- Neurons communicate with each other via electrical events called ‘action potentials’ and chemical neurotransmitters.
- At the junction between two neurons (a synapse), an action potential causes neuron A to release a chemical neurotransmitter.
- The neurotransmitter can either help (excite) or hinder (inhibit) neuron B from firing its own action potential.
- In an intact brain, the balance of hundreds of excitatory and inhibitory inputs to a neuron determines whether an action potential will result.
Neurons are the fundamental units of the brain and nervous system, the cells responsible for receiving sensory input from the external world, for sending motor commands to our muscles, and for transforming and relaying the electrical signals at every step in between. More than that, their interactions define who we are as people. Having said that, our 86 billion neurons do interact closely with other cell types, broadly classified as glia (these may actually outnumber neurons, although it’s not really known). Still, as a quick introduction to neuroscience fundamentals, it’s probably best to start by knowing what neurons are and how they talk to each other. To understand this, we’re going to need a little bit of jargon, but knowing what these concepts are is essential to appreciating exactly how the brain works at the level of its constituent neurons.
There are two main types of neurons in the brain – excitatory neurons, which make up ~80% of the population, and inhibitory neurons, which comprise the remaining 20%. We’ll get to why some are called excitatory and others inhibitory a bit later, but it basically has to do with the effect they have on their target neurons.
A feature of neurons that distinguishes them from many other cell types of the body is the numerous long, thin processes that emanate from their cell bodies (Fig. 1). These processes are of two types: axons and dendrites. Broadly speaking (there are exceptions), axons are the transmitters of information, dendrites the receivers; the axon of one neuron will transmit information to the dendrite of a second neuron. Typically, a given axon will contact 100–1000 other neurons, and a neuron’s dendrites will receive inputs from a similarly large number of partners.
Dendrites look a lot like the branches of a tree, which is apt given the word’s origins (from Greek “dendron”, tree). Continuing with the tree analogy, the dendrites of most cells have spines, whose form and function aren’t too dissimilar from the leaves of a tree; spines are small structures (<4 um in length) that serve as the actual site for much of the axonal input to dendrites (Fig. 1), a bit like how leaves are the site for receiving the light needed for photosynthesis. In most neurons, two or more dendrites extend from the cell body (also called the ‘soma’), and each branches a few times before terminating 0.1–1 mm from the cell body. In contrast, a neuron only ever has one axon, but this axon can branch many, many times. Axons can also be very long, much longer than the 1 mm of a dendrite – within the brain, axons connect different brain regions centimetres apart, and axons from the spinal cord must reach the toes. The array of shapes taken by dendrites and axons is truly impressive, even beautiful, and the shape of a neuron can be used to distinguish many cell types in the nervous system (Fig. 2). It can also give clues to the function of a given type of neuron.
Action potentials, aka spikes
So we’ve covered the basic hardware, but how do the axons and dendrites actually work to let neurons communicate? To understand this, we’re going to have to get a little bit technical. Neurons are essentially electrical devices. There are loads of channels sitting in the cell membrane (the boundary between a cell’s inside and outside) that allow positive or negative ions to flow into and out of the cell. Normally, the inside of the cell is more negative than the outside; neuroscientists say that the inside is around -70 mV with respect to the outside, or that the cell’s membrane potential is -70 mV. This membrane potential isn’t static. It’s constantly going up and down (see Fig. 3), depending mostly on the inputs coming from the axons of other neurons. Some inputs make the neuron’s membrane potential become more positive (or less negative, e.g. from -70 mV to -65 mV), and others do the opposite. These are respectively termed excitatory and inhibitory inputs, as they promote or inhibit the generation of action potentials (the reason some inputs are excitatory and others inhibitory is that different types of neuron release different neurotransmitters; the neurotransmitter used by a neuron determines its effect).
