Lecture 3: Microanatomy and Neurotransmission

Question 1

How did early anatomists see neurons?

Answer 1

• -early anatomists would dehydrate brains in formaldehyde or alcohol so they would be easier to work with
• -What they found was that if you took these dehydrated brains and put them in solutions that contained metal, then the functional units (neurons) would suck up the metal so that you would be able to see them. This was by chance. We have no idea why they suck up the metal
• -we have no idea how golgi stains work, we just know they work

Question 2

What is the soma?

Answer 2

• -Core region;

- -Processes information

Question 3

What are dendrites and dendritic spines?

Answer 3

Dendrites
–Branching extensions;
–Receive information;
–# of dendrites = amount of incoming information.
Dendritic Spines
–Protrusions from a dendrite that serves as point of contact with other axons

Question 4

What is the axon/axon hillock/axon collateral?

Answer 4

Axon
--Carries information to other neurons;
--White matter.
Axon Hillock 
--Point at which the axon leaves the soma (cell body).
Axon Collateral(s)
--Point at which axon branches out;
--Allows message to be sent in multiple directions simultaneously

Question 5

What is the myelin sheath?

Answer 5

• -Insulates axons;

- -Signal travels further, faster, stronger

Question 6

What is the terminal button/synapse?

Answer 6

Terminal Button
–Stops extremely close to dendritic spine of another neuron;
–Does not touch other neurons.
Synapse
–Junction between one neuron and the other;
Space between terminal button & dendritic spine.

Question 7

What are neurons and glia cells?

Answer 7

Neurons

• -Carry out brain’s major functions;
• -Many different types; Can be very specialized.
• -sensory neurons, interneurons, motor neurons

Glia Cells

• -Aid and modulate activity of neurons;
• -nourishment, support, synthesis & clean-up.

Question 8

What are sensory neurons?

Answer 8

–Brings sensory information to the brain (afferent). monitor what’s going on I the outside world
–Structurally, they are the simplest type of neuron.
–One single dendrite on one side, cell body, and single axon on the other side
Subtypes of sensory neurons:
1) Bipolar neurons; E.g. retinal bipolar cell: light hits the receptor and then the retinal bipolar cell fires an action potential
2) Somatosensory neurons.E.g. multipolar cell (can receive more information and send more information)

Question 9

What are interneurons?

Answer 9

–Link sensory & motor neurons
–Ensures that you don’t have movements when you don’t want any
–Lots of dendritic branching so that it can receive lots of information from the sensory neurons to see if it should fire or not and signal to the motor neuron
–Subtypes of interneurons:
1) Stellate cell (star shaped):
Very small, many dendrites extending around entire cell body.
2) Pyramidal cell (pyramid shaped):
Long axon with multiple sets of dendrites.
–axon could be really long (from head to toe)
3) Purkinje cell:
Output cell; Extremely branched dendrites - Cerebellar purkinje cells receive information from every part of the brain so that you can coordinate your motor movements precisely (decides whether to fire or not and which motor neuron/muscle fiber to stimulate)

Question 10

What are motor neurons?

Answer 10

• -Carry information (motor instructions) from brain into spinal cord and muscles (efferent)
• -Not only muscles; internal organs as well like the stomach and intestines (GI tract) to get them moving so you can digest your food
• -Large cell bodies to process information.
• -All outgoing information must pass through motor neurons to reach target muscles.
• -The purkinje cells influence activity of the motor neurons (fine tune them and correct previous mistakes)

Question 11

What are the subtypes of glia cells?

Answer 11

1) Ependymal cell – located on walls of ventricles, produce CSF;
2) Astrocyte – provides structural support, regulates blood brain barrier; wrapped around the blood vessels -the symmetry allows it to provide structural support
3) Microglia – immune function, engulfs foreign substances; Microglia help get rid of toxins that passed the BBB. They lie dormant and are in their large state but then shrink up when there’s a foreign substance for easier and faster navigation (so it can fit through in between all the other cells and thus move faster)
4) Oligodendroglia – insulates axons in the CNS;
5) Schwann cell – insulates axons in the PNS

Question 12

How do neurons in the PNS repair themselves when you get a cut?

Answer 12

–Microglia remove debris
–Schwann cells will divide and form a breadcrumb trail for new axons to follow
–Schwann cells envelop the new axon, forming new myelin
Unfortunately, repair is much less common in CNS damage.

