6: Chemical Control of the Brain and Behaviour

During past week and the lecture six we went through some important topics of chemical control of brain and behavior. The components of the nervous system that operate in expanded space and time are secretory hypothalamus (periventricular area of hypothalamus), autonomic nervous system (sympathetic and parasympathetic nervous systems) and the diffuse modulatory systems of the brain (cell groups that differ with respect to the neurotransmitter they use.)

Hypothalamus maintains the homeostasis of the body by regulating the temperature and blood consumption. The secretory hypothalamus affects their many targets by releasing hormones directly into the bloodstream. It is connected to the pituitary gland by a stalk. It controls the anterior and posterior pituitary gland different ways. Magnocellular cells in the periventricular area of hypothalamus extend axons down the stalk to pituitary and into the posterior lobe. It releases oxytocin and vasopressin. The anterior pituitary is an actual gland which secretes wide range of hormones that regulate secretion from other glands. The anterior lobe is controlled by the parvocellular cells of hypothalamus. There is no axons; they communicate via bloodstream, tiny vessels run down the stalk to anterior lobe where the hormones bind to specific receptors and secrete or stop secreting hormones into the general circulation. Anterior pituitary gland releases hormones such as ACTH, LS, FSH and GH.

In addition, we talked studied the structure and effects of sympathetic and parasympathetic nervous systems. Unlike somatic nervous system, autonomic nervous system is a disynaptic pathway, it contains two neurons from the beginning to the target. It innervates three types of tissues; glands, smooth muscles and cardiac muscles. It also regulates digestive, metabolic functions of the liver, functions of kidney, urinary bladder, large intestine and rectum. It is also essential to the sexual responses of the genitals and interacts with body’s immune system. Sympathetic NS is most active during stress and fight-or-flight situations and parasympathetic during rest.

The diffuse modulatory systems are cores, which each system has a small set of neurons. The neurons arise from the central core of the brain, most from the brain stem. The focus modulatory systems activate specific metabotropic receptors.They use neurotransmitters such as NE, 5-HT, DA and ACh. The neurons of NE arises from Locus Coeruleus and spread vast areas of CNS. Serotonergic nerves arise from Raphe Nuclei and innervates most of the brain. The cholinergic diffuse modulatory systems arise from Pontomesencephalotegmental complex and basal nucleus of Meynert and Medial septal nuclei. Substantia nigra innervates the striatum and Ventral tegmental area the frontal lobe. These use dopaminergic neurons. Many drugs affect on these pathways.

Alexandra & Alisa


5: The neurotransmitter systems

In the fifth lecture we talked about neurotransmitter systems. There are more than 100 different neurotransmitters and some neurotransmitter candidates. The major classes of these are amino acids, amines and peptides. To be called neurotransmitter, there are three conditions that has to be filled: it need to be synthesized and stored in presynaptic neuron, needs to be released from the presynaptic axon terminal following stimulation and need to produce a response in the postsynaptic cell.

We went a bit deeper in the function of synapses than previously in this course. The transmitter system can be divided into chain of events. The neurotransmitters are synthesized by the synthesizing enzymes and transferred to vesicles by vesicle transporters. This all happens in the presynaptic axon terminal. After releasing these neurotransmitters they can be reuptaken or degraded by degradative enzymes. In the postsynaptic membrane there are receptors, transmitter-gated receptors and G-protein-coupled receptors. They affect directly on the ion channels, either opening or closing them, or  they can work as a signal cascade through G-protein-coupled receptors. The G-protein-receptors activate the G-proteins, which then again activates the effector enzymes (like adenylyl cyclase in case of ACh). Effector enzymes use ATP to create second messenger cascade which leads to opening and closing many ion channels. This one G-protein-coupled receptors affecting on many ion channels is called signal amplification. These opening and closing of channels have an excitatory or inhibitory effect on the postsynaptic dendrite. Finally, we learned about the different types of neurons depending on which neurotransmitter they use:  catecholaminergic, amino acidergic, serotonergic, cholinergic neurons. 



4: Chemical Senses, Eye, Central Visual System

This week lecture was about the chemical senses (taste = gustatory and smell = olfactory), the eye and the central visual system. This week we learned a lot of new things, since each of these topics involve huge amount of information in order to understand their function. Especially, learning how our vision works was literally eye opening. 

