1
00:00:10.120 --> 00:00:11.680
Welcome back to what a plant knows.

2
00:00:11.680 --> 00:00:15.930
In today's lecture, we're going to talk
about what do plants feel.

3
00:00:15.930 --> 00:00:18.560
Now, we feel plants all the time.

4
00:00:18.560 --> 00:00:21.780
Sometimes they're soft and comforting,
like grass or lying on

5
00:00:21.780 --> 00:00:25.440
grass in a park, or the softness of rose
petals.

6
00:00:25.440 --> 00:00:29.390
Other times, plants, we feel them as rough
and thorny, prickly like the thorns

7
00:00:29.390 --> 00:00:30.910
on a blackberry bush that are getting

8
00:00:30.910 --> 00:00:33.050
in the way of us harvesting some
blackberries.

9
00:00:34.880 --> 00:00:35.140
but in

10
00:00:35.140 --> 00:00:39.040
most cases, plants for us are passive
objects,

11
00:00:39.040 --> 00:00:41.830
like props that we interact with, that we
ignore.

12
00:00:41.830 --> 00:00:43.960
We pluck petals off of daisies.

13
00:00:43.960 --> 00:00:45.390
We saw the limbs off of trees.

14
00:00:45.390 --> 00:00:49.840
But what if the plants knew that we were
touching them.

15
00:00:51.160 --> 00:00:54.020
You know, it's probably a bit surprising,
and maybe even a

16
00:00:54.020 --> 00:00:58.790
bit disconcerting to discover that plants
know when they're being touched.

17
00:00:58.790 --> 00:01:00.270
For example, when I take

18
00:01:00.270 --> 00:01:04.990
this Mimosa plant, and I touch it, it's
leaves close.

19
00:01:04.990 --> 00:01:08.790
Somehow or another, this plant knew that I
was touching

20
00:01:08.790 --> 00:01:11.390
it, or that something was touching it, and
it responded.

21
00:01:12.740 --> 00:01:16.230
Other plants, like vines, when their
tendrils, the long part

22
00:01:16.230 --> 00:01:20.150
that grab onto the fences, when they're
touched, they start coiling.

23
00:01:20.150 --> 00:01:22.640
And we've all seen the Venus Fly Trap
close

24
00:01:22.640 --> 00:01:25.650
in on an unsuspecting fly, as it's
crawling across

25
00:01:25.650 --> 00:01:26.520
its open leaves.

26
00:01:28.780 --> 00:01:32.000
Trees also know when their branches are
being, are swaying

27
00:01:32.000 --> 00:01:34.600
in the wind, when they're being pushed by
the wind.

28
00:01:34.600 --> 00:01:38.020
And there are some plants, that if you
touch them so much, can even die.

29
00:01:39.310 --> 00:01:41.710
So of course, when I'm using the word feel
here, that

30
00:01:41.710 --> 00:01:45.250
plants feel when they're being touched,
I'm not talking about emotion feeling.

31
00:01:45.250 --> 00:01:50.080
I'm talking about the ability to perceive
tactile sensation.

32
00:01:50.080 --> 00:01:54.510
And in this case, some plants actually
feel better, or are

33
00:01:54.510 --> 00:01:59.640
more sensitive, than we are.
How sensitive are plants to touch?

34
00:01:59.640 --> 00:02:02.540
Well, most humans can feel the weight of
about

35
00:02:02.540 --> 00:02:05.590
two micrograms of a thread going across
your skin.

36
00:02:07.310 --> 00:02:11.830
Some plants, for example, the burr
cucumber, where their, its tendrils,

37
00:02:11.830 --> 00:02:17.520
start coiling under the weight of only a
quarter of a microgram.

38
00:02:17.520 --> 00:02:19.690
That's ten times more sensitive

39
00:02:19.690 --> 00:02:20.370
than humans.

40
00:02:21.990 --> 00:02:25.920
So, although plants are more sensitive to
humans when it comes to touch, plants

41
00:02:25.920 --> 00:02:28.970
and animals, including humans, share some
surprising

42
00:02:28.970 --> 00:02:32.540
similarities when it comes to feeling that
touch.

