Neurotansmission 2

Question 1

Myelin Sheath

Answer 1

fatty tube placed around the axon by either an oligodendrocyte or a Schwann cell.

Question 2

Oligodendrocytes and Schwann Cells

Answer 2

Olig: forms mylin around axons in the brain and spinal cord.
Schwann: wraps around peripheral nerves to form myelin

Question 3

Axon unmylinated with an action potential

Answer 3

It is the same along all points, all or nothing response.

Question 4

Axon mylinated with an action potential

Answer 4

The depolarisation is built up at the Nodes of Ranvier (a bit like charging a battery), and then it decays through the myelinated sections. Needs less action potentials than an unmyelinated one.

Question 5

Multiple Sclerosis

Answer 5

Due to a damaged myelin sheath.

Symptoms: loss of sensitivity, muscle weakness, difficulty with coordination and balance

Question 6

Types of Synapse

Answer 6

1. Electrical synapse:
   Very rare in adult mammalian neurons
   Junction between the neurons is very small (3nm – gap junction
   Gap is spanned by proteins (connexins) which are used to communicate between the neurons (ions move freely)
2. Chemical synapses:
   Common in adult mammalian neurons
   Junction between the neurons 20-50nm (synaptic cleft)
   Chemicals (neurotransmitters) are released from the presynaptic neuron to communicate with the postsynaptic neurones

Question 7

Why does synapse location matter?

Answer 7

The closer the synapse is to the soma the greater its influence on the production of an action potential in the axon.

Activation of an excitatory synapse leads to local and small (1mV) depolarisation of the postsynaptic cell known as an EPSP (excitatory postsynaptic potential).

EPSP decays over the length of the dendrite (decremental decay).

Question 8

Process and the Chemical Synapse

Answer 8

1) Action potential travels down the axon
2) When it gets to the synapse, depolarisation opens voltage-dependent calcium channels
3) Influx of calcium leads to neurotransmitter release
4) Neurotransmitter binds to and activates receptors on the dendrites of the postsynaptic cell
5) This leads to depolarisation or hyperpolarisation of the postsynaptic cells.
6) This spreads to the postsynaptic soma, where summation occurs.
7) If there is enough depolarisation, then an action potential is generated at the axon hillock.

Question 9

Dales Law

Answer 9

If a particular neurotransmitter is released by one of a neuron’s synaptic endings, the same chemical is released at all synaptic ending of that neuron.

Question 10

Ionotropic Receptors: Excititory Fast Transmission and Inhibitory fast transmisson

Answer 10

Excitatory fast transmission:
Ion channel opens
Movement of positive ions into the neurone (Na+)
(e.g. Glutamate receptors)
Depolarisation
Excitatory post synaptic potential (EPSP)

Inhibitory fast transmission:
Ion channel opens
Movement of negative ions into the neurone (Cl-)
(e.g. GABAA receptors)
Hyperpolarisation
Inhibitory post synaptic potential (IPSP)

Question 11

How a Metabotropic Receptor is activated

Answer 11

Activation of a G-protein coupled receptor:

Neurotransmitter binds to receptor and activates the G-protein (exchange GDP for GTP)

G protein splits and activates other enzymes

The breakdown of GTP turns off G protein activity

Series of chemical reactions that leads to an amplification of the signal – second messenger system

Question 12

Glutamate

Answer 12

Activates different types of receptors: mGluR, NMDA, AMPA, Kainate. Major fast excitatory neurotransmitter in the CNS

Question 13

GABA

Answer 13

GABA (gamma aminobutyric acid)
Major inhibitory neurotransmitter
Activates an ionotropic receptor (GABAA receptor) which opens a chloride channel (Cl-) leading to hyperpolarisation (IPSP).