Neural signalling

📚The biology

Neurons

Neurons are cells of the nervous system that carry electrical impulses. Each has:

The resting potential

A membrane potential is a difference in electrical charge (voltage) across a membrane. When a neuron is not transmitting an impulse, its membrane is polarized: the inside is negative relative to the outside, at about −70 mV. This is the resting potential.

It is set up by sodium–potassium pumps, which use energy from ATP to pump ions in opposite directions: 3 Na⁺ out of the neuron and 2 K⁺ in for each ATP used. This produces concentration gradients: a high concentration of Na⁺ outside and of K⁺ inside. The resting potential is negative because:

• the pump moves more positive charges out than in (three for every two);
• the membrane is much more permeable to K⁺ than to Na⁺, because some potassium channels are open at rest, so K⁺ diffuses out down its gradient, taking positive charge with it, while little Na⁺ leaks back in;
• the cytoplasm contains many negatively charged proteins and other ions that cannot leave.

Nerve impulses are action potentials

A nerve impulse is an action potential: a rapid, temporary reversal of the membrane potential, from negative inside to positive inside and back again, that travels along the nerve fibre. It is electrical because it involves the movement of positively charged ions — Na⁺ into the fibre, then K⁺ out — not a flow of electrons. The action potential at one point triggers the next point along, so it is propagated from one end of the fibre to the other.

Speed of nerve impulses

Comparative data of this kind are analysed using correlation. A positive correlation means both variables increase together (conduction speed and axon diameter); a negative correlation means one decreases as the other increases (data sets comparing species can show conduction speed falling as animal size increases). The correlation coefficient \( r \) (from −1 to +1) measures the strength of a linear correlation. The coefficient of determination \( R^{2} \) shows the proportion of the variation in the dependent variable that is accounted for by the independent variable: \( r = 0.9 \) gives \( R^{2} = 0.81 \), so 81% of the variation is explained.

Synapses

A synapse is a junction between two neurons, or between a neuron and an effector cell such as a muscle fibre or gland cell. The cells are separated by a narrow gap, the synaptic cleft, which the impulse cannot jump; a chemical, the neurotransmitter, carries the signal across. (There are also electrical synapses, but the syllabus deals only with chemical ones.)

A signal can only pass one way across a typical synapse, because neurotransmitter is released only from the presynaptic neuron and its receptors are only on the postsynaptic membrane.

Release of neurotransmitter

1. An action potential arrives at the presynaptic terminal and depolarizes its membrane.
2. Voltage-gated calcium channels open and Ca²⁺ ions diffuse into the terminal.
3. Calcium acts as a signalling chemical inside the neuron: it causes vesicles containing neurotransmitter to move to and fuse with the presynaptic membrane.
4. The neurotransmitter is released into the synaptic cleft by exocytosis.

An excitatory postsynaptic potential

Acetylcholine is a neurotransmitter used at many synapses, including neuromuscular junctions.

1. Acetylcholine diffuses across the synaptic cleft.
2. It binds to transmembrane receptors on the postsynaptic membrane. These receptors are ion channels, which open.
3. Na⁺ ions diffuse into the postsynaptic cell, making its inside less negative: a depolarization called an excitatory postsynaptic potential (EPSP).
4. If the depolarization reaches the threshold, an action potential is triggered in the postsynaptic neuron (or the muscle fibre contracts).
5. Acetylcholine is quickly broken down by the enzyme acetylcholinesterase in the cleft, so the signal is brief.

Depolarization and repolarization AHL

An action potential happens in a fixed sequence, controlled by voltage-gated channels:

1. Threshold. A stimulus makes the membrane potential rise from −70 mV. If it reaches the threshold potential (about −55 mV), voltage-gated sodium channels open. Below threshold, nothing happens: the action potential is all-or-nothing.
2. Depolarization. Na⁺ ions rush into the axon down their concentration gradient. Their positive charge makes the inside rapidly positive, to about +30 mV.
3. Repolarization. The sodium channels close, and voltage-gated potassium channels open. K⁺ ions diffuse out, and the inside becomes negative again.
4. The potassium channels are slow to close, so the potential briefly overshoots below the resting level (hyperpolarization). During this refractory period, another action potential cannot start at that point.
5. The sodium–potassium pump restores the original ion concentrations.

Propagation by local currents AHL

At the point of an action potential, the inside of the axon has a high concentration of Na⁺ that has just entered. These ions diffuse along the inside of the axon to the neighbouring, still-resting region, while outside the membrane Na⁺ ions diffuse the other way, towards the region that has lost them. These movements of charge are local currents. They make the adjacent membrane less negative, until it reaches the threshold potential; its sodium channels open, and an action potential occurs there. The process repeats, so the impulse moves along the fibre. Because the region just behind is refractory, the impulse travels in one direction only.

Oscilloscope traces AHL

An oscilloscope trace plots membrane potential (mV) against time (ms). You should be able to relate each part of a trace to cellular events:

The number of action potentials in a given time gives the frequency of impulses. A stronger stimulus does not produce a bigger action potential — they are all the same size — but a higher frequency.

Saltatory conduction AHL

In a myelinated fibre, the myelin sheath is interrupted at intervals by small gaps called nodes of Ranvier. The ion channels and pumps are clustered at the nodes; the myelin between them insulates the membrane so ions cannot cross there. Local currents therefore travel inside the axon from one node to the next, and the action potential occurs only at nodes, appearing to jump from node to node. This is saltatory conduction (from Latin saltare, to jump). It is much faster, and uses less ATP, because ion movement and pumping happen only at the nodes.

Exogenous chemicals at synapses AHL

Inhibitory neurotransmitters AHL

Some neurotransmitters, such as GABA, are inhibitory. They open channels that let negative ions (Cl⁻) into the postsynaptic neuron, or positive ions (K⁺) out. The inside becomes more negative than the resting potential: the postsynaptic membrane is hyperpolarized. This is an inhibitory postsynaptic potential (IPSP), and it moves the neuron further from threshold, making an action potential less likely.

Summation AHL

A single postsynaptic neuron usually receives synapses from many presynaptic neurons, some excitatory and some inhibitory. A single EPSP is usually too small to reach threshold. The postsynaptic neuron adds together all the EPSPs and IPSPs arriving at the same time (and in rapid succession): summation. If the combined effect raises the membrane to threshold, an action potential is fired; if not, nothing happens. This all-or-nothing outcome is how neurons integrate information — in effect, making decisions.

Perception of pain AHL

Pain receptors are free nerve endings of sensory neurons in the skin. Their membranes contain channels for positively charged ions that open in response to stimuli such as high temperature, acid, or chemicals such as capsaicin in chilli peppers. (Capsaicin binds to the same channel that opens at high temperature, which is why chilli feels hot.) Positive ions enter, the membrane reaches the threshold potential, and nerve impulses pass along sensory neurons to the brain, where pain is perceived.

Consciousness as an emergent property AHL

No individual neuron is conscious. Each one simply sums its inputs and fires or does not. Yet the interactions of around 86 billion neurons, each with thousands of synapses, produce consciousness: awareness, thought and experience. Consciousness is an emergent property — a property of the system as a whole that cannot be found in, or predicted from, its individual parts — and another example of how interaction between components creates new properties at a higher level of organization.