Week 3 - Nervous System Basics

Question 1

What is cell membrane potential?

Answer 1

All communications between neurons and other cells
occur through their membrane surfaces
These membrane changes are electrical events that
proceed with great speed

Question 2

What charge is the membrane of an undisturbed cell?

Answer 2

+ve on the outside and -ve on the inside.
The uneven distribution of charges is called potential difference.

Question 3

What is the cell membranes resting potential?

Answer 3

0.07V(-70mV)

Question 4

What are the contents of the intracellular and extracellular fluid?

Answer 4

Extracellular: High concentrations of sodium (Na+) and Chloride Ions (Cl-)
Intracellular: High conc of potassium Ions (K+) and negatively charged proteins (Pr-)
Membrane is selectively permeable.

Question 5

What are the different types of channels in the membrane?

Answer 5

Leak = always open
Gated = open/close depending on circumstances

Question 6

What happens during passive movement in the cell?

Answer 6

Chemical Concentration Gradient: move K+ out of the cell;
Na+ into the cell
• K+ moves out of cell faster than Na+ can move into cell
Electrical gradient: +ve charged outer membrane surface
repels +ve K+ ions
• At same time, -ve charge inner membrane attracts +ve
Na+
• Chemical gradient is stronger than electrical repelling
force
• K+ still move
Net Loss of +ve ions

Question 7

What happens in the sodium potassium pump?

Answer 7

Pumps 3 intacellular sodium ions for 2 extracellular potassium ions.

Question 8

What happens in a change in membrane potential

Answer 8

Any stimulus that (1) alters membrane permeability to
sodium or potassium or (2) alters the activity of the
exchange pump will disturb the resting potential of the cell
Any change in the resting potential can have an immediate
effect on the cell
A stimulus opens gated ion channels that are normally
closed during resting potential
Opening of these channels accelerates the movement of
ions across the cell membrane
This movement changes the membrane potential

Question 9

What happens to chemically gated Chanel’s during a change in membrane potential?

Answer 9

OPENING OF GATED NA+ CHANNELS INC SODIUM ENTRY INTO CELL, INCREASING NUMBER OF +VE IONS SHIFTING THE MEMBRANE POTENTIAL TO 0MV(depolarisation)

When stimulus is removed depolarisation occurs which restores resting membrane potential
A stimulus that opens gated K+ channels will move membrane potential away from )my which may take it to -80mv (hyperpolarisation)

Question 10

What are graded potentials?

Answer 10

Also called local potentials
Only affect a limited portion of the cell membrane –
cannot spread far from the site of stimulation
Affect too small an area to have an effect on the activities
of relatively enormous cells
Produced by any stimulus that opens a gated channel
Occur in the membranes of all cells in response to
environmental stimuli

Question 11

Explain the process of graded potentials

Answer 11

1. Resting membrane exposed to chemical, sodium channels opens, Na+ enters cell, membrane potential rises, depolarisation occurs.
2. Movement of Na+ through channel produces local current. Depolarises nearby cell membrane (graded potentials) Change in potential is proportionate to stimulus.
3. When stimulus removed, membrane potential returns to normal - depolarisation. Graded potential can influence operations in distant portions of the cell if they lead to the production of an action potential.

Question 12

Describe an action potential

Answer 12

An electrical signal that affects the entire membrane
surface
An action potential is the change in the permeability of the
cell membrane
Skeletal muscle fibres and axons have excitable
membranes that will conduct action potentials

Question 13

What is the all or none principle?

Answer 13

Every stimulus – minor or extreme – that brings the
membrane to threshold will generate an identical action
potential
This principle applies to excitable membranes

Question 14

Describe how an action potential is generated

Answer 14

1. Depolarisation of membrane to threshold potential.
2. Activation of Na channels and rapid depolarisation.
3. Inactivation of Na channels and activation of K channels
4. Brief Hyperpolarisation, return to normal permeability and resting state
5. Return to resting potential

Question 15

What are the two types of propagation of an action potential?

Answer 15

Continuous Propagation
Saltatory Propagation

Question 16

Describe Continuous propagation

Answer 16

Occurs in unmyelenated axon:
1. Action potential depolarises membrane to +30mv
2. Local current: Na ions spread away from channel Depolarises next segment to threshold
3. This segment develops an action potential, original segment enters refractory period.
4. Local current continues to depolarise the next segment and the cycle repeats it self. Action potential travels in 1 direction at a speed of 1m/sec

Question 17

What is a myelinated axon

Answer 17

The axon is wrapped in layers of myelinated except at the nodes.

Question 18

Describe Saltatory Propagation

Answer 18

Depolarisation only occurs at the nodes so action potential ‘jumps’ from node to node.
Faster than continuous propagation, 18-240m/sec

Question 19

What is a synapse?

Answer 19

The site of communication between a nerve cell and some other cell. Information transfer occurs through the release of chemicals called neurotransmitters from the synaptic terminal. Chemical synapses are the most common type of synapse.

Question 20

2 types of neurotransmitter

Answer 20

Excitatory neurotransmitters:
• cause depolarization of postsynaptic membranes
• promote action potentials

Inhibitory neurotransmitters:
• cause hyperpolarization of postsynaptic membranes
• suppress action potentials

Question 21

What neurotransmitters is released at a cholinergenic synapse?

Answer 21

Acetylcholine(ACh)

Question 22

What are the events at a cholinergenic synapse?

Answer 22

1. An arriving action potential depolarizes the axon terminal of a
   presynaptic neuron.
2. Calcium ions (Ca2+) enter the cytosol of the axon terminal. This
   results in ACh release from the synaptic vesicles by exocytosis.
3. ACh diffuses across the synaptic cleft and binds to receptors on the
   postsynaptic membrane. Sodium channels open, producing a
   graded depolarization.
4. Depolarization ends as ACh is broken down into acetate and choline
   by AChE. The axon terminal reabsorbs choline from the synaptic
   cleft and uses it to resynthesize ACh.

Question 23

What are other important neurotransmitters?

Answer 23

Norepinephrine/Noradrenaline
Dopamine
Serotonin
Gamma aminobutyric acid (GABA)

Question 24

Describe Norepinephrine (NE)

Answer 24

Usually has an excitatory effect, due to depolarisation of postsynaptic neurons.
Important in brain and in portions of the ANS
Released by adrenergic synapses.
NE broken down by monoamine oxidase.

Question 25

Describe Dopamine

Answer 25

CNS neurotransmitters that is released i many areas of the brain.
Can be excitatory or inhibitory.
Inhibition + important for precise control of movements
Released by dopaminergic synapses
Lack of dopamine can lead to muscle stiffness, characteristic of Parkinson’s Disease.

Question 26

Describe Serotonin

Answer 26

Neurotransmitter found in the CNS
Interactions among serotonin, NE and other neurotransmitters are thought to be involved in regulation of sleep/wake cycles. Evidence that inadequate serotonin production can be important in context of depression.
Released by serotonergic synapses.

Question 27

Describe GABA

Answer 27

Generally has inhibitory effect
Functions remain incompletely understood
In the CNS GABA release appears to reduce anxiety.