Neurotransmitter systems (weeks 40 & 41)

Neurotransmitters are molecules that play an important role in neural communication. They are chemical messengers that carry information from neural cells to other cells. Amino acids, amines and  peptides are the three major classes of neurotransmitters. There have been found around 100 different neurotransmitter molecules. However, the most work is done by only tenth of the transmitters. First neurotransmitter discovered was the acetylcholine which helped to understand the cholinergic system.

Neurotransmitter is determined as a molecule which needs to be synthesized and stored in presynaptic neuron. In addition, it needs to be released from the presynaptic axon terminal following stimulation. Also, it needs to produce  a response in the post-synaptic cell.

Neurotransmitters can be roughly divided into two classes based on how they affect to the post-synaptic cell. Neurotransmitter can be either excitatory or inhibitory. Excitatory neurotransmitters have excitatory effects on the neuron. This means they increase the probability that the neuron will fire an action potential in the post-synaptic neuron. Glutamate is the most common excitatory neurotransmitter in the brain (AMPA receptor, NMDA receptor, kainata receptor). Inhibitory neurotransmitters decrease the probability of the post-synaptic action potential. GABA is the most common inhibitor. Other common neurotransmitters are acetylcholine (nicotinic receptor, muscaranic receptor) and serotonin.

Neurotransmitters can act through a second messenger neurotransmission. In the second messenger neurotransmission the first messengers cause target cell to release the second messengers. The first messengers are extracellular factors, often neurotransmitters or hormones. Serotonin is one neurotransmitter that acts as the first messenger. The type of the second messengers can vary a lot. cAMP is a one example of the second messenger molecule.

The brain has diffuse modulatory systems that are neurotransmitter systems which perform regulatory functions, i.e. modulate postsynaptic neurons in various areas of the brain by altering their activity, e.g. making them more or less excitable or synchronous. The diffuse modulatory systems tend to consists of a small core set of neurons that are located in the brain stem, and the activity of this small subset of neurons affects the activity of a much broader set of postsynaptic neurons.

The norepinephrine system located in the locus coerelus in the pons is one of the diffuse modulatory systems. The axons leaving from locus coeruleus innervate almost every part of the brain, i.e. all of the cortex, the cerebellum, the midbrain, the thalamus and so on. Activity in locus coeruleus has been linked to mood and depression, as well as attention and arousal.

Another of the diffuse modulatory systems is the serotonergnic system. The serotonin containing neurons are located within the raphe nuclei in the brain stem. These nuclei innervate most of the brain similarly to locus coeruleus neurons. Also similarly to locus coeruleus, the raphe nuclei seem to activate most during wakefulness and arousal.

The cholinergic system located in the basal forebrain complex is one of the two major diffuse modulatory cholinergic systems. This system has two major nuclei: medial septal nuclei and the basal nucleus of Meynert. The medial septal nuclei innervates hippocampus, whereas the nucleus of Meynert innervates the neocortex. Like the other systems in the brain stem, the cholinergnic system seems to be part of regulating arousal and sleep-wake cycles. It also seems to be important for learning and memory.

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Sensation

The lecture examined sensory perception in the nervous system. The sensory systems that were examined were the chemical senses, which include both taste and smell. The sense of taste is produced on the tongue. The surface of the tongue has papillae, which consists of taste buds, and the taste buds consist of taste cells. There are different types of taste cells for the different types of basic tastes (salt, sour, bitter, sweet, umami). While there are a quite small amount of taste receptor, there are thousands of different olfactory receptors, and what one usually experiences as a taste is the combination of taste and smell. 

Retina is a multilayer structure with different cell types, such  as photoreceptors, bipolar cells and ganglion cells. Photoreceptors are the cells that react to light by changing the configuration of a retinal molecule. Photoreceptors then signal to bipolar cells, which convey information to ganglion cells. Ganglion cells form receptive fields of the visual space. Receptive fields are center-surround structures, where the center portion reacts either to a spot of light or darkness and the surrounding region reacts to darkness if the center reacts to light and to light if the center reacts to darkness. 

