What you'll learn
Nervous and hormonal coordination covers the two systems by which organisms detect change and respond to it. At CAPE level the nervous system is treated at the level of membrane potentials: you must explain how a resting potential is established, how an action potential is generated and propagated, why the refractory period matters, and how synapses transmit and integrate signals. The endocrine system is treated alongside it, with the two compared systematically. By the end of this topic you should be able to describe neurone structure, explain resting and action potentials in terms of ion movements, explain saltatory conduction, describe synaptic transmission and summation, compare nervous and hormonal coordination, and describe the action of adrenaline and the second messenger model.
Key terms and definitions
Neurone — a nerve cell specialised to transmit electrical impulses
Resting potential — the potential difference across the membrane of a neurone at rest, about minus 70 millivolts
Action potential — the brief reversal of membrane potential that constitutes a nerve impulse
Depolarisation — the reduction and reversal of the potential difference towards positive values
Repolarisation — the restoration of the negative potential difference
Hyperpolarisation — an overshoot making the inside more negative than the resting potential
Threshold — the level of depolarisation that must be reached for an action potential to be generated
All-or-nothing principle — an action potential is either generated at full size or not at all
Refractory period — the period following an action potential during which another cannot be generated
Myelin sheath — the insulating layer formed by Schwann cells around some axons
Node of Ranvier — a gap in the myelin sheath where the membrane is exposed
Saltatory conduction — the jumping of the impulse from node to node along a myelinated axon
Synapse — the junction between two neurones
Neurotransmitter — the chemical released at a synapse to transmit the signal
Second messenger — a molecule inside the cell that relays a hormonal signal from a surface receptor
Core concepts
Neurone structure
A sensory neurone carries impulses from receptor to central nervous system, with a cell body on a side branch. A motor neurone carries impulses from central nervous system to effector, with a cell body at one end bearing many dendrites. A relay or intermediate neurone connects the two within the central nervous system and is typically short with no myelin.
All have a cell body containing the nucleus and numerous mitochondria and ribosomes; dendrites receiving signals; and an axon conducting the impulse.
Many vertebrate axons are wrapped in a myelin sheath formed by Schwann cells, interrupted at intervals by nodes of Ranvier.
The resting potential
At rest the inside of the axon is negative relative to the outside, typically by about 70 millivolts, and the membrane is described as polarised.
Two mechanisms establish this.
The sodium–potassium pump actively transports three sodium ions out of the axon for every two potassium ions it moves in, using ATP. Since more positive ions leave than enter, the inside becomes more negative, and a concentration gradient is established with sodium high outside and potassium high inside.
Differential membrane permeability then reinforces it. The membrane is far more permeable to potassium than to sodium at rest, because many potassium channels are open while most sodium channels are closed. Potassium therefore diffuses out down its concentration gradient faster than sodium leaks in, adding to the negative charge inside.
Large negatively charged proteins within the axon, which cannot cross the membrane, also contribute.
The action potential
An action potential is a brief reversal of the membrane potential, and its stages must be given in order with the ion movements specified.
A stimulus causes some sodium voltage-gated channels to open, and sodium ions diffuse in, making the inside less negative. This is depolarisation.
If the depolarisation reaches the threshold of about minus 55 millivolts, more sodium voltage-gated channels open. This is positive feedback: sodium entry causes further depolarisation, which opens more channels, which admits more sodium. Sodium floods in and the potential rises rapidly to about plus 40 millivolts, so the membrane is briefly reversed.
At this point the sodium channels close and potassium voltage-gated channels open. Potassium ions diffuse out down their concentration gradient, and the inside becomes negative again. This is repolarisation.
The potassium channels are slow to close, so potassium continues to leave and the potential briefly falls below the resting value. This is hyperpolarisation.
The sodium–potassium pump then restores the original ion distribution and the resting potential is re-established.
The all-or-nothing principle states that if the threshold is reached an action potential of full size is generated, and if it is not reached no action potential occurs at all. The size of the action potential does not vary with the strength of the stimulus.
Stimulus strength is instead encoded in two ways: a stronger stimulus generates action potentials at a higher frequency, and it stimulates a greater number of neurones. This is a standard examination point and candidates frequently miss the second mechanism.
The refractory period
Immediately after an action potential the sodium channels are inactivated and cannot reopen, so no further action potential can be generated however strong the stimulus. This is the absolute refractory period, followed by a relative refractory period in which a stronger than normal stimulus is required.
The refractory period has three functional consequences that are commonly examined.
It ensures the impulse travels in one direction only, since the region behind the action potential cannot be re-excited.
It ensures action potentials remain discrete and do not merge into one another.
It limits the frequency of impulses, which sets an upper limit on the intensity that can be signalled.
