Editor: This article is the second of a two-part series. Check out Part One here.
Previously, we have touched upon the basic components of the nervous system and also learnt about how the ‘electrical’ part of ‘electrochemical’ signals are transmitted, as well as how impairments in this signalling component may adversely affect our own health.
In the next part of this series, we will be talking about the components and process of the ‘chemical’ part of electrochemical signalling, as well as provide a brief introduction into the brain anatomy.
Synapses and neurotransmitters
Synapses
A synapse is a gap at the end of a neuron that allows an electrochemical signal to be passed on from one neuron to the next. The structure of a synapse can be divided into three parts - presynaptic ending, synaptic cleft , and post-synaptic ending.
The presynaptic ending of the synapse contains neurotransmitters, which are released when an action potential has been generated and reached the end of the axon. The synaptic cleft is the gap between the nerve cells, and the neurotransmitters that are released from the presynaptic ending will diffuse across this cleft to reach the postsynaptic ending of the other neuron. The post-synaptic ending contains receptors that bind to their respective neurotransmitters. This binding initiates the generation of an action potential at the other neuron, thereby passing on the signal.
Figure 1: A diagram of the synapse, showing the presynaptic ending that contains neurotransmitters in their vesicles, the synaptic cleft in which the neurotransmitters diffuse across, and the postsynaptic ending containing receptors which bind their respective neurotransmitters. Figure adapted from Yang et al. (2018)’s publication Universal and Convenient Optimisation strategies for Three-terminal Memristors.
Neurotransmitters
Neurotransmitters are chemical messengers that transmit messages from neurons to other neurons, or from neurons to other target cells. They are involved in the maintenance of many bodily functions such as our appetite and muscle movement, and they do this by binding to the receptors on target cells. When bound to receptors, neurotransmitters can either cause electrical signals to travel down the recipient cell or stop the signalling entirely.
If there is an imbalance in the amount of neurotransmitters in the body, this can lead to the development of several diseases such as Alzheimer’s or Parkinson’s, or even other mental health disorders such as OCD (obsessive-compulsive disorder) and depression.
Neurotransmitters can be classified by the type of function they elicit when bound to their receptor, and these functions can be split into three main groups: excitatory, inhibitory and modulatory.
Excitatory neurotransmitters increase the likelihood of a neuron firing off an action potential when they are bound to their receptor, therefore passing on any relevant electrochemical signals received. Inhibitory neurotransmitters, on the other hand, perform the opposite function - they decrease the likelihood of an action potential occurring when bound to their receptor, thereby preventing the electrochemical signal from being transmitted. Interestingly, some neurotransmitters such as dopamine can perform both functions.
Meanwhile, modulatory neurotransmitters have several unique functions. They can affect a large number of neurons or a whole section of neural tissue at one time by using one or more chemicals to regulate a diverse population of neurons, leading to the release of chemical messengers involved in the second messenger signalling system. Although they can technically target a wider area, they are more slow-acting than excitatory and inhibitory neurotransmitters as they target slower G-protein neuroreceptors. This allows for a longer, more sustained duration of action potential- the ion channels stay open for longer, mediating a larger amount of depolarisation.
Putting the ‘chemical’ in ‘electrochemical signalling’ - the final piece of the puzzle
Now that we’ve gotten an idea of what synapses and neurotransmitters are, we can finally start to piece together how they aid in the transmission of electrochemical signals from one neuron to another.
When an action potential arrives at the synaptic knob, it leads to the opening of voltage-gated calcium ion channels (Ca2+) that are present in the membrane of the synaptic knob. The opening of this gate allows for Ca2+ ions to move into the synaptic knob via the channels. The movement of Ca2+ ions triggers the migration of vesicles filled with neurotransmitters to the presynaptic membrane. These vesicles then fuse with the presynaptic membrane in a process called exocytosis.
After exocytosis, neurotransmitters are released into the synaptic cleft. They diffuse across the cleft and bind to specific receptors that are present on the postsynaptic membrane. Once bound, neurotransmitters can either elicit excitatory or inhibitory responses in the recipient cell.
