Neuronal communication (5.1.3)
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Sensory receptors are specialised cells that detect specific types of stimuli or changes in the environment.
They are located in sensory organs, for example, olfactory receptors in the nose and thermoreceptors in the skin.
Sensory receptors are specific to one type of stimulus; for example, rod and cone cells in the eye respond to changes in light. This ensures the brain can accurately identify and interpret different types of stimuli.
Sensory receptors act as transducers, converting a stimulus into an electrical signal (a nerve impulse). Neurones transmit electrical impulses to the CNS (central nervous system) where the brain coordinates a response via an effector (e.g., muscle).
There are many types of sensory receptors, each responding to a specific stimulus. These receptors can occur alone or be grouped together in sensory organs to increase efficiency.
Pacinian Corpuscles are mechanoreceptors (pressure receptors) that detect pressure changes in the skin.
They are made up of rings of connective tissue and gel wrapped around the end of a sensory neurone.
Changes in pressure deform the rings of connective tissue, which compress the nerve ending, generating a nerve impulse.
The mechanism of action of a Pacinian corpuscle is:
- Pressure applied to the skin deforms the layers of gel and connective tissue surrounding the sensory neurone ending.
- This deformation stretches the membrane of the sensory neurone, causing stretch–mediated sodium ion channels to open.
- Sodium ions diffuse into the neurone, causing depolarisation of the membrane and production of a generator potential.
- If the generator potential reaches the threshold, voltage–gated sodium channels open.
- An action potential is generated and travels along the sensory neurone to the central nervous system.
The nervous system enables the detection of and response to changes in an organism’s internal and external environment.
It coordinates responses to these stimuli by sending electrical impulses from receptors, along specialised nerve cells (neurones), to effectors.
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 receptors to the central nervous system, where they synapse with relay or motor neurones.
They have long dendrons that carry an impulse from the receptor to the cell body. The short axon transmits an impulse to the CNS.
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 neurones carry impulses from relay neurones (and sometimes directly from sensory neurones) to effectors such as muscles or glands.
They have many short dendrites that carry nerve impulses towards the cell body. They have a single long axon that carries nerve impulses away from the cell body in the CNS to an effector.
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 voltage across its membrane.
The membrane is in one of two possible states, either resting potential or action potential.
When a neurone is not transmitting an action potential, it is at its resting potential.
Neurones maintain a resting potential of approximately In this state, the membrane is polarised.
The outside of the neurone membrane is more positively charged than the inside of the membrane.
Some ion channels are gated, and others are always open across the cell membrane of neurones.
The sodium–potassium pump uses ATP to move three ions out for every two ions pumped in, resulting in more sodium outside the cell and more inside the cytoplasm.
ions diffuse out of the neurone down the electrochemical gradient, but ions cannot diffuse in because the gated sodium ion channels are closed.
More positively charged ions are found outside the axon, so the inside of the neurone is negatively charged relative to the outside at resting potential.
Depolarisation is when the energy of a stimulus reverses the axon membrane charge. The stimulus changes the shape of voltage-gated ion channels, causing the membrane potential to rise to
At the resting potential, voltage–gated sodium channels are closed, and potassium ion channels are open.
Energy from a stimulus opens voltage–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–gated sodium channels close and voltage–gated potassium channels open, so diffuses out of the axon. continues to move out of the axon causing hyperpolarisation. Potassium channels then close, and the sodium–potassium pump restores the resting potential (repolarisation) by moving out and into the axon.
An action potential in the sensory receptor is triggered by a stimulus.
A wave of depolarisation spreads along the axon.
Sodium ions diffuse along the axon, triggering depolarisation in the next section. This is how action potentials are propagated along a neurone.
In non-myelinated neurones, at the resting potential, there are sodium ions outside and potassium ions inside the axon.
An action potential depolarises the membrane, as sodium ions enter the axon. This causes voltage–gated sodium channels to open (positive feedback) further along the axon. More sodium ions enter, resulting in further depolarisation. This wave travels along the axon.
Behind the new depolarised region, voltage–gated sodium ion channels close and voltage–gated potassium ion channels open. leaves the axon down its concentration gradient and repolarisation occurs as the resting potential is restored.
The neurone is now ready for a new stimulus.
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.
In a myelinated neurone, depolarisation of an axon membrane occurs at the nodes of Ranvier.
The action potentials jump between nodes; this is known as saltatory conduction. This is more efficient and speeds up transmission as it is faster than a wave of depolarisation going along the whole length of the axon.
Repolarisation needs ATP, so this also reduces the amount of ATP that is required.
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 between two neurones or a neurone and an effector. An impulse is transmitted across a synapse by chemical neurotransmitters.
Cholinergic synapses are made up of several parts, including:
- Synaptic cleft – small gap (~20–30 ) between neurones.
- Presynaptic neurone – ends in a presynaptic bulb / knob that contains synaptic vesicles filled with neurotransmitter (e.g., acetylcholine, GABA).
- Postsynaptic neurone – with specific receptor proteins for neurotransmitters.
A neurotransmitter is a type of chemical messenger produced by a neurone and released from the presynaptic knob when a nerve impulse arrives.
At a synapse, the electrical impulse is converted into a chemical signal carried across the synaptic cleft by neurotransmitters.
Acetylcholine is an example of an excitatory neurotransmitter.
GABA is an example of an inhibitory transmitter.
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 ensure the unidirectional transmission of impulses.
An impulse can be transmitted from one neurone to several, resulting in several responses arising from one stimulus.
An impulse can also be transmitted from several neurones to one, meaning that several stimuli can trigger a single coordinated response.
Spatial summation occurs when neurotransmitters released from multiple presynaptic neurones act on a single postsynaptic neurone at the same time. The combined effect of their generator potentials may reach the threshold and trigger an action potential.
Temporal summation occurs when a single presynaptic neurone releases neurotransmitter repeatedly in rapid succession. The generator potentials build up over time and, if the threshold is reached, an action potential is triggered.