Brain cells

Question 1

Brain cell types

Answer 1

Neurons are grouped together to form gray matter (cell bodies) and white matter (axons and other cells).
Other supporting cells are called glia:
Microglia
Astrocytes
Oligodendrocytes

Question 2

Neuron structure

Answer 2

Neuron is a type of cell that makes up the nervous system and supports, among other things, cognitive function.
They consist of 3 components:
- Cell body (soma): part of the neuron containing the nucleus and other organelles
- Dentrites: branching structures that carry information from other neurons in close proximity.
- Axon: a branching structure that carries information to other neurons and transmits an action potential.

Question 3

axon hillock

Answer 3

The axon hillock is essentially the “root” or beginning of the axon. From the hillock, the axon stretches outward until ending in the terminal, which synapses unto the dendrites of other neurons.
The hillock also contains the spike-triggering zone, where excitatory postsynaptic potentials (or EPSPs) converge and accumulate in order to trigger an action potential.

Question 4

Neuron types

Answer 4

There are some significant differences between different types of neurons in terms of spatial arrangements of dentrites and axon.
- Bipolar neuron e.g. in retina
- Unipolar neuron e.g. ganglion cell of dorsal root carry touch and pain signal
- Multipolar neuron e.g. Motor neuron of spinal cord, Purkinje cell of cerebellum(
- Anaxonic neuron e.g. Amacrine cell of retina?
Each neuron consists of many dendrites but only a single axon, although the axon may be divided into several branches called collaterals.

Question 5

Neuron activation

Answer 5

Neuron activation by input that change neuron state
- Excitatory state (green): input as electrical current that brings neuron closer to activation state (from resting state) → depolarization
- Inhibitory state (red): input as electrical current that brings neuron far away from activation → hyperpolarization
The single excitatory input can not activate the neuron but the sum of all the excitatory inputs and 1 inhibitory input exceeds the threshold and generate an action potential.
The first ever measured and published intercellular recording of an action potential is from Hodgkin and Huxley 1939

Question 6

Synapse

Answer 6

the small gap between neurons in which neurotransmitters are released, permitting signaling between neurons.

Question 7

Neurotransmitters

Answer 7

chemical signals that are released by one neuron and affect the properties of other neurons.

Question 8

Action potential

Answer 8

a sudden change (depolarization and repolarization) in the electrical properties of the neuron membrane in an axon, which forms the basis for how neurons code information (in the form of the rate and synchrony of action potentials.

Question 9

How do neurons communicate with each other?

Answer 9

The two neurons forming the synapse are referred to as presynaptic (before the synapse) and postsynaptic (after the synapse), reflecting the direction of information flow (from axon to dendrite).

• When a presynaptic neuron is active, and electrical current (termed an action potential) is propagated down the length of the axon. When the action potential reaches the axon terminal, chemicals are released into the synaptic cleft. These chemicals are termed neurotransmitters. (Note that a small proportion of synapses, such as retinal gap junctions, signal electrically and not chemically).
• Neurotransmitters bind to receptors on the dendrites or cell body of the postsynaptic neuron and create a synaptic potential. The synaptic potential is conducted passively through the dendrites and soma of the postsynaptic neuron. These passive currents form the basis of EEG.
• These different passive currents are sum#–med together and if their summed activity exceeds a certain threshold when they reach the beginning of the axon in the postsynaptic neuron, then an action potential will be triggered in this neuron. In this way different neurons can be said to be “communicating” with each other.

Question 10

electrical signalling and the action potential in axon of the presynaptic neuron

Answer 10

If a passive current of sufficient strength flows across the axon membrane, this begins to open the voltage-gated Na+ channels. →Na+ may enter the cell
The negative potential normally found on the inside reduced → depolarized. If the neuron receives enough stimulus (passive current) to reach a threshold at about -50 mV, the cell membrane becomes completely permeable and the charge on the inside of the cell momentarily reverses. This sudden depolarization and subsequent repolarization in electrical charge across the membrane is the action potential. If the threshold is not met, no action potential is generated.
As the membrane potential becomes more positive and reaches a value of roughly +50 mV. The negative potential of the cell is restored via the outward flow of K+ through voltage-gated K+ channels and closing of the voltage-gated Na+ channels. → repolarization
There is a brief period in which hyperpolarization occurs (the inside is more negative than at rest). This makes it more difficult for the axon to depolarizes straight away and prevents the action potential from traveling backwards. the cell hyperpolarizes at a voltage more negative than its baseline (-80 to -90 mV) due to the increased outward K+ flow. Eventually the K+ channels return to their baseline state and the membrane potential reaches -70 mV again until another stimulus surpasses threshhold.
An action potential in one part of the axon opens adjacent voltage-sensitive Na+ channels, and so the action potential move progressively down the length of the axon, starting from the cell body and ending at the axon terminal. The conduction of the action potential along the axon may be speeded up if the axon myelinated.

