Brain Cells & Nerve Impulses ( + Textbook Chapter 1 )

Question 1

Contextualise:

How did Santiago Ramón y Cajal (1852 - 1934) influence the field of neuroscience?

What was his main ‘revelation’?

Answer 1

He utilised staining techniques with silver salts (developed by Camillo Golgi) on infant brain cells to show that each neuron is separate from one another - our neuronal activity is the sum of many interacting parts

(rather than a ‘single unit’ of combined brain cells)

His original aspirations of being an artist helped shape his remarkable illustration abilities in the medicinal field too!

Question 2

List:

Identify some of the key animal cell aparatus neurons have in common with all other somatic cells:

(disregarding exceptions like red blood cells)

Answer 2

• Cell Membrane (Phospholipid Bilayer)
• Nucleus
• Mitochondria
• Ribosomes
• Endoplasmic Reticulum

Question 3

Identify:

What are the common structural features of most neurons?

What are some exceptions though?

Answer 3

• Soma (Cell Body)
• Axons ( + Axon Hillock)
• Dendrites
• Presynaptic Terminals

Note: There are some variations in which cells lack axons or clear dendrites

Question 4

Define:

Dendritic Spines

Purpose?

Answer 4

Small projections along dendrites that help increase the surface area available for receiving synaptic signalling

Question 5

Describe:

A neuron’s soma (cell body) and one of its primary functions:

Answer 5

The soma contains important structures like the nucleus, mitochondria, and ribosomes - it is the location in the cell at which most of the metobolic work is conducted

Metabolic work may include important functions such as synthesising proteins to be carried in vesicles along microtubules in the axon by transport proteins so that they may be used as neurotransmitters (signals) at the presynaptic terminals

Question 6

True or False:

The soma (cell body) is also a location of a neuron with synapses along its surface?

Answer 6

True

Question 7

Contrast:

How are invertebrate neuronal cells different to vertebrates’?

Answer 7

Invertebrates lack any myelinated axons

This means action potentials propogate on average much less quickly compared to those within myelinated vertebrate axons (note: not all vertebrate axons are myelinated though)

Question 8

Fill-in-the-Blank:

Myelin sheaths are an ‘insulating’ cover along the axons of neurons that have gaps along them known as ____, which allow for ____ conduction. Whereas unmyelinated axons just carry out ____ conduction.

Answer 8

1. Nodes of Ranvier
2. Saltatory
3. Continuous

Question 9

Explain:

How do myelin sheaths act to increase the speed of action potential conduction along the axon?

Identify the two main mechanisms/factors:

Answer 9

1. Increasing Membrane Resistance (i.e. Decreasing Ion Permeability)
2. Reducing Membrane Capacitance

1. Since ions cannot cross the membrane where the sheath is, they diffuse along to the nodes of Ranvier (where volatge-gated ion channels are at their highest concentration) and continue action potential propagation through rapid depolarisation
2. Capicitance relates to how much charge may be stored and therefore how much is required to cause an action potential (lowering this allows easier induction of electrical impulses)

Question 10

Define & List:

State what a presynaptic terminal is, and give two alternative names for it:

Answer 10

Presynaptic terminals are the terminal/end regions of the branches projecting out from a neuron’s axon at which synapses with other cells are formed and signalling occurs.

Alternative names are ‘end bulb’ and ‘bouton’

Question 11

Fill-in-the-Blank:

1. “An ____ neuron brings information into a structure” (typically referring to PNS > CNS)
2. “An ____ neuron carries information away from a structure” (typically referring to CNS > PNS)
3. “An ____ neuron [has] dendrites and [an] axon [that] are entirely contained within a single structure”

Source: ‘Biological Psychology’ James W. Kalat 13th Edition

Answer 11

1. Afferent
2. Efferent
3. Inter-/Intrinsic

For example, the cited textbook states an intrinsic neuron in the thalamus would thus have its axon and all its dendrites contained within this brain region

Note: Afferent and efferent are relative descriptors dependent on perspective and which direction an electrical impulse is going in the context etc.

Question 12

Explain:

Would an interneuron have a comparatively shorter/longer/same-sized axon compared to other neurons?
Why?

Answer 12

It would, on average, be shorter as their structures are constrained to one brain region

Contrast: Some motor neurons can have axons that are metres long!

