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Transcript
Answers to Test Your Knowledge questions for
Chapter 10 The control of movement
Question 10.1
As a general point, feedback defines a system in which A influences B and, in turn, B
influences A. Feedback can be negative or positive. In the case of negative feedback,
disturbances from a state tend to cause action such as to correct the disturbance. Since
disturbances are self-eliminating, the term 'negative' is used. Negative feedback was
described in Chapter 2 in the context of homeostasis, where for example, a disturbance to
body temperature (A) causes action such as sweating or shivering (B) such as to oppose
the disturbance to A. The same principle is applicable here. A disturbance to equilibrium
(A) instigates action, e.g. changing position of the arms (B), which tends to oppose the
disturbance.
Question 10.2
(a) Under these conditions, the automatic response to the word RED written in green ink
would be to utter the word 'red'. In order to comply with instructions and utter 'green'
there would have to be inhibition exerted on this tendency.
(b) To react to the ink colour and respond with 'green' would require the controlled mode.
Question 10.3
The action of performing a punch can be anticipated and bodily adjustments would be
made simultaneously. These would compensate for the change of weight distribution
arising from the extension of the arm. Such a change made in anticipation of a
disturbance (and thereby pre-empting it) constitutes 'feedforward'. Corrective actions
made in response to a disturbance to body equilibrium, e.g. that caused by receiving a
punch, constitute 'negative feedback'.
Question 10.4
If you are unsure about the precise mode of action of neurotransmission and
neuromodulation, you might like to consult Chapter 3, where these terms are explained.
Neurotransmitter would be employed where ballistic action is called for as in the brain
rapidly instigating a response or in inhibiting a response. Neuromodulation would be
involved, for example, where a reflex is sensitized by an input outside the direct stimulusresponse link. With reference to Figure 10.11, you can imagine that the neuron 'from
brain' that synapses on an interneuron is one that releases neurotransmitter. Through this
route, the skeletal muscle is activated, as it can also be via the input from 'sensory
neuron'. You can imagine that the neuron 'from brain' which synapses upon motor neuron
A releases neuromodulator. Note the sensory neuron that synapses on a short interneuron,
which, in turn, forms a link with A. The efficacy of this route, i.e. the strength of
stimulus-response link could be modulated by activity in the pathway descending from
the brain.
Question 10.5
As was described in Chapter 4, 'Neurons', neurotransmitter, following its occupation of
receptors is very quickly eliminated from the synaptic cleft, either by reuptake or by
break down. By this means, the postsynaptic cell is able to track closely changes in
activity in the presynaptic neuron. The role of acetylcholinesterase is described in the
present Chapter as one of breaking down acetylcholine, the neurotransmitter employed at
the neuron-muscle junction of skeletal muscle. If a poison inactivates
acetylcholinesterase, released acetylcholine will not be broken down. It will remain
attached to receptors at the skeletal muscle inducing contraction, even though there is no
neural command to do this. The muscle is described as going into 'spasm'.
Question 10.6
Both types of muscle control system are equipped with processes for contraction and
relaxation, i.e. 'antagonistic' control. Chapter 3 described the control of smooth muscle
and cardiac muscle in terms of the opposing influences of the sympathetic and
parasympathetic branches of the autonomic nervous system. Whereas activity in one
branch would tend to cause contraction of smooth muscle, activity in the other would
tend to cause relaxation of smooth muscle.
In the case of skeletal muscle, there is only one type of neural input, that of excitation.
However, antagonism to this is achieved in two ways. First, there is both excitation and
inhibition of the motor neurons that innervate skeletal muscle (see Figure 10.11).
Contraction is induced by excitation of motor neurons, whereas inhibition of the motor
neurons causes relaxation. The other type of antagonism in skeletal muscle control is that
between paired extensor and flexor muscles (Figure 10.13).
Question 10.7
(a) In Figure 10.1, through the box marked 'neuromuscular system', commands to action
are effected as behaviour. An important part of executing action is the control exerted
over working muscle fibres, the extrafusal fibres, by means of alpha motor neurons. Note
that, as shown here, the 'state of muscle' is information fed back to perceptual systems.