43
00:02:35.000 --> 00:02:39.930
So, how do we feel?
How do we sense that we are being touched?

44
00:02:41.320 --> 00:02:47.100
Humans, all animals, sense touch through
what's called our somatic sensory system.

45
00:02:47.100 --> 00:02:49.360
This is the nervous system that responds
to different

46
00:02:49.360 --> 00:02:53.610
types of physical stimulations and it's
actually quite diverse.

47
00:02:53.610 --> 00:02:57.260
But in general, it consists of various
types of receptors

48
00:02:57.260 --> 00:03:00.520
that respond to touch or to pain or to
temperature.

49
00:03:01.850 --> 00:03:06.890
Neurons that mediate that connect between
these receptors in the skin or

50
00:03:06.890 --> 00:03:10.830
in our muscles through our spinal cord and
up to the brain.

51
00:03:10.830 --> 00:03:13.410
And our brain which processes the
information and

52
00:03:13.410 --> 00:03:16.200
defines it as this particular type of
sensation.

53
00:03:17.860 --> 00:03:21.310
Well here we're going to see now, a cross
section through our skin, to get a better

54
00:03:21.310 --> 00:03:25.040
idea of what these, sensors, what are the

55
00:03:25.040 --> 00:03:28.430
receptors, what are the structures for
different sensations.

56
00:03:28.430 --> 00:03:30.590
In our skin there are different types of
nerves,

57
00:03:30.590 --> 00:03:34.870
different types of structures for
different types of sensations.

58
00:03:34.870 --> 00:03:37.640
For example, close to the tips, close to
the top of

59
00:03:37.640 --> 00:03:42.730
the skin there are nerves that are
specific only for pain.

60
00:03:42.730 --> 00:03:47.150
And they contain specific receptors which
are called nociceptors.

61
00:03:47.150 --> 00:03:50.210
These are the receptors that mediate our
sensation of pain.

62
00:03:51.910 --> 00:03:53.860
There are about three different types

63
00:03:53.860 --> 00:03:58.920
of nerves that have receptors that feel
different types of pressure.

64
00:03:58.920 --> 00:04:01.610
Whether it be light pressure, or a deep
pressure, and they

65
00:04:01.610 --> 00:04:05.710
respond differently, again, to what type
of pressure's going on the skin.

66
00:04:05.710 --> 00:04:08.510
These nerves contain what we call
mechanoreceptors.

67
00:04:10.100 --> 00:04:11.590
And other parts of our skin contain

68
00:04:11.590 --> 00:04:14.520
receptors that respond to changes in
temperature.

69
00:04:14.520 --> 00:04:17.790
These contains receptors that are called
thermal receptors.

70
00:04:17.790 --> 00:04:19.020
And there's different thermal receptors

71
00:04:19.020 --> 00:04:21.610
for heat and different thermal receptors
for cold.

72
00:04:23.850 --> 00:04:27.510
So what happens once one of these
receptors is stimulated.

73
00:04:29.020 --> 00:04:32.030
So once the receptor is stimulated, the
principle involved

74
00:04:32.030 --> 00:04:35.740
in neural communication is the same for
all nerve cells.

75
00:04:35.740 --> 00:04:36.460
It's electricity.

76
00:04:37.470 --> 00:04:42.390
The initial stimulus, whether it be touch
from a canoreceptor or cold from one

77
00:04:42.390 --> 00:04:45.095
of the thermal receptors, starts a rapid

78
00:04:45.095 --> 00:04:49.690
electro-chemical reaction which is called
a depolarization.

79
00:04:49.690 --> 00:04:53.250
And we'll define that soon, what this
depolarization is.

80
00:04:53.250 --> 00:04:55.970
And this depolarization is propagated
along the

81
00:04:55.970 --> 00:04:58.880
nerve, much like a wave going through
water.

82
00:04:58.880 --> 00:05:03.900
And this electrical signal can then
proceed from nerve

83
00:05:03.900 --> 00:05:08.630
to nerve until it reaches our brain where
it elicits the reaction.

84
00:05:10.440 --> 00:05:15.370
So the depolarization is a result of what
we call an action potential.