Question 13

Identify:

From Left to Right, name each of the basic neuron types and give an example found in the body of each one:

Answer 13

1. Unipolar (e.g. afferent/sensory nerves in spinal/cranial regions)
2. Bipolar (e.g. some retina nerves)

3.Pseudounipolar (e.g. afferent/sensory nerves in ‘more complex’ nervous systems like humans)

4.Multipolar (e.g. majority in the CNS and some in the ANS)

Note: Unipolar nerves are found in simpler nervous systems/not in humans

Question 14

True or False:

Glia outnumber neurons in the nervous system

Answer 14

False - recent scientific findings have contradicted this originally accepted notion

Glial cells have greater numbers in the cerebral cortex whilst neurons are the predominant cell of the two in places like the cerebellum

Question 15

What are actin filaments and how do they contribute to cell function?

Answer 15

They are components of the cytoskeleton that plays vital roles in movement and stability of cells.

Actin is actually one of the smallest structural components of the cytoskeleton, and is a globular protein that binds tightly to other actin proteins in order to make up actin filaments.

Question 16

Describe:

Microtubules and the role play in cellular functions.

Answer 16

They are hollow tube-like structures that are part of the cytoskeleton network and facilitate the movement of substances/organelles by acting as ‘tracks’ for motor proteins.

(These motor proteins are known as kinesins and dyneins).

They have other functions beyond this too such as assembling the mitotic spindle for cell division, and axon extension in neurons (i.e. providing structural support for both these and dendrites).

Question 17

What is meant by ‘neurons are post-mitotic’?

Answer 17

This means neuronal cells cannot replicate itself via mitosis.

This has greater connotations for the adult brain, which once fully developed has a CNS completely comprised of post-mitotic neurons.

Question 18

List:

The key ion(s) with a higher concentration in the intracellular environment.

Answer 18

• Potassium (K+)

There are also organic anions within the cell, which help contribute to its negative charge comparative to the extracellular environment.

Question 19

List:

The key ion(s) with a higher concentration in the extracellular environment.

Answer 19

• Sodium (Na+)
• Calcium (Ca2+)
• Chloride (Cl-)

The higher concentration of these ions outside the cell plays an important role in excitatory and inhibitory post-synaptic potential changes.

Question 20

Define:

‘Excitable’ cells.

Answer 20

Cells that can be stimulated by and conduct electrical charges.

This allows them to propagate action potentials.

Somatic cells that are ‘excitable’ include neurons, cardiac cells, and some types of endocrine cells.

Question 21

What are the TWO key communication mechanisms used by neurons?

Extra: Describe each of the mechanisms briefly.

Answer 21

Chemical Signalling
&
Electrical Signalling

Both involve synapses between two cells.

Chemical signalling requires the release of neurotransmitters, their diffusion across the synaptic cleft, and binding with membrane receptors of the post-synaptic cell. This triggers localised depolarisation/hyperpolarisations.

Electrical signalling involves a more ‘direct’ transfer via gap junctions between cells, which allow passive diffusion of ions/ionic current. This essentially passes an action potential from one cell to the next.

Question 22

Why do neurons die within minutes without oxygen?

Answer 22

• Neurons are constantly carrying out high-energy consumptive tasks.
• ATP is the ‘fuel’ molecule that powers these, and its synthesis requires oxygen.
• They are unable to store sufficient energy molecules and so ‘run out’ very quickly.

Despite only comprising ~2% of our mass, our brains use ~20% of the oxygen taken up by our system!

Question 23

List:

FIVE key functions of astrocytes.

Answer 23

• Maintaining homeostasis.
• Regulating chemical concentrations.
• Supporting microglia when fighting infection.
• Contributing to neural repair/glial scarring.
• Maintaining a functional blood-brain barrier.

In relation to homeostasis, it is particularly vital for astrocytes to regulate the uptake and release of glutamate.

Question 24

How do astrocytes contribute to the blood-brain barrier?

Answer 24

They secrete factors that regulate the tight junctions between endothelial cells of blood vessels.

This may either cause biogenesis/reinforcement of tight junctions, or sometimes even disruption depending on the permeability/response required.

Question 25

What is the primary function of oligodendrocytes?

Answer 25

Forming myelin sheaths around axons of CNS neurons.

Myelin sheath allow for much more rapid conduction of action potentials!

Schwann cells are the equivalent for the PNS.

Question 26

Where do microglia derive from?

Answer 26

Bone marrow cells - much like other immune cells.

Question 27

List:

TWO primary functions of microglia.

Answer 27

• Resident immune cells/’first line of defence’ for the CNS.
• Gliosis.

-

Gliosis is the removal of ‘debris’ (i.e. dead/dying cells etc.).

They achieve this through actively surveying the CNS, and carrying out phagocytosis when necessary.

Microglia may also release pro-inflammatory cytokines to activate astrocytes to support in fighting infections.

Question 28

How might astrocytes respond if the CNS had dysfunctional microglia?

