The nervous system and nerve impulses (8.1, 8.3, 8.4)
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Nerve cells are called neurones.
They have a cell body, dendrites which carry nerve impulses towards the cell body, and an axon that carries nerve impulses away from the cell body.
Nerves are bundles of axons.
Sensory neurones carry impulses from sensory cells to the CNS (central nervous system).
Relay neurones connect sensory neurones to motor neurones within the CNS.
They have short dendrites that carry nerve impulses towards the cell body and a short axon that carries nerve impulses away from the cell body. Their short length allows rapid transmission of nerve impulses over the short distances within the CNS.
Motor neurons carry impulses from the CNS to effectors (such as muscles or glands). For this reason, they are also known as effector neurones.
The cell body is located inside the CNS.
Myelin (layers of plasma membrane) is an electrical insulator.
The myelin sheath around a neurone’s axon is made up of Schwann cell membranes. They insulate the axon and speed up impulse transmission.
Myelinated neurones conduct up to 100× faster than neurones that are unmyelinated.
Nodes of Ranvier are gaps between the Schwann cells making up the myelin sheath.
Electrical impulses jump between nodes, which speeds up signal transmission. This is known as saltatory conduction.
Electrical impulses move along a neurone by changing the potential difference (electrical voltage) across its membrane.
When a neurone is not transmitting an action potential, it is at its resting potential.
Neurones maintain a resting potential difference of approximately In this state, the membrane is polarised.
The inside of the neurone membrane is more negatively charged than the inside of the membrane.
Neurone membranes contain sodium–potassium pumps which create high potassium ion () concentration inside the axon and high sodium ion () concentration outside.
ions diffuse out of the axon down their concentration gradient. This increases the electrochemical gradient, attracting ions back into the axon.
At the concentration gradient balances the electrochemical gradient, meaning there is no net movement of ions and the membrane is polarised.
Energy from a stimulus opens voltage–dependent gated sodium channels, allowing to enter the axon down the electrochemical gradient. This triggers more sodium channels to open (positive feedback), and the membrane’s potential difference reaches
The voltage–dependent gated sodium channels close and voltage–dependent gated potassium channels open, so diffuses out of the axon down the electrochemical and concentration gradient. The cell becomes negatively charged again — it is hyperpolarised.
Potassium channels then close, and the resting potential is restored as ions diffuse back into the axon via channel proteins.
After an action potential, voltage–gated sodium channels remain closed for a short time so there can’t be another action potential. This is called the refractory period.
Action potentials travel in one direction and cannot overlap because of the refractory period.
There are several factors that can affect the speed of an impulse, including:
- Axon diameter: the larger the diameter, the faster the transmission, as there is less resistance to the movement of ions in the axon.
- Temperature: the higher the temperature, the faster the transmission, as the rate of diffusion is increased.
- Myelin sheath: when present, transmission is faster owing to saltatory conduction.
If the threshold value is reached by a stimulus then an action potential is triggered. If this value is not met, there is no action potential.
The size of an action potential is always the same, regardless of the stimulus size.
This is known as the all-or-nothing principle, there is either an action potential or not.
A more intense stimulus results in more frequent action potentials, not bigger ones.
Threshold value in neurones can vary, so some are easier to depolarise than others.
A synapse is a junction where two neurones meet.
Synapses separate presynaptic and postsynaptic membranes, with a gap in between known as the synaptic cleft. Impulses cannot jump across this cleft.
The presynaptic membrane contains synaptic vesicles that contain neurotransmitters.
Impulse transmission at a synapse happens in the following way:
- Action potential arrives at the presynaptic bulb / knob.
- Voltage–gated channels open and calcium ions enter the presynaptic bulb / knob.
- Vesicles containing neurotransmitter fuse with the presynaptic bulb / knob membrane, releasing neurotransmitter into the synapse via exocytosis.
- Neurotransmitter moves across the synapse and fuses with receptors on the postsynaptic membrane.
- channels open, leading to depolarisation and generation of a new action potential in the postsynaptic neurone.
- If depolarisation exceeds the threshold at the postsynaptic membrane, then an action potential will be produced and propagated.
Acetylcholinesterase is an enzyme that breaks down acetylcholine that is left in the synaptic cleft. This prevents continuous stimulation.
The neurotransmitter is hydrolysed, and products are returned to the presynaptic bulb / knob to be reformed and reused. This process requires ATP.
Synapses are responsible for:
- Coordination of responses based on information received from multiple neurones.
- Control of nerve pathways.
The extent to which depolarisation occurs at the postsynaptic membrane is dependent on:
- The frequency of the impulses / number of impulses received.
- The type of synapse: inhibitory (less likely to depolarise) or excitatory (more likely to depolarise).
Excitatory synapses make the postsynaptic membrane more permeable to sodium ions, causing depolarisation.
At excitatory synapses, summation can occur:
- Spatial summation occurs when neurotransmitters released from multiple presynaptic neurones act on a single postsynaptic neurone at the same time. The combined depolarisations may reach the threshold and trigger an action potential.
- Temporal summation occurs when one presynaptic neurone releases neurotransmitter repeatedly in rapid succession. The depolarisations build up over time and, if the threshold is reached, an action potential is triggered.
Impulses arriving at inhibitory synapses open channels and channels; chloride ions will diffuse into the postsynaptic membrane and potassium ions will diffuse out.
The potential difference across the membrane is increased (hyperpolarisation), making it less likely for the membrane to reach the threshold required for depolarisation and generating an action potential.