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179

Nervous coordination

Chapter 9

The human nervous system
The human nervous system is made up of the brain
and spinal cord, which form the central nervous
system; and nerves, which form the peripheral
nervous system. Nerves themselves, and also much
of the central nervous system, are made up of
highly specialised cells called neurones.

Information is transferred along neurones in
the form of action potentials, sometimes known
as nerve impulses. These are fleeting changes in
the electrical charge on either side of the plasma
membranes.

Neurones
Figure 9.1 shows the structure of a motor neurone.
This type of neurone transmits action potentials

from the central nervous system to an effector such
as a muscle or a gland.

The cell body of a motor neurone lies within the
spinal cord or the brain. The nucleus of a neurone
is in the cell body (Figure 9.2). Often, dark specks
can be seen in the cytoplasm. These are groups of
ribosomes involved in protein synthesis.

Many thin cytoplasmic processes extend from
the cell body. In a motor neurone, all but one
of these are quite short. These short processes
conduct impulses towards the cell body, and they
are called dendrites. One process is much longer,
and this conducts impulses away from the cell
body. This is called the axon. A motor neurone
with its cell body in your spinal cord might have its
axon running all the way to a toe, so axons can be
very long.

By the end of this chapter you should be able to:

a describe the structure of motor and sensory
neurones;

b explain the role of nerve cell membranes
in establishing and maintaining the resting
potential;

c describe the conduction of an action potential
along the nerve cell membrane, including
the role of myelin in increasing the speed of
transmission;

d explain synaptic transmission, including the
structure of a cholinergic synapse;

e outline the roles of synapses.

axon

terminal
branches

synaptic knob

nucleus of
Schwann cell

Schwann cell

node of
Ranvier

nucleus

dendrite

cytoplasm containing many mitochondria
and rough endoplasmic reticulum

Figure 9.1 A motor neurone.

cell body

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Chapter 9: Nervous coordination

180

Within the cytoplasm, all the usual organelles,
such as endoplasmic reticulum, Golgi body and
mitochondria, are present. Particularly large
numbers of mitochondria are found at the tips
of the terminal branches of the axon, together
with many vesicles containing chemicals called
transmitter substances. These are involved in
passing nerve impulses from the neurone to a
muscle.

Sensory neurones (Figure 9.3) carry impulses via
a dendron from sense organs to the brain or spinal
cord. Their cell bodies are inside structures called
dorsal root ganglia, just outside the spinal cord.

Intermediate neurones, sometimes called relay
neurones (Figure 9.3), have their cell bodies and
their cytoplasmic processes inside the brain or
spinal cord. They are adapted to carry impulses
from and to numerous other neurones.

SAQ
1 Describe two differences between the structures

of a motor neurone and a sensory neurone.

Myelin
In some neurones, cells called Schwann cells wrap
themselves around the axon all along its length.
Figure 9.4 shows one such cell, viewed as the
axon is cut transversely. The Schwann cell spirals
around, enclosing the axon in many layers of its
plasma membrane. This enclosing sheath, called
the myelin sheath, is made largely of lipid, together
with some proteins.

There are small uncovered gaps along the axons,
where there are spaces between the Schwann cells.
These are known as nodes of Ranvier. They occur
about every 1–3 mm.

About one third of our motor and sensory
neurones are myelinated. The sheath increases the
speed of conduction of the nerve impulses, and
this is described on pages 186–187.

A reflex arc
Figure 9.5 shows how sensory, intermediate and
motor neurones are arranged in the body to form
a reflex arc. In the example in Figure 9.5, a spinal

Figure 9.2 An electron micrograph of the cell
body of a motor neurone within the spinal cord
(× 1390).

axon

cytoplasm dendrites

nucleus plasma membrane

Figure 9.3 Types of neurones.

Motor neurone

Sensory neurone

Intermediate neurone

cell body

axon

direction of
conduction of
nerve impulse

axondendron

axon

cell body

cell body

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Chapter 9: Nervous coordination

188

action potential in the plasma membrane of the
postsynaptic neurone.