Question 13

What is the difference between a nerve and a tract?

Answer 13

Axons tend to project in bundles

• -Nerve when outside the CNS;
• -Tract within the CNS.

Question 14

How do neurons communicate?

Answer 14

For one neuron to communicate with another neuron, it must use both electrical and chemical signals (mostly, there are always exceptions).

Question 15

Explain the electrical communication of a neuron.

Answer 15

–Each neuron has a resting membrane potential of about -70mV
–This occurs because the inside of the cell is negatively charged, relative to the outside of the cell
=Large negatively charged proteins (A-) inside;
=Potassium ions (K+) - inside;
=Large Sodium ions (Na+) – outside;
=Chloride ions (Cl-) – outside.
–We rely entirely on sodium-potassium pumps to keep this balance.
–Exchanges 3 Na+ (outbound) for 2 K+ (inbound);
–Use up ~2/3 of a cell’s energy expenditure
–How can membrane potentials change?
–Channels and pores on the cell membrane surface allow ions in and out of the cell;
–Different stimulations will open different pores/channels and allow ions to enter/exit the cell (when a neurotransmitter binds to receptors, it opens these channels)

Question 16

What is hyperpolarization/depolarization?

Answer 16

• -Hyperpolarization: membrane potential is exaggerated, so difference between inside and outside are greater; cell becomes more negative
• -Depolarization: membrane potential is diminished, so difference between inside and outside are lessened. Cell moves toward the positive side

Question 17

Explain the action potential.

Answer 17

–An action potential is a brief (~1ms), but very large, reversal in polarity of an axon’s membrane.
–Inside the cell (i.e. intracellular) becomes positive, relative to the outside (i.e. extracellular);
–Reversal of membrane polarity is due to an influx of Na+ and efflux of K+.
–Stimulation of cell is required to depolarize a membrane to -50mV;
—Each neuron will receive excitatory and inhibitory input from pre-synaptic cells.
Excitatory Post Synaptic Potentials (EPSP)
Inhibitory Post Synaptic Potentials (IPSP)
–Once threshold is met, no further stimulation is required, and depolarization continues until the inside of the cell reaches +30mV (or +40mV), relative to the outside.
–Na+ channels are sensitive to small changes in membrane potential, so any ion fluctuations will cause them to open;
–Voltage gated K+ channels are attuned to -50mV. Once this threshold is met, they all open.
–Most of these voltage gated ion channels are at the axon hillock which is why the stimulation needs to propagate down to the axon hillock. Once it does, these voltage-gated ion channels open and an action potential occurs that propagates down the entire length of the neuron by jumping from one node of ranvier to the next
–This change is abruptly reversed, thanks to the Na+/K+ pumps, and the resting membrane potential (-70mV) is restored

Question 18

What is spatial summation and temporal summation?

Answer 18

• -Spatial summation: EPSPs & IPSPs that happen close together in space will be summed;
• -Temporal summation: EPSPs & IPSPs that happen close together in time will be summed.

Question 19

How does chemical transmission occur?

Answer 19

–Synthesis:
some neurotransmitters are synthesized from DNA/mRNA and stored in vesicles located in the axon terminal. Others are synthesized from building blocks imported into the terminal.
–Release:
Transported to pre-synaptic membrane, an electrical signal arrives and triggers the release of neurotransmitters into the synaptic cleft
–Receptor Action:
Activate target receptors on post-synaptic membrane;
Depending on the receptor, the post-synaptic cell can be excited or inhibited.
–Inactivation (X4):
Must be inactivated or will continuously stimulate (or inhibit) post-synaptic neuron;
Modes of inactivation are i) glial cell uptake, ii) enzymatic degradation, iii) diffusion and/or iv) reuptake.

Question 20

How many NT and what are the different classes?

Answer 20

–The exact number of NTs is unknown, but over 200 unique transmitters have been identified
–Classes of NTs:
1) Monoamines NT’s
Dopamine (DA), Norepinephrine (NE), Epinephrine (aka Adrenaline); Serotonin (5-HT); and Histamine.
2) Amino Acid NT’s
Glutamate, GABA, glycine and D-serine;
3) Peptide NT’s
Somatostatin, Substance P, Opioid peptides;
4) Transmitter Gases
Nitric oxide, carbon monoxide.