Nerve cells, otherwise known as neurones, use nerve impulses to communicate with one another and pass information.
Nerve impulses are waves of electrochemical changes across neurones that assist in the formation of an action potential in response to a stimulus.
A nerve impulse will cause a set of physiological changes that occur in a neurone due to a stimulus' mechanical, chemical, or electrical disturbance. This will be propagated across an axon. The nerve impulse is passed to neighbouring neurones over the synaptic cleft and to muscle fibres via neuromuscular junctions through the release of neurotransmitters.
A stimulus describes an internal or external change in the environment. Examples include heat, pressure, and sound.
A synaptic cleft (also known as a synapse) describes the junction between two or more neurones. Meanwhile, a neuromuscular junction describes the gap between a neurone and a muscle fibre.
The structure of a neurone
There are several types of neurons in the body, including sensory, motor, and interneurones. Below are some key structural features:
- Cell body: contains the cell's organelles, namely the nucleus, rough endoplasmic reticulum (rER), and mitochondria.
- Dendrites: located in the 'receiving' end of a neurone, dendrites branch out from the cell body and create synapses with other neurones. This is where the receptors of a neurone in charge of starting the nerve impulse will usually be located.
- Myelin sheath and Schwann cells: Schwann cells cover the length of the axon to create the myelin sheath. This myelination provides insulation for the propagation of action potentials.
- Nodes of Ranvier: junctions between Schwann cells.
- Axon: a fibre extension which joins the cell body at an axon hillock.
- Axon terminals: the terminals where action potentials are transmitted to other neurones.
Fig. 1 - Basic neuron structure
Interneurones are also known as relay neurones. They are the same thing!
Difference between sensory and motor neurones
Let's summarise some of the differences between sensory and motor neurones.
| Sensory neurone | Motor neurone |
| Carries sensory impulses from the sensory organ to the CNS | Carries motor impulses from CNS to effectors |
| Unipolar - contains a single extension from the cell body | Multipolar - many dendrites |
| Relatively short axon | Relatively long axon |
| Found in skin, nose, ears, tongue, and eyes | Found in glands and muscles |
Interneurones have a cell body in the middle of the axon, unlike motor and sensory neurones. They are found exclusively in the CNS and are the link point between sensory and motor neurones.
Fig. 2 - Difference between motor, sensory and interneurons
Factors affecting nerve impulse conduction
Let's cover the main factors affecting nerve impulse propagation speed.
- Neurone myelination: this increases the speed of signal transmission, in comparison to unmyelinated neurones, by allowing the nerve impulse to effectively 'jump' from one Node of Ranvier to another Node of Ranvier, where no myelin sheath is present. This is known as saltatory conduction.
An impulse will travel at a speed of up to 150 m/s in myelinated neurones, while only 0.5 to 10 m/s in unmyelinated neurones.
So why would you have neurones that are unmyelinated? Surely this would be inefficient? Some neurones will pass signals that do not travel very far, and myelination would not increase the speed by much. Exerting energy to myelinate cells is costly and "not worth" the energy required. As well as this, if a neuron's axon diameter is really small, the myelin sheath will add an unnecessary layer. Lastly, some signals do not need to be propagated at such high speeds, for example, long-lasting pain.
- Temperature: an increase in temperature increases the conduction speed because the membrane becomes more permeable to ions. The movement of certain ions from the outside to the inside of the neurone and vice versa is what starts the nerve impulse.
Warm-blooded animals maintain a constant body temperature by homeostasis (the maintenance of a constant internal environment), so temperature is not too important as a factor. However, in cold-blooded animals, when the environmental temperatures decrease, the propagation of nerve impulses will be comparatively slower.
- Axon diameter: axons that have larger diameters will conduct electrical impulses faster than ones with smaller diameters. The surface area to volume ratio is the culprit! Axons that are larger in diameter will have lower diffusion rates of ions (less leakage and, therefore, faster conduction).
