Neural signalling

The nervous system is made of cells called neurons. The brain and spinal cord form the central nervous system, and peripheral nerves link it with the rest of the body.

A neuron has a cell body, containing cytoplasm and a nucleus, with long nerve fibres projecting from it. Dendrites are many short fibres that bring impulses to the cell body. An axon is one long fibre that carries impulses away. Electrical impulses travel along these fibres.

Sensory neurons bring signals from receptors, relay neurons connect neurons in the CNS, and motor neurons take signals out to effectors, which are muscles and glands.

Watch out: receptors detect stimuli and effectors respond to them.

A membrane potential is a difference in charge across a membrane. A membrane with positive charge on one side and negative on the other is polarized. In a neuron that is not transmitting an impulse, the inside is negative at about −70 mV. This is the resting potential.

• The pump: sodium-potassium pumps use ATP to move three Na⁺ out for every two K⁺ in. This active transport builds a concentration gradient for each ion, and removes more positive charge than it brings in.
• Leakage: the membrane is more permeable to K⁺, so K⁺ leaks out faster than Na⁺ leaks in.
• Proteins: large, negatively charged proteins stay inside the axon.

So a "resting" neuron is not idle. It uses ATP all the time to keep the gradients in place.

A stimulus lets positively charged sodium ions flow into the axon, and the potential reverses from about −70 mV to about +40 mV. Potassium ions then flow out, restoring a negative potential. This brief change is an action potential.

An action potential at one point triggers the next region of the fibre, so the impulse is propagated along it. At any one point it lasts only about two milliseconds.

The impulse is electrical because it involves the movement of charged ions, which changes the voltage across the membrane. It is not a flow of electrons along a wire.

Diameter: a narrow fibre has greater resistance to the flow of ions inside it, so a wider fibre conducts faster. Giant axons help a squid to escape quickly.

Myelin: Schwann cells wrap around the axon, forming a lipid-rich sheath with high electrical resistance. Ions cross the membrane only at the gaps, or nodes of Ranvier, so the impulse jumps from node to node. This is far faster, even in much narrower fibres.

1. An action potential arrives and depolarizes the presynaptic membrane.
2. Calcium channels open, and Ca²⁺ diffuses into the knob.
3. Calcium acts as a signalling chemical inside the neuron, making vesicles move to the membrane.
4. Vesicles fuse with the membrane, releasing neurotransmitter into the cleft.

Use acetylcholine (ACh) as your example. It is used at many synapses, including the neuromuscular junction.

5. ACh diffuses across the cleft, from high to low concentration.
6. It binds to specific transmembrane receptors on the postsynaptic membrane.
7. These receptors are also ion channels. They open, and Na⁺ flows in.
8. This depolarizes the membrane: an excitatory postsynaptic potential. If it is large enough, it starts an action potential in the next cell.
9. The enzyme acetylcholinesterase breaks ACh down to choline and acetate. The channel closes, and the choline is reabsorbed and reused.

Watch out: always say that the neurotransmitter crosses the cleft by diffusion. Remember to mention its removal by an enzyme.