Question 11

Myelin

Answer 11

a fatty substance that is deposited around the axon of some cells ( especially those that carry motor signals) that speeds conduction.

Question 12

chemical signaling and passive conduction in the postsynaptic neuron

Answer 12

• When the action potential reaches the axon terminal, the electrical signal initiates a sequence of events leading to the release of neurotransmitters into the synaptic cleft.
• Protein receptors in the membrane of the postsynaptic neurons bind to the neurotransmitters. Many of the receptors are transmitter-gated ion channels. This sets up a localized flow of charged Na+, K+ or chloride (Cl-), which creates the synaptic potential.
• These synaptic potentials are then passively conducted in the postsynaptic neuron.

Question 13

McCulloch-Pitts model of neuron

Answer 13

A theoretical neuron with 2 input x1, x2 và 1 output y
value of input/output of either 0 or 1 → binary.
Neuron is characterized by 1 parameter theta Θ ← → activation threshold

Question 14

Perceptron

Answer 14

Perceptron - a theoretical neutron
introduces weight to each input
(Frank Rosenblatt)

Question 15

XOR affair in Perceptrons (1969)

Answer 15

A single neuron can´t implement the XOR function
For realising XOR using perceptron/neurons an OR gate, an AND gate and an NOT AND gate are required.
Solution →deep neural network

Question 16

Oligodendrocytes

Answer 16

are a type of neuroglia whose main functions are to provide support and insulation to axons in the central nervous system (CNS), equivalent to the function performed by Schwann cells in the peripheral nervous system.

• Creating myelin sheath
• extend its processes up to 50 axons, wrapping approximately 1 μm of myelin sheath around each axon

Question 17

Microglia

Answer 17

neuroglia (glial cell) located throughout the brain and spinal cord
account for 10–15% of all cells found within the brain
key cells in overall brain maintenance
act as the first and main form of active immune defense in the CNS
scavenging the CNS for plaques, damaged or unnecessary neurons and synapses, and infectious agents
Can react quickly to decrease inflammation and destroy the infectious agents before any infectious agents damage the sensitive neural tissue
Can transform in various types → versatile brain cells

Question 18

Astrocytes

Answer 18

characteristic star-shaped glial cells in the brain and spinal cord
astrocytic endfoot processes that physically connect the cells to the outside of capillary and to neuron
function:
biochemical control of endothelial cells that form the blood–brain barrier*
provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, regulation of cerebral blood flow (store glucose and use it to fuel the neuron)
repair and scarring process of the brain and spinal cord following infection and traumatic injuries

Question 19

The blood–brain barrier (BBB)

Answer 19

is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of the central nervous system where neurons reside.The blood–brain barrier is formed by endothelial cells of the capillary wall, astrocyte end-feet ensheathing the capillary. The BBB limits which materials in the blood can gain access to neurons in the nervous system.

Question 20

The tripartite synapse

Answer 20

refers to the functional integration and physical proximity of the presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding glia as well as the combined contributions of these three synaptic components to the production of activity at the chemical synapse.
- many processes envelop synapses made by neurons. In humans, a single astrocyte cell can interact with up to 2 million synapses at a time. → tripartite synapse → astrocyte helps with homeostasis function

Question 21

Electrical gradient

Answer 21

A force that develops when a charge distribution across the neuronal membrane develops such that the charge inside is more positive or negative than the one outside. Electrical gradients result from asymmetrical distributions of ions across the membrane. In neurons, an electrical gradient occurs when ion pumps move three sodium ions (Na+) out of the cell, and two potassium ions (K+) into the cell. As the net amount of positively charged ions outside the neuronal membrane becomes greater than on the inside, an electrical gradient – i.e. a difference in electrical charge – is created.

Question 22

Equivalent circuit of a neural membrane

Answer 22

At resting place, net currents for Na+ and K+ is 0.

Membrane voltage Vm depends on the equilibrium potentials and the conductance of Na+ and K+ charges

Question 23

Voltage-gated channels

Answer 23

opens only when a certain voltage is applied

Question 24

Hodgkin-Huxley model

Answer 24

cell as an electrical circuit