85
00:05:15.370 --> 00:05:18.020
So while the mechanisms involved in these

86
00:05:18.020 --> 00:05:20.602
electrical chemical signaling are actually
quite complex, and

87
00:05:20.602 --> 00:05:22.570
there could actually be an entire course,
just

88
00:05:22.570 --> 00:05:25.910
on neurosignaling, the basic principles
are relatively simple.

89
00:05:27.200 --> 00:05:32.120
Just as a battery, for example, maintains
a charge, an electric current,

90
00:05:32.120 --> 00:05:34.830
by keeping different electrolites,
different ions

91
00:05:34.830 --> 00:05:37.420
in different compartments of the battery.

92
00:05:37.420 --> 00:05:40.550
A cell also has a charge due to the
different

93
00:05:40.550 --> 00:05:42.530
amounts of ions, the different amounts of

94
00:05:42.530 --> 00:05:45.600
salts that are inside and outside the
cell.

95
00:05:46.710 --> 00:05:49.300
For example, there are more sodium ions on
the

96
00:05:49.300 --> 00:05:53.570
outside of cells and more potassium ions
on the inside.

97
00:05:53.570 --> 00:05:57.400
And this change in the ions is what gives
cells an electrical charge.

98
00:05:59.150 --> 00:06:02.470
So when a mechanoreceptor is activated for
example, or

99
00:06:02.470 --> 00:06:05.680
any type of receptor for that matter, this
opens

100
00:06:05.680 --> 00:06:08.930
up specific holes, channels in the
membranes of

101
00:06:08.930 --> 00:06:12.560
the cells which lets ions pass through the
channel.

102
00:06:12.560 --> 00:06:14.320
This happens near the point of contact for

103
00:06:14.320 --> 00:06:18.410
mechanoreceptor, and this change in the
ions leads

104
00:06:18.410 --> 00:06:20.830
to an electric charge, which leads to the

105
00:06:20.830 --> 00:06:24.450
opening of more channels further down the
same nerve.

106
00:06:24.450 --> 00:06:26.930
Which leads to another change of charge.

107
00:06:26.930 --> 00:06:30.690
And this change of charge again progresses
as a wave down

108
00:06:30.690 --> 00:06:34.430
the length of the neuron.
Again like a wave going through the ocean.

109
00:06:36.770 --> 00:06:38.810
When this charge then reaches the end of
the

110
00:06:38.810 --> 00:06:42.090
neuron at the junction where one nerve
meets another

111
00:06:42.090 --> 00:06:45.860
nerve, this leads to the release of a
second

112
00:06:45.860 --> 00:06:48.490
type of ion, another ion, which is called
calcium.

113
00:06:49.570 --> 00:06:54.970
And this calcium change causes a chemical
change at the junction between the two

114
00:06:54.970 --> 00:06:58.000
neurons which leads to the electrical
chemical

115
00:06:58.000 --> 00:07:00.809
signal preceding again to the next neuron.

116
00:07:02.090 --> 00:07:04.980
What the calcium ions do as they're
released, leads to

117
00:07:04.980 --> 00:07:06.420
the release of chemicals that

118
00:07:06.420 --> 00:07:09.260
are called neuro-recep-, are called
neurotransmitters.

119
00:07:09.260 --> 00:07:10.940
And maybe you've heard of these type of

120
00:07:10.940 --> 00:07:14.790
chemicals, neurotransmitters, because they
transmit a signal between neurons.

121
00:07:15.790 --> 00:07:18.660
When one nerve releases a
neurotransmitter,

122
00:07:18.660 --> 00:07:20.660
and it's caught by another nerve,

123
00:07:20.660 --> 00:07:24.755
this again causes the action potential to
propagate through the second nerve.

124
00:07:24.755 --> 00:07:26.850
And this continues until it reaches the
brain.

125
00:07:28.140 --> 00:07:34.070
Okay, now I know that this was a very
complex series of of explanations.

126
00:07:34.070 --> 00:07:36.750
So let's take a short break and review
action

127
00:07:36.750 --> 00:07:40.400
potentials and neural communication again,
before we move on.