Propagation and saltatory conduction
In an unmyelinated axon, the influx of sodium ions at one point causes local currents that depolarise the adjacent region to threshold, generating an action potential there. The process repeats along the axon, so the impulse travels as a continuous wave.
In a myelinated axon, the myelin sheath is an electrical insulator and ions cannot cross it. Depolarisation can therefore occur only at the nodes of Ranvier, where the membrane is exposed.
Local currents flow from one node to the next, so the action potential effectively jumps from node to node. This is saltatory conduction, and it is very much faster than continuous conduction because far fewer action potentials need to be generated over the same distance. It is also more energy-efficient, since less ion pumping is required to restore the resting potential.
Three factors affect conduction speed: myelination, which increases it greatly; axon diameter, with wider axons conducting faster because they offer less resistance to the flow of local currents; and temperature, with higher temperatures increasing the rate of diffusion of ions and of active transport, up to the point at which proteins denature.
Synapses
A synapse is the junction between two neurones, separated by a gap called the synaptic cleft.
Transmission proceeds as follows. An action potential arrives at the presynaptic knob, causing voltage-gated calcium channels to open. Calcium ions diffuse in.
The influx of calcium causes synaptic vesicles containing neurotransmitter to move to and fuse with the presynaptic membrane, releasing neurotransmitter into the cleft by exocytosis.
The neurotransmitter diffuses across the cleft and binds to specific receptor proteins on the postsynaptic membrane.
Binding opens sodium channels in the postsynaptic membrane, so sodium diffuses in and depolarises it. If the threshold is reached, an action potential is generated in the postsynaptic neurone.
The neurotransmitter is then removed, either hydrolysed by an enzyme or reabsorbed into the presynaptic knob, which prevents continuous stimulation. For acetylcholine the enzyme is acetylcholinesterase, and the products are reabsorbed and resynthesised using ATP from the numerous mitochondria in the presynaptic knob.
Synapses have several important functions. They ensure one-way transmission, since only the presynaptic knob contains vesicles and only the postsynaptic membrane has receptors. They allow one neurone to communicate with many, and many to converge on one. They allow integration of signals, and they are the basis of learning through the strengthening of frequently used pathways.
Summation explains how sub-threshold signals can still produce a response. In temporal summation, several impulses arrive in quick succession from the same presynaptic neurone, and the neurotransmitter accumulates until the threshold is reached. In spatial summation, several different presynaptic neurones release neurotransmitter simultaneously onto the same postsynaptic neurone, and their combined effect reaches the threshold.
Inhibitory synapses act differently, opening chloride or potassium channels so that the postsynaptic membrane becomes hyperpolarised and is less likely to reach the threshold.
Comparing nervous and hormonal coordination
The two systems differ systematically, and a comparison table is worth holding ready.
Nervous coordination uses electrical impulses along neurones, with chemical transmission only at synapses. Hormonal coordination uses chemicals carried in the blood.
Nervous responses are very rapid, within milliseconds; hormonal responses are slower, taking seconds to days.
Nervous effects are short-lived; hormonal effects are longer-lasting.
Nervous signals are precisely targeted to specific effectors; hormones reach all tissues but affect only those with the appropriate receptors, so the effect is more widespread.
Nervous coordination suits rapid responses to sudden change; hormonal coordination suits longer-term processes such as growth, development and reproduction.
Hormones and the second messenger model
A hormone is produced by an endocrine gland and secreted directly into the blood, travelling to target cells that possess complementary receptors.
Steroid hormones are lipid-soluble and pass through the cell surface membrane, binding to receptors inside the cell and acting directly on the DNA to alter gene expression.
Protein and peptide hormones cannot cross the membrane and act through a second messenger. Adrenaline is the standard example.
Adrenaline binds to a receptor on the cell surface membrane of a liver cell. The receptor changes shape, activating the enzyme adenylyl cyclase on the inner surface of the membrane. Adenylyl cyclase converts ATP into cyclic AMP, which acts as the second messenger inside the cell. Cyclic AMP activates a cascade of enzymes, ultimately activating glycogen phosphorylase, which hydrolyses glycogen to glucose. Glucose is released into the blood.
The cascade amplifies the signal enormously: a single adrenaline molecule leads to the production of many cyclic AMP molecules, each activating enzymes that act on many substrate molecules, so a very small hormone concentration produces a very large response. This amplification is the key advantage of the second messenger system and is what questions on it usually target.
Adrenaline's other effects prepare the body for activity: increased heart rate and stroke volume, dilation of airways, vasoconstriction in the gut and vasodilation in muscle, and dilation of the pupils.
Worked examples
Example 1: Explaining the resting potential (5 marks)
Explain how a resting potential of about minus 70 millivolts is established and maintained.
