neurons and neural communication- lecture 3

Question 1

what are neurons (nerve cells)

Answer 1

-specialised cells that recieve and transmit info throughout the central nervous system
- vary in size but all have the same basic structure

Question 2

parts of a neuron

Answer 2

soma
dendrites
axon
presynaptic terminals
plasma membrane

Question 3

soma

Answer 3

• contains cell nucleus (houses chromosomes and DNA)
  bulk of the cell is made of the cytoplasm which houses variety of structures

Question 4

dendrites

Answer 4

-all info recieved here
- recieved from other neurons across synapse (line the surface of the dendrites)
- outgrowths called dendritic spines increase SA available for synaptic communication

Question 5

axon

Answer 5

• all info set along the axon
• info sent as electrical impuse called action potential
• covered with layer of insulating material called myelin sheath (vertebrates only)
• in vertebrates there are breaks in myelin sheath called nodes of ranvier

Question 6

presynaptic terminals

Answer 6

• action potential passes from cell body along the axon to presynaptic terminals
• when action potential reaches terminal they secrete neurotransmitters across synapse to next neuron in next chain, this then either excites or inhibits the post synaptic receptors on dendrites o another neuron

Question 7

efferent axons

Answer 7

send info out from nervous system e.g. motor neurons

Question 8

afferent axons

Answer 8

send info in e.g. sensory neurons

Question 9

plasma membrane

Answer 9

separates the inside of the cell

Question 10

support cells in the nervous system

Answer 10

glia/glial cells provide support to development and activity of neurons

Question 10

features of support cells

Answer 10

• astrocytes
• schwann cells
• oligodendrocytes

Question 10

astrocytes

Answer 10

wrap around related neurons helping to synchronise activity, preventblood-borne toxins enterig neurons (e.g. blood-brain barrier)

Question 11

schwann cells

Answer 11

build myelin sheaths in peripheral nervous system, help neuron regrowth and guide to appropriate targets

Question 11

oligodendrocytes

Answer 11

build myelin sheaths in brain and spinal cord (CNS)

Question 12

electrical activity in the neuron

Answer 12

the inside of an inactive axon is more negatively charged than outside
this is called resting potential (-70mV)

Question 13

probing a squid

Answer 13

microelectrodes are placed inside and outside the axon
both are connected to a voltmeter and voltage inside and out recorded

Question 14

resting potential

Answer 14

• caused by concentrations of ions inside and outside cell
• more sodium outside the cell
• causes a negative charge (-70mV) inside the cell

Question 15

movements of ions in and out the cell

Answer 15

• axon membrane is selectively permeable to potassium and sodium
• at rest- potassium and sodium difuse slowly through ion channels in axon membrane
• but sodium tries to sneak into the cell to balane concentration and charge
• the balance is maintained by ion pumps- 3 sodium out 2 potassium in

Question 16

maintaining resting potential

Answer 16

• neuron is polrised during resting potential (a difference in electrical charge between inside and outside)
• polarisation is essential (ensures neuron is ready to fire when it recieves impulse)

Question 17

all or nothing principle

Answer 17

neurons have diff thresholds of excitation (the strength of the trigger required to respond)
but once threshold is reached an action potential is triggered

Question 18

passing info- the action potential

Answer 18

1. a positive charge applied to the inside of axon makes it more positive
2. membrane potential rapidly reversed, inside becomes strongly positive- up to 40 mV
3. membrane potential quickly returns to normal but 1st it briefly overshoots
4. this all takes 2 milliseconds

Question 19

0- resting potential (-70 mV)

Answer 19

more sodium outsie than in so inside is more negative, inside and outside is polarised which is maintained by ion pumps

Question 20

1- depolarisation

Answer 20

when stimulated past threshold sodium channels open and sodium rushes into axon causing a region of positive charge

Question 21

2- repolarisation

Answer 21

sodium channels close, potassium channels open and potassium exits axon

Question 22

3- hyperpolarisation

Answer 22

potassium continues to exit after repolarisation causing brief undershoot in charge (too negative) so ion pumps restore resting balance (sodium out, potassium in)

Question 23

after action potential

Answer 23

the axon cant cope with repeated excitation as the sodium-potassium pump cant keep up

Question 24

absolute refractory period

Answer 24

after action potential sodium channels remain closed for about 1ms during this time no stimulus can excite the neuron

Question 25

relative refractory period

Answer 25

sodium channels open ut potassium channels remain closed for 2-4ms only an extremely strong stimulus can excite the neuron

Question 26

movement of action potential

Answer 26

• unmyelinated axons (intervertebrates and small neurons)
• each oint along the axon membrane generates axon potential
• the next area of membrane is depolarised, reaches its threshold and generates another action potential
• so the action potential passes down the axon like a wave

Question 27

where are myelinated axons

Answer 27

only in vertebrates

Question 28

advantages of myelinated axons

Answer 28

• energy saving- sodium potassium pumps are only required at specific points along the axon
• speed