Nervous System

Question 1

Describe the general structure of a motor neuron

Answer 1

— Cell body
— Dendrons: carries impulses towards cell body
— Axon: carries impulses away from cell body

Question 2

Describe the additional features of a myelinated motor neuron?

Answer 2

— Schwann cells: Wrap around axon many times
— Myelin Sheath: Made from Schwann cells
— Nodes of fancier: Short gaps between Schwann cells where there is no myelin 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 neuron?

Answer 4

— Stimulus leads to sodium ions entering the cell membrane causing first section of membrane depolarises
— Stretch mediated sodium ion channels open causing more depolarisation further along the membrane
— Previous section repolarises
— 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 cannot occur where myelin sheath ascots as electrical insulator. So impulse does not travel along whole axon length.

Question 6

What is resting potential?

Answer 6

Potential difference across neuron membrane when not stimulated

Question 7

How is resting potential established?

Answer 7

— Membrane is more permeable to potassium ions than sodium ions
— Sodium potassium pump actively transports three sodium ions out of cell and two potassium ions into cell
— Establishes electrochemical gradient

Question 8

Stages in generating an action potential?

Answer 8

— Depolarisation
— Depolarisation
— Hyper polarisation
— resting potential

Question 9

What happens during depolarisation?

Answer 9

— Stimulus leading to facilitated diffusion of sodium ions into cell down electrochemical gradient
— Potential difference across membrane becomes more positive
— If membrane reached threshold stretch mediated sodium ion channels open

Question 10

What happens during repolarisation?

Answer 10

— Stretch mediated sodium ion channels close and potassium channels open
— facilitated diffusion of potassium ions out of cell down their electrochemical gradient
— potential difference across membrane becomes more negative

Question 11

What happens during hyperpolarisation?

Answer 11

— ‘overshoot’ when potassium ions diffuse out
— potential difference becomes more negative than resting potential
— refractory period
— potassium channels close and sodium potassium pump re-establishes resting potential

Question 12

Explain the importance of the refractory period?

Answer 12

— No action potential can be generated
— Ensures unidirectional impulse
— Ensure 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 action potentials have some 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
— less leakage of ions

Question 16

How does temperature affect speed of conductance?

Answer 16

Faster rate of diffusion and respiration

Question 17

Suggest an appropriate stats test to determine whether a factor has a significant effect on the speed of conductance?

Answer 17

Students T test

Question 18

Units for the frequency of impulse conduction?

Answer 18

Hz

Question 19

How can an organism detect the strength of a stimulus?

Answer 19

Larger stimuli raises membrane to threshold more quickly after hyper polarisation = greater frequency of impulses

Question 20

What is the function of synapses?

Answer 20

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

Question 21

Describe the structure of a synapse?

Answer 21

— Presynoptic neuron ends in synaptic knob
— Synaptic cleft
— Postsynaptic neuron

Question 22

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

Answer 22

— Wave of depolarisation travels down presynaptic neuron causing voltage gated Ca^2+ channels to open
— Vesicles move towards and fuse with presynaptic membrane
— Exocytosis of neurotransmitter into synaptic cleft

Question 23

How do neurotransmitters cross the synaptic cleft?

Answer 23

Simple diffusion

Question 24

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

Answer 24

— Neurotransmitter bind to specific receptor on postsynaptic membrane
— Ligand-gated sodium channels open
— If influx of sodium ions raises membrane to threshold, action potential is generated

Question 25

Explain why synaptic transmission is unidirectional?

Answer 25

— Only presynaptic neuron contains vesicles of neurotransmitters and only postsynaptic membrane has complementary receptors
— So impulse always travels presynaptic to postsynaptic

Question 26

Define summation and name the 2 types ?

Answer 26

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential:
— Temporal summation
— Spatial summation