NEUR321 - Cells of the nervous system

What is a good way to visualize cells?

• Fixed tissue (fixation defined on next card)

• Make thin slices

• Stain it to reveal features

• *if tissue is unfixed (alive), can see other features such as electrical activity

Define fixation (of tissues)

Fixation consists of two steps: cessation of normal life functions in the tissue (killing) and stabilization of the structure of the tissue (preservation)

For all the following terms, say whether they are present in all cells or are neuron specific: Golgi body, microfilaments, lysosomes, axon, cell membrane, dendrites, endoplasmic reticulum, dendritic spine, mitochondria, nucleus, nuclear membrane, intracellular fluid, microtubules

Only dendrites, dendritic spine and axon are neuron specific membranes; all other structures are present in all cells

The cells are enclosed in a __________; it forms a __________ between inside and outside. The fluid inside cells is called ____________, while the fluid outside cells is called ______________________.

Membrane, boundary, cytoplasm, extra cellular fluid

Membranes are semi-permeable. What does that mean?

Only lipid-soluble or small molecules can get through without help (from proteins that act as selective channels)

Explain the structure of cell membranes

It is a phospholipid bilayer → phospholipids have a hydrophilic head and hydrophobic tail, and are arranged in two layers, with heads pointing to the exterior (of the bilayer)

• The hydrophobic interior of the membrane prevents free diffusion of polar molecules — but small uncharged, lipid-soluble molecules are free to pass through it (except they won’t like the aqueous environment)

What are the functions of: Golgi complex, mitochondria, emdoplasmic reticulum and ribosomes

• Golgi complex: packaging and processing of proteins

• Mitochondria: convert sugar and oxygen into energy (in the form of ATP)

• Endoplasmic reticulum and proteins: site of protein synthesis

What is the nucleus? What does it contain?

A membrane bound structure inside the cell body — contains DNA (genetic info)

Name some animals used in neuroscience research

• Primates

• Cats

• Dogs

• Mice

• Rats

• Rabbits

• Pigs, Guinea pigs

• Sheep

• Ferrets

• Chicks

• Pigeons

• Lobsters, electric eels, fish, zebra fish, squid, slug, snails

• Drosophila

• Etc.

Neurons are the … of the brain

Information processing units

T/F: neurons are the functional units of the CNS; they relay, process, integrate, and relay information

Neurons play many roles, name a few

• receive information from sensory receptors

• Relay information to other neurons

• Control muscles → behavior

• Release hormones

• Regulate local blood flow

• Hold instructions for behavior → memories

• Regulates thoughts and emotions

• Regulates unconscious physiology → homeostasis

What are the 3 main parts of neurons?

1. Soma — core region, also called cell body

2. Dendrites — branching extensions, collect information from other cells

3. Axon — main root, carries messages to other neurons

A neuron has ________ axon that can branch, most neurons have ______ dendrites

Only one, many

The axon starts at the ______________, and it can branch into ________________ after. Near the end of the axon, there are branches called __________. Each of them ends with a pre-synaptic terminal, a.k.a _________. It is close to the ________________ of the next neuron, but it doesn’t ________ the dendrite: there’s a small gap called a __________.

Axon hillock, axon collaterals, teleodendria, button, dendritic spine, touch, synapse

How do dendrites increase the surface area of a cell? Why does it matter?

There is 1-20 primary dendrites per cell; each branches numerous times, and each is covered in dendritic spines. Both of those together increase the surface area of the cell greatly

• Dendrites collect information from other cells: More surface area thus means more information exchange

What are the 3 main classes of neurons ? (Just name)

Sensory, motor, and interneurons

T/F: within each main class of neurons, there are many many different types of neurons

True

There is a wide variety of neurons in terms of sizes and shapes. Why?

Each neuron has a job to do. The morphology of a neuron can tel, us about its function:

• Size: neurons with long extensions likely relay information, whereas neurons with short extensions likely engage in local processing

• Shape: complex dendrites integrate information from a wide area, while simple dendrites integrate on a much smaller scale and often act as relays

Give characteristics of sensory neurons.

• “input”

• Bring info TO the CNS

• Afferent/Dorsal

• Efficient relay cells

• One simple dendrite that can be small (like bipolar cells in the retina) or large (like sensory neurons for skin)

• Can be among the largest in the body (sciatic nerve ~ 1 meter)

Describe interneurons.

