Nervous Coordination

Question 1

Describe the general structure of a motor neuron

Answer 1

Cell Body:Contains organelles and a high proportion of RER
Dendrons:Branch into dendrites which carry impulses towards cell body
Axon:Long unbranched fibre carries nerve impulse away from cell body

Question 2

Describe the additional features of a myelinated motor neurone

Answer 2

•Schwann Cells:Wrap around axon many times
•Myelin Sheath:Made from myelin rich membranes of Schwann cells
•Nodes of Ranvier:Very short gaps between neighbouring Schwann cells where there is no mylein sheath

Question 3

Name 3 processes Schwann Cells are involved in

Answer 3

•Electrical insulation
•Phagocytosis
•Nerve regeneration

Question 4

How does an action potential pass along an unmyelinated neurone?

Answer 4

1.Stimulus leads to influx of Na+.1st Section of membrane depolarises
2.Local electrical currents causes Na+ VG channels further along membrane to open.Meanwhile,section behind repolarises.
3.Sequential wave of depolarisation

Question 5

Explain why myelinated axons conduct impulses faster than unmyelinated axons

Answer 5

Saltatory Conduction:Impulse jumps from one node of ranvier to another. Depolarisation cant occur when myelin sheath acts as an electrical insulator. So impulse doesn’t travel along whole axon length

Question 6

What is resting potential?

Answer 6

Potential difference across neurone membrane when not stimulated. Usually about -70mv in humans

Question 7

How is resting potential established?

Answer 7

1.Membrane is more permeable to K+ to Na+
2.Na/K pump actively transports 3Na+ out of cell and 2K+ into cell
Establishes electrochemical gradient:Cell contents more negative than extracellular environment

Question 8

Name the stages in generating an action potential

Answer 8

1.Depolarisation
2.Repolarisation
3.Hyperpolarisation
4.Return to resting potential

Question 9

What happens during depolarisation?

Answer 9

1.Facilitated diffusion of Na+ ions into cell down electrochemical gradient
2.P.d across membrane becomes more positive
3.If membrane reaches threshold potential VG Na+ channels open
4.Significant influx of Na+ reverses p.d to +40mv

Question 10

What happens during repolarisation

Answer 10

1.VG Na+ channels close and VG K+ channels open
2.Facilitated diffusion of K+ out of cell down their electrochemical gradient
3.P.d across membrane becomes more negative

Question 11

What happens during hyperpolarisation?

Answer 11

1.’Overshoot’ when K+ ions diffuse out =p.d becomes more - than RP
2.Refractory period:no stimulus is large enough to raise membrane potential to threshold
3.VG K+ channels close & Na/K pump reestablishes resting potential (RP)

Question 12

Explain the importance of the refractory period

Answer 12

No AP can be generated in hyper-polarised sections of membrane:
•Ensures unidirectional impulse
•Ensures discrete impulses
•Limits frequency of impulse transmission

Question 13

What is the “all or nothing” principle?

Answer 13

Any stimulus that causes the membrane to reach threshold potential will generate an action potential
All APs have same magnitude

Question 14

Name the factors that affect the speed of conductance

Answer 14

•Myelin Sheath
•Axon diameter
•Temperature

Question 15

How does axon diameter affect the speed of conductance?

Answer 15

greater diameter = faster
•Less resistance to flow of ions (depolarisation & repolarisation)
•Less leakage of ions (easier to maintain membrane potential)

Question 16

How does temperature affect speed of conductance?

Answer 16

Higher temperature = faster
•Faster rate of diffusion (depolarisation & repolarisation)
•Faster rate of respiration (enzyme - controlled) = more ATP for active transport to reestablish resting potential

Question 17

How can an organism detect the strength of a stimulus?

Answer 17

Larger stimulus raises to threshold potential more quickly after hyperpolarisation = greater frequency of impulses

Question 18

What is the function of synapses?

Answer 18

Electrical impulse cannot travel over junction between neurones
Neurotransmitters send impulses between neurons/from neurons to effectors.
New impulses can be initiated in several different neurons from multiple simultaneous responses

Question 19

Describe the structure of a synapse

Answer 19

Presynaptic neuron ends in synaptic knob:contains lots mitochondria, ER & vesicles of neurotransmitter
Synaptic Cleft: 20-30nm gap between neurons
Postsynaptic neuron:Has complementary receptors to neurotransmitters

Question 20

Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another

Answer 20

1.Wave of depolarisation travels down presynaptic neuron,causing VG Ca2+ channels to open
2.Vesicles move towards & fuse with presynaptic membrane
3.Exocytosis of neurotransmitter into synaptic cleft