Excitable Tissue : Nerve

Question 1

What is special about Glial Cells in comparison to Neurons?

Answer 1

Glial cells continue cellular division into adulthood, neurons do not.
Neurons are finite and actually make up the least amount of nerve tissue.
When there is damage, the Glial cells divide and fill in the space that the damaged nerve took up.

Question 2

Glial cell types and actions

Answer 2

Microglia : brain macrophages

Macroglia :

a) Oligodendrocytes : Provide myelin for the CNS. Multi-branched, synapse with a number of different neurons.
b) Schwann cells : Provide myelin for neurons in the PNS (peripheral). Isolated to 1 branch for 1 neuron and sometimes only one small location on that neuron.
c) Astrocytes : Neurotransmitter vacuum cleaner.

Question 3

Astrocyte function ( specific)

Answer 3

• Neurotransmitter vacuum cleaner. (particularly glutamate and GABA). Important because Glutatmate is our most excitatory neurotransmitter, and GABA is the most inhibitory neurotransmitter, and too much of that stimulus hanging around can be damaging
• Induce capillary endpthelial cells to make up the blood brain barrier. (tell the endothelial cells to make up the blood brain barrier - send out projections and act as overseers.
• Regulate extracellular ion concentrations - like an ion vacuum. Particularly for K and Na
• Produce trophic substances to neurons ( glycogen, lactate) : provide food so that the neurons have a source of energy to carry out their functions

Question 4

Myelin sheath

Answer 4

wrapped around the axon. Made of many layers. has a number of gap junctions within so that signals don’t take a large amount of time to make it around all of the layers.

Allows for unidirectional movement of the action potential. Usually the AP starts at the end, sometimes we put intentional blockers so the AP doesn’t fire.
ex) during biceps flexion, we put a blocker on triceps flexion so that we don’t tug/tear the muscle.

Question 5

Node of Ranvier

Answer 5

small exposed section of the axon which is used to conduct electrical signal : hop along the nodes of ranvier. Concentrates all the voltage gated channels which generate action potentials

Question 6

Cell body of a neuron

Answer 6

houses the nucleus, contains organelles, cell machinery for protein synthesis and other functions.

Question 7

terminal buttons

Answer 7

where we release neurotransmitters : conversion of electrical signal to a chemical one

Question 8

Neurotransmitter Recycling

Answer 8

a number of the neurotransmitters and taken back up again and recycled. However, the neurons still need a constant supply of material and fuel to make that happen.

Question 9

Where do action potentials happen?

Answer 9

Between the Axon hillock and the initial segment of the axon.

(like an equation ; need enough positive numbers to overcome the negative numbers and reach the threshold.

Question 10

Abourization

Answer 10

Increasing sophistication of brances in the neurons. Most sophiticated are in the cerebellum, the regulation centre, which helps with coordination and balance. If this was not so complex, we would all seem drunk all of the time.

Question 11

4 important functional zones of neurons.

Answer 11

1) Receptor/ Dendritic zone : 
site where summartion of signals occurs
2) Site where propogation of action potentials are generated : 
initial segment in motor neurons, node of ranvier in cutaneous sensory neurons
3)Axon Process : 
transmits impulses to nerve endings
4) Nerve endings :
site of neurotransmitter release.

Question 12

Axonal Transport (general)

Answer 12

• connects cell body with terminal buttons : could be up to 1 meter.
• protein synthesis happens in the cell body , needs to be communicated to the terminal buttons
• Axoplasmic flow

Question 13

Axoplasmic flow

Answer 13

• Axoplasmic flow : movement of materials from the cell body, to the axon terminals and back.

Question 14

Kinesins

Answer 14

Facilitate orthograde transport ( movement of materials from the cell body to axon terminals)

like little protein people. they walk along the actin filaments with a backpack, and carry things to the axon terminals.
Fast ones : 400 mm a day
Slow ones : 0.5-10.. a day

ATP dependent process

Question 15

Dynein

Answer 15

Faciliate retrograde transport ( movement of materials from the axon terminals to the cell body)

ie) like recycled membrane vesicle

200mm per day

ATP dependent process

Question 16

Explain the Action Potential

Answer 16

1 - Neuron starts at resting membrane potential ( RMP)
NA and K channels are closed.

2- Summation of synaptic potentials depolarizes membrane to threshold. Beginning of absolute refractory period ( once we reach threshold, the floodgates open and everyone does the same thing)

3- Widespread opening of Na gated channels , which inititates the positive feedback loop causing rapid depolarization phase. (Na reacts the fastest and floods into the neuron)

4 - Peak depolarization ( facilitated by K) undo the whole process, positive charges leave the cell.
5 - Na Channels enter inactivated state and repolarization occurs due to slower, still open K channels.

6 - K channels are slow to close resulting in the membrane being hyperpolarized.

7 - Na channels reset marking the beginning of the relative refractory period. The membrane remains slightly hyperpolarized ( defense mechanism : want to make sure we really want to fire b/c it can cause damage from constant unnecessary action)

This all happens very quickly : between 3.5-4 miliseconds.

Question 17

How do we restore the Na and K pool levels?

Answer 17

The action potential only takes a fraction of the pools.

But, Na/ K ATPase helps push them against their gradients. * ATP dependent process.

Question 18

Role of Calcium

Answer 18

Not much in Action Potentials, largely involved in muscular contraction.

Changes in extracellular calcium will influence neural action poetntials :
too much Ca : hyperpolarized cell.
too little Ca : depolarizes cell ( neurons fire more easily)

Question 19

Chloride

Answer 19

Associated with Gaba, it serves as an inhibitory molecule : it makes it harder for the Action potential to happen b/c it’s more negative, harder to reach threshild. it hyperpolarizes the cell.

Seizure medications work in this way - enhance GABA