The sodium–potassium pump actively transports three sodium ions out of the axon for every two potassium ions moved in, using ATP. Because more positive ions are removed than are brought in, the inside becomes more negative than the outside.
This also establishes concentration gradients, with a high concentration of sodium outside the axon and a high concentration of potassium inside.
The membrane is far more permeable to potassium than to sodium at rest, since most potassium channels are open while most sodium channels are closed. Potassium therefore diffuses out down its concentration gradient more rapidly than sodium leaks in, which makes the inside still more negative.
Large negatively charged protein molecules inside the axon cannot cross the membrane and add to the negative charge. The combined effect is a potential difference of about minus 70 millivolts, with the inside negative relative to the outside.
Example 2: Explaining saltatory conduction (4 marks)
Explain why a myelinated axon conducts impulses faster than an unmyelinated axon of the same diameter.
The myelin sheath is an electrical insulator, so ions cannot cross the membrane where it is present. Depolarisation can therefore occur only at the nodes of Ranvier, where the axon membrane is exposed.
Local currents flow from one node to the next, so an action potential generated at one node depolarises the next node to threshold directly. The impulse effectively jumps from node to node rather than being regenerated continuously along the whole membrane.
Because far fewer action potentials have to be generated over a given distance, and each takes time, the impulse travels much faster. In an unmyelinated axon an action potential must be generated at every point along the membrane, which is considerably slower.
Example 3: Explaining summation (4 marks)
Explain how a stimulus too weak to trigger an action potential at a single synapse can nevertheless produce a response.
A single weak impulse releases only a small quantity of neurotransmitter, which produces insufficient depolarisation of the postsynaptic membrane to reach the threshold, so no action potential is generated.
In temporal summation, several impulses arrive from the same presynaptic neurone in rapid succession. Each releases neurotransmitter before the previous quantity has been removed, so the neurotransmitter accumulates in the cleft, more receptors are occupied, more sodium channels open and the depolarisation reaches the threshold.
In spatial summation, several different presynaptic neurones release neurotransmitter onto the same postsynaptic neurone at the same time. Their individual contributions add together, and the combined depolarisation reaches the threshold.
In either case an action potential is then generated, so a response occurs even though no single input was sufficient alone.
Common mistakes and how to avoid them
The most frequent error is stating that a stronger stimulus produces a larger action potential. Action potentials are all-or-nothing; strength is encoded by frequency and by the number of neurones stimulated.
Students often say that sodium ions are actively transported into the axon during depolarisation. They diffuse in through voltage-gated channels; active transport occurs only in the pump restoring the resting state.
Another common slip is describing the impulse as travelling along the axon as a current. The action potential is regenerated at each point, or at each node in a myelinated axon.
Many candidates omit calcium ions from synaptic transmission, which is a reliable mark.
In comparison questions, answers frequently give only speed. Duration, precision of targeting and the mode of transmission are separate points.
Finally, candidates often describe cyclic AMP as the hormone. It is the second messenger produced inside the cell in response to the hormone binding at the surface.
Exam technique for "Nervous and hormonal coordination"
Describe potentials in terms of specific ions and specific channels. Saying that charged particles move is not enough; name sodium, potassium and the type of channel.
When describing an action potential, follow the sequence of the graph and label each phase, since marks are allocated per phase.
For synapse questions, give the full sequence including calcium influx, vesicle fusion, exocytosis, receptor binding and neurotransmitter removal.
In comparison questions, use the same criteria for both systems and present them in parallel.
For the second messenger model, name each component in the cascade and finish with the amplification, which is usually the final mark.
Quick revision summary
The resting potential of about minus 70 millivolts arises from the sodium–potassium pump moving three sodium out for every two potassium in, and from the membrane's greater permeability to potassium. An action potential begins when a stimulus depolarises the membrane to a threshold of about minus 55 millivolts, opening voltage-gated sodium channels so sodium floods in and the potential reaches about plus 40 millivolts; sodium channels then close, potassium channels open and potassium leaves, repolarising and briefly hyperpolarising the membrane before the pump restores the resting state. Action potentials are all-or-nothing, with stimulus strength encoded by frequency and by the number of neurones recruited, and the refractory period ensures one-way travel and discrete impulses. Myelination allows saltatory conduction between nodes of Ranvier, greatly increasing speed, as do larger diameter and higher temperature. At a synapse, calcium influx triggers vesicle fusion and neurotransmitter release, binding opens sodium channels in the postsynaptic membrane, and the transmitter is then hydrolysed or reabsorbed; temporal and spatial summation allow sub-threshold inputs to combine. Nervous coordination is fast, brief and precisely targeted, while hormonal coordination is slower, longer-lasting and widespread. Adrenaline acts through adenylyl cyclase and cyclic AMP as second messenger, producing an amplifying enzyme cascade.