Neurons And Synapses And Impulses


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what are neurons
nerve cells that make up the nervous system often elongated and carry messages at high speed in form of electrical impulses
what are the 2 parts of the nervous system
central nervous system (CNS): brain and spinal cord peripheral nerves that connect all patrs of the body to the central nervous system
how do neurons carry electrical impulses long distances in the body
using elongated structures called nerve fibres/ axons
what are sensory neurons
carry nerve impulses from receptors to CNS
what are motor neurons
carry impulses from CNS to effectors (muscle and gland cells)
what are relay neurons
carry impulses within CNS from one neuron to another
what is a synapse
a junction between 2 neurons
what is the synaptic cleft
the plasma membranes of 2 neurons are separated by a narrow flui-filled gap called the synaptic cleft
what are neurotransmitters
messages passed across the synapse in the form of chemicals 
what direction do neurotransmitters always travel in
from the pre-synaptic neuron to the post-synaptic neuron
what initiates the stages of synaptic transmission
a nerve impulse reaches the end of the pre-synaptic neuron
what does the arrival of the impulse do to the membrane of the neuron what does this result in
depolarization of the pre-synaptic membrane causes voltage-gated calcium channels to open  so calcium ions diffuse into pre-synaptic neuron
what does an influx of calcium ions cause
vesicles of neurotransmitter move to pre-synaptic membrane and fuse with it meaning neurotransmitter is released into the synaptic cleft by exocytosis
what happens when the neurotransmitter diffuses across synaptic cleft
neurotransmitter binds to receptors in the post-synpatic membrane
describe the receptors on the post-synaptic membrane what happens with them
they are transmiited-gated ion channels they open when neurotransmitter binds sodium and other positively charged ions diffuse into post-synaptic neuron 
what does soduim diffusion into post-synaptic neuron cause
depolarisatin of this membrane (post synaptic)
what does depolarisation pass on as
an action potential
what occurs to prevent continous synaptic transmission
neurotransmitter in synaptic cleft is rapidly broken down calcium ions are pumped out of the pre synaptic neuron into synaptic cleft
what is a resting potential
the electrical potential across the plasma membrane of a cell that is not conducting an impulse
how do neurons pump ions across their plasma membranes what is pumped in and what is pumped out
by active transport sodium is pumped out of neuron potassium is pumped into neuron
what are the charges in an outside the neurons and why is this what does this cause
the inside develops a net negative charge because of the presence of chloride and other negatively charged ions there is therefore an electrical potential or voltage across the membrane:...
what is an action potential
the reversal and restoration of the electrical potential across the plasma membrane of a cell, as an electrical impulse passes along it 
what happens when an impulse passes along a neuron
sodium and potassium ions are allowed to diffuse across the membrane through voltage-gated ion channels
what happens to the electrical potential across the membrane what is this called
it is initially reversed but is then restored an action potential
what does an action in one part of the neuron cause
causes an action potential to develop in the next section of the neuron
why does this happen
because of diffusion of sodium ions between the region with an action potential amd the region at the resting region
what does these ion movements, local currents, cause
they reduce the resting potential
if the potential rises above the threshold level what happens
voltage-gated channels open
when the channels open and sodium ions diffuse into the neuron down the concentration gradient, what happens next
this reduces the membrane potential and causes more sodium channels to open
what does the entry of more positively charged sodium ions cause
the inside of the neuron develops a net positive chrge compared to the outside - the potential across the membrane is reversed: called depolarization
what does this polarisation causes
potassium channels open after a short delay ions diffuse out of neuron down concentration gradient
what does the exit of positively charged potassium ions cause
the inside of the neuron develops a neg negative charge again compared with the outside - the potential across membrane is restored: called repolarisation
how are the concentration gradients of sodium and potassium across the membrane restored
by the active transport of sodium ions out of neuron and potassium into neuron
what does it mean when the resting potential has been restored
the neuron is then ready to conduct another nerve impulse 

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