Module 2: Neurons, Synapses and Nervous Systems Flashcards
what are the three stages of information processing?
1) sensory input - eye
2) Integration- central nervous system
3) Motor output - peripheral nervous system - effector
describe the structure of a neuron
>nucleus > dendrites > cell body > axon hillock > axon > synaptic terminals
what are the different types of neurons and their structure?
> sensory neuron - small, few dendrites, long axon, cell body is half way down the axon
Interneuron - lots and lots of dendrites and synaptic terminals, used for collecting information, can form huge tree like structures
motor neuron - long and multiple dendrites, cell body located next to the dendrites, regular axon
describe the structure of a nerve
> a collection of neurons
axons
connective tissue
blood vessels
what are glia cells?
> supporting cells
vital for structural integrity and normal function
10-50 times more glia then neurons in the mammalian brain
supply nutrients
three types:
1) astrocytes
2) oligodendrocytes and Schwann cells
what are astrocytes
> located in the CNS
structural support
regulate extracellular concentrations of ions and neurotransmitters
formation of the blood-brain barrier
prevents things from getting into the brain.
what are oligodendrocytes and Schwann cells?
> oli in the CNS
Schwann in the PNS
form myelin shealths around axons - fatty insulators
lipid membranes: insulator
when defective cause multiple sclerosis
have nodes of ranvier between each schwann cells on the axon of a neuron.
what is sodium/potassium ATPase?
> maintains the resting potential of the neuron
pumps out 3 x Sodium out of the cell
pumps 2 x potassium into the cell
for each ATP molecule
what are the relative concentrations of ions inside and outside the cell?
Extra cellular concentration: > overall slightly postive > 5mM K+ > 150mM Na+ >120mM Cl- Intracellular concentration: > overall slightly negative > 140mM K+ > 15mM Na+ >10mM Cl- >100mM A- (large anions)
why is the resting membrane potential negative?
> Na+/K+-ATPase pumps 3Na+ out and 2 K+ in
the membrane at rest has many open K+ channels and few open Na+ or Cl- channels
K+ outflow leads to a net negative charge inside the cell.
the excess negative charges inside the cell exert attractive force that stops too many K+ ions from leaving the cell
the chemical gradient of K+ is thus opposed by the electrical gradient
this gradient forms -70mV equilibrium potential in human cells.
Which cells are excitable?
neurons
myocytes - muscle cells
pancreatic beta cells - release insulin
describe hyperpolarisation
inside of membrane becomes more negative
> opening of voltage-gated K+ channels
> K+ travels out
describe depolarisation
inside of membrane becomes more positive
> opening of voltage-gated Na+ channels
> Na+ in
what is a graded potential?
> can be depolarisation and/or hyperpolarisation
vary in magnitude with the strength of the stimulus
local and die out
what is an action potential?
> always depolarisation
reach a certain threshold (-50mV in humans) then all or nothing response
travel along axons
magnitude is independent of the strength of the original stimulus
1-2 millisecond and high frequency
what are voltage-gated ion channels?
open when a certain voltage reaches them Na+: > closed --depolarisation--> open --> inactivated by peptide tail ---> closed again > fast > into cell K+: > closed --depolarisation--> open ---> closed > slower > out of cell.
describe the process of producing an action potential
1) resting state - voltage-gated ion channels are closed
2) a stimulus cause a few Na+ channels to open. Na+ rushes in
3) depolarisation occurs if threshold is reached by incoming Na+ ions. lots of Na+ channels open. lots of Na+ rushes in
4) repolarisation. K+ channels open and K+ rushes out. Na+ channels inactivated and then start to close
5) undershoot. Small hyperpolarisation. also need Na+/K+-ATPase to restore Na+ and K+ concentrations.
what are refractory periods?
> Absolute refractory period:
- voltage-gated Na+ channels inactivated during repolarisation
- no action potential can be generated because Na+ channels open then inactivate
> relative refractory period
- an action potential can only be generated if a large enough stimulus is applied
- because some Na+ channels have closed again and are ready to be opened to let in Na+ to create a depolarisation event.
limits firing frequency
the action potential can only travel in one direction
what is the mechanism for local anaesthetics?
> local anaesthetics prevent action potentials by blocking voltage-gated Na+ channels from returning to the closed state, leaving them as inactive.
how does the conduction of action potentials work?
1) an action potential is generated as Na+ flows inward across the membrane at one location
2) the depolarisation of the action potential spreads to the neighbouring region of the membrane, reinitiating the action potential there. to the left of this region, the membrane is repolarising as K+ flows outward.
3) the depolarisation-repolarisation process is repeated in the next region of the membrane. in this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.
how does axon diameter effect action potentials?
> the larger the diameter, the less resistance = faster conduction
invertebrates: speeds vary from a few cm/s to 100m/s in giant squid axon
what are the factors that effect the speed of action potentials?
1) axon diameter
2) temperature
3) degree of myelination
how does temperature effect action potential speed?
> any chemical reaction, i.e., an action potential, occurs faster at warmer temperatures
how does the degree of myelination of the axon effect action potentials?
> myelin insulates the axon membrane in vertebrates = faster conduction speed
conduction speed is affected more by myelination than axon diameter
unmyelinated nerve fibre - smooth, slower conduction
myelinated nerve fibre - saltatory conduction
- action potential only has to be propagated at the nodes of ranvier
what is saltatory conduction?
> Schwann cells produce a myelin sheath around the axon
the depolarising current during an action potential at one node of Ranvier spreads along the interior of the axon to the next node, where voltage-gated sodium channels enable reinitiation. Thus, the action potential jumps from node to node to node as it travels along the axon.
can reach conduction speeds of 120m/s
list and explain the two types of communication that can occur at the synapses
Electrical synapses > at gap junctions > direct electric current between cells > relatively few synapses of this type Chemical synapses > involves release of a chemical neurotransmitter > neurotransmitter released by presynaptic neuron > vast majority of synapses
what are the steps that occur at a chemical synapse in direct synaptic transmission?
1) an action potential arrives, depolarising the presynaptic membrane
2) the depolarisation opens voltage-gated channels triggering an influx of Ca2+.
3) the elevated Ca2+ concentration causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft.
4) the neurotransmitter binds to the ligand-gated ion channels in the postsynaptic membrane. In the example, binding triggers opening, allowing Na+ and K+ to diffuse through.
