6.5 Neurones and Synapses Flashcards
what are neurons?
neurons transmit electrical impulses - 85 billion in the human body
have a cell body wiht cytoplasm and a nucleus but also narrow outgrowths called nerve fibres along whihc nerve impulses travel
- dendrites are short branched nerve fibres used to transmit nerves betweeen neurones in one part of the brain and the spinal chord
- axons are elongated nerve fibres that transmit to a nerve from e.g. the tips of toes to teh spinal chord
what does myelination of nerve fibres do?
it allows for saltatory conduction
the nerve fibre is cylindrical in shape with plsma membrane enclosing a narrow region of cytoplasm, teh diameter of most cases is 1 micrometer – speed about1 meter per second
coating of a nerve fibre my myelin sheat which is made of many layers of phospholipid bilayer, and is called schwann cells which grow around teh nerve axon many times making it have those many layers of phospholipid bilayer - 20+ layers
node of ranvier is the gap between myelin sheaths which is where the schwann cells jump to in saltatory conduction, much quicker up to 100 meters per second
what is a resting potential
neurones pump sodium and potassium ions across their membranes to generate a resting potential - it is created due to an imbalance of positive and negative charges
- na/k pumps put 2k+ in and 3Na+ out of the cell - conc gradient
- membrane is 50 more permeable to K+ ions than na+ ions so K+ ions leak back in faster tahn Na+ ions - conc gradient, charge imbalance
- proteins inside nerve fibers are anions so increase charge imbalance
resting potential is -70mv
what is an action potential
action potentials are the depolarisation and repolarisation of neurones,
depolarisation - negative to postitive
as na channels open na+ ions diffuse into the neurone so the membrane potential is raised to 30mv
repolarisation - postive to negative
na channels close and k channels open so potassium channels diffuse out fo the neurone so cell is negative again (about 70mv) - a couple of milliseconds for conc gradietn to reestablish and then the neurone can transmit another nerve impulse
how is the nervous system divided up
how are action potentials propagated
the ion movements that depolarize one part of the neruone trigger depolarisation in teh neighbouring parts of the neuron
nerve impulses always move in one direction and can only be initiated by the terminal of a neuron
refractory period prevents this
what are local currents
local currents cause each successive part of the axon to reach the threshold potential
because of depolarisation and the conc of sodium ions that is established an axon will have a different sodium conc to a neighbouring axon and sodium ions will diffuse between these regions both inside and outside the axon
inside the axon there is a higher na conc on teh depolarised part of the axon so the the ions diffuse along the axon
outside sodium ions diffuse from teh polarised part to the depolarised part - forms local currents
these reduce the conc gradients of neurones that are about ot be depolarised so the membrane potential rise from -70 to -50, teh voltage gates sodium channgles then open as this is the threshold potential causing depolarisation
what are synapses
synapses are junctions betweee nuerones and between neurons and receptor or effector cells
in muscles or glands there are synpases between neurons and muscle fibres or secretory cells
if hte pre synaptic and post synaptic are seperated by fluid filled gap (synaptic cleft 20 nanometers wide) then neurotransmitters are needed
how do synapses work
- nerve impulse arrives at pre synaptic membrane
- depolarization causes calcium ions to diffuse through channels in teh membrane into the neurone
- influx of calcium causes vesciles contianing neurotransmitters to move to teh pre synaptic membrane and fuse wiht it
- exocytosis
- diffusion of neurotransmitteer to the receptor where they bind with the post synaptic membrane
- causes adjacent sodium ion channels to open
- this cause the post synaptic membrane to reach threshold potential
- action potential is triggered
- neurtransmitters are broken down and removed from synaptic cleft
what is acetylcholine
a neurotransmitter often used between neurons and muscle fibres
produced in teh pre-synaptic neurone by combining choline, absorbed from teh diet, with an acetyl group producef during aerobic respiration - loaded into vesicles
acetycholine remaind boudn ot receptros on teh post synaptic neuron for a short amount of time intitating only one action potential across the neurone
acetylcholinesterase in the synaptic cleft rapidly breaks down acetylcholien and the choline is reabsorbed in the presynaptic neuron where it is converted back into active neurotransmitters
what are neonicotinoids
these block synaptic transmission at cholinergic synapses in insects by binding of neonicotinoids pesticides to acetylcholine receptors
- synthetic compounds similar to nicotine
- binding is irreversible
synaptic transmission is prevented - paralysis and death
not toxic to humans and other mammals - insects have many more cholinergic synpases and neonicotinoids bind less strongly to acetylcholine receptors in humans
imidacloprid a neonicitinoid is used very widely
- effects on honeybees and other beneficial insects
what is a threshold potential
all or nothing principle in nerves - postive feedback loop
the voltage needed for the voltage gated sodium channels to open - 50 mv
if there is not enough neurotransmitter secreted then threshold potential may not be reach and a return to resting state occurs
a typical synpase in teh brain will have synapses with many pre synaptic neurons so it may be necessary for many of these to be active to trigger a threshold potentia; - type of mechanism used for decision making