Action potentials are the fundamental units of communication between neurons and occur when the sum total of all of the excitatory and inhibitory inputs makes the neuron’s membrane potential reach around -50 mV (Fig. 3), a value called the action potential threshold. To keep things short, neuroscientists often refer to action potentials as ‘spikes’, or say a neuron has ‘fired a spike’ or ‘spiked’. The term is a reference to the shape of an action potential as recorded using sensitive electrical equipment (Fig. 3)
The action potential starts in the axon at a specific site near the cell body, and travels quickly through the axonal branches towards the axon terminals, where it causes neurotransmitter to be released into the synapse. This is the textbook description of an action potential, but it turns out that spikes can also travel backwards; as well as going down the axon, a spike can at the same time go back through the soma and, depending on the cell type, all the way to the dendritic tips. This ‘backpropagation’ has important implications for how neurons—in particular the dendrites—process or integrate all the inputs they receive, a fundamental but complicated and intriguing process investigated at QBI by Professor Stephen Williams.
Neurons talk to each other across synapses. When an action potential reaches the presynaptic terminal, it causes neurotransmitter to be released from the neuron into the synaptic cleft, a 20–40 nm gap between the presynaptic axon terminal and the postsynaptic dendrite (often a spine). After travelling across the synaptic cleft, the transmitter will attach to neurotransmitter receptors on the postsynaptic side (Fig. 4), and depending on the neurotransmitter released (which is dependent on the type of neuron releasing it), particular positive (e.g. Na+, K+, Ca+) or negative ions (e.g. Cl-) will travel through channels that span the membrane. Synapses can be thought of as converting an electrical signal (the action potential) into a chemical signal in the form of neurotransmitter release, and then, upon binding of the transmitter to the postsynaptic receptor, switching the signal back again into an electrical form, as charged ions flow into or out of the postsynaptic neuron.
The brain works largely through a combination of electrical and chemical signalling within and between its neurons. Neurons generate action potentials and release neurotransmitter into synapses. The transmitter travels a short distance across to the postsynaptic neuron, attaches to receptors specific for that transmitter, and causes movement of ions into or out of the cell. In a given neuron, this happens at hundreds of sites at any given moment, reflecting the activity of the many other neurons it’s listening to. The total flux of ions in a neuron from all of its presynaptic partners determines whether it will fire its own spike, starting the process anew. Fundamentally then, neuronal communication is based on neurons “deciding” whether, based on all of the inputs they receive from hundreds of partners, they should fire a spike of their own.
Axon – The long, thin structure in which action potentials are generated; the transmitting part of the neuron. After initiation, action potentials travel down axons to cause release of neurotransmitter.
Dendrite – The receiving part of the neuron. Dendrites receive synaptic inputs from axons, with the sum total of dendritic inputs determining whether the neuron will fire an action potential.
Spine – The small protrusions found on dendrites that are, for many synapses, the postsynaptic contact site.
Membrane potential – The electrical potential across the neuron's cell membrane, which arises due to different distributions of positively and negatively charged ions within and outside of the cell. The value inside of the cell is always stated relative to the outside: -70 mV means the inside is 70 mV more negative than the outside (which is given a value of 0 mV).
Action potential – Brief (~1 ms) electrical event typically generated in the axon that signals the neuron as 'active'. An action potential travels the length of the axon and causes release of neurotransmitter into the synapse. The action potential and consequent transmitter release allow the neuron to communicate with other neurons.
Neurotransmitter – A chemical released from a neuron following an action potential. The neurotransmitter travels across the synapse to excite or inhibit the target neuron. Different types of neurons use different neurotransmitters and therefore have different effects on their targets.
Synapse – The junction between the axon of one neuron and the dendrite of another, through which the two neurons communicate.
Figure 1 (right) – Nägerl UV and Bonhoeffer T (2010) J Neurosci. Jul 14;30(28):9341-6
Figure 2 – Parekh R and Ascoli GA (2013) Neuron Mar 20;77(6):1017-38
Figure 3 (right) – Gentet LJ et al. (2010) Neuron Feb 11;65(3):422-35
Figure 4 (right) – www.cime.epfl.ch/old-news