Nerve Impulses

Nerve cells, otherwise known as neurones, use nerve impulses to communicate with one another and pass information.

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.

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

• 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.

• 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.

• 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).

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

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.

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.

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.

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.

1. A stimulus is detected by a receptor (remember that they are usually on the dendrites of the neurones).
2. 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.
3. 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.
4. The CNS generates a response (in the form of nerve impulses)
5. The effectors react to the signals from the CNS by releasing substances (glands) or contracting or relaxing (muscles)