1. After you have been infected once by a particular bug (bacterium,
virus), you become immune to it in the future (you don't catch measles
twice). Because your immune system learns from the first exposure, the
bug is swiftly killed off next time you become infected. That is the principle
of immunisation. (Section 5.5.2)
2. Identical twins have the same genotype, and so their body chemistry
is identical. We can have difficulty distinguishing them, because they
look so alike; their immune systems can't distinguish either and so don't
regard the graft as 'non-self'. However, siblings have different genotypes
and therefore different antigens, and so their immune systems react to
the graft as 'foreign'. (Section 5.2.3)
3. T-helper cells promote (help) the action of other white cells, e.g.
B-lymphocytes, to release antibodies in response to invading bugs, etc.
In the absence of T-helper cells, the immune response is suppressed. With
lowered defences, the body becomes very susceptible to infections, etc.
that are normally easily controlled by the immune system. (Beyond BBB,
but not irrelevant.) (See Section 5.4.2)
4. Neurons and blood cells have a common embryonic origin, and share
certain characteristics. Lymphokines (LK) share the properties of neurotransmitters,
and can have excitatory/inhibitory actions (see foot of p106). Some neurons
can synthesise LKs; however, the action of LKs is very localised, unlike
the more 'distant action' of neurotransmitters. (In fact, LKs are more
like hormones that act locally.)
1. In a nerve net (See Section 6.2.3, and Ch. 6 Summary) there are no
chemical synapses, and information can pass freely between neurons in either
direction. The neuron junctions are in fact forms of electrical synapses,
such as the junctions between some types of muscle cells. e.g. cardiac
and smooth muscle.
2. See Summary to Chapter 6 (p54) for the main points.
3. Myelin has not evolved in invertebrates, and so the only way to increase
axonal conduction velocity is to increase axon diameter. Some 'giant axons',
which are involved in the pathways for fast escape reflexes, have axons
up to 1mm (1,000m) in diameter. In mammals (and other vertebrates) myelination
allows greater axonal conduction velocity for a given size. The largest
mammalian axons, 20m in diameter, have conduction velocities around l20ms-1.
In contrast, a 1,000m squid axon has a conduction velocity of only 33ms-1.
Myelin is good stuff! (But if myelin is so good why are not all mammalian
nerves myelinated?)
4. Habituation was described in Book 1, Chapter 6 (see also, Book 2,
p118). It is a gradual reduction in the size of a response to repeated
presentation of a harmless stimulus. It is a simple form of learning, which
can save energy.
1. The main components in a reflex are named in the boxes in Fig 7.1,
and an actual example of a reflex pathway is given in Fig 9.7. N.B. This
diagram is slightly wrong. Both the sensory neuron and the motor neuron
pathways should link to the same muscle: the upper of the two shown, as
the knee-jerk reflex involves the extensor muscles (e.g. quadriceps femoris)
of the thigh.
2. A twitch is a single, brief contraction produced by one action potential
in a motor neuron; a tetanus (or tetanic contraction) is due to the summation
of many twitches occurring in succession. (See Section 7.6.3). The force
produced by a tetanus is greater than a twitch, in spite of the appearance
in Fig 7.18.
3. Force output can be increased by (a) activating more muscle fibres
(which operate in functional groupings called motor units) (b) increasing
the frequency of action potential firing, so producing tetani (plural of
tetanus) rather than twitches. (See section 7.6.4)
4. Not strictly. The rhythmic (up and down) movements of the wing muscles
are produced (generated) by the activity in a small network of interneurons
in the central nervous system (a central pattern generator). The Central
Pattern Generator (CPG) can do this without any outside influence (See
Section 7.7; Summary on p165). However, the frequency of the neural activity
generated by the CPG is different from natural wingbeats, and for normal
flight movements (e.g. in wind and rain), inputs from sensory receptors
are necessary to modify the basic rhythm of the CPG.
5. A CPG is a network of neurons in the central nervous system. whose
output is 'pulsed', thus generating a neural pattern that can be used to
drive motor neurons. The system acts as a switch alternating its outputs
first to one set of motor neurons and then to another set. There is evidence
that many forms of rhythmic behaviour in mammals, such as breathing, chewing
and walking are at least partly controlled by CPGs. But as with locust
flight, sensory inputs are used to modulate performance.
1. The sagittal plane is the midline plane, the one the arrow passes
through (see Fig 8.1d)
2. See Fig 8.35 (p342) and also Fig 8.4 for additional labels. Afferent
axons of sensory neurons enter the spinal cord by the dorsal roots, which
if damaged would cause numbness. Paralysis would result from damage to
ventral roots, which contain efferent (motor) axons.
