The Nervous System

Question 1

What are the main functions of the nervous tissue?

Answer 1

Excitability
Conductivity - ability to receive an excitation.
Transmissibility - ability to transfer the information to another site.

Question 2

Describe the general features of a neuron.

Answer 2

Very heterogeneous, they can be very small or big: 5-100 μm.

Different shapes but common plan of cytological organization.

Question 3

How does a neuron transmit signal?

Answer 3

A stimulus arrived at the dendrites. If this is strong enough to reach the cell soma, then it is transmitted to the end of the neuron.

Question 4

Describe the chracteristics of the cell body of neurons.

Answer 4

Large euchromatic nucleus with prominent nucleolus surrounded by perinuclear cytoplasm.

Nissl bodies can be seen which are the cisternae of RER.

Under TEM:
RER with free ribosomes
Mitochondria 
Barr body if female.
Cajal body (arean containing snRNPs to splice pre-mRNA).
Well developed GA.

Question 5

Describe the appearance of motor neurons.

Answer 5

Motor neurons are big so very easily studied.

A big pale nucleus is observed with a dark spot inside, which is the nucleolus. The nucleus is lightly stained as the chromatin is dispersed. This is highly transcribing chromatin as the neuron makes the neurotransmitter.

Nissl bodies are seen in the cytoplasm.

Question 6

Describe the classifications of neurons based on their functions.

Answer 6

• Sensory neurones transmit signal from the receptors to the CNS.
• Motor neurons transmit signals from the CNS or ganglia to effector cells.
• Interneurons (intercalated neurons) form communication and integrating network between sensory and motor neurons.

Question 7

Describe bipolar neurons.

Answer 7

1 dendrite, 1 axon.

Often associated with receptors for special senses.

Generally found within retina or vestibulocochlear of ear.

Some neurons don’t fit into above generalization, e.g. amacrine neurons have no axons and only dendrites.

Question 8

Describe pseudounipolar neurons.

Answer 8

1 process, axon divided into 2 branches.

One branch extends to periphery, the other to CNS.

Developed from bipolar neurons.

Mostly sensory neurons close to the CNS.

Sensory neuron cell bodies located in the dorsal root ganglia and cranial nerve ganglia.

Question 9

Describe main features of dendrites.

Answer 9

Multiple sensory extensions (afferent) of the neuron cell body.

Form dendritic trees that increase neuron surface area.

Contains many cytoplasmic organelles, similar to cell body except the GA.

Small protrusions called spines on the surface.

Question 10

Describe main features of axons.

Answer 10

Single extension which can divide into branches after emergence from cell somar.

Axons can have different lengths, this depends on the type of neurons. They can be up to 1m long.

Not rich in cytoplasmic organelles, there is very little SER. Contains small tubules of SER, mitochondria and cytoskeleton.

Brings nerve impulse from the cell body to the periphery (efferent function).

Constant flow of molecules in the axon (axoplasmic flow).

Originates from the axon hillock.

Question 11

What is the main function of dendrites?

Answer 11

Receive information from other neurons or the environment and transmit to the cell body.

Question 12

What is the main function of axons?

Answer 12

Convey information away from the cell body or to another neuron or effector cell.

Question 13

Describe axonal transport.

Answer 13

Anterograde - transport from the cell soma to the axon terminal, this involves the motor protein kinesin.

Retrograde - transport from the axon terminal to the cell soma, this involves motor protein dynein.

Question 14

What are the layers of connective tissue covering nervous tissue at each level?

Answer 14

There’s also a layer of connective tissue surrounding the whole cell called the endoneurium.

Then another layer of connective tissue surrounds groups of nerve fibers called the perineurium.

Lastly a third covering surrounds the whole nerve called the epineurium.

Question 15

What does a nerve fiber consist of?

Answer 15

A nerve fiber is the axon including its coating of myelin sheath.

Question 16

Describe the formation of the myelin sheath (myelination).

Answer 16

Schwann cells surround the axon, its cell membranes become polarized.

The

Question 17

What is the myelin sheath made of?

Answer 17

Made up lipids and proteins, the PM of schwann cells.

Question 18

Describe the formation of the myelin sheath (myelination).

Answer 18

Schwann cells surround the axon, its cell membranes become polarized.

The schwann cell begins wrapping around the axon in a spiralling motion.

A mesaxon domain is formed from the connection between abaxonal PM (external) adaxonal PM (internal).