This derives from muscle spindles and sensory neurons that detect proprioceptive
information. (Though the section focused on the use to which this information is put in
the calculation of local actions at a spinal level, this information is also exploited at
higher levels of the system).
(b) Ian Waterman suffered damage to the myelinated axons that carry proprioceptive
information. Their input would normally derive from muscle spindles.
Question 10.8
See Figure 10.19. The delay is partly the time taken for action potentials to travel along
the axons of the sensory (nociceptive) neuron, motor neuron and 2 interneurons. It is also
the synaptic delay at 3 neuron-neuron synapses and one neuron-muscle synapse.
Question 10.9
If the system is disturbed from the value set, it automatically tends to resist the
disturbance, the defining feature of a negative feedback system. In more physiological
terms, information on muscle stretch is fed back to the spinal cord, where compensatory
alterations in the output to the motor neuron are produced.
Question 10.10
Let us first consider the situation shown in Figure 10.20a. A weight unexpectedly lands
on the arm and displaces it through angle This decreases the contraction in the arm's
biceps muscle (Figure 10.13). In terms of Figure 10.11, the increased stretch of a biceps
muscle would trigger an increased activity in a sensory neuron such as that shown
synapsing upon neuron B. This would increase the activity in B, which would increase
the degree of contraction of the biceps muscle. This would go some way towards
decreasing angle , and thereby restoring the previous arm position. Control through
descending pathways from the brain would complete the restoration.
Now let us consider the situation shown in part (b). A command is sent from the brain to
alter the angle by the amount . There is an increase in the strength of the signal sent to
the motor neurons controlling the contraction of the extensor muscle. By means of
inhibition, this is accompanied by a decrease in the strength of the signal causing
contraction of the flexor muscle. Corresponding signals are sent in alpha and gamma
motor neurons to extrafusal and intrafusal muscle fibres.
Question 10.11
As described in the present Chapter, L-Dopa owes its efficacy to the fact that it forms
part of the synthetic pathway for dopamine. For it to have an effect, there must be some
DA neurons left in which DA can be synthesized.
Question 10.12
A time of physical exertion is one in which relatively high levels of energy and oxygen
are required by the skeletal muscles. A parallel activation of somatic and autonomic
nervous systems means that delays in getting elevated levels of energy and oxygen to
skeletal muscles are avoided. This is a kind of feedforward system. The alternative
mentioned, for the reaction to respond only as the need arises for increased resources, is a
negative feedback mode of operation. It would suffer from an inevitable time-lag.
Question 10.13
A tract is a pathway of neurons within the CNS. The corticospinal tract is so-called
because the cell bodies that form this tract are located in the cortex and their axons
project down the spinal cord. Other pathways are defined 'by exclusion' as
noncorticospinal since they do not arise in the cortex, i.e. the cell bodies of their neurons
are not located in the cortex. However, they are influenced by cortical neurons and their
axons, of course, project in the spinal cord.
Question 10.14
The motor homunculus (described in Chapter 5, 'The brain') corresponds to the areas of
the body over which some control is exerted by neurons in the associated regions of
motor cortex. Some regions of the body have a relatively large amount of associated
cortex. Regions with a large representation have a fine resolution of motor ability (e.g.
fingers). Such areas are associated with fine-grained control involving the corticospinal
tract.
Question 10.15
Real action would involve some preparation and planning, as well as switching in the
plans and effecting motor control. It might be that once the motor cortex is engaged,
control has gone beyond the mode of planning and is in the 'go' mode. Thinking about an
action could engage all of the planning region but the 'brake is then put on' before the
plans get switched into action by the motor cortex
Question 10.16
Motor skills involve a high speed of action potential transmission over relatively long
distances. For neurons to show such characteristics requires myelination (Chapter 4,
'Neurons') , and so the development of motor skill would be expected to be associated
with the maturation of glial cells and myelination. Myelin provides a wrapping around
axons, which speeds the transmission of action potentials.
COREDCT NUMBERS
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