Aantekeningen hoorcollege 3 - Development, Learning & Behavior - Universiteit Utrecht (2022/2023)

H C   3   -   M E I   2 0 2 3  

Brain development

The brain is one of the most complex systems in our body.

What the brain is made of

• 100 billion neurons
• Neurons consist of:
  
  • Cell body (soma): contains the nucleus (in which all genetic materials are stored) and most cell process take place in the cell body
  • Dendrites: the receiving end of the neuron
  • Axon: it’s much longer than the dendrites and is the sending part of the neuron (it sends signals from one neuron to another)
  • Myeline sheet: cover of the axons. Two important functions:
    
    • to keep the neuron healthy
    • to speed op signal transduction along the axon
  • axon terminals: where the signal ends up and is transmitted to other cells

How do neurons communicate

• Resting potential: when a neuron is not sending any signals, it has a negative potential of 70 millivolts (-70 mV). The negative charge is caused by the distribution of ions (particles with a certain charge, positive or negative) outside and inside the neuron.
  • During the resting state, there are a lot of positive charged sodium ions outside the neuron, while inside the neuron, there are relatively much proteins (that have a negative charge) and some positive charged sodium ions and some positive charged potassium ions.
  • Because there are more positive charged ions outside the neuron, the result is that the neuron has a negative charge of about 70 millivolts
• In the cell-membrane (walls of the neuron) are a lot of channels through which ions can go in- and outside the neuron » causes the polarity of the neuron to change
• When the neuron is at rest, the distribution of ions in- and outside is kept relatively stable by the sodium-potassium-pump (consists of special ion channels that always transport three positive sodium ions out of the neuron for every two positive potassium ions that are transported into the neuron)
• If the amount of positive ions increases so much that the polarity is decreased to threshold value of -55 mV, special voltage gated ion channels are opened through which a lot of positive ions can rush into the neuron. This causes the polarity of the neuron to switch to a positive polarity of 40 mV (called action potential). The spike in positive charge is the signal in the neuron.
• As the action potential is reached, immediately another channel of voltage gated ions are opened, causing a lot of positive ions to rush out again. This causes the polarity to switch back to -90 mV (repolarization)
• The sodium-potassium-pump causes the negative potential to go back to it’s resting potential
• Potential propagation: how does the signal move from one place on the axon to the other side? The changing of the polarization of the neuron happens at the openings in the myeline sheet. The signal ‘hops’ from one opening to another, and by every opening the process is repeated.
• The better the myelination of an axon, the faster the signal transduction
• When the signal moves ‘over’ the axon, it ends on the axon terminal.
  
  • In the axon terminal are neurotransmitters
  • In a neuron is a electrical signal, between neurons is a chemical signal
  • In the axon terminal are synaptic vesicles which contain neurotransmitters and those transmitters are synthesized from elements within the axon terminal.
  • When a signal reaches the axon terminal the vesicles move to cell membrane and they release their neurotransmitters into to synaptic cleft. There they diffuse and ‘hook up’ with receptors on the dendrites (there are specific receptors for specific neurotransmitters)
  •  When neurotransmitters bind on the dendrite, they change the polarity of the cell membrane a little bit. If the polarity is disturbed enough so that the threshold of -55 mV is reached, the dendrite will ‘fire’ and will continue the signal in the next cell
• Neurotransmitters that are left in the synaptic cleft are cleaned up
  
  • Reuptake: recycling the neurotransmitters into the axon
  • Synthesis: breaking down of neurotransmitters in the synaptic cleft by certain chemicals
• Each neuron can be connected to 10-15.000 other neurons so they need a lot of stimulation before they can continue the signal
• Different kinds of neurotransmitters:
  
  • Excitatory: increase the change that the neuron will fire
  • Inhibitory: the neuron can give a signal to the other neuron not to fire

How to influence neurotransmitters (to influence the brain)

• Target neurotransmission (increasing or decreasing so you get more signals)
• Target:
  
  • synthesis of transmitters by diet
  • presynaptic receptors (the receptors on the axons) » blocking the reuptake » more neurotransmitter are available in the synaptic cleft » more change that it will bind to the post synaptic neuron
  • postsynaptic receptors » block / improve binding » the signal that is send from the first neuron, never reaches the second neuron   
  • metabolization (breakdown) of neurotransmitters in the synaptic cleft » more / less

How is the brain organized

• How do you find out:
  • Brain damage
  • Brain stimulation
  • Electral recording (EEG) » identifying when certain activity occurs
  • Neuro imaging » identifying where certain activity occurs
• Three major subdivisions:
  
  • Forebrain » biggest part of the brain.
    
