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6.2 nervous coordination - Coggle Diagram
6.2 nervous coordination
lesson 1: neuron structure
cell body
in the cell body the rough endoplasmic reticulum is associated with the production of proteins and neurotransmitters
contains all the useful organelles- nucleus and Golgi apparatus
dendrons
extensions of the cell body which subdivide into smaller branched fibres called dendrites
axon
axon surrounded by Schwann cells which protect it and provide electrical insulation
single long fibre that carries nerve impulses away from the cell body
Schwann cells
carry out phagocytosis to remove cell debris and play a part in nerve regeneration
wrap around the axon many times so layers of there membrane build up around it
membrane is rich in myelin
shcwann cell envelopes and rotates around the axon forming the myelin sheath
gaps between adjacent Schwann cells are called nodes of ranvier
lesson 2: resting potential
nerve impulses
self-propagating wave of electrical activity that travels along the axon membrane
temporary reversal of the electrical potential difference across the membrane
reversal between two states: resting potential and action potential
the electrical potential difference is due to unequal ion concentration
movement of ions across the phos[holipid bilayer
the phospholipid bilayer of the axon prevents sodium and potassium from moving across by simple diffusion (as they are charged)
they instead move across the membrane in two ways: facillitated diffusion (through channel proteins down a concentration gradient) and antive transport (via carrier protien against concentration gradient)
two main factors contribute to establishing and maintaining the resting potential
differential membrane permeability
axon membrane is not equally permeable to all ions, allows some ions to pass through more easily than others due to the distribution of (open) ion channel proteins in the membrane
active transport of sodium and potassium via the sodium potassium pump
voltage-gated channel proteins
will be either:
open - axon depolarised (becoming more positive)
closed - axon polarised/repolarsied (becoming more negative)
maintaining the resting potential
Sodium-potassium pumps actively transports three sodium out of the axon and two potassium into the axon.
as more sodium are transported out of the axon than potassium are transported in an electrochemical gradient is created, the tissue fluid if more positive and the cytoplasm is more negative
sodium and potassium ions diffuse down their respective concentration gradients, sodium diffuses back into the axon while potassium diffuse back out into the tissue fluid.
as the membrane is more permeable to potassium ions than sodium ion (more open potassium channels than sodium channels) so potassium diffuses out of the membrane faster than sodium diffuses in
axon membrane potential = -70mV
lesson 3: action potential
what is an action potential?
brief change in the distribution of electrical charge across the cell surface membrane caused by the rapid influx of sodium ions and potasssium ions across the membrane of the axon
key points
an action potenial is a large, rapid, all or nothing depolarisation of the axon membrane that can travel along the entire length of the neuron
all or nothing
once the treshold value (-55mV) is reached the action potential always has the same size regardless of of stimulus length
self-propogating
it does not fade, it travels along the axon without decreasing in size
caused by the opening of voltage-gated sodium and potassium channels
strength of stimulus is instead coded by the frequency of action potentials not their size
generating an action potential
1) stimulus
stimulus triggers sodium ion channels in the membrane to open, sodium ions flood into the neuron, the potential difference changes to become more positive inside the neuron
2) depolarisation
if the threshold value (-55mV) is met then the membrane will become depolarised, depolarisation causes voltage gated sodium channels to open allowing more sodium ions to enter causing more depolarisation
3) repolarisation
after the neuron membrane has depolarised to +40mV the sodium ion channels close and potassium ion channels open, potassium ions are transported back out of the neuron and the potential difference across the axon membrane becomes more negative
4) hyperpolarisation
potassium ion channels are too slow to close, too many potassium ions diffuse out of the neuron causing hyperpolarisation, the potential difference briefly becomes more negative than the normal resting pontiac (>-70mV)
5) return to resting potential
once the potassium voltage-gated channel proteins are closed the sodium potassium pump restores the resting potential, the sodium ion channel proteins this section of the membrane become responsive to depolarisation again
the refractory period
period where the neurone membrane cannot be stimulated, because sodium ion channels are recovering and can't be opened
ensures that impulses only travel in one direction - they cannot reopen until resting potential has been established
what factors effect the transmission of impulses
the presence of myelin
in unmyelinated neurons the speed of conduction is slow because depolarisation must occur along the whole membrane of the axon
myelin acts as an electrical insulator - action potential is not formed in the section of axon covered by the sheath
when myelin is present the action potential is able to 'jump' between nodes of ranvier as these are the only sections of the axon where an action potential is established - this is called saltatory conduction
the diameter of the axon
the greater the diameter of the axon the faster the rate of conduction
thicker axon = greater surface area over which diffusion of ions occurs - increases rate of diffusion of potassium and sodium ions through channel proteins so increases the rate at which depolarisation and action potentials can occur
temperature
increase in temp increases ions kinetic energy so more movement of ions through channel proteins - increases speed of transmission
temperatures above about 40 degrees slow down speed of transmission as channel proteins in axon membrane start to denature
lesson 4: cholinergic synapses
function of mitochondria and endoplasmic reticulum in the presynaptic neuron
required for the manufacturing of neurotransmitter
mitochondria provides ATP/energy
endoplasmic reticulum makes and packages the neurotransmitters
neurotransmitters are stored in synaptic vesicles - action potential stimulates the synaptic vesicles to fuse with the presynaptic neurons membrane and release neurotransmitters via exocytosis
role of neurotransmitters
temporarily bind to to receptor molecules in the postsynaptic neurons membrane - this causes sodium ion channels in the post synaptic neurone to open which stimulates a new action potential in the post synaptic neurone
synaptic transmission in cholinergic synapses
1) action potential arrives, depolarising presynaptic neurone/membrane
2) calcium ion protein channels open, calcium ions diffuse into presynaptic neuron
3) synaptic vesicles fuse with membrane releasing neurotransmitter (acetylcholine) by exocytosis
4) acetylcholine diffuses across synaptic cleft
5) acetylcholine binds to receptor proteins on postsynaptic membrane
6) Receptor proteins open. Sodium ions diffuse through the membrane depolarising the postsynaptic membrane
7) Postsynaptic membrane is depolarised.