The mesaxon further rolls around the axon, creating an inner and outer mesaxon.

The cytoplasm and nucleus is squeezed to the periphery.

Question 19

Describe the structure of the myelin sheath.

Answer 19

The major dense line is observable as regions of fusion between the inner surfaces of the PM.

In between are the intraperiod lines as regions of fusion between outer surfaces of the membrane.

Question 20

What are the functions of the myelin sheath?

Answer 20

Increase velocity of impulse.

Insulate the axon.

Question 21

What are Schmidt-Lanterman clefts?

Answer 21

There are lager spaces in between the major dense lines and the intrapedial lines in which a higher amount of cytoplasm contained.

These structures appear as incisors, or clefts. They are called Schmidt-Lanterman clefts.

They are the consequence of the interaction between the internal faces of the schwann cell. There are some areas in the cytoplasm where there are higher amounts of cytoplasm contained.

Question 22

What is the function of connexin at the level of the cleft?

Answer 22

At the level of the cleft, there is the expression of Cx32.

These connexins allow the passage of molecules from one layer to another, as the folded layers allow little molecules into the middle.

Question 23

Describe myelination in the CNS.

Answer 23

In the CNS, oligodendrocytes form the myelin sheath of more than one axon (up to 50). This is because the oligodendrocytes have a specific shape.

Question 24

Describe the structure of non-myelinated fibers in the PNS.

Answer 24

There are also non myelinated fibers in the PNS. they are slower in transmission. Although the axons are not naked and are allocated in the invagination of schwann cells.

Question 25

When does myelination usually occur?

Answer 25

Myelination occurs in the last phases of foetal development and continues in postnatal life.

Question 26

How do schwann cells contribute to nervous tissue regeneration?

Answer 26

Schwann cells can proliferate and organise a path over which the axon is reconstructing.

There are signals released by the schwann cells and target cells that drive the regeneration of the nerve fiber.

Question 27

Describe the condition of multiple sclerosis.

Answer 27

Multiple sclerosis is due to an autoimmune response against components of the myelin sheath. This means that the immune system of the patient is stimulated to produce antibodies against their own myelin.

There is an inflammation in the CNS which leads to the destruction of the oligodendrocyte and demyelination of fibers, thus progressive loss of for instance motor activities.

Question 28

How does a neuron transmit signal?

Answer 28

If a dendrite receives information that reaches a specific threshold, an action potential is started that stimulates Na+ voltage gated channels to open.

The membrane potential at rest is negative, when there is an opening, Na+, usually more abundant in the extracellular matrix, will flow inside the axon and invert the action potential to become + (depolarization).

Then K+ voltage gated channels open to allow K+ to flow out into the extracellular compartment, they are usually more abundant in the intracellular compartment (repolarization).

But in order to repolarize, the Na+ needs to be actively pumped out and K+ pumped in. Since this would occur against the concentration gradient, active transport is needed in the form of sodium potassium pumps.

Question 29

Describe main features of a synapse.

Answer 29

The interconnection between neurons occurs through structure called synapses.

Synapses can be chemical or electrical.

In chemical synapses are the release of vesicles carrying the signal across the synaptic junction.

While in electrical synapses, there is just the formation of gap junctions with the flow of ions responsible for transmitting the impulse from one synapse to another. Electrical synapses are faster as there is a direct flow of ions.

Question 30

What is neuroligin? Why is it important?

Answer 30

A cell adhesion protein on the postsynaptic membrane that mediates the formation and maintenance of synapses between neurons.

Neuroligins are essential for the proper development of cognitive functions. Abnormal genes for this function are accompanied by autism and mental retardism.

Question 31

How does a synapse work?

Answer 31

An action potential arrives at the axon terminal, which induces Ca2+ voltage gated channels in the PM to open.

This causes synaptic vesicles to migrate, dock and fuse with the presynaptic membrane, therby releasing their neurotransmitters via exocytosis.

Neurotransmitters diffuse across the synaptic cleft and binds to post-synaptic cleft (transmiter gated channels), therby changing its conformation and allowing their pores to open.

Question 32

How does Ach work?

Answer 32

Ach is released at the axon terminal, binds to specific receptors which activates depolarisation of the PM that will open the sarcoplasmic reticulum and release the Ca2+ to activate contraction.

After Ach has bound the receptor, it needs to be removed. An enzyme called acetylcholine esterase cuts ach into choline and acetic acid, then recycled to make new ach.

Question 33

How do nerve agents work at the level of synapses?