    • Thalamus: organizes input from sensory organs and routes them to appropriate areas of the brain
    • Hypothalamus: motivation and emotion. Control autonomic nervous system (fluid/food intake, temperature control) and endocrine systems (e.g. sexual behavior)
    • Limbic system: motivation, emotion, learning, memory. Contains the hypothalamus, amygdala (social processing, aggression and fear) and hippocampus (forming and retrieving memories)
    • Cerebral cortex: largest part of our brain 
• • Hindbrain » the part above the end of the spinal cord and contains the medulla oblongata, pons and cerebellum (lower part of the brainstem)
    
    • Medulla oblongata: is very critical for life functions (heart rate, respiration, blood pressure)
    • Pons: bridge between cerebrum & cerebellum and spine, sleep and arousal
    • Cerebellum (kleine hersenen): (smooth) muscular movement coordination, learning and memory
  • Midbrain » upper part of the brainstem and on top of the hindbrain. It regulates eye movements and muscle movements
  • The midbrain and hindbrain together compose the brainstem. The brainstem connects the spinal cord with the cerebrum
• Motor cortex (part of cerebral cortex):
  
  • Controls voluntary body movements
  • Lateralized » everything you control on the left side of your body is regulated by the right hemisphere and visa versa
  • Amount of cortex for each body area = complexity of movement » the size of the cortex says something about the complexity of movement that you can achieve with that specific body part
• Somatic sensory cortex (part of the cerebral cortex): processes al the information from our senses
  
  • Receives input from our sensory receptors (heat, cold, touch, sense of balance and movement)
  • It is lateralized
  • The amount of area allotted is correspondent to complexity and sensitivity of that particular sensory system

How the brain develops

• Neurogenesis: ‘birth’ of new cells
  • Most of the neurons are formed in week 5-25
  • +/- 250.000 new neurons per minute
  • Neural stem cells (haven’t differentiated yet) » when they divide, some become neurons, others become glial cells
• Migration
  
  • More cells are created, and the newer cells push the other cells to the outer layers of the brain
  • Neurons using glial cells to migrate (using it as a ‘rope’)
  • The cell itself grows out a protrusion, which connects with the outer layer of the brain and it releases and sort of ‘pulls’ itself to their new position
• Axonal/dendritic outgrowth » neurons start growing axons and dendrites
• Apoptosis: programmed cell death » the brain of a newborn has way more neurons then a newborn. A lot of those neurons are cleaned up. Within the genetic code of each cell, there’s a small part of the instruction that says ‘destroy yourself’ and every cell will do that unless it’s deactivated  
  
  • Target cells release a certain chemical which are called the ‘survival factor’. They make just enough survival factor for the optimal number of neurons          in the end (neural Darwinism). The cells that get enough of the survival factor, for them the self-destruct button is deactivated
  • Overproduction is done to make sure that within development the embryo can cope with certain types of damages
• Synaptic production » making of connections between neurons. They make certain types of connections by chemical processes (chemo-attraction, chemo-repulsion)
• Myelination (= insulation of neurons)
• Synaptic elimination/pruning » it makes our brain very adaptive (it’s for the organism easier to make a load of connections at first and then see which connections are used the most/are the most stable » those connections that are most functional in the context which the child is growing up in, survive)

The brain grows until we have 6 layers of distinct types of cells. They layer in which the neuron ends up in determines it’s function

Sensitive periods

• In the sensitive periods, the system expects some sort of input » experience dependent synapse formation
• Experience expectant synapse formation: the organism expects input. When it doesn’t receive it, that part of the brain won’t develop