Read our Size to Volume Ratio article to learn more about the phenomenon.
Nerve impulse transmission
Neurones send and receive impulses to and from the CNS and are important in coordinating a response to a stimulus. Nerve impulses (the way neurones coordinate this response) are transmitted as action potentials along the neurone's axon. The steps of an action potential include:
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Resting phase
Fig. 3 - Action potential scheme
The membrane potential is the difference in charge between both sides of the plasma membrane.
Because the membrane is non-polar, ions cannot flow through freely and need proteins (most commonly ion channels) to let them through. The stimulus that starts a nerve impulse activates certain proteins in the membrane that allow certain ions to move across it, causing a change in membrane potential. Depending on the phase of the action potential, different proteins will be activated, and the ions will flow differently, making the membrane potential change in different directions.
Nerve impulse transmission mechanism
Nerve impulses can travel in two ways along the neurone's axon:
- Saltatory conduction
- Continuous conduction
Saltatory conduction only occurs in myelinated neurones where the impulse jumps from one Node of Ranvier to the next. Conversely, continuous conduction occurs in unmyelinated axons, where the impulse travels along the whole length of the axon.
When you consider the steps of an action potential, saltatory conduction is more energy efficient. This is because repolarisation requires ATP, and due to the presence of the nodes, you essentially reduce the amount of repolarisation needed compared to continuous conduction.
Transmission over the synapse
As we mentioned earlier, a synapse describes the junction between a neurone and another neurone. This site contains three components:
- Pre-synapse (the axon terminal site)
- Synaptic cleft (the gap between the neurones)
- Post-synaptic membrane (the dendrite membrane of the receiving neurone)
For the information to be transferred, neurotransmitters are needed.
Neurotransmitters are produced in the neurone cell body. They are chemical messengers that transmit the nerve impulse across the synaptic cleft. They are released from the pre-synaptic membrane and diffuse across the synaptic cleft to reach the post-synaptic membrane, where they bind to specific receptors.
As a result of the neurotransmitters being released from the pre-synaptic membrane and receptors located on the post-synaptic membrane, nerve impulse transmission occurs in one direction only! This means the nerve impulse cannot travel back to the originating neurone.
Have a read of our articles on Types of Synapse and Transmission across a Synapse for a deeper dive!
Nerve impulses and the stimulus-response model
Let's quickly recap how a nerve impulse travels to produce a response in effector cells and link that to the molecular aspect of nervous impulse that we just covered. A stimulus-response model can be used to describe this.
- A stimulus is detected by a receptor (remember that they are usually on the dendrites of the neurones).
- Receptors transform the stimulus to a nerve impulse (if the stimulus reaches a certain threshold value) by letting certain ions flow in or out of the neurone.
- The nerve impulse travels to the central nervous system (CNS). This consists of the brain and the spinal chord. It travels by "jumping" from one neurone to the next: the stimulus that the first neurone receives is changed into an electrical charge (the nerve impulse) that travels through the axon. In the axon terminal, the electrical change is transformed into a chemical change by the release of neurotransmitters, which are then the stimulus for the next neurone. The process repeats itself until the nerve impulse reaches the effector cells.
- The CNS generates a response (in the form of nerve impulses)
- The effectors react to the signals from the CNS by releasing substances (glands) or contracting or relaxing (muscles)
Effectors are cells that actively respond to a stimulus. This includes muscle cells and glands.
Nerve Impulses - Key takeaways
- Nerve impulses are waves of electrochemical changes across neurones that assist the formation of an action potential in response to a stimulus.
- Neurones send and receive impulses to and from the central nervous system.
- Factors affecting impulse conduction include temperature, axon diameter, and the myelination of neurones.
- The main phases of an action potential include depolarisation, repolarisation, hyperpolarisation, and the resting phase.
- Nerve impulses are transmitted over a synapse to other neurones via neurotransmitters.
Explanations 
Exams 
Magazine 
