3.6.2 Nervous Coordination Flashcards
Structure of a myelinated motor neurone
Cell body, contains organelles eg nucleus and lots of rough endoplasmic reticulum, produces proteins and neurotransmitters
Dendrons, extensions of cell body, divide into smaller branches called dendrites, carry nerve impulse towards cell body
Axon, a single long fibre carry nerve impulses away from cell body
Schwann cells, surround axon, provide electrical insulation, carry out phagocytosis and regenerate nerves, wrap around generating many layers
Myelin sheath, covers axon made up of the Schwann cell membranes (lipid called myelin)
Nodes of ranvier, areas between adjacent Schwann cells where there is no myelin sheath, transmission of impulse is faster
Explain resting potential
3Na+ actively transported out of axon, 2K+ actively transported into axon both using carrier proteins, occurs due to sodium potassium pump
More Na+ in tissue fluid, more K+ in cytoplasm of axon, an electrochemical gradient is created
Facilitated diffusion moves Na+ into axon and K+ out of axon, channel protein has gates which are closed for Na+ movement (membrane is less permeable to Na+)
Inside of axon is -ve charged compared to tissue fluid making it polarised
What’s a nerve impulse
Reversal of electrical potential difference across the axon membrane
Has resting potential and action potential
What happens during depolarisation
Energy from a stimulus causes some sodium voltage gated channels in axon to open, Na+ diffuses (facilitated diffusion)into axon, reverse potential difference across membrane
Threshold value 40mV action potential is established
What happens during repolarisation
40mV action potential is established, voltage gates on Na+ channels close, voltage gates of K+ open, electrical gradient allows K+ to diffuse out of axon
What happens during hyperpolarisation
K+ ions diffusing out of axon cause an overshoot of electrical gradient, inside of axon is more negative than tissue fluid
Gates on K+ ion channels are closed
Sodium potassium pump continues, resting potential reestablished
What’s action potential
Neurones voltage increase beyond a set point, continual wave of depolarisation, generates nervous impulse
What’s the passage of an action potential across an unmyelinated axon
Resting potential, Na+ in tissue fluid, -ve charge inside axon, axon membrane is polarised
Depolarisation, Na+ moves into axon, inside is now +ve, membrane is depolarised
Localised electrical current by Na+ cause sodium voltage gates channels to close, potassium ones open, moves along axon membrane
Axon membrane behind action potential is repolarised, sodium potassium pump continues across the axon
What’s the all or nothing principal
If depolarisation doesn’t exceed threshold voltage then an action potential isn’t generated
If a stimulus triggers depolarisation, there will be a peak to the same max voltage, larger stimuli increase frequency of action potentials
Ensures animals only respond to large stimuli
What’s the passage of an action potential across a myelinated axon
Action potential can occur at nodes of ranvier, action potential jumps between adjacent nodes this is called saltatory conduction
Action potential passes along a myelinated neurone faster
What’s the refractory period
After an action potential is created Na+ voltage gated channels close prevents Na+ moving into the cell
What’s the importance of the refractory period
Action potentials propagated in one direction, action potential move from active to resting, cannot be propagated into a refractory region, move in one direction
Production of discreet impulses, new action potential can’t be formed immediately after another due to refractory period, ensures action potentials are separated
Limits number of action potentials, action potentials are separated from each other, only a certain amount can pass in a given time, strength of stimulus that can be detected is limited
What shows the strength of a stimulus
Frequency of action potentials
What factors effect the speed of conduction of an action potential
Myelination and saltatory conduction
Axon diameter
Tenperature
How does myelination and saltatory conduction effect speed of an action potential
Myeline sheath acts as electrical insulator, causes action potential to jump between adjacent nodes of ranvier this is called saltatory conduction
How does the diameter of an axon effect the speed of an action potential
Greater axon diameter means faster speed of conductance, less leakage of ions, easier to maintain membrane potential
How does temperature effect the speed of an action potential
Only in cold blooded animals
Higher temperature increases rate of diffusion for ions
Enzymes work faster at higher temps up to an optimal temp, respiration releases energy for active transport, faster movement of ions
Higher temps increase the speed and strength of muscle contractions
What’s a synapse
Where one neurone communicates with another or with an effector
Structure of a synapse
Synaptic cleft, small gap
Presynaptic neurones, releases neurotransmitter
Synaptic knob, swollen end of Presynaptic neurone, lord of mitochondria, manufactures neurotransmitters
Synaptic vesicles, stows neurotransmitters
Postsynaptic neurones, specific receptor proteins on membrane to receive neurotransmitter
What’s a neuromuscular junction
Motor neurone meters skeletal muscle fibre, many of these junctions spread across muscle for rapid and coordinated contraction
Nerve impulse received at junction, synaptic vesicles fuse with Presynaptic membrane releasing acetylcholine, diffuses into Postsynaptic membrane, more permeable to Na+, Na+ enters, membrane depolarised, acetylcholine broken down by acetylcholinesterase, choline and acetyl diffuse back into neurone, recombine using energy from mitochondria
Transmission in a cholinergic synapse
Action potential at synaptic knob (summation), depolarisation at knob, Ca2+ channels open, Ca2+ diffuses into synaptic knob
Vesicles containing acetylcholine move towards and fuse with Presynaptic membrane, acetylcholine released to synaptic cleft
Acetylcholine diffuses down a conc gradient across synaptic cleft to Postsynaptic membrane, acetylcholine binds to complimentary shaped receptors, Na+ protein channels open, Na+ diffuses in, generates new action potential in Postsynaptic neurone
Acetylcholinesterase hydrolyses acetylcholine into acetyl and choline, diffuse back across synaptic cleft to Presynaptic neurone, prevent continuous generation of action potentials, discreet transfer of info (unidirectionality)
ATP released by mitochondria recombine choline and acetyl to acetyl choline, stored in synaptic vesicles, Na+ protein channels close
What’s unidirectionality
Synapses only pass info from Presynaptic neurone to Postsynaptic neurone
What’s summation
Spatial summation, different Presynaptic neurones release enough neurotransmitter to exceed threshold of Postsynaptic neurone, trigger new action potential
Temporal summation, single Presynaptic neurone releases neurotransmitter lots of times in a short period, if the conc of neurotransmitter exceeds threshold of Postsynaptic neurone, new action potential is triggered
What’s inhibition by inhibitory synapses
Presynaptic neurone releases neurotransmitter binds to Cl- protein channels on Postsynaptic neurone, Cl- protein channel opens, Cl- moves into Postsynaptic neurone by facilitated diffusion,
K+ channels also open, K+ moves out of Postsynaptic neurone into synapse, inside of Postsynaptic membrane is more -ve than outside, hyperpolarisation, more Na+ ions needed to produce action potential
Cholinergic synapse vs neuromuscular junction
Both neurotransmitters acetylcholine transported by diffusion
Both have receptors on which bind to acetylcholine causing influx of Na+
Both use Na+ K+ pump to depolarise axon
Both use acetylcholinesterase to hydrolyse acetylcholine
NMJ
Links neurones to muscle
Only involves motor neurones
Action potential ends
CS
Links neurones to neurones, or neurones to effectors
Uses motor, sensory and intermediate neurones
A new action potential can be produced
How movement of K+ and Na+ controlled
Channel proteins across the phospholipid bilayer, contain specific gates,if open all the time ions can move through by facilitated diffusion
Carrier proteins actively transport Na+ and K+ ions through the sodium potassium pump
