Principles of Neuroscience

Lecture 2:


Primarily focused on what happens on the cellular level (this lesson is a summary of the cellular mechanisms which lie underneath)

CogSci focuses more on the higher level of the “systems” level (language, thought etc) , this lesson is a background of the systems level

The Neuron-Soma (the cell body, maintains the metabolic functions of the cell, the bulbous end of a neuron cell, can be considered the cell body )The Nucleus is contained within the Soma.

Neurons have processes (bodies which extend from the soma): the sending process or axon (how information flows from the soma to other neuron cells, the dendrites (the receptors or receive branch of the neuron cell), the dendrites then have spines or knobs which contain receptors, the synapse is the gap between any pair of neurons, neurons are linked by these physical extensions from the soma called processes, they link in such a way that they are not connected but leave a small gap called a synapse.

Information is transferred between neurons when a nucleus changes its electrical properties in some way. This electrical change causes some kind of chemical release into the synapse space between the dendrites of two neurons this process is then repeated in the dendrites of the receiving neuron, thus transferring information to the second neuron cell.

The physiological changes that allow for communication in neurons: a cell has a membrane, this prevents the flow of some ions (either from inside to outside of the cell membrane or across the membrane, the main point is that it is regulated), in the resting potential or electrical difference between the outside with respect to the inside of the cell is -70 milivolts (a very small amount of electrical energy, or change across the membrane) it is negative because of the trapped or captured positively charged ions (Na in the example Ivry gives) against the membrane of the cell.

During Neural transmission there are gradual changes in the membrane physiology, as different receptor events take place (it could get “excited” meaning that the negative charge across the ion is increased, or inhibitory processes or hyperpolarized) As these changes accumulate something called an action potential happens, this is where the changes in the membrane are so great that the charge is reversed to +50 millivolts and channels in the membrane are opened up which allow for Na to enter . This triggers the release of neurotransmitters. The Action Potential is generated right along the top of the axon non the dendrite.

De-Polarization, reaches threshold, channels open, physiological processes push the sodium back out, release electrical pulse towards the dendrites and the process is resolved and the cell membrane repolarizes.

Synaptic Transmission is the result of an Action Potential. When there is an action potential there are terminal points of the axon controlled by calcium (which is another ion, ions like to bond to other ions which makes these “bonding events” which change the chemistry of the neuron cell more likely) , these calcium channels open up at the Axon terminal. This causes transmitters to bind to the membrane of the axon either pre synaptic or post synaptic they are able to release transmitters into the synapse.

A presynaptic event takes place as part of the axon process or the process of the sending neuron cell

A post synaptic event takes place as part of the dendrite process or the process of the receiving cell

Little vesicles at the end of the axon, which contain neuron transmitters, allow these transmitters to be sent across the synapse to receptors on the dendrite by way of calcium channels formed by ions bound to the membrane.

This is actually two processes. The axon passively empties its neurotransmitters into the synapse because of the calcium bond pathway. In a separate event the Dendrite will have receptors for that specific neurotransmitter which allow the
neurotransmitter to bond with the dendrite as it is transferred out of the synapse.
This will then change a cause in the permeability of the membrane post synoptically (on the dendrite of the receiving neuron) This can be either a depolarization (excitatory, increase in membrane potential) or hyper polarization (inhibitory, decrease in membrane potential), this is very local process which happens only the dendrites but it can accumulate to the membrane of the entire receiving cell and the process starts over.

Neurotransmitters are specific chemicals contained in the vesicles of the cells which are able to travel over the synapse between two different cells. There are over 100 types of neurotransmitters and more are added every year. Scientists debate what the difference between a hormone and neurotransmitter are in some cases they are able to resemble each other. Ex: dopamine, serotonin, glutamine, epinephrine.
The receptors of dendrites have very specific “affinities” for neurotransmitters. Specific neurons are sensitive to receive specific neurotransmitters and send specific neurotransmitters. Generally cells are only able to transmit a single type of neurotransmitters but their receptors may be able to receive more than one type of neurotransmitter.