• Technically every neuron between a sensory neuron and a motor neuron is considered an interneuron

• 2 types:

  • Long-distance projection: possesses long axons that allow them to connect circuits of neurons in different parts of the brain

  • Local projection: short axons, form circuits only with nearby neurons (within one brain region)

• Shapes: chandelier, arcade, cajal retzius, common basket, large basket, neurogliaform, horsetail, martinotti

Give and describe 3 examples of interneurons

1. Stellate (star-shaped) cell: small; many short dendrites extend around the cell body; short axon

2. Pyramidal cell: has a long axon, a pyramid-shaped cell body, and two sets of dendrites, apical and basal; carries info from the cortex to the rest of the brain and spinal cord

3. Purkinje cell: a distinctive interneuron with extremely branched dendrites that form a fan shape; carries info from the cerebellum to the rest of the brain and spinal cord; one of the largest cells in the brain

Give characteristics of motor neurons.

• “Output”

• Signal FROM the CNS to the muscles/other organs

• Efferent/ventral

• Located in the cerebral cortex, brainstem and spinal cord

• Bushy dendrites to collect info

• Usually, large cell body and large axons that connects to muscles

• ALL efferent (outgoing) neural information must pass through them to reach muscles

Sensory neurons collect _________ information from the _____ and connect it to _____________ that process the information and pass it on to _________ neurons. Those neurons then move muscles and so produce behaviour.

Afferent (incoming), body, interneurons, motor

What are the three terms used in a sequence to describe neuronal networks?

Input → association → output

Within a neuron: information feeds into the ___________, which feed this information into the ____________, before it is relayed down the ______ which branches at its end.

Dendrites, cell body, axon

T/F: neurons conduct all of the signals they receive (i.e. all signals received by the dendrites will go down the axon of the neuron). Explain

False

• A neuron has thousands of inputs, both negative and positive

• It processes and integrates the inputs, then potentially relays this information onwards depending on the average summary of the inputs (later flashcards will explain the principle)

T/F: the pre-synaptic axon terminals come in contact with the dendrites of the post-synaptic neuron

False. There is a gap between the two neurons called the synapse

What is contained into the presynaptic terminals?

Chemicals → neurotransmitters and neuropeptides

• they are released onto the next neuron to relay the signal

Are chemical synapses unidirectional or bidirectional?

In general, they are unidirectional, meaning that only the presynaptic cell influences the postsynaptic cell

Signals from a presynaptic cell can either _________ or __________ the next neuron

Excite or inhibit

• More excitation than inhibition = signal relayed

• More inhibition than excitation = neuron stays silent

What are the two views on how neurons code for behaviour?

1. Each neuron codes for specific aspects of behaviour “grandmother cell” — Jerome Lettvin, 1969

2. Neurons work together in complex networks to code for specific aspects of behaviour — loss of one or more neurons would not affect behaviour as the network can still function

Accepted = network

Review the cricket / simple network example

T/F: neurons are static

False. They can change, retire, make new connections, etc.

Neurons are post-mitotic. What does this mean?

Means that they cannot be replaced (most); they live with us throughout our whole life

Give main characteristics of Glial cells

• Between a 1:1 and 10:1 ratio with neurons

• Like neurons, They have multiple forms and play multiple roles

• Unlike neurons, they are constantly renewed throughout life

• They play the supporting actor role in the nervous system

Name the 6 main types of Glia

1. Ependymal cells (CNS)

2. Microglia (CNS)

3. Astrocytes (CNS)

4. Oligodendrocytes (CNS)

5. Satellite Glial cells (PNS)

6. Schwann cells (PNS)

Describe ependymal cells

• Cells that line the ventricles

• They help generate cerebrospinal fluid (CSF) and create/assist the flow of the fluid

Describe microglia

• Immune system and repair in CNS

• Clear dead cells (including apoptotic neurons)

• Similar functions as white blood cells (monocytes) and macrophages

• Recent evidence shows extensive and essential communication between microglia and neurons

Describe astrocyte/astroglia

• Star shaped

• Provide structural support

• Convey nutrients between blood vessels and neurons

• Regulate blood flow

• Part of the blood brain barrier

• Can play a role in healing the brain after damage

Describe satellite glial cells

• Small cells that surround neurons cell bodies located in the autonomic nervous system (PNS)

• They resemble the astrocytes and assist in regulating the external chemical environment

• Very sensitive to injury and may exacerbate pathological pain

Describe oligodendrocytes

• Myelinate axons in the CNS

• Branch and myelinate several axon segments of different cells

Describe Schwann cells

• Myelinate axons located in the PNS

• Myelinate only one segment of only one cell (wraps around the axon)

What wraps around axons? What does it do?

