15 Transmission across a synapse
15.1 Neurones and nervous coordination
Principles of coordination
Nervous system: uses nerve cells to pass electrical impulses along their length
o Stimulate target cells by secreting chemicals, neurotransmitters, directly on
them
o Leads to rapid communication btw specific parts of an organism
o Responses are short lived and restricted to localised region
Example of nervous coordination is reflex action: eg withdrawal of hand from heat stimulus
Hormonal system: produce chemicals/hormones
that are transported in blood plasma to their target
cells (have specific receptors on cell surf
membrane, change in hormone conc stimulates
them ∴slower, less specific form of communication
btw parts of organisms)
o Responses long lasting and widespread,
slower
Example of hormonal coordination is control of blood glucose conc
Neurones
Specialised cells adapted to rapidly carrying electrochemical changes, nerve impulses, from one part of body another
Mammalian motor neurone made up of:
o Cell body: has all usual cell organelles, including a nucleus and lots of RER.
Associated w/ synthesis proteins and neurotransmitters
o Dendrons: extensions of cell body, subdivide into smaller branched fibres,
dendrites, that carry nerve impulses to cell body
o Axon: single long fibre carrying nerve impulses away from cell body
o Schwann cells: surround axon, protecting and providing electrical
insulation. Carry out phagocytosis (removal of cell debris) and involved in
nerve regeneration. Wrap themselves around axon many times ∴ layers of
their membranes build up around it
o Myelin sheath: forms axon covering, made up of schwann cell membranes
(rich in myelin lipid). Myelinated neurones: neurones w/ myelin sheath
o Nodes of Ranvier: constrictions btw adj schwann cells where there is no myelin sheath (2-3µm long, occur
every 1-3mm in humans)
Sensory neurones: transmit nerve impulses from receptor to
relay/motor neurone. Have 1 long dendron. Carries impulse to cell
body and 1 axon that carries it away from cell body
Motor neurones: transmit nerve impulses from relay to
effector(gland/muscle). Have long axon and many short dendrites
Relay neurones: transmit impulses btw neurones. Have numerous
short processes
15.2 Nerve impulse
Nerve impulse: self propagating wave of electrical activity that
travels along axon membrane, temporary reversal of electrical
potential difference across axon membrane
Resting potential
Movement of ions:
o Phospholipid bilayer of axon plasma membrane prevents Na + and K+ diffusing across
o Ion channel proteins (specific to ion) span phospholipid bilayer, some have gates that can be open/closed at
specific times for facilitated diffusion of Na+ and K+ (some gates are always open, facilitated diffusion can
continuously occur)
o Na+ /K+ pump: carrier protein that actively transports Na + out of axon and K+ in
axon
Resting potential (maintained by active transport): inside of axon is -ve relative to outside
(-50 to -90 mV, -70 in humans), axon polarised
Potential difference due to:
o Na+ actively transported out of axon by Na+ /K+ pump
o K+ actively transported in axon by Na+ /K+ pump
o 3Na+ out 2K+ in, electrochemical gradient made from more Na + in tissue fluid,
more K+ in cytoplasm
o Sodium diffuses back in naturally, potassium back out
, Most channels open to K+, most Na+ channels closed
Action potential (passive diffusion, membrane transmitting nerve impulse)
Stimulus of sufficient size detected by receptors in nervous system, energy cause temporary reversal of changes either
side of axon membrane
o If stimulus is great enough, -65mV inside membrane becomes +40mV (action potential), membrane
depolarised: axon membrane channels (voltage gated) change shape ∴ open/close depending on voltage
across membrane
1. Resting potential: some K voltage gated channels open, Na ones are closed
2. Stimulus energy causes some NA channels to open ∴ Na diffuses into axon, along electrochemical gradient (Na is +ve,
so triggers reversal in potential difference across membrane)
3. Causes more Na to diffuse in, greater influx by
diffusion
4. Action potential goes to +40mV, Na channels
close and K channels open
5. K channels opening, reverses electrical gradient
∴ more K channels open, more K+ diffuses out,
starting repolarisation
6. Outward diffusion of K+ causes temporary
overshoot of electrical gradient (more -ve inside
than outside, than usual = hyperpolarisation)
7. K channels close and Na+ /K+ pump continues to
re-establish resting potential (repolarised)
15.3 Passage of an action potential
action potential remains the same along axon
nothing physically moves, just a depolarisation
acting as stimulus for another one in the next region (conduction)(travelling wave of depolarisation)
Unmyelinated axon
1. at resting potential, Na conc outside axon membrane is high relative to inside, vice v. for K conc,
overall +ve outside than inside ∴axon membrane polarised
2. stimulus causes Na influx ∴ reversal charge on axon membrane ∴ depolarised (action potential)
3. localised electrical currents established by Na influx cause Na channels to open further along axon.
