Chapter 2 - The notion of Neuron

Question 1

What is a neuron?

Answer 1

A cell capable of transmitting information coded with electrical impulses.

Question 2

Brain - Number facts

Answer 2

Average weight - 3 lbs, 2% of body
Number of neurons - 10 - 100 billion
Grey matter (cell bodies - 40%), white matter (cell extensions (60%)
Glial cell (other cells) 10 - 50 times the number of neurons
Time until unconsciousness after loss of blood supply to brain = 8-10 sec
Time until reflex loss after loss of blood supply to brain = 40-110 sec
Time until irreversible damage has occurred after loss of blood supply = 120-240 sec
Total surface area of the cerebral cortex = 2.5 sq. ft.
Thickness of cerebral cortex = 1.5 - 4.5 mm
Average number of neurons in cortex = 10 to 20 billion arranged in 6 layers

Question 3

What is Glia?

Answer 3

“Nerve glue”. Constitute about half of the brain mass, necessary for the wellness of neurons.

Question 4

What are Astrocytes?

Answer 4

“Star cells”. Support neurons physically, provide them with vital chemicals, clean up dead cell remains.

Question 5

What are Oligodendrocytes?

Answer 5

“cells with several treelike extensions”. Wrap around the main extensions of neurons and provide electrical insulation.

Question 6

What are Microglial cells?

Answer 6

Smallest cells of the glia, can destroy remains of dead cells and protect brain against infection by viruses or bacteria

Question 7

What is the soma?

Answer 7

Also known as cell body, part of the neuron that contains the nucleus and other necessary cellular equipment.

Question 8

What are dendrites?

Answer 8

Membrane extensions involved in the reception of messages from other neurons. Form the dendritic system.

Question 9

What is the axon?

Answer 9

Main extension of a neuron, transports signals towards other neurons, muscle cells, or glands.

Question 10

What are terminal buttons?

Answer 10

Small knobs at the extremity of the axon that branch out and facilitate communication with other cells.

Question 11

What is the myelin?

Answer 11

Multi-layered sheath of membrane from supporting cells that covers the axon, help speed up signal transmission.

Question 12

What are nodes of Ranvier?

Answer 12

Small portions of naked axon found between successive myelin sheaths along the axon.

Question 13

What are synapses?

Answer 13

Regions of communication between neurons, occurring between the terminal buttons of one neuron and the dendrites of another neuron.

Question 14

What is the size of a typical neuron and axon?

Answer 14

Body of large neuron is approximately 50 µm (50 millionths of a meter), the diameter of an axon is classically around 1 µm. Although the axon can be over a meter in length (eg. the axon of a motor neuron from the precentral gyrus to a toe muscle).

Question 15

What are the 3 main types of neurons and their functions?

Answer 15

Sensory neuron - convey information about the external world and the inner body towards the central nervous system.
Motor neuron - command muscles and glands of the body.
Interneurons - communication informations between two other neurons.

Question 16

Describe the three principle types of neurons based on shape and organisation.

Answer 16

Multipolar neurons (mostly motor and interneurons, most common) - large bunch of dendrites extending around the soma and one axon capable of reaching far away neurons or other cells.
Bipolar neurons (mostly convey visual or auditory sensory information) - have two main extensions on either side of the soma: one connected to a generally compact dendrite tree and one axon.
Unipolar neurons (mostly convey somatosensory information, like touch, pain etc.) - have one stalk connecting them to a single tube joining the dendritic tree directly to an axon.

Question 17

What is the resting potential (or membrane potential) of a neuron?

Answer 17

-70 µV, indicating a negative charge inside the axon compared to the outside.

Question 18

What are ion channels?

Answer 18

Protein molecules that span across the cell membrane allowing the passage of ions from one side of the membrane to the other.

Question 19

What are the two key principles of signal conduction along the axon?

Answer 19

1. Ions move towards areas where they are less concentrated (diffusion) but are also driven by electrical potential (electrostatic pressure).
2. The resting potential is actively maintained by specialised molecules (proteins) in the axon membrane. Such molecules (ion transporters) force ions in and out of the neuron, others (ion channels) let them enter or exit.
   When the neuron is at rest, ion channels are closed and transporters pump continuously to maintain resting potential.

Question 20

What is the process of action potential in an axon?

Answer 20

1. Tiny electrical chock opens an ion channel (sodium channel) which is voltage-dependent (electro-sensitive).
   Sodium ions (charged positively) flow inside the axon through the membrane because (a) sodium is less concentrated inside the axon (diffusion force) and (b) the inside of the axon is negatively charged (electrostatic force). Causing membrane potential to change rapidly (from -70 µV to +40 µV) because positive ions are going in.
2. After sodium channels open, potassium ions (also voltage-dependent, react slower) open letting potassium ions flow outside axon. Causes membrane potential to revert from +40 µV to -70 µV and even -80 µV because positive ions are going out.
3. Ion transporters pump sodium out and potassium in to re-establish original configuration.

Question 21

What is action potential in an axon?

Answer 21

Change of potential of the cell membrane (depolarisation). Happens in around 2 milliseconds.

Question 22

What are myelinated axons?

Answer 22

Most axons covered in myelinated sheaths, act as insulators. Prevent exchange in myelinated sections of the axon.
Action potential decreases in myelinated axons as it travels under myelin sheath, remains strong enough to activate voltage-dependent sodium channels at the next node of Ranvier.

Question 23

What is saltatory conduction and its advantages?

Answer 23

Action potential “jumps” from one node of Ranvier to the next.
Allows for faster transmission of action potentials.
Myelin sheaths decrease risk of interference between nearby axons (safer signal transmission).

Question 24

What is frequency coding?

Answer 24

Number of action potentials (spikes) transmitted per unit of time, conveys intensity of stimuli.

Question 25

All-or-none response

Answer 25

Neuron either fires action potential or it doesn’t.

Question 26

Variable intensity responses

Answer 26

Nervous systems ability to interpret levels of stimulus intensity or muscle contractions, managed through variations in frequency coding.

Question 27

Stimulus strength coding

Answer 27

Based on the variation in the frequency of action potentials; higher frequency correlates with stronger stimuli and vice versa.

Question 28

Synapse

Answer 28

Exchange between neurons (or between neuron and muscle cell), where terminal buttons of one neuron come close to the dendrite of another neuron or muscle cell.

Question 29

Presumptive membrane

Answer 29

Membrane of the terminal button of the source neuron; before the synapse.

Question 30

Postsynaptic membrane

Answer 30

Membrane of the dendrite of the target neuron, after the synapse.

Question 31

Synaptic cleft

Answer 31

Narrow space between pre- and post-synaptic membranes.

Question 32

Neurotransmitters

Answer 32

Communication molecules released by terminal buttons, travel across synaptic cleft and attach to receptors in the postsynaptic membrane.