In the exercise session we discussed how senses are adapted to different situations. For example, what is the difference between the human eye and octopus eye. It is clear that both structures are developed to sense electromagnetic stimuli but how relevant is a good sight and colour separation in the depths of the sea.  All mammals have the same set of senses but their sensitivity is based on the habitat. For example, hawks have a very good vision because they have to spot their prey from far above and wolves have a good sense of smell for recognizing their pack and avoiding possible threats.

In some cases a sensory organs do not operate well or an organ has damaged so bad that it can not transfer stimuli to the brain. For ill-working organs we have found different solutions like eyeglasses to fix bad sight or hearing aid to allow us to hear clearly again, but if the sensory organ has failed completely it is very hard or impossible to repair.

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Transmission and structure

This weeks lecture examined information transfer in the brain, i.e. the principles of neurotransmission in synapses and the conduction of action potential in the axon.  Action potential in a presynaptic neuron results in the release of a neurotransmitters at the axon terminal. The transmitter molecules bind to the receptors of a postsynaptic neuron. The binding of a neurotransmitter can excite or inhibit the postsynaptic neuron, i.e. increase or decrease its membrane potential. If the signal is excitatory, the membrane potential is moved closer to the action potential generation threshold. There are multiple different types of neurotransmitters and whether the signal is excitatory or inhibitory depends on the transmitter that is released and the region of brain it is released at.

In the exercise session this week the focus was on the structure of the brain. We were given play dough and our tasks was to reconstruct human brain from it. The aim of the exercise was to give some hands on experience regarding the brain structure. Creating a 3D object gave us a different perspective on the real life structure of the brain than just viewing 2D images from a textbook. It was suprisingly hard to connect different parts of the model to each other. Forming the cortex was the most challenging part with all its sulci and gyri.

– Timo and Joonas

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Second week

During the second week, we were introduced to how the nerve cell membrane operates and how it contributes to the communication between nerve cells. The membrane consists of phospholipids and proteins. The lipids form a bilayer structure which segregates the cell organelles and the cytosol from the external environment. The membrane proteins form passages that allow electrically charged particles to transport through the membrane. These passages are called ion channels.

Ions on the opposite sides of the membrane create two gradients: a concentration gradient and an electrical gradient. Proteins in the membrane, such as ion channels and ion pumps, help to maintain these gradients at equilibrium. In equilibrium, the cytosol is slightly negatively charged in comparison to the external environment (-65 mV). Changes in the equilibrium of the ion gradients produce an action potential, which is the conduction of charge along the axon, and this is the foundation of information transfer in the nervous system.

Some nerve cells have myelin sheaths. Myelin sheath is a fatty substance that surrounds the axon of a nerve cell. Myelin sheath insulates the axon and increases the propagation speed of the action potential. Myelin can be found in both the central nervous system and peripheral nervous system. Myelin is made up of glia cells. In central nervous system, these glia cells are called oligodendrocytes, and in peripheral nervous system, they are named Schwann cells.

– Joonas & Timo

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First steps

This week our exciting journey to understand the human brain began. The brain is the most complex object in the known universe, making it an truly intriguing object of study. It is a subject that certainly fascinates us both, since the brain is the root of all the big questions of life. What is consciousness? What are emotions? What is creativity? And, importantly, how do these impressive phenomena arise? These are just some of the questions we hope to come closer to understanding on our journey to understand the brain.

During the first week of the course we were introduced to the basic components that forms the human brain, the neurons. We studied different neuron types and how they are connected to each other. Basic physics behind the neural activity were also talked about. It is fascinating how different cell types working together can form such a high complexity structure such as the brain. The human brain can be divided to Brodmann’s areas according to the morphology of the neural cells. Some Brodmann areas correspond to functional divisions that can be found in the brain, e.g. Brodmann area 17 is the primary visual cortex and the Brodmann areas 1, 2 and 3 are equivalent to the somatosensory cortex.

– Timo and Joonas

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