This is shown in Figure 9.16. The cytoplasm
of the presynaptic neurone contains vesicles of
transmitter substance. More than 40 different
transmitter substances are known. Noradrenaline
and acetylcholine (sometimes abbreviated to ACh)
are found throughout the nervous system, while
others such as dopamine and glutamate occur only
in the brain. We will look at synapses which use
acetylcholine as the transmitter substance; they are
known as cholinergic synapses.

You will remember that, as an action potential
sweeps along the plasma membrane of a neurone,
local circuits depolarise the next piece of
membrane. This opens voltage-gated Na+ channels
and propagates the action potential. In the part of
the membrane of the presynaptic neurone that is
next to the synaptic cleft, the action potential also
causes calcium ion channels to open. So the action
potential causes not only sodium ions but also
calcium ions to flood into the cytoplasm.

This influx of calcium ions causes vesicles
of acetylcholine to move to the presynaptic
membrane and fuse with it, emptying their
contents into the synaptic cleft. (This is an example
of exocytosis.) Each action potential causes just a
few vesicles to do this, and each vesicle contains
up to 10 000 molecules of acetylcholine. The
acetylcholine rapidly diffuses across the cleft,
usually in less than 0.5 ms.

The plasma membrane of the postsynaptic
neurone contains receptor proteins. Part of the
receptor protein molecule has a complementary
shape to part of the acetylcholine molecule, so
that the acetylcholine molecules can bind with
the receptors. This changes the shape of the
protein, opening channels through which sodium
ions can pass (Figure 9.17). Sodium ions rush
into the cytoplasm of the postsynaptic neurone,
depolarising the membrane and starting off an
action potential.

1 An action
potential arrives.

2 Calcium ion
channels open.

3 Vesicles containing
acetylcholine move
to the presynaptic
membrane.

5 Acetylcholine
diffuses across the
synaptic cleft to
the postsynaptic
membrane.

6 Acetylcholine
binds to receptors
in the postsynaptic
membrane.

7 Sodium ion channels
open – the membrane
is depolarised and
an action potential is
produced.

4 Vesicles
fuse with the
presynaptic
membrane
and release
acetylcholine
into the
synaptic cleft.

Ca2+

Na+

+

+





Figure 9.16 How an impulse crosses a synapse.

Figure 9.15 A synapse.

synaptic bulb

mitochondrion

synaptic cleft

synaptic vesicle
containing transmitter
substance

presynaptic membrane
postsynaptic
membrane containing
transmitter receptors

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Chapter 9: Nervous coordination

189

A neuromuscular junction is a synapse between
the end of a motor neurone and a muscle. Here,
the plasma membrane of the muscle fibre is the
postsynaptic membrane, and acetylcholine sets up
an action potential in it in just the same way as in a
postsynaptic neurone. The action potential sweeps
along the plasma membrane of the muscle fibre
and causes the fibre to contract.

Recharging the synapse
If the acetylcholine remained bound to the
postsynaptic receptors, the sodium ion channels
would remain open. Action potentials might fire
continuously, or it might be impossible to reinstate
the resting potential across the membrane, so that
there could be no new action potentials generated.

acetylcholine diffusing
across the synaptic cleft

channel protein and
acetylcholine receptor

bound
acetylcholine

channel open after
binding acetylcholine

channel closed

Na+

Figure 9.17 How an acetylcholine receptor works.

Plants have action potentials, too. They do not
have specific ‘nerve cells’, but many of their cells
transmit waves of electrical activity that are very
similar to those transmitted along the neurones
of animals. The action potentials generally last
much longer and travel more slowly than in
animal neurones (see graph below).

Almost all animal and plant cells have
sodium–potassium pumps, which maintain
an electrochemical gradient across the plasma
membrane, and it is this that produces the resting
potential. As in animals, plant action potentials
are triggered when the membrane is depolarised.
Just as in animals, there is a refractory period
following each action potential.