Receptors may be coupled to Na+ or Ca2+ channels; when these bind to their respective neurotransmitters, they produce an excitatory response. In contrast, receptors that are coupled to chloride (Cl-) or K+ channels may produce an inhibitory response- when Cl- or K+ ions enter the neuron excitability is reduced, therefore the probability of a successful action potential occurring decreases.
Figure 2: An overview of how synaptic transmission occurs. When the action potential arrives at the presynaptic ending (dark blue outline), it triggers the opening of voltage-gated Ca2+ channels (light blue rectangles). The influx of Ca2+ through these channels then triggers the exocytosis of vesicles containing neurotransmitters (dark blue circles) with the presynaptic ending membrane. Once this occurs, the neurotransmitters are released into the synaptic cleft, where they diffuse across and bind to their receptors (yellow rectangles) on the postsynaptic ending (yellow outline).
Instead of being coupled to ionic channels, other receptors may be linked to ‘second messenger’ systems. These are systems that require a small molecule called a ‘secondary messenger’ to initiate a series of downstream reactions that amplify a signal until it reaches its intended recipient, ultimately triggering a response.
The amplification of a signal via this method is called a ‘signalling cascade’, and receptors that are linked to these systems may be coupled to ion channels that trigger the release of secondary messengers, or they may be indirectly involved in the synthesis of secondary messengers as well.
Figure 3: A flowchart showing how receptors and neurotransmitters may be coupled to secondary messenger systems, which work in tandem with other components such as coupling factors and tertiary messengers to help amplify and transmit a signal to the target cell. As a result, this leads to the production of a physiological response. Examples of first messengers, secondary messengers, coupling factors, and tertiary messengers are stated above.
Although there are various ways in which synapses, neurotransmitters, and receptors transmit a signal to various recipient cells, the end result is the same - the signal reaches its target cell, and an appropriate response is produced towards the stimuli.
Besides neurons, there’s another crucial part of the nervous system that has not been discussed - the brain.
In the following sections, we’ll be talking about brain function and anatomy, and how all of these components function in tandem with the rest of the nervous system to aid the execution of everyday functions.
Brain function
The brain is a complex organ made up of 100 billion nerves that communicate information to each other via synapses. It is responsible for a variety of behaviours such as memory and emotion, and it also controls movement and other organ functions throughout our body.
The brain is able to react and respond to changes in the environment by receiving input from sensory nerves that detect these changes. Once this information has been received, the brain processes it, then generates a response - for example, by dictating the movement of motor neurons in your hand and foot.
How does the brain carry out so many complex functions?
To answer this question, we will have to look deeper into the structure of the brain, and how each component carries out their own function independently and simultaneously with other regions of the brain.
Brain anatomy
The brain is split into three main parts, namely the cerebrum, the cerebellum, and the brainstem.
Figure 4: The anatomy of the brain, with the cerebrum, cerebellum, and brainstem shown. The cerebrum is the largest part of the brain responsible for complex functions, the cerebellum receives input from sensory nerves and coordinates muscle movement, whereas the brainstem processes visual and auditory information, as well as controls our heart and lung function. This figure is adapted from the Australian Academy of Science (2015)’s Getting our head around the brain.
Cerebrum
The cerebrum is the largest part of the brain and is responsible for more complex functions such as speech, reasoning, emotions, as well as processing touch, vision, and sound.
The cerebrum can be divided into two halves: the left and right hemisphere. Both hemispheres are joined together by fibres called corpus callosum, and these fibres allow for messages to be transmitted from one side to the other. Each hemisphere controls the opposite side of the body - this is why functions on the right-hand side of the body are affected when a stroke occurs in the left hemisphere.
The hemispheres carry out different functions. The left hemisphere generally controls speech, arithmetic, and writing, whilst the right hemisphere controls creativity, spatial ability, as well as any artistic or musical skills.
Figure 5: A brief anatomy of the cerebrum (in pink) with the left and right hemispheres labelled. The left hemisphere is responsible for a variety of functions such as speech, arithmetic and writing, whereas the right hemisphere is more centered around creativity, artistic abilities, and spatial abilities. This figure is adapted from Blausen.com staff (2014)’s Medical Gallery of Blausen Medical 2014.