Behind region of depolarisation, Na channels close, K channels open. K + leave axon along
electrochemical gradient
4. action potential/depolarisation is propagated in same way further along axon. K + continue to move
out, to the point that axon membrane behind has returned to original charges state/repolarised
5. Repolarisation allows Na+ to actively transport out, returning axon to its resting potential
Myelinated axon
Fatty myelin sheath around axon acts as an electrical
insulator, preventing action potential forming
Nodes of Ranvier: breaks in myelin, where action
potential can occur ∴ localised circuits arise btw adj
nodes and action potential jumps from node to node
(saltatory conduction)
o Action potential passes along myelinated
neurone faster than unmyelinated neurone of same
diameter
Bc in unmyelinated. Depolarisation can happen all way along ∴ takes longer
15.4 Nerve impulse speed
Action potential doesn’t change size across axon
Factors affecting action potential speed
Myelin sheath: acts as an electrical insulator, preventing action potential forming, action potential jumps from node to
node (saltatory conduction), increases speed of conductance
Axon diameter: greater diameter, faster speed of conductance bc less leakage of ions from a larger axon (leakage
makes membrane potentials harder to maintain)
Temp: affects rate of diffusion of ions ∴higher temp=faster nerve impulse. Affects enzyme controlled reactions, eg
respiration (to make ATP needed for active transport and Na/K pump). Affects speed and strength of muscle
contractions
o Important for ectotherms (cold blooded, body temp varies w/ enviro)
Thresholds “all or nothing”
Nerve impulses: all or nothing responses
Certain level of stimulus/threshold value that triggers action potential
o Below threshold (of any stimulus strength): no action potential/impulse generated ∴ nerve impulse travels
15.1 Neurones and nervous coordination
Principles of coordination
Nervous system: uses nerve cells to pass electrical impulses along their length
o Stimulate target cells by secreting chemicals, neurotransmitters, directly on
them
o Leads to rapid communication btw specific parts of an organism
o Responses are short lived and restricted to localised region
Example of nervous coordination is reflex action: eg withdrawal of hand from heat stimulus
Hormonal system: produce chemicals/hormones
that are transported in blood plasma to their target
cells (have specific receptors on cell surf
membrane, change in hormone conc stimulates
them ∴slower, less specific form of communication
btw parts of organisms)
o Responses long lasting and widespread,
slower
Example of hormonal coordination is control of blood glucose conc
Neurones
Specialised cells adapted to rapidly carrying electrochemical changes, nerve impulses, from one part of body another
Mammalian motor neurone made up of:
o Cell body: has all usual cell organelles, including a nucleus and lots of RER.
