
what is is called when a synapse is synapsed onto another synapse which is synapsed onto the neurone
axo-axonic synapse
what is it called when synapse is synapsed at end of dendrite (further away from cell body)
axo-dendritic synapse
what’s it called when a synapse is synapsed onto the cell body/at beginning of dendrite next to cell body
axo-somatic synapse
what are the different values in an action potential
- graded potential makes the cell depolarise to threshold (around -55mV)
- once it reaches -55mV then you get a massive depolarising phase where the inside of the cell becomes positive (around +40mV)
- but then very quickly the cell repolarises and goes back to a bit more negative than it was before (phase of hyperpolaristation) and then eventually it’ll go back to resting membrane potential
how do we get the depolarisation in the action potential
- Na+ voltage gated channels open and the inside of the cell becomes more positive
- these open very quickly and shut very quickly
- K+ leaky channels are also still open because they are usually always open
how do we get the hyper polarisation (or just depolarisation) in the action potential
- the voltage gated channels from the depolarisation stage will shut
- voltage gated potassium channels (different from leaky K+ channels) will open so cell becomes more positive
- eventually those will shut too and we are back to resting membrane potential
what are the properties of the action potential
- have a threshold
- all or none (you either reach threshold and get action potential or you don’t)
- have a refractory period (break between firing)
- they are self-propagating (grows new action potential as it continues along membrane)
- they travel slowly
- they have these properties because they are mediated by voltage gated channels as opposed to ligand ones which generate graded potential
why can the action potential never go backwards along axon
due to the refractory period
what are two ways to speed up action potential
- large axons
- myelination
self propagation of action potentials is very slow but these two methods speed it up
how do large axons speed up action potential
- just like how water flows more easily through a bigger tube
- electric current flows more easily down a large axon (axial resistance is lower)
- large axon allows Na+ channels to be more spaced out along axon so requires fewer channels
- mammals don’t have large axons but squids are very good at it
how does myelination speed up action potential
- Schwann cells in PNS and oligodendrocytes in the CNS
- myeline increases membrane resistance and reduces membrane capacitance (so less capacity for ion channels and more of them shut too)
- so less current is wasted, the axons are less leaky
what are the consequences of demyelination
- multiple sclerosis in the CNS
- Guillain-barre syndrome in the PNS
- they attack the myeline sheath
- membrane resistance is decreased and capacitance is increased
- more current is lost and conduction fails
what is the compound action potential
- humans have small and large unmyelinated and myelinated axons, all conducting at different velocities
- extracellular recording from a nerve (bundle of axons) will therefore generate a ‘compound’ action potential
- this gives classification of axons based on their conduction velocity and this correlates with their anatomy and their function
- e.g. large myelinated will have a larger velocity than small unmyelinated
what is the neuromuscular junction
the synapse between the motor neurone and skeletal muscle
how is the action potential in the neuromuscular junction generated
- the action potential in motor neuron opens voltage gated Ca2+ channels in presynaptic terminal
- this triggers fusion of vesicles containing ACh (Ca2+ dependent exocytosis)
- acetylcholine is released (ACh) and diffuses across synaptic cleft
- ACh binds to ACh receptors (nicotinic) on post synaptic plate
- this opens Na+/K+ ligand gated channels
- this evokes a graded potential (the end plate potential) which ALWAYS depolarises membrane to threshold
- this opens voltage gated Na+ channels and evokes new action potential
- ACh is removed by acetylcholinesterase
what does the ligand-gated Na+/K+ channel do in the neuromuscular junction
evokes the graded potential which is unusually large and ALWAYS REACHES THRESHOLD
what do the voltage gated Na+ channels on the post synaptic plate do
generate the new action potential
is there synaptic integration in the neuromuscular junction’s new action potential
no
- there are post-junctional folds that pack voltage gated Na+ channels close to where the graded potential is evoked so no synaptic integration
so what are the order of the ion channels which open in the neuromuscular junction
- voltage gated calcium channels open (pre-synaptic)
- acetylcholine binds to nicotinic receptors post-synaptically after diffusing across synapse
- ligand gated sodium potassium channel opens (triggers graded potential)
- voltage gated sodium channel opens (triggers action potential)
- Act removed by acetylocholinesterase
what are the differences between the CNS synapses and the neuromuscular junction
- same sequence of events but range of neurotransmitters
- range of post-synaptic potentials
- small post-synaptic potentials (synaptic integration)
- anatomical arrangement of synapses different (axon-dendritic, axon-somatic, axon-axonal)
- synaptic connectivity (convergence, divergence, feedback inhibition, monosynaptic and polysynaptic pathways)
- synaptic plasticity
what are some of the common neurotransmitters in CNS synapses
- most important excitatory one in CNS is glutamate (mediates nearly all fast excitatory transmission in CNS)
- most important inhibitory would be GABA (mediates nearly all fast inhibitory transmission in the CNS)
- glycine=mediates all fast inhibitory in spinal cord
- nitric oxide is transmitter that doesn’t go in vesicle because too liipophilic
what is the difference between the post synaptic potentials in the CNS and the neuromuscular junction
- in CNS there is range of post synaptic potentials (fast and slow EPSP, fast and slow IPSP) whereas in the NMJ you just have one massive fast EPSP (endplate potential)
- also in CNS post synaptic potentials are small and often have to be added together=synaptic integration (why CNS synapse is less predictable, might not always reach threshold unlike NMJ)
what are the different types of anatomical arrangements of synapses in the CNS
- axo-dendritic
- axo-somatic
- axo-axonal
- in the NMJ there is just the motor neurone synapsing directly onto skeletal muscle
what is convergent synaptic connectivity in the CNS
- activity of one cell if being influenced by activity of lots of cells
what is divergent synaptic connectivity in the CNS
- one neuron is synapsing onto several different ones and affecting the activity of several different post synaptic neurones
(you get this is in the NMJ too because one motor neurone will synapse onto several different muscle fibres and activate them all and form the motor unit
what is feedback inhibition in synaptic connectivity in the CNS
- action potential initiated at axon hillock, send action potential down neurone
- sometimes on the way there is axon collateral which when activated activates an inhibitory interneurone
- inhibitory transmitter released which hyperpolarises the cell
what are monosynaptic and polysynaptic pathways
monosynaptic = simple reflex activités could be mediated by a pathway of just two neurones
polysynaptic = most of our reflexes, can turn excitatory reflex into inhibitory one by turning one of the many neurones into an inhibitory neurone
what is synaptic plasticity in the CNS
- using synapse intensively can change their strength
- can effect long term depression
- one of the reasons neurones tend no to use calcium to depolarise cells
