3.6 Organisms Respond to Changes

Question 1

structure of a myelinated motor neurone

Answer 1

cell body, dendrons, axons, Schwann cells, myelin sheath, nodes of Ranvier

Question 2

cell body

Answer 2

contains all the cell organelles, including the nucleus and rough endoplasmic reticulum
production of proteins and neurotransmitters

Question 3

dendrons

Answer 3

extensions of the cell body which divide into dendrites
Transmit/carry nerve impulses towards the cell body

Question 4

axons

Answer 4

a single long fibre
Nerve fibres carry nerve impulses away from the cell body

Question 5

schwann cells

Answer 5

form the myelin sheath wrapped around the axon
Protect the axon, electrically insulate it, phagocytosis of cell debris + involved in nerve regeneration

Question 6

myelin sheath

Answer 6

made up of the membranes of Schwann cells
Schwann membranes are rich in myelin (lipid)

Question 7

nodes of ranvier

Answer 7

gaps between Schawnn cells where there is no myelin sheath
2-3 micrometers long + occur every 1-3 mm in humans

Question 8

sensory neurones

Answer 8

Nerve impulses from a receptor to an intermediate/motor neurone
One long dendron + one axon
Cell body in the middle

Question 9

motor neurones

Answer 9

Nerve impulses from an intermediate/relay neurone to an effector
Long axon + many short dendrites
Cell body at one end

Question 10

intermediate/relay neurones

Answer 10

Impulses between neurones with numerous short processes

Question 11

define resting potential

Answer 11

when the inside of the axon is negatively charged relative to the outside of the axon, when this occurs the axon is polarised

Question 12

establishment of resting potential

Answer 12

Sodium ions are actively transported out of the axon by the sodium potassium pumps
Potassium ions are actively transported into the axon by the sodium potassium pumps
Active transport of sodium is greater than that of potassium because 3 sodium ions move out for every two potassium ions that move in
As a result, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm. More potassium ions are in the cytoplasm than in the tissue fluid. This creates an electrochemical gradient.
Sodium ions begin to diffuse back naturally into the axon while the potassium ions diffuse back out of the axon
Most of the gated channels for diffusion of sodium ions are closed but for potassium are open, maintaining the electrochemical gradient causing resting potential

Question 13

define action potential

Answer 13

when the negative charge inside the membrane becomes the positive charge, depolarising the axon, occurs when transmitting a nerve impulse

Question 14

establishment of action potential

Answer 14

At resting potential, some potassium voltage gated channels are open but the sodium voltage gated channels are closed.
The energy of the stimulus causes some of the sodium voltage gated channels in the axon membrane to open so sodium ions diffuse into the axon along their electrochemical gradient.
Sodium ions are positively charged, so this diffusion causes a reversal in the potential difference across the membrane
As more ions diffuse, more sodium ion channels open, causing a greater influx of the ions
Once the action potential of around +40 mv exists, voltage gates close on the sodium ion channels and those on potassium ion channels open
The electrical gradient which was preventing further outward movement of the potassium ions is now reversed so even more channels open, starting repolarisation of the axon due to movement of K+
This causes hyperpolarisation of the axon due to the inside being more negative than usual
Potassium ion channel gates close and the sodium potassium pump moves sodium out and potassiums in.
This reestablishes the resting potential so the axon membrane is repolarised

Question 15

Passage of action potential along an unmyelinated neuron

Answer 15

The axon membrane is polarised because there is greater concentration of positive ions on the outside than the inside
Stimulus causes sudden influx of sodium ions into the cytoplasm so a reversal of charge on the membrane, this is the action potential and depolarises the membrane
Influx of sodium ions cause localised electrical currents which open voltage gated sodium ion channels a little further along the axon. This causes depolarisation to move along the membrane. Behind the moving depolarisation, sodium channels close and potassium ones open
The outward movement of potassium ions which occurs behind the action potential has caused the axon to become repolarised.
Repolarisation of the axon allows sodium ions to be actively transported out, returning the axon to resting potential in preparation for a new stimulus.