Associated w/ synthesis proteins and neurotransmitters
o Dendrons: extensions of cell body, subdivide into smaller branched fibres,
dendrites, that carry nerve impulses to cell body
o Axon: single long fibre carrying nerve impulses away from cell body
o Schwann cells: surround axon, protecting and providing electrical
insulation. Carry out phagocytosis (removal of cell debris) and involved in
nerve regeneration. Wrap themselves around axon many times ∴ layers of
their membranes build up around it
o Myelin sheath: forms axon covering, made up of schwann cell membranes
(rich in myelin lipid). Myelinated neurones: neurones w/ myelin sheath
o Nodes of Ranvier: constrictions btw adj schwann cells where there is no myelin sheath (2-3µm long, occur
every 1-3mm in humans)
Sensory neurones: transmit nerve impulses from receptor to
relay/motor neurone. Have 1 long dendron. Carries impulse to cell
body and 1 axon that carries it away from cell body
Motor neurones: transmit nerve impulses from relay to
effector(gland/muscle). Have long axon and many short dendrites
Relay neurones: transmit impulses btw neurones. Have numerous
short processes
15.2 Nerve impulse
Nerve impulse: self propagating wave of electrical activity that
travels along axon membrane, temporary reversal of electrical
potential difference across axon membrane
Resting potential
Movement of ions:
o Phospholipid bilayer of axon plasma membrane prevents Na + and K+ diffusing across
o Ion channel proteins (specific to ion) span phospholipid bilayer, some have gates that can be open/closed at
specific times for facilitated diffusion of Na+ and K+ (some gates are always open, facilitated diffusion can
continuously occur)
o Na+ /K+ pump: carrier protein that actively transports Na + out of axon and K+ in
axon
Resting potential (maintained by active transport): inside of axon is -ve relative to outside
(-50 to -90 mV, -70 in humans), axon polarised
Potential difference due to:
o Na+ actively transported out of axon by Na+ /K+ pump
o K+ actively transported in axon by Na+ /K+ pump
o 3Na+ out 2K+ in, electrochemical gradient made from more Na + in tissue fluid,
more K+ in cytoplasm
o Sodium diffuses back in naturally, potassium back out
, Most channels open to K+, most Na+ channels closed
Action potential (passive diffusion, membrane transmitting nerve impulse)
Stimulus of sufficient size detected by receptors in nervous system, energy cause temporary reversal of changes either
side of axon membrane
o If stimulus is great enough, -65mV inside membrane becomes +40mV (action potential), membrane
depolarised: axon membrane channels (voltage gated) change shape ∴ open/close depending on voltage
across membrane
1. Resting potential: some K voltage gated channels open, Na ones are closed
2. Stimulus energy causes some NA channels to open ∴ Na diffuses into axon, along electrochemical gradient (Na is +ve,
so triggers reversal in potential difference across membrane)
3. Causes more Na to diffuse in, greater influx by
diffusion
4. Action potential goes to +40mV, Na channels
close and K channels open
5. K channels opening, reverses electrical gradient
∴ more K channels open, more K+ diffuses out,
starting repolarisation
6. Outward diffusion of K+ causes temporary
overshoot of electrical gradient (more -ve inside
than outside, than usual = hyperpolarisation)
7. K channels close and Na+ /K+ pump continues to
re-establish resting potential (repolarised)
15.3 Passage of an action potential
action potential remains the same along axon
nothing physically moves, just a depolarisation
acting as stimulus for another one in the next region (conduction)(travelling wave of depolarisation)
Unmyelinated axon
1. at resting potential, Na conc outside axon membrane is high relative to inside, vice v. for K conc,
overall +ve outside than inside ∴axon membrane polarised
2. stimulus causes Na influx ∴ reversal charge on axon membrane ∴ depolarised (action potential)
3. localised electrical currents established by Na influx cause Na channels to open further along axon.
Behind region of depolarisation, Na channels close, K channels open. K + leave axon along
electrochemical gradient
4. action potential/depolarisation is propagated in same way further along axon. K + continue to move
out, to the point that axon membrane behind has returned to original charges state/repolarised
5. Repolarisation allows Na+ to actively transport out, returning axon to its resting potential
Myelinated axon
Fatty myelin sheath around axon acts as an electrical
insulator, preventing action potential forming
Nodes of Ranvier: breaks in myelin, where action
potential can occur ∴ localised circuits arise btw adj
nodes and action potential jumps from node to node
(saltatory conduction)
o Action potential passes along myelinated
neurone faster than unmyelinated neurone of same
diameter
Bc in unmyelinated. Depolarisation can happen all way along ∴ takes longer
15.4 Nerve impulse speed
Action potential doesn’t change size across axon
Factors affecting action potential speed
Myelin sheath: acts as an electrical insulator, preventing action potential forming, action potential jumps from node to
node (saltatory conduction), increases speed of conductance
Axon diameter: greater diameter, faster speed of conductance bc less leakage of ions from a larger axon (leakage
makes membrane potentials harder to maintain)
Temp: affects rate of diffusion of ions ∴higher temp=faster nerve impulse. Affects enzyme controlled reactions, eg
respiration (to make ATP needed for active transport and Na/K pump). Affects speed and strength of muscle
contractions
o Important for ectotherms (cold blooded, body temp varies w/ enviro)
Thresholds “all or nothing”
Nerve impulses: all or nothing responses
Certain level of stimulus/threshold value that triggers action potential
o Below threshold (of any stimulus strength): no action potential/impulse generated ∴ nerve impulse travels