8) Acetylcholine broken down into acetate and choline by acetylcholinesterase.
9) Choline recycle into acetylcholine
recyling of neurotransmitters
after diffusing across the synaptic cleft neurotransmitters are destroyed or recycled
this prevents continuous stimulation of the postsynaptic neurone
in cholingeric synapses acetylcholine is broken down into choline and acetylCOA and in the synapse acetylcholine is broken down by acetylcholinesterase into its two components - choline is transported back into the axon terminal and used to make more acetylcholine
lesson 5: summation and inhibition
summation
the process by which multiple signals (excitatory and inhibitory) together are able to generate an action potential
when not enough neurotransmitter is released from the presynaptic neurone the effect of multiple impulses can be added together to overcome this
two types of summation: temporal and spatial
temporal summation (frequency)
high frequency impulses in the presynaptic neurone
cause enough neurotransmitter to be released in quick succession to generate an action potential in the postsynaptic neurone
spatial summation (multiple neurones)
more than one presynaptic neurone releases neurotransmitter into the same synapse or onto the same postsynaptic neurone at the same time
enough total neurotransmitter is released to generate an action potential
benefits of summation
allows for the effect of a stimulus to be magnified
combination of different stimuli can trigger a response
avoids the nervous system being overwhelmed by impulses
the synapse acts as a barrier and slows down the rate of transmission of a nerve impulse that has travelled along two more neurons
inhibition
some inhibitory synpapses work independently by using a specific neurotransmitter (e.g GABA)
others work by counteracting the action of an excitatory synapse -
inhibitory and excitatory synapses work antagonsitically
if the cell body of a motor neuron is subject to both excitatory and inhibitory synapses the following happens:
1) sodium ions enter cell body following stimulation by the excitatory synapse
2) stimulation of the inhibitory synapse causes potassium ions to diffuse out of the cell body
3) this cancels out the effect of the sodium ions entering
4) threshhold value is not reached - no action potential generated
how independent inhibitory synapses work
action potential arrives causing calcium ions to enter presynaptic neuron
this causes release of GABA into the synaptic cleft
GABA binds to receptor sites on chloride ion channels in postsynaptic neuron membrane, chlorine channels open and negatively charged chloride ions diffuse into postsynaptic neuron
GABA binds to receptor sites on potassium ion channels in postsynaptic membrane. potassium ion channels open and positively charged potassium ions diffuse out
this hyper polarises the neuron
lesson 6: neuromuscular junctions
what is a neuromuscular junction and how do they work
synaptic connection between the terminal end of a motor neuron and muscle (skeletal/cardiac)
work in a similar way to synapses but are located between neuron and muscle cell - striated muscle contracts when it receives and impulse
defintions
sarcolemma
plasma membrane of the muscle cell
myofibrils
long contractile fibres, groups of which run parallel to each other on the long axis of the muscle cells
sarcoplasmic reticulum
specialised form of the endoplasmic reticulum dedicated to calcium ion handling, necessary for muscle contraction and relaxation
T-tubules
allow an action potential to reach the centre part of the cell
events at a neuromuscular junction
1) An action potential, travelling along the axon of a motor neurone, arrives at the presynaptic membrane.
2) This causes calcium ions to diffuse into the neurone, through the voltage-gated calcium ion channel proteins which were opened due to the change in the membrane potential
3) Vesicles containing the neurotransmitter acetylcholine (ACh) are stimulated to fuse with the presynaptic membrane.
4) ACh diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma
5) Sodium ion channels in the sarcolemma open, allowing sodium ions to diffuse in.
6) The sarcolemma depolarises, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre.
7) Voltage-gated calcium ion channel proteins in the membranes of the sarcoplasmic reticulum (which lie very close to the T-tubules) open.
8) Calcium ions diffuse out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the myofibrils.
9) Calcium ions bind to troponin molecules, stimulating (tropomyosin) them to change shape. This is the start of the process of muscle contraction (the sliding filament model).
similarities between neuromuscular junctions and cholinergic synapses
cholinergic synapses
found between neurons
can be excitatory or inhibitory
neuromuscular junctions
found between motor neurone and muscle
only excitatory
both
stimulated by an action potential on the presynaptic membrane
use acetylcholine as a neurotransmitter