3. The spinal cord is thicker in the cervical and lumbar regions, which
'bulge' to accommodate the extra neurons serving the limbs. (See Section
8.3)
4. Cranial nerves are attached to the brain! (as opposed to the spinal
cord) Most spinal nerves have both motor and sensory components; some cranial
nerves also have motor and sensory components; some are purely sensory
(e.g. olfactory nerve, optic nerve); and others purely motor. (Beyond BBB,
but details are given in Table 8.1)
5. See Fig 8.5
6. The ANS operates 'beneath' voluntary (or conscious) control to regulate
the function of visceral/internal organs that are responsible for homeostasis.
There are anatomical differences between the two divisions (sympathetic
& parasympathetic), such as their different origins in the CNS, and
the relative lengths of the pre- and post-ganglionic neurons are different,
and there are some differences in the transmitters involved. Some of the
effects of the two divisions are given on p 190. The parasympathetic system
is essentially conservative and energy building; its actions are generally
not widespread, unlike those of the sympathetic division, which can be
thought of as acting to prepare the body for 'fight or flight'. The action
of the sympathetic nerves is augmented by adrenaline and noradrenaline
released from the adrenal medulla, which is effectively part of the sympathetic
nervous system. The epitome of the action of the parasympathetic NS is
a cow in a field (think of what cows do in fields!); the effects of the
sympathetic system are soon experienced by you when you walk past the cow
and discover that it is an angry bull! (See Section 8.5)
7. Motor cortex (precentral gyrus) is the origin of descending voluntary
motor pathways (e.g. corticospinal tract). The sensory cortex is the end-point
of most sensory pathways. Association cortex is neither purely sensory
nor motor, but acts to integrate (or associate) sensory information and
to plan appropriate actions, which are then effected by the motor systems.
(See p 184) The white regions in the brains shown in Fig 8.27 are areas
of association cortex.
8. Basically that sensory inputs from different parts of the body are
represented in an orderly manner in the cortex, so that neighbouring body
parts are represented by adjacent areas of cortex. (See Sect 8.4.3, 9.5.2
& 9.5.3, and Ch. 5 of Book 3)
9 Binocular vision is due to the overlapping fields of view (visual
fields) of the right and left eyes (see Fig 8.29). It allows us to judge
distance. Try to put one small object on top of another, or even a peg
on a clothes line, with one eye shut. If you can manage that, try threading
a needle!
10. If you can't work this out, just look through a set of (properly
adjusted) binoculars, and draw an outline of your field of view: it should
be circular, because it lies within the binocular (visual) field and the
images from the two eyes are superimposed in the brain.
1. White matter contains axons, which appear whitish because of the
myelin around them.
2. A receptive field applies to a single sensory neuron and its terminal
branches; examples of receptive fields are shown in Figure 9.21. Note how
they increase in size from the finger tips to the forearm. In contrast,
a dermatome applies to a spinal nerve (see Fig 9.4), which contains many
sensory neurons, and so the dermatome is made up of the combined areas
of all the receptive fields of the neurons in that spinal nerve and which
have their cell bodies in the dorsal root ganglion.
3. See Figure 9.7, but note the error in the origin of the sensory neuron.
A more accurate example is the pathway involving the biceps muscle in Fig
9.9(a)
4. The principle is shown in Figure 9.8, except that it should be applied
to the flexor and extensor muscles of the legs (see also Fig 9.9, although
this applies to the arm).
5. See Section 9.4 for the details, and the Summary (pp. 247-8) for
an overview. Basically, the motor cortex sends the 'movement commands'
to the muscles; the basal ganglia are involved in initiating and planning
movements and also in regulating the output of the motor cortex; the cerebellum
receives inputs from balancing organs and proprioceptors, and also helps
to coordinate the activity of different muscle groups.
6. See pp. 258-9. The main point is that individual peripheral receptive
fields are represented in the sensory cortex. Regions with small, densely
packed receptive fields tend to occupy more cortical space. Compare the
size of receptive fields in the arm (Fig 9.21) with the cortical representation
of these same areas (see Fig 9.25 and Fig 8.l0b - although the latter is
actually the motor cortex, the representation in the sensory cortex is
dimensionally similar) A further explanation is given in Book 3, Section
5.3
7. The receptive fields of peripheral sensory neurons (those with cell
bodies in the dorsal root ganglia) are uniform; stimulation of any part
of the receptive field (RF) will increase the neuron's firing rate. The
red coloured concentrically arranged receptive field shown in Fig 9.23(a)
is that of a cell in the (ventrobasal) thalamus. See Fig 9.24. Stimulation
of the central region (crosses) will excite the thalamic cell (cell A),
and stimulation of the outer part of the RF (triangles) will inhibit the
thalamic cell. The receptive field of this thalamic neuron comprises the
sum of the receptive fields of its input neurons. Thus, neurons a, b, and
c all provide input to the ventrobasal cell (Cell A), but, while the input
from a (central region of RF) is excitatory, the inputs from b and C (peripheral
surround) are inhibitory to cell A because they project via inhibitory
interneurons (the red neurons in the Fig.)