The myelin sheath

• Insulates the neurons

• Speeds up transmission of the neural signal

• Each segment is separated by a small gap called node of Ranvier

T/F: In the CNS, several myelinated and/or unmyelinated axons together with the blood supply and other connective tissue will form what’s called a nerve bundle

False, it’s in the PNS

Many brain tumours arise from abhorrent glial cells. What are those diseases called?

Gliomas

What disease affects myelination

Multiple Sclerosis

When neurons are injured in the nervous system, it can lead to loss of ___________ and loss of ____________ in the affected areas. Is recovery (to near normal function) possible ?

Sensation and movement

• Possible sometimes in the PNS, because of glia

• Not possible in CNS because of the formation of glia scars (astrogliosis)

look up peripheral nerve regeneration for better explanation on slide 50

Define gene

A gene is a portion of DNA that codes for the structure of a protein. When produced, these proteins direct cell development and function

It is estimated that humans have ~______ protein coding genes (Pertea and Salzberg)

22,300

DNA is stored in the cell nucleus in ___ pairs: ___ pair(s) of autosomal and ___ pair(s) of sex chromosomes. A person with trisomy has ___ chromosomes instead of ___. Each chromosome is one long coiled ____________ molecule made up of 4 simple _______________ (name them). The paring is __ & G and __ & T

23, 22, 1, 47, double-helix, nitrogenous bases (adenine, guanine, cytosine, thymine), C&G, A&T

How does DNA code for proteins?

• First, DNA is transcribed to messenger RNA (mRNA)

• mRNA is moved to the cytoplasm, where it interacts with ribosomes on the endoplasmic reticulum

• The ribosome reads the mRNA code and builds a protein according to the instructions (becomes string of amino acids = protein)

Ribosomes read sets of three mRNA bases, called _______. One translated (previous term) = one _______________, following the RNA code. The RNA code contains ___ combinations, but there is only ___ amino acids.

Codon, amino acid, 64, 20

T/F: proteins are strings of amino acids

True

Describe the structure of an amino acid

All have:

• amino group (NH3+)

• Carboxyl group (COO-)

• A side chain that determines the nature of the amino acid

• One hydrogen atom

T/F: proteins are large, complex molecules that play many critical roles in the body; They are essential for the structure, function, and regulation of the body’s cells, tissues and organs

True

A proteins function depends on its _________

Structure

Proteins can be found at 3 locations. Name them. How do they know where to be?

1. Bound to membranes

2. Floating free in the cytoplasm

3. Released outside the cell

Prepared for these locations in the ER and Golgi

When bound to membranes, proteins can act as … (2) What do these do?

Channels (gated or non-gated) or pumps

They regulate the transport of molecules, such as ions, in and out of cells

Give examples of proteins that can change their shape when they interact with other molecules

1. Cell surface receptors : allows a cell to respond to its environment

2. Enzymes: regulate biochemical reactions inside the cell

What’s the function of the following (membrane proteins):

1. Ion channels

2. Receptors

3. Transport molecules

4. Membrane-bound enzymes

5. Adhesion molecules

1. Provide diffusion pathway for charged ions across the membrane

2. Recognize and bind extracellular signalling molecules and transmit message across the membrane

3. Pumps/carriers move membrane impermeable molecules across the membrane

4. Synthesize biochemical reactions (both on the inside and outside of the membrane)

5. Mediate adhesion between cells or between cells and their environments

If all of our cells have the same genes, how come we have different types of cells, tissues, or organs?

Not every cell expresses the same genes — thus, not every cell can generate every protein, making them different (e.g. a liver cell will only express the genes needed to form liver tissues, etc.)

It is the genes that the cell expresses that give the cell its unique identity

T/F: only a small proportion of our genes are expressed in the brain

False, large proportion — underlies the diversity and complexity of neurons