Answer 28

Astrocytes possess ‘compensatory’ phagocytic machinery, and may take on the role of clearing cell debris.

Question 29

Name the PNS ‘equivalent’ of an astrocyte:

Answer 29

Satelite cells.

Although their full range of functions are unknown, new studies show satellite cells may be involved in things such as the repair and maintenance of muscle fibres.

Question 30

What importance do ‘leaky’ K+ ion channels have in regards to resting membrane potentials?

Answer 30

Helping establish and maintain the voltage difference between the intracellular (-) and extracellular (+) environment.

These ion channels are always open and so allow passive diffusion of potassium ions across the cell membrane.

The higher concentration of K+ inside the cell causes them to diffuse down their concentration/chemical gradient to the extracellular space, thus causing the intracellular environment to lose positive charge/become more negative compared to outside the cell.

Eventually the electrostatic repulsion of the built-up positive charge outside the cell causes K+ to diffuse back into the cell down their electrical gradient at the same rate as the chemical gradient drives them out.

This combined forms the electrochemical gradient that helps establish and maintain the resting membrane potential.

Question 31

Why does potassium contribute the most to the resting membrane potential?

Answer 31

The cell membrane is far more permeable to K+ overall, and thus the resting membrane potential is closest to potassium’s equilibrium potential.

Its high permeability is established by leaky potassium channels that are always open and allow passive diffusion of K+ down its electrochemical gradient.

Question 32

True or False:

All cells have the exact same resting membrane potential.

Answer 32

False

Although RMPs are ubiquitously negative, their exact value of voltage difference varies depending on the cell.

Question 33

Define:

Depolarisation

(In relation to neurons)

Answer 33

When the intracellular environment becomes less negative.

This is caused by Excitatory Post-Synaptic Potentials (EPSPs).

Question 34

Define:

Hyperpolarisation

(In relation to neurons)

Answer 34

When the intracellular environment becomes more negative.

This is caused by Inhibitory Post-Synaptic Potentials (IPSPs).

Question 35

Compare & Contrast:

Sodium vs. potassium voltage-gated ion channels.

Answer 35

• Sodium voltage-gated channels have TWO ‘gates’ (an activation and inactivation gate).
• Potassium voltage-gated channels have only ONE ‘gate’.

Question 36

Describe:

The THREE states a voltage-gated sodium channel can be in.

Answer 36

1. Resting/Closed: the activation gate is closed and no ions flow through.
2. Activated: the activation gate is open and ions flow freely.
3. Inactivated: the inactivation gate ‘swings shut’ and ions are blocked from entering the cell.

(Source: Khan Academy MCAT Unit 7)

Note: during the inactivated state, the activation gate is still open, but it’s the inactivation gate that stops further influx of sodium into the cell.

Question 37

Define:

The absolute refractory period of a neuron.

Answer 37

The point during an ‘action potential cycle’ at which the membrane can produce no new action potentials, regardless of the magnitude of stimulation.

Question 38

Define:

The relative refractory period of a neuron.

Answer 38

The point during an ‘action potential cycle’ in which a high magnitude stimulation is required for any new action potentials to be initiated.

Note this coincides mostly with the hyperpolarisation phase of the action potential.

Question 39

Describe:

The main factor contributing to the absolute refractory period of a neuron.

Answer 39

The inactivation gate of voltage-gated sodium channels are closed and so even if the activation gate is open, no sodium can flow into the cell.

Question 40

Describe:

The main factor contributing to the relative refractory period of a neuron.

Answer 40

Hyperpolarisation due to the lag in voltage-gated potassium channels closing and thus excess efflux of their positive charge, leading to the intracellular space being more negative than RMP.

Note: the inactivation gates of voltage-gated sodium channels are open at this stage though, and so unlike an absolute refractory period, another action potential could fire during this stage (so long as the stimulus is strong enough).

Question 41

How is the RMP restored after an action potential has been triggered?

(Resting Membrane Potential)

Answer 41

Sodium-potassium pumps restore the RMP ion concentrations by using ATP to actively ‘pump’ sodium out and potassium back into the cell.

For each cycle of the sodium-potassium pump, 3 Na+ are pushed out and 2 K+ are brought in.

Question 42

What causes the one-directional propagation of action potentials?

Answer 42

• Local depolarisations along the axon trigger voltage-gated sodium channels to open and a Na+ influx in that region.
• The Na+ diffuse along the axon and depolarise other regions, causing channels further along to open.
• The absolute refractory period and closing of inactivation gates on channels that have just been depolarised prevents any ‘backflow’ and ensures forwards ‘propagation of local depolarisations’.