Many different types of stimuli have been
shown to trigger action potentials in plants. In
Venus fly traps, for example, the touch of a fly
on one of the hairs on the leaf starts an action
potential that travels across the leaf and causes
it to fold over and trap the fly. This is quite fast
as plant responses go, taking only about 0.5 s
between the stimulus and the action.

Chemicals coming into contact with the
plant’s surface also trigger action potentials.
For example, dripping a solution of acid of a
similar pH to acid rain onto soya bean leaves

Action potentials in plants
causes action potentials to sweep across them.
In potato plants, Colorado beetle larvae feeding
on the leaves has been shown to induce action
potentials, of the shape shown in the graph here.
These travel only slowly, from the leaves down
the stem and all the way to the tubers beneath
the soil. At the moment, we don’t know what
effect, if any, these action potentials have, but it
is thought that they might bring about changes
in the metabolic reactions taking place in some
parts of the plant.

–80

0 100

Time / s

200 300 400

–60

–40

–20

0

20

40

P
o
te

n
tia

l
d
iff

e
re

n
ce

/
m

V

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Chapter 9: Nervous coordination

197

e State the functions of the structures labelled 3, 5 and 6. [3 marks]
f Based on your knowledge of the structure of a neurone, draw a labelled transverse

section of the neurone across the line labelled A to B. [2 marks]

12 The diagram below shows an action potential.

a What is meant by the term ‘threshold’ as shown on the graph? [1 mark]
b Why were there failed initiations after the stimulus? [2 marks]
c State the names of stages 1 to 5 and explain what is happening to the sodium and

potassium channels in the neurone at:
i 1 ii 2 iii 3 iv 4 v 5 [10 marks]
d Explain what takes the membrane of an axon from the resting potential to the

threshold potential as an action potential approaches. [2 marks]

13 The diagram below shows a section through a cholinergic synapse between two neurones.

continued ...

– 80

0 1
Time / ms

2 3 4

– 60

– 40

– 20

0

20

40

P
o
te

n
tia

l
d

iff
e

re
n

ce
/

m
V

threshold

stimulus

1 3

4

5

2

I
II

III
IV

V

VI

VII

VIII

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Chapter 9: Nervous coordination

198

14 a By means of an annotated diagram only, describe the structure of a myelinated
sensory neurone. [5 marks]

b Describe the function of the myelin sheath. [3 marks]
c Describe how a nerve impulse is transmitted along a non-myelinated neurone. [4 marks]
d Both the nervous and endocrine systems coordinate the transmission of information

in the body of a mammal. Explain three di�erences between the two systems. [3 marks]

15 a Describe how a resting potential is maintained in a neurone. [4 marks]
b Describe the events that lead to the generation of an action potential and the

subsequent return to the resting potential in a neurone after a stimulus is applied. [6 marks]
c Action potentials can travel along axons at speeds of 0.1�100 ms�1 . Suggest three

factors which may in�uence their speed. [3 marks]
d Explain why there is a delay in transmission of an impulse across a synapse. [2 marks]

16 a Describe the sequence of events in the transmission of an impulse across a synapse
(you may use an annotated diagram). [7 marks]

b The transmission of impulses across a synapse may be modi�ed in various ways.
Explain the role of the synapse in the following:

i unidirectional transmission of an impulse
ii summation of impulses
iii inhibition of impulses [6 marks]
c Botulin is a neurotoxin which attaches to the presynaptic membrane and prevents

the release of acetylcholine. Suggest what e�ect it may have on the transmission of
an impulse across the synapse. [2 marks]

Essay questions

a Name the structures labelled I to VIII. [4 marks]
b Describe the direction of the impulse across the synapse. [1 mark]
c Give two roles of the synapse in the nervous system. [2 marks]
d Describe the roles of the following in the transmission of an impulse across a

synapse in the nervous system of a mammal:
i structure II
ii structure V
iii structure VI [6 marks]
e What is the role of calcium ions in the passage of impulses across the synapse? [3 marks]

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