Each hemisphere can be further divided into four lobes: the frontal, temporal, parietal, and occipital lobe. These lobes can also be sectioned off into regions that perform specific functions. Nonetheless, although each lobe may perform a distinct function, it is important to remember that no lobe functions alone - they all form complex relationships with each other to execute a variety of tasks.
Figure 6: A diagram of the lobes in our brain. The frontal lobe (blue) is located towards the front of the brain and controls emotional expression, memory, and problem-solving skills, but it is also involved in muscle movement and speech. On the other hand, the parietal lobe (yellow) processes sensory information in the environment, mainly those regarding touch, sense, and temperature. Meanwhile, the occipital lobe (red) is in charge of visual processing. And finally, the temporal lobe (green) is involved in the process of memory, such as the recognition of faces and learning of languages. The cerebellum is also shown below the temporal lobe. This figure is adapted from Gaurav Das on clipartkey.com.
Frontal lobe
The frontal lobe is located towards the front of the brain, as implied by its name. This area of the brain controls a variety of cognitive skills in humans such as emotional expression, problem-solving, and memory. Essentially, it is in charge of our personality, but it also carries out other functions such as muscle movement and speech.
Parietal lobe
The parietal lobe is located behind the frontal lobe, separated by a boundary called the central sulcus. This region processes sensory information in the environment sent to them by nerves in the skin, mainly focusing on information regarding touch, taste, and temperature. It may also pass on information to other regions of the brain for processing.
Occipital lobe
The occipital lobe is sort of sandwiched between the parietal lobe and the temporal lobe. It is mostly responsible for processing and perceiving visual information - this means that it allows us to process and understand what we see everyday.
Temporal lobe
The temporal lobe is located around the bottom-middle position in the brain, which happens to also be around our ear level. It is involved in processing auditory information received from the ears, and it is also responsible for interpreting different frequencies, pitches, and noises. Besides that, it is also in charge of selective hearing, allowing us to filter out unimportant information on a day-to-day basis.
Additionally, the temporal lobe helps us recognise and remember objects, such as faces. It also helps us comprehend language, and, in this process, creates and preserves long-term memory.
Cerebellum
The cerebellum is located beneath the cerebrum. It receives information from sensory systems, the spinal cord or the brain, and processes it in order to coordinate muscle movement, balance, and the maintenance of posture.
Brainstem
The brainstem is located in front of the cerebellum and can be split into three regions: the midbrain, pons, and medulla oblongata.
The midbrain is in charge of controlling eye movement, as well as processing visual and auditory information. The pons is located below the midbrain, and is essentially a group of nerves that helps connect various parts of the brain together. These groups of nerves are also involved in controlling facial movement and transmitting sensory information. Finally, the medulla oblongata is located in the lowest part of the brain. It controls the function of the heart and lungs, such as but not limited to - sneezing, breathing, and swallowing.
Figure 7: A diagram of the brainstem, which is split into the midbrain (blue), pons (red), and medulla (purple). The functions of each part are outlined briefly in the diagram. This figure is adapted from OpenStax.
Chemical imbalances, memory, and sleep - real life implications of neurobiology
Although this overview is a cursory look at the role neurotransmitters play in the nervous system, their action on the body and the brain may have large repercussions on their functions.
An imbalance in certain neurotransmitters may lead to the development of mental health issues, and it may even tamper with our ability to remember things. For example, low levels of serotonin have been linked to the development of depression. This is why certain antidepressants that are prescribed - Prozac, for instance - act to increase the amount of serotonin in the brain.
Moreover, the brain controls a wide variety of functions that are crucial for everyday life. Therefore, to say that brain damage or malfunction would have a large impact on our day-to-day life is an understatement.
Even though this article has outlined some of the basic functions of different regions of the brain, it is still unclear how certain regions carry out their tasks, or the role that the brain plays in certain bodily functions. A good example of this would be sleep. But all in all, it is stunning to see how this organ controls so much of our life without us even consciously knowing.
Author: Zoe Cheah, BSc Biological Sciences