1. No. The reflex is caused by tactile stimulation of the female's rear
(see Fig 10.6). However, this stimulus is not sufficient to elicit the
reflex unless the female is in a receptive state (oestrus). Oestrogen facilitates
the reflex, by activating a descending pathway from the hypothalamus to
the motor neurons. The excitability of the motor neurons is increased in
this sensitised state, so that they now fire in response to the sensory
input, producing the lordosis posture.
2. There are many problems. Electrical stimulation is 'unnatural', and
nerve thresholds to electrical stimuli are determined by the size (rather
than function) of the neuron. It is possible that the stimulus current
could spread to affect cells some distance away from the electrode. Also,
any effects could be due to stimulation of axons that happen to pass through
the hypothalamus but which link two other brain areas. So, whilst electrical
stimulation methods do provide some information, it is sensible to do other
experiments (e.g. lesioning) to check the conclusions.
3. Since animals will 'work' (increasing the rate of producing an operant)
to obtain the brain stimulation (a reward), this is an instance of positive
reinforcement.
4. A cognitive map is a form of memory (spatial memory) that stores
information about the spatial relationships of an animal's environment
(Book 1, 8.2.1). Place cells in the hippocampus are location specific,
and tire when the animal is in a particular location (place) within its
environment. (p282) It seems plausible that place cells contribute to the
biological basis of spatial memory (they may even be directly responsible
for the spatial memory).
5. Normal rats quickly learn to find a submerged platform in a pool
of murky water, and within a few trials, will swim directly to it. Hippocampally-lesioned
rats do not display this learning ability, and would swim around at random.
(see paragraph 2 on p281). Their behaviour on successive trials would be
repetitions of the patterns shown in panel 1 in Fig 10.8, and they would
show no signs of learning even over many trials.
1. In simple terms, when one area is damaged, the residual language
abilities reflect the 'function' of the intact (but effectively disconnected)
area. Thus, in Broca's dysphasia, the remaining language reflects what
Wernicke's area can do on its own - language that is generally contextually
correct, but which lacks fluency. In Wernicke's dysphasia, the intact Broca's
area confers fluency, but the language is generally meaningless, since
there is no integration of the sensory inputs, etc. by Wernicke's area.(see
11.2.3)
2. (a) Within hemispheric differences. Roles of different (primary) areas of the neocortex: e.g. motor, somatosensory, visual, auditory cortices etc. (See 8.8). Specialized areas of association cortex, e.g. Broca's and Wernicke's areas involved in language. (See 11.2) Different areas of temporal cortex and hippocampal cortex serving different memory functions. (Sections 11.4.2 and 11.4.3)
(b) Between hemispheric differences. General pattern of motor
and sensory cortices involved in control of functions of contralateral
side of body. (Sections 9.4, 9.5) Lateralisation of cognitive functions
(Section 11.3). Left hemisphere is language dominant in majority of individuals
(Table 11.1) Left hemisphere 'controls' verbal abilities; right hemisphere
'controls' spatial ones (Section 11.3.2) Left hippocampus associated more
with verbal memories; the right hippocampus more with non-verbal (spatial)
memories. (Section 11.4.3)
3. Since most tests of cerebral dominance rely on language expression
(see top of p303) the dominant hemisphere is really the one which is dominant
for language - in most people this is the left hemisphere. But the (language)
dominant hemisphere does not 'rule' the other one, it only appears to do
so since it does the talking. However, the 'silent' hemisphere is better
at other functions, such as spatial abilities. (This theme is developed
further in Book 6, Section 6.3.1)
4. Amnesia is loss of memory. Retrograde amnesia is a loss of memory
of events preceding the circumstances (e.g. injury) that caused the amnesia.
Anterograde amnesia is a loss of ability to form new long term memories.
(see 11.4.2)
5. There is no easy answer to this one!! The various types of memory
are discussed in Section 11.4, but the important point is that the different
classes are not alternatives, nor are they different words for the same
things. Figure 11.11 provides one possible summary, but where should short-
and long-term memories fit into this scheme of things? The Summary to Section
11.4 (pp3 17-8) is worth looking at.