However, regarding the chemical senses, what we found interesting was, how the information from the olfactory bulb to olfactory cortex differs from other sensory axons, since it doesn’t go through thalamus. It is also highly interesting how we detect all the different odors in our orbitofrontal cortex. It is discussed that the olfactory maps may be used the distinguish different chemicals, but there must be something that reads and understand it. Also the temporal coding in olfactory system might have a part in distinguishing the chemicals. From the side of gustatory sense, what we found most interesting, were the transduction mechanisms of the different tastes (sweetness, saltiness, bitterness, sourness and  umami). We learned that saltiness and sourness have similar transduction mechanism, whereas sweetness, bitterness and umami have similar transduction mechanisms.

Regarding our vision, there were very many new things we learned. Especially interesting was the whole process how the photoreceptors of our retina can convert light energy into neural activity. Other highly interesting thing was, how the image is then formed in our brain, and also more deeper, how we can for example recognize objects and movements (the dorsal and ventral streams). 

Alexandra & Alisa

3: Synaptic transmission

In the third lecture we talked about synapses and synaptic transmissions. There are two kind of transmissions, electrical and chemical. What was new, were especially the electrical synapses. They occur at gap junctions (which then again occur between cells in nearly every part of the body). The speciality of electrical synapses is, that they allow direct transfer of ionic current from one cell to the next and they function bidirectionally (to both directions, unlike chemical synapses). Also, transmission at these is very fast, which is logical since they don’t require the electrical-chemical-electrical transformation that chemical synapses do. For what are these then needed for? They are important in locations, where normal function requires synchronized activity of neighboring cells. 

The chemical synapses are only between neurons. Compared to electrical synapses, they are slow. It takes time to convert the electrical signal to a chemical signal in the synaptic cleft and then back to an electrical signal. In the chemical transmission the action potential opens Ca2+ channels in the axon terminals membrane which then again allows the vesicles to release the neurotransmitters to the synaptic cleft. These neurotransmitters either excite or inhibit the action potential in the postsynaptic cell depending on the neurotransmitter and the receptors of the postsynaptic membrane.  

Additionally, in the exercise session of this week, we learned about neuroanatomy, which was highly interesting!

Alexandra & Alisa

2: Neuronal membrane at rest & action potential

In the second lecture we learned about the neuronal membrane at rest, and about the action potential. The theory behind these focused on the intra- and extracellular fluids, the ions in them as well as on the phospholipid membrane (bilayer) in between of the intra- and extracellular fluids. Again, some things were already familiar to us, but many new findings came along. 

One thing that was quite familiar to us was that the basic structure of the neuronal membrane: it is formed of two molecules thick sheet of phospholipids and therefore called as the phospholipid bilayer. However, after studying the structure of the membrane more deeply we learned many new, especially electrochemistry related facts, that helped us understand more deeply the resting membrane and action potential. 

We learned also what different functions proteins have in the neuron: they distinguish neurons from other types of cells. Enzymes, cytoskeleton and receptors are all made up of protein molecules. One especially interesting part in this chapter was how the equilibrium potentials are established by the ionic concentrations and electrical forces between ions. Also, one new thing we learned was the importance of the astrocytes regulating extracellular potassium concentration in the brain which is called potassium spatial buffering.

Alexandra & Alisa

1: Introduction, Neurons and Glia

The first lecture was mainly an introductory lecture to the course, in which we went through all the practical matters of the course, motivational aspects and also some basics of the brain’s structure and functions.

Well, why is it important to study the brain? Two aspects were presented: first of all it is highly interesting how does the brain work (e.g. how can we store things in our memory or use languages). Secondly, the burden of brain disorders is huge. For example, alone dementia causes globally approximately $1000 billion costs to the society per year. Therefore, even a tiny achievement towards the cure of some brain disorder could have huge impacts.

After understanding the importance of studying the brain, we started to learn the basics about brain anatomy, neurons and glia, and the organelles inside the cells. Many of these things were already quite familiar, but some things were new. For example we learned how to classify neurons. They can be classified for example based on 1) the number of neurites (unipolar, bipolar, multipolar), 2) the dendrites (pyramidical/stellate & spiny/aspinous), 3) the connections formed (primary sensory neurons, motor neurons, interneurons), 4) axon length (golgi type I, golgi type II) and 5) gene expression (use of different neurotransmitters). Another relatively new thing we learned was the different glia types: astrocytes (most of the glia in brain are astrocytes) and myelinating glia (oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system).

Alexandra & Alisa