Answer 33

Nerve agents can bind to acetylcholine esterase and stop its action, this leads to the accumulation of ach and therefore a continuous stimulation of muscle fibers.

Question 34

How can nerve agents be blocked?

Answer 34

Atropine can be used to block the effect of nerve agents.

It binds ach receptors and blocks the large amounts of ach that has accumulated in the synaptic space to bind to receptors.

Question 35

How can some drugs work at the level of the synapses?

Answer 35

When drugs are taken that function at the level of the nervous system, most function as agonists or antagonists of the neurotransmitters.

Question 36

What is porocytosis?

Answer 36

Neurotransmitter are released without fusion.

Presynaptic vesicles anchored to the presynaptic membrane next to Ca2+ selective channels by Snare and synaptotagmin proteins.

Ca2+ caused vesicle to change conformation and form a 1nm transient pore connecting its lumen to the synaptic cleft. Neurotransmitters are released through here.

Question 37

What are neuroglia cells?

Answer 37

Family of non-neuronal cells, found in the nervous tissue generally around and between neurons.

Cells of the neuroglia can be divided into 2 groups: the neurologia of the CNS and the PNS.

Question 38

What are the functions of neuroglia cells?

Answer 38

Support the functions of neurons.

Intervene in the organisation of nerve tissue, e.g. astrocytes important in removing neurotransmitters in the synaptic cleft.

Responsible for making faster or slower transmission.

Involved in blood brain barrier.

Question 39

Describe the functions of astrocytes.

Answer 39

They are important in forming the blood brain barrier, which is formed due to tight junctions between endothelial cells. Though there are also pericytes involved, the astrocytes lie on top of the vessels and form an additional layer surrounding the capillaries.

Another important function is to support the right precursors for energy for the neurons as neurons prefer lactate. Astrocytes transform glucose from the capillaries into lactate and pass on to the neurons.

Movement of metabolities and waste to and from neurons.

Question 40

Describe the main features of astrocytes.

Answer 40

Largest of neuroglial cells.

Forms a network of cells in the CNS to communicate and support neurons.

2 types - the fibrous (in white matter) and the protoplasmic (in gray matter)

Question 41

Describe the main features of microgila cells.

Answer 41

They are part of the monocyte macrophage lineage (phagocytic cells).

Smallest of neurogilal cells with relatively small, elongated nuclei.

They can be easily identified by the expression of specific proteins like the iba1.

Question 42

What are the functions of microglia cells?

Answer 42

Part of the immune surveillance at the level of the CNS.

They mediate neuroimmune reactions, they are involved in chronic pain after spinal cord injuries.

They usually remove molecules that can pass the brain blood barrier.

Question 43

What are the functions of ependymal cells?

Answer 43

Epithelial cells which line the cavities of the brain and the spinal cord, so represent the epithelium which covers these cavities.

Question 44

Describe the main features of ependymal cells.

Answer 44

Single layer of cuboidal to collumnar cells.

These are ciliated cells, the movement of the cilia are important in mixing the brain fluid in the spinal canal.

Question 45

What are the functions of satellite cells?

Answer 45

A role in nutrition and modulation of the function of neurons in the ganglia.

Electrical insulation.

Pathway for metabolic exchanges.

Question 46

Describe the stem cells of nervous tissue.

Answer 46

There are stem cells in the nervous tissue, the nervous system has regenerative ability, although this is very limited.

There are neural stem cells present in the ventricular zone. These can differentiate and give rise to some of the glial cells.

Question 47

What unusual conditions occur in alzheimers?

Answer 47

In alzheimers, there’s a precursor of amyloid which is degraded in an unconditional way, causing an accumulation of this incorrectly processed protein in the extracellular environment, which forms plugs in the nervous system.

At the same time, there is a hyperphosphorylation in proteins involved in the stabilisation of the neurotubules.

Question 48

Why do the unusual conditions in alzheimers have a detrimental effect?

Answer 48

This amyloid precursor is probably important in the stabilization of synapses, therefore the unconventional processing causes unstable synaptic junctions.

Inside the cell, microtubules are stabilized by microtubule stabilizing proteins, one of which are tau proteins. If tau is hyperphosphorylated, it is detached from the tubulin molecules. So microtubules dissociate and become unstable, forming neuro defibrillating tangles.

In an alzheimer brain, there are aggregates of cytoskeleton formed inside the cells, as well as the formation of amyloid plugs in the extracellular space.