Nervous Coordination (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

Information is passed through the nervous system as nerve impulses, electrical signals that travel along neurones.

A neurone that is not transmitting an impulse has a difference in charge across its cell surface membrane, called the resting potential; the inside of the axon is negative relative to the outside, at around -70 mV.

There are different types of ion channel in the axon membrane: some open and close depending on the voltage across the membrane, others do not. The open potassium channels that help set the resting potential are not voltage-dependent and should not be confused with the voltage-gated potassium channels involved in action potentials.

In a resting axon the potential difference is about -70 mV. When a neurone is stimulated the membrane potential changes; if the stimulus is large enough to reach the threshold potential, an action potential results. Action potentials passed along neurones are nerve impulses.

Action potentials are all-or-nothing, so the strength of a stimulus is coded by frequency: a stronger stimulus gives a higher frequency of action potentials, a weaker stimulus gives a lower frequency. The refractory period ensures action potentials are separate and travel in one direction.

Action potentials occur at the point of stimulation and are transmitted along the axon as nerve impulses.

Some neurones are surrounded by a fatty myelin sheath formed from Schwann cells.

Use language carefully

• action potentials appear to jump between nodes of Ranvier, not between neurones.
• Explain that action potentials occur only at the nodes, so the impulse appears to jump between them in saltatory conduction, which is faster.

Be prepared to apply this to unfamiliar contexts, for example impulse conduction data from different species.

After an action potential the axon recovers and is unresponsive; this is the refractory period, so there is a minimum time between action potentials.

The junctions between cells in the nervous system are synapses. A synapse has a presynaptic cell (the neurone that carries a nerve impulse towards the synapse), the synaptic cleft, and the postsynaptic cell.

Synapses that use acetylcholine (ACh) are cholinergic synapses. Sequence: an action potential arrives at the presynaptic cell; voltage-gated calcium ion channels open and Ca²⁺ diffuse in; vesicles of ACh fuse with the presynaptic membrane and ACh is released by exocytosis into the synaptic cleft; ACh diffuses across and binds to receptors on the postsynaptic membrane; sodium ion channels open, Na⁺ enter and the membrane depolarises; if threshold is reached an action potential is generated; acetylcholinesterase then breaks down ACh so the response stops and the products are reabsorbed.

Use precise language

• say acetylcholine is released from the presynaptic membrane into the synaptic cleft, not 'chemicals are released into the synapse'.
• Impulses pass in one direction because neurotransmitter is released only from the presynaptic side and receptors are only on the postsynaptic membrane.

The first few impulses may not release enough neurotransmitter, but if stimulation continues the quantity in the cleft builds up until threshold is reached.

Some synapses are excitatory, causing depolarisation; others are inhibitory: they prevent a new action potential in the postsynaptic cell by causing hyperpolarisation, e.g. by opening chloride or potassium channels.

Drugs may affect synaptic transmission, for example by mimicking a neurotransmitter, blocking receptors, stopping reabsorption or breakdown of neurotransmitter, or changing its release. For example, when the postsynaptic neurone is overstimulated, producing feelings of pleasure. Cannabinoids bind to receptors on the presynaptic membranes of neuromuscular junctions, causing calcium ion channels to close; release of neurotransmitter is reduced, weakening muscle contraction.

Neuromuscular junctions are specialised synapses at the junctions between neurones and muscles. The presynaptic cell is a motor neurone and the postsynaptic cell is a muscle fibre; the neurotransmitter is ACh.

A neuromuscular junction is a specialised cholinergic synapse between a motor neurone and a muscle fibre. Transmission follows the same early steps as other cholinergic synapses (action potential arrives, calcium ions enter, ACh is released into the cleft and binds to receptors on the postsynaptic membrane, the sarcolemma). Then:

As neuromuscular junctions are a type of synapse, they share many features with cholinergic synapses: in both, calcium ions trigger the release of neurotransmitter, acetylcholine is the neurotransmitter, and AChE removes it. Differences are summarised below.

Because every action potential in the motor neurone releases enough ACh to depolarise the sarcolemma, a signal from the nervous system reliably leads to a muscle response.