Because of this difference in affinities and the physical structure of the neuron cell it is possible that the locality of the production and distribution of certain neurotransmitters/chemicals differ. Dopamine for example is produced in one part of the brain. The central and stem structures contain most of the neurons which produce dopamine, but the axons of the cells may extend and as a result there are many cells throughout the brain which are able to receive dopamine. The same is true of serotonin. The central structures of the brain produce the chemical, but it is distributed throughout the brain.

Other cells in the brain: blood vessels, the glial cells (metabolic functions, repair, maintenance of the neuron cells in many ways, these are the cells the ensure that the “society” of neuron cells overall is functioning, they are charged with “cleaning up” the dead neuron cells) glial cells may improve the conduction of signals, this may be because there are two different types of glial cells which form a myelin, a covering or sheath of fatty tissue around the axon which speeds the transmitter. It does this by essentially being a membrane which conducts electricity better than the axon.

Another set of glial cells are called astrocytes which from the blood-brain barrier. Glial cells prevent the blood system from directly coming in contact with neurons. Astrocytes do this by allowing oxygen to pass from the blood over their cell bodies to the neuron cells.

There is an interesting field of research that studies the specifics in the relationship between the roles of the astrocyte and the neuron. Astrocytes and Neurons reacted similarly to outside stimuli and scientists therefore wonder if the astrocyte plays are role in the transfer of neurotransmitters that is greater than previously thought. Does this come from the fact that increased activity in the neurons as a result of outside stimuli simply means that the neurons then require more oxygen thus activating the astrocytes to play this intermediary role between the neurons and the blood cells? Other studies have selectively shut down the astrocytes because of differences between the chemical properties of the two cells. The paper written about this showed that this increased the neuron activity. Glial cells may modulate the sensitivity of the cells they are associated with. These cells regulate the activity of the neuron cell in some way.

Neurons cannot fire indefinitely, when the release neurotransmitters they must enter a refractory “recovery” period before they fire again.

There are probably 11 billion neurons in the cerebral cortex (twice as much if you include the sub-cerebral cortex and older parts of the brain). How do all of these small processes which happen at a sub-atomic level in the synapse control a complicated process like cognition? There are many types of size and shape of neurons. All neurons are very different. Some axons can be three feet long. Some have large dendritic arbors but few axons. Neurons are not more densely packed in humans than other mammals, we have more neurons only be virtue of the size of the human brain.

Interconnectivity is an important part of the process. There is more than a one to one connection. Massive dendritic arbors have many axons connecting them. Some neurons have 5,000 synapses. Each neuron is therefore only a few synapses away. Because of the intense interconnectivity between dendrites and axons information can travel to any part of the brain very quickly. Each neuron only contributes a tiny part to this process. Large cognitive events only take place because of massive numbers of neurons acting at the same time.

Parallelism- Many to Many communication between neurons. Any single neuron influences many neurons and is influenced by many many neurons. Parallelism at the system level: inputs at any part of the brain travels in a parallel way throughout the brain simultaneously because the neuron architecture, not from one region to another, more like to all regions all the time. Not a serial chain

Plasticity- there is some sort of change in the neuron. Learning is a structural change in the neuron. Development constantly reshapes the structure of the neurons. The receptors/neuron structures/ change the physical shape of the nervous system all the time, this allows for changes in the relationship between different parts of the brain. This can be talked about in terms of hardware (more dendrites, neurons, axons) or software (neurotransmitters, chemicals, receptors) the line between them is not clear at times because the receptors are physical part of the brain which corresponds directly to a specific “software” chemical in the form of a neurotransmitter.
(Optical neurons on infant and adult felines) Infant has more dendrites than the adult, but the adult dendrites are larger. Vision may require the pruning of many dendrites in order to develop more fully certain specific ones.