Question 16

Passage of action potential along a myelinated neuron/saltatory conduction

Answer 16

When sodium channels are open, sodium ions move into the axon from the higher concentration outside to the lower inside
The concentration of sodium ions rises where the channels are open
The ions continue to diffuse but this time sideways along the neuron
They diffuse away from the increased concentration and towards a lower concentration down the neuron, nearer the next node
This causes slight depolarisation because the voltage gated channels are opened by changes in the potential difference, causing an influx of sodium ions
The continued influx causes a full depolarisation which is further along the neuron and this moves the action potential along the neuron as well

Question 17

nodes of ranvier

Answer 17

gaps between the Schwann cells which form the myelin sheath

Question 18

why is saltatory conduction faster than along unmyelinated neurons

Answer 18

action potential can jump from one node to another

Question 19

how does the myelin sheath affect speed of action potential

Answer 19

Travels faster because impulses are transferred more efficiently so a response is coordinated faster and the organism is better protected

Question 20

how does axon diameter affect speed of action potential

Answer 20

Greater the diameter the faster the speed of conductance because there is less leakage of ions from a large axon which would make membrane potentials harder to maintain
There is a reduced internal resistance

Question 21

how does temperature affect speed of action potential

Answer 21

Higher the temperature, higher the rate of diffusion of ions, the faster the nerve impulse
Enzymes involved in respiration for active transport in the sodium potassium pump function more rapidly at higher temperatures so nerve impulse quicker when it is warmer
Too high a temperature and the enzymes/plasma membrane proteins denature so impulses fail to be conducted
Affects speed + strength of muscle contractions

Question 22

explain the all or nothing principle

Answer 22

There is a certain level of stimulus, the threshold value, which triggers action potential. Any stimulus below the threshold value, there is no action potential and therefore no impulse is generated. This is the nothing principle.
Any stimulus above the threshold will generate an action potential so a nerve impulse can travel.

Question 23

how to detect the size of the stimulus

Answer 23

The number of impulses passing in a given time. The larger the stimulus, the more impulses that are generated in a given time
By having different neurons with different threshold values. The number and type of neurons used to pass impulses to the brain determines the size of the stimulus.

Question 24

define refractory period

Answer 24

Occurs after action potential when inward movement of sodium ions is prevented because sodium ion voltage gated channels are closed so it is impossible for a further action potential to be generated.

Question 25

purposes of the refractory period

Answer 25

Ensures action potentials are propagated in one direction only
Produces discrete impulses so action potentials are separated from one another
Limits the number of action potentials that can pass along the axon in a given time so it limits the strength of stimulus that can be detected

Question 26

Features of sensory reception common to all receptors

Answer 26

Is specific to a single type of stimulus (will not respond to any other kind of stimulus)
Produces generator potential (converting the energy of stimulus into a nervous impulse) by acting as a transducer

Question 27

structure + description of pacinian corpuscle

Answer 27

Sensory receptors found in the skin, joints, ligaments + tendons
A single sensory neuron is in the centre of layers of connective tissue
Oval shaped
detects changes in pressure (mechanical stimuli)

Question 28

how does a pacinian corpuscle produce a generator potential

Answer 28

In resting, stretch mediated sodium channels are too narrow for sodium ions to pass along them so the neuron has a resting potential (PD is negative)
Pressure is applied causing the corpuscle to deform which stretches the membrane around the neuron
The stretching widens the sodium ion channels so sodium ions can diffuse into the neuron down their concentration gradient via facilitated diffusion
An influx of sodium ions depolarises the membrane and produces a generator potential

Question 29

explain the low intensity response of rod cells

Answer 29

rhodopsin can break down even with low energy to create a generator potential
Stimulation of multiple rod cells can be combined to exceed threshold value + stimulate a generator potential

Question 30

explain the visual acuity of rod cells

Answer 30

Low visual acuity because the brain cannot distinguish between separate sources of light due to multiple rod cells linking to one bipolar cell and therefore only being able to send a single nervous impulse

Question 31

location of the rod cells

Answer 31

At the peripheries, rod cells are in abundance because light intensity is lowest

Question 32

explain the high intensity response of cone cells

Answer 32

Each cone cell has their own separate bipolar cells so the stimulation of multiple cone cells cannot be combined to exceed the threshold value and thus produce a generator potential
Iodopsin requires a higher intensity for its break-down and create a generator potential

Question 33

explain the visual acuity of cone cells

Answer 33

High visual acuity because each cone cell is connected to its own bipolar cell/sensory neuron, meaning two generator potentials are produced and two nerve impulses can be sent to the brain

Question 34

location of the cone cells

Answer 34

Fovea receives the highest intensity of light so cone cells are present

Question 35

define visual acuity

Answer 35

the clarity of vision, determined by the individual’s ability to recognise small details with precision

Question 36

colour seen by rod and cone cells

Answer 36

rod - Cannot distinguish different wavelengths of light so black + white only
cone - Each cone cell is sensitive to a different